Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: scstill on April 27, 2025, 11:52:40 am
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Some questions about operation of pentode in Preamp and 2nd stage position.
Prew42 schematic below.
The 6D6 is used since part of the original prewar radio
The 6D7 (cool metal clad) chosen to be period correct replacement for the original radio 6A7 pentagrid that I couldn't figure out how to use in this amp.
1) 6D6 sometimes I see the Screen bypass cap connected to the cathode and sometimes to ground. Is there a reason for either application?
2) 6D6 sometimes I see the grid leak resistor in front of the grid stopper and sometimes behind. Is there a difference?
3) 6D7 I have used this design "successfully" in Phirty6 amp, without the screen bypass cap. Is it a mistake not including a screen bypass cap?
4) In an earlier design there was not a 2nd stage, the volume pot fed directly into the grid stopper of the 75 PI, thus the need for the .022 cap to protect the Vol pot. Can that cap now be removed?
5) Since this is a planned to be a head, does it make any sense to add a high power resistor to the speaker jack to protect against unplugged speaker? Never seen this done so maybe not.
Additional discussion (and pictures) here
https://el34world.com/Forum/index.php?topic=29232.msg321542#msg321542
https://stillampd.com/prew42
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What stands out immediately; 0.022uF between TS and 6D7 is not needed; 470Ω 5W will burn up with parameters provided, will dissipate just over 7W at idle - Use a 10-15W part.
1) Does not matter for AF. Cap is usually grounded in RF ckts to aid in suppression of parasitic oscillations. Connecting to cathode prevents Ig2 a current loop through Ck likely to help with stability, just a guess. I've always just grounded it out of convenience. If you're having stability issues, then try attaching to the cathode.
2) With AF it doesn't matter, however, if you use an input coupling cap, grid leak needs a DC path to ground, so after input cap.
3) Lower gain and lower high freq. roll-off knee without the screen bypass cap.
4) The cap between the gain stage and the self-biased cathodyne needs to stay - Removing will upset the cathodynes bias.
5) Protection resistor for a head is wise. Use a switched jack so resistor is only in-circuit if nothing is plugged into output jack.
--Pete
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470Ω 5W will burn up with parameters provided, will dissipate just over 7W at idle - Use a 10-15W part.
Pete - Appreciate the wisdom provided to my questions...
Did I miss something on the dropping resistor?
The 470 drops 35 volts (385-350); 35^2 /470 = 2.6watt
I calculate the vdc from 700vct as Vdc = 0.637 * 700 - 60 = 385vdc
~60v drop on the rectifier tube
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470Ω 5W will burn up with parameters provided, will dissipate just over 7W at idle - Use a 10-15W part.
Pete - Appreciate the wisdom provided to my questions...
Did I miss something on the dropping resistor?
The 470 drops 35 volts (385-350); 35^2 /470 = 2.6watt
I calculate the vdc from 700vct as Vdc = 0.637 * 700 - 60 = 385vdc
~60v drop on the rectifier tube
Nah you're good to go, I locked on the 125mA transformer rating and based calcs on that number. Sorry.
--Pete