6G2 uneven bias wiggle

[hylaphone]

Built a 6G2 clone a while back, it has been a fantastic amp.
One thing keeps bothering me however, at deeper trem settings the distortion can get a little raspy.
I figured this was just a characteristic of the cold end of the "wiggle".

Today I fired it up and watched the swing on each tube with an analog meter. Noticed something peculiar - one power tube (V3) swings about 20mA, while V4 only swings 10mA.
These are a matched pair, with the trem off they draw well within 1 mA of eachother.
Switched them around, tried different tubes, and the discrepancy stays at the socket. V3 always wiggles with twice the range of V4.

Any ideas why this would happen, and could this have something to do with the raspy distortion?

[hylaphone]

Sorry, here is the schematic:

[g1]

These are a matched pair, with the trem off they draw well within 1 mA of eachother.

Is that with signal present or just at idle? If you can you should look at it with a scope and see whether the signal is symmetrical at either end of the phase splitter with and without the trem.

[loudthud]

Is V3 the tube that gets driven from the phase inverter plate? (top one on the schematic)

The flaw in that circuit is that the amount of tremolo signal that gets to the output tube grid is attenuated by the output impedance of the phase inverter. Since the impedance is different on the two outputs of the phase inverter, the signal level at the grids of the output tubes is different. If you disconnect the two 0.1uF caps that go to the phase inverter, the signal will be the same on both output tubes. This situation might be improved if the 0.1uF caps are reduced to 0.022uF or even 0.01uF.

[hylaphone]

Good call loudthud.
.01uF caps evened things out within a few mA. Sounds a lot smoother, still slightly raspy but I am beginning to wonder if the concertina PI is just not my cup of tea...

Any other tips to smooth out the distortion of this circuit?

[loudthud]

Merlin B recommends a big grid stopper and a couple of other fixes for the Concertina. He has a website (I forget the name) you can look at and a book you can buy. Hunt around for him on this site and you will find that stuff. There is also the so called Paul C mod. It imbalances the Concertina in a quircky way. Go to my profile and look at the Visitor Messages. There is a link to a thread where problems and solutions are discussed.