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Post by John on Mar 2, 2006 12:22:27 GMT -7
Myles,
My Route 66 just arrived (ebay purchase) The previous owner took the tubes out for shipping. But I don't know which socket is for the ef86 and which is the 12ax7.
Help! I'm dyin' to try it out, but I can't!
As you've said in the past, the tube closest to the power tubes is usually the phase inverter (in this case the 12ax7), but at the risk of frying anything, I think I'll wait.
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Post by John on Mar 2, 2006 12:34:09 GMT -7
The Dr. himself returned my call about 15 minutes after I left a voice message.
Just to let everyone know, that he gave good customer service. Especially considering I didn't purchase the amp new. He even took the time to give me some pointers on the amp.
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Post by myles on Mar 2, 2006 13:59:44 GMT -7
Myles, My Route 66 just arrived (ebay purchase) The previous owner took the tubes out for shipping. But I don't know which socket is for the ef86 and which is the 12ax7. Help! I'm dyin' to try it out, but I can't! As you've said in the past, the tube closest to the power tubes is usually the phase inverter (in this case the 12ax7), but at the risk of frying anything, I think I'll wait. V1 is most close to the input jack ... most far from the power tubes. That is where the EF86 goes. V2 is the 12AX7 phase inverter. Happy playing! |
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Post by myles on Mar 2, 2006 14:01:09 GMT -7
The Dr. himself returned my call about 15 minutes after I left a voice message. Just to let everyone know, that he gave good customer service. Especially considering I didn't purchase the amp new. He even took the time to give me some pointers on the amp. When the good Doctor is not flooded and has some free time he will always take extra time for pointers and hints. Try to get that sort of service from most other folks. We can only wish it was that way. |
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Post by John on Mar 3, 2006 8:34:52 GMT -7
Myles, does a Route 66 (or any other amp) have to be rebiased when a new GZ34 is installed? I read in one of these threads that it does, but I've never heard of it before. (not rebiased for the power tubes, but rebiased to match the GZ34)
I'm searching for solutions to make my 66 sound better (see posting in Route 66 section). I received it yesterday and I'm very disappointed in the amp. I've got a new GZ34 here, and new KT-66's with a matched phase inverter arriving this afternoon.
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Post by myles on Mar 3, 2006 13:01:48 GMT -7
Myles, does a Route 66 (or any other amp) have to be rebiased when a new GZ34 is installed? I read in one of these threads that it does, but I've never heard of it before. (not rebiased for the power tubes, but rebiased to match the GZ34) I'm searching for solutions to make my 66 sound better (see posting in Route 66 section). I received it yesterday and I'm very disappointed in the amp. I've got a new GZ34 here, and new KT-66's with a matched phase inverter arriving this afternoon. There can be a slight difference in plate voltage but not enough to bother with as the Route 66 sounds great over a very wide bias range and a matter of +/- B+ volts is not going to amount to anything. Make sure the tubes are KT66HP or KT66C in a 5-6 rating. |
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Post by John on Mar 6, 2006 9:19:32 GMT -7
Myles,
What is your opinion of the Weber Biasrite tool? I have a very basic education when it comes to electronics....but I can open up an amp and know the difference between resistors (BBROYGBVGW), capacitors, diodes, transformers and the like. I know how to read a voltmeter. I know to be scared of voltage stored in large capacitors.
I'm getting sick and tired of having to pay money to have my amps biased. The local shop is getting expensive and arrogant. I had to wait three weeks to have repair work done on one of my amps. They neglected to tell me the tech went on vacation. I also have to drive one hour to get there. I'm a repeat customer who has to drive a long way to get there, and when I called to ask if they could quickly bias an amp (5 minutes worth of work) when I pick up the repaired amp, they told me it would be $50 for a 'next on bench' fee. Or, I get to leave it there, wait 3 weeks and pay $80/hr bench time....and still have to drive 2 hours round trip to pick it up.
I'd like to purchase something along the lines of a Weber (or GT) bias tool and make my own adjustments.
Any comments?
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Post by myles on Mar 6, 2006 18:54:32 GMT -7
Myles, What is your opinion of the Weber Biasrite tool? I have a very basic education when it comes to electronics....but I can open up an amp and know the difference between resistors (BBROYGBVGW), capacitors, diodes, transformers and the like. I know how to read a voltmeter. I know to be scared of voltage stored in large capacitors. I'm getting sick and tired of having to pay money to have my amps biased. The local shop is getting expensive and arrogant. I had to wait three weeks to have repair work done on one of my amps. They neglected to tell me the tech went on vacation. I also have to drive one hour to get there. I'm a repeat customer who has to drive a long way to get there, and when I called to ask if they could quickly bias an amp (5 minutes worth of work) when I pick up the repaired amp, they told me it would be $50 for a 'next on bench' fee. Or, I get to leave it there, wait 3 weeks and pay $80/hr bench time....and still have to drive 2 hours round trip to pick it up. I'd like to purchase something along the lines of a Weber (or GT) bias tool and make my own adjustments. Any comments? These are great devices and pay for themselves the first few times you use them. As an example ... Fender blackface amps have the bias pot access under the chassis so you can do a bias job in about 30 seconds. So if you charge somebody $25 bucks to do it ... it is fast and easy for them, it is 1/2 or 1/4 of what a bench charge in L.A. would be, and it is done right. I use the GT tool that does not have an integral meter as it fits in my toolbox more easily and has little to go wrong ... and I already have three meters in my toolbox. They are also cheaper. But .... the Weber is a great unit and having the integral meter can be great for a lot of folks. They all work the same and if you get any of them and want me to step you through it's first use feel free to email me and I will give you my home number so we can walk it through in a late afternoon, evening or weekend. I will also direct you to an area on my website where it will give you all tube types and where the bias should be set for any amp after one knows the plate voltage. |
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Post by John on Mar 7, 2006 7:16:59 GMT -7
I already have a multi-meter, so I'm just looking for the tube attachement. Below is the web page for the Weber products: weberspeakerscom.secure.powweb.com/biasrite/br_page.htmCan you expand upon the need (if any) for the 'V1" option? Since there's only one bias trim pot in the amp, I don't know if dissipation rate is something I can do anything about anyway. I'm not going to be comparing truckloads of tubes, I just want to bias the tubes I have and get back to playing. I'm looking to purchase the BR-AH, the one socket version (8 pin for EL34 and 6L6) and hook it up to my multimeter. Any comments? Do I need anything more? And thanks for the offer for telephone assistance for my first attempt. I'll take you up on this in a couple of weeks when the unit arrives. |
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Post by myles on Mar 7, 2006 11:48:55 GMT -7
I already have a multi-meter, so I'm just looking for the tube attachement. Below is the web page for the Weber products: weberspeakerscom.secure.powweb.com/biasrite/br_page.htmCan you expand upon the need (if any) for the 'V1" option? Since there's only one bias trim pot in the amp, I don't know if dissipation rate is something I can do anything about anyway. I'm not going to be comparing truckloads of tubes, I just want to bias the tubes I have and get back to playing. I'm looking to purchase the BR-AH, the one socket version (8 pin for EL34 and 6L6) and hook it up to my multimeter. Any comments? Do I need anything more? And thanks for the offer for telephone assistance for my first attempt. I'll take you up on this in a couple of weeks when the unit arrives. The unit I use is this: www.groovetubes.com/product.cfm?Product_ID=1191It is small as it does not have a meter. The reason for using matched tubes is so you do not have to look at both of them but then again .... it is great to be able to do so if your tube vendor only tests at one point on the tube. For dynamic matching this is not as much of an issue. The single tube bias tools are easy .... just swap the sockets and measure there too if you want to do that. Maybe not as fast as two leads but we are only talking a few seconds here. The Weber unit for the single socket is really nice ... and that with any meter is all you need. It is much akin to the GT unit and they would work exactly the same. |
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Post by kruzty on Mar 8, 2006 6:54:34 GMT -7
Myles, when I turn on my Carmen Ghia and it isn't warm, one of the EL84's makes a bright flash. I have Mullard 12AT7s that do this all the time and I've heard that is normal for them. Is that something that is "normal" for an EL84 (they are GT EL84S tubes)? If it makes a difference, it is the one closest to the rectifier.
Thanks,
Andy
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Post by myles on Mar 8, 2006 12:51:47 GMT -7
Myles, when I turn on my Carmen Ghia and it isn't warm, one of the EL84's makes a bright flash. I have Mullard 12AT7s that do this all the time and I've heard that is normal for them. Is that something that is "normal" for an EL84 (they are GT EL84S tubes)? If it makes a difference, it is the one closest to the rectifier. Thanks, Andy Andy, That is pretty common and has nothing to do with whether the tube is good or not. In fact .... on some preamp tubes .... especially Telefunkens and Mullards ... there can be such a bright flash on power up that people think they blew up the tube and then panic and call me. I tell them to turn the amp back on and ask .... does it play? They say yes. I say then all is well. |
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Post by samuelson on Mar 10, 2006 0:09:19 GMT -7
a question for Myles. I want to buy a cabinet for my Maz-38, either go with the z-best at $750 or save alot of cash and just get an avatar at $300. Is the Z-best worth that much extra money? same speakers in each. I asked this question at guitar player magazine's forum but I can't find where I asked it now. any input would be welcomed.
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Post by myles on Mar 10, 2006 10:53:26 GMT -7
a question for Myles. I want to buy a cabinet for my Maz-38, either go with the z-best at $750 or save alot of cash and just get an avatar at $300. Is the Z-best worth that much extra money? same speakers in each. I asked this question at guitar player magazine's forum but I can't find where I asked it now. any input would be welcomed. There are a number of aspects to a cabinet. Sound is the main aspect and the Z-Best is a ported cabinet of dimensions that match and balance. But ... CONSTRUCTION is a huge aspect in how the cab holds up, tone and projection (if it is particle board then there can be a lot of issues) and other factors. Years ago I was on a tour when there was an accident off the back end of a truck lift gate. Three 4x12 cabs fell face down. Two of the three cabs had ALL their speakers rip out of the soft baffle board as the mounting screws were done in the common way of doing things. One cab was a Rivera 4z12 cab. Baltic Birch baffle board and t-nutted speaker hardware. All speakers stayed in place and not not even budge. Cheap cabs will rattle and have all sorts of weird resonances come and go. In any case ... as is the general rule in life ... you get what you pay for. If you want a cab that will last for decades it is a lot more cost effective in the long run to buy a better piece of gear and you will be happier each time you play it as it sounds better too. $300 for a cab with a V30 and H-30 seems to be next to impossible .... it is sort of like you bought the raw speakers in their own cardboard boxes and got the cab for free. Sometimes free stuff is worth what you paid for it. Actually this is usually the case. Even if these things were assembled by slave labor in some 5th world country it has to make me wonder if the screws are cast zinc, the jackplate and connector are 10 cent items and the wood? Wow ... what wood? Is it even wood? Even compressed particle board and glue would be more pricy. But to be fair .... I have not heard this cab. It may sound great! But at that cost, that would be more than enough of a factor for me to never take it anywhere or move it from one room to another if when it was first played it had no rattles or buzzes. I would count my blessings and leave it right where it was. |
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Post by fishman on Mar 10, 2006 17:45:24 GMT -7
myles,
what are your recommendations for retubing my MAZ 38 NR 1-12 combo?
Mostly the power tubes ... this is a new amp, made about a month ago or so and just aquired from humbucker music....not that the tubes in it are bad, its just that I think the amp could open up some more....I always played around with tubes on my old fenders....and I found some great sounds that way....
I also have a Tonker on order as I am not a big celestion person, although the stock G12H 70th anniversary sounds pretty good....thanks....mike
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Post by samuelson on Mar 10, 2006 17:53:27 GMT -7
Myles: check out the Avatar web site www.avatarspeakers.com and see the construction methods and such. For an $80 or so upgrade it comes with better wiring, wing nuts and the like.It's almost worth taking a chance on the speaker cabinet and if it's junk then at least I have the vintage 30 speakers to use in other situations. but I'm most likely going to wild west guitar tomorrow morning and getting the z-best cab to go with the maz-38. At 49 years of age I completely understand the quality thing, yet sometimes the deal is too good to pass up. thanks for your input. |
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Post by myles on Mar 10, 2006 20:26:06 GMT -7
myles, what are your recommendations for retubing my MAZ 38 NR 1-12 combo? Mostly the power tubes ... this is a new amp, made about a month ago or so and just aquired from humbucker music....not that the tubes in it are bad, its just that I think the amp could open up some more....I always played around with tubes on my old fenders....and I found some great sounds that way.... I also have a Tonker on order as I am not a big celestion person, although the stock G12H 70th anniversary sounds pretty good....thanks....mike Right off the bat toss in a matched quad of EL84S in a #5 or #6 rating. That is what we use in all my clients Z amps and Matchless amps. After that is done we can talk preamp tubes and phase inverters. |
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Post by myles on Mar 10, 2006 20:28:51 GMT -7
Myles: check out the Avatar web site www.avatarspeakers.com and see the construction methods and such. For an $80 or so upgrade it comes with better wiring, wing nuts and the like.It's almost worth taking a chance on the speaker cabinet and if it's junk then at least I have the vintage 30 speakers to use in other situations. but I'm most likely going to wild west guitar tomorrow morning and getting the z-best cab to go with the maz-38. At 49 years of age I completely understand the quality thing, yet sometimes the deal is too good to pass up. thanks for your input. I need to look into these. Thanks for the link! They actually look great and their description .... well, I can't find any fault with their methods. This may be a great deal that was something I missed. I will look into these cabs. I need to see how they sound too as different construction, baffle board fastening, materials, volume, etc all affect tone. But on paper they look quite nice! |
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Post by samuelson on Mar 11, 2006 18:08:16 GMT -7
Myles: I bought the z-best cabinet today. went to wild west guitars and couldn't help myself it sounded so nice and gave the 1x12 Maz-38 that bigger sound. now my question is this: on the back of the Maz-38 there are outputs for 4,8 and 16, and that is OHM I am presuming. do I plug in the amp speaker into 8 and the cabinet into 4? (the cabinet says 4 ohm in back). Or do I get a y split cable which I bought at Orvac Electronics for about 2 dollars today and plug that into the 4 and then split to the 4 on the cabinet and the amp speaker to the other 4? what is the difference in 4 and 8? is there more power coming from one than another? I have no clue how this ohm thing works. does it matter if the amp speaker is in the 4 or 8. and did the amp come stock plugged into the 4 or 8 ohm? I may have plugged the amp speaker into the wrong one as I was playing around with connecting the amp to my Vox 212 cabinet. I certainly don't want to mess up my amp.
thanks for all your time in answering all of our questions here in this and the other forums.
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Post by myles on Mar 12, 2006 11:25:43 GMT -7
Myles: I bought the z-best cabinet today. went to wild west guitars and couldn't help myself it sounded so nice and gave the 1x12 Maz-38 that bigger sound. now my question is this: on the back of the Maz-38 there are outputs for 4,8 and 16, and that is OHM I am presuming. do I plug in the amp speaker into 8 and the cabinet into 4? (the cabinet says 4 ohm in back). Or do I get a y split cable which I bought at Orvac Electronics for about 2 dollars today and plug that into the 4 and then split to the 4 on the cabinet and the amp speaker to the other 4? what is the difference in 4 and 8? is there more power coming from one than another? I have no clue how this ohm thing works. does it matter if the amp speaker is in the 4 or 8. and did the amp come stock plugged into the 4 or 8 ohm? I may have plugged the amp speaker into the wrong one as I was playing around with connecting the amp to my Vox 212 cabinet. I certainly don't want to mess up my amp. thanks for all your time in answering all of our questions here in this and the other forums. I'd unplug the interal speakers and plug the Z-Best into the 8 ohm tap if the Z best is an 8 ohm cab. If you want to use both at the same time and the internal speakers were plugged into the 8 ohm jack already.... then plug both sets of speakers into a Y adaptor and plug the Y adaptor into the 4 ohm jack. |
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Post by foxx on Mar 13, 2006 22:22:09 GMT -7
Hi Myles, I have a Mazerati 1X12 combo w/ celestion V30. I have owned this 1 year and have changed V1 pre-amp tube, the stock JJ EC83S to a GT12ax7m, left the stock 5751 in and replaced the original el84's with some new GT el84's with a rating of 7.
I noticed a power increase with the new el84's, as suspected. The original tubes had 6 months of home use, and had a bit of a rattle.
The new el84's did not rattle, until I cranked the amp. They rattle at higher volumes but slightly, no big deal. I bought the rating at 7 because that was the only matched quads they had at my Z dealer at the time. I meant to get around to a new 5751 and just never did.
At this time I am only playing at home, I don't need such a loud amp and would like to have it break up a little earlier. I know that I have several options to achieve similar results.
1 lower rating on the el84's
2 use a 12ax7(m) in V2
3 less efficient speaker maybe?
4 airbrake
5 mismatched pairs of el84's? (what affect would that have?)
I mostly am interested in a little more grit not heavy distortion. I have a budda Phatman, a beano boost, and a fuzz to compliment the rati's tone. but I can't really turn my amp up past 1/4 volume at home, what a shame.
What is your suggestion as to get some more grit? Oh, I have a LP style w/ p-90's (not too harsh) and a semi-hollow body w/ humbuckers. I like the tone of both guitars and like the p-90's for rock and the sheraton for clean to mild gain. I just want to use the pedals less often, but it's too clean even at 9:00. this is how the amp is supposed to be I guess. I had a really cheap guitar with harsh p-90's before I changed the tubes, so the 7 rating was welcome. thanks for any input.
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Post by myles on Mar 14, 2006 14:43:44 GMT -7
See in the body of your post ... Hi Myles, I have a Mazerati 1X12 combo w/ celestion V30. I have owned this 1 year and have changed V1 pre-amp tube, the stock JJ EC83S to a GT12ax7m, left the stock 5751 in and replaced the original el84's with some new GT el84's with a rating of 7. ........ 7's will give a lot of headroom. I noticed a power increase with the new el84's, as suspected. The original tubes had 6 months of home use, and had a bit of a rattle. The new el84's did not rattle, until I cranked the amp. They rattle at higher volumes but slightly, no big deal. I bought the rating at 7 because that was the only matched quads they had at my Z dealer at the time. You should ask a dealer to order what you need rather than what they might have.I meant to get around to a new 5751 and just never did. At this time I am only playing at home, I don't need such a loud amp and would like to have it break up a little earlier. I know that I have several options to achieve similar results. 1 lower rating on the el84's This is the way to go ... #2-32 use a 12ax7(m) in V2 That will give you a more agressive amp at all levels and that is not the right direction here.3 less efficient speaker maybe? This will work nicely4 airbrake This will work the best5 mismatched pairs of el84's? (what affect would that have?) This would be awful ... you will loose sustain at all levels and have very fast tube wearI mostly am interested in a little more grit not heavy distortion. I have a budda Phatman, a beano boost, and a fuzz to compliment the rati's tone. but I can't really turn my amp up past 1/4 volume at home, what a shame. What is your suggestion as to get some more grit? Oh, I have a LP style w/ p-90's (not too harsh) and a semi-hollow body w/ humbuckers. I like the tone of both guitars and like the p-90's for rock and the sheraton for clean to mild gain. I just want to use the pedals less often, but it's too clean even at 9:00. this is how the amp is supposed to be I guess. I had a really cheap guitar with harsh p-90's before I changed the tubes, so the 7 rating was welcome. thanks for any input. |
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Post by dickieg on Mar 14, 2006 17:21:57 GMT -7
I have a couple of 7025s -they're supposed to be fantastic. (We fell in love with one when we put it in a Fender Concert in place of one of the 12ax7s) I want to try one or two in my MAzJr. V-1 and V-2 are the candidates - what is the difference between "V-1-1st preamp" and "V-2 tone stack driver?" in terms of what I might hear using a different tube? (did that make any sense?)
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Post by foxx on Mar 14, 2006 18:16:03 GMT -7
Thanks alot Myles +1 to your Karma
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Post by myles on Mar 15, 2006 11:14:43 GMT -7
I have a couple of 7025s -they're supposed to be fantastic. (We fell in love with one when we put it in a Fender Concert in place of one of the 12ax7s) I want to try one or two in my MAzJr. V-1 and V-2 are the candidates - what is the difference between "V-1-1st preamp" and "V-2 tone stack driver?" in terms of what I might hear using a different tube? (did that make any sense?) V1 is the first tone and gain stage and this is where 85% of the tone and gain is developed from tube changes and is where you see the most change on the input section. a long smooth plate 7025 will be brighter than other 12AX7, ECC83 tubes. The other important tube but it effects the back end and how the output section sounds is the phase inverter. |
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Post by myles on Mar 15, 2006 11:15:43 GMT -7
Thanks alot Myles +1 to your Karma Well thank you very much! |
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Post by dickieg on Mar 15, 2006 13:54:24 GMT -7
Thanks Myles! What would be the effect of putting a 7025 in V2? (or both 1 & 2) Also, what does "effects the back end" mean tonally??
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Post by myles on Mar 15, 2006 13:58:02 GMT -7
Thanks Myles! What would be the effect of putting a 7025 in V2? (or both 1 & 2) Also, what does "effects the back end" mean tonally?? A long smooth plate 7025 in V2 when there is already one in V1 will brighten things up a touch more. The "back end" is the output section ... phase inverter and output tubes. I have a post somewhere here and also on my website in regard to how a phase inverter impacts things. |
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Post by myles on Mar 15, 2006 14:01:49 GMT -7
Actually.... here is what I wrote about phase inverters:
The most important tube in your amp? The Phase inverter!
Many people think that V1 (the first gain stage) is the most important tube in an amp. This is true in some cases but not in all cases. V1 (usually the preamp tube closest to the input jack) has the largest impact on your tone and gain but has less impact on your output distortion touch dynamics and output stage distortion than the phase inverter. The phase inverter is generally the preamp tube that is the most close to your output tubes in most amps.
Let’s think about this for a moment. Today’s amps come in many “flavors”. There are three basic amp topologies looking at things from one viewpoint.
• Non Master volume amplifiers
• Master volume amplifiers
• Channel switching amplifiers
In master volume amps we have pre and post phase inverter master volume controls. These work differently but for this piece of writing I will put them in the same master volume category. Rolling down the master does what? It allows the front end to be driven harder and thus we hear our front end distort. At some point we can drive some amps so hard in the front end that the tone becomes so compressed and distorted that even I can sound like a decent player! Your mistakes are covered up in the mush and distortion of ti all. This distortion is passed down the signal chain where it is reproduced and amplified by the output stage of the amp. This has nothing to do with output stage distortion. This type of distortion is not touch sensitive. This type of distortion is not something that most articulate players would favor for a sweet tone, blues tone, or even classic rock tones. This is NOT what people refer to as the “brown sound”.
Channel switching amps. Many of these amps have so much “junk” in the signal path that hearing tube changes in V1 is a pretty hopeless endeavor. When you do hear a change it is because the tube is closer to industry spec than another tube may have been. If you want to hear different great tones from swapping out V1 then listen to the tube under test in a more classic amp design.
Channel switchers continued. I get calls and letters all the time where people have a “bonzo-3 channel gripmaster Mk III recto” or whatever. Many owners of these amps say: “the owner’s manual says that V3 is used for the turbo channel” or whatever. Remember, V1 feeds EVERYTHING else down the line, EVEN IN THESE AMPS. The “turbo channel” generally does not use JUST V3. The chain is fed by V1. V1, even in multi channel amps is still the most important tube in the TONE AND GAIN stage of most amps. If you want to change the ratio of preamp tube distortion to output tube distortion then we change V1 in some cases. Going from a 12AX7 to a 5751 will reduce front end gain. Going down to a 12AY7 will reduce the front end gain further and generally give one more clean headroom.
Back to the phase inverter. Taking a simple classis non-master volume amp (but this is the same for master volume amps as well actually).
I will try to keep things simple here with a few basics. If you have questions on all the complex versions feel free to contact me.
Fender Tolex era amps – These generally used a 12AT7 in the phase inverter.
Marshall type amps – These generally used a 12AX7 in the phase inverter.
There are many other differences in these amps but I will stick to the PI (phase inverter).
Some basic tube facts
• 12AX7
o Has a published spec gain of 100
o Has a publishes spec current output of 1.2 milliamps
• 12AT7
o Has a published spec gain of 60-70
o Has a published spec current output of 10.0 milliamps (ten times that of the 12AX7 as a side note)
As a third example, a 5751 has a gain which is almost identical to the 12AT7 but it’s standing current is 1.0 milliamps (about the same as the 12AX7). But, there is a third factor, transconductance, to be considered. The 5751 has a transconductance of about 1200. A 12AX7 has a “TC” of about 1600 and a 12AT7 has a TC over 5000. These three tubes act quite differently. A 5751 and 12AX7 are much more closely related than the 5751 and 12AT7.
We will stick to the basic 12AX7 and 12AT7.
When you push your amp hard it is not as much the output tubes distorting as it is the phase inverter breaking down and distorting. We are talking output stage distortion here. We are not talking about how you may have messed up the signal with preamp tube distortion and compression already. The phase inverter may be the hardest worked tube in most amps. I cannot begin to count the times when I have found phase inverters that were long past their service life. When you change your output tubes change that phase inverter. At the least change it every other output tube change.
Many folks think that when they want to have their amp have more clean headroom they can insert a 12AT7 in place of the 12AX7. Very true. (By the way, the 12AT7 in a first gain stage is an awful tone generator in a guitar amp. If you want to drop front end gain use a 5751 (gain of 60-70) or a 12AY7 (gain of 40). The 12AY7 was the first gain stage in the classic Fender Tweed Bassman, Deluxe, etc.
Going from a 12AX7 to a 12AT7 in the PI (phase inverter) will yield a change in output tube distortion, touch, and output dynamics in most amps. Is this because the gain is lower in the 12AT7? 10% yes perhaps. The lower gain is a factor but the larger factors are:
• We have almost 10 times the current available to drive the output tubes before the phase inverter starts to break down.
• We have a transconductance of 5500 vs 1600 of the 12AX7. Keeping this simple, it means it takes a lot less input signal for a given output signal.
The output tubes are less important than many folks may think. Think about this. In the Hi-Fi world there have been many amplifier designs. Some such as the Scott and Fisher lines used EL-84 output tubes. McIntosh used 6L6 and 6550 / KT88 tubes. Dynaco and some British amps used EL-34 tubes. All of these amps, when operated at the rated specs generally stated that from 20-20,000 cycles at .5% distortion or less they were considered “flat” by industry spec. The output tube type had very little to do with anything. In guitar amps we purposely push the output tube beyond their design limits to make them distort. The difference between a Svetlana 6L6 and an RCA 6L6 is the difference in the way the tube sounds when it is pushed beyond it’s design limits. Being in the “tube business” this is not a great subject. What I am basically saying here is: Before you go to a pricy output set of tubes and a possible need to rebias the amp think about a simple phase inverter change. There are no amp adjustments necessary when you change the phase inverter.
In real life we rarely get to the point of pushing our output tubes to their limits. Our front end is going into distortion. Our phase inverter is breaking down too. The ratio of this front end distortion to phase inverter drive and breakdown is determined by amplifier design topology. You cannot make a Marshall into a Fender no matter how many people tell you that this can be done. Putting 6L6 tubes in a Marshall will NOT make it sound like a Fender either.
There are many 12AX7 types of tubes available. They are all different even though they are supposed to have the same specs. Even when one looks at the same tube type from the same maker out of the same production run we find HUGE variances. +/- 50% off spec is common. Most 12AX7s today show a current output of 0.6-0.8 milliamps where 1.2 milliamps is expected. You throw a 12AX7 in your PI slot with a 0.8mA output and you are 30% down on what the amp can do right off the bat. Your amp is not as full, tight, responsive, or just plain “powerful” or dynamic. Many of today’s high production amps use the Sovtek 12AX7WA short plate as a generic 12AX7. I have issues with these in the tone and gain stage but staying with the topic of phase inverters, these are just awful (12AX7WA Sovtek) for the most part. In tests these show very low standing current. The Ei long smooth plate also shows low standing current, even though a long plate. Generally, large plate tubes will show higher standing current but this is not always the case. The JJ ECC83S has the highest standing current of any current production tube. The JJ is a short plate tube. The JJ is an exception. You might think the JJ would be a good PI. Not from my personal taste. The way the JJ breaks down is not as musical to my ear in most amps and I do not care for the touch response and dynamics of the tube in the PI position in most amps. What do I like? I like the Sovtek 12AX7LPS and the GT 12AX7M. The GT 12AX7M is also available as a matched phase inverter from the SAG over at GT as the SAG-AX7-MPI. There is also an SAG-AT7-MPI. I have talked about matched phase inverters in other places prior to this piece of writing. The 12AX7M and 12AX7R2 (Sovtek 12AX7LPS) are both long plate designs. In either case I check these for specs because in all cases there is a wide range of variables from tube to tube and run to run in production.
There are a lot of great NOS tubes. They have advantages in the tone and gain stage but they are not as available as production tubes made today. When you are on the road or on tour these are harder to find. In the case of the phase inverter we do not want to stock a bunch of Mullards or Telefunkens to burn up every output tube set change. There are great current production tubes that give us all we can ask for.
Other great phase inverters to consider are the 5751, 12AY7, and 12AU7.
The bottom line here is simple. The phase inverter is one of the most important tubes in your amp and the hardest worked tube in the preamp section of your amp. It is how this tube breaks down that provides your output stage distortion tone, character, and amp feel.
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Post by dickieg on Mar 15, 2006 15:13:24 GMT -7
Wow -I tried to understand but didn't absorb much.
Let me ask this. I'm going to try one of the 7025s in V1 (I'm looking for headroom) SHould I also try one of them in the Phase Inverter ? Which one is that on my Maz Jr Reverb? How should the 7025 sound different from what came with the amp? (Sorry if I sound dumb - I really only know how to work the front of the amp and the guitar! - this is all new)
Thanks so much for your patience and care.
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