Tube Amps / Music Electronics
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|5/25/2000 3:51 PM|
||Filament and bias supply questions...|
Restoring an old Traynor YBA-1.
The filament supply has a virtual ground established by 2, 100 ohm resistors that were fried. Replacing these was painless and they, as would be expected, are not warm to the touch. The filament voltage is a little on the high side at 6.7 volts, but that shouldn't cause the resistors to fail. I would assume that these failed due to a filament shorting to a plate at some point. I can't think of another failure mode that would take out these resistors. Any ideas? Is there something else that I should check?
Also, I need to install an adjustable bias. Does anyone know where I can get the panel mount pots that have a screwdriver adjustment instead of a shaft? I would like to mount this on the back panel of the amp, but I don't want a knob that can get twisted by accident. Any ideas?
Thanks in advance.
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|5/25/2000 4:04 PM|
Doug, I don't know you amp, but I can tell you that the 100 ohms failed in Fenders too. The reason was that they were carbon comp resistors, and they were placed an inch away from the amp's pilot light. So they were exposed to constant heat and would eventually scorch -- they would simply fall apart, turn to crumbs, you know? ) The most any of those resistors could ever see is, subjected to a full 6.3V (which they are not, they each see half of that) would be about .38W. I use 100 ohm 1/2W, flamepoof as replacements in Fenders. Nominally, each resistor dissipates about .1W (3.15V * 3.15V/100 ohm), so 1/2W is plenty of margin.
Perhaps in your amp they are close to a heat source and roasted over time?
What value do you need? Smart Parts sells a 10K bias pot similar to the ones that Fender used in the days of old. Unfortunately, the pots looks extremely cheesy (CTS made in ROC, I believe), and the ones I bought came without a start washer or a nut...
There is something else you can do: get a quality pot, one that you can take apart and put back together without having it fall apart on you (most CTSs fall in this category). It would be great if you could get one with a slot on the back, and CTS's 10KA pots happen to fall in this category (don't worry about the pot being audio VS linear taper, many people have done this trick and it works well). Saw off the pot's shaft flush to the bushing, then take the pot apart and using a file make a groove on the surface of the shaft. That way you have access to the pot both from the top and bottom.
Lastly, Angela sells -- I have heard -- some bias pots that have a slotted shaft.
|5/25/2000 6:32 PM|
I've seen these resistors blow due to a bad power tube. I had a set of Sovtek EL34EH's (brand new) that popped these resistors when a transient (from a guitar cord being pulled out) arced the tubes. I use 2-watt 100 ohms (I use them in another loacation anyway), and they smoked ! I agree under normal circumstances, they should not pop.
Another couple of ideas for the screwdriver adjustable trimmers: Either use a conventional pot with a split shaft (or a solid one, and you cut a slot in it), and mount them on a bracket, so the shaft can be reached by a small hole in the chassis. The other alternative is using multiturn trimmers (Digitkey has them) with an attachment that allows you to mount them through a panel.
|5/25/2000 9:23 PM|
I've tried the multiturn and have glued them to the bottom of the chassis, but that little slot is just hard to hit though a hole. Guess I am looking for a little easier solution. I like the idea of the little clamp to hold the recessed pot. Thanks for the help. Doug.
|5/25/2000 9:15 PM|
I should have mentioned (Dooh!) that the filament wires coming from the last output tube were a little crispy too. I replaced the resistors with 100R 1/2 watters mounted sort of vertical and away from everything else. I'll check out Smart Parts and Angela. I had thought of disassembling a pot, but wasn't sure if that would work. That sounds like a great way to go and I'll certainly try that. Thanks for help.
|5/26/2000 7:42 PM|
sometimes on the power tubes a high voltage arc occurs from the plate pin 3 to the heater pin 2. This can give the heater supply and any components like balancing resistors a little more stress than they might normally be subject to. Always a good idea to check for burned residue on the socket & tube base between these pins.
|5/29/2000 3:05 AM|
||For Pots, Try Newark Electronics|
Newark Electronics has practically any pot for this purpose (or many others) your heart could desire. 10-turn, locking-shaft, screwdriver adjust, conductive plastic, wirewound, cermet, surface-mount, trimmers, etc. They are at www.newark.com or (800) 463-9275.
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