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Post by toneseeker on Apr 3, 2006 15:06:04 GMT -7
Hey My Route 66 (s/n 10039) is equipped with a set of GTKT66-HP grade 5 tubes. The recomended center-tap voltage, according the Doctor's bias recommandations, should be app. 475 V for optimum. Mine reads app. 435 V (a bit cold I suppose). My problem is: Where do I locate the mentioned bias trim pot. I can only locate one trim pot which is on the circuit-board. Is this the one to use (I just want to 123% sure)...or have I missed a detail?? Note; my amp has green/white wires going to the center-tap (just a detail I guess...). Please help (Myles?)
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Post by prowler on Apr 3, 2006 15:45:02 GMT -7
I don't have a Rt. 66 but I do have a photo of my SRZ. The bias trim pot is circled. Hope this helps...
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Post by Phil (aka Phil) on Apr 3, 2006 16:30:43 GMT -7
Don't worry about the center tap voltage. You have to have a pretty high voltage coming out of your wall socket to achieve 475V; probably 125 or 126V. I know all about this; I'm an engineer at an electric utility. The "wall" voltage where you live is probably lower; your amp is probably not biased cold. I've never read anywhere close to 475V on my Route 66. The only trim pot in the amp will be for the bias voltage. I would use a Weber Bias Rite or equivalent to measure cathode current. My Route 66 came set at 32mA, which sounds good to me. I had mine at 36mA for a while but I like it a little lower. Anywhere from 28mA to 40mA or so is acceptable but most folks seem to like 32-36mA. I'm not Myles but I've been down this road a few times PDW
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Post by myles on Apr 3, 2006 21:30:57 GMT -7
I prefer 34mA or so and the two Route 66's I had Saturday each as 460b+ off pin 3.
I use a bias tool .... the same method as the Weber tool .... and I prefer this method to the transformer shunt method or any other method.
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Post by toneseeker on Apr 4, 2006 11:53:10 GMT -7
Thanks to all three of you...gona buy a set of bias probes... The output voltage of my step-down trafo reads 115 VAC, so that explains why I only meassure app. 430 V... By the way; is it normal that the EF86 pre-amp tube doesn't glow at all?? nor do it get particular hot (luke warm only...)..??
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Post by myles on May 3, 2006 13:15:04 GMT -7
Thanks to all three of you...gona buy a set of bias probes... The output voltage of my step-down trafo reads 115 VAC, so that explains why I only meassure app. 430 V... By the way; is it normal that the EF86 pre-amp tube doesn't glow at all?? nor do it get particular hot (luke warm only...)..?? Some EF86's have the heaters more encased then others so some will glow more than others. This means nothing. You may be getting low plate voltage because you are biased so hot that the tubes are sucking too much of the power supply and dropping your B+.
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Post by cementman on May 3, 2006 14:44:34 GMT -7
hey myles and also pdw,,, my amp tech is old school, (maybe older than you hah) he solders a temporary resistor at the correct place and biases the amp that way, then after bias he just solders it back off. i have watched him do this a few times. he simply says he can't see paying over a hundred bucks for the bias rite or other. by the way myles i biased my 6545 just as you said, got the sag 12zx7 for the phase inverter and the set of #6's from groove tubes and biased it accordingly to 36 ma and it sounds smooth as silk,,, just as you said. thanks for your kind info and time... have any of you had time to take a look at the mods that dr z has put forth for the maz 38? i want to do one of them but cant decide which one to try first. probably the cut control mod... any suggestions about those mods?
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Post by zpilot on May 12, 2012 23:42:06 GMT -7
I used to have a Sr and I did the mod on the treble control. It tuned it to better suit my ears butt I seldom had an opportunity to really turn that amp up and wound up getting a Jr. The great thing about all of the Doc's mod's is they are cheap and easy to do, they do what he says they will, and if you don't like them you can put the amp back to stock without having damaged the amp. I have an opinion on modding "Z" amps. I have found that a lot of manufacturers engineer their amps and effects to sound good when played alone in a store so you will buy them. The Doc's stuff sounds best when played in a mix, either live or recorded. Among other things I have tried different speakers in my "Z's" and more times than not I have returned to whatever came stock. I hope you give the amp a chance just the way it is at a few performances before you change anything.
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Post by Deleted on May 13, 2012 0:01:51 GMT -7
I turn the pot all the way clock wise so I don't red plate the tubes right off the bat.
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Post by KeithA on May 13, 2012 15:57:22 GMT -7
Hmmmm, I never thought about the adjustable bias on the 66 before I ordered mine. I've been spoiled by my Reinhardts since they have the test points jacks on the outside of the chassis and the adjustment knob on the top of the chassis. It makes a bias adjustment easy and the chassis doesn't need to be pulled. I assume it's not that straight-forward with the 66 Keith
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Post by prspastor on May 13, 2012 18:57:40 GMT -7
Hi Keith, It is not that big of a problem. Really simple with a bias probe and you don't have to do it that often. Those KT66 last a good long time. 4,000 hours I believe... at 5 hours/week, that is 15 1/2 years.
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Post by Deleted on May 13, 2012 20:05:29 GMT -7
Keith, I've heard of guys drilling a hole in the bottom of the head and then plugging it with something easily removed, for access to the pot when they need it.
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Post by KeithA on May 14, 2012 2:56:29 GMT -7
Hi Keith, It is not that big of a problem. Really simple with a bias probe and you don't have to do it that often. Those KT66 last a good long time. 4,000 hours I believe... at 5 hours/week, that is 15 1/2 years. I don't think any tube today will last that long ! It's no biggie, it would just make sense to put the test points on the back of the chassis and an adjustment pot on the top of the chassis so that the bias could be adjusted without pulling the chassis. My Reinhardts are done this way (and Komets are as well) and its a quick check with a standard multimeter to check/set bias. Keith
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Post by prspastor on May 14, 2012 16:04:09 GMT -7
^^^^^ ;D One can always hope and dream.
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Post by Deleted on May 14, 2012 16:27:40 GMT -7
They reissue Gold Lions stayed in my route for quite awhile. I was never in a situation where I could crank the Route, so tubes lasted a good while for me. Removing the chassis wasn't my favorite thing to do. It beat paying someone else to do it. That way I could set the bias where I wanted it and check it with the chassis out.
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Post by pintail78 on May 14, 2012 16:36:38 GMT -7
you can buy generic "bias rite" testers on ebay for 30$ or so they work fine. bias is a pretty ballpark thing anyway, it doesnt have to be perfect but close.
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Post by Phil (aka Phil) on May 14, 2012 16:53:20 GMT -7
The KT66 "thousands of hours" thing was based on Genelex KT-66's installed per their operating specs (4" center-to-center spacing minimum, I believe). When run down at the bias point Z runs 'em thousands of hours on Genelexes are definitely possible. I don't know about the GT HP's that the OP's and my 66 shipped with (I've got #8). They should definitely last A LOT longer than say, JJ EL84's in a MAZ JR or Mazerati GT. I'd hate to put a solid number on it but over a thousand hours wouldn't surprise me. If the 66 in question has a new production 5AR4 I'll bet the recto blows first Not going out on a limb there am I
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Post by prspastor on May 14, 2012 21:30:57 GMT -7
Good to have company out on this limb Phil. ;D
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Post by KeithA on May 15, 2012 3:09:00 GMT -7
Hey, I'm not saying that 'in theory' tubes shouldn't last that long All I'm saying is that tube quality today, in general, seems to be a crap shoot and tubes seem to be giving up the ghost earlier and earlier due to less than stellar build quality, etc. Keith
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Post by Phil (aka Phil) on May 15, 2012 3:51:29 GMT -7
Here's the original GEC KT66 data sheet... "at least 10,000 hours" at the 21W level.
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