ST70 7199 board component change question

Hi, I'm new to the forum, lots of great information.  I have a ST70 that I'm restoring and the original 7199 board has had a few changes by a previous owner.  The 4 coupling caps have been changed from .1uf to .2 uf. and the .05 uf caps to .1 uf.  I understand that is not an issue but a recommended change.  However, in addition the two 390 pf caps have been removed and it looks like the adjacent 1K resistors replaced with 2.2K ohm units.  To my ears, the amp sounds good and performs well.
My question:  Were the elimination of the 390 uf caps and resistor's replacement an OK change for this amp?  I would just as soon keep the 7199 board.
Thanks in advance, see pic.

https://s1169.photobucket.com/user/camaroguy62/media/IMG_2240.jpg.html?sort=3&o=0

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No, nothing under the board, no additional caps on the amp, looks like they were just removed and the resistors changed.

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camaroguy62 wrote:Hi, I'm new to the forum, lots of great information.  I have a ST70 that I'm restoring and the original 7199 board has had a few changes by a previous owner.  The 4 coupling caps have been changed from .1uf to .2 uf. and the .05 uf caps to .1 uf.  I understand that is not an issue but a recommended change.  However, in addition the two 390 pf caps have been removed and it looks like the adjacent 1K resistors replaced with 2.2K ohm units.  To my ears, the amp sounds good and performs well.
My question:  Were the elimination of the 390 uf caps and resistor's replacement an OK change for this amp?  I would just as soon keep the 7199 board.
Thanks in advance, see pic.

https://s1169.photobucket.com/user/camaroguy62/media/IMG_2240.jpg.html?sort=3&o=0

I'll start off by saying that sound is extremely subjective.
However, I do recommend, that you perhaps consider updating to the VTA driver board. Apart from it sounding better, (many here can attest to that), one really good benefit is that with the new VTA driver, you are going from BIAS adjustment of two output tubes together to individual output tube BIAS adjustment. This enables you to 'match the performance' of each output tube as closely as possible. Some additional mods will be required, such as adding two BIAS resistors to two output tunes.
Another benefit is that you would open up the possibility of using a number of different more commonly available driver tubes, 12AU7, 12BH7, 5963, etc., mix and match them to get the best sound for your ears, or even going with the octal VTA using 6SN7 tubes.

Thanks for all the good advice, I am leaning toward making this amp as it was originally designed, with the exception of replacing the selenium rectifier. I'm not sure when the Sprague Vitamin Q caps were installed, but I would expect it was decades ago, and I'm concerned about their longevity--I have had good success in other amps (not Dynaco) with Sprague/Vishay 716 series orange drops, and may go that route, going back to the original values, .1uf and .05uf. (I also have available Russian PIO K40's in the proper values). I already have replaced the can cap with a new, same value unit, as well as two new 50uf bias caps.  I then would reinstall new 390 uf caps (1000V) and replace the two resistors with original value units, essentially making the amp as designed except for the diode.  I do appreciate the value of a replacement board, both in terms of tube availability and also individual bias adjustment, but I guess I'm more of a restorer at heart than a 'modder'.

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I have replaced the bias supply caps with ones I had on hand and they are rated at 160V, so that should not be a problem.  I did read on a thread in this forum by Bob Latino that the silicon based bias voltage may be a little too high to control with the pots, and if so, to parallel the two bias 10K resistors, one with a 10K and parallel the other one with a 20K.


"The only minor thing that could happen is that in some amps after a change from the stock selenium rectifier to a diode the bias range will change somewhat. You won't get your "1.56 volts DC" bias voltage near the center of the bias control anymore. The standard thing to do if this does happen is to alter the values of the two 10K resistors on the 7 lug terminal strip. The usual cure is to add another 10K resistor in parallel with the 10K resistor between lugs 1 and 2 (to give a 5K effective resistance) and a larger 20K or so resistor in parallel with the 10K resistor that is now between lugs 3 and 4. A 20K resitor here will give you an effective 6.66K resistance between lugs 3 and 4. This should now give you the correct bias somewhere near the center of the bias control. The reason for the necessity of the resistor alterations is due to the greater forward flow of current by a diode as opposed to a selenium rectifier.

Bob"

I have a variac which is a great help on slowly bringing up the amp voltage when checking the bias settings.  Also, thanks for the caution on the pc board, I'll be careful.
Tom