Upgrade all original ST-70?

After many years of considering an ST 70 finally acquired one. Seller had upgraded to the VTA board, larger capacity filter cap, Solen output coupling caps. ceramic tube sockets, original transformers, new RCA inputs, and output 5 way binding posts. The sound is best described as smooth with fine detail clarity. The solid state amp used for years is no slouch, but the fine details just not as good as the ST 70. Before making the purchased asked for guidance on another tube forum. The majority of responses warned that the ST 70 was not powerful enough to drive 87dB sensitivity speakers. No problem at all reaching bleeding ear levels.

The bug has bitten. What appears to be an all original ST 70 is on the way. Thought is to rebuild/upgrade myself. That path would include a VTA board, new sockets, new caps, etc. Also considering turning that unit into an ST 120 for comparison with the just acquired ST 70. Eventually only keep the one I like best. This is all probably nuts, but can't help myself.

texbychoice wrote:After many years of considering an ST 70 finally acquired one.  Seller had upgraded to the VTA board, larger capacity filter cap,  Solen output coupling caps. ceramic tube sockets, original transformers, new RCA inputs, and output 5 way binding posts.  The sound is best described as smooth with fine detail clarity.  The solid state amp used for years is no slouch, but the fine details just not as good as the ST 70.  Before making the purchased asked for guidance on another tube forum.  The majority of responses warned that the ST 70 was not powerful enough to drive 87dB sensitivity speakers.  No problem at all reaching bleeding ear levels.

The bug has bitten. What appears to be an all original ST 70 is on the way.  Thought is to rebuild/upgrade myself.  That path would include a VTA board, new sockets, new caps, etc.  Also considering turning that unit into an ST 120 for comparison with the just acquired ST 70.  Eventually only keep the one I like best.  This is all probably nuts, but can't help myself.    

I would rather pass on an unmolested st70 and purchase a st120 kit . You get a better
amp with less fuzz and you save one st70 for the world.

I am curious what you mean by "fuzz"? The ST70 with the VTA board in use currently is clear and detailed. Absolutely no hiss, hum or "fuzz". An all original unit might have some issues.

texbychoice wrote:I am curious what you mean by "fuzz"?  The ST70 with the VTA board in use currently is clear and detailed.  Absolutely no hiss, hum or "fuzz".  An all original unit might have some issues.

 

By "fuzz" i mean less work and trouble. ( i'm not english speaking)

Buying a complete kit that has all parts and instructions is certainly easier
then trying to get all the parts around the board, still you won't get
a stainless chassies ( which reduces hum and is more stable). You will
end up with a documented unit that has excellent support.
With the rebuilt st70 route you are basically on your own.
Selling the ST-70 will cover up for the price difference, and you could get
the VTA kit cheaper then the sum of ST70 + parts

An original unmolested st70 is a endangered species and should be preserved

Thanks for the clarification. I don't disagree about preserving a vintage ST70 if it is in operating condition and good physical condition. Build a kit or refurbish a unit, no worry about complexity, sourcing parts or support. At this point, real original units are likely to need some care. If that means an upgrade to keep them out of the recycle stream, that is the preferred approach.

texbychoice wrote:Thanks for the clarification.  I don't disagree about preserving a vintage ST70 if it is in operating condition and good physical condition.  Build a kit or refurbish a unit, no worry about complexity, sourcing parts or support.  At this point, real original units are likely to need some care.  If that means an upgrade to keep them out of the recycle stream, that is the preferred approach.    

To preserve is NOT to change. Replacing parts with equivalent is one thing,
building a different amp with the remnants is another.

Make the st70 shine, then sell it and but a vta70 ( or 120) for the money is my
recommendation.

Regarding the preservation of the old Dynas... my Dynas include an original ST70 and two Mark III mono blocks. Unfortunately in the 60' s we used them on the road to drive A7s for a PA system and we drilled out the phono plugs and replaced them with 1/4 inch jacks for compatibility with other PA stuff. Also added two 1/4 inch jacks for speaker outputs.. so, we pretty much trashed the steel cases with these mods. Other than this they are original except for new multicaps and selenium mod. Had I known then...well. This is really a cosmetic issue I guess. I think I saw Bob has some newly fabricated cases for the ST70. Anything for the Mark IIIs? BTW, the Mark IIIs were the absolute perfect amp to drive the A7s. And never died on a gig, not once.

The "original" ST70 arrived. Claimed to have been powered on without issue, but two output tubes broken and the rest of the tubes ook used up. Light rust on the transformers. Light corrosion on the chassis. PCB looks original with Black Cat Caps. A couple PCB traces appear to be separating from the board. Cloth covered transformer wires likely the source of musty smell. Does not appear to be a good candidate for preservation as an original. Will be moving ahead with a VTA Octal Board and other new parts.

Or sell it on and buy an all new kit from Bob. Much better solution and less work in the end.

Work required is not a concern. It would be unreliable if I attempted to keep it original. Too many things need to be replaced. Could pass it on for someone else to work on. At some point somebody has to put forth some effort, otherwise could end up in the trash. That would be a waste.

texbychoice wrote:Work required is not a concern.  It would be unreliable if I attempted to keep it original.  Too many things need to be replaced.  Could pass it on for someone else to work on.  At some point somebody has to put forth some effort, otherwise could end up in the trash.  That would be a waste.    

If concerned about the pc3 board, then replace it with a new , available from dynakitparts and
others. That and a bias kit + a new can cap will have it ready for another 50 years service.

A caution about trying to build an ST120 on your own (not from Bob's kit). You will be very hard pressed to find suitable output transformers.

Best to just get a kit from Bob.

audiobill wrote:A caution about trying to build an ST120 on your own (not from Bob's kit).  You will be very hard pressed to find suitable output transformers.

Best to just get a kit from Bob.

Parts on order from tubes4hifi to rebuild the original condition ST 70. An ST 120 was considered due to so many cautions an ST 70 did not supply enough Watts. The ST 70 in my system currently with VTA board assembled by another gentleman has more than enough power, so ST 120 is off the table.

Could keep both ST 70s and run each in mono mode for more power. Don't think more is better and too much is just right makes sense in this situation.