Amp tech help please

jaybird

jaybird

New member
Hi guys,

Been a long while since I posted but could use some help. I had a output tube go bad in my modded Marshall 1959. Blew both the fuses. I replaced the fuses and all the tubes - both pre and power, but it's not working. In standby I see the tubes are glowing from the heaters, but dead silence when switching out of standby and no other visible change on the output tubes. I put my Weber Bias rite in, and reading all 0.0's for plate voltage, current, etc.

Any ideas as to what to look at next?

Thanks!

Jason
 
If you're handy with a multimeter start checking resistors like the ones across the tube sockets to see if they got taken out.
 
jaybird":27hoqzg9 said:
Hi guys,

Been a long while since I posted but could use some help. I had a output tube go bad in my modded Marshall 1959. Blew both the fuses. I replaced the fuses and all the tubes - both pre and power, but it's not working. In standby I see the tubes are glowing from the heaters, but dead silence when switching out of standby and no other visible change on the output tubes. I put my Weber Bias rite in, and reading all 0.0's for plate voltage, current, etc.

Any ideas as to what to look at next?

Thanks!

Jason
Click to expand...

Remove the power tubes and chassis. Read bias supply voltage from pin 4 to ground on each socket with the amp off standby. You should see a negative voltage of -30 to -40V. go from there. Sounds like there may have been an arc across the sockets and back to the bias supply. Look for anything burnt too, like screen resistors.
 
blackba":bg74lkm3 said:
Please discharge the caps before you start poking around with a multimeter.
Click to expand...

I can bias a couple amps but that's as far as my knowledge of amps go. How does one discharge the caps?
 
Fuego":32orpqne said:
blackba":32orpqne said:
Please discharge the caps before you start poking around with a multimeter.
Click to expand...

I can bias a couple amps but that's as far as my knowledge of amps go. How does one discharge the caps?
Click to expand...

I'll also show my lack of knowledge.....shouldn't the caps be discharged prior to biasing power tubes (safety of you and the amp)?
 
With most modern amps, even not so modern, the caps do drain by themselves. Turn the amp off without turning the standby back on. Unplug the amp. Let it sit overnight. Clip the negative lead to the chassis and touch the positive lead to the positive side of each cap. Make sure the meter can handle whatever voltage is indicated on the cap, usually it's down to harmless voltages or none. But if you are not doing any work on the caps, you don't need to mess with them unless you are probing around ungrounded like testing resistors or caps. There are many tutorials on how to drain caps on youtube and such. It is a serious issue. I'm working on a friends amp actually and I let it sit for a few days off standby and then tested the caps voltages and they are at zero. I'm going to do a cap job though and that is why I made sure. If you are just biasing an amp obviously it will be on. Testing a resistor you do want to make sure there are no big voltages left in the circuit so definitely make sure the caps are drained first because your meter is not grounded when testing resistors.
 
Thank you for the answer but due to my lack of amp knowledge, it raises more questions for me. Turn the amp off but leave the standby in the playing position? As in, only move one switch at shutdown?
 
I'm about to start a '69 100w Plexi build from the MetroAmp instructions. To drain the filter caps, you put an alligator clip on the chassis and the other end on pin 1 of V1 and wait a few min. Then test with your multimeter.
 
Fuego":3at2umch said:
Thank you for the answer but due to my lack of amp knowledge, it raises more questions for me. Turn the amp off but leave the standby in the playing position? As in, only move one switch at shutdown?
Click to expand...
Yes. Turn the power off but leave the standby on.
 
jaybird said:
Hi guys,

Been a long while since I posted but could use some help. I had a output tube go bad in my modded Marshall 1959. Blew both the fuses. I replaced the fuses and all the tubes - both pre and power, but it's not working. In standby I see the tubes are glowing from the heaters, but dead silence when switching out of standby and no other visible change on the output tubes. I put my Weber Bias rite in, and reading all 0.0's for plate voltage, current, etc.

Any ideas as to what to look at next?

Thanks!

Jason[/quote
i am new here and help me My guess is that when the tube blew, it took out a screen grid resisto
Click to expand...
 
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