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ST-70 ident. Never saw this before.....

Strgazr1967

Strgazr1967

Active Member
85373FCD-EB68-4FB7-BDB7-C00ABAB5E15C.jpeg 4602CB44-C530-442E-BFCA-1E3ECBB6EE5C.jpeg B75B8B26-10E0-4563-8CAA-342FFD6C04FC.jpeg I have a chance to grab this ST-70 but was curious what mod has been done. I’ve never seen one w 4 tubes on the board. Waiting for an answer from the seller. Says it sounds amazing, all new tubes and will demo. Any thoughts?

Thanks!
Bobby
 
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@petercapo is correct. Speaking with the owner he says he purchased the amp about 10 years ago already modified. He recently replaced all the tubes and says it has worked flawlessly for him since purchasing it. He is including the owners manual but does not think he has any schematics. He has also said it is one of the best sounding tube amps he has ever heard. Something I see mentioned quite a bit in regards to this mod. The elephant in the room though is all the negative posts in regards to what was the company and it’s owner. A fact that does make me very nervous being honest. If it were not for the price I may pay for it I would likely continue looking for a more “original” or better supported modified ST-70. If it is as stated is the amp worth $350? Anyone with experience that can back up all the claims I have read about it sounding “stunning”? I have 2-3 days to do more research so any help
Is appreciated.

Bobby
 
Honestly, for $350 I don't think you could go wrong. Worst case, you could replace the driver board with something from Bob Latino.
 
The main value lies in the transformers and chassis. If these are good and the present board is working, you are well set. The new tubes are a bonus, and I’m guessing that the replacement board saves you from the cost of expensive 7199 driver tubes.
 
I would definitely keep it as is and seek out any/all dox on this build re Wellborn.
Fairly rare, most other mod examples are a dime a dozen as are oems, this is supposed to be the best of the best.
 
petercapo said:
Curious that the sonically stunning, ostensible Rolls Royce of Stereo 70 mods is selling for only $350. And, yes, I recall the problem reports about the company that I agree were troubling. The point has been made in other discussions that most of these vaunted "mods" for the Stereo 70 do not have testing results from an independent lab to verify their designers' claims.

Further, did Welborne Labs' idea of a good-sounding amplifier match with your idea, today? Even if you would find it a good-sounding amplifier, is there a way to know if it is functioning properly or not? I don't mean whether it powers on and plays music - a tube amp can do that but still not be working properly and still not be performing to its potential. Also, the Welborne mod was available as a kit - did the kit builder do a good job building it?

If you want to pursue this, IIRC Holger at Erhard Audio has a copy of the paperwork for it. The idea that you can't go wrong buying it, because the price is worth the transformers alone, depends on a few things like whether the transformer leads have been trimmed too short for rework, if the chassis has been cut up, etc. There are some variables.

Might be helpful if there were additional, larger, detailed images. If you could get a totally complete set of documentation, namely the complete assembly manual, schematics and any other documentation it would have originally come with, then it might be worth a try. If not, seems to me like a roll of the dice.

The best documented, independently tested and verified, time tested Stereo 70 is the classic, original 1959 version that doesn't actually need modification to sound really good. In case you haven't seen it, here is a relatively recent, landmark article on the subject - note in particular the first two paragraphs: http://www.audioregenesis.com/documents/ST-70 Base Line Testing.pdf Here's another, brief article - note in particular "Q7:" https://www.dagogo.com/interview-mike-sanders-quicksilver-audio/
Click to expand...

Pete,

The gentleman selling it works with my wife and is retiring. He is packing up to move south and he has known about my interest in audio for awhile now. He offered me the amp and said he wouldn’t take any more than that because of all the times my wife has helped him out.

He was able to find both the original owners manual as well as the Welbourne paperwork minus the schematic. I am hoping that hunting around here could yield me a copy.
 
Pio1980 said:
I would definitely keep it as is and seek out any/all dox on this build re Wellborn.
Fairly rare, most other mod examples are a dime a dozen as are oems, this is supposed to be the best of the best.
Click to expand...

Pio definitely going to give it a shot. It may not turn out to be the best of the best but it is something to work with hopefully. It appears to be in mint condition. The new tubes alone are probably worth almost $250-300. I’ll start a thread on it hopefully Wednesday night/ Thursday morning.
 
Strgazr1967 said:
Pio definitely going to give it a shot. It may not turn out to be the best of the best but it is something to work with hopefully. It appears to be in mint condition. The new tubes alone are probably worth almost $250-300. I’ll start a thread on it hopefully Wednesday night/ Thursday morning.
Click to expand...
PM @BuzzK about the skiz.
 
Curious what the circuit is. Knowing what the tubes are could narrow down the guesses somewhat, but at the end of the day there are really only so many ways to drive a pair of output tubes. It generally comes down to one of two commonly used phase inverters, the only real difference is where and how the voltage amp stages are placed. I doubt anyone bothers with paraphase splitters, but I guess that could count as a third style.
 
Welborne had a big catalog at the end of 1994 that promoted his projects and offered technical advise. As implausible as it might sound, I found my copy and indeed it does include the entire nine page build manual for the Alan Kimmel designed Dynaco ST-70 driver board. At the time, Alan Kimmel was a big proponent of the Mu follower topology and in the early 1990s, he wrote technical articles in Glass Audio promoting it. He also designed a complex driver stage for the ST-70 using a 5751 and 6U8A that was marketed by Welborne. The kit also included a new regulated bias supply board and power supply filtering. Welborne recommended and sold the Magnequest replacement output transformers for the ST-70. Some considered the Kimmel designed driver board the best of those available at the time. I'll scan the article some time today; so if anyone needs it they can PM me.
 
gadget73 said:
Curious what the circuit is. Knowing what the tubes are could narrow down the guesses somewhat, but at the end of the day there are really only so many ways to drive a pair of output tubes. It generally comes down to one of two commonly used phase inverters, the only real difference is where and how the voltage amp stages are placed. I doubt anyone bothers with paraphase splitters, but I guess that could count as a third style.
Click to expand...
Long tail differential?
 
That's definitely a Welbourne. I have the docs if you need them. I started to re-do their schematic using component values instead of part numbers, but haven't completed it. My goal was to try to figure out how many dB of feedback was used.

Minority opinion here, but I think the Welbourne mod is the best sounding driver stage I've heard on an ST-70. I'll put my ST-70 against any other within the same power and PP class. It's my benchmark.

$350 is a steal. The driver kit cost at least that.
 
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@BuzzK PM sent, Thank you for the offer.

I originally was a bit leary but the more I read and the more input you guys offer it seems I may have a pretty desirable ST on my hands soon. Provided it's as nice as the owner says. Feel free to keep the info coming.

I will keep everyone posted. Pick up is set for Wednesday morning.
 
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