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In Reply to: RE: ST-70 Output Tubes posted by Triode_Kingdom on August 11, 2021 at 22:19:41
"The primary impedance on the original Dynaco A-470 output transformers was 4300 ohms. This works out well for EL34, 6L6, KT66, KT77, KT88 or 6550 output tubes."
The above is a quote from Bob Latino. I think that is the source of the confusion.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Follow Ups:
Mike Samra was a big advocate of using a 6L6 in VTA's version of the ST-70 (link to but one post, below).
I put their (stuffed with parts by me) 12AU7 board into one of my ST-70s years ago, and was never in love with it. I describe the sound as "too solid-stateish" and have been told by more than a few that the non-linear 12AU7 was the prime suspect. I tried 12BH7s, but that was largely a lateral move.
After reading Mike's frequent recommendations, I bought some 6P3S-E from Jim McShane, back when he still sold them. I'm not going to claim the amp was "transformed", but it certainly was a bit more pleasant to listen to music with than with the (original but used) Mullard EL-34 or reissue Gold Lion KT-77 I tried.
I'll leave it to those with more circuit knowledge than me to explain it (Canceling distortions? He asks, very tongue in cheek) but whatever it was, I liked the VTA better with the Soviet 6L6 variant.
"I tried 12BH7s, but that was largely a lateral move."
IMO the 6cg7 would have been a better choice. The 6cg7 is just as linear as a 6sn7, one of the most linear signal tubes ever made outside of some direct heated signal tubes.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Apparently I didn't do enough research. Since the 6CG7 isn't a direct plug 'n play replacement for a 12AU7, I must have missed posts recommending it.
I did some more homework just now and found a couple of comments on the 12AU7. One, by former Inmate Neff (who I always appreciated) said, "As I stated the only worse thing [than] having a 12AU7 in an amp is a chassis fire." which brought me a good laugh over my morning coffee. I was aware that the 12AU7 wasn't a respected tube, but humor like that almost always gets my attention.
The second comment was by Mike Samra, who wrote, "I will tell you and I know others feel this way, I don't like the 12au7 in an amplifier, period! The 6cg7 is so much better and it's more or less a drop in replacement for a 12au7 by just moving the filament wires when you have a 6.3v fil source. The tube is much more linear and makes more power and the end result is lower distortion with a more detailed and open sound.
For the life of me i can't understand why any manufacturer or builder would still use the 12au7 and the only thing I can think of is, they are cheap, and available."
End quote. (link to Mike's post below)
My unenthusiastic idea was to convert the amp to 12A77s, since it only involves a few resistor changes, and because I have five NOS JAN 12AT7s sitting here, doing nothing. Triode Kingdom informed me that the 12AT7 would only be marginally better than the 12AU7 though, hence the lack of enthusiasm.
Converting the VTA board to 6CG7s will require cutting and rewiring a few filament traces, but I had thought about hacking into it anyway, to add the CCS they now use, and eliminating the AC balance pots. I've got nothing to lose, since the amp's just been sitting here collecting dust for a long time.
Once again Tre', thanks for the recommendation. You brought some hope and possibilities to an amp I had no idea what to do with, and it's always nice to have a plan.
.
Have Fun and Enjoy the Music
"Still Working the Problem"
Tre', have you tried any of the reissue 6CG7s? I wonder if they're as linear as NOS.
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Buy Chinese. Bury freedom.
I have a good stock of NOS and OS and my system doesn't use them anymore.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
I'm also going to email Jim McShane to see what his recommendations are.
On the other hand, NOS 6CG7s and 6FQ7s are still somewhat available. This issue of using better tube types and other parts is exactly why I hate PCBs. You and I have discussed this in the past. If I had a ST-70, I would do everything possible to replace that board with an aluminum panel that could be wired point-to-point. Dynaco used PCBs in their products to save production cost and make kit building easier. We don't have those priorities. :)
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Buy Chinese. Bury freedom.
Edits: 08/14/21
"Hacking" PCBs isn't the best way to go, long term. But, on the other hand, it's easy enough to do, and the 6CG7s should give me an idea of whether or not I'd be moving in the right direction.
If I feel it's enough of an improvement, then I could make a turret board, perhaps with octal sockets for 6SN7s, which would enlarge my tube choices.
Don't tell anybody but I once took a hole punch to a Citation II and put a 6sn7 based Williamson front end with low NFB. I up-tapped the output transformer to get a higher impedance for the triode strapped 6550s.
Less power but it's a great sounding amp in a bi-amped situation (crossover at 100Hz) with the lows filtered out at the input. (when you up-tap an output transformer the reflected impedance goes up but the primary inductance doesn't change so the LF -3db point goes up an octave)
My cousin still uses that amp and loves it.
Looking back, I could have just built the thing with 6cg7s and kept the value instead of "cutting up" the chassis but at the time Cit IIs weren't worth that much. I think I paid $100 for that one. I owned 4 of them at one point.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
I think I felt a collective chill from all the Citation II fans while reading your post.
That might make an interesting thread, asking "What did you wreck [or devalue] in an attempt to improve things?". I'm betting there's some early Marantz or other tube gear out there with holes where they don't belong or a chassis painted red.
I was going to tear into my Audio Research SP3-A-1 preamp around thirty years ago, and the advice I got was "Leave it alone". Given what they're going for now on ebay, it was good advice.
There are a number of things I'd like to change in my Citation V. Much of it would be hidden, like the use of larger PS caps and maybe the addition of a screen choke. However, rewiring a socket for a different tube type hurts resale value, even when socket size stays the same. The mod I want most - a front panel power switch - would really kill its market value, no matter how professionally it's done. Wish I had bought a dozen of these back when they were $100 each. That's the only way I could justify making one of them "my own."
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Buy Chinese. Bury freedom.
I had a running Cit.ii and a pair of W6m carcasses...I kept the Heathkits and sold the restored Cit.ii. Too much money got waved under my nose.
Cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
Ain't that the truth? Pre internet, I got to see a lot of great old hi-fi equipment. But being a Rust Belt dweller at the time, I had little disposable cash to buy any of it.
That was when I bought my modest Dynaco collection, and even that has increased in value.
There's more to life than money. :-)
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
That's true, and I probably should have left the monitary value out of my comment.
My other interest, in addition to hi-fi, has always been motorcycles. Finding an old, 1940s original paint and parts Harley or Indian is rare. Everybody was customizing them in the 1960s/1970s, not thinking a thing about it. But now, finding one in its original state is a thing of beauty.
I'd be happy to find a modified Marantz 8B, some extra holes drilled in its chassis, but I'd be a lot happier to find one that hasn't been touched, simply for its aesthetic value.
In the summer of 1974 my good friend Ron Finuf one year out of high school, while working for David "Huggy Bear" Hansen at "The Shop" bought an old police bike. I think it was a pan head. He ripped it apart and chopped it finishing it just in time to ride it to Idaho to watch Evil do his thing.
Huggy Bear and a bunch of other guys from The Shop were going but changed their minds at the last minute and headed off in a different direction. Ron rode all the way there and back solo.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
We have a legendary Harley drag racer named Pete Hill from the area I now live. Pete was in his seventies when he jumped on a rigid frame (no rear shocks) Harley Knucklehead (which preceded Panheads) and rode it to Sturgis SD and back to SC, about 3300 miles, round trip.
That was back in the days when men were men, and sheep were nervous, as the old saying goes.
I've ridden bikes all over Texas, Colorado, Pennsylvania and upstate New York, but I don't have any stories like that to tell. Except maybe the time I rode from Colorado Springs to Dallas on New Year's Day when it was 12 degrees and a foot of new snow on the ground. It was so bad, I didn't ride for nearly ten years after that experience. :(
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Buy Chinese. Bury freedom.
Punch that 12 ºF into a wind chill calculator on line, add your average speed, and it's no wonder you weren't anxious to get back in the saddle, not to mention the potential for ice on the road. Hypothermia is a very real thing on a bike.
My most grueling ride was when a few of us left for New Hampshire about six years ago. We did 1003 miles in 17 hours that first day. We'd have pushed further, but one guy's eyes were so beat up by pollen that he was having trouble seeing, so we stopped for the night and finished the run the next morning.
Wow...and I thought riding 100 miles in 7 hours was pretty sweet. It was in the summer...lots of fun in -7F with studded snow tires, riding trails and rivers. Best epic ride ever.
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
can't say that I did not occasionally get jealous of the motorized folks...LOL
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
.
Have Fun and Enjoy the Music
"Still Working the Problem"
.
Have Fun and Enjoy the Music
"Still Working the Problem"
That was prior to the release of the 6sn7 board which is what is in my Don Sachs amp which is the custom version of the ST-120.
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You paid HOW MUCH for that electrical receptacle?!!! Are YOU nuts?
when Mike wrote that he must have been using the high gain version driver board using 12AT7's. Not the newer low gain 12AU7 board.
What leads you to assume that it was the earlier, 12AT7 version Mike was talking about?
Below is a link to one of his 12AU7 VTA posts, but a search will turn up others. If I had to speculate, I think Mike had tried both, because in one post he listed the resistors to change to convert the circuit from the AT7 to the AU7.
He's not correct, the EL34 that these were designed for is absolutely the best match. Thats true from the standpoint of both output power and distortion. This doesn't mean the other tubes can't be used in a pinch, but the design of these amps is really optimized for the EL34
.
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Buy Chinese. Bury freedom.
.
Have Fun and Enjoy the Music
"Still Working the Problem"
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