Tube amp noise diagnosing help!

For those building and repairing amps on their own. Learna and share ideas.

Re: Tube amp noise diagnosing help!

Postby Unit_1 on Tue Dec 31, 2013 10:08 pm

your bias is TOO high!

your power tubes are red plating - they are running too hot and will *blow* if you don't reduce the bias - they should not be glowing red at the bottom of the tube like that. be sure to turn the bias all the way down before turning the amp on, then adjust the bias up until you hit the right milliamps. any questions *ask*!!!

also kinda sounds like a ground loop hum as well

when grounding, it's best to use a star ground. and the order you ground is important. start with the ground from the transformers, then the ground from the power tubes, then the ground from the power filter caps, then the ground from the pots, then the ground from the shielded wires, and finally the ground from the input jacks.

when ever possible twist the wires that have signal in them around each other, and the ground wires around each other, but don't twist a wire that has ground around a wire that has signal. and power around power, but not around signal or ground.

this takes advance planning. see the circuit board at the bottom of this post for a fender 6g9-b where i've twisted the brown and blue wires around each other before every putting a single component on the board.

on the original 6g9-b the pot wires are twisted around each other as well. everywhere you can twist, do it.

if you follow this order you will not have a ground loop hum, if you reverse the order you'll have a nasty ground loop hum! be sure all grounds are 18ga wire so the flow of electrons will be easier to go down to ground than back into the circuit.

as LUC.key said it would be a good time to desolder your components and maybe get a pre-made turret board to solder everything to. a lot of us look at amps very frequently, to the point where we can spot errors in resistor values as long as the resistor is where it's supposed to be layout wise.

otherwise we can't help without having the amp, and a schematic and going over every single resistor on the layout, a very time consuming activity. like we can look at the layout and spot an error in 20 seconds or so, compared to probably five hours.... :shock:

i can understand how you might not want to undo and redo the amp, so before you do that try to run the grounds as i said, be sure to rebias your tubes, reflow the connections on the input jacks as that can be a major source of grounding hum and then see how it sounds.

also be sure to check that the switches on the input jacks are working properly. i got a whole batch of brand new switchcraft jacks once and *every single one of them* the switches needed to be *GENTLY* sanded, ONE pass only, with 600 grit sand paper folded in half so both sides of the switch get sanded at the same time before the switch worked right. you don't want any of the plating on the switch to be removed, so be very careful.

boy was i ticked, but i found it before i used them because i always check my jacks before using to be sure the switches are operating as designed. the switch was so bad it was actually acting like a capacitor, since a capacitor in it's most simple form are two plates of metal close to each other separated by air. as i checked for continuity i watched the dmm acting crazy. i switched to voltage and saw it gaining volts like a capacitor charging and finally figured out the switch contact was not contacting!

if you're still getting that popping run away gating sound, you'll need to shield all of the inputs and outputs from V1 and V2, or use the chopstick method to move the wires around the way i described in my previous post.
GOOD LUCK! and be sure to come back if you have *any* questions about anything!!! :cheers:

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Re: Tube amp noise diagnosing help!

Postby Papa Dog on Wed Jan 01, 2014 9:01 am

izzit just possible that those two in-line fuse holders in the middle of the pre-amp section might be causing some hum?
call me silly or harsh or whatever, but it seems that AC that close to the pre might just be problematical.
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Re: Tube amp noise diagnosing help!

Postby LUC.key on Wed Jan 01, 2014 4:29 pm

Unit_1 wrote:your bias is TOO high!

your power tubes are red plating - they are running too hot and will *blow* if you don't reduce the bias - they should not be glowing red at the bottom of the tube like that. be sure to turn the bias all the way down before turning the amp on, then adjust the bias up until you hit the right milliamps. any questions *ask*!!!


hmm, doesnt look like redplating to me, seems like "normal" glow from the filament wiring inside the tube, camera and the chasis reflection makes it look weird tho
anyway, bias is definitely worth to check!
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Re: Tube amp noise diagnosing help!

Postby Vortexion on Wed Jan 01, 2014 4:59 pm

LUC.key wrote:hmm, doesnt look like redplating to me, seems like "normal" glow from the filament wiring inside the tube, camera and the chasis reflection makes it look weird tho

Agreed. The glow is present before the amp is switched on from standby, and remains the same regardless of what is happening. Apart from that, though, everything Unit_1 and the others have said is correct: your lead dress and layout are such a cat's cradle, it's a miracle that this amp doesn't squeal uncontrollably as soon as it's switched on. :shock:

This is a steep learning curve for you, msca8h, but stick with it - you've come to the right place for advice, and the stuff you learn in the process of solving the problems in this amp will stay with you for life. :thumbsup:
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Re: Tube amp noise diagnosing help!

Postby msca8h on Fri Jan 17, 2014 12:46 pm

Thanks everyone for supporting me!
All of the problems got solved by just moving the wires to the Primary tap of the OT.
It was a rough feedback.

The input jacks were messed up.
I bought new ones and rewired them vintage style and they work perfectly.


However, about bias.
The AC series is fixed cathod bias.
It is fixed.
All of the voltage ratings are ok

http://www.buzzbee.co.kr/shop/goods/goo ... =001002032


as you guys can see here,
The ACs' EL84 were designed to be overbiased.
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Re: Tube amp noise diagnosing help!

Postby gusfinley on Sat Jan 18, 2014 12:24 am

bias terms are pretty confusing!

"Fixed" bias means that a negative voltage is applied to the input grid of the output tubes. This is sometimes also called "adjustable" bias because there is usually a potentiometer in the circuit to adjust the "fixed" voltage. This was not the bias method used on the AC15.

Cathode bias means that a positive voltage is created at the cathode, usually by the current running through a cathode resistor. This resistor is almost always a "fixed" resistor value, athough I have built an amp with an adjustable cathode resistor.

Its more about where the bias voltage is applied than about the ability of the bias voltage to be varied or not....
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