Just a comment regarding the 6AS7. i have used this valve in various designs for about 6 years at this stage and it can sound real good - clean and punchy.
However it can be very prone to instability and this can set off microphonic ringing. They can suddenly start singing and carry on all by themselves for minuets on end. I really started to notice this when my main speakers jumped up in sensitivity to somwhere over 105db/w. This was a particular problem when using CCS's on the cathode or the anode of the 6AS7. I needed to place stopper resistors between the CCS and the 6AS7 to bring it under control, but it never goes totally away.
Shoog
However it can be very prone to instability and this can set off microphonic ringing. They can suddenly start singing and carry on all by themselves for minuets on end. I really started to notice this when my main speakers jumped up in sensitivity to somwhere over 105db/w. This was a particular problem when using CCS's on the cathode or the anode of the 6AS7. I needed to place stopper resistors between the CCS and the 6AS7 to bring it under control, but it never goes totally away.
Shoog
With this circuit there is no tendency to ringing. The first voltage amplifying tube (6SL7) is most sensitive for mechanical knocking. Rest of the tubes are very unsensitive.
I have no experience of CCS together with 6AS7G.
Hopefully our test procedures are similar.
I simply measured the output voltage change between 8 and 4 ohms load and calculated output impedance as dU/dI.
Other users of the 6AS7 have reported similar issue with microphony and high frequency instability - but I would hazard a guess that many people simply would not have speakers sensitive enough to hear it.
Shoog
In the article, he did not mention how the output impedances were measured, but I believe they were done using the ON-OFF method since Audio had several articles on the subject. While your method wasn't ON-OFF, the results should still be in the same ballpark, I think...
I found an modification that essentially inproved the bass-end performance of my 6AS7G PP amplifier.
In original circuit the cathode capacitors of 6AS7G were 47 uF. After changing those to 330 uF the bass end now begins from 20 Hz with much lower distortion than before.
I also modified the driver stage a bit to get some more gain.
Attached the new test results and modified circuit diagram.
In original circuit the cathode capacitors of 6AS7G were 47 uF. After changing those to 330 uF the bass end now begins from 20 Hz with much lower distortion than before.
I also modified the driver stage a bit to get some more gain.
Attached the new test results and modified circuit diagram.
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- nominal power 40 W
- impedance 5k/8 and 4 ohms
- Ia=200 mA
- this opt also has UL taps (43%)
Leakage inductance 14,8 mH ( my measurement).
I have not measured the primary inductance, but it is obviously high.
Size: dia 105 mm, height 55 mm.
- impedance 5k/8 and 4 ohms
- Ia=200 mA
- this opt also has UL taps (43%)
Leakage inductance 14,8 mH ( my measurement).
I have not measured the primary inductance, but it is obviously high.
Size: dia 105 mm, height 55 mm.
This opt is very close to present production models of Toroidy.
They already produce 4k and 6k6 opt with almost similar specs.
Therefore the price was only some 50% above the list price of standard models.
They already produce 4k and 6k6 opt with almost similar specs.
Therefore the price was only some 50% above the list price of standard models.
This transformer seems to be already in production and available.
TTG-6AS7PP - Tube output transformer [5kOhm] 2x6AS7G / 2x6N13S Push-pull or similar - Shop Toroidy.pl
TTG-6AS7PP - Tube output transformer [5kOhm] 2x6AS7G / 2x6N13S Push-pull or similar - Shop Toroidy.pl
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