5e3 noise troubleshoot

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5e3 noise troubleshoot

Postby nbn792 on Tue Mar 10, 2009 3:01 pm

Hi I have a stock 5e3 circuit running canadian rca 6v6, rca ax7, and ge ay7. The normal channel sounds spectacular and I have been playing it for about a year. My bright channel is unusable. please help.

The noise occurs with volume about halfway up or higher. Higher tone settings cause the noise at lower volumes. It does not make the noise unless you play. Amp dimed no input is pretty quiet. If I play on the bright channel I get a loud ugly distortion that comes in and out along with the volume of the note. It is really horrendous.

I have rewired the jacks, this time using shielded cable straight to the tube. Swapped preamp tubes. Tried different speaker cabs. Installed a new set of sprague filter caps. Checked coupling caps for leaking dc on grids.

I was trying to narrow down what section of amp its coming from, but bc I need to play to make the noise pulling tubes and grounding grids isnt really telling me much. It does sound assymetric, but I am unfamiliar with how this PI system works.

The only variation from a stock circuit is that I added a ~230 ohm resistor in series between the power tube cathode and the stock 250ohm/25uf cathode bypass in order to bias my tubes. I don't know if this could cause my problem.

Any help is appreciated, I have two other amps in waiting! :)
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Re: 5e3 noise troubleshoot

Postby NatStrat79 on Tue Mar 10, 2009 3:38 pm

Since you said that the normal channel works fine, I am going to assume for now that the problem is specifically in the bright channel itself. If there was a problem with the phase inverter it would also affect the normal channel as the two channels are blended together before the phase inverter. I would first double check all of the wiring, capacitor values, and resistor values. In the bright channel make doubly sure that you didn't make any mistakes putting the amp together. Then if you have a multimeter maybe think about posting some voltage readings. It sounds like you have some sort of parasitic oscillation to me, could be caused by a number of things but since I get the idea from your post that this is a homebrew amp I think its best to check for mistakes in wiring first and also make sure you have neat and tidy wiring. Poor lead dress could also be the problem. Make sure that any wires that cross, cross at 90 degree angles and try to keep signal wires away from others.
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Re: 5e3 noise troubleshoot

Postby nbn792 on Tue Mar 10, 2009 9:28 pm

yes it is homebrew. I can post some voltages later, but last I checked they are all good, otherwise I would not be using the amp. I will check over things when I get home from work and post. I do have an oscilloscope as well as a dmm, and was using the oscope today to peek after the coupling caps following v1. the bright channel did show some odd things happening at frequencies much higher then the guitar signal. Unfortunately I dont have a sig generator for clean listening to be sure this is oscillation.

thx
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Re: 5e3 noise troubleshoot

Postby Watto on Tue Mar 10, 2009 11:23 pm

Hi There
I have built a few deluxes and I occasionally found some oscillation in the Bright channel. I ended up designing my own board to get rid of the problem.

If you have checked all voltages and made sure all components are correct, then I'd try a couple of things :-
*I'd change the volume pot out on the offending channel to eliminate that as a problem.
*when you built it did you put the 68k resistors of the inputs on the board or are they wired directly to the jacks (I always wire to the jacks)
*with the shielded cable, is the input jack end of the shielding sealed and not touching anything, and the tube end of the shielding attached to the cathode
( I made a mistake once of letting one stray strand of the shielding touch another pin :roll: that's another story)
*did you add shielded cable between pin 2 of V2 and the volume pot. (again attaching the shielding to the cathode)
* one mod to help eliminate oscilation is to totally remove the 0.022 cap off the board and wire it directly between pins 1 and 7 of the PI V2. you'll have to remove the wire that was under the board originally attached between this cap and the 1 meg resistor on pin 7 (hope this makes sense) what you are doing is removing a few inches of wire from the circuit which can cause oscillation, you are also moving the position of the PI cap which is originally close to and running in the same direction as the bright channel cap. It's hard to move the bright channel cap so it's easier to move to PI cap.
* the final oscillation fix, and may not be needed after the previous mod. But the .1 cap coming off pin 6 of V2 needs to be turned 90 degrees to the way it is currently sitting. I actually build my boards to accommodate this but you can use some wire to extend one end of the cap and turn it 90 degrees. I have done this to many customers boutique deluxes to fix the problem.

Not having the amp in front of me is difficult, but I do all these mods as a rule and haven't had oscillation problems.
I hope this info is clear enough for you, I'm not good at describing stuff.

Cheers
Watto
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Re: 5e3 noise troubleshoot

Postby nbn792 on Wed Mar 11, 2009 1:47 am

the 68k resistors are combined and soldered directly to v1

I spent about an hour moving wires around with plastic kntting needle, but no luck.

I swapped in a pair of known good 6l6s. Same.

I changed the wire from the bright chan coupling cap to vol pot into shielded cable. Same.

I removed the wire for the PI cap and wired it directly from v2 to the 1 meg junction. next time I will be moving that other cap. at this rate I might as well rebuild the thing. Same,

I did notice that you mention your shields are grounded to the cathodes.... is there a reason you dont ground to chassis?

When I ground out my grid of v3 i get a big volume drop of guitar and very very little of this problem noise. When I ground out grid of v4 i get an increase in hum, about the same amount of problem noise, and only a slight volume drop of guitar. in my past experience, I should get similar resluts by grounding either grid... so I am looking at my PI section. I will try and post voltages soon. thanks.
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Re: 5e3 noise troubleshoot

Postby Watto on Wed Mar 11, 2009 2:16 am

have you got the V3 and V4 1.5k grid resistors with the shortest amount of lead on the pin. I assume you are using pin 6 as a terminal, I always try and have almost zero wire between pin 5 and the resistor,
then there's the old dry solder joint but I'm sure you'd have found that by now . I always look at each solder joint with a magnifying class either way, just for a visual check.
as an experiment you may want to swap the brown and blue wires of the OT, it may or may not work. It has for me before.

I'll keep trying to think of what else it could be. but I think you're right in narrowing it down to the PI

the shields grounding to the cathode is a trick I read in an article or forum post from Bruce Collins of Mission Amps, it makes it easier to install for one as the pins are next to each other. I'll try and find out article/post I read and the exact reasoning behind it, I wouldn't want to give you the wrong info :thumbsup:

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