6G2 intermittent power drop when played loudly

[Muttly]

Hello Everyone,

I am attempting to troubleshoot a friend's 1962 Fender 6G2 Brown Princeton.

There has been some previous work done to the amp including a replacement CE cap can. The factory stock replacement should be a three section 30uf-30uf-30uf. Previous tech installed a CE 40-20-20-20.

The amp also has a recent replacement speaker.

The amp basically works well when played with the volume set at about 6 and below. When turned up to 7 or higher, there seems to be an intermittent power drop almost as if the power switch was shut off for a second.

The problem seems to only happen with a high input signal...In other words, even if the amp volume pot set to 10, if the guitar volume is set low, the amp works fine. The problem arises with a strong signal going into the amp.

Also, if a booster is used while the amp volume is set low, the problem re-appears.

I replaced the rectifier with a known to be working well old American 5y3 and replaced the 6v6 tubes with a matched pair and biased to .023A with 370V at the plates.

I also noticed some Blobby looking solder joints at the B+ dropping resistors, even though they measured Ok, I replaced them with new 2 watt metal oxides.

There was a previous modification consisting of a 47uf cap paralleled to the 40uf section of filter can.

I removed that cap because I've read that 40uf is the Max recommended first filter on a 5y3.

I am now guessing that may have been a previous tech suspected that the 40uf can section was no good?

Can anyone give me some ideas on what to check next?

Thanks,
Chris

[sgelectric]

https://el34world.com/charts/Schemat..._6g2_schem.pdf

[Muttly]

https://el34world.com/charts/Schemat..._6g2_schem.pdf

Thanks for the Schematic.
I 've been doing some more trouble shooting. I swapped out all tubes, re-checked bias, and I made up a filter cap array with three 22uf filter caps grounded to the power transformer bolts, and completely bypassed the existing filter cap can, as I was "suspicious" of that cap can.

Still having the issue.

I happen to have a "spare" Princeton Reverb output transformer with the correct part# , but the wrong date code.

I think that I'll try swapping out the OT as a test. After that I'm afraid that the power transformer might be at fault.

[Helmholtz]

Whenever there is an intermittent drop out, it is most likely related to a contact problem (contact resistance changing by vibration). So look for broken solder joints, bad tube pin to socket contacts and the like. Don't forget the speaker wiring and jack.
No reason to suspect caps and transformers.

[Jazz P Bass]

A few voltage measurements would help narrow it down.

I see none posted.

[Muttly]

Whenever there is an intermittent drop out, it is most likely related to a contact problem (contact resistance changing by vibration). So look for broken solder joints, bad tube pin to socket contacts and the like. Don't forget the speaker wiring and jack.
No reason to suspect caps and transformers.

You are correct Sir!
I was barking up the wrong tree. I had directed my attention to something in the power supply because when I opened up the amp I found a 47uf cap paralleled to the 40uf section of filter can.

From what I've read, that could damage a 5y3 rectifier tube. I wondered if the rectifier was intermittent due to some kind of damage caused by the 87uf 1st filter.

I removed the 47uf cap and replaced the rectifier tube with an old American made 5y3.

The tubes were also biased pretty cold at about .012A.

Anyway after trying a few things, I just started looking at some of the previous work that was done to it such as Orange Drop caps of non-factory-stock value, a few resistors replaced with beige carbon films, and the output jack wiring looked crappy.

I was shaking the output jack wiring with a chop stick, and it wasn't making any noises so I figured it's solid, but crappy looking.

I checked my box of stuff and found a couple Blue Molded caps of the correct value and a few carbon comp resistors in order to at least keep the original appearance of the 1962 Princeton kind of correct...and while I was at it, I re-did the output jack wiring.

I didn't expect to fix the power drop outs because the amp sounded good at quieter non -distorted volumes.

Anyway, I'm not sure which solder joint may have been the culprit, but it looks like I accidentally fixed the amp.

voltages to Ground...

B+ 375
screens 368
3rd filter cap 333
6v6 plates 371
negative bias volts -32

voltage drop across OT primary windings (155 ohms and 156 ohms)
blue 3.6v
brown 3.32v

I figure 21 mA and 23 max at idle...about 8 watts and 8.5W

Well, that's my story

[trobbins]

That symptom is consistent with the start of internal arcing in the OT - so along with all the other possible causes, imho I wouldn't rule it out.

[Helmholtz]

That symptom is consistent with the start of internal arcing in the OT - so along with all the other possible causes, imho I wouldn't rule it out.

Very good point, makes a lot of sense.
I only thought of mechanical/vibrational reasons.
But high ouput means large OT primary voltages and thus arcing is a possibility, too.