AC vs DC Balance in PI, Relative importance

[David Root]

I added an ODS section to an existing amp (actually replaced the reverb with the ODS and put in a footswitch w/relay). The tubes are 6SL7 not 12AX7.

For the PI I selected a Phillips ECG 6SL7 that indicated good DC balance between triodes. PI plate resistors are 82k and 95.4k (out of spec 91k carbon comp). I get 246.7 and 237.7 DC on the plates (3.8% imbalance) and 3.7% AC imbalance after the coupling caps (eliminates DC ripple from the measurement). This is after 60 minutes or so operation.

The PI is a straight 5F6A/5F8 with 10K tail.

I also tried a 5691, which is a toughened up industrial 6SL7, and got lower but very close DC plate voltages, 233.8 and 235.7, for 0.8% DC imbalance. AC imbalance was 5.1% however.

B+ is 385 VDC.

There is no plate voltage trimmer. I have been led to believe that AC balance is more important in a guitar amp, whereas if anything DC balance is more important in a bass amp (which this is not).

If I put in a plate voltage trimmer I think I could completely balance both AC and DC with the Phillips ECG.

Is this trip really necessary or should I stop being so anal about it and tweak the ODS circuit instead? (Doesn't sound great yet). Clean circuit sounds very good. (not Dumble design)

[talbany]

For the PI I selected a Phillips ECG 6SL7 that indicated good DC balance between triodes. PI plate resistors are 82k and 95.4k (out of spec 91k carbon comp). I get 246.7 and 237.7 DC on the plates (3.8% imbalance) and 3.7% AC imbalance after the coupling caps (eliminates DC ripple from the measurement). This is after 60 minutes or so operation.

The PI is a straight 5F6A/5F8 with 10K tail.

I also tried a 5691, which is a toughened up industrial 6SL7, and got lower but very close DC plate voltages, 233.8 and 235.7, for 0.8% DC imbalance. AC imbalance was 5.1% however.

B+ is 385 VDC.

There is no plate voltage trimmer. I have been led to believe that AC balance is more important in a guitar amp, whereas if anything DC balance is more important in a bass amp (which this is not).

If I put in a plate voltage trimmer I think I could completely balance both AC and DC with the Phillips ECG.

The tail resistor in the LTP effects the balance between inverted and non inverted output voltage gain on each side...(inverted being the same side as the input) ..The lower the tail the greater in balance raising the inverted side thus the need for the lower (82k in your amp) values to compensate.. For instance 6sn7 has an inverted output gain of.........................-9.5853335674479
and in phase of......8.2545805367318
The same calculations with 22k tail and you get
Inverted...................-8.9580042428105
In phase...................8.891445289119

Better matched..
this is with a 6sn7 w/82k and 100k plate 820 ohm cathode
The trimmer pot will help a bit with better balancing of the voltage drop across the plate resistors which sets the rails so they clip evenly however the gain in balance still exists...The result of the lower tail will be more asymmetrical adding more harmonic content but hearing a grittier type of distortion associated with PI clipping.. This is of coarse with output section being pushed..
In Conclusion if it's balance you are after you need to raise the tail..22 or even 24k..and a 10k pot to balance the swing...Should do nicely
Tony