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Old 13th April 2011, 02:19 AM   #1
rman is offline rman  Canada
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Default Did bootstrapping pot change it's operation

Hi.

I built this guitar amp over a year ago. Now I am wondering about how the volume control works. It reaches almost full volume at less than one quarter rotation. At the time I thought I put in a linear pot by mistake, but now I am not so sure. the master volume in front of the phase splitter is returned to the upper end of the concertina's cathode instead of to ground. If I remember right, I had read that this "bootstraps" the resistor making it look like a higher impedance to make an easy load for the cascode. Does that make sense? I don't really understand bootstrapping. Can this connection make a log pot act this way?

Other than that I am happy with this amp and think it sounds great, however I may not be much of a judge since it is the only one I have used.

The other change I have made is replacing the tone bypass switch with a double switch that disconnects the tail of the tone network from ground as well as shorting across to the next stage.

Cheers.
Rolf.
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Old 13th April 2011, 08:40 AM   #2
Merlinb is offline Merlinb  United Kingdom
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Yes, a bootstrapped MV on a cathodyne inverter is kinda silly. Even if it is a log pot, it is not surprising that it gets to full volume very quickly (a linear pot would make this worse).

Consider: if the pot is 1Meg and is turned a quarter way up then the resistance from grid to cathode will be about 50k if it is a log pot. That resistance gets bootstrapped to perhaps as much as 500k. The remaining resistance in series with the grid (which isn't bootstrapped) is 1Meg-50k = 950k. Therefore the attenuation is 500/(950+500)=0.3 (-9dB). As far as your ears are concerned, that isn't that much quieter than full volume. So yes, bootstrapping does have the effect of changing the taper.
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Old 13th April 2011, 09:37 AM   #3
DF96 is offline DF96  England
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If the 390pF caps were not there, the pot would be almost a volume on/off switch. With the caps it becomes a tone control. Not a volume control! The real volume control is the 1M befoe the cascode. BTW is that really a 55K biasing the upper grid - that would make the lower valve run with very low anode voltage.
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Old 14th April 2011, 01:09 AM   #4
rman is offline rman  Canada
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Thanks guys. I will put the pot on ground.

Df96. I designed the cascode with the help of this web page. http://www.freewebs.com/valvewizard/cascode.html
Apparently it is normal for the bottom of the cascode to have low anode voltage.
If I understand correctly, It doesn't really swing output voltage but makes the most of it's gm. The strong current swing drives the top cathode and the top tubes rp with the changing current provides it's input current.

Maybe someone with a good handle on cascode operation can say if that is correct?

Cheers.
Rolf.
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Old 14th April 2011, 05:31 AM   #5
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Quote:
Originally Posted by rman View Post
Thanks guys. I will put the pot on ground.
Rolf.
Do that and you'll shut off the tube. The grid bias is set by the 715ohm resistor as long as the pot is connected to the low end of the 715ohm resistor. If you ground the pot, you'll have the extra voltage across the 15k ohm resistor to deal with.
I would suggest replacing the pot with a fixed resistor eg 470k.
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Old 14th April 2011, 07:10 PM   #6
DF96 is offline DF96  England
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I can't really comment on what might be normal for a guitar amp, but normally an audio or RF cascode would be run with something like one-third to one-half of the HT voltage at the lower anode. Running at too low a voltage can mean that the lower triode is running into its grid current region which can create distortion due to non-linear loading of whatever comes before - in your case a 1M pot. It is true that the anode swing will be small. Fortunately the 6DJ8 can run at low HT voltages, but 30V or so is a bit too small in my opinion. However, if you are happy with how it sounds then fine.
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Old 15th April 2011, 12:03 AM   #7
rman is offline rman  Canada
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Hi.

Payitforwardeddie: I meant I would disconnect the pot from the bias (715 ohm)
resistor and put that leg on ground. That should be ok.

Df96: Thanks for the info on cascode operation. the site I got the info from is guitar amp oriented so I guess that is how guitar cascode stages differ. after all
harmonic distortion are what g-amps are all about.

I used 6dj8 tubes because I already had them. I knew that one challenge would
be to get these nice linear tubes to distort enough for good tone. It does sound
nice but could probably use more distortion yet.
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