Best output tubes for Dynaco ST-70

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Best output tubes for Dynaco ST-70

Postby darkhawk22 » Fri Apr 29, 2016 9:02 am

Ok I know this question is a bit subjective, of course, but what would be the best replacement output tubes for a basically stock Dynaco ST-70.

Backstory:
I purchased an early 1960's model not long ago and upon receipt I replaced the Multi-stage cap with the 80-40-30-20 from dynakitparts and also the Rectifier due to a hum issue. The output tubes that are on there must be fairly generic as they have no text on them at all. One red-plated at one point prior to replacing the rectifier but has since worked fine, though the readings are surely different from the other three at this point. The biasing holds at approx. 1.5 to 1.6 and the output sounds fairly even.

I'd like a nice full range Output tube

What i'm considering:
JJ/Tesla 6CA7
Electro Harmonix Fat Bottle 6CA7

Any other suggestions?
Thanks everyone!
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Re: Best output tubes for Dynaco ST-70

Postby TomMcNally » Fri Apr 29, 2016 11:33 am

I'd stay away from the JJ's ... I build a new amp that had 8 of them in PPP and I think all but two of them went into 4th of July Mode, internal sparks and fireworks. I believe they also have very skinny pins, which tend to come loose in the sockets. I replaced them with Svetlana's and have no issues since.

... tom
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Re: Best output tubes for Dynaco ST-70

Postby petercapo » Fri Apr 29, 2016 9:14 pm

I thus far feel that the baseline character of my Stereo 70 (newer build, but original circuit topology) can have some emphasis in the bass region. And, to bring out more of the highs, I need to pick the right tubes.

Of the two choices you are considering, I have read a couple of different sources indicating that the JJ 6CA7 may be on the warm side with a pronounced bottom end. If this is true, then I would not think it would be a good choice for a stock Stereo 70 if you are looking for a more balanced sound top-to-bottom. The question of JJ reliability is a consideration, as well.

My impression from reading other comments is that the EH 6CA7 is less like a “typical” EL34, and sounds more lively and clear, and, I think, with more top end, all of which would be better for a stock Stereo 70.

In my experience, the EH EL34 also is lively with more top end than other EL34s, though not as overall refined as other EL34s. A good choice, IMO.

I have tried the reissue Mullard EL34, which I feel is a good choice, though it is not as lively as the EH and has a moderately extended top end and a less prominent bottom end.

One surprise I had recently was when I tried the Tung Sol EL34B. I read a lot of positive comments about it. But, I have not found it to be a good choice for my Stereo 70. To me, it sounds congested, too laid-back overall, the top end is too recessed and the imaging/soundstage tanked. Maybe this is a good tube in other amps, but it doesn’t work for me in my Stereo 70.

The overall best current production tube I have thus far tried is the Gold Lion KT77, though I hate to admit it because IMO it’s overpriced.

You know, I’d really address the red-plating thing further. It can have different, sometimes intermittent causes, and if the problem is not stabilized you’ll keep wrecking good tubes.
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Re: Best output tubes for Dynaco ST-70

Postby Geek » Fri Apr 29, 2016 10:39 pm

Heard pretty much every tube in these.

+1 for stay away from JJ and add TAD to that list.

Best bang-for-the-buck: EH EL34.
Best sound I personally found was winged =C= Svetlana's... the old ones, not the new ones.
Worst experience: Shuguang (sick) half will work, half will not. It's a crapshoot.
(the half that won't, go Chernobyl quite spectacularly =:o )

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Re: Best output tubes for Dynaco ST-70

Postby cdmawolf » Sat Apr 30, 2016 10:53 am

I also prefer the Flying =C=, Gold Lion, and EH Big Bottle, not the skinny EH that is more for guitar amps.
In that order...

I have a hard time using tubes that cost the same as the amp itself.
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Re: Best output tubes for Dynaco ST-70

Postby petercapo » Sun May 01, 2016 6:55 am

Wish the =C= were still generally available. The few places that still have some are fetching high prices for them, and they may be late production runs that reportedly were not too reliable, which I also observed for myself - tried buying some from a seller in Russia in the past year - almost sent my amp to an early grave.
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Re: Best output tubes for Dynaco ST-70

Postby burnedfingers » Fri May 06, 2016 5:52 am

I always enjoy the word "BEST" in any thread. I'm a believer in that an amplifier I will call "A" will sound different with a different preamp and different speakers. Not a real believer in "Best" tubes. My opinon is tube "A" will possibly sound different when coupled with preamp "A" than with preamp "B" What I have learned is to leave the J J tubes sitting on the shelf and purchase anything other than them. The same is true with speakers... Tube "A" may sound different with speaker "A" than it does with speaker "B ".
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Re: Best output tubes for Dynaco ST-70

Postby Geek » Fri May 06, 2016 6:20 am

Funny, had a similar discussion today with a customer who was frustrated with any S.E.T. he tried sounded like garbage on his fav pair of speakers, but fine on the rest.... turned out his fav speakers were on the list of "fuggeddabbout" using S.E.T's on ;)
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Re: Best output tubes for Dynaco ST-70

Postby petercapo » Fri May 06, 2016 8:26 am

Yes, but in this case the O.P. qualified his question by stating he wanted a “…full range Output tube” for his stock Stereo 70, not merely asking which one was "best." I took this to mean he is looking for a tube that would have a balanced, more complete spectrum from top-to-bottom, which I know not all EL34s have in my Stereo 70 - some tubes have this, others less so.

Synergy among system components is, of course, a huge factor. But, I would think the baseline sonic character of a given tube, or the relative difference in sound between different tubes, is likely to be fairly consistent. Unless, of course, there are differences in circuit design that would result in a given tube sounding substantially different across different kinds of amplifiers.
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Re: Best output tubes for Dynaco ST-70

Postby darkhawk22 » Fri May 06, 2016 12:44 pm

Thanks for your input everyone. Currently, it has winged C's in it. The ones installed are the older version with whitish looking letters and logo. Honestly, I didn't pay that much attention to it originally. After further listening, I've not had any further issues with red-plating and the tubes seem to be biasing as they should. I've really been enjoying the amp so far and have cleaned it up slightly with some Flitz metal cleaner and it looks great. Next up is to repaint the transformers.
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Re: Best output tubes for Dynaco ST-70

Postby rmyauck » Fri May 06, 2016 8:18 pm

How about 6L6 types like the new TS fat bottle C type?
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Re: Best output tubes for Dynaco ST-70

Postby skidave » Mon May 09, 2016 7:47 am

rmyauck wrote:How about 6L6 types like the new TS fat bottle C type?


More details please. TS is which vendor?

Thanks
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Re: Best output tubes for Dynaco ST-70

Postby petercapo » Mon May 09, 2016 7:56 am

I believe rmyauck was referring to Tung Sol. A number of folks go to other tube types, like the 6L6 family, for the Stereo 70. However, you’re likely to run into biasing issues with the stock St-70 that would require either changing the bias circuit resistors and/or changing the rectifier to a 5U4, which would reduce overall power output. Additionally, Dave Gillespie studied 6L6 in the stock Stereo 70 and noted an increase in distortion. Based upon this, if you want best power output with lowest distortion, the original Stereo 70 was really designed for EL34/6CA7 with the KT77 apparently also working fine. But, again, some folks nevertheless like to use the 6L6 family – just be prepared to possibly change some other things about the amp along with it.
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Re: Best output tubes for Dynaco ST-70

Postby dannyr » Mon May 30, 2016 7:26 pm

My favorite in this amp is the EH 6CA7 but I haven't tried the gold lion kt77's or the new Tungsols. I'm so happy with the 6CA7 I doubt I ever will. I have had JJ KT77's. Mullard XF2s, EH EL34's, Simmens EL34's, SED el34's and 6L6GC's, Reflector 6P3S-EV and probably others that I can't recall and 6CA7 sounds best to me and is affordable.
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