converting Bandmaster to Tweed

[Fred G.]

I'm considering converting my Bandmaster's vibrato channel to a tweed Bassman clone (the vibrato was removed years ago for a seldom-used effects loop, I have the extra needed tube stages).

I'm looking at the old Bassman schematic (5F6-A), I see that there is no plate resistor on the last preamp tube stage (the one feeding the tone stack). Will this cause me any problems duplicating this in my Bandmaster (i.e., direct plate/B+ connection in this stage)? Any hidden pitfalls, etc?

I just don't think this is going to be that difficult of a modification (converting to tweed), it appears pretty straightforward. Please correct me if I'm wrong!

Also, any insights, caveats, or tips before I try this would be greatly appreciated!

Thanks in advance,

Fred G.

[Satamax]

What you're talking about is the cathode follower. No it shouldn't be too hard to do what you want. I'd buy a replacement trany for the bandmaster with a 5V tap so i could use a tube recto. (actualy i have one sitting here, but it only to switch the BM to tube recto and 6V6) Cos the voltages are bit high for a tweed.

[Fred G.]

Hmmm.... I was under the impression that the term "cathode follower" referred to the type of tone stack (following the preamp tube cathode) rather than to the tube stage it "followed".

What I'm really hoping to learn more about is why there is no plate resistor on the tube stage in question, and why such a high value cathode resistor.

Thanks,

Fred G.

[Steve Conner]

No, cathode follower refers to the tube stage itself. The input is to the grid, the output is taken from the cathode, and the plate is just connected to B+.

The name comes about because the cathode voltage just follows whatever you do to the grid voltage. Hence, the voltage gain is a little less than 1, unlike the ordinary hook-up for a preamp tube (technically called a common cathode stage) which amplifies by a factor of about 60.

A cathode follower is used when the designer doesn't want the voltage of the signal amplified any more, but wants it buffered so it can drive a heavier load.

[Fred G.]

Thanks Steve (and Satamax too).

I finally gathered as much after doing a little more reading after posting this, but it's still nice to get it condensed down into a paragraph or so.

I noticed the Marshall version uses a 100k cathode resistor while the Fender Bassman uses 56k - is this because of lower B+ at the plate, or ?

Also, I want to drop the B+ at the plate a little bit (to get closer to the Tweed values), I want to do this by increasing resistance values in the power supply from the Bandmaster values to the 5F6-A values. Am I over-simplifying here, or is this all I need to do?

Thanks,

Fred G.

[stokes]

The 5F6-A Bassman schem I have shows a 100k cathode resistor not a 56k.The first Marshalls were a copy of the Fender Bassman.You can lower plate voltages like you describe,by adjusting the dropping resistors in the B+ rail,just be aware that it will drop the volts in the other channel as well,in case you wanted to keep the normal channel stock.

[Fred G.]

Thanks, Stokes!

That's EXACTLY what I want to do. Regarding the cathode resistor, you're absolutely right, the Fender schem shows 100k. I was thinking of the somewhat generic Tweed preamp schematic on the Ampage reference page.

I appreciate your feedback (no pun intended)!

Fred G.

[Fred G.]

Well, this project is just about completed.

I had to stray from the Tweed values (I wanted to use the more readily-available 12AX7 in the first preamp tube slot), so I essentially used Marshall JCM800 values (as well as adding an extra optional gain stage up front).

I have not yet changed the tone pot values to the Tweed/Marshall values.

This amp is sounding pretty dang good...

I have not dropped the B+ yet, and there are currently a whooping 398 volts on the plate of the cathode follower stage - yikes!

Next step is to drop this B+ in the preamp DOWN some, I guess I'm going to completely convert to Tweed resistance values.

I have an extra, unused pot position - any ideas for a useful additional preamp control (other than the usual Marshall/Tweed complement)?

All advice, suggestions, comments welcome!

Fred G.

[isaac]

I know this is old, but how's the amp now, sounds like a very interesting project, what did you end up doing with the extra hole? Clips? I have the habit of finding old 2007 posts, but some of them are really interesting, so I though I'd ask. Also a schematic would be cool.