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VTA mod board

Category: Amplifiers

Hi,

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. My favorite, until now, was the Kevin Kennedy(KTA HiFi) board until I tried on a whim, a VTA board from Tubes4HiFi. Immediately, with no warm up, this 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 very 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. I then had the urge to try this schematic on a pair of Heathkit W7's. If the Dynaco ST-70 is great, then the Heathkits are world class. These amps smoked the Dynaco from the beginning of the bandwith to the last and final note. There is a difference between the two amps besides the transformers. The Hathkits have solid state rectification where the Dynaco is tube rectified. I, at some point, just to be fair to Dynaco and its followers, am going to change the rectifier to solid state, just to compare apples to apples. I doubt it will make that much difference but one never knows. I am not a solid state lover in the recitifier but you just can't tell the Heathkits are solid state rectified with this board. This is a great mod board. Just try it!
theschwrtz
2 responses Add your response
stereovinyl
1 posts
 
I have tried the board in a pair of Heathkit W-7's. I bought two of them and didn't use the third 12AT7 of each board because the W7's are monoblocks. Also, when stuffung the boards only use pins 1,2, and 3 of the middle tube and all the parts associated with them because the input circuit shares both halves of the middle 12AT for the input of each channel on a Dynaco or any stereo amp. I also didn't use the bias supply on the board. I used the stock bias supply from the original Heathkit circuit. I put the board on standoffs and built a small wood sub chassis around the board for looks. It looks unusual but it looks good in my opinion. I didn't put any parts on the bottom of the board as described in the original picture on the VTA website. I was able to get the board down a bit lower to the chassis. If the new bias supply built on the board is used then you must put the bigger coupling caps on the bottom so you have access to all the bias pots when needed. That means you will need taller standoffs.

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 on the Heathkits 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 on the Heathkits. I felt it was a bit bloated on the Dynaco. Using a different feedback resistor and capacitor value helped but not like the Heathkits.

Try this board! You will be glad you did and it is easy to install.
merccougar89
3 posts
 
This board is like christmas anytime of the year.I to have many dynaco mods from the top of the line curcio to a welborne labs circuit to a get this PUKE Vanalstine that I had bought used.The VTA 70 is unquestionably the best circuit ever put in a dynaco st70 PERIOD!! Whats so nice about the board is the fact you have a separate bias pot for each ouput tube right on the board plus 120uf plus of filtering on the driver board alone..This enables you to take out the old bias pots and filter cap and bias caps and install up to 500uf of capacitance after the choke underneath the chasis because it frees up so much room.Im using poly pro filter caps in mine and pio coupling caps and this amp is world class..Im able to power my martin logan monolith 2 electrostatic speakers with conviction and realism thats exponential.If anyone wants photos or lives in the michigan area ,your welcome to come to my home and hear it for yourself against the curcio and the vanalstine.No I dont sell them but I have put them in quite a few st70s for people.
mike
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