Bassman input stage

[Bernardduur]

Hey all!

I am in the process of trying several preamps in my build

now I wanted to try to add a Bassman 5F6 preamp to this build; the orginal has 2 cathodes onto 1 820 ohm resistor (w/250 uF cap).

As I will only be using 1 channel I wanted to know what to do to make it into a one channel version. On some sites I read to enlarge the resistor to 1k5 like the normal Fender values, on others I read that I should lower it to 750 ohm.

Thanks!

Edit; second question; I would like to remove the 270k grid resistor and place it on the end of the preamp to better mix with the other preamp in the head (Blackface Bassman). Would this removal alter something to the sound? Would it be wise to do this or wouldn't it be needed.......

[tubeswell]

A 1k5 resistor bypassed by a 22-25uF cap is the 'standard' 'Fender' value for a single 12AY7 or 12AX7 triode in an input stage. But if you want to be adventurous, anything cathode resistor between 820R to about 5k will work - you just get different results. You can even go up to around 40k unbypassed, but the triode will be behaving way differently.

My understanding is that when Fender put 2 x 12A_7 triodes side by side off a common cathode he went to 820R bypassed with 25uF to get the same performance out of each triode as he would out of a single one (because in theory the current is now going to flow through both triodes, so you've got to more or less halve the resistance to get twice the current at the same voltage. Okay so 1k5 isn't quite 2 x 820R, but its close enough because a tube ain't that a-finickity an electrical device)

[MWJB]

That 270K is a mixer resistor, doing the same job as the 220K's in a BF Fender, you can keep it there (as well as the 220K right before the PI), or remove it for a slight improvement in fidelity...whichever you prefer the sound of.

Ideally you are going to need 3 triodes to allow the use of the cathode follower tone circuit that was used in the 5f6a.

[Bernardduur]

Thanks all!

I am using 2 12AX7's so I have enough stages; sounds nice already (blackface side)....... hope the tweed side is gonna sound as sweet!

[Bernardduur]

Thanks! The unit works fine and sounds really fine!

Small question..............

Voltage; I used the Blackface bassman power section. I run the preamp on 385V.............. The 5F6 schematic says the preamp runs on 325V...... is this an issue? Or, how can I drop it down to this voltage?

[MWJB]

385v at the B+ rail should be giving you about 260vdc on the 12AX7 plates...fine for a BF styleamp but a bit much for a 5F6A. If you want to keep the blackface channel true to type AND run the tweed channel at typical voltages, then add another dropping resistor & filter cap between the two preamp tubes. For the tweed side you want 220-230vdc on a 12AX7 plate (330-340v on the tweed pream B+ node).

[Bernardduur]

Thanks!