New to the forum: guidance needed

Forty-five years ago, I was a penniless student and, to round-off corners, I used to built Dynakits for a newly opened audio store that was offering the wired products at the kit price to attract customers. So, I built Mark IIIs, Mark IVs, ST-70s, PAS-2s, PAS-3s, FM-2s, FM-3s, SCA-35s (a lot of those), Stereo 120s and PAT-4s.

About a month ago, I found to my utter surprise an original ST-70 box inside of which sat an un-built ST-70 kit complete except for one of the output transformers that was missing. I then vaguely remembered that, on testing a unit that I had just finished building, one of its output transformers had shorted and I had replaced it by taking one from an un-built kit box. Obviously, this was the kit I had taken the transformer from. What a strange experience to see again the transformers in padded paper material, the cardboard-wrapped output tubes, the chassis! I was back in the sixties! A bit like a time warp indeed!

So, I decided to built the kit and I started looking around on the Web World to find resources and information. I was somewhat shocked to witness the legend that has been built around this amplifier and the sheer availability of parts and modification kits.

So much so that after a while, I became confused about what to do.

• Should I try to get an original output transformer to complement the one I have or go for the new ones being offered ? I have just found an old ST70 in horrible shape (very dirty, tubes missing and driver board broken) but the owner offers to sell me one of its output transformers. Will it match the one I have? I suppose that throughout the life of the ST-70 there were different output transformers; how can I recognize if the one being offered is the same?
• Can I use the electrolytic capacitors after all those years ?
• The PC board is the original phenolic type; is it better to buy an identical but modern one or to acquire the new board their different drivers? Opinions here seem widely divergent.
• I notice that the output tubes (the four of them) have the number "3" written on top of each one. Anybody knows what it is for?
• Apart from the electrolytics, should any other discrete components, such as resistors, tube sockets, etc. be replaced?

I am relieved to have found this forum! I would very much appreciate receiving some guidance as to how to proceed to put together this piece of equipment and have it sound even better than the original.

Best Regards,

Yvon Massicotte

Welcome to the board. What a great find. Being of a certain age (over 50) the ST70 reminds me of the old Volkswagon Bug. There are an incredible amount of after-market parts available.

Best,
Doug

A quick observation: If you have the original output tubes, they are most likely Mullard EL-34's labeled for Dynaco. These tubes in original, unused condition could sell for many hundreds of dollars in an auction on Ebay, if you can claim that they test well. The sale of those tubes could easily finance a new set of outputs, and your new driver board, and all related expenses...and leave you with a healthy profit. Food for thought. Best of luck, I envy your project!

kevco wrote:A quick observation: If you have the original output tubes, they are most likely Mullard EL-34's labeled for Dynaco. These tubes in original, unused condition could sell for many hundreds of dollars in an auction on Ebay, if you can claim that they test well. The sale of those tubes could easily finance a new set of outputs, and your new driver board, and all related expenses...and leave you with a healthy profit...

Hi Yvon

Use the profits to buy a set of Genalex Gold Lion KT-666 or KT-88 tubes (assuming that you beef up the power transformer of course). Some of us on this forum believe that those tubes may just be the best-sounding OP tubes available for the ST-70.

-- JunkyJan
BC, Canada

Use the profits to buy a set of Genalex Gold Lion KT-666 or KT-88 tubes (assuming that you beef up the power transformer of course). Some of us on this forum believe that those tubes may just be the best-sounding OP tubes available for the ST-70.

-- JunkyJan
BC, Canada[/quote]

Hmmm...KT666? I've heard those tubes sound like the "devil". Did you mean KT66?

bnc31 wrote: Hmmm...KT666? I've heard those tubes sound like the "devil". Did you mean KT66?

ROFLMAO ! Yes,sorry, typo!

-- 'Jan

A Freudian typo! Suggests it's a H3ll of a tube!

I'm glad to be back on the forum. My computer's hard disk crashed and it's the first time in 3 weeks that I can sit and read the topics at leasure.

Kevco suggests that I sell the NOS El-34s, if I can claim that they test well.

How can I test those tubes and still claim they are unused?

And, does that mean that, in reality, using the old Mullard EL-34s will not make the St-70 sound better than if one uses some of the new tubes available today? In other words, is the price one can get for NOS Mullard EL-34s merely the result of a myth or fad?

Yvon

SOME say the original Mullard EL34s are among the best-sounding EL34 tubes ever made.

Whether or not this is true depends on whom you ask, what kind of amplifier they are used in, the speakers being listened to, the phase of the moon, what side of the bed you got off this morning, etc., etc...

However, bearing in mind that the EL34 specification includes a maximum plate voltage of 800 volts...I would never subject a Russian, Chinese or other current EL34 to that kind of plate voltage; but I have seen experiments in which original Mullards were run at close to that plate voltage, and survived.

Not in a Stereo 70, of course!

Yvon- I merely suggest that the perceived value of those Mullard tubes is so incredibly high that you could sell them, use the cash return to buy a fine set of new Russian tubes and still have enough left over to take the wife out for a delicious dinner every night for a week. And your amp would sound great! Perhaps they are the best El-34's out there, but they're not so much better as to warrant a potential 1000% increase in cost over the best new El-34's in the game. By merely claiming in an Ebay auction that they "were recently discovered in an unbuilt Dynaco ST70 kit" and are to the best of your knowledge unused, I would not be surprised to see them bring $500(American). If however, you were able to test them in a reliable tube tester, and document the results as 100% new...$2000 would not be out of the question for these. Brent Jessee lists these at $200 per pair used but testing good or $300 per pair new. Double that for two pairs. Let the guys with the $10,000 amplifiers brag about their NOS Mullards that they scored on Ebay. My ST70 sounds wonderful with Russian tubes. I would wager that are very few members of this site are using NOS Mullards regularly in their Dynacos. My point being: the tubes are worth more than the amp, sell them, pocket the return and enjoy the music!

kevco wrote: I would wager that are very few members of this site are using NOS Mullards regularly in their Dynacos.

How about OOS Mullards? After I had an arcover in an old Mullard in a Mark II I replaced the outputs with a pair of JJ EL34. I can't say I can notice enough difference to speak of. Then I found a couple of even older Mullards (the ones with the copper shell around the base) and put them into the amplifier. I'm happy either way but I'm careful not to let the amplifier get too far out of its linear operating range; since I run subwoofers anyway, the Mark II are bandwidth-limited to 100Hz on the low end.

I must say that I hear more differences between tubes when used in guitar amps than in hi-fi amps. Since guitar amps often sound the way they do because the tubes are intentionally being driven into their nonlinear range, this makes sense.