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Loud hum, and high AC at idle on one output tube.
Loud hum, and high AC at idle on one output tube.
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Old 14th November 2019, 08:16 PM   #11
Enzo is offline Enzo  United States
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Pull the power tubes and check each s0cket pin. Is there CLEAN B+ at both plate and screen pins? Is there CLEAN bias voltage at the grids?
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Old 14th November 2019, 08:25 PM   #12
bendyha is offline bendyha  Sark
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Quote:
Originally Posted by Enzo View Post
Pull the power tubes and check each s0cket pin. Is there CLEAN B+ at both plate and screen pins? Is there CLEAN bias voltage at the grids?
Been done, and yes, it measures up as ripple free as one would want to expect from the simple RC filter in the amp.

As "Clean" as tens of thousands of similar amps that don't exibit the problem described.
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Old 15th November 2019, 03:30 AM   #13
wiseoldtech is offline wiseoldtech  United States
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OK...
Things like endless speculation can be eliminated by posting a useable schematic of this unit.
The link previously provided does not work.
And so many of us just hate to dig blindly.
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Old 15th November 2019, 05:45 AM   #14
baudouin0 is offline baudouin0
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Yes that appears to be a link to a forum we don't have access to.
The fluke goes up to about 2KHz so the AC is not oscillation.
You have not got your PT and OT connections muddled up?
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Old 15th November 2019, 09:15 AM   #15
bendyha is offline bendyha  Sark
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Quote:
Originally Posted by wiseoldtech View Post
OK...
Things like endless speculation can be eliminated by posting a useable schematic of this unit.
The link previously provided does not work.
And so many of us just hate to dig blindly.
Okay.. i'll see what I can do...I don't quiet understand the reason, but when I open this page I see the schematic I posted big and clear..?
I've always had troubles posting images here - in other forums one can usually just drag and drop an image, copy and insert, or at least upload from your computer. Here it seems on can only link to an image uploadingn site, and even then it is dificult to get the link working...I have given up trying a few times before, and this is one of the reasons im not around here so often to post.
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Old 15th November 2019, 09:23 AM   #16
bendyha is offline bendyha  Sark
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This is the full schem, not as neat as the cutout i was trying to post.Click the image to open in full size.
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Last edited by bendyha; 15th November 2019 at 09:28 AM.
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Old 15th November 2019, 10:28 AM   #17
Alan4411 is offline Alan4411  Wales
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Try this.
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Old 15th November 2019, 12:30 PM   #18
baudouin0 is offline baudouin0
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OK your statements are not really consistent with any fault. So start again, measure the DC voltages on the power supply to check they are all OK (without OP valves). I would then suggest removing the driver valve v38 and seeing if the output stage is the problem. If you measure DC voltage across screen grid resistors it should indicate if the plate current is very roughly OK. Its just possible there's a fault in the power supply low voltage rails which put massive amounts of hum on the PA input. I still don't understand by probing the V5 plate stops the hum unless there is poor connection or HF oscillation.
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Old 15th November 2019, 05:38 PM   #19
Mark Tillotson is offline Mark Tillotson
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Well its clearly oscillating at high power levels, probably ultrasonically and maybe pulling down the supply rail - did you measure the supply rail?



First thing is check the supply decoupling caps aren't dried out. Then check if it behaves with the driver tube removed - if so then the whole amp is likely oscillating from unwanted positive feedback somehow.


And take care with those voltages of course...
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Old 15th November 2019, 06:19 PM   #20
baudouin0 is offline baudouin0
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Oscillating it could well be although the fluke I did not think could measure high frequencies.

I can think of one other thing. Its modern solid state bridge rectifier on the HT. If the EL84 went dead short (which it will have done) and the amp stayed in that condition for a while then it may be enough to cook one half of the primary winding in the output transformer. This could produce a shorted turn causing all sorts of HF strange behaviour. It would explain why all the DC voltages checkout and why there is such a difference in plate AC levels when being a transformer you would expect them to be similar at least up to 100KHz.

Looking at the mains fuse and power supply I reckon a shorted EL84 could put >500ma down one side of the transformer primary without blowing the fuse. That's enough to damage it.

You could check DCR on each transformer primary, but you may have to measure inductance. You could put the transformer across a signal gen I guess.

Last edited by baudouin0; 15th November 2019 at 06:44 PM.
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