troubleshooting a 5E3 with distorted sound

[mhraposo]

Hi all,

I’m having a problem in my 5E3. Here is the description:
As turned on, the amp makes several scratches sounds and static, with some pops. After a while, it normalizes operations.
While, the operation is not normal, the sound is distorted, very overdriven and low.

I’m made the following debug operations so far:
- Changed the 5Y3
- New set of JJ 6v6
- Unplugged preamp tubes to check if the problem was on the preamp stage (the sound remains)
- I’ve checked PT voltages and they are ok. A little bit higher due to European power grid, but ok.

When the problem is active, I have strange readings in the plate (pin 3):

Tube 1: 379v
Tube 2: 354v

In the screen (pin 4) both voltages are equal in abnormal operation: 286v

In normal operation both plates have same voltages.

I was considering some sources of the problem:

- Plate resistor. This would not explain the difference in voltages
- Grid resistor. This would make sense since it would induce distinct voltages.
- Caps. The popping sound and resuming of normal operation could indicate a cap problem?
- Coupling cap. This would not explain the different voltage.

Any suggestions or directions that you would like to point me out are most welcome.

Thank you in advance.

[Amp Kat]

Is this a homebrew brand new amp ? Has it ever worked right before ?

[mhraposo]

Is this a homebrew brand new amp ? Has it ever worked right before ?

It is a homebrew amp, but not new. Has been working for more than two years.

[Chuck H]

It seems like when the problem is happening that the preamp tubes aren't drawing proper current. I'll wager there is a ground connection that is failing. Focus on the preamp cathode ground connections first. Don't even bother to test them. Just re-flow all the preamp cathode circuit grounds all the way to the chassis. Then repost back.

[g1]

Chuck, I think he is calling the power tubes V1 and V2, as he mentions screen voltage.

When the problem is active, I have strange readings in the plate (pin 3):

Tube 1: 379v
Tube 2: 354v

In the screen (pin 4) both voltages are equal in abnormal operation: 286v

In normal operation both plates have same voltages

So, in normal operation are both plates at 379V or 354V ?
I'm guessing they are both near the 350V when working normal, and that when the fault occurs, one of the power tubes is not conducting.

[mhraposo]

Chuck, I think he is calling the power tubes V1 and V2, as he mentions screen voltage.


So, in normal operation are both plates at 379V or 354V ?
I'm guessing they are both near the 350V when working normal, and that when the fault occurs, one of the power tubes is not conducting.

Exaclty. In normal operation both plates are at arround 380v. When working in abnormal operation, one of the tubes is not conducting. problem is not on the tubes, since i've replaced them.

[mhraposo]

It seems like when the problem is happening that the preamp tubes aren't drawing proper current. I'll wager there is a ground connection that is failing. Focus on the preamp cathode ground connections first. Don't even bother to test them. Just re-flow all the preamp cathode circuit grounds all the way to the chassis. Then repost back.

chuck, thank you for the reply. But if it is a problem on the preamp, with the pre amp tubes off for troubleshooting, the noise wouls not happen. Or am I wrong? BR

[Amp Kat]

chuck, thank you for the reply. But if it is a problem on the preamp, with the pre amp tubes off for troubleshooting, the noise wouls not happen. Or am I wrong? BR

You know you can run 1 tube at a time and that should give you a good indication if the distortion is indeed coming from the bad tube with the low voltage. Swap tubes and see if it's tube related or circuit related. The bad solder connection could be on the power tube socket and not the preamp or at one of the .1uf coupling caps or 220k resistors or the ground connection to them.

[g1]

Exaclty. In normal operation both plates are at arround 380v. When working in abnormal operation, one of the tubes is not conducting. problem is not on the tubes, since i've replaced them.

If they are both around 380V at the plates, but one goes down to 350V when the fault occurs, then the problem is that one tube is OVER conducting (rather than one tube not conducting). Most likely suspect is probably the coupling cap for that tube.

[Chuck H]

If they are both around 380V at the plates, but one goes down to 350V when the fault occurs, then the problem is that one tube is OVER conducting (rather than one tube not conducting). Most likely suspect is probably the coupling cap for that tube.

+1
Leaky cap possible.

[mhraposo]

If they are both around 380V at the plates, but one goes down to 350V when the fault occurs, then the problem is that one tube is OVER conducting (rather than one tube not conducting). Most likely suspect is probably the coupling cap for that tube.

That was a good clue. In fact with the voltage dropping, it pointed to over conducting on that tube.

I've checked connections with chop sticks and spotted a broken connection on the over conducting tube. It has probably got broken with some tube change movement. Maybe the initial soldering was not solid enough.

For now, it is singing again. I will go through more exhaustive testing during the weekend to try to confirm that this was the problem.

Thank you for the comments, suggestions and ideas. Really appreciated. BR