Bassman Micro: V1 Cathode Resistor?

Lancer X

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I'm laying out my Bassman Micro build, and in reading up on the @robrob "Lead" Voicing Mod, I encountered something I don't understand.

The V1 cathode resistor in the original 5F6A was 820 ohms, but Rob explains that:

"when we separate the cathode resistors we have to double their value to retain the same bias so V1 pin 8 (my unmodified channel) needs a 1.5K 1/2 watt cathode resistor (original was 820 ohms) to keep the same bias and tone."

Since the Bassman Micro is a single channel amp, I expected to see a 1.5k resistor for the V1 cathode. But the Bassman Micro design shows an 820 ohm. Can anyone explain this to me?

Thanks in advance. I want to include the "Lead" Voicing mod in my build, and I have to determine the proper "stock" V1 resistor before proceeding.
 

2L man

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When both triodes share one cathode resistor its value should be about 0,5 what a single triode cathode resistor is. Then double current make same voltage loss and bias comes wanted.
 

King Fan

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You're comparing this V1 topology in the 5E3 and full-size 5F6a (where Rob describes the Lead Channel mod):

1666127162987.png


to this in the Bassman Micro:

1666127233901.png


Notice they both have a shared cathode (pins 3 and 8 are wired together) -- so they both have an 820Ω cathode resistor. They're shared for somewhat different reasons (the 5F6a ties the two channels to one bias; the Micro runs both triodes in parallel on one channel), but the voltages are the same.

Since you only have the one channel (ie, you can't mod one of two channels like Rob does on the original) do you plan to make the 'Lead' channel the only channel? Or are you gonna split the cathodes and try to create two separate channels? I don't know what effect it would have to 'un-parallel' the two triodes. Slightly less gain? Slightly more noise, more high frequencies? Beyond my pay grade... :)
 

Lancer X

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Ok, gotcha and thanks! I should have been able to puzzle that out.

Since you only have the one channel (ie, you can't mod one of two channels like Rob does on the original) do you plan to make the 'Lead' channel the only channel? Or are you gonna split the cathodes and try to create two separate channels? I don't know what effect it would have to 'un-parallel' the two triodes. Slightly less gain? Slightly more noise, more high frequencies? Beyond my pay grade... :)

I was planning to work with the one channel: use a 3PDT switch to swap between all the parts for the lead voice:

Group 1) .022 uF coupling cap --> .0047 cap
Group 2) 47k V1 load resistor --> 94K (probably 100K actually)
Group 3) 250 uF bypass cap and 820 ohm cathode resistor --> 0.68 uF cap and 1.5K resistor


Does this sound like a valid approach?
 
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