One of the ST-70s I have, a provocative event happen. I was playing Heart's
"Magic Man" while monitoring the B+. When the song went way down in
frequency, the B+ dropped down by 42 volts! I know that this demonstrates
the weakness of the PS. Are the caps too small or is the Xfr inadequate?
What would you do to rectify (no pun) this problem. Thanks.
Cordially,
west

west wrote:

One of the ST-70s I have, a provocative event happen. I was playing Heart's
"Magic Man" while monitoring the B+. When the song went way down in
frequency, the B+ dropped down by 42 volts! I know that this demonstrates
the weakness of the PS. Are the caps too small or is the Xfr inadequate?
What would you do to rectify (no pun) this problem. Thanks.
Cordially,
west



Beefing up the PS caps couldn't hurt,just mind the value of the cap
after the rectifier.(~40uf max,for Tube rect.)
I put the SDS labs cap board in mine to replace the old can cap,it
helped the low-end a bit.
I think it's both,X-fmr and caps.I've heard that the stock ST-70 is a
bit wimpy for the job..Mine seems okay,low-end was okay,but it's better
now with SDS labs board..However the power tranny does seem run a bit
hot.(cloth lead PA-060)

west wrote:

One of the ST-70s I have, a provocative event happen. I was playing Heart's
"Magic Man" while monitoring the B+. When the song went way down in
frequency, the B+ dropped down by 42 volts! I know that this demonstrates
the weakness of the PS. Are the caps too small or is the Xfr inadequate?
What would you do to rectify (no pun) this problem. Thanks.
Cordially,
west

Like nearly all pre 1970 tube amps the PS used in ST70
isn't the world's stiffest supply.

A drop of 42v in the B+ is only around 10%.

I revised an ST70 and used 450v for B+, with about 35 v at the cathodes
using cathode bias, so Ea = 415 v, and you get 35 watts class AB1 to the rated
load,
without a huge shift in the B+ and with musical signals where virtually no
clipping occured.
But with a sine wave the supplies all alter by 10%, especially
with a 4 ohm load on the 8 ohm Z match outlet.

The original ST70 has fixed bias, but when RL is reduced below optimum
the B+ rail will droop more than 10% like it does in all these old amps
when a sine wave is taken up to clipping.
Usually music does not cause anywhere near the same
droop in the B+, because the average power isn't as great as a sine wave.
If you are getting a huge droop in the B+ like happens with a sine wave
then the load could be low, or you are running the music in so hard that
the signal is grossly clipped.

The only way to prevent the droop is to use SS diodes, and
get a higher B+ to start with, and then series regulate with SS
down to the voltage normally seen at idle as the original amp has.
You would fair reg without the tube rectifier, and just the SS diodes, but then

the B+ will be too high, and biasing the UL circuit will be a problem
and the loads too low for the B+ used to get the same amount of class A % of
power.

Patrick Turner.

....the plot thickens.. Heart's Magic Man synthesizer downrun was used in
the 70's for checking out loudspeakers ...st-70s, you say ?:-)
Rudy

"west" wrote in message
m...
: One of the ST-70s I have, a provocative event happen. I was playing Heart's
: "Magic Man" while monitoring the B+. When the song went way down in
: frequency, the B+ dropped down by 42 volts! I know that this demonstrates
: the weakness of the PS. Are the caps too small or is the Xfr inadequate?
: What would you do to rectify (no pun) this problem. Thanks.
: Cordially,
: west
:
:

Yes, the ST-70 power supply is pretty wimpy. The GZ34 is taxed to the
limit, the PS caps are quite small, and the two share a common supply.
If you push the amp hard the limitations are quite apparent.

You can beef it up somewhat with modern caps and silicon rectifiers.

One of my first mods was to build a pair of outboard choke input power supplies for an ST-70, one for each channel. I was surprised how much power and bass the bascially stock circuit could deliver with a beefier supply. Separating the power supplies for each channel greatly improved the separation and stereo effect as well. This was a nutty approach, but I learned a lot about power supplies and their effect in the process.

- Paul


west wrote:
One of the ST-70s I have, a provocative event happen. I was playing Heart's
"Magic Man" while monitoring the B+. When the song went way down in
frequency, the B+ dropped down by 42 volts! I know that this demonstrates
the weakness of the PS. Are the caps too small or is the Xfr inadequate?
What would you do to rectify (no pun) this problem. Thanks.
Cordially,
west