Adjusting pentode tube screen voltage?

[patricks]

Ah, that's a neat way to do it!

A couple more questions, though...
1) Do you know what difference (if any) there will be on screen voltage with the pot on either end of the screen filter cap, or does the screen just "see" the total resistance between Rg2 (R8 in the schematic earlier in the thread) and ground (regardless of whether the resistors are before or after the cap)?

2) Does the name "squish" mean anything, or is it just a label? From what I've gleaned so far, changing Rg2 will affect the amount of gain from V2, but "squish" implies to me that that the pot will compress the signal. Which I suppose could just refer to the natural compression that happens when gain is increased...

This is great, I'm learning heaps!

[DougH]

Actually, I was incorrect. You are not varying the screen voltage with this trick, because the cap is blocking DC. The larger that series resistance is, the less effective the cap will be at bypassing the screen signal. The unbypassed screen acts similarly to an unbypassed cathode, and will compress the signal more, hence the "squish" name for the control. Because you are on the other side of the cap, you are dealing with low voltage signals instead of high voltage DC, so you don't really have the same concerns about the pot.

So I guess this doesn't actually vary the screen voltage, but it still might be a fun trick to try.

[patricks]

Ah, good to know, thanks. It'll be something more to experiment with, though

[PRR]

> Check the "Squish" control

Don't see why it matters much what type of pentode you try this on.

[patricks]

Cool. I'll just have to test it out and see whether I like the sound better with the pot before R8 or after the cap

[DougH]

> Check the "Squish" control

Don't see why it matters much what type of pentode you try this on.

I just remember a few years ago when they designed this, and there was some reason they liked the 7199, but I can't remember what it was. For some reason I thought it might have had something to do with that control, but it's been a while.

[teemuk]

Well, for starters...

The 7199 is a dual triode/pentode so you get two tubes in one... unlike in EF86, which is just a single pentode.

...and despite being designed for low microphonics the EF86 is infamously nothing but and it hasn't been uncommon for people to search for more robust alternatives.

[DougH]

Well, for starters...

The 7199 is a dual triode/pentode so you get two tubes in one... unlike in EF86, which is just a single pentode.

There you go. Forgot that detail...

[patricks]

This is very cool, it's giving me some more ideas...

So, I was reading up on the 7199 and many sources said they're getting scarce and it's possible to sub the 6U8 instead. The 6U8 is a combined triode-pentode in a single tube, but both are independent. That got me thinking, could I use the 6U8 pentode stage in place of the EF86, then cascade the output from the pentode into the triode as an extra mini gain stage (maybe with a better tone stack between the two stages than the one currently in the AC4 schematic)?
Or would the triode be better used as a cathodyne phase inverter to drive an extra power tube (for moar power!!)?

This is all hypothetical so far, just exploring ideas. Even if the triode stage goes unused, the 6U8 might be a better bet than the EF86 anyway, since the 6U8 is apparently less microphonic than the EF86

[Johan]

All the good ef86 got selected and picked long time ago, so the"selected" nos tubes today are yesterdays rejects. For stages that doesn't clip,(like the first stage in a vox amp) I've have good experience with 6ak5/ef95, cheap, low noise, 6,3volt heater, 7pin socket....but dont tell anyone or they will be gone soon too...
J

[patricks]

Cool, I'll keep that between us

Had a couple more ideas for the dual pentode/triode tube (6U8 or similar) -
Maybe the triode gain stage could be set up in parallel with the pentode stage, with a switch to select between the two. Or select both and have a "blend" pot after the triode to control the amount of triode signal that's mixed with the pentode signal.
Or use the triode as a cathode follower to act as a buffer for a tone stack.

Now we're getting a long way away from the original, fairly simple mod and into redesigning the circuit...

[amptramp]

The 7199 has a unique pinout which means you will have to move some things if you want to install a substitute.  I would suggest a 6GH8 in place of a 6U8 - same pinout, more gain and lower noise.  It was common to use the 6GH8 as a horizontal oscillator in televisions, so it is one of the most plentiful tubes around.  You need to move connections to three pins to get a 6U8 / 6GH8 replace a 7199.

[patricks]

Excellent! Higher, gain, lower noise and more plentiful - I like
I've spent the last few days reading through the valve wizard site, so I might try drawing some load lines and playing with some of the options for the pentode and triode I mentioned before...