Grid Leak and Volume Control -- Question about

[kagliostro]

On my 5C1 derived amp thread DummyLoad pointed my attention to the fact that 1625 (807) didn't allow grid leak resistor higher than 500k
(may be 470k ?)

so I tried to think a way to have both Volume Control and Grid Leak of appropiate value without waste of signal (this in my poor technical mind)

What will happen if I arrange a Volume pot as grid leak resistor and connect the wiper to the incoming signal instead of to the grid of the power tube ?



Thanks

Franco

[sluckey]

That will work. I would rather connect the wiper to the 1625 grid.

[kagliostro]

Thanks Steve

but connecting the wiper to the grid of the 1625 diden't affect the correct leak value for the 1625 on rotating the shaft ?

Franco

[sluckey]

Sure it does. You don't want to EXCEED the MAXIMUM resistance of 500K. There is no MINIMUM resistance specification. Even zero ohms is valid.

This is not a grid leak bias circuit. The grid simply needs to be held at zero volts. So, 0Ω, or 100KΩ, or 500KΩ all accomplish that goal. Just don't exceed 500K max spec.

[Lauri]

If you wire the potentiometer that way it will change the AC load of the 6SL7 depending on how much you have turned up the volume.

[kagliostro]

Many thanks Steve & Lauri

Now the situation is much more clear on my mind

Franco

[shooter]

Just don't exceed 500K max spec

What happens, or could happen if this is exceeded?  I just do it, but never grasped the why

[Lauri]

What happens, or could happen if this is exceeded? 

Worst case scenario is that reverse grid current starts biasing the tube hotter, causes thermal run away and destroys the tube.

[tubeswell]

The original Marshall 18Wer schematic has the vol control into the PI input grid set up this way FWIW


http://www.drtube.com/schematics/marshall/18w.gif

[kagliostro]

Thanks Tubeswell

That is interesting

we never invented anything, someone has always preceded us

May be there the use of a double pot is studied as to don't affect the previous stage load




Franco

[sluckey]

May be there the use of a double pot is studied as to don't affect the previous stage load

That's not a double pot. It's two separate pots. The one on the left is a tone pot. The one on the right is the volume pot.

[kagliostro]

Oh, I see


Franco

[xm52]

It's interesting to look at a GE 6L6GC data sheet, they specify under Maximum Ratings, Grid No 1 circuit resistance of 100K for fixed bias, 500K for cathode bias for both pentode and triode connection.


A lot of designs seem to get away with exceeding that 100K maximum.


I suppose one has to determine why they set that maximum. Is it to simply to ensure a long service life.

[PRR]

> A lot of designs seem to get away with exceeding that 100K maximum.

100K is a cover-our-butt spec. 99% of Golden-Age tubes would be fine with 101K even (obviously) 220K. But the tube makers did not want a bad day in the factory to cause a rush of field failures and complaints.

Additionally: in Guitar Amps we often bias the tube somewhat below maximum dissipation. That gives a little leeway for grid-leakage before things go seriously wrong.

Finally: Tube stage amps fail. Abuse, cheap-brand tubes, playing louder than the designer ever considered. Users know how to replace tubes. A dead amplifier doesn't kill anybody. This isn't rocket science. (Note that the B-52 bomber used tubes into the 1990s; that's where you don't dare cheat the specs.)

[Fresh_Start]

The original Marshall 18Wer schematic has the vol control into the PI input grid set up this way FWIW

http://www.drtube.com/schematics/marshall/18w.gif

The Tremolo channel does. In fact, it looks just like a 5E3. However, the other channel has a "normal" volume pot. Maybe there was some reason the PI needs to see a constant resistance up front for the Tremolo channel. It's not the tone control because Tweed Champs etc. also have normal volume pot arrangements.

Cheers,
Chip

[sluckey]

Don't go by that 18 watt schematic! It has errors, especially with how the trem channel volume pot connects to the LTP PI. The pot is wired wrong and there is a missing coupling cap. These errors have been verified by reverse engineering an original 1974 circuit. Here's the corrected schematic from the same website...

     http://www.drtube.com/schematics/marshall/18wr.gif