Blackvibe 6L6 Strange behavior on tone pots

[retodellatorre]

Hi guys
I build a 6L6 blackvibe no trem, no verb.
For safety reasons I gave it to a tube tech, he says everything is fine.
What i really wonder is how the tone pots react, in my understanding weirdly.
On any blackpanel amp if you turn all the tone pots to 0, there will be no sound at all.
This seems, even if rob robinettes says 100% vibroverb, very different on the blackvibe. All Tone pots have a minimal scope/range of action.
When i turn bass from 0 to 10 this would equal like 3-4 on my super, or princeton. Same for treble, the middle would equal 3-5 on a super or Princeton. So the scope is very limited. It is so limitied, that i still think there might be a mistake in my soldering, even tough my amp tech, which is a real tube electronics pro , says ist ok.

Any suggestions or tips would be much appreciated!!

Thx reto

[martin manning]

The 6k8 tail resistor on the mid pot is what keeps the tone controls from killing the sound altogether. If you want to go back to the AAB763 functionality, delete that resistor and consider replacing the 100k mid/raw pot with a 10k audio.

[retodellatorre]

Thx so much ! Will the scope then also be as on a ab763?

Thx!

[martin manning]

Possibly. Unless you have the 100k mid pot turned way down, to zero, you have lifted the tone stack to a significant degree. Do the treble and bass controls seem more normal when the mid pot is at zero?

[retodellatorre]

Hmm i did not try that. I will go to the 10k a pot on the middle and kill the 6.8k resistor. Instead of the resitor i go to ground with the lug directly right?
Thx martin!

[martin manning]

Correct. Try it with the mid pot at zero, which would be 6k8 resistance to ground, like a 10kA mid pot set at ~3:00.

[retodellatorre]

Will do so in abfew days and keep this treath updated.
Thx !!

[pdf64]

... On any blackpanel amp if you turn all the tone pots to 0 ...

Fender knobs for these amps are 1-10, no 0 available, CCW = 1, CW = 10.

... 6k8 resistance to ground, like a 10kA mid pot set at ~3:00.

With a regular 1-10 knob and 10k log pot, I recall a fairly high setting is required, maybe around '8', to achieve 6k8 resistance.
Maybe you mean 3 o'clock, where noon is 50% mechanical rotation?

[martin manning]

Exactly. Using Alpha 15% log taper, 270 deg rotation, I make it 02:52:48 on the clock, or 8.38 on a Fender-type 1-10 knob to get 6k8 of 10k.

[retodellatorre]

Ok - I changed the tone pot to a AK10 removed the 6.8k resistor. Now it sounds like my blackpanels, in my sense muuuuch better control in bass treble and middle. It still sounds a little icepicky and „new“ but i guess it will break in a little too.
Thx for your help!
Reto

[Reeltarded]

What is the bright cap value? 250p? Try 100p. Break-in isn't probably going to retire ice picky.

[retodellatorre]

Honestly … i did not install the bright switch, since i never used it on my super reverb - that said, the cap is used if the switch would be off (not bright) and be bypassed if on (bright) right? If so i made a thinking mistake and left the cap also off
So i have to solder in the cap anyways even if i dont use the switch right?
Thx so much for your help!!

[Reeltarded]

Bright cap makes brighter.

Turn mid control up slightly, and turn treble down the same amount.

If you do add the bright cap it peaks ^ the treble so you turn treble control down. The natural curve might work better.. or might not. You should test.

[retodellatorre]

Oh dang sorry… and thanks!

So since i dont have the switch and the cap, another Method that rob robinettes has on his page is placing a cap over the 820 ohm nfb resistor to reduce icepick.
Have you guys done that before?
Thx!

[martin manning]

A cap across the FB resistor will introduce a phase shift, and might even cause oscillation, but you can give it a try. If that doesn't work, there are several other ways to reduce treble content, including placing a cap across a plate load, from plate to cathode, or across phase inverter anodes.