'68 to '59 Bassman mod w/master Vol...help?

[bren-hur]

Hi! I am new here and therefore extend my greetings to all...HELLO!

First off I want to state my amp tech skills as PROFICIENT...not lame but definately not legendary. I have a '68 Bassman 50 to which I did the standard mods/fixes... Bias fix, electrolytics changed, nonsense capacitors removed from power tubes and elsewhere, basically reverted it to pretty much AA864 specs...I was happy BBUUUTT... the mod bug hit me again...which led me to Thundertweak amps and the most legendary Mr Ace. He did a bassman mod which sounded KILLER...normal channel pure blackface, Bass channel '59 tweed with Master volume and Switchable gain stage. RAD! He didnt have a schematic but pointed me in the correct direction (Transatlantic Bassman and Hoffman hotswitch)....SO....I did my best to put together a schematic...hopefully it's attached to this post...if not email me, if you are interested! Like I said...my skills are proficient..so I was hoping some legendary folks could look at my schem and make sure I'm not gonna go and bugger some stuff up...I love my amp...and dont want it buggered.

I did a pretty much straight 5f6a circuit on the bass channel and my main worry was the power going into the tubes...I left all the resistors at the standard 100K...but I'm pretty sure the '59 tweed had different trannies, and therefore the preamp circuit which I copied might run a bit different. Any thoughts anyone? any values that the people here think might make a better circuit?

Sorry this is so long! I probably lost most of you in the first paragraph! Feel free to email me or post your thoughts!

Thanks....BrenHur

[PRNDL]

Do you know why they used 100K / 4.7K instead of 100K / 1.5 K in the gain stage?

The 470K to ground with the 270K to the grid looks somewhat like a voltage divider to lower the gain of the boost channel, but not really (I believe it's to keep the grid at zero). It could be reviewed and revised, if you'd like to be able to set the gain.

[hasserl]

Do you know why they used 100K / 4.7K instead of 100K / 1.5 K in the gain stage?

Assymetry of the clipped signal? Reduced gain?


I think the schematic is missing a 220k mixing resistor from the 59 channel.

I did a similar amp with a Weber 6A20 chassis. I built it without reverb, gave it an AB763 circuit in one channel and the 59 Bassman in the other channel, but no extra gain stage. It's nice to have both circuits in one amp.

[bren-hur]

Thanks for the repsonses! Very helpful....my n00bness quickly becomes apparent...

The 470K to ground with the 270K to the grid looks somewhat like a voltage divider to lower the gain of the boost channel

This was me combining the 5f6a circuit and the Hoffman hotswitch (JCM800 preamp) the 5f6a had the 270 going to the grid and the JCM800 had the 470k... and I have low understanding!

I modified the schematic (attached to this post...) matching it more to the JCM800 preamp.....so hopefully it makes more sense now! I also added the 220k mixing resistor.

Any more thoughts? I appreciate your help!

[txstrat]

Hi bren,

I built an amp similar to this schematic a long time ago and I still play it. It has similar features as you described. Blackface channel and british channel with master volume (I didn't use the hotswitch).
I use only one input and go to the first half of a 12AX7. From here I switch with a relay to the different channels.
I use a 1.5k resistor and a 22/25 cap on the first cathode. That's not the values you would use for the british channel but I thought. "what the heck". It makes the clean channel sound "blackface" and the british channel sound british (maybe not if directly compared to a Marshall, but I use different speakers anyway).
This leaves one half of a 12AX7 free and I used this for a switchable third gain stage in the blackface channel (see Fender Champ II schematic). It adds more grind to the sound up to crunchy distortion when you crank it and gives sort of a more dynamic feel if you leave the volume at about half up.
Besides I left out the mixing resistors. I used the free pole of the (DPDT) relay to switch between the channel outputs to the PI.
Those are my suggestions so far. Hope this helps.

Matt

[cucei]

hi!

hello, I'm new here ...
My question is: is listening very well your amplifier?
Trying to end the mod?
I would like to make this mod on my amp!
help

[txstrat]

It sounds like two amps in one - blackface and bassman (with master volume) switchable via relay.
What mod exactly do you mean?
To share the first half of the first tube with two different preamps?

[chocopower]

Question for Bren-Hur (and every body)

I can see you added a 27K resistor to drop voltage in V1 (A and B)... Why donīt use the same voltage to feed the V2?

This way it would be closer to the Bassman voltages. Isnīt?

Maybe iīll do something similar in my Bassman head clone...


bassman