Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: kagliostro on March 01, 2020, 06:52:49 am
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A circuit arranged like the one in the red box can be considered as an effective MV ?
(https://i.imgur.com/SbfNwNG.jpg)
This way, I suppose, the Power Tube has his Grid Leak Resistor, the previous stage a Fixed Load (1M, the pot) and AC signal is routed to ground via the 1uF capacitor, depending on pot wiper position
:w2: :w2: :w2:
Thanks
Franco
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and AC signal is routed to ground via the 1uF capacitor, depending on pot wiper position
that's how I'm "seeing" it, not sure if it's ALL the frequencies, it's too early for that math :laugh:
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There may be some bassy rumble when set to minimum - what's the 1uF supposed to be doing in the circuit?
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what's the 1uF supposed to be doing in the circuit?
to avoid grid of the Power Tube to be directly connected to ground
1uF value, I suppose, not sure, is enough to route to ground all AC frequencies (if necessary the value can be higher)
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All this is a my personal supposition
Franco
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what's the 1uF supposed to be doing in the circuit?
to avoid grid of the Power Tube to be directly connected to ground...
As it’s cathode biased that’s a total non issue; most all gain/vol controls connect the grid to 0V.
Just add a safety resistor, in case the pot fails and leaves the grid floating, which would be a problem for a power tube.
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Thanks Pdf64
Franco
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1µF cap in series with .022µF will be less than .022µF overall, so 1µF cap is moot. put the wiper on the grid of the 1614 output tube with a 1M R hanging off the wiper to ground in case of lost continuity of the wiper.
1614 is a transmitting tube and g1 can be driven positive, so the grid 1 circuit maximum resistance spec is not given for audio modulator use in class A1 or AB1 - if you follow the 6L6 rule the tube will last longer: for fixed bias grid 1 circuit maximum resistance is 100K, and for cathode bias it is 500K. i'd use a 500K pot instead of the 1M pot if wired with wiper to grid.
ciao
--pete
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The triode gain stage will be loaded down when the pot is turned down, reducing its gain. Is that intentional.
Also, as the pot is turned down, the resistance to ground (via the 1uF) forms a high-pass filter with the 0.2u cap, reducing bass as you turn it down. Is that intentional?
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Ciao Pete
Thanks
OK for all what you say, only the tube was a 1625 not 1614, however it was only an example to represent a generical Power Tube
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Ciao d95err
The triode gain stage will be loaded down when the pot is turned down, reducing its gain. Is that intentional.
Also, as the pot is turned down, the resistance to ground (via the 1uF) forms a high-pass filter with the 0.2u cap, reducing bass as you turn it down. Is that intentional?
No, those are unwanted collater effects, I was only wondering if that way was a feasible manner to avoid the load and the wiper disconnection problems (wiper connected to G1 of the power tube) and have an effective Master Volume
Seems that it is absolutely not a viable way
Franco
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I see that rather as a one knob tone control where you lead some frequencies (low) to ground.
/Leevi
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You could of course deliberately mess with the frequency response of the master volume to get a desired effect.
For example, George at Metropolous amplification wrote somewhere about the master volume of the Metro-Plex amps having a network of resistors and capacitors to implement a ”loudness” compensation function when turning down the volume.