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here is our Dynaco ST70 page

click here to go to Bob's amps (ST70-ST120-M125)

our famous VTA70 driver PCB upgrade for your amp!


we've been selling this modification/upgrade for the ST70 since 1989 - 32 years now!!
we now offer 2 choices

our classic version using 12AU7 tubes
our NEW octal version using 6SN7 tubes
how to choose if you're not sure which one to buy . . . ?? 

* If you have an older stock version ST70 choose the "classic" version.
* The new octal version can be used on ST70s that have an upgraded power transformer
    (to handle the extra current needed by the larger octal tubes) 


(photo courtesy of customer Tevian)




see our customer comments on this mod/upgrade at the bottom of this page!!

non-USA buyers, please contact me by email first before placing your order!!

shipping to non-USA buyers will cost an additional $10 to $35 for shipping and customs paperwork!!

email "tubes4hifi" for info


"classic" version bare PCB with parts list, schematic, and manual is $35.00 FREE shipping in USA  
our "classic version" VTA70 modification is newly upgraded with a CCS (constant-current-source)
in the "long-tailed" phase inverter for perfect AC phase balance,
and is still the premium ST70 modification available on the market.
It has an upgraded bias supply and 4 individual bias adjustments, one for each output tube.

top view of PCB

bottom view of PCB
"classic" version PCB and parts kit is $85
(NOT including 3 driver tubes) FREE shipping in USA
"classic" version assembled and ready to install for $130
(FREE shipping in USA)(NOT including 3 driver tubes,),
Plugs right into your ST70 amp.

octal driver for ST70

This board uses three 6SN7 tubes rather than the smaller 12AU7 type tubes,
with a CCS (constant-current-source) for perfect phase-splitter balance.
Same exact size, parts layout, and hookup connections as the boards above.

lower photo shows bottom side with optional K-42 PIO caps

better set of three tubes for new octal driver
$86 for set of three
VTA70 octal driver, bare PCB only $39
(free shipping in USA)
VTA0 octal driver, PCB and parts kit (no tubes) $99
(free shipping in USA)
VTA70 octal driver, assembled, tested, ready to install (no tubes) $139
free shipping in USA)

here's a comment from a recent "VTA70 octal" buyer . . . .

"Hi Roy, 
Just wanted to let you know that I got the amp up and running, and it sounds great!
I'm very happy with the 6SN7 board.  I did some quick measurements on the completed amp 
and they were better than I expected. Here's what I got:
Max power before clipping 40W 
Response @ 40W +/- 1db  20Hz-30kHz
THD @ 20W  .12% @ 1kHz  .15% @ 20kHz  .45% @ 20Hz
As far as sound Vocals are outstanding. The overall sound has a nice balance of warmth without loosing detail. 
Very good soundstage in both width and depth. 
I congratulate you on a fine upgrade to the old Dynaco work horse and I thank you for your quick responses 
to my Emails during the assembly process.    Bill K"


NOTE: the following items do NOT including shipping unless ordered with a kit above,
if ordering separately please add $5 to your order for shipping

add $5 for shipping fees
add $10 for shipping fees
add $20 for shipping fees
add $30 for shipping fees

Another option is a capacitance upgrade from our standard 188uF on board,
to 400uF on board, for an additional $12 !!
(upgrade price is a substitute for standard caps)

K42-Y PIO capacitors
In our opinion these provide a better sound than even the expensive AuriCaps.
Possible shipping delay as we get these directly from Russia.
$25 for a set of 4

add 3 JJ 12AU7 tubes for $40,
these are good tubes for the "classic version"!

add 3 EH 12AU7 tubes for $50,
these are better tubes for the "classic version"!

add 3 mil-spec 6189 tubes (12AU7) for $60,
these are excellent tubes for the "classic version"!

add 4 JJ EL34 output tubes for $80 - good quality at a low price

add 4 Tungsol EL34B output tubes for $125 - high quality tube at a great price

new input jacks $16 pair GOLD PLATED (price includes USA shipping)

original type UPGRADED quad cap (80+40+30+20uF@525v) $50

original type Dynaco switch (power, mono-stereo) $4 each (only with orders over $20 total)

dual speaker posts $18 pair - these are standard 3/4" spacing
and must be used with single plugs or terminals, fits ST70 and MK3 only

new TRIPLE speaker posts $24pair - note these do NOT use standard 3/4" spacing
and must be used with single plugs or terminals, fits ST70 and MK3 only
Triode/pentode switching kit
This kit may be added to any amp driven with the VTA mod PCB to allow your amp to operate
in either the normal pentode ultralinear mode, or switch to sweet sounding triode mode.
Simply remove the old bias pots and install the switches in the empty 3/8" holes.
Note - Kit cannot be installed in a stock amp which still uses the old original bias controls.
Comes with all parts and instructions. $30 with FREE First Class mail delivery in the USA.

Digital Voltmeter

check your bias !
one for $14 free shipping
two for $24 free shipping


This is a "time delay relay" board" for tube amps and preamps.
The time delay allows the tube filament heaters to come up before any high voltage is applied.
The time delay is nominally about 17 seconds but may be adjusted for more or less delay
at the discretion of the user. There are complete instructions for the assembly and installation
of the board. In a stock ST-70 the board replaces lugs 5, 6 and 7 of the 7 lug terminal strip.
In a VTA ST-70/ST-120 tube amp kit, the VTA-TDB replaces the 3 lug terminal strip.
The VTA-TDR may also be easily retrofitted to any VTA ST-70/ST-120 tube amp kit.

The board is 1 7/8 inches wide by 2 1/2 inches long. It takes about 1/2 hour to assemble and
is powered by a 6.3 volt AC line taken off V2. When the board is powered up a RED LED lights up
to show that the board has power. About 17 seconds after power is applied, a GREEN LED comes on,
and the relay kicks in to apply high voltage to your amp.
If you are in an area that has quick ON/OFF/ON power interruptions the board will automatically reset itself.

This board is to be used ONLY for solid state rectifiers. If used with a TUBE rectifier,
the TDR can cause problems with the normal slow warm up of the tube rectifier and may damage a tube rectifier.
If you intend to use a TUBE rectifier ONLY with your amp, you do not need the TDR.
If you do use a tube rectifier with the TDR, you should BYPASS the TDR.
Instructions are included with the TDR on how to bypass the TDR.

This board is a relay switch that will not apply the high voltage power to the amplifier until after the
tube filaments are warmed up, the board has a 17 second delay that can be changed.

This is a very heavy duty industrial relay used for 280vac 3 phase and rated at 8 amps.
This is a great addition to your tube amp. It can run on 6vAC or 12vDC.

TDR assembled $45


click here to go to complete amplifier kits click here to go to our MK3 page click here to go to our downloadable Dynaco manuals page

visit our new "Dynaco Tube Audio Forum"

Written Articles

ST70 modifications comparison test - Glass Audio magazine article


here's a link to a wiring diagram for our VTA amp

here's a photo of the internal wiring on a VTA70 amp, with the Triode/Pentode switches . . .


see Bob's page to order an amp like this !!


contact us for any parts you may need for your Dynaco ST70
email "tubes4hifi" for info

our dealer on the east coast is Bob Latino. . . he offers NEW Dynaco ST70 KITS and ready-build amps

click here for his "buy-it-now" page

 brand NEW ST70 amplifiers, in KIT or ready to go! 

you can send email to Bob Latino directly here for info on these complete kits or amps . . .


some additional info on our VTA70 PCB . . .

Our driver circuit is a modern all triode driver design, for totally neutral uncolored sound.  
One half of the first tube is used for each channel as a voltage amplifier.  
One each of the other tubes is used per channel as a combined phase splitter/driver 
in a long-tailed pair configuration.  13 db total loop feedback is used. 
Parts quality is top notch, with mil-spec tubes, metal film resistors, and propylene caps.
There is an individual bias pot for each output tube, as well as a signal balance pot for the 
phase splitters. 
The bias is set for 40 ma, about 20% less than stock to reduce heat and conserve tube life. 
Open loop response of this design is incredible.  It has flat frequency response from 
7 Hz to 70 KHz, with good square waves from 7 Hz to 30 KHz.  Open loop gain is 40. 
The PCB is 8.0" x 3.3".  Max power output is 40 wpc. 

This board compares very favorably with the best of the competition, and is a better quality PCB for less money.
For example, the Curcio board with comparable options (DC & AC balance, propylene caps) is about $40 more, and it is single sided.
Ours is double-sided with plated thru holes. Now available in our improved design using 12AU7 or 12BH7 tubes




     customer comments

I bought and built this amp and have compared it to gear costing many times what this did and they cant beat them. 
GREAT JOB on your circuits and amplfier mods. Thank you.   Mike

You are amazing. I placed an order on saturday and the parts arrived at my home with mondays mail !
Thanks again,  Steve

Awesome amp.  I am really happy with the results.  This is my final product for my system.   
We are using the amp now and we are extremely happy with it.  
Thank you for your awesome design and I will recommend your kits for anyone else that wants this quality product.  
I may have a couple of people that want you products.   I will be keeping an eye on your web site for any future products.
Thanks for you fine products.    Mark 

I am very pleased!!  I don't think I have ever had such good sound.  
I am using your phono stage, SP14 preamp,  a pair of M130 monoblocks, and B&W 803 speakers.
Thanks for your very fine equiipment.    Allan

I just wanted you to know how much I appreciate this upgrade.  
I have 2 ST 70s. One I rebuilt to the original specifications and the other I did using your board.  
The original amp sounds muddy in comparison to the upgrade. 
Your circuit is quieter and there is much better bass definition. In addition it is a very quiet amplifier. 
I enjoyed assembling and installing it! Such a pleasing result is quite rewarding.
Thank you,   Kip 

Thanks so much!  It's always a true pleasure doing business with you  
You're an absolute gentleman.  Have an excellent evening.
Regards,   Mark

It is this kind of great service that will make people come back and buy more from you! Thanks!
/ Anders

Hey Roy,  Got the mod board done.
This amp simply kicks ass sonically and i look forward to recommending your products.  
thanks for the good after sale bedside manner for dopes like me.   Mike

All-in-all, I am pleased with how your circuits perform and would not hesitate to purchase other kits from you. 
Your technical support when I had a problem has also been excellent. 
I do not know how much positive feedback you receive, but I thought I would let you know we are happy 
with your circuits. My wife said this is the best sounding system we have had so far. 
Best regards,   Ralph 

"I finished your mod and this is honest to god the first time 
I ever enjoyed listening to an ST70..The ST70s to me have always 
sounded kind of sloppy, but this mod tightened it up and the overall 
tonal balance is much much much better.  The biggest thing I noticed 
was the lininerity of the sound and the focus of the instruments 
possess an exponential difference now as opposed to the stock ST70.
The amp is tighter and much faster."
    Bill     (VTA70 mod)

"I have only run my ST-70 a few hours (w/ all tubes new), and let me 
tell you how happy I am from the sweet sound I'm hearing.  
Compared to stock, well no comparison.  
Better highs, deeper and wider soundstage and tauter bass, more 
transparent and better detail by a factor of 10.  I heard an ST-70 
with a Curcio upgrade last night driving the same speaker I have 
and using a higly regarded preamp, and in my ears, 
there is no contest.  Thanks very much.  A very happy customer."
  John R.  M.D.  (VTA70 mod)

"hi roy
I put together the board of yours and then I wired it to my Dynaco ST70.
Let me tell you this is the most stable, accurate, fast, solid sounding 
mod I've ever heard on a Dynaco... WOW.  
I couldn't believe the drive they have.  Oh by the way a friend of mine 
had the Curcio modded amps and he heard these tonite and he said this is 
a much better sounding design than the Curico." 
     mike    (ST70 mod)


Superior packing, very honest seller, A+'s all the way, very smooth transaction

Quality workmanship,quick shipping.Recommended!Thanks!

Nice amplifer, works great. It sounds better than my MC240! no problems!

Awesome product as promised!!!!! Nice and quick delivery!!!!!!


September 2005 comments
. . . posted on AudioGon by recent customers . . . .


 VTA mod board  

I know all of you out there who are do it yourselfers(DIYer's) have done a mod or two 
on the most recongnized modded amps around, the Dynaco ST-70 and the Dynaco Mark III's. 
Well, I have bought just about every mod board available and have even done a few on 
others not sold mainstream from time to time. 
  Immediately, with no warm up, this VTA board was showing much promise and once it warmed up 
it was all there, period. After looking at the schematic and seeing what was done I was 
saying to myself and a close friend who is an accomplished tech, was saying the same thing. 
  "This board shouldn't be doing to the amp what it is doing." It took a marginal amp 
and made it great. This is a great mod board. Just try it!

Now for the sound. Let's just say I used the board on a few Dynaco's and I was happy with the results as compared to other mod boards and stock boards I tried. But when I put the boards in it took on a new life form. Dynamics in spades. Musical? None better! This is the best mod to an EL-34 amp I ever heard. The music is about as real as it gets with lifelike images that make you feel the musicians and singers are in the room for a personal perfomance just for the listener. The bass is extremely accurate and tight. I felt it was a bit bloated on the original Dynaco. Try this board! You will be glad you did and it is easy to install. stereovinyl 09-23-05

RE: VTA70 PCB kit It is a real winner - finished the amp in one day and everything worked perfectly and all was very quiet. All four 6ca7s balance at the same point on the pots (.40v after 12hrs of burn in) and remain stable. I put in some classic Eric Clapton, Eagles, Joe Cocker and David Allen Coe in the CD changer and WOW!! This thing is pure and really belts out a tight bass, with expanded mids and brilliant highs. It's been on continuously now for 3days playing a variety of old vinyl, along with DATs of CDs and live performances. I really think it outshines my MAC 225, and certainly beats any ST70 I have ever heard.

Roy, just a follow up to my recent order of a VTA board with the upgraded audio caps. I installed it last Sunday, which took no more than an hour. Fired it up, biased it per directions and everything immediately checked out O.K. (after warm up, the B+ voltage is settling at 439, 1 volt below the norm you set in your directions). It's driving a new set of Svetlana "winged C" EL34s, employing vintage Mullards. To be succinct, the sound is incredible - much more robust, with depth and detail far, far beyond what I previously was hearing from the amp (that isn't just me, even my thirteen year old Ipod-centric daughter commented on how "three dimensional" my system now sounds). It is even bringing out a decent soundstage from a satellite radio signal!!! Given that, listening to analog sources (vinyl and a vintage Kenwood tuner) is pure heaven. Thanks again for a great product.

Roy - WOW - yours is some pc board !! > It makes vanalstines ultimate 70 board > sound terrible by comparison. Much thanks!! > John F. I have used over 20 of Roy Mottram's VTA driver boards in Dynaco ST-70 builds for customers. They are extremely well built, have no open solder traces and in comparison to the Triode and Curcio boards - IMHO the VTA simply sounds better. The VTA board has individual bias controls (DC balance) for each output tube so a matched quad of output tubes is not really necessary. I have the VTA MOD Dynaco ST70, and a freind of mine has the VTA MK3 mod, as far as I am concerned it is the best Dynaco upgrade ever, and it doesnt take a rocket scientist to install. check it out at Dynaco tube amps and Klipsch are made to be together. Just installed the board in my ST70 and I have to say it sounds superb! I had a previous upgraded board that's well regarded but sounds dull in comparison to your board. N. Haider, MD

Hi Roy, A note to let you know that I received the PCB and the parts I ordered. The PCB is absolutely outstanding as compared to the Curcio! It's clear that your intent was to give your customers a high quality PCB at a very reasonable cost.

I have only run my ST-70 a few hours (w/ all tubes new), and let me tell you how happy I am from the sweet sound I'm hearing. Compared to stock, well no comparison. Better highs, deeper and wider soundstage and tauter bass, more transparent and better detail by a factor of 10. I heard an ST-70 with a Curcio upgrade last night driving the same speaker I have and using a higly regarded preamp, and in my ears, there is no contest. Thanks very much. A very happy customer. John R. M.D.

Hi Roy, Received your VTA ST70 mod, and I am thrilled. I must say, the quality and craftsmanship that have gone into this board are truly exceptional. You may be interested to know that before I ordered this board from you I did my homework, and investigated most of the mod boards being offered by other companies. Time and again, yours was rated by far the best, and now I can see why. I have loved the sound of tube amps since the 1950's, and have always strived to have the best sounding equipment(and generally LOUDEST) in my entire town. I didn't realize that part of your mod board INCLUDES additional power supply filter caps, When I finished the project, and powered it up according to instructions, and everything seemed just fine, but at first I thought there was something wrong because I couldn't hear any hum at all through the speakers. Then the music started. WOW! This is an amazing board you've designed. It has tons of power, plus great range. Amazing !! Thanks for going the extra mile for me -Peter S

This amp is absolutely LIQUIED. The sound stage is large and has wonderful detail! I would recomend this MOD for any ST70 Steve (VTA70 mod) Your board sounds really nice! I will recommend it to my friends with Dynaco Stereo 70's. To be honest the Dynaco just didn't cut the mustard in stock form but now it�s a whole new animal. Joe When I finished the project, and powered it up according to instructions, and everything seemed just fine, but at first I thought there was something wrong because I couldn't hear any hum at all through the speakers. Then the music started. WOW! This is an amazing board you've designed. It has tons of power, plus great range. Peter

Roy Just a note to let you know how much I enjoy the driver board I purchased from you. It made a night and day difference in the way the Dynaco sounded. I had heard people rave about the stock Dynaco ST70 so I bought one only to be disappointed by its sonic signature. I was assuming that it would have some bass and authority but it actually sounded very bad instead. Your circuit changes and your board brought it new life and authority that it should have had if it had been designed correctly in the first place. It ranks up with my "quicksilver" amps now and frankly if I had had the modified Dynaco first I never would have bought them. They sit gathering dust now while I use the Dynaco with your VTA70 modification. Best Regards, Joe T

Roy, Your ST-70 mod board is by far the best sounding one I've ever heard. I'm truly impressed by the improvement in sound and the other benefits of the board, like seperate biasing for the EL34's. I didn't expect anything good when I bought it, but had to try it as I've tried everything for the ST-70. I'll spread the word. And Thanks! Audiodyssey Anthony Cordero

Good afternoon, The amp is getting better and better, I put about 12 hours on over the weekend. I'm ruined. This was a second system. My main system is a pr of Magnepan MGIIIa's with Oddesey Mono amp's (high Current). My other system is a SET FI X 2a3 with fostex BLH. After listening to the Lascalla's and your upgraded amp board I cannot listen to the other systems.. It's like going to AM radio. I mean the bass the punch the dynamics the imaging the clarity. It's outstanding.

join the growing list of people that have been amazed and blown away by the incredible transformation this seemingly simple modification makes. It is a TOTAL TRANSFORMATION of the amplifer and the sound quality is unbelievably unbeatable!!


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 tubes 4 hifi
 "the path of least resistance is through a vacuum"