Hi,
I Bought an amp that have just had the output transformer replaced. The owner said she used it once fine and then it started blowing fuses.
I set up a current limiter and everything started up fine after five minutes of playing without the limiter I got blue sparks through one of the power tubes and the slow blow fuse blew.
The PT and the OT are not original. I have sketch out the power supply So not sure of which windings are connected Because I haven’t disconnected.
I research as much as possible to could be bad tube Or loose socket Or poor OT connection or Intermittent voice coil problem.
*I notice when taking readings with no power tubes pin3 Create scratching sound in speaker.
*I test for vac on bias circuit. noticed 22 vac On negative bias Circuit Close to diode Then no vac 10mm up circuit futher , Only the -40vdc negative dc bias.
*I question the power supply. original schem. has a lot higher voltage730v vs 495vdc to OT primary centre tap (ht1 on schematic) than amp has now.
*the two 220k resistors Jumping the filter caps straight after the rectifier.
Any advice what to check for would be great thanks.
I have a pair spare El34, do not want to put them in in case the problem could be not just bad tube.
I will upload the schematics now, thanks
I Bought an amp that have just had the output transformer replaced. The owner said she used it once fine and then it started blowing fuses.
I set up a current limiter and everything started up fine after five minutes of playing without the limiter I got blue sparks through one of the power tubes and the slow blow fuse blew.
The PT and the OT are not original. I have sketch out the power supply So not sure of which windings are connected Because I haven’t disconnected.
I research as much as possible to could be bad tube Or loose socket Or poor OT connection or Intermittent voice coil problem.
*I notice when taking readings with no power tubes pin3 Create scratching sound in speaker.
*I test for vac on bias circuit. noticed 22 vac On negative bias Circuit Close to diode Then no vac 10mm up circuit futher , Only the -40vdc negative dc bias.
*I question the power supply. original schem. has a lot higher voltage730v vs 495vdc to OT primary centre tap (ht1 on schematic) than amp has now.
*the two 220k resistors Jumping the filter caps straight after the rectifier.
Any advice what to check for would be great thanks.
I have a pair spare El34, do not want to put them in in case the problem could be not just bad tube.
I will upload the schematics now, thanks
If you have 730v on the anodes, (EL34 is happy up to 800v), -40v bias is not enough. Nearer -50v before the current is correct.
Check what you have available and of course, ensure the main smoothing capacitors are rated to a high enough voltage.
Check what you have available and of course, ensure the main smoothing capacitors are rated to a high enough voltage.
Hi,
with the PT and ps design that is in the amp now I only have 492v pin3 at the anodes and 482v at screen pin 5.
I dont Know how long the Amp has had this different power supply in it.
I wonder because the original design has a larger voltage difference Between anode and Screen.
Is it likely to affect the Amp only having 10 V difference between the two?
A bad tube is far more likely than the entire list of other possibilities combined. Any time I see sparks inside a tube, I can pretty much count on that tube being bad.
Thanks, I hope that’s the case maybe the bad tube destroyed the OT That was just replaced and the tech Didn’t realise a bad tube had caused the problem and left the bad Tube in
The sparks could be due to plate over current causing it to bend.
The sparks you see could be plate touching the screen drawing more current and fuse blow out.
The over current could be too low or no bias.
Disconnect the B+ from the rectifiers, check if the grid bias voltage is stable and not fluctuating at the socket
Pull the output tube out from the socket and see if the pins on the socket are broken, particularly the grid.
Replace if necessary.
Or You can bring the amp to a tech and check plate current and set the bias properly.
Take all precautions if you work on high voltage DC. One hand working is good. Keep someone around if required to shut off power..
Regards.
The sparks you see could be plate touching the screen drawing more current and fuse blow out.
The over current could be too low or no bias.
Disconnect the B+ from the rectifiers, check if the grid bias voltage is stable and not fluctuating at the socket
Pull the output tube out from the socket and see if the pins on the socket are broken, particularly the grid.
Replace if necessary.
Or You can bring the amp to a tech and check plate current and set the bias properly.
Take all precautions if you work on high voltage DC. One hand working is good. Keep someone around if required to shut off power..
Regards.
Thank you the power supply and voltages and grid resistors checked out fine.
I think it may have been a bad tube and/or bias.
I rewired The series parallel Heater wire combinations So it would run at 50w with two power tubes at half output impedance and reset the bias. It is working fine now. I don’t want to put the other tube back in because I have no way of testing it and I think it may have been what originally took out the output transformer.
Do you think the tube might still be alright or is it better not to try installing it after it’s Been blowing fuses?
I would certainly not do that. I once had to repair a similar 4xEL 34 guitar amp of which one of the output tubes (an JJ EL 34) had developed a short (g1 and cathode).
Fortunately it had blown the fuse at once thereby saving its brother ( the g1's were connected) and the ot.
So I won't take the risk. If you're lucky the fuse will blow at once if not it might cost you the ot.
Fortunately it had blown the fuse at once thereby saving its brother ( the g1's were connected) and the ot.
So I won't take the risk. If you're lucky the fuse will blow at once if not it might cost you the ot.