Adding tube/gainstage to BF Bassman?

[JohnnyCrash]

I built a BF Bassman a while back, now I’m considering turning the preamp into a two channel Mesa MkI (I already have other BF type amps).

I’d like to add another 12AX7 tube and socket.

Will the power transformer accomodate the extra plate and heater current?

 

[printer2]

Plate current no problem. The heater current should not be either as long as you do not go much below 5% of 6.3V. The heater supply can go down below 10% but you start getting a loss in cathode emissions. But the short answer, should be fine.

 

[JohnnyCrash]

I think I’m leaving it as-is after all.

I had already tweaked the circuit to give both channels an extra gainstage (3 stages for each channel).

The first gainstage for each channel is from the input jack (22k grid resistors) to the grid, the plate then feeds a “Gain” control (I replaced one input jack on each channel with a 1 meg pot). The cathode resistors are 820 ohms for this stage, bypassed with a 2.2uF for the Bass channel and 1uF for the Normal channel.

The Gain pot feeds the next stage, which is the typical Fender tonestack.

The output of each tonestack feeds into a third stage each, which each feeds into the phase inverter (via 220k summing resistors).

It’s not a high gain beast, but it crunches AND each channel has a Gain and Volume control.

I put a fatter midrange resistor on the “Deep” and “Bright” switches, which can go from scooped BF Fender mids to Marshall fatness.

I like to use the Normal channel for rhythm and the Bass channel for solos... or, conversely, I like having a clean and a dirty channel in one amp — in either case, my old Morley ABY switch acts as my channel selector

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My only problem now is the Master volume is very abrupt. It goes from quiet to loud very quickly on the dial. Both are audio taper 1 meg pots.

I also haven’t dimed it for testing purposes (it’s 11pm and I just put down the soldering iron, my neighbors would kill me at this hour).

 

[JohnnyCrash]

Scratch that... I have enough spare parts in my junk drawer to add a gain stage per channel, so I guess I’m doing it

 

[Mark the Moose]

I was just contemplating this same mod for a 5f11 last night. Please update us how it works out.

 

[Commodore 64]

Put it on a switch and you can swap from parallel to cascaded gain, too.

 

[Natima]

Out of curiosity which type of master volume are you using?

 

[Mark the Moose]

Would this be comparable to the gain channel on the super-sonic? Isn't that a bassman preamp with an extra gain stage? Maybe check the supersonic schematic.

 

[Phrygian77]

There are two tubes for the bass channel. Is an Express style preamp not enough gain? That's what I did on my BF Bassman build. You could also do a 2203/2204 style preamp.

Sent from my HTC U Ultra using Tapatalk

 

[JohnnyCrash]

OK, I just drilled the chassis and installed another 12AX7. I might need some advice (Mesa MkI preamp schematic included for reference)...

Each channel is as follows (with different voicings for each channel):

1. Input jack > 22k grid resistor > 1st gainstage grid. Stage 1’s plate feeds a 1 meg Gain pot, which feeds into the next stage’s grid.

2. This second stage is the plate fed tonestack. The Volume (actually a “Gain #2” control) at the end of the BF Bass/Treble stack feeds into the next stage’s grid.

3. This is where I diverge from the MkI. The plate/output of each channel’s third stage is fed to a 1 meg Master Volume pot (note: the MkI puts the MV after the next stage... I don’t do this). Each pot feeds into each channel’s next stage’s grid.

4. Each plate feeds into the Phase Inverter via a 220k summing resistor each. Instead of the MkI’s 0.047uF cap and MV, I simply feed the plates to the summing resistors (per the original BF Bassman circuit).

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As you can see, the third gainstage is different than a MkI’s. This may be the cause (or part of the cause) for the problems I have with both MVs (see below).

Also the input to the PI is different than a MkI since I have two identical MkI preamp channels. I’m using summing resistors rather than a single channel’s MV pot into the PI.

I had to work on my lead dress to eliminate oscillation in the Normal channel due to high gain and this channel’s increased treble. I think I’ve fixed this part.

ADVICE:
1. The MVs seem to freak out (squealing, noise, general face-melting chaos) when trying to use them both at the same time, regardless of which channel I am plugged into.

2. I would like to have two MkI channels and the Negative Feedback. But the summing resistors might not be ideal. Would eliminating the NF to free up a second PI input be a better approach?

3. If so, I can reconfigure the third stage to be identical to a MkI’s and each MV can feed its own PI input.

However, I’d prefer to keep the NF if at all possible.

4. The noise is pretty high, even for a high gain amp. Might this be due to the summing resistors and MV situation I’m using?

 

[JohnnyCrash]

Out of curiosity which type of master volume are you using?

See my updated post above.