Help with AB763 Fender Showman preamp gain adjustment

[sonolink]

Hi

I built a Fender Showman AB763 preamp clone. Sounds great but it's VERY gain. I want to plug it in a 18w Plexi power amp but it's way too loud and breaks up much too soon. I guess the design was made to fit a bigger power amp.

Can someone tell me what is the best way to adjust this possibly without altering the tone? I guess it must be some resistor related adjustment but I'm wary of changing things blindly.

Thanks
Sono

 

[CJ]

try a 12AY7 instead of the 7025/12AX7a, this was the original tube in the early Fenders and it has superior sound to the 7025.

a 6072 is the MIL version and can be less problematic (noisy - microphonic)

no $$$?
a simple resistor ladder will work,

is this the right schematic?

i bet PRR can offer some alternate advice besides the tube swap,

12AY7 has the same base/pinout, so it drops right in with no re-wiring,

7025 has a gain of 100, so 2 stages = 100 x 100 = 10,000

12AY7  has a gain of 44 so yo get 44 x 44 = 1936 for 2 stages,

 

[Dr.Banzai]

How much of the pre-amp did you build? You should really only have the 1 12AX7 with the tone and volume between the sections. You can pad that resistively to get it down to a level the phase inverter in the 18W circuit needs.  The Showman (well, any BF amp right? was driving a much bigger swing, and has a lot more signal, yes.  You can try alternate tubes as was suggested, but I think you'll still be too hot even then. Just whip up a T-pad or something. If you're not sure of the math you can just drive the PI directly to see what you need for full output, and then check that against the output of your pre-amp and adjust from there.

 

[sonolink]

Thanks to both for your comments. I have tried other tubes like the 12AT7 and the 12AY7 but on v1 they're very microphonic and on v2 still very gainy.

This is the schematic of what I've built. Essentially the A763 pre with a Pro Junior Vol/Tone (I don't have room for the full TMB) into a a 18w Plexi power amp.

I finished the build yesterday night and another thing that happens is that it hisses like a snake when opening the vol.  Could be because of the gain though.

I thought of wiring a 1M B pot to the preamp output to adjust gain, then substitute the pot with a permanent resistor but I don't know if that would be "proper".

no $$$?
a simple resistor ladder will work,

Excuse my language barrier. Ehat do you mean by resistor LADDER?

You can pad that resistively to get it down to a level the phase inverter in the 18W circuit needs.

Would you please tell me how? Where to begin?

Just whip up a T-pad or something. If you're not sure of the math you can just drive the PI directly to see what you need for full output, and then check that against the output of your pre-amp and adjust from there.

I don't know what a T-pad is. By driving the PI directly do you mean plugging a guitar directly into V2?

Could you please elaborate a bit more? I would be most grateful

Thanks
Sono

 

[sonolink]

I've been reading a bit about split load resistors.  Is that what you mesnt by T-pads, Dr.Banzai?

Cheers
Sono

 

[CJ]

you could try a 1 meg pot as shown, either leave it as a Master VOL or find a level you like with the pot and then read the ohms off the pot and sub in 2 resistors as a voltage divider,

 

[sonolink]

CJ
Thanks a lot! I'll try it 

 

[sonolink]

Just tried it and it works fine. I just realized it's a split load resistor but instead of splitting the plate resistor we're splitting the 1M, which I don't know what it's called (output resistor? )
Thanks for the tip!

I still have some hiss though and a faint hum. Any ideas on how to tame that?

Cheers
Sono

 

[PRR]

This should be all you need with a 18W output stage......

 

[sonolink]

PRR
Thanks for your answer
Will that keep the Blackface character of the preamp?
Cheers
Sono

 

[CJ]

if you want the total Blackface character then you probably need the whole amp,  RCA black plate 6L6GC tubes, some Special Design speaks, and the floating baffle board open back cabinet,

Jerry Garcia used a Showman preamp into a Macintosh power amp, kind of a unique setup,

 

[sonolink]

CJ
Of corse you're right. My aim is to build a BF for club gigs with a clean headroom.  I've only built Plexis so far (about a dozen) and this build is like getting acquainted with the beast

My idea is to use 2 6L6s but I still have to adapt the Supply to be able to use the TXs I like. So I thought in the nean time I could see how it sounds coupled with an 18w plexi. The topology seems very similar.

So my question should probably be, if  I use only half V1 with the original TMB and 6L6s in the power amp will it change the BF character or just reduce gain? I hope this makes sense

Thanks for your great input
Cheers
Sono

 

[andyfromdenver]

CJ
Of corse you're right. My aim is to build a BF for club gigs with a clean headroom.  I've only built Plexis so far (about a dozen) and this build is like getting acquainted with the beast

My idea is to use 2 6L6s but I still have to adapt the Supply to be able to use the TXs I like. So I thought in the nean time I could see how it sounds coupled with an 18w plexi. The topology seems very similar.

So my question should probably be, if  I use only half V1 with the original TMB and 6L6s in the power amp will it change the BF character or just reduce gain? I hope this makes sense

Thanks for your great input
Cheers
Sono

Couple questions/ suggestions.
What do you mean by "plexi" (I have my understanding but want to clarify what you mean)?
If you want to simulate input 1 on a typical fender parallel another 68k off your input.
If you are doing a full plate driven TMB tone stack, I think you should utilize the 2nd triode for makeup gain. 
And more if you have a more complete scheme.
What are you voltages at the preamp prior to the 100k?

Oh and don't ground your B+ just kidding

 

[andyfromdenver]

Also, there's no reason that preamp won't work with any power section.
If you are referring to the scheme designation in the title and not the scheme shown (which is hard to see fully on iPhone)

 

[sonolink]

Hi again

Both solutions worked really great. PRR's  is probably more "authentic" since the amp never breaks up and sounds really Fenderish  8)
CJs solution is very handy though because you can set the resistors to have say a Fender sound from 0 to halfway and then different colors between Vox and Marshall and the breakup sounds wicked!

Since this is a prototype to try out the preamp section (new to me), my aim now is to build a power amp using two 6L6s. I'd like to use the same TX and Bridge Rectifier if possible. I guess I'll have to adapt a bit.
I'll open a new thread on that with voltages etc.

Thanks again to all for your help and time

Cheers
Sono

 

[Andy Peters]

Hi

I built a Fender Showman AB763 preamp clone. Sounds great but it's VERY gain. I want to plug it in a 18w Plexi power amp but it's way too loud and breaks up much too soon. I guess the design was made to fit a bigger power amp.

Well, the Showman amp was designed so that it wouldn't blow up when Dick Dale cranked it to 11. How do I know this? Well, I've seen Dick Dale play many times (did sound for him a couple of times, too), and that's what he always tells the audience.

He also told me that I should start with the guitar channel on the mixer all the way down because, you know, he's Dick Dale and plays loud. I kinda chuckled at that, while recalling the Dinosaur Jr shows I did.

-a

 

[CJ]

when Dick was poor and needed new strings, he used to sneak down to the Santa Monica Yacht Harbor and steal the ropes that they used to tie up the boats,