A Classic Design with Unusual Tubes [EF85 Vox AC10]

[jewishjay]

I'm trying to build (something like) a Vox AC10 with tubes I have on hand. Here's my schematic, the voltage readings I get cold at start up and after 15 minutes of warm up, and the original schematic for reference. The amp works, and the tremolo is beautiful, BUT is V2 is extremely microphonic. I tried playing around with the plate and cathode resistors and lifting the bypass cap. But that didnt help much and the tremolo only seems to work if V2 is starving.  Any suggestions? Or am I trying something impossible?

[sluckey]

Microphonics is a mechanical issue. Changing components doesn't help. Swap V2 and V3. Any better? Or grab a handful of tubes and start rolling until the microphonics is gone. Lot's of people use a shock mounted tube socket. Sometimes even a tube shield will help. I've also seen rubber/silicon rings put on the tubes.

     https://www.tubesandmore.com/products/damper-rings-high-temperature-silicone-red
 

[jewishjay]

Alright! I tried swapping V1 &V2 ...it was worse. I added rubber o-rings to suspend the tube socket, and the microphonics are ....not gone, but something I can live with. Thanks Steve!

Now, can I ask for some help with the depth control? It seems like the range is wrong. Theres no tremolo from zero to 75% and then suddenly there's ALL THE TREMOLO. And the over all volume goes slightly up too. Should I adjust the value of the R between the screen and the pot? and if so, higher or lower?

[shooter]

maybe change the pot's taper

[Williamblake]

The pentodes anodes are DC coupled in your schematic. If this is on purpose you wpild only need one plate resistor and one cathode resistor for both but maybe there is just a cap missing in the schematic.

[jewishjay]

Once it was  mounted in a cabinet those EF85s screamed bloody murder! Just too microphonic to use. So I redesigned for EF86s and now it plays and sounds great. Still working on the tremolo depth....

It seems to work by lowering the screen voltage of V2. So I just need to hone in on the exact range. I tried a few different screen resistors but that didnt make the depth knob more useful. I might try a 250k pot.....

[jewishjay]

And boom goes the dynamite! A 250k pot brought the screen closer to ground and made the depth range more useful. Now the knob acts like you expect it to. This style of tremolo sounds so good! Now I want to just fine tune the interstage Rs...or should I say grid Rs... at the phase inverter and into the power tubes. This thing is LOUD and theres plenty of room to experiment.

What value grid R do you like going into EL84....or in this case 6973 ?
What about the grid R / voltage divider into the PI ?