Weird Problem I can figure out...help

[Contech]

Hello Fellow Techs,

I am in the middle of repairing and upgrading a 67’ Fender Twin Reverb Amp. This poor amp was tortured with multiple mods over the years. I’ve got it back to most of its original specification. I replaced all the filter caps, installed new tubes, re-biased, and replaced few bypass and coupling caps. I’ve now got it working pretty well. However, something just happened and I have the strangest problem that just popped up and I simply can’t figure it out. The problem occurred when I tried to change the input jack on the Vibrato channel.

Essentially, if I plug into the Normal channel, signal still leaks into the circuit when the volume is 100% down. You can hear muffled signal (almost microphonic) ring through the amp. What’s even more odd is that when you turn up the Vibrato Channel volume, it is active on the Normal channel. It’s almost like there is a grounding problem.

I know the signal path well and I’ve looked everywhere. I just can’t find the issue. I need some help. Any ideas? Thanks in advance.

[nickb]

Check the cathode bypass cap that is common to the Normal and Vibrato channel second stage. If that goes bad you'll get crosstalk between the channels

[Contech]

Thanks nickb. I just check both bypass caps and they are good with solid solder joints. I checked all the grounding around the jacks and all seems good. I still get signal bleed coming through the circuit with both volumes down to 0. Very odd. I thought maybe the preamp tube was the problem and switch it out with another new tube and the problem persists. Thanks for your advice. Still looking...

[Dave H]

Thanks nickb. I just check both bypass caps and they are good with solid solder joints. I checked all the grounding around the jacks and all seems good.

Are those the two caps on the input stages? You need to check the cap across the 820R common cathode resistor on the second stages after the volume pots.

[nickb]

Thanks nickb. I just check both bypass caps and they are good with solid solder joints. I checked all the grounding around the jacks and all seems good. I still get signal bleed coming through the circuit with both volumes down to 0. Very odd. I thought maybe the preamp tube was the problem and switch it out with another new tube and the problem persists. Thanks for your advice. Still looking...

It sounds like you checked the power supply bypass caps. I was referring to the common cathode cap:

[Contech]

Hi again nickb. Thanks again for your help. I've checked all the common and independent cathode caps. I've actually replaced the individual bypass caps on V1a & V2a (Input Preamps), I just replaced the common bypass cap on the second preamp stage (V1b and V2b). I also replaced the common bypass cap on V3 (reverb input stage) as well as the common bypass cap on both stages of V4. My problem still exists. I'm chasing electronics....It's gotta be here somewhere. I suspect some kind of grounding problem but I'm open to more thoughts. See yellow highlights on attached scheme.

[Dave H]

"Invalid attachment specified"

[nevetslab]

It sounds like your Ground plane (brass plate where all the control's grounds tie to) isn't making good ground to the system. As nickb pointed out, the common cathode bypass cap for V2, having the 820 ohm cathode resistor & bypass cap, which is common to both channels...that stage is usually where this sort of leakage crops up.

If we all can't help back-seat drive you to the solution, I'm nearby at CenterStaging in Burbank.

[Chuck H]

I'm with Contech on this one. I think it's a failed ground connection. I think this because if it were limited to a single point of interface the volume control should still mute the signal UNLESS it too has a ground fault. So... You're probably looking for a ground fault associated with the preamp.

And, just for comic relief I plucked what MUST be an auto correct error from your original post that implies a a more lurid use of amplifier.

Vibrator Channel

[Contech]

OK gang, thanks for the great input. I'm headed back out to my shop to investigate grounding. Very helpful guys! Stay tuned. I gotta tackle this one....

[Contech]

Viola! That was it! It was a faulty ground issue. All of the grounds for each bypass cap included a buss wire to the brass grounding plate. They all were solid connections. However, there was NOT a good solid ground connection from the brass plate to the chassis. I just added a buss wire from the brass plate to a ground tab near the transform and viola! It worked! The volume pots now drain the audio signal to 0. No guitar bleeding through the circuit...cool!

Thanks so much guys. I'm not that experienced with working on tube amps but I'm pretty familiar with tube amp electronics. Troubleshooting is an art form and experience certainly helps. I really appreciate everyone's advice.

Cheers.

P.S. I plugged in my old Deluxe Reverb today and it too has a small issue with bleed. I'm going to fix that one too!

Thanks again.

[Chuck H]

nevetslab gets the cookie! Though he really should share it with Nick.

That brass grounding plate should probably be grounded near the input. I know it "feels" like a big, industrial looking solder blob should go on the other end, but electronically it should be quieter to ground that plate near the input. Don't ground it in both places though. Just one end or the other.

[nickb]

nevetslab gets the cookie! Though he really should share it with Nick.

That brass grounding plate should probably be grounded near the input. I know it "feels" like a big, industrial looking solder blob should go on the other end, but electronically it should be quieter to ground that plate near the input. Don't ground it in both places though. Just one end or the other.

That's very generous, but undeserved . You called it right yourself above. So enjoy one half of a well passed around cookie.

[Contech]

Yep, I made sure this the brass plate was solidly ground to the chassis down near the input jacks. It's only grounded a one end. It really cleaned this up nicely. Thanks again.