Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Rockranger on May 22, 2023, 12:02:04 pm
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Hey Everyone
A friend wants me to look at his 64 Vibroverb. He says that the amp sounds great but it has very little volume compared to what it used to have, in fact he says his Princeton is louder. He has verified that all tubes are good and speaker are good, all voltages are looking great as well. I have a fair bit of experience with these these amps and have built a blackface deluxe reverb, but he has quite a bit of amp knowledge himself so I’m guessing that it may not be a simple fix( I don’t have the amp yet). When I get it I will go over everything and see if there is anything obvious just wondering if it is possible that it may be the OT?
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Its possible, but it would not be high on the list of usual suspects IMO. OTs are pretty robust.
Tubes - despite thinking they are fine
Capacitors
Resistors
Potentiometers
Hearing loss :icon_biggrin:
Would all be more likely causes in my book. But impossible to tell until you get the amp and run some tests.
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bmccowan has it right.
Building an amp and troubleshooting a problem are two totally different things.
I read all too often that valuable amps are damaged by the inexperience of a repairman despite his good will. And a 64 vibroverb is very valuable.
Tubes; a tube tester can very often fail to detect this kind of subtle problem. You have to put in new tubes to know.
I once had a similar problem on a Fender channel and the cause was the volume knob. Its total resistance was too low and sent part of the signal to ground
An audio transformer can be checked very well with an audio signal generator.
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Tubes - despite thinking they are fine
And if its a 64 Vibroverb with original tubes, the output tubes AND the rectifier tube could be well past use-by-date.
Tube swap is always first thing to try - Got some new tubes to roll?
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Rockranger wrote abut his friend;
"...He has verified that all tubes are good and speaker are good, all voltages are looking great as well."
In this case, the only way to know if the tubes are good is to put in new ones. Test tubes are not reliable. And without equipment (signal generator and ocilloscope ....) and a good knowledge of this will be very difficult to repair.
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And if the filter capacitors are original, they should be retired and go on Medicare. Fresh F&Ts might do wonders for the amp.
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And if the filter capacitors are original, they should be retired and go on Medicare. Fresh F&Ts might do wonders for the amp.
100 % right
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Sorry for the late response but I finally got this amp at my home. As always I truly appreciate the responses, the knowledge and information on this form are incredible. I am very aware of the value of this amp and that is why I posted this before I even had the amp, I don’t pretend to be a repair man but I don’t mind looking at something if someone is stuck and has no where else to take their amp. That being said I don’t touch anything I am not confident that I can fix.
When I tried the amp out everything was working but the owner was definitely right it was a bit lower volume than I expected and just had no guts. I cleaned all pots and tube sockets then put all new tubes in and hooked up the signal generator and oscilloscope. I noticed right away that the volume pot on both channels were bad and replaced them. This really cleaned up the sound, took away a hisss that was present, but I still wasn’t getting the volume I wanted. I am new to using an oscilloscope but it seemed like there wasn’t enough change in amplification between gain stages so I replaced the coupling cap in the normal channel just as a test and found my problem. Proceeded to replace all coupling caps and the amp came back to life, lots of volume and character and sweet breakup that wasn’t possible before. Personally I’m a fan of much smaller amps but I must say I’m jealous of this one, I’ve had the good fortune of being able to play lots of vintage amps but to my ear this one is special. There is something about the blackface through that 15” altec speaker for sure! Again thanks for the responses guys, glad this wasn’t too hard to fix but it’s great having this form for backup. While I have the amp here I will be replacing the filter caps with fresh F&Ts.
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Glad it worked out. You likely know this, but if the amp had its original blue Ajax Mallory tone/coupling caps, people seek those out. Its not likely that all of them were bad and they can be tested.
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Absolutely they are in a baggie and will be given back to the owner.