Tube vs SS B+

[west]

First of all, I would like to ask opinions for the quantitative and
subjective differences between tube rectification and solid state
rectification for, let's say... an ST-70amp or similar.
I have a brand new Sound Valve Super ST-70i that uses SS rectification.
Do you think it would be sacrilegious to convert the Sound Valve's B+ supply
to a GZ-34 tube rectifier? Is there more involved than what appears to be a
straight forward conversion? BTW: I enjoy mostly chamber, light
instrumental jazz, and ambience music, if that means anything. Thanks for
your consideration.

Cordially,
west

[Patrick Turner]

west wrote:

> First of all, I would like to ask opinions for the quantitative and
> subjective differences between tube rectification and solid state
> rectification for, let's say... an ST-70amp or similar.
> I have a brand new Sound Valve Super ST-70i that uses SS rectification.
> Do you think it would be sacrilegious to convert the Sound Valve's B+ supply
> to a GZ-34 tube rectifier? Is there more involved than what appears to be a
> straight forward conversion? BTW: I enjoy mostly chamber, light
> instrumental jazz, and ambience music, if that means anything. Thanks for
> your consideration.
>
> Cordially,
> west

The change from SS diodes to a tube rectifier may result in a
lower B+ because of the poorer conversion ratio between the AV and the DV,
since the tube rectifier has much more series resistance than the
silicon diodes. Also, the value of the capacitors being charged off the
SS diodes may be too high to suit the tube rectifier, which requires
the cap to be no more than about 47 uF.
Using SS diodes, if the B+ is 450v, the AV from the power tranny would be 335
vrms,
and if a tube rectifier was fitted, expect only +400v.
The voltage across the tubes from anode to cathode would be then about
+370v, allowing for the drop across the cathode resistors which sets the bias.

I don't know what exact existing filter arrangement you have in the B+ rail;
the old ST70s had about a 1.5H choke, and about 40 uF caps, pretty poor,
by today's standards.
If a tube rectifier was fitted, the first cap in the CLC filter could be
47 uF, and the L can be 3 H and the second cap could be 220 uF.

Although the B+ will be lower, but you could increase the bias current,
and run the tubes to get more class A % in the power output.
It presently runs class AB, and the last one I tested gave about
35 watts, but with lower B+, and higher Ia, expect 25 watts,
but more class A, like a Quad II amp operating condition.

Perhaps you'd hear a better sound, but I think you would need exceptional ears.
I think the sweetness and clarity of the pure class A may enhance
what your tastes are for music, since you don't seem to want to
play rock and roll loud, and perhaps you never use more than
2 watts if your speakers are about an average 90 dB/watt/metre sensitivity.

Patrick Turner.