Has anyone built this tone circuit?

[Casey4s]

Someone here recommended the Weber VST Active tone circuit to me a few weeks ago, so I installed it recently into my DIY 5F6A. I was wondering if anyone else has tried it and what did you think?

I installed it in two variations, and I decided that I like it better with 500K L pots instead of 500K A pots.  It seems to behave better with the linear pot, I guess because it is essentially working from a variable voltage divider idea. 

The other thing is that I had to add a by-pass cap (22/35) to the cathode of the amplifier triode.  The controls did almost nothing without a bypass cap in place, kinda weak and lifeless.



I am getting a very mild "motor boating" with the controls in certian combinations, and the signal is getting a bit distorted when cranked. But I haven't decided if it is related to this circuit or if it just emphasises a pre-existing condition I was unaware of.

I am just curious if anyone else has any expierience with this tone circuit.

Weber's Schemetic
https://taweber.powweb.com/store/tca_schem.jpg


Thanks for any comments...

[Casey4s]

I found the motor-boating trouble and the amp is pretty quiet now.

Nobody else has tried this tone circuit?

[PRR]

I hesitated to criticize Weber, but _I_ don't like the plan.

The response with Audio taper is uneven and not in a good way.

The basic circuit, Baxandall feedback, is usually implemented with Linear pots and gives unity-gain with pots centered. You need to think how this works with the amp's gain-structure. It is similar to a 5F6a's tone-driver, but the 5F6a has some gain through driver and tone-stack.

To my eye, without context, I see VERY low input impedance, near 10K at higher frequencies. You can't drop such a treble-sucker into most tube amps without additional buffering. Feeding it from the 5F6a's 270K mix resistors seems wrong, very limited gain and EQ-range. The tone-section also loads this tube, severely limiting max boost.

And the tubes are biased on the edge of distortion. V2 cathode is nearly as high as (or higher than!) V2 grid, meaning on any positive swing V2 degenerates from buffer to diode. If you look at the true 5F6a plan, this amp-buffer features 820 ohms in the first cathode, not 1K5.... pulls plate down from 2/3rd of supply nearer 1/2 supply, so the 2nd tube is not starved for plate-cathode voltage. Some amps use this mis-bias deliberately for "flavor". However doing that within NFB and tone-control is dubious.

But hey, if it works for you, enjoy.