Dynaco ST-70 mod poll

Ive been looking for some Mullard EL34's for my Fisher Amp and ended up buying an entire ST-70 for the tubes.. Its all stock as far as I can tell, and is in decent shape, including the cage. Since the previous owner had it running a month ago i powered it up. Surprisingly, this amp is very quiet with all the old components still in it. Im considering just replacing the selenium rectifier with an IN4007 and just be done with it. I'm starting to really like its simple layout. Got a lot of polishing to do, as you can see I've started that process.

back to the title of the post: If I were to decide to upgrade this amp, what is the best mod in your opinion? Should I? There seem to be many many upgrades out there. Just curious what the consensus is here?








Thanks,
Tom

As far as "the best" mod out there, you will get no consensus at all. However, IMO if you want for the amp to sound great, I'd look into one of any number of Mullard topology mods which replaces the input board and implements a long-tailed inverter/driver stage. 7199s are a very marginal-sounding tube to base an amp around IMO.

Dave Gillespie recently did a thread on the ST 70. IIRC He made some suggestions regarding the PS (though I don't remember exactly what) . Its worth reading in any event before you move forward.
Like goldear said you wont find consensus. My understanding is if you follow that thread it would at the very least be an excellent starting point. (they have a pretty stellar reputation already).

Just keep in mind that everything you do is better than any other thing you do, except the ones that make it worse than all the others.

My opinion: Listen to it stock for awhile and then decide what you want to do.

My opinion ( which is just an opinion) :
replace the selenium rectifer and be done about it.

Whenever an 7199 fails there will be problems, at that point get a "converter" for an alternative tube ( 6gh8 ).

If the rectifier tube starts flashing, consider replacing the big cap to a simular one.

They sound darn good stock. Nary it for a while as mentioned. You can always mod later.

What preamp are you going to use

Tom,
Good question. I used an older VTA board with the three 12au7 tubes when I did mine a few years ago. At the time, I really liked it. It wasn't, isn't and will never be the most detailed amp I've ever heard-I think that's just an output tranny limitation. It is however a very musical and enjoyable amp to listen to, but I think it will never earn a spot in your main listening system. It does make a nice back up amp though...so that's something to consider.
So many ways to do it. Leave it alone, keep the original circuit but use a modern and more reliable PCB, replace the driver board with a metal plate punched for tube sockets and do it point to point, or try one of the many replacement boards out there. There is no "best" way from what I've read.

The only consensus I've seen is that the mains transformer is a little underpowered and the output trannys are not the best.

Two things I would keep in mind:
1. Engineering is the art of the compromise
2. You can't make a silk purse out of a sow's ears.

mfm22 said:

They sound darn good stock. Nary it for a while as mentioned. You can always mod later.

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I could not possibly agree less with this statement. Stock stereo 70s combine everything bad which tubes have to offer (very mushy-bass, and no HF-detail) combined with none of the good aspects (no glorious mids, no sweet highs, and no 3D imaging).

These amps are a good platform for building a decent sounding amp on top of. But unless pure mush is the kind of sound which you love, I'd make some significant changes.

However the advice to listen to the amp stock for a while first is good advice.

I couldn't disagree more with goldear. My stock 70 has all the atributes he claims they lack. I run them on Klipsch Forte'(+90db spl) speakers and enjoy glorius clarity, soundstage, bass performance, mids and highs. Couldnt ask for more. I use a custome demo disc for speaker/system tests and the 70 always performs excellently on all the various musical styles on the disc.
Maybe goldear has not heard the 70 on the right speakers. I dont doubt he has had bad experience with it.
My experience has been totally contradictory. But we all have different ears and so hear things differently.

Replace the crispy driver board, consider the bigger power tranny from Triode, consider if the main cap needs replacing.

Oh...yeah...the selenium rectifier too.

goldear said:

I could not possibly agree less with this statement. Stock stereo 70s combine everything bad which tubes have to offer (very mushy-bass, and no HF-detail) combined with none of the good aspects (no glorious mids, no sweet highs, and no 3D imaging).

These amps are a good platform for building a decent sounding amp on top of. But unless pure mush is the kind of sound which you love, I'd make some significant changes.

However the advice to listen to the amp stock for a while first is good advice.

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ferninando said:

I couldn't disagree more with goldear. My stock 70 has all the atributes he claims they lack. I run them on Klipsch Forte'(+90db spl) speakers and enjoy glorius clarity, soundstage, bass performance, mids and highs. Couldnt ask for more. I use a custome demo disc for speaker/system tests and the 70 always performs excellently on all the various musical styles on the disc.
Maybe goldear has not heard the 70 on the right speakers. I dont doubt he has had bad experience with it.
My experience has been totally contradictory. But we all have different ears and so hear things differently.

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I stand somewhat in the middle. Though I no longer use a Dynaco Stereo 70 or MK IV amps, I loved them. The highs were softened and not fully extended, but nevertheless sounded terrific. They were not dull, muted, etc. The bass was rounded and certainly not prodigious, but yet sounded ample and realistic. The midrange of these units was beautiful, lacking the nth degree of detail but sounding like music. More interesting I think, is that in my systems in which Pas 3 and an Audio Research preamps were used as well as both Magnepans and coned speakers, the imaging was both broad and deep.

I set one up with a Dynakitparts 7199 reproduction board with new componants,(cc resistors) bias balance kit, and k42 caps,painted black to look original.
Looks original. I like how that one sounds.

People wonder why I don't own an ST70. Owning one is like watching football. Everyone knows better than everyone else what needs to be done with it. I don't like football for that matter either.


Amusingly, if you look at Dave's measurements of a dead stock ST70, they bear out almost none of the complaints about weak bass, weak highs, etc. That frequency response is about as flat and clean as you can reasonably expect to see, especially given the rather modest budget point these were sold at some 50 years ago.

http://www.audiokarma.org/forums/showpost.php?p=7608491&postcount=5

mind you those are single channel driven measurements. Both channels might come out somewhat worse, but its probably power supply related.

The power supply really is the weak point, not the rest of the guts. They beat the ever living snot out of the rectifier, which goes along with how often you see people burning them out and railing against the quality of modern 5AR4 tubes. Sand rectify it, maybe add a second choke and split the channels so each got 1/2 the load, and thats probably as good as its getting short of upgrading the transformer. You'd probably also be able to get much closer to single channel driven performance out of both channels with a power supply that can keep up.

I totally agree that the "complaints" are NOT backed up by any tests/data on ST70s in original "new" condition. Certainly, a 40+ year old amp's components MAY NOT be at spec any more so saying that an original amp doesn't sound as good as some new or just-modified amp shouldn't be too surprising. But there have been a fair number of people who have modded these amps and then returned them to oem circuitry, preferring that sound.

I think something that is often overlooked about the ST70's "reputation" is that it was mostly offered as a kit. Therefore, much of it's reputation depended on the skill of the assembler.
If you know what you are doing, how to dress wires, component layout, etc., it will sound as designed. If not...it will still work, but maybe not so well. The little things seem to add up and become noticeable.

I'm running my st70 with the current VTA driver board with russian pio and rca black plate 5963s and GL reissue kt66s. It sounds fantastic.

I'd love to see how many modern AVRs will be up and running at all in 50-60 years. Personally, I'd bet against them.

valveglow said:

i'd love to see how many modern avrs will be up and running at all in 50-60 years. Personally, i'd bet against them.

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"avr" ??