Paoli 60M / VTA ST-120 mashup

Hello all! Long time visitor, first time poster.

I recently stumbled on this website http://www.blackdahlia.com/html/tip_84.html. I had never heard of Paoli, but was intrigued by their modifications to the venerable MkIII, namely the additional independent power supply for the output tube screens. Apparently they got some good press in the early 70's but were taken down along with the demise of dynaco. There is shockingly little information about these amps online.

As I am in the midst of building my own variant of a VTA ST-120 and have a natural propensity to not "leave well enough alone", anyone see any quantifiable advantage to adding a few 2000uf caps and a second power transformer to my build?

Thanks Bob! Yes, I had seen that thread, though not much more information there than on the black dahlia page.

Really looking for

1. Why giving the output tubes a separate supply is an improvement on the original schematic? If anyone understands the theory, please fill me in or point me to some reading material.

2. Spec of the secondary power transformer.

3. Will this in any way be an upgrade to my ST-120 build, or am I just wasting my money.

I'm thinking the second transformer would be a crutch to support a power transformer that wasn't up to the task when it came to driving some of the more popular tubes that the amp wasn't originally designed to use. Like KT88's in an ST70 fer instance. Sure, you can do it, but you're really pushing things with the original iron when you crank it.

Not a problem with the ST120 though. MUCH beefier iron. Compare the stack size between the original Dynaco units and what ships with the kits ... BIG difference. I've been running a quad of KT120's as well as a 6SN7 in mine with nary a complaint. That's with the wall voltage stepped down to 117vac with a bucker, the result of which is a slightly lower B+.

jho wrote:Thanks Bob! Yes, I had seen that thread, though not much more information there than on the black dahlia page.

Really looking for

1. Why giving the output tubes a separate supply is an improvement on the original schematic? If anyone understands the theory, please fill me in or point me to some reading material.

2. Spec of the secondary power transformer.

3. Will this in any way be an upgrade to my ST-120 build, or am I just wasting my money.

(3)
An excellent and complicated way of wasting money!

You your money to a good (tubed) pre-amp !

Nice to have you actively onboard, jho.
The design of the VTA ST-120, as others have said, has made such mods as an extra power supply superfluous, I believe.
If I were to make any changes to a self-built ST-120 from stock, I would address several very small design issues, and one medium-sized one.
First, the placement of input jacks and power switch would be reversed. Inputs in rear, switch in front. Little things which make life more pleasant.
The bigger (but certainly not huge) change that I would make concerns the rectifier circuit. The stock 5AR4 glass rectifier here is stretched fairly thin - same capacity as in the ST-70 but twice (or so) the requirements. The most common failure in the ST-120 is the rectifier tube. There are two theoretical fixes. One is to move to plug-in SS rectifier - but of the usual choices here the Weber copper-cap option is often the source of meltdown too. Permanent, heavy duty installed SS rect. has been described several times in this forum. Me, I like sticking with tubular mode - it just sounds right to me (maybe it's the slightly reduced B+ or maybe the ever-so-softened attack). The realistic solution here is to utilize TWO 5AR4s in parallel. Here's the rub - there's barely enough breathing room for the one in the stock design, let alone two. The solution is a bigger chassis. But that opens up an expensive kettle of fish.
The only other fix is to seek  vintage 5AR4s, or equivalent (discussed many times in this forum) but that's also expensive (good ones are way over $100 a pop now and going up as we speak) not to mention the crap-shoot of the ancient tube that's probably as old as you are.
Just my penny-and-a-half on prioritizing modifications to the VTA ST-120. You may ignore me henceforth.
And I do agree with peterh - the addition of a VTA preamp - go for the SP-14 and you'll never regret it - will do more for you than any mods you could make. Weawy, Doc.

I think Kently means 5AR4's. They are kind of expensive. Here's a list from one dealer....

Mullard 5AR4/GZ34's…$165.00
Philips 5AR4/GZ34's, metal base, 1957 tv2 production in crisp original boxes…$395.00
GE 5AR4's…$125.00
Sylvania 5AR4's, fat bottles…$120.00
Sylvania 5AR4's, mis-labeled as 5V4GA's…$95.00
Hitachi 5AR4/GZ34's, high quality Japanese rectifier tubes…$70.00

Dave_in_Va wrote:I think Kently means 5AR4's. They are kind of expensive. Here's a list from one dealer....

Mullard 5AR4/GZ34's…$165.00
Philips 5AR4/GZ34's, metal base, 1957 tv2 production in crisp original boxes…$395.00
GE 5AR4's…$125.00
Sylvania 5AR4's, fat bottles…$120.00
Sylvania 5AR4's, mis-labeled as 5V4GA's…$95.00
Hitachi 5AR4/GZ34's, high quality Japanese rectifier tubes…$70.00

Oopsies. Yup. Fixed it.
The insane markup on the Philips is due, no doubt, to the "crisp cardboard boxes".

at those prices you can buy ten half decent chinese rectifiers, which overall should give you about the same life.
Don't forget to add the "yellow sheet" diode mod, that's about 40c worth of diodes that will likely triple or quaduple your rectifier life anyway!!
THEN, yes, buy a good tube preamp!! A tube amp makes music sound really good, a tube preamp makes music sound more than incredible !!

skizo, your ST-120 build is one of my inspirations, but I didn't start with the kit, just the bare driver board. All components are hand picked and will be fit to a custom chassis. I have a quad of KT120s and a few RCA cleartops waiting on the shelf.  

As to the stock iron not being up to the task, may be, but take a look at the schematic. Mr. Key must have thought it was worth rejecting the UL taps on the OPT and creating the secondary 'stiff' supply to the grids. And those 2000uf caps couldn't have been cheap. Even if I don't end up trying this on my amp, I'd like to understand the supposed performance advantage.

As for you guys who somehow made this thread about tube rectifiers, just like in the 60M, my rectifier is solid state. Though I do like the thought of a pair 5AR4s!

Kentley wrote:

Dave_in_Va wrote:I think Kently means 5AR4's.

Oopsies. Yup. Fixed it.

OOpsie ... missed one ...

Kentley wrote: The stock 5AU4 glass rectifier here is stretched fairly thin

No issues using the Mullard GZ37 here. Got a spare AND a couple old Mullard GZ34's locked in the vault.

tubes4hifi wrote:at those prices you can buy ten half decent chinese rectifiers, which overall should give you about the same life.

Problem being, a rectifier can cause some serious damage down the line if it blows. I imagine having 10 blow would really be pushing yer luck.

jho wrote:skizo, your ST-120 build is one of my inspirations, but I didn't start with the kit, just the bare driver board. All components are hand picked and will be fit to a custom chassis. I have a quad of KT120s and a few RCA cleartops waiting on the shelf.  

As to the stock iron not being up to the task, may be, but take a look at the schematic. Mr. Key must have thought it was worth rejecting the UL taps on the OPT and creating the secondary 'stiff' supply to the grids. And those 2000uf caps couldn't have been cheap. Even if I don't end up trying this on my amp, I'd like to understand the supposed performance advantage.

I deleted the last part of your quote about rectifiers ... wouldn't want to make this a thread about rectifiers, eh.

Maybe a bit of confusion on my earlier post ... I mentioned the "stock iron" being weak, but that was in reference to the original Dynacos. All the ST120's have had the beefed up iron, therefore, my comment about unnecessary redundancy when it comes to a adding a second transformer. I don't expect there'd be any other benefits either - noise, isolation, that sort of thing. One way to find out though.

* No comment about the additional caps. Filtering in the stock build seems more than adequate. My only concern would be that too much capacitance could slow response/transient times, causing increased distortion. If anything, I might consider going with upgrade components with the same ratings.

Example of possible overkill ... I remember a few folk scratching their heads about the larger chokes I used in my build ... my thoughts were that even if the chokes supplied with the kit were more than adequate, the larger mass would result in better heat dissipation if nothing else. Can't really say they made a difference one way or another, but hey ... I had a lot of room to play with, ok.

(And I have to say I'm +1 on preferring a bottle rectifier. Didn't much care for the sound of the Copper Cap. I suppose a lot depends on whether you're in the "you CAN hear the difference" camp when it comes to rectifiers. In any case, I have also done the yellow page mod, so there IS some silicon involved.)

Oh. And thanx for the flowers! I'm a big fan of my work as well ...   ;-}

PS ... current driver set is a 6SN7 center and a triode matched pair of Westinghouse Star 5363's in the channels. Those seem to have taken root - can't imagine it getting any better!

*******  >> Nice looking build taking shape here if you're still looking for ideas:

https://dynacotubeaudio.forumotion.com/t3067-swedish-vta-st-70

Let me weigh in here as, I believe, the first customer for Roy's VTA SP14... Or at least I tried to be. Ordered it as soon as he came in line to report what he had cooked up.

Since then, of course, I've dumped this piece of crap....

Heh heh heh JUST kidding!

KIDDING!

You know, over on the Transcendent forum, go check... You will often read assertions that the Grounded Grid is about as good a pre amp as you'll find under anywhere between $5k to infinite.

Well, ai have one, and it is great, but I replaced with an SP10 (was it a "10" Roy?), and it was much better, but at about the same price.

Then came the SP 14, and this was before oil went to $9.95, taking Alberta's economy with it.

To cease making a short story too long, the SP 14 is not just "way better", it's stunningly better. Transcendent would never sell another GG if customers heard this. It's truly a staggeringly good pre amp.

Make no mistake, though. The SP 14 is merely part of a broader conspiracy. The same people are responsible, culpable indeed for other acts of audio sedition.

Between Roy and Bob, they have brought uodates and modifications to the original Dynaco design that will simply make you weap when you hear real music played through these products.

I say experiment, by all means, but I sugest after, not before or instead of building a VTA kit. Start with the best kit available, then improve on it if you can.

jho, I know we've kinda sorta gone off-topic. But not really - we attempt to put the picture into focus. Tube Nube, in his inimitable fashion, has perhaps clarified the Big Picture better than anyone. Until you taste the steak, don't douse it with A-1 sauce.