Changing a Blackface tonestack to Bassman

[chocopower]

Hi!!!

I,m modifying the Normamal channel of my Guyatone GA-1050 (Pro-Reverb like) to Bassman. I had change some tone stack values and itīs fine, but i had read somewere that moving tonestack to oposite side of circuit make it sound closer to a tweed amp. Is this right? How could i do it?

Thanks!!!!

[MWJB]

You really need to find a spare half a 12AX7 from somewhere as the tweeds use a cathode follower tone circuit. This is quite an involved mod and there are a couple of ways of achieving it, but they mean losing your vibrato or something else to free up that triode.

Tweed Bassmans have 4x10"s, a presence control and a floating baffle board - does your amp?

What I would do, in the first instance, is...

the tone stack mods (that you have covered), what middle resistor value did you end up with, can you fit a 25K middle pot?

swap the PI tube for a 12AX7

replace reverb channel preamp cathode resistor & bypass cap at V2 pin 3 with 2.7K & 15uf, or 3.3K & 10uf (this will restore the Fendery chime to the reverb channel that you may lose by using a 12AX7 in the PI - in fact in conjuction with the 12AX PI tube, I prefer this to the stock BF circuit).

replace the power supply dropping resistor between screen supply and PI with 4.7K 3W

tweak the power supply dropping resistor between PI & preamp nodes until you get 220v to 230v on the plates of V1 with a 12AX7 installed. Use a 3W resistor.

This is a compromise and you still don't get the tweed cathode follower tone stack, but it's reversable and doesn't affect the reverb channel that much.

[chocopower]

Tweed Bassmans have 4x10"s, a presence control and a floating baffle board - does your amp?



the tone stack mods (that you have covered), what middle resistor value did you end up with, can you fit a 25K middle pot?

Is a 2x12 amp (Weber 12F150) and i add a variable NFB (whit a bypass cap).
I don,t know what "floating baffle board" mean.....

I had put .022 for mids and testing .022, .047 and .1 for bass.
10K mids pot is grounded whith a 5K resistor for uper value (just provisional, waiting for a 25K pot )

Iīll try to understand all your mods and in some days iīll be back whit more questions........

sorry....

[MWJB]

BF/SF Fenders have a 12mm plywood (or 1/2") or chipboard baffle compared to the 9mm (or 3/8") ply baffle, secured on just 2 sides, in Tweed amps. The BF style is much stiffer & sounds different.

[chocopower]

replace the power supply dropping resistor between screen supply and PI with 4.7K 3W

tweak the power supply dropping resistor between PI & preamp nodes until you get 220v to 230v on the plates of V1 with a 12AX7 installed. Use a 3W resistor.

I can understand all the rest of the mods, but i,m having problems locating this resistor (my poor inglish.....)

Could you mark it in a lyout..?

[MWJB]

It's not on that layout, as it's located on the filter cap pan under the chassis. On Pro Reverb, remove the cap pan, drain caps and look at the underside of the chassis with the PT to your left, speaker jacks & rear fascia towards you and the front control panel facing away from you.

The resistor in question is the one at the bottom right of the filter cap board, bridging the "+" ends of the 2 caps at the far right.

If your amp is not laid out like this then you'll have to find it. Start at the choke - one side connects to B+, the other to the first power supply dropping resistor (at least 1 W rating 1K to 4.7K, most probably 1K or 2.2K, this is the one we are changing to 4.7K). The other end of this resistor connects to the preamp supply dropping resistor (most likely 1W 10K), this resistor sits between PI filter cap and preamp filter cap and the other end connects directly to the upstream ends of V1 & V2's 100K plate resistors.

[chocopower]

MWJB said:

"Replace the 1K with a 4.7K 3W. Replace the original 4.7K with a value that will result in 220-230v on the 12AX7 plates in V1...I'd guess at 27K for a starting point.

With a 12AX7 phase inverter this will give you more of a pre-blackface vibe. If you need to restore the BF style "chime" to the reverb channel replace the cathode cap & resistor at V2, pin 3 with 2.7K/15uf or 3.3K/10uf (depending on how easy it is for you to find a 15uf cap).

100uf filters - There's no reason why the caps can't be fitted with "+" to "+" AS LONG AS THERE IS THE JUMPER that I mentioned (missing from your diagram but should be in the actual amp). Fender arranged them in a more logical sequence but schematically they are the same thing. I often fit series filter caps the way that they are shown in your diagram, if there are restrictions in the amount of space/eyelets that you have to play with."

no 3 watts resistors in my area. (isn,t 2 watts enougth??) I gonna order by mail.
You told me about star with a 27k resistor for the voltage drop in V1...
What values should i buy?

best regards!!!!!

[Fred G.]

Hmmm....I thought 2W was plenty? That's what I just ordered from Mouser for my Bandmaster-to-Bassman/Marshall conversion. The schematic I have of a Marshall JCM 800 2203 specifies 2W resistors in the power supply.

As far as the 27k, the '59 Tweed Bassman used a 4.7k and a 10K.

And yes, converting to a cathode follower tone stack is the only way to REALLY get there.

Fred G.

[MWJB]

Yeah, 2W should be fine. I suggested 27K as the PI to preamp nodes dropper as Chocopwer's B+ voltages were similar to bigger 6G_A amps (higher than typical 5F6A) and their preamp voltages fall in the desired (as a starting point anyway) range.

[Fred G.]

Hi MJWB,

I'm involved in a similar (but more thorough) modification myself - converting a Bandmaster to a Bassman/Marshall, and I'm at the point where I'm looking to drop the preamp voltages, so this thread is getting interesting!

I'm waiting on the resistors I ordered from Mouser. I'm concerned with the high voltage on the plate of the cathode follower (almost 400 volts), but I haven't dropped the preamp voltage yet. Also when I tested this node with my multimeter, I got sparking at the probe similar to discharging filter caps with a screwdriver across the chassis. Is this normal? I've never had it happen before!

[chocopower]

And yes, converting to a cathode follower tone stack is the only way to REALLY get there.

Fred G.

Yes...... i know it.... For the extra triode I just need to choose between remove tremolo or reverb.......

What values do you put in tone stack and in V1 cathodes??

[Fred G.]

Choco,

It's quite a piece of work to do this to a Fender - I wouldn't attempt it unless you have a lot of confidence in your skills, and some experience at it.

I used the vibrato tube - but my Bandmaster didn't have reverb. You'd need to get your B+ from the last preamp node, as well, and use this with the vib tube instead of the current B+ source.

I couldn't even BEGIN to tell you how to incorporate reverb into a tweed-style preamp, but there IS a web site out there where this mod is presented.

[MWJB]

Hi Fred,

Even the original 5F6A ran the pin 6 plate of the cathode follower at 325v (if you believe the schematics, it's obviously higher now), The RI

[chocopower]

Choco,

It's quite a piece of work to do this to a Fender - I wouldn't attempt it unless you have a lot of confidence in your skills, and some experience at it.

I used the vibrato tube - but my Bandmaster didn't have reverb. You'd need to get your B+ from the last preamp node, as well, and use this with the vib tube instead of the current B+ source.

I couldn't even BEGIN to tell you how to incorporate reverb into a tweed-style preamp, but there IS a web site out there where this mod is presented.

Ohh!! my amp have reverb and tremolo (it,s a 2 6L6 Twin Reverb)
Iīm just have to decide wich one sacrifice..... (but i,m gonna wait until i learn some more about this)

Iīm gonna try the droping voltage mod and some more changes in tone stack and droping cathodeīs resistor/cap.
Could you tell me wich values do you put there?

[MWJB]

...sorry about that fat fingers on my part...as I was saying...

...the RI runs the cathode follower pin 6 at 383v, or more, so "almost 400v" is ball park. Avoid the EH12AX7LPS (spiral filament) here, reports of them not coping well. Check with tube vendors in case they know of a tube that might struggle with this kind of voltage. I've had no problems with JJ, don't care much for Sovtek 12AX7WA but they're sturdy.

I know of a builder who has a design that can run a 12AX7 PI tube (the amp I saw had EI here) at 450v+ on the plate! Sounds scary but no reports of failures in 10 yrs of production.

Blue sparking - yes sometimes you might see a 'little' spark, I might be concerned if it was very violent and accompanied with loud pops/arcs with the controls turned down (assuming that you've double checked that you haven't accidentally got the meter set to current).