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. 
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. 
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.
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.
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.
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.
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
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.
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.
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.
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.
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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...
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...
I can warm my hands on the D90 if I want to.It remains an exciting project.
Now that I know that the amp is orginal (not modified) I started to dig a little deeper in the adjustments. One of the things that bother me a lot, is that the bias is unstable.
I am not sure if this rececently happened. What I see, is that on all 8 tubes, the bias continuously varies by as much as 6mA per tube.
My meter seems okay and just to be sure I tried another one. Same story.
Anyone an idea?
The suply Voltages are absolutely stable.
i have not seen the schematics but i suspect that there are interactions in the bias settings, adjust one pot and the others need re-adjustments...you have to be patient, a 3 to 6 mA difference i would say is not such a big a big deal...but get them as close to each other is always a good idea..
Warning!! Most polyurethane are water based. DO NOT USE water based varnish. You would be better off using insulation varnish made for the purpose of transformers. Most require you to bake the transformer or use the windings as a heat source. If you can't find insulation varnish then use shellac. Its alcohol based so you won't have shorting issues afterwards.you can also use wax to quite it down but its very messy. But it works well. But if the transformers been vacuum impregnated you mind find all of this to be a waste of time. If you noise is coming from the windings then you'll be hard pressed to remedy it without pulling a vacuum on it yourself.
Have you checked to make sure there isn't some loose laminations on it?
Also check out arcdb.as. Great site for arc info.
Nick
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