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In Reply to: RE: SEP 6L6G posted by hifipaul on April 02, 2015 at 15:46:53
Guys:
Thank you for your comments and help. The need for adequate gain became very important as feedback was applied. The selected gain tube is a 7f7. The circuit is similar to the RH design.
Some years ago I modified a ST70 to take just about every output tube with a 6l6 type pin out. I found the 6l6g to have the most compelling midrange of all of them; and that proved to be true here again. Note all of the tubes in the pic.
Surprises: All of the 6l6s sounded at least good. The Soveteks were way better than I expected. RCA 6L6GBs (black plates) had the best balance top to bottom, really not a surprise though.
I am now into the Frack with it stage. The feedback needs to be dialed in, parts selection, and I need to decipher of a cathode follower is needed between stages (as in the RH schematic).
Follow Ups:
The 7F7 is essentially a loctal 6SL7. This makes your amp very similar to the one DAK has built.
When you mention the RH amp, I guess you should be referring to the RH Universal. I expect that you have applied the LM317 cathode solution?
If the above is true you don't need to "tune in" the resistor ratio, just follow the schematics: with 6SL7 you should have a n anode resistor of 100k and an Rfb of 200k.
You don't need the cathode follower on this amp.
Depending on the current set for the output tubes, and the primary resistance of the OPTs, you might play with other tubes as well. Either 6V6, 6F6 etc or EL34, 6550, 8417 ...
******
http://rh-amps.blogspot.com/
Alex: Thanx for your comments.
I am using a cathode resistor of 500ohms to bias the 6L6 in UL. The CCS is a great idea, but simplicity ruled the day here. It runs 50ma at 300v across the tube. All versions of the 6l6 family work here + 6550s. They all sound different.
The dialing in that I referred to involved the feedback scheme. After experimenting (by ear) I settled on 147Kohm feedback resistor to the anode + some global feedback. In general I kept the FB to a minimum.
This project was a test to find out whether I like the sound ( I do like ) before I embark on a bigger version using PSE 6550s or KT88s.... and real SE outputs. A test-bed chassis will be set up over the summer.
A friend of mine now has the amp. He thinks it sounds wonderful.
I wonder how long that Rubycon multisection cap will last ?
Al
Good point. Sometimes the obvious is hard to see.
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