Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: kagliostro on March 18, 2016, 05:08:20 am
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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 ?
(http://i.imgur.com/RASLxr5.jpg)
Thanks
Franco
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That will work. I would rather connect the wiper to the 1625 grid.
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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
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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.
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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.
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Many thanks Steve & Lauri
Now the situation is much more clear on my mind
Franco
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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 :dontknow:
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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.
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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 (http://www.drtube.com/schematics/marshall/18w.gif)
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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
(http://www.drtube.com/schematics/marshall/18w.gif)
Franco
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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.
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Oh, I see
Franco
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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.
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> 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.)
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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 (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
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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 (http://www.drtube.com/schematics/marshall/18wr.gif)