redpalting...need suggestions please

[jjman]

Considering that the bias grid voltage is good on all 4 tubes I would carefully try the primary leads switcharoo. But only one red side seems strange if you have PFB. I'm guessing some oscillation is occurring from a lead dress issue.

I recently put shielded grid wires on my '71 Fender. (I had removed the grid-to-ground caps long ago.) Although merely moving them around may have worked, dressing them properly and with shielded wire ended the oscillation that had developed during very loud volume strumming.

Can you post a pic showing the full length of grid wires to the OP tubes?

[paulster]

I would suggest either a leaky coupling cap, so you're offsetting your negative bias voltage with a positive DC voltage, a problem with getting the bias voltage to those tube sockets (splitter resistors intact? no nicks in the grid wire for that side?), or a parasitic oscillation.

If you've got a scope then check for high frequency oscillation above 20kHz and up to around 1MHz.

[guitarsnguns04]

im going to check the caps today when my wifes get back home to watch our little one and I will check all the wires and solder joints again too. If it persists afet this Ill try the primary lead flip flop and see what happens. I do have a scope but I am not really sure how to use it yet. Thanks for all the help everyone. Hopefully I can get this ironed out so I can enjoy this amp

[guitarsnguns04]

I switched the ot leads and she howled so I switched them back. I also lifted the 2 caps in question and read .005vdc across them. I resoldered them. I also switched the polarity on the cap before the presence pot as I did have it orientated backwards. All the voltage read good but I still have +10vdc on the nfb?? Could this be a grounding issue? I am thinking about redressing all my grounds as I am stumped. I looked over the whole amp as well power supply,preamp and bias board etc? I cant figure this one yet

[smokin' tone]

I apologize If I missed something here since I skipped over some of the posts. Does this only happen when it's cranked? Any chance that you have the 82k and 100k plate resistors reversed?

[guitarsnguns04]

it only redplates after I run it hard for about 10-15 minutes. It has positive voltage on the nfb all the time though. Ill double check the resistors in question though. Thanks for the suggestion

[martin manning]

it only redplates after I run it hard for about 10-15 minutes. It has positive voltage on the nfb all the time though. Ill double check the resistors in question though. Thanks for the suggestion

Some thoughts: Where are you measuring positive voltage on the NFB, and how? Is it present with no input signal? DC or AC? If AC, can your DMM measure its frequency?

The voltage at the speaker should be ~zero with no input signal. If it's not, then perhaps there is a HF parasitic oscillation going on? Which pair of output tubes is red-plating? Is it the pair being fed by the non-inverting side of the PI (the side that gets the NFB, which is the pair on the left in the layout)? What happens if you disconnect the NFB?

MPM

[gbruzzi]

I switched the ot leads and she howled so I switched them back. I also lifted the 2 caps in question and read .005vdc across them. I resoldered them. I also switched the polarity on the cap before the presence pot as I did have it orientated backwards. All the voltage read good but I still have +10vdc on the nfb?? Could this be a grounding issue? I am thinking about redressing all my grounds as I am stumped. I looked over the whole amp as well power supply,preamp and bias board etc? I cant figure this one yet

Paper caps sometimes drive you crazy: they look good but are not. The only way is to use an insulation tester or you could replace the suspect one with a good one.
The 10V DC is normal: it means that the phase-splitter's current is 2 mA (if the resistor is 5.000 Ohm . By an oscilloscope you could see it and the AC feedback.

gio

[guitarsnguns04]

I am reading the nfb from ground to the res on the presence pot..10vdc+. The pair of tubes that redplate are the ones closest to the transformer in the layout posted..the right side looking at the layout. I lifted the cap the.01 between the 10k and the 1meg and it tested good with no leakage as well. I also rechecked all the resistors on the entire amp before installation and again today and they all read spot on. The only voltage readings that differ from the express voltage chart are as follows:
v3/pi
pin2=21 should be 28
pin3=31 should be 42
pin7=20 should be 27
pin8=31 should be 42

everything else in the amp is spot on voltage wise and suggestions?

[martin manning]

So those voltages on the PI are all about 25% low... Is the B+ on that node good? What about plate voltages (pins 1 and 6)? Have you tried swapping out the PI tube?

MPM

[guitarsnguns04]

heres the plate voltage for v3/pi
pin 1 is 189v
pin 6 is 195

I have tries a couple of brand new 12ax7s in v3 also with no difference..hmm

[guitarsnguns04]

I think I have the problem solved. I shortened the grids up today and put the resisters back to 100k..I also adding some shielded wire and it seems to be spot on now. I played it for about an hour with my attenuator..hard and didnt get any redplating etc. Must have been a parasitic problem that was inaudible. Ill be playing it alot in the next few days to make sure its fixed. Thanks for all the advice and help!

[Roe]

redplating on v4-5 is a common problem on 100w plexis. lowering the 82k plate resistor often helps

[guitarsnguns04]

thanks roe! any suggestions what value to change it too?

[Roe]

try as low as 65k or 68k.