Changing a Blackface tonestack to Bassman

[Fred G.]

Choco,

Which tube(s) are you referring to with respect to dropping the cathode resistor/cap values? That's not really going to "Tweed" your amp, if you're talking about preamp tubes.

Can you read a schematic? You might get more mileage out of converting the PI circuit to that of a Tweed bassman/Marshall and replacing the 12AT7 with a 12AX7. It's not difficult. I'd start with this and the tone stack values. Change the slope resistor from 100k to 56k, put .o22's in the bass and mid caps. Dropping the preamp voltage might prove interesting, too.

Changing the PI and tone stack are good places for you to start.

Ripping out the vibrato circuit, reconfiguring the vib tube and tone stack to cathode follower, and changing out the B+ node are NOT for the faint-hearted, nor for a beginner!

Have you thought about converting this amp to Blonde or Brownface specs? This would be a LOT easier, and you'll get a VERY cool-sounding amp, believe me!

To "Blonde" it, all you'd have to do would be to redo the PI, change PI tube to a 12AX7, change the feedback and shunt resistors, and maybe drop the preamp voltage some. Check out the schematics - visit the Fender Amp Field Guide web page for COMPLETE fender schems.

[Fred G.]

MJWB,

I was hoping you'd return and finish that post!

Maybe the higher voltage on the preamp plates has something to do with the RI Bassman not sounding authentic, as some claim...

Regarding the sparking - it WAS popping, indeed. I'll recheck it, and double-check that I had my meter set correctly - that's a possibility.

What would something like this be indicative of? Like I said, I've NEVER had that happen before - except when I've discharged filter caps with the "screwdriver across the chassis" method -

[chocopower]

Which tube(s) are you referring to with respect to dropping the cathode resistor/cap values? That's not really going to "Tweed" your amp, if you're talking about preamp tubes.

I,m talking about V1. I,m just looking for a god base for my overdrive pedals.

Can you read a schematic? You might get more mileage out of converting the PI circuit to that of a Tweed bassman/Marshall and replacing the 12AT7 with a 12AX7. It's not difficult. I'd start with this and the tone stack values. Change the slope resistor from 100k to 56k, put .o22's in the bass and mid caps. Dropping the preamp voltage might prove interesting, too.

I,m learning to read schematic.
12AT7 for a 12AX7, done. What more changes need i do in P.I.???.
Slope resistor from 100k to 56k, done.
.022uf in bass and mids, done.
Voltage drop in V1 plate..... Next Saturday morning... (I,m a working man )

Ripping out the vibrato circuit, reconfiguring the vib tube and tone stack to cathode follower, and changing out the B+ node are NOT for the faint-hearted, nor for a beginner!

ok.... i gonna look for a tech for this mod...


Have you thought about converting this amp to Blonde or Brownface specs? This would be a LOT easier, and you'll get a VERY cool-sounding amp, believe me!

I gonna study this option..... too.... Wich model do you recomend?


thanks a lot for your help!!

[Fred G.]

Choco,

Have you been satisfied with the changes you've made to date? These should have made quite a difference in your amp.

As for brown/blonde Fenders to look at - Bandmaster or Bassman (models beginning with a "6G" on the schemetic.

I'd hold off on having the cathode follower modifications done for now. The Brown/blonde phase inverter resistance values might be worth trying out, too.

If you're just looking to set up a good-sounding amp for your foot pedals, then I really don't think you need to go to the extreme step of converting this amp to have a cathode-follower tone stack. It's really not necessary.

Fred G.

[chocopower]

ok!

I´m gonna try the voltage droop and next week, i come agin with a lot more questions.....

by!!!

[MWJB]

RI Bassman preamp voltages aren't necessarily any higher than original bassmans, many originals run 475v or a little more depending on bias/brand of tube (did you see my post of 04:46 yesterday?). There are lots of other differences apart from B+ (earlier RI cabinets, baffle material, brand & composition of just about every component, Legend 102s in the earlier RIs etc., etc).

I guess the violent popping/arcing would be indicative of connecting the B+ momentarily to ground, hence the Q about meter settings.

[chocopower]

hello again!

cheking voltage drop plates...

with a 27k resistor: 168 volts in V1´s pin 1


with a 18k resistor...

V1
pin 1, 192 volts
pin 6, 196 volts

is this safe?

best regards from spain!!

V2
pin 1, 226 volts
pin 6, 200 volts

[chocopower]

no conment?

[MWJB]

Yes, the 18K seems to do the trick, close enough, if you like the sound.

Do you have a different size cathode resistor at V1 pin 3 compared to V2 pin 3? V1 & V2 plate voltages don't always match exactly but 25v difference would seem a lot (I know I suggested a larger value for V2,3 just want to confirm).

[chocopower]

Yes, the 18K seems to do the trick, close enough, if you like the sound.

Do you have a different size cathode resistor at V1 pin 3 compared to V2 pin 3? V1 & V2 plate voltages don't always match exactly but 25v difference would seem a lot (I know I suggested a larger value for V2,3 just want to confirm).

In those moments, a 27K is doing the work... I just cheked for a minutes (i live in a 7 floor... ) I´m gonna check it in my friday and saturday gigs....


V1 cathode resistor in pin 3 is as stock (1500 ohm) and in V2, pin3, a 2700 ohm is put as you sugested for a stocker sound in Vibrato channel.

Thanks for you response and sorry for my inpatiente (and for my inglish)

[MWJB]

Cool. No worries.

[chocopower]

This afternon i cheked the amp at concert volumen..

With both voltages (168 and 200 volts) the sound break a lot easyer than the Vibrato channel.
Finally i keep it in the 200 volts option for my next gig because at 168 volts basses get out of control if i jump volumen uper 3 or 4.
Maybe next week i chek some changes in tone stack to try cut the problem.
Now the tone stack is Bassman 59 like (.022/.022/250pf) with a .047 coupling cap.
A 1.5k/22uf are in both grounded cathodes (the amp comes from seller without grounded cap in V1,pin 8)

Maybe a .022 coupling cap is a god star for tweak...

any sugestion?

[MWJB]

Yes, try a smaller coupling cap by all means.

Yes, I agree that 168v would be too low for most people's tastes.

Maybe also try a GE5751 in V1, this will soften the tone slightly, increase headroom and reduce stodgyness regarding the bass?

[chocopower]

stodgyness???

Ufff..... what it means?

To much for my poor vocabulary....

[MWJB]

Bassy, flat, dull, too thick, woofy.