trying to figure out Bassman schematic

[chcjunior]

I'm somewhere between the schematic looking like a bunch of circles and lines versus.....well, I'm not too sure what lay on the other side.

I'm looking at the 5F6-A circuit. Keep in mind I've never played a Bassman. I see where it has two volumes and 2 inputs for both a Bright and Normal channel. Is this a low gain/high gain input set up for two seperate non-switchable channels with their own respective volume? Or am I way off here?

I apologize to the forum for my new interest in Fender amps, as I will be pestering the same with questions over the next month or so. Thanks!

 

[ES350]

Re: trying to figure out Bassman schematic

V1 is split between two 'channels', 1/2 triode for normal, 1/2 for bright. The inputs for each 'channel' are the typical Fender 68K/136K hi/lo setup and the volumes more or less act independently (they do interact in subtle ways that you need to hear)...the only difference between the two is a 100pF cap across the bright volume wiper.

 

[Scott_F]

Re: trying to figure out Bassman schematic

I think you've got it right. Each pair of inputs is a hi/lo design controlled by one volume. Each channel is voiced slightly different, Mostly it's a "brightness" issue. Or if you want to get adventurous, you can split the first resistor/capacitor using seperate values for each channel.

 

[chcjunior]

Re: trying to figure out Bassman schematic

So could you join the two channels to one volume, and then use the extra volume knob as a master volume? Or am I talking crazy?

 

[Scott_F]

Re: trying to figure out Bassman schematic

Hmm.... maybe a more efficient thing to do would be to get rid of the presence pot and make it into an MV?

Otherwise, you could just kill off one channel. Hmm.... how about using that other 1/2 of the 12AX7 for some more gain? Haha! Except the amp sounds better as a 12AY7 in there.

You, my friend, are hooked!

 

[chcjunior]

Re: trying to figure out Bassman schematic

Hmm.... maybe a more efficient thing to do would be to get rid of the presence pot and make it into an MV?

Otherwise, you could just kill off one channel. Hmm.... how about using that other 1/2 of the 12AX7 for some more gain? Haha! Except the amp sounds better as a 12AY7 in there.

You, my friend, are hooked!

Definitely like the idea of losing the presence pot, keeping a volume for each channel intact, and dropping in a master volume.

Yeah, I'm hooked in a bad way! I believe I'm going to hold off from building for just a little while to acquaint myself a little better with schematics and options I have. Last night I picked up the tube amp book by Pittman and have almost finished "Electricity Demystified" in order to give myself a layman's overview.

 

[Dirt123]

Re: trying to figure out Bassman schematic

Definitely like the idea of losing the presence pot, keeping a volume for each channel intact, and dropping in a master volume.

Yeah, I'm hooked in a bad way! I believe I'm going to hold off from building for just a little while to acquaint myself a little better with schematics and options I have. Last night I picked up the tube amp book by Pittman and have almost finished "Electricity Demystified" in order to give myself a layman's overview.

I think you are on the way to inventing a Boogie Mk I.

 

[Lake Placid Blues]

Re: trying to figure out Bassman schematic

So could you join the two channels to one volume, and then use the extra volume knob as a master volume? Or am I talking crazy?

Running the two channels in series and making the spare knob a master is essentially what Marshall did in creating the 2204/2203 Master volume amps in 1975.

 

[Scott_F]

Re: trying to figure out Bassman schematic

cool stuff

 

[Lewguitar]

Re: trying to figure out Bassman schematic

I wouldn't get rid of the prescence control. It's a big part of the 5F6-A tweed Bassman tone, IMO. That and the cathode follower is where alot of the glassy crunch comes from...just as it does in a 4 input Marshall. The prescence control is part of the negative feedback circuit. I wouldn't add a master volume either...I doubt you could get enough gain out of the preamp to make it worthwhile. If a 40 watt tweed Bassman is to loud for you, I'd go for smaller amp. Maybe build a 20 watt version with two 6V6's or something. Sort of like a tweed Super. Just my 2 cents...

 

[BigDaddy]

Re: trying to figure out Bassman schematic

I'm almost finished on my low powered 6V6 bassman/Marshall/DR hybrid.

It's a BFDR power section and a bassman/Marshall preamp.

It took a lot of work getting the values correct including using 500k volume pots and a .001mfd coupling cap instead of a .022 going into the phase inverter. I was really surprised at how much bass the amp produced and am still trying to tame it without losing the fullness of the tone.

It's in a combo cab with 2 P10RJ speakers reconed to 16 ohms. They have a 1.5" VC and can handle a little more power than a regular P10R.

So far the amp sounds like a low powered tweed bassman.

I think you could achieve the same tone with a deluxe reverb and some tweaks. Maybe a pair of P10R's or Q's instead of the 12. Also going from a 6.8K resistor on the bass pot to a 10k would give it a little more tweed mid tone. That Duncan tone stack calculator is great. The vibrato defeat switch also really fattens the sound into bassman territory.

A tweed bassman is a loud amp, much louder than I expected when I bought a reissue about 12 years ago. I tamed it with a different rec tube and a Hoffman board kit with some changed values. It was a lot more usable then.

They can be detuned power wise without losing their tone. A different rec tube or Webervst copper cap to lower the B+, real 5881's or 6l6 power tubes instead of the 6L6GC's, maybe a 5751 or 12AY7 in one of the slots. And a few cap changes to limit the bass and it can be a very usable amp.

A master volume is a waste in that amp and the presence control is very useful.

It was designed for the 12AY7 and 5881 tubes, it's much louder and distorted with all 12AX7's and real 6L6GC's. I think that's one reason the reissues added the 4.7K resistor on the presence control. I guess you can also lower the value on the NFB resistor too, that would tame it.

 

[ES350]

Re: trying to figure out Bassman schematic

Ditto---the Bassman RI is a different critter; much higher plate voltages (60 watts claimed), harder filtering, etc, etc. They need a few tweaks and parts subs to sound warm like the originals...The BFDR/Bassman clone sounds cool. I'm going to try a 4x6V6 fixed bias/long tail PI circuit in a tweed twin chassis one of these days; got a nice Holland 4x10 cab that needs a head.