Audio Research D75A restoration hints

david winter

Classical music lover.
Hi,

I would like to restore an Audio Research D75A amp which has weak tubes.
The double triodes are all 6CG7 except for the 12AX7.
Since this is not so old I wanted to know if there are specific parts to replace:
- Whether the original electrolytics can be kept or are known to fail / leak (note that the quad in the middle of the bias pots are already replaced).
- Among the resistors, some have increased in value. Are there any spefici values to sort / match ?

Any help appreciated. Thanks !
 
Not so old!?!? It was discontinued in 1976o_O

This is a great amplifier,and is well worth your best restoration efforts.

-You should replace all of the electrolytic caps. By now,they're done and can no longer be trusted. These amps run hot,and heat is the electrolytics biggest enemy.
-All of the carbon composition resistors should be replaced as well.Notorious for drifting upward in value and generating lots of noise.

Here is the restoration thread on my own D76A which is from the same era; for an electrolytic cap reality check,look at the cap photos in my first two posts.


http://audiokarma.org/forums/index.php?threads/audio-research-score.675148/
 
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Thanks !
It's just that some caps replaced sometime ago made me think it was more recent. Indeed it's quite a restoration job.
I can see most of the big caps you used on your photo, but a parts list would save me some time finding the others, as well as replacement diodes (no big deal there, but knowing the types from someone who did a nice job is always helpful).
So if you still have a parts list that will be really appreciated.
 
I'm in the same boat. I have a D75A that I bought about 6 years ago from a friend who bought it trying to change his whole system
Mine is in very good shape , Svetlana output tubes, but caps are all silver originals and most likely should be replaced.
I have enough repair experience, but not so much with tube equipment
Also recommendation on types of ceramic and main cans caps and resistors to be updated would be great
 
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Reading the threads on the D-76 and D-75, I once had a D-76 and I traded up to a D-79. This was a massive thing as big as a big microwave oven and much heavier. It used only 4 6550s but drove the heck out of them. Interestingly, the amps had screen-grid fuses on the front panel, something I had never seen before. I was listening along one day when there was a loud “crack” from the amp. It was dead and the screen grid fuses had blown. On looking inside I noticed one of the 6550s had broken. The failure mode was amazing. The output tubes were some variety of stock 6550s that some audiophile outfit had rebranded with a lot of gold paint on the tube envelopes. Near the bottom was some text that included a small letter “o” about a couple of mm wide. The tube envelope had broken clean through the center of the “o” as if someone had drilled (or torched) a clean circular hole from which apparently the air rushed in and voila. The amp still sits on my floor after several years. I intended to fix it but the screen fuses were a type which were semi-permanent and very expensive. I bought them them and a new quad of 6550s but in the meantime I substituted a couple of small Fisher 100s in my system and it was an eye-opener. The Fishers did the job much better. The D79, Gordon Holt’s favorite back in the day, played loud and clear with a deep, deep soundstage that separated all the instruments convincingly. Unfortunately, it didn’t really sound much like music. A clarinet sounded pretty much the same as a flute, and the music never got past the speakers. If anybody wants to come and get it for any reasonable price, you can haul it out and we’ll both be happy for a while.
 
Reading the threads on the D-76 and D-75, I once had a D-76 and I traded up to a D-79. This was a massive thing as big as a big microwave oven and much heavier. It used only 4 6550s but drove the heck out of them. Interestingly, the amps had screen-grid fuses on the front panel, something I had never seen before. I was listening along one day when there was a loud “crack” from the amp. It was dead and the screen grid fuses had blown. On looking inside I noticed one of the 6550s had broken. The failure mode was amazing. The output tubes were some variety of stock 6550s that some audiophile outfit had rebranded with a lot of gold paint on the tube envelopes. Near the bottom was some text that included a small letter “o” about a couple of mm wide. The tube envelope had broken clean through the center of the “o” as if someone had drilled (or torched) a clean circular hole from which apparently the air rushed in and voila. The amp still sits on my floor after several years. I intended to fix it but the screen fuses were a type which were semi-permanent and very expensive. I bought them them and a new quad of 6550s but in the meantime I substituted a couple of small Fisher 100s in my system and it was an eye-opener. The Fishers did the job much better. The D79, Gordon Holt’s favorite back in the day, played loud and clear with a deep, deep soundstage that separated all the instruments convincingly. Unfortunately, it didn’t really sound much like music. A clarinet sounded pretty much the same as a flute, and the music never got past the speakers. If anybody wants to come and get it for any reasonable price, you can haul it out and we’ll both be happy for a while.


Where are you located?
I have sent you a pm.
 
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