Single Channel Dyna/VTA 70 - Can I Remove A Driver Tube?

[knotscott]

I have a pair of Dyna/VTA 70 "twins" that I use as single channel mono blocks. Can I safely remove the driver tube from the side that has no output tubes, or will that cause an issue with the rest of the circuit?

 

[rayma]

That will be ok, since there are both input and output coupling capacitors for that stage.
The only effect will be a slight rise in the power supply DC voltage for the remaining small tubes.

 

[knotscott]

Thanks rayma! No sense putting hundreds of hours on what could be good spare tubes.

 

[rayma]

Yes, good point.
Bear in mind that the B+ will rise somewhat from spec, due to only one pair of output tubes.

Here is the schematic I found. If you have a newer, different one, post it here.

 

[knotscott]

Understood, but was hoping the increase in B+ would be insignificant.

I do have a more recent schematic of the VTA 70 pcb.

 

[rayma]

I would guess the B+ would rise by around 20V without one pair of output tubes.
As long as the bias is readjusted after removing the tubes, it should be ok.

 

[knotscott]

Thanks again. Your info is much appreciated.

 

[6A3sUMMER]

The original ST70 used 50mA cathode current (plate + screen) for each EL34 (200mA total).

The KT88 (shown in Post # 1?) would have to be adjusted to a more negative fixed bias voltage, or the current per KT88 would be more than 50mA.
Perhaps that is OK, since there is only one channel operating.
More current into the 2 KT88s, will tend to bring the B+ back down closer to its original voltage (with 4 EL34s).
One question is: What sounds better, KT88s at 50mA each and higher B+; or KT88s at 70mA each and slightly lower B+?

. . . Just thinking out loud.

Those were not KT66 tubes, were they?

 

[jxdking]

BTW, what’s the voltage gain of VTA-70?
7.5K/100 Ohm=75 times? It seems too big for an amp.

 

[grovergardner]

Excuse me, but can't you bridge these amps like the original? That would make more sense.

 

[Sorento]

BTW, what’s the voltage gain of VTA-70?
7.5K/100 Ohm=75 times? It seems too big for an amp.

12AU7 in both input and splitter have low gain; assuming a 6.6K UL transformer
open loop gain: 27dB
closed loop gain: 2.4dB
so only 3dB of FB, just enough to halve THD and reduce output impedance from 3 to 2 ohms.

 

[jgf]

I went with the mono amps in 2 stereo 70 chassis route, I think it has the advantage of lower power supply sag, avoiding the compression effect of the original at high power. I run them from a 6.3V bucking transformer to keep voltages where they should be.

 

[nerdorama]

12AU7 in both input and splitter have low gain; assuming a 6.6K UL transformer
open loop gain: 27dB
closed loop gain: 2.4dB
so only 3dB of FB, just enough to halve THD and reduce output impedance from 3 to 2 ohms.

Do you mean 24dB closed loop?

 

[jxdking]

Do you mean 24dB closed loop?

24dB is about right.
Based on the VTA-70 review, the gain at 8 Ohm is about 22dB.

 

[knotscott]

Excuse me, but can't you bridge these amps like the original? That would make more sense.

These amps can be bridged, but running two output transformers in series tends to not sound as good.

 

[knotscott]

The original ST70 used 50mA cathode current (plate + screen) for each EL34 (200mA total).

The KT88 (shown in Post # 1?) would have to be adjusted to a more negative fixed bias voltage, or the current per KT88 would be more than 50mA.
Perhaps that is OK, since there is only one channel operating.
More current into the 2 KT88s, will tend to bring the B+ back down closer to its original voltage (with 4 EL34s).
One question is: What sounds better, KT88s at 50mA each and higher B+; or KT88s at 70mA each and slightly lower B+?

. . . Just thinking out loud.

Those were not KT66 tubes, were they?

Yes, those were KT66. This amp won't run KT88 as-is.