Audio Research D90... a HOT, buzzing amplifier - diyAudio
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Old 1st December 2015, 10:24 AM   #1
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Default Audio Research D90... a HOT, buzzing amplifier

Hi everyone,
I've just posted my introduction and here's my first thread on this great forum.

A few days ago I aquired an Audio research D90 (not a D90B) which is populated with Electro Harmonix KT90 power tubes and an EH 6550 regulator tube.
The amp was in a doubtful condition:
-aged soldering connections (already solved since yesterday)
-Lousy aftermarket loudspeaker sockets (I fixed it up to the point where at least it's safe to operate; should do more later)
-It was badly mis-adjusted. (especially the DC balance was farrrrr off...

I did a full adjustment as per the guidelines of ARC and found the amp to be working well, with a few things I like to ask advice on from more experienced users:

-Heat: Even without the top cover, the power tubes get blazing hot. And not only them, the power transformer gets very hot as well. (too hot to touch after a few hours of playing.) Is this normal?

-With the above, please note that I biassed the KT90's on 65mA (ARC suggestes an even higher bias of 75mA. Would this make a big difference on the sound?

-The power transformer is buzzing / rattling loudly. I can hear it through soft passages in the music. I'd like to make it more quiet. Plastic spray came to my mind, but I think that given the temperature of the transformer, this isn't wise. Any suggestions?

Thanks for reading this :-) I'm looking forward to know ya all a bit better and to get my D90 in perfect shape as well.
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Old 1st December 2015, 11:09 AM   #2
AJT is offline AJT  Philippines
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bias the tubes at 45 mA in the meantime and see if the transformer quiets down and run cooler....find out also if the sound quality takes a hit... please tell us what you find...
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Old 1st December 2015, 11:19 AM   #3
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Hi AJT,
Thanks for the suggestion. I will do it and report back.
At the same time, I wonder that if this solves the problem, there might be an underlaying issue in the amp?
I mean: It was constructed for 75mA bias so it should be able to work like that. What would cause the tranny to run this hot and buzz this loud?

Cheers,
J.
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Old 1st December 2015, 11:48 AM   #4
AJT is offline AJT  Philippines
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was in a similar situation once, the owner of the amp complained about the heat,
so backing down on bias current fixed it...may work for you too..

increasing bias current, increases the class A window but increases heat....
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Old 1st December 2015, 12:08 PM   #5
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Hmmm. Very logical.
I'm actually not complaining that much, I just woder if the heat (especially the transformer heat) is normal.

Right now I'm digging a bit deeper into the amp and I see that more things are changed.
There's a small winding on the secundary side of the output transformers, with Violet and Green wires. These should be connected to one of the grids of the output valves through 10 Ohm resitors. But they're disconnected instead. Hmmm.
The 10ohm grid resistors are all connected together (even from both channels!) and end up in a fuse. The other end of the fuse is connected to the center of the diconnected (violet/green) winding. Here I'm short of knowledge. Did someone change from Penthode to Triode mode? Any help is appreciated.
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Old 1st December 2015, 12:32 PM   #6
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UPDATE:
This fuse and connection method seems to be present on more D90's. It might be an undocumented upgrade from ARC. I will email them and let you all know.
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Old 1st December 2015, 08:00 PM   #7
DAK808 is offline DAK808  United States
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Quote:
Originally Posted by JacquesPlafond View Post
There's a small winding on the secundary side of the output transformers, with Violet and Green wires. These should be connected to one of the grids of the output valves through 10 Ohm resitors. But they're disconnected instead. Hmmm.
The 10ohm grid resistors are all connected together (even from both channels!) and end up in a fuse. The other end of the fuse is connected to the center of the diconnected (violet/green) winding. Here I'm short of knowledge. Did someone change from Penthode to Triode mode? Any help is appreciated.
If the wires connect to the grid it could be for ultralinear operation. How do you know where those wires connect? In UL mode the wires should connect to P4 of the tube socket. Sometimes that connection is fused but I am not familiar with AR equipment. Photos would be more helpful. regards, 808
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Old 2nd December 2015, 08:47 AM   #8
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Thanks for the hint, DAK808. I am still awaiting an answer from ARC, but I figured out most of what's going on myself already.
Seemingly the amp was initially designed as a UL amp. (hence the topology found in ARC's original schematics.)
But somewhere they must have figured that Tetrode operation was more desirable. (reason?)
The fuse is all original and is just to protect the B+ supply in case of valve failure.
I know now that this fuse is original, as a) It's wired with the exact same wire as the rest of the amp and b) It's present on each and every picture of a D90 I can find on internet.
So for now: This issue is off my list.

As for heating and buzzing transformer:
I did another round of adjustments yesterday (AC balance and inverter balance) and things are a tad better now. My next step will be to lower the bias as per the suggestion of AJT. I'll keep you guys posted. It's definitely a very interesting amp.

Cheers,
J.
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Old 7th December 2015, 10:36 AM   #9
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Here's what I've managed to pull up:
The schematics of the D90 on the Audio Research website are a non-exhaustive version. I've found a more complete diagram where all changes over time are noted on.
It appears that the screen tap on the transformer was left unused as from mid 1981 production on. The screens were connected to the B+ directly instead.
One or two months later, the B+ was increased from 310V to 410V and the Bias current was lowered to 60mA (As by then, ARC started to use 6L6GC output tubes instead of 6550's.) This is still BEFORE the introduction of the D90-B.
I hope this information can be of help for other ARC D90 owners.

Last edited by JacquesPlafond; 7th December 2015 at 10:39 AM.
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Old 7th December 2015, 10:49 AM   #10
AJT is offline AJT  Philippines
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in a pentode, it s not the plate voltage that determines cathode current, but the screen voltage...
so increasing the screen voltage only means that grid1 negative bias was increased....
so assuming a plate voltage of 400 volts, each tube is then dissipating 24 watts....
you can also do that on the 6550...the 6550 is around 30 watts si it should run cooler at 60mA...
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