An audio friend showed me some pics of a VTA 70 he picked up (without tubes), but his driver board looks a bit different than current versions. Can someone tell me which version this is, and what driver tubes are compatible with it?
5 posters
Which Version VTA 70 Is This?
knotscott- Posts : 63
Join date : 2019-12-28
Location : Rochester, NY
peterh
peterh- Posts : 1869
Join date : 2012-12-25
Location : gothenburg, sweden
There seems not to be any CCS ( transistor-looking devices), thus this is an olderknotscott wrote:An audio friend showed me some pics of a VTA 70 he picked up (without tubes), but his driver board looks a bit different than current versions. Can someone tell me which version this is, and what driver tubes are compatible with it?
for ECC81 / 12AT7 type tubes
knotscott likes this post
knotscott- Posts : 63
Join date : 2019-12-28
Location : Rochester, NY
Thanks Peter. Agreed...there are no LM334s or similar that I can see.
Bob Latino- Admin
- Posts : 3277
Join date : 2008-11-26
Location : Massachusetts
Yes - that amp has the older VTA ST-70 driver board from 2007 - 2011. In early 2012 we changed the VTA ST-70 driver board to the CCS version that uses the LM334 IC to give the driver circuit a constant current source. Nearly all of those amps without the LM334's use 12AT7 driver tubes. The newer VTA amps (2012 - to the present) use 12AU7 type driver tubes. The newer board using the 12AU7 driver tubes has a very slightly lower signal to noise ratio.
Bob
Bob
knotscott likes this post
1973shovel- Posts : 45
Join date : 2009-12-13
While most pre-CCS boards may have used 12AT7s, that doesn't mean that they all did. If your friend wants to be sure which tubes his amp is configured for, have him check specific resistors which converted the 12AT7 version to use 12AU7s.
Below is a link to an old VTA Forum thread, one where, in post number 6, Roy identifies the resistors in question, and specifies their values. Have your friend identify those resistor values, either by their color bands, or by lifting one leg of those resistors and taking a measurement. That's really the only way to know which tubes the older VTA board is configured for.
https://dynacotubeaudio.forumotion.com/t1860-what-are-the-current-resistor-values-and-power-ratings-for-the-hi-gain-12at7-and-lo-gain-12au7-12bh7-vta-mkiii-iv-upgrade-boards#13292
Below is a link to an old VTA Forum thread, one where, in post number 6, Roy identifies the resistors in question, and specifies their values. Have your friend identify those resistor values, either by their color bands, or by lifting one leg of those resistors and taking a measurement. That's really the only way to know which tubes the older VTA board is configured for.
https://dynacotubeaudio.forumotion.com/t1860-what-are-the-current-resistor-values-and-power-ratings-for-the-hi-gain-12at7-and-lo-gain-12au7-12bh7-vta-mkiii-iv-upgrade-boards#13292
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gener8r- Posts : 61
Join date : 2012-04-28
I wouldn't want to bias that board, flipping the amp over and over to get the two back tubes sync'd
Bob Latino- Admin
- Posts : 3277
Join date : 2008-11-26
Location : Massachusetts
gener8r wrote:I wouldn't want to bias that board, flipping the amp over and over to get the two back tubes sync'd
Those two 10K pots on the bottom of the OLD board were NOT USED TO BIAS the amp's back tubes. Those two pots were used to set the the phase splitter section of the OLD driver board. What you did before you installed them was to measure the resistance from the center pin to each of the two outer pins and adjust the pot until the you got about 5000 ohms on each side. These pots were installed on the bottom so that once set, you could not adjust them without taking off the bottom cover and once set properly, these two pots never needed any adjustment.
The newer VTA driver boards from 2012 to the present use two LM334 IC's to do the job. The LM334's are current regulators that do the job that the two 10K pots once did.
Bob
10-E-C likes this post