I got a Fender Twin Reverb Silverface a 2 months ago and the power tubes were dead. I replaced them and there was quite a loud hum which got quieter only when I turned the bias balance pot all the way to the left. So I got to switching the tubes but it wouldn’t go away. I heard that you can run these amps on the two inner or the two outer tube so I thought that I’d give that a try and sure enough when I pulled out the two outer tubes the hum got quieter and with a quick adjustment of the bias pot it went completely. Then I moved the same tubes to the outer sockets and the hum came back. Really loud when turned to one side and slowly grew quieter as I turned it towards the other side but it was still very noticeable through the whole range of the pot.
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In fact, just pull all four power tubes, turn the amp on, and measure voltage at pins 3,4,and 5 of each socket. We want to see B+ voltage at 3 and 4 (450-500), and bias voltage at 5 (-50). (approximates)
And for that matter, verify there is no ripple (AC component) on the bias voltage.
And for that matter, verify there is no ripple (AC component) on the bias voltage.
Sorry for posting in the wrong section Kevin. I'm new here.
Loudthud and Enzo thanks for the quick reply. I'll be home again on the weekend. I'll check those things then and get back to you!
Loudthud and Enzo thanks for the quick reply. I'll be home again on the weekend. I'll check those things then and get back to you!
I hope you're not just randomly turning the bias trim-pot around without measuring plate current. Fender ran the bias pretty hot and you could cook your tubes by playing with the bias without measuring your idle current. I would imagine that the hum gets quieter with fewer tubes since your output power was reduced, and same with playing with bias, you're adjusting the potential output power.
While making these changes did you play through the amp? If so, how does it sound? I would expect that turning the bias pot in either direction would introduce distortion and affect your output volume.
If the amp still has the original B+ decoupling caps they're probably in need of replacement. To add to what Enzo said, check for AC ripple on the B+ lines too. Use a small, high voltage cap (I use 47nF ceramic) at your scope probe tip so you're not putting 400+V on your scope cables, they're likely 300VDC rated.
While making these changes did you play through the amp? If so, how does it sound? I would expect that turning the bias pot in either direction would introduce distortion and affect your output volume.
If the amp still has the original B+ decoupling caps they're probably in need of replacement. To add to what Enzo said, check for AC ripple on the B+ lines too. Use a small, high voltage cap (I use 47nF ceramic) at your scope probe tip so you're not putting 400+V on your scope cables, they're likely 300VDC rated.
While making the measurements with the tubes pulled, also check that the bias voltage is present on all the sockets. A blown tube, as you have had, can take out the grid resistor. This causes the tube to sink a load of current and the power supply hum goes sky high which the, now out of balance, output section cannot cancel.
I doubt this has anything to do with your problem, but:
I've always modded old Fender amps with the two-prong power cord to have a three-prong cord, with the ground connecting to chassis. It did wonders for reducing hum on my blackface champ, as well as on an old vibrochamp.
Replacing all the caps didn't seem to fix hum in my circumstance, but it did cut out some nasty distortion I was having with the champ. If it's a 60s amp, you can bet those old caps are about to give up the ghost. I've seen some exploded caps in old Fender amps.
Ground that chassis if it isn't already. It will at least get rid of RFI. Keep the old power cord so you can resell the amp as original if you desire. It really was a miracle fix for a couple of amps.
All that being said, you probably have one of the problems laid out by the other posters. I don't know what it will do to resale value, but I do recommend replacing ALL caps in older amps. Just from experience, it's helped the tone out tremendously.
I've always modded old Fender amps with the two-prong power cord to have a three-prong cord, with the ground connecting to chassis. It did wonders for reducing hum on my blackface champ, as well as on an old vibrochamp.
Replacing all the caps didn't seem to fix hum in my circumstance, but it did cut out some nasty distortion I was having with the champ. If it's a 60s amp, you can bet those old caps are about to give up the ghost. I've seen some exploded caps in old Fender amps.
Ground that chassis if it isn't already. It will at least get rid of RFI. Keep the old power cord so you can resell the amp as original if you desire. It really was a miracle fix for a couple of amps.
All that being said, you probably have one of the problems laid out by the other posters. I don't know what it will do to resale value, but I do recommend replacing ALL caps in older amps. Just from experience, it's helped the tone out tremendously.
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Flip polarity switch if equipped because that's what they are there for to get rid of hum.
If that doesn't work and the filter caps are old I would replace the old filter caps.
Another thing to look at is if the heater circuit has no center tap create an artificial one using two 100 ohm resistors to ground.
If that doesn't work and the filter caps are old I would replace the old filter caps.
Another thing to look at is if the heater circuit has no center tap create an artificial one using two 100 ohm resistors to ground.
Ok, problem solved! Thanks for your help everybody. I finally got time to open her up and sure enough one of the screen resistors had split in 2. All the others had drifted significantly as well so I replaced them all and now she sounds like a dream.
P.S Doozer Dave I’m pretty sure that fender made the parameters of the bias and hum balance pots so that you couldn’t fry your tube just by turning them only by running it unbalance for a longer period of time. This is why they are accessible unlike the older models where it is hidden away inside. At least this is what I have been told and have experienced to be true.
P.S Doozer Dave I’m pretty sure that fender made the parameters of the bias and hum balance pots so that you couldn’t fry your tube just by turning them only by running it unbalance for a longer period of time. This is why they are accessible unlike the older models where it is hidden away inside. At least this is what I have been told and have experienced to be true.
Glad to hear you found the problem.
My silver face Super Reverb has them inside, so I assumed your TR was the same.
My silver face Super Reverb has them inside, so I assumed your TR was the same.
Cheers! That makes sense. Mine was the final SF design TR so maybe your SR is an earlier production as well
Dick Deviant,
Those 470 Ohm 2W Carbon screen resistors have a bad habit of going open circuit, usually after they have drifted in value (usually high) by as much as 200 or 300 Ohms.
One of the 1st things I do on any Fender is strip out those screen resistors and fit 1K 5 Watt "Rats Coffin" style resistors in their place. This helps protect the output tubes and some claim it gives you a little more "touch responsivity" when playing. I never noticed that but I do think the amp breaks up a little earlier, that is you don't have turn it up quite as loud to get a little power amp compression.
I know that many Fender techs do the same thing.
Cheers,
Ian
Those 470 Ohm 2W Carbon screen resistors have a bad habit of going open circuit, usually after they have drifted in value (usually high) by as much as 200 or 300 Ohms.
One of the 1st things I do on any Fender is strip out those screen resistors and fit 1K 5 Watt "Rats Coffin" style resistors in their place. This helps protect the output tubes and some claim it gives you a little more "touch responsivity" when playing. I never noticed that but I do think the amp breaks up a little earlier, that is you don't have turn it up quite as loud to get a little power amp compression.
I know that many Fender techs do the same thing.
Cheers,
Ian
Sounds good Ian.
This twin I'm running now sound perfect to me though so I want to keep it stock. If I ever come across a cheap one that is dead or someone else has started modding then I will be very interested in testing out all these little tricks for break up and tone.
All the same thanks for the tip!
This twin I'm running now sound perfect to me though so I want to keep it stock. If I ever come across a cheap one that is dead or someone else has started modding then I will be very interested in testing out all these little tricks for break up and tone.
All the same thanks for the tip!
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