New member with a V4-B

[poorboywilliams]

Just joined the forum and thought I'd introduce myself and my project(s). I recently brought an original B-25 back into service – my first attempt at tube amp repair, and it went pretty well (apart from a hum I’m still trying to trace). Now I decided to tackle the other, more complicated project in my basement, a 1971 V4-B.

This thing probably lived a hard life before I took it in - dirty, road worn, zero ink left on the face plate but surprisingly original inside. What took it out of service was that it blows fuses (not right away, but after playing for a few minutes). Inside, it looks like the only components that have ever been replaced were one of the rectifier diodes, one resistor (R54), and the power cord. So I figured it was time for a more complete refresh.

I ended up buying the power supply kit from Fliptops, along with some fresh diodes, and placed an order with Mouser for new plate and screen grid resistors, bias resistor/trimpot, couple other odds and ends. Installed the kit this weekend, and now I'm waiting for the Mouser order to arrive. While I wait, I thought I'd share a couple pics - hoping to get a little reassurance that the filter caps look wired correctly? Mostly concerned with C18 and C20. I think I kept the connections in line with the stock setup...
   
   

Two questions, while I'm at it...
1. Fly back diodes. I know there is some debate on their usefulness. I bought replacements, but now I'm wondering if I should just remove them. Do I really just snip them out, with no other wiring rerouting?

2. The new power cord is grounded, so assume this makes the polarity switch useless? I noticed that the death cap is still there - can this just be snipped out as well? Any wiring rerouting necessary?

Anything else I should be thinking about updating while I'm under the hood?

Thanks all.

[Hangman]

My opinion on the Flyback diodes is pretty well known. Ive seen blown OTs in amps that had flyback diodes and in amps that did not. But Ive never seen an amp without flyback diodes have a blown flyback diode.
I understand the purpose of the diodes, but from my perspective as a repair technician, The flyback diodes always seemed more likely to fail than the transformer, and that could be at a performance.
Is replacing the OT more expensive? of course... but Im not convinced the flyback will make a difference in the likelihood of the OT failing.

The "death cap" isn't a huge deal. Replace it with a X2 safety cap, and leave it in there.

Replace C16 if it hasn't been already. upgrade R41,R42, R47 and R48 to 5W resistors.

[poorboywilliams]

Hangman, thanks.

I finally had a chance to button everything up this weekend and test it out, and now I have a new problem to solve. The amp works fine, but the output is really low. It's not distorted, just really (really) quiet.

Cutting to the chase...
How can I test my OT to make sure it isn't blown?

For anyone who wants the details (sorry for the long post)...
I fired it up first on a dim bulb tester - all good, no shorts. Plugged it into the wall, and it works fine but it's really quiet. I can turn it up well past 12:00 and it's still not loud enough to irritate my wife upstairs. Sounds like a 20w practice amp - probably quieter. That ain't good!

Here's what has been replaced:
Power supply (C17-20; D1-4, D5; R56,57,52,53)
Bias supply (C16; R50; R49, C15 [early '71 model, no C23]; D6)
Other: R39,40,45,46; R35,36; R41,42,47,48; R37,38,43,44; D7-10, C21 with an X2 safety cap.

With it running, I took some measurements around the amp. I don't have them in front of me, but they all looked normal...
B+ from OT center tap at C20 = 550v
At pos side of C19 = 540v
Plate voltage, 540-545v on all 4 tubes.
Plate current, all 4 tubes in the high-20s mA.
Plate dissipation somewhere around 53-55% on all tubes.

All tubes are checked and known to be good (tested 7027 pairs in my B25, swapped all others, including the 6K11). I ended up replacing the flyback diodes - for what it's worth, the old ones were reading OL in both directions, so they had gone open.

Anything else I should be looking for, checking, or replacing? My fear is a bad OT - is there a way to test it?

Thanks in advance.

[Hangman]

can you test the Ext amp out?

I've seen a number of V4s with resistors that fail open (or really high. like a 47k would measure 700k) in every case, the amp was super quiet.

checking the external amp out will allow us to isolate the problem either to the preamp side or power amp side of things.

[poorboywilliams]

Tested the external amp out - plugged the bass directly in there (tried both jacks), and with the volume maxed on the bass, it was definitely louder. Still relatively quiet and nowhere near as loud as it should be, but far better than with the bass in one of the regular inputs. Not sure exactly what I was expecting, but I assume this points to an issue in the preamp section?

I guess my next step will be testing every resistor that I can get to - hopefully I find something that has drifted way out. Let me know if there's anything else I should be considering. Still wondering about that OT..

[Hangman]

to test the ext amp out you will need to plug your bass into the amp as you normally would, but then plug the ext amp jack into the input of another amp, or into a mixer. be sure you use the ext amp jack and not the ext spkr jack, as the latter could definitely damage an amp/mixer.

alternately you could plug your bass into a mixer or some kind of boost pedal, and then plug into output of the mixer or pedal into the ext amp jack, that would test your power amp. if you can get a loud signal out of the power amp this way, then your problem is in the preamp.

when you plug a bass directly into the ext amp jack you would expect it to be very quiet. but the fact that it was louder than when you plug into the input of the amp, leads me to believe that you could have a problem in the preamp. test the ext amp jack as I just described and we'll know for sure.