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Post by gtrnstuff on Dec 2, 2008 10:21:17 GMT -7
Hi all. After searching "bias" I didn't find my answer. Will using a bias measurement adaptor such as bias probe, bias rite, etc provide the same info as the shunt method?
Is it an accurate way to bias my Z-28?
I have a Z-28 biased conservatively, I thought,at 25ma, by raising the voltage at the red wire to 406. Yet my JJ 6V6s don't last more than a few months before developing red plating or failure. I'm using the formula 12w/406v=29ma. Measuring plate voltage on the red wire on the cap can, right?
Thanks in advance. Paul
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Post by smolder on Dec 2, 2008 10:50:10 GMT -7
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Post by gtrnstuff on Dec 2, 2008 12:15:09 GMT -7
Thanks, but that's where I started, and that's what my unanswered questions refer to.
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Post by smolder on Dec 2, 2008 12:38:22 GMT -7
doh! All these months and I just found that page yesterday... but I never had an amp that required bias until this week.
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Post by arniez on Dec 2, 2008 13:08:07 GMT -7
I think the Z28 was designed to run the plates around 370. I have no expertise on this but 25ma sounds a little high. Try running em around 22ma. ArnieZ
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Post by Curt on Dec 2, 2008 16:10:30 GMT -7
For safety and convienience I highly recommend the dual socket probe from Amp-Head;
amp-head.com
Quality product, fast shipping, great prices on tubes as well.
Your wall voltage will have an effect on your plate voltage therefore bias setting, as will/should your ears, if I recall 20 to 26 should be a manageable range for most 6V6's.
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Post by gtrnstuff on Dec 2, 2008 17:40:21 GMT -7
Thanks for the replies. I'm still waiting for the 'expert confirmation' that tells me these bias tools (I'm using two Weber Bias Rite adaptors and two DVMs) are telling me the truth with this amp. I'm no designer/engineer/real tech, but this is the only fixed bias amp I've worked on where changing the plate voltage is the method of adjusting the bias. I know GT 6V6 #5 are supposed to run well with the Z28 at 375v. I can't always find those, and wanted to try some other brands. I assumed I could bias for current draw. Then, using the formula in my OP, re-check plate voltage, and make sure I'm within spec for a 6V6. Maybe I just got lucky and found a couple bad sets of JJs, but they are supposed to be tough, aren't they?
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Post by myles on Dec 2, 2008 17:46:53 GMT -7
The Z-28 can have a B+ (plate voltage) above 375.
Set at 22mA and then measure the plate voltage off pin 3 of one of the output tube sockets with the amp connected to a speaker, volume all the way off, nothing in the input jack and the amp out of standby ready to play.
Then set at 26mA and measure again.
Then post the two measurements here at 22 and 26 mA so I can see what is going on with the power supply.
Once I know what is going on I can give you a nice range.
DO NOT adjust the bias to drop the voltage to 375 as in many cases that is way too hot. Use a bias probe.
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Post by myles on Dec 2, 2008 17:51:32 GMT -7
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Post by Curt on Dec 2, 2008 18:03:05 GMT -7
Thanks for the replies. I'm still waiting for the 'expert confirmation' that tells me these bias tools (I'm using two Weber Bias Rite adaptors and two DVMs) are telling me the truth with this amp. [glow=red,2,300]I'm no designer/engineer/real tech[/glow], but this is the only fixed bias amp I've worked on where changing the plate voltage is the method of adjusting the bias. I know GT 6V6 #5 are supposed to run well with the Z28 at 375v. I can't always find those, and wanted to try some other brands. I assumed I could bias for current draw. Then, using the formula in my OP, re-check plate voltage, and make sure I'm within spec for a 6V6. Maybe I just got lucky and found a couple bad sets of JJs, but they are supposed to be tough, aren't they? Exactly, so stay away from the shunt method, go with a bias tool. They are telling you the truth as long as you are following directions and have your meter set properly.
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Post by Phil (aka Phil) on Dec 2, 2008 18:55:47 GMT -7
In addition to the excellent advice from Curt and Myles, I'll just add that the shunt method is dangerous from the standpoint that when you put your mulitmeter in "ammeter" mode to do a shunt reading, it's essentially a straight piece of wire, *not* the same thing as when you're taking a voltage reading *across* a component. That means if you touch the wrong things while attempting a shunt reading, you're *shorting something* that might not take kindly to being shorted. If you're lucky all that will happen is that the fuse in your meter will blow... if it's a good meter with fuse protection.
I've got both the Amp Head unit that I use with my Fluke meter and a Weber Bias Rite.
Also, when you use the "bias probes" from any manufacturer, remember that you're not just measuring cathode current, you're also measuring screen current, which could be as much as 5% of the reading. It's usually a couple of mA. For that reason I usually calculate screen current and figure that into my bias setting, or just add a couple of mA and tweak by ear for best results if I'm in a hurry or lazy ;D
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Post by gtrnstuff on Dec 2, 2008 20:19:50 GMT -7
Not too pricey, considering what I've spent on short lived tubes in the past few years. Thanks Myles!
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Post by gtrnstuff on Dec 2, 2008 20:25:32 GMT -7
Thanks everybody. Myles, I won't be able to get it back on the bench for a few days. I am curious to know more. Right now I've got a pair of NOS JAN Phillips running at 20 and 23 with 'red wire' reading 406v. Seems happy.
Phil--yeah, the shunt method scares me a little. I'm careful, but why take chances? And I forgot about screen current.
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Post by myles on Dec 3, 2008 9:46:03 GMT -7
Thanks everybody. Myles, I won't be able to get it back on the bench for a few days. I am curious to know more. Right now I've got a pair of NOS JAN Phillips running at 20 and 23 with 'red wire' reading 406v. Seems happy. Phil--yeah, the shunt method scares me a little. I'm careful, but why take chances? And I forgot about screen current. Do not do that red wire thing ... measure off the output tube socket ... pin 3.
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Post by Phil (aka Phil) on Dec 3, 2008 18:10:45 GMT -7
Thanks everybody. Myles, I won't be able to get it back on the bench for a few days. I am curious to know more. Right now I've got a pair of NOS JAN Phillips running at 20 and 23 with 'red wire' reading 406v. Seems happy. Phil--yeah, the shunt method scares me a little. I'm careful, but why take chances? And I forgot about screen current. Do not do that red wire thing ... measure off the output tube socket ... pin 3. Cut the red wire!!! No, the blue wire! Are you sure?? Of course I'm sure!! Ok, cutting the blue wire!! No wait!!!! BOOM
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Post by gtrnstuff on Dec 4, 2008 9:21:22 GMT -7
The Z-28 can have a B+ (plate voltage) above 375. Set at 22mA and then measure the plate voltage off pin 3 of one of the output tube sockets with the amp connected to a speaker, volume all the way off, nothing in the input jack and the amp out of standby ready to play. Then set at 26mA and measure again. Then post the two measurements here at 22 and 26 mA so I can see what is going on with the power supply. Once I know what is going on I can give you a nice range. DO NOT adjust the bias to drop the voltage to 375 as in many cases that is way too hot. Use a bias probe. Myles, Thanks for your help. OK, here's what I measured today. AC wall voltage 124.7. With the JAN Phillips 6V6 pair, measuring one tube I got: at 22ma, pin 3 is 406vdc at 26ma, pin 3 is 402vdc I used it yesterday for about 6 hours with the hotter tube running 23ma, the cooler one at 21 and it seems happy, sounds good. And these tubes were in my Deluxe Reverb for about 4 years. I'm tempted to leave them in until it sounds too mushy or one of them fails. Thanks again, Paul
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Post by myles on Dec 4, 2008 9:51:36 GMT -7
The Z-28 can have a B+ (plate voltage) above 375. Set at 22mA and then measure the plate voltage off pin 3 of one of the output tube sockets with the amp connected to a speaker, volume all the way off, nothing in the input jack and the amp out of standby ready to play. Then set at 26mA and measure again. Then post the two measurements here at 22 and 26 mA so I can see what is going on with the power supply. Once I know what is going on I can give you a nice range. DO NOT adjust the bias to drop the voltage to 375 as in many cases that is way too hot. Use a bias probe. Myles, Thanks for your help. OK, here's what I measured today. AC wall voltage 124.7. With the JAN Phillips 6V6 pair, measuring one tube I got: at 22ma, pin 3 is 406vdc at 26ma, pin 3 is 402vdc I used it yesterday for about 6 hours with the hotter tube running 23ma, the cooler one at 21 and it seems happy, sounds good. And these tubes were in my Deluxe Reverb for about 4 years. I'm tempted to leave them in until it sounds too mushy or one of them fails. Thanks again, Paul Paul, Here is what you want.... 405 V 14 W 40% 50% 60% 70% 80% 14 17 21 24 28 You want to set things between 21-24mA. 26mA is getting close to class A ... too hot, shorter tube life, harsh and sterile. The amp will also not sound as "big" at low and medium levels. Sort of like how a car would run if the idle was set too high when you tried to drive 50mph and the idle was set at 5,000rpm. Below 20mA or so the amp would run cold, sound grainy and not develop full power. Using the shunt method (which I hate as all tubes have different draw on the power supply even if they are the same "rating" or tubes with the same test specs) can vary all over the place. To drag the power supply down 25+ more volts to get that "375" number you would have to bias at something over 30mA and that would be just awful.
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Post by gtrnstuff on Dec 4, 2008 10:20:18 GMT -7
Thanks Myles. I guess my gut and my ears were telling me right, then:-) Thanks for giving me the math to support it. So 6V6 is a 14 watt max dissipation? Somewhere along the way I got 12.
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Post by myles on Dec 4, 2008 10:43:56 GMT -7
Thanks Myles. I guess my gut and my ears were telling me right, then:-) Thanks for giving me the math to support it. So 6V6 is a 14 watt max dissipation? Somewhere along the way I got 12. The spec varied between manufacturers a bit but the standard spec is 14 watts today for most of the newer tubes. Many folks list the 6V6GT as a 12 watt tube as in the book standard: Maximum Ratings (Design Center Values) PentodePlate Voltage ................................. 315 V Grid No. 2 Voltage ............................ 285 V Plate Dissipation ............................. 12 W Grid No. 2 Dissipation ........................ 2 W If you add the plate and grid wattage you get 14 watts as many folks quote. As a side note, you can see that most amps today run these beyond design voltage maximums Many Fender BF deluxe reverbs have B+ above 425v and some Jim Kelly amps used to run over 500v but kept the grid voltages much lower.
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Post by Deleted on Dec 7, 2008 20:25:04 GMT -7
I have Hytrons in my '73 Deluxe Reverb. Great sounding tube. I threw a pair in my Z28 last night, but the bias was way too low (around 17 mA according to my Bias Rite). I didn't feel like taking the chassis out to adjust the bias, but perhaps I should based on your post! I also put in a Copper Cap based on your recommendations. The Z28 is definitely my go-to amp. Ches
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Post by myles on Dec 8, 2008 10:03:17 GMT -7
I have Hytrons in my '73 Deluxe Reverb. Great sounding tube. I threw a pair in my Z28 last night, but the bias was way too low (around 17 mA according to my Bias Rite). I didn't feel like taking the chassis out to adjust the bias, but perhaps I should based on your post! I also put in a Copper Cap based on your recommendations. The Z28 is definitely my go-to amp. Ches You may have a decent set of tubes here but they may just be a very weak set. If you could send them back to the vendor and state they will not bias properly and are weak and ask for a set that meets design spec maybe they will honor the request. First try adjust the bias to get into the proper range. At this setting the amp would sound and feel pretty bad but on the flip side, the tubes would last forever and if your hands were used to heat they probably would not even burn you as they would be running so cold.
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Post by Deleted on Dec 8, 2008 14:47:04 GMT -7
I believe I bought them as a matched quad from Mike at KCA several years ago so I could carry the second pair as a back up. I biased them properly in my Deluxe Reverb and they sound great.
I'll either put this pair in the Z28 and get the bias set properly, or I'll just buy another quad from Mike, bias them for the Hytron's, and have a spare pair for both amps. They're great sounding tubes!
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