Voltage spike?

[SacDave]

I just joined the forum; I’ve built three 5E3’s and a G15 reverb.  Now I’m having a problem with one my 5E3’s. How it started I was checking the voltages
When I tested the voltage on pin #3 on V3 (6V6) I noticed if I turned the Tone or Volumes past 9 the voltage would spike up to 425VDC, from 0-9 the voltage stays at about 376VDC.  Also I will get a high pitch squeal after 9 on the controls. Squeals only when I’m testing the voltage.   V4 the voltage stays stable, my other two 5E3’s do not have this problem.  So what I’ve tried, multiple sets of tubes (all tubes), replaced the filter caps, tried another or replaced every cap. Replaced most every resistor, replaced the pots, today I wired in a new OP.  I’ve gone over the grounding made some changes.  I also pulled the board and checked the jumpers, checked and rechecked all wiring. So at this point I’m at a complete loss.  The amp still sounds good but with the voltage spike I’m not using it, as for changing some of the components I was going to try deferent types of caps, pots, OP  just to see what the deference’s would be.  So dose anyone has any idea what might be causing the voltage spike?  I plan I building a new turret board and do rebuild on this Amp after I decide on components, I just want to find what’s going on first,

[eleventeen]

I would have to guess you have some sort of oscillation going on. The 5E3 does not have a negative feedback loop so the phasing of your OT should not matter. So how this could be the cause, I have no idea. OTOH....even knowing that it *can't* be the problem....I would flip around either the plate wires or the speaker wires before I ripped the whole darn thing apart/replaced the parts board. Prefer flipping two wires than disconnecting/reconnecting 40-50 wires. 


The way you could know whether the above guess/theory is valid would be to throw a scope on the output (and/or various other places in the  amp) as you place your meter probe on the 6V6 plate terminal.

[SacDave]

I did flip the OP wires, no help I really wasn't expecting  putting a new OP would improve things, I'm just running out of suggestions  heck I even changed the knobs no help.  I don't want to pull the amp apart until I find the problem I'm also waiting on more parts  for when I do rebuild the amp. Every now and then I mess with it trying to find what's going on.  Thanks anyway

[sluckey]

When I tested the voltage on pin #3 on V3 (6V6) I noticed if I turned the Tone or Volumes past 9 the voltage would spike up to 425VDC, from 0-9 the voltage stays at about 376VDC.  Also I will get a high pitch squeal after 9 on the controls. Squeals only when I’m testing the voltage.

I've seen several perfectly healthy amps do some weird squeals and static when probing the plates of the output tubes. Many times it only does it on one of the tubes.

If the amp works fine under normal playing conditions, I would not worry about it. There's really no reason to be probing the output tube plates with volume or tone controls turned up.

[SacDave]

Thanks,  I'm concerned about the Voltage spike more than anything else . I'm pretty new to amp building but I'm guessing the voltage spiking to 425 can't be good for components ?  Being new at this Where do you have the Volume and tone set when testing voltage? My other Amps the voltage is pretty stable regardless of where the controls are set.

[sluckey]

I don't think your voltage "spike" is a jump in dc voltage. I think your meter is seeing that oscillation (squeal) and doesn't know how to interpret it correctly.

Or, it could be that the squeal (HF oscillation) is there even without the probe connected and only becomes audible when you connect the probe. You really need an o'scope to see what's going on.

My controls are always set to max CCW when I'm measuring dc voltages in an amp. Sometimes I even put that note on my schematics.

[SacDave]

That might be the case,  I don't have a scope and really would be lost. At this point i'll go with gremlin. This was my first amp build , it was a kit so I learned a "little " more since my first build  so I want to rebuild this one I like making turret boards, sourcing  my own parts and I have read quiet a bit on grounding  so I want to put my touches to it.  Thanks again I appreciate the help.

[PRR]

When you attach a long probe-wire to the plate, it acts as a "transmitting antenna", spraying more signal around your preampery, encouraging an oscillation.

With many tube stages, a big oscillation rectifies in the grid R-C network, drives the grid negative, cuts-off the tube, "blocking".

If a large power tube blocks to cut-off, current drops from 40mA to few-mA, it is not unlikely a soft power supply will un-sag from 360V to 425V.

[SacDave]

Thanks  PRR ,  Don't quiet understand to much of the technical stuff still learning.  Out of curiosity have you ever seen this happen before ?