No sound tube audio circuit.

Thread Starter

Skylar Coy

Joined Apr 29, 2017
133
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 yetIMG_1692.JPGimage.jpg
 

crutschow

Joined Mar 14, 2008
25,260
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.
 

Thread Starter

Skylar Coy

Joined Apr 29, 2017
133
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.
 

crutschow

Joined Mar 14, 2008
25,260
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.
 

Thread Starter

Skylar Coy

Joined Apr 29, 2017
133
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.
 
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
 

Kermit2

Joined Feb 5, 2010
4,162
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.
 

ian field

Joined Oct 27, 2012
6,539
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.
 

Thread Starter

Skylar Coy

Joined Apr 29, 2017
133
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:)
 
Last edited:

Thread Starter

Skylar Coy

Joined Apr 29, 2017
133
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!
 

Thread Starter

Skylar Coy

Joined Apr 29, 2017
133
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.
 
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