So let me take you back. I have this amplifier from 75' the output transformer I thought was going bad. Lots of fizzing noises, low output, bad distortion. Etc. then one day it basically made a non stop fizz and bad noises and I decided to check plate voltage. I forgot my meter was in current mode and shorted the plate to ground. No sound. I thought to myself. Crap! I killed the transformer but i don't care because I needed a new one anyway. Well. My new one came in and still. No sound. Now when I check plate voltage it is at 940mv. Soooo. Not good. The led lights up but kinda dim at that. But no more dim than before. what shall I check? Sorry for the messy output transformer wires I haven't Cut them yet

 

mind you that before the whole plate to ground thing i was getting about 500v on the plate...

 

I would check the rectifier diodes generating the 500V at the power transformer output.
Sounds like one or more of them may have been zapped.

 

I would check the rectifier diodes generating the 500V at the power transformer output.
Sounds like one or more of them may have been zapped.

what leads you to think that? I beleive you are correct however. on further examination of the diode (leads black and charred) and my diode tester on my meter giving me a reading of 390 and 750.. i could only find two of those 1n 4005 diodes.

 

what leads you to think that? I beleive you are correct however. on further examination of the diode (leads black and charred) and my diode tester on my meter giving me a reading of 390 and 750.. i could only find two of those 1n 4005 diodes.

Those diodes rectify the high voltage you are missing.
Those diodes are not 1N4005 but are 2.5A, 1000Vpiv types.

 

Those diodes rectify the high voltage you are missing.
Those diodes are not 1N4005 but are 2.5A, 1000Vpiv types.

That's the rectifier tube. Replaced. I now have 9 volts on the plate

 

Ok I'm confused! Is the proprietor of this inquiry @Skylar Coy or @Pochie45566? And what's with the new thread?

With regards
HP

 

Ok I'm confused! Is the proprietor of this inquiry @Skylar Coy or @Pochie45566? And what's with the new thread?

With regards
HP

Agh my account will not work for some reason and I could not access it. Same person. New thread. Just continued here bc I could not access the old lol.

 

What voltages are you getting at points A,B,C,D,E at the psu output?

 

What voltages are you getting at points A,B,C,D,E?

I'll test it tomorrow morning. I suspect one of the capacitors is going bad. Original from 75. I think that a is especially screwed. The one connected to the b+ because that would impact the plate voltage.

 

hat's the rectifier tube.

I don't see a rectifier tube.
Where is it on the schematic?

 

I don't see a rectifier tube.
Where is it on the schematic?

Got me beat. Kinda a crappy schematic they wrote it as 4 diodes I guess.

 

Got me beat. Kinda a crappy schematic they wrote it as 4 diodes I guess.

Can you see the diodes on the other side of the chassis, or is there a valve?

 

Got me beat. Kinda a crappy schematic

Indeed!

FWIW the schematic of the power transformer's primary circuit is daft in that it has the amplifier's power input unconditionally bypassed! --- Said observation, taken with previously discovered inaccuracies (i.e. omission of additional power transformer primary tap{s}) are cause for dubiety as regards schematic integrity/completeness! While I'm tempted to ascribe the discrepancies to a non-original power transformer, there is no scenario wherein the the circuit, as drawn, is viable...

Point being; It may behoove you determine the actual PSU configuration prior to proceeding...

Best regards and, again, good luck!
HP

 

I too, feel this may be an attempt to work on highly modified equipment using a (error filled) schematic of the original unmodified amp.

YOU sir, must start at the power cord, and draw out the circuit you find in the amp and compare it to schematic. The transformer is first. It will be the hardest to fathom due to its complexity.

Photograph of the quadrant in question will aid us in helping you.

 

mind you that before the whole plate to ground thing i was getting about 500v on the plate...

Some PAs run at that sort of voltage - I've put silicon rectifiers in to eliminate most of the volt drop - its much louder and you get a pretty blue glow around the edges of the plates.

A fizzing output transformer could be parasitic oscillation - the first thing to try is stopper resistors in series with the output tube grids.

Running it with no load can allow voltage peaks that cause breakdown - carbonised insulation can arc more frequently at normal operating peaks.

 

Some PAs run at that sort of voltage - I've put silicon rectifiers in to eliminate most of the volt drop - its much louder and you get a pretty blue glow around the edges of the plates.

A fizzing output transformer could be parasitic oscillation - the first thing to try is stopper resistors in series with the output tube grids.

Running it with no load can allow voltage peaks that cause breakdown - carbonised insulation can arc more frequently at normal operating peaks.

Plate on this amplifier should be 450 ish.

 

Plate on this amplifier should be 450 ish

In which case 500V is an anticipatable/acceptable unloaded EMF -- That said, if the amplifier indeed employs solid state diodes (there seems to be some ambiguity on this point?) good practice will employ filter caps specified at maximum working EMFs ≥ 550V...

But I understand your immediate problem is very low/no B+?

So....
A few items to check/consider

-Are you seeing appropriate (AC) EMF on the secondary? (Warning! -- Please be mindful of your indicator's maximum limits in contemplation of such measurement!)
-Is the "B+ Switch" (i.e. the switch intervening the rectifier cathodes and the filter chain) closed and making good contact?

→Correct secondary EMF coincident with low rectifier output 'indicts' the rectifiers...
→Low Secondary output may owe to an overload condition -- in such case please disconnect the rectifiers then re-measure the secondary EMF...

Please measure the following (relative to ground) then report your findings here:
1) The (DC) EMF apparent at the (common) rectifier cathode point.
2) The (DC) EMF at the 'load side' of the 'B+ switch'.
3) The (DC) EMF at each of PSU output points A,B,C,D

→'Passage' of step #1 coincident with 'failure' of step #2 implicates the 'B+ switch'
→Should steps #1 and #2 'Pass' but Step #3 'Fail' your problem is almost certainly an open filter inductor (L506)

Best regards and good luck!
HP

 

In which case 500V is an anticipatable/acceptable unloaded EMF -- That said, if the amplifier indeed employs solid state diodes (there seems to be some ambiguity on this point?) good practice will employ filter caps specified at maximum working EMFs ≥ 550V...

But I understand your immediate problem is very low/no B+?

So....
A few items to check/consider

-Are you seeing appropriate (AC) EMF on the secondary? (Warning! -- Please be mindful of your indicator's maximum limits in contemplation of such measurement!)
-Is the "B+ Switch" (i.e. the switch intervening the rectifier cathodes and the filter chain) closed and making good contact?

→Correct secondary EMF coincident with low rectifier output 'indicts' the rectifiers...
→Low Secondary output may owe to an overload condition -- in such case please disconnect the rectifiers then re-measure the secondary EMF...

Please measure the following (relative to ground) then report your findings here:
1) The (DC) EMF apparent at the (common) rectifier cathode point.
2) The (DC) EMF at the 'load side' of the 'B+ switch'.
3) The (DC) EMF at each of PSU output points A,B,C,D

→'Passage' of step #1 coincident with 'failure' of step #2 implicates the 'B+ switch'
→Should steps #1 and #2 'Pass' but Step #3 'Fail' your problem almost certainly an open filter inductor (L506)

Best regards and good luck!
HP

Rather sure the filter capacitors are open. It indeed is tube rectified. At least A is. That's directly connected to the b+ I am going to buy new caps tomorrow. Will get back to you guys soon. Thanks a ton!

 

In which case 500V is an anticipatable/acceptable unloaded EMF -- That said, if the amplifier indeed employs solid state diodes (there seems to be some ambiguity on this point?) good practice will employ filter caps specified at maximum working EMFs ≥ 550V...

But I understand your immediate problem is very low/no B+?

So....
A few items to check/consider

-Are you seeing appropriate (AC) EMF on the secondary? (Warning! -- Please be mindful of your indicator's maximum limits in contemplation of such measurement!)
-Is the "B+ Switch" (i.e. the switch intervening the rectifier cathodes and the filter chain) closed and making good contact?

→Correct secondary EMF coincident with low rectifier output 'indicts' the rectifiers...
→Low Secondary output may owe to an overload condition -- in such case please disconnect the rectifiers then re-measure the secondary EMF...

Please measure the following (relative to ground) then report your findings here:
1) The (DC) EMF apparent at the (common) rectifier cathode point.
2) The (DC) EMF at the 'load side' of the 'B+ switch'.
3) The (DC) EMF at each of PSU output points A,B,C,D

→'Passage' of step #1 coincident with 'failure' of step #2 implicates the 'B+ switch'
→Should steps #1 and #2 'Pass' but Step #3 'Fail' your problem is almost certainly an open filter inductor (L506)

Best regards and good luck!
HP

I'm not comfortable powering it up at the moment due to brand new transformers and those filter caps are very old.