Specifications

measure, the usual screen bypass cap used in all three models is also connected to the cathode terminal as well, for the same
purpose of maximizing stage gain. While both approaches work well for their intended purpose, the addition of the fixed bias
effort in the ST-70 makes it susceptible to low frequency oscillation (@~ 3 Hz), if specific amounts of low voltage DC is applied to
the input jack. Who would do such a thing? Why early versions of Dynaco's own PAS preamp did!
Dynaco resolved this issue by eliminating the DC from appearing at the output of the PAS, so all was well when the later PAS
edition was used with the ST-70. But it still left the ST-70 susceptible to possible oscillation with other sources that contained low
levels of DC voltage in the signal.
The easiest solution to this is:
1. Remove the two 470K resistors currently at the input jacks. If you do not have replacements, remove them carefully, as you
will reuse them in step 3.
2. Remove the two wires currently connected between eyelets 7 & 17, and their respective input jack. Install two .22 uF 100 volt
caps of your choice in their place. The voltage rating of the caps should be kept low to keep the size small.
3. Reconnect one of the removed 470K resistors directly between eyelets 10 & 17 for the Right channel, and the other between
eyelets 7 & 8 for the Left channel.
4. Install two 1 meg 1/4 watt resistors in the positions originally occupied by the two 470K resistors in the original build.
This procedure effectively places an appropriate coupling cap in series with each input of the ST-70, so that any low level DC
that might be part of the input signal is blocked, and can no longer induce any LF instability.
7199 PERFORMANCE:
This tube was designed near the end of the vacuum tube era, with its primary attribute being reduced hum and noise over that of
similar tubes offered previously, making its use in high fidelity equipment attractive. But experience has shown that different
examples of the 7199 can deliver varying degrees of performance. This is not just between those manufactured from different
tooling, but those within the same tooling production as well.
When I first received my ST-70, I mentioned it had a full complement of new, or nearly new Zenith labeled tubes installed in all
the audio positions, all of which tested excellent/new under dynamic testing. During the final checkout however, I noted that the
right channel had significantly lower THD levels than the left channel produced (~ .75% vrs <.20% at 1 kHz at 1 db below 35
watts). Since all the passive components had already passed muster, I knew it was either a tube problem, or a transformer
problem. It was a tube problem. Swapping the two 7199s between their sockets swapped the THD levels nearly figure for figure
between the channels. Hello inconsistency.
Two other very good 7199s were available and tried in the ST-70, with both producing good performance as well. So while three
7199s produced various levels of good performance in the ST-70, the one Zenith tube was clearly sub par, even though it tested
similar to the others. Therefore, I set up some tests to try and determine what the issue was behind the problem tube.
A chart was set up with the following information gathered from each tube, to look for possible clues:
Tube #1 -- THD = .135%, Ep(p) = 108.9, Eg2 = 31.4, Ek(t) = 117.9
This HH Scott tube provided the LEAST Open Loop Gain.
Tube #2 -- THD = .75%, Ep(p) = 83.0, Eg2 = 23.0, Ek(t) = 95.0
This Zenith tube provided the GREATEST OLG.
Tube #3 -- THD = .21%, Ep(p) = 89.4, Eg2 = 25.5, Ek(t) = 100.0
This Zenith tube provided next to the highest OLG
Tube #4 -- THD = .16%, Ep(p) = 110.9, Eg2 = 30.2, Ek(t) = 120.7
This HH Scott tube provided the next to lowest OLG
Observations:
1. Both Zenith tubes had the highest Open Loop Gain, meaning that the distortion reduction created by the NFB loop would be
greatest with these tubes.
2. Both Zenith tubes also had the highest THD levels.
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