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VTA or DIY Tube ST70 Board???

Rocjr

Rocjr

Active Member
It's been a long while since, and after trying a number of different driver boards, that I conceded that I'd rather just let the ST-70 be what it is. Many of the boards and upgrades seem to only make the 70 better in some ways while nearly always killing what made the amp appealing at all. In the end the amp always seem'd to come up short. Better, but so-what. Having tubeamp-itis, I have lots of amps that I like to rotate in. The 70 has been relegated to an output tube burn in mule. I'd rather have an infant output tube short out and wreak havoc in a 70 than an 8b or Fairchild 255As.

A few weeks ago I visited a pal that has an AES pre, phono, and Altec Valencias. That day he had a ST70, completely rebuilt, with a DIY tube board. OP tubes were SED EL34s. I know his system well. It never sounded better, and I've heard it with a number of amps. I know better than to simply single out the amp for the good sound. I'm not quite a novice. But, I don't need but a nudge to get an amp on the bench, and I'm inspired again! Now, I know what you're thinking--Wouldn't time be better spent stocking the seed vault, panning for gold, or spackling the bomb shelter? I say NO! It's Stereo 70 time again :nutz:

So, has anyone tried both boards and have an opinion? If you have a preference does any one or more things need to be done in addition for best sound? A must do? I have a luverly, clean, cloth version number. The thought of individual bias pots again, finally...makes me smile.

Rich
 
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I use a version of the VTA board, that board with the low gain tubes I do up.

It sounds really really good, plenty of tube rolling, separate bias for all 4 tubes,
plenty to tinker with, all in all a solid performer, can't say against the other one.

For power supply I use about 220uf under the amp after the choke to beef it up.
 
Kegger,

Thanks for your comments. Is there anything meaningful sonically to be gained by adding the higher value decoupling caps on the board?

Does your favorite tube selection require resistor changes, or are you just talking about specific tube rolling with what's available /plug in and play?

I'm leaning heavily towards the VTA board. I can borrow my buds DIY Tube'd ST70 to do a shootout down the line.

Rich
 
great timing on this thread. i may be coming into (4) st70's that are all original. im looking into replacement boards myself. i wondered about those mod bards but am leaning towards getting the dynaco doctor stock replacement board and just populating with my choice of parts. all this hangs in the balance to see if i get the amps though.
 
ke4mcl said:
great timing on this thread. i may be coming into (4) st70's that are all original. im looking into replacement boards myself. i wondered about those mod bards but am leaning towards getting the dynaco doctor stock replacement board and just populating with my choice of parts. all this hangs in the balance to see if i get the amps though.
Click to expand...


The problem with the stock board is that 7199s are in thin supply, 6u8s and an adapter are the option. Plus the stock board/circuit has no DC balance option which is a pain over time, along with other issues.
 
Yes, this is good timing. I'm also considering a new board for one of my ST-70's and boards for my Mark III's. So far I'm leaning toward a VTA board for the 70 and DIYtube / Poseidon boards for the III's. All will get under chassis cap boards.

Ron
 
Rocjr said:
Kegger,

Thanks for your comments. Is there anything meaningful sonically to be gained by adding the higher value decoupling caps on the board?

Does your favorite tube selection require resistor changes, or are you just talking about specific tube rolling with what's available /plug in and play?

I'm leaning heavily towards the VTA board. I can borrow my buds DIY Tube'd ST70 to do a shootout down the line.

Rich
Click to expand...

The HUGE caps used on the board are not really doing anything there, you are better
off doing more at the output tubes that way the whole amp gets it. 33uf on top is fine.

I redid the circuitry for the low gain version, I'd suggest buying blank board/populate it.
(If you get one I can give you any tweaked values needed)
 
Rocjr said:
The problem with the stock board is that 7199s are in thin supply, 6u8s and an adapter are the option. Plus the stock board/circuit has no DC balance option which is a pain over time, along with other issues.
Click to expand...

7199's are not a problem here. been hoarding them for years. never intentionally bought one but always set them aside when one randomly popped up in a tube lot i bought.

the stock boards look appealing to me as i personally dont like buying "modded" stuff.
 
A couple of friends have the VTA boards. A few months ago I had the chance to listen in at a comparison session. A bone stock ST70, one with a VTA board but otherwise near stock, and a clone which used the VTA board and had an uprated power tranny and new manufacture clone OPT's. The stock amp didn't have a chance. It was muddy, slow and thick compared to the others. We compared with EL34's, original Genelex KT66's, and some Russian 6p3s coin base types with appropriate rebiasing for each on the VTA board having the 4 bias pots. The Russian tubes were great and the crowd favorite that day. I know this isn't scientific, but I would buy the VTA board and have no regrets if I had an ST70.

Best,
John
 
i dont think xicon caps are exactly quality stuff. i've gotten them in cheap grab bag deals numerous times. yeah i know, just change them. by the same token youd think they could spend an extra buck or two and put something a little better known on there.
 
I only have the DIY baord and love it.
No indiv bias... but allows for bal and bias of the pair.
I then used C. Chongs cap baord... very very nice unit.

I am building a 2nd st70 and chose to keep all the same.. new power trany and alike, but use triode Elec.'s board... It uses EF86 tubes and a 12AX7.

I like both over the VTA.

I know the VTA is realy nice, but I think their advertising is misleading, and I hate the power suply in on the PCB. I like 2 seperate sections... thus the DIY and Triode choice.
 
Your right the "advertising is misleading" in a way, the power supply IS NOT on the
board for the VTA version, just the "RC" stages for the front end are, 4 each for all
of the triode sections, actually it is a good thing to have them as close to the tube
section using them as you can, so that is a nice thing, I use 4 33uf decoupling caps
in those stages, then supplement underneath for the "power supply" after the choke.

Technically the RC stages are "part" of that power supply, but they do nothing there
for adding capacitance where it counts, the output tubes is where it should be upped.
 
justinsweber said:
I only have the DIY baord and love it.
No indiv bias... but allows for bal and bias of the pair.
I then used C. Chongs cap baord... very very nice unit.

I am building a 2nd st70 and chose to keep all the same.. new power trany and alike, but use triode Elec.'s board... It uses EF86 tubes and a 12AX7.

I like both over the VTA.

I know the VTA is realy nice, but I think their advertising is misleading, and I hate the power suply in on the PCB. I like 2 seperate sections... thus the DIY and Triode choice.
Click to expand...


Justin,

Are your objections based purely on aesthetic considerations, whether subjective or practical, with regard to any of these boards? I only care about the sound. Can you comment on your impressions, sonically, concerning the VTA board against the others given that it's the odd man out in your response? That's really what I'm hoping for.

Rich
 
nerdorama said:
A couple of friends have the VTA boards. A few months ago I had the chance to listen in at a comparison session. A bone stock ST70, one with a VTA board but otherwise near stock, and a clone which used the VTA board and had an uprated power tranny and new manufacture clone OPT's. The stock amp didn't have a chance. It was muddy, slow and thick compared to the others. We compared with EL34's, original Genelex KT66's, and some Russian 6p3s coin base types with appropriate rebiasing for each on the VTA board having the 4 bias pots. The Russian tubes were great and the crowd favorite that day. I know this isn't scientific, but I would buy the VTA board and have no regrets if I had an ST70.

Best,
John
Click to expand...

was the bone stock st70 recapped or was it really bone stock as in the day it left the factory?
 
ke4mcl said:
7199's are not a problem here. been hoarding them for years. never intentionally bought one but always set them aside when one randomly popped up in a tube lot i bought.

the stock boards look appealing to me as i personally dont like buying "modded" stuff.
Click to expand...

The basic ST 70, (actually that's a misnomer, as the ST 70 is an Eico amp, the Dynaco is a "Stereo 70") is a good performer, but was a price point Item.
The use of the 7199 and the circuit is genius, but hardly cutting edge sonically. The 7199 has both a pentode and a triode for phase splitting. Nice and cheap, also compact, but hardly the last word in drive.

Thus, the stock circuit is compromised. Also, the power transformer is right on the edge of barely having enough juice, another design point that allows this to be tolerated is how heavily the tubes are biased towards Class A, making the power drain on the transformer at least fairly constant.

If the stock circuit makes you hard, go with the SDS or Triode labs mod, basically the same circuit but with a real pentode (EF86 I believe it was?) and 12 AU7A triode tube replacing the triode section of the weeny 7199.
I used to put a 220 Cerafine under the chassis like Kegger suggests, AFTER the choke. The stock board works, but is hardly high end....new parts help, but the entire circuit is a compromise.

While opinions vary, the best I built was the Welbourne conversion, which sounded excellent. In fact, that was my first tube amp project, fired it up and worked for over 10 years. Many have had a lot of problems getting that board to work, its hardly a beginner board.

Good luck, it will be great fun no matter what you do, and dont misunderstand, the stock amp is very pleasant, but as one poster posted, listening to his friend reworked amp and commenting it sounded great in his friends sysytem, yes it can sound that good!

Russellc
 
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