1967 Pro Reverb BF brief oddity after recap

[ETMusic777]

Quick question for the amp pros here. I just finished putting new Caps in the Doghouse, Bias cap, bias resistor, screen resistors, grid stoppers and a new 3 prong cable in to the 67 Pro Reverb blackface. Everything went well and I powered up using an Amp Maniac as a variac to slightly raise the voltage on the new caps to break them in. Everything tested well and sounded great on my bench (aka coffee table) so I buttoned it up and put it back in the amp cabinet.

Here's where the weird thing happened. I plugged in to the "Vibrato" section input 1 and started playing it and noticed that when I turned the knob for the Reverb down, the amp volume got louder when I turned the Reverb knob up..and the amp volume got quieter. NOT just the amount of Reverb in the circuit, but the entire amp volume as well, and it acted in reverse where turning down made it louder and turning up made it quieter 1 became 10 for VOLUME in other words, on the Reverb knob.

So I powered everything down and unhooked the reverb cables in the back and reversed them. I powered it up again and of course I had no reverb function at all at that point. So I put the Reverb cables in the proper RCAs and powered it up and the issue was gone and has not come back. And all is good now and it acts as it should. The Reverb knob only controls the amount of reverb in the circuit as it should...but it was weird for a moment ...any thoughts as to why that happened?

 

[Wally]

You may have a bad ground on one of those cables????

 

[Phrygian77]

How much of a volume change are we talking about here? The reverb control does have an effect on the impedance of the reverb mixing network. Also, the second stage cathode is shared, which causes some interaction. Try playing through the normal channel and crank the reverb up to 10 to see what I mean.

 

[Ricky D.]

Ghost in the machine.

 

[schmee]

I have not had that happen, but will say ; clean the rca jacks , especially the outside of the jack on the amp itself where the "claw" on the cable jack presses on/grips it. I have had amps do weird things from those being dirty.

Are you sure the amp didnt have that issue before you did the cap job?

 

[ETMusic777]

You may have a bad ground on one of those cables????

OK maybe...I did not change anything on the amp except new doghouse caps, bias cap and resistor, screen resistors, grid stoppers and grounded 3 prong. All of that came out great with nice new solder joints. I did not change any grounds except the power cable, which I grounded properly to the lug on the PT. I totally took the 2 prong aux power jack and ground switch out of the loop and wired the new power cord directly to the fuse/powerswitch with the black lead and white lead to PT. Snipped out the death cap etc.

Whatever it was, it was a totally a 100% weird thing. The Reverb knob became the volume knob, but worked in reverse. The entire volume of the amp went up when turning the knob down, and the reverb volume went with it. When turning that knob up, everything went down.

It only happened for that moment in time after powering the amp back up following the service. I should have taken a video of it. No I was not smoking any illegal substances at the time lol.

I played the amp for an hour last night and everything seems fine. The amp is dead quiet now and seems a bit "tighter" since I swapped out the caps. I am going to leave the cathode bypass caps in there for now and not change them because the look great and tested fine. This amp was all original when I got it. I am going to get in there again and check all of the voltages.

Off topic question: I don't like how the fiber board has warped up over time (see photo). I have 6 other BF amps and none of them have this issue to this degree, only this 67 Pro. Does anyone know if that can be straightened out? I suppose I would need to remove all solder joints, heat somehow and press it out which would be a major PITA. Everything fine on the amp but that is the one thing that I wish I could change.

I love this amp! (other than the weird gremlin last night), its very clean but has a nice breakup when cranked. I will post back here if it happens again.

 

[ETMusic777]

I have not had that happen, but will say ; clean the rca jacks , especially the outside of the jack on the amp itself where the "claw" on the cable jack presses on/grips it. I have had amps do weird things from those being dirty.

Are you sure the amp didnt have that issue before you did the cap job?

Nope never had the issue, it was a one time thing. I think you may be on to something regarding those RCAs.

 

[Wally]

So I powered everything down and unhooked the reverb cables in the back and reversed them. I powered it up again and of course I had no reverb function at all at that point. So I put the Reverb cables in the proper RCAs and powered it up and the issue was gone and has not come back. And

Nope never had the issue, it was a one time thing. I think you may be on to something regarding those RCAs.

The first statement above is why I suspected a problem with one or both of those cables. @schmee is suspicious of those connections. I have witnessed problems with those Rac connecti9ns that were due simply to oxidized surfaces. I have seen other situations where there was an issue with the ground connections to the RCA plugs…the reverb could function but there was another problem that was caused by a ground connection problem. One situation was with the Vibroking that the late,great Bugs Henderson owned. The amp was functioning..including the reverb. However, there was a trailing noise on every note. A fair number of techs had not found the problem, and out of frustration Henderson allowed his last bass player to bring the amp home to Lubbock and let me give it a shot. I got lucky. Simple things first, right? I checked the speaker cable…no apparent problem. I then checked the RCA connections for the reverb. Voila….there was a ‘make and break’ problem that necessitated resoldering the ground on one RAC connector. It is nice when one finds simple problems quickly. I checked the amp out, left the bias where it was, and sent it back with my friend.
I suspect that your contact surfaces were oxidized. Good find….

 

[ETMusic777]

Thats good information thanks Wally I will look at the connections and grounds.

 

[Wally]

Thats good information thanks Wally I will look at the connections and grounds.

It would seem that simply unplugging those RCA plugs and reconnecting them cleaned the problem up, it seems to me.

 

[schmee]

.....
I played the amp for an hour last night and everything seems fine. The amp is dead quiet now and seems a bit "tighter" since I swapped out the caps. I am going to leave the cathode bypass caps in there for now and not change them because the look great and tested fine. This amp was all original when I got it. I am going to get in there again and check all of the voltages.
Off topic question: I don't like how the fiber board has warped up over time (see photo). I have 6 other BF amps and none of them have this issue to this degree, only this 67 Pro. Does anyone know if that can be straightened out? I suppose I would need to remove all solder joints, heat somehow and press it out which would be a major PITA. Everything fine on the amp but that is the one thing that I wish I could change.

I love this amp! (other than the weird gremlin last night), its very clean but has a nice breakup when cranked. I will post back here if it happens again.

Just leave the fiber board as is. I have had several that are warped badly. They work just fine. My 71 Princeton I still have is fairly warped.Yours looks mild by comparison!
If the "gremlin" shows up, twist those rca jacks back and forth on the amp end and see if it stops.

 

[RetiredUnit1]

Almost sounds like run away oscillation but it normally doesn't vanish. You didn't move any of the wires running to tubes while you were in there did you?

They need to cross at 90 degree angles as much as possible. I built a 5e3 once that got quieter as you turned the volume up. Hooked it up to the oscilloscope and you could watch the signal change from a wave into a box full of squiggles as the volume went up

Started chop sticking and when I moved one little wire it was gone...

 

[ETMusic777]

Almost sounds like run away oscillation but it normally doesn't vanish. You didn't move any of the wires running to tubes while you were in there did you?

They need to cross at 90 degree angles as much as possible. I built a 5e3 once that got quieter as you turned the volume up. Hooked it up to the oscilloscope and you could watch the signal change from a wave into a box full of squiggles as the volume went up

Started chop sticking and when I moved one little wire it was gone...

Thanks for the response and good suggestion. No, I did not move any wires away other than the power cord wires when I took it to a grounded 3 prong. I've run the amp for several hours over the past 3 days and its been fine with no problems. One additional thing that I did was I swapped out the old 5AR4 rectifier tube, which was the original from 1967 for a newer one.

I had this Pro Reverb "dimed" last night running through the Fryette Powerstation attenuator and it sounds great and has a nice overdriven creamy distortion. Its not as bass heavy as the Bassmen and is 'cleaner'. Sounds awesome.