Bassman 100 help: screen resistors fried

[charlie_p]

I agree.. i had it biased to 33mA with the tubes that just blew. One good thing though, this time the screen resistors didn't fry.. it just took 2 tubes out..

I'll let you know the range of the bias adjustment tonight when i have time to spend on it..

I think i am going to put everything back to factory specs and see if it still blows the old 6l6's. If they hold up i will just change it out for el34's and see if that holds up. I found another method to add the bias adjustment pot while leaving the bias balance intact. I may try that and see if that is any better.. and while i am in there i am going to upgrade all the filter caps, bias caps and resistors..

I just don't want it to be a transformer issue. It doesn't seem like it to me thought.. but at the same time i don't understand why it's blowing tubes either.. so ...

[txstrat]

The voltages seem OK even though they're a little on the high side.
Around 450v on the plates and grids shouldn't harm either 6L6 or EL34.
The power transformer is able to PROVIDE an amount of current when NEEDED.
It wouldn't PUT OUT more current to harm the tubes. If the tubes drew more current than the PT was able to provide the fuse should blow. What's happening here is, that the tubes somehow draw too much current (within the range the PT can provide) and blow. We have to find wout WHY they draw too much current.
Using EL34 you should keep the cathode current at around 5-10mA less than for 6L6.
Have you checked the bias pot? Sometimes the center lug doesn't do it's work anymore. In this case the bias voltage might be missing on the power tubes and they blow.
I prefer to wire the bias pot as a variable resistor (see pic). That way the bias voltage would go more to the negative side if the center lug fails. The values of the pot and resistors have to be adjusted for your circuit.

[Steve Conner]

I'd want to know: Has anyone ever successfully converted a silverface Bassman 100 to EL34s before? The instructions on Regis' site just say "Bassman", and there were about 10 different amps with that name, not counting the recent solid-state ones.

Maybe it works with an old blackface one, but the screen voltage on the new ones is just too high. Remember that when Jim Marshall made his first amps, he ripped off the original Bassman but substituted EL34s, and the early Marshalls suffered from poor reliability when overdriven. Supposedly the only way Hendrix could get his to survive a show was to ditch the EL34s for 6550s.

I don't think there is anything wrong with it. Bad bias, excessive current in the power tubes, would just cause red-plating, to blow screen grids you need excess screen voltage and/or too high load impedance. And the only "fault" that can cause either of these is a bad speaker connection or wrong impedance of speaker cabinet.

One more thing though, parasitic oscillations... Does it have grid stopper resistors on the power tube sockets?

[txstrat]

My line of thought was, when the tube redplates (due to wrong bias where the control grid lets more electrons through, so to speak) the screen grid would also collect more electrons which causes current flow in the screen grid circuit. This could cause the screen grid to overheat and melt and/or fry the grid resistor.
I'm not sure if my thoughts are correct, though.

[charlie_p]

Steve: YouTube - Fender Bassman 100 (modified) .

Mine has the stock 1500 ohm grid stopper on pin 5 of the tubes.. In most of the el34's i have seen have a 5.6k but someone said that shouldn't cause this problem. My thinking was if i am going to "mimic" a marshall type amp with the el34's then i should use those values in my circuit. Maybe i am incorrect about that..maybe i should throw some 6550's in there and see what happens.. it does only seem to give me a problem when the volume gets up into the 8 9 10 range when the amp is pushed hard.

Txstrat: I will give your bias adjust a try this weekend before i put everything back to stock. I am also going to put in new tube sockets.

I have never seem the tubes red plate though.. a little blue inside when i hit notes or chords hard.. everything looks normal then a flash of light and boom.. the tubes go.. like i said this time around the 2k screen resistors didn't fry. just the fuse and 2 tubes.

BTW.. thanks for taking the time to help me figure this out guys!!!

[jrfrond]

"Decided to put the bias mod back to the original balance, and when i went to unsolder the wires i found a that the wire that went to the center lug on the pot coming from the two 220k resister that go to pin 5 was broken and wasn't attached. Could that be all that was causing the problem?"

This would be a "no-bias" condition. Could it cause a PROBLEM??? Hell yes!!!

Also, the resistors should definitely be at least 1.5K/5W wirewound for EL34's. EL34's really need some screen current limiting to keep from redplating, and even then, if you close the lights and view the tubes in the dark with the amp on, you are liable to see a faint cherry glow on the plates, especially at the inside bend.

[charlie_p]

jrfrond: thanks for the input.. i didn't ask if it could cause a problem.. i knew that it was a problem, i asked if that could be ALL that was causing the problem..obviously it wasn't since it's still doing it even after that problem was discovered and fixed. I have 2k 5w wirewound resistors on there..