Need help with Caswell #39 preamp schematic

[Icarus]

Ok
I took it to a tech that is a little familiar with the Caswell mods.

But he was not sure about how to make the three different sounds footswitchable .

[erigm]

Ok
I took it to a tech that is a little familiar with the Caswell mods.

But he was not sure about how to make the three different sounds footswitchable .

Took what to a tech ... the idea of three footswitchable sounds?

The simplest way is to use relays. One footswitch controls the first relay which selects clean or distortion (crunch/highgain). The second footswitch selects cruch or highgain. You can also use relays to bring different gain controls and master volumes in and out as well.

Look at my schematic and use relays instead of the double throw mechanical switches. The footswitch would operate like most Peavey footswitches. On selects "clean" or not, and the other selects "crunch" or "ultra".

[Icarus]

erigm , I ment the amp 1959 SLP

I haven't spoken much to the tech as he hasn't started on the mods as yet.

Im just trying to get as much info before he gets started , to save time &
$$Money

Forgive my technical ignorance , Im not a tech
I just gig alot & would really like to have thee good sounds out of one of my marshalls without have to use extra pedals boost etc..

Im not sure footswitching is the issue as this point
but thanks I'll look into the peavey footswitch

My main objective is working out the easiest way to have
3 sperate volumes & masters
And looking for a high gain Mod that is NOT very Metal sounding

Cheers

[erigm]

erigm , I ment the amp 1959 SLP

I don't know what you mean here. I was also talking about modding your 1959 to a caswell type spec.

I haven't spoken much to the tech as he hasn't started on the mods as yet.

Im just trying to get as much info before he gets started , to save time &
$$Money

Good idea!

Forgive my technical ignorance , Im not a tech
I just gig alot & would really like to have thee good sounds out of one of my marshalls without have to use extra pedals boost etc..

Im not sure footswitching is the issue as this point
but thanks I'll look into the peavey footswitch

My main objective is working out the easiest way to have
3 sperate volumes & masters
And looking for a high gain Mod that is NOT very Metal sounding

Cheers

Well, if you want to use these three sounds in a live situation I think the footswitching is a very integral part of this mod. I didn't mean you would actually use a Peavey footswitch. I just meant yours could operate in the same manner, if you were familiar with those.

Using the schematic I posted as a blueprint a tech should be able to figure out a way to use relays and make it all footswitchable. If not, you may want to find a new tech. You don't want to pay a bunch of money for an end product you don't want. Just my $.02.

[Icarus]

yeah Thays what i ment , as in looking at the peavey footswitch function
not buying one.

Im assuming you have built something similar, do you have any sound clips ?

[erigm]

yeah Thays what i ment , as in looking at the peavey footswitch function
not buying one.

Im assuming you have built something similar, do you have any sound clips ?

I've built the circuit without the switching, like the original #39 amp. It sounded like Appetite For Destruction. It was a little nasally or peaked for my flavor. I tend to like tones with a little more meat and chloresterol. More VH1 than AFD, but it was very nice.

I haven't built the switching part yet. I've only built one relay switching circuit ever. I usually use jFet shunt switching. I do plan on rebuilding this amp though. I've built it in a Laney AOR50H chassis. I'll keep the architecture, but probably change the values a little bit. I'm sure I'll post on it when I'm finished, I've got two amps in the queue before that one. Keep us informed on your progress though.

[paulster]

I've got one switchable between 1959/2203/Caswell-esque and the huge difference in gain between the three modes means you'd have to have individual gain and master controls for each mode in order to make it usable live as a 3-channel amp. I've only got a single gain and master because I didn't intend using it as a channel switcher, but wanted the flexibility of the different modes.

That means adding 5 pots to the front of your amp, which is likely to be a real-estate struggle, and also finding room to cram in two channel switching relays and control logic, without introducing loads of noise.

It's probably better to build an amp from the ground up for this sort of thing.

[beaulieu]

anybody got this mod in a Layout form??

[erigm]

anybody got this mod in a Layout form??

layout schmayout!!!

[erigm]

I've got one switchable between 1959/2203/Caswell-esque and the huge difference in gain between the three modes means you'd have to have individual gain and master controls for each mode in order to make it usable live as a 3-channel amp. I've only got a single gain and master because I didn't intend using it as a channel switcher, but wanted the flexibility of the different modes.

That means adding 5 pots to the front of your amp, which is likely to be a real-estate struggle, and also finding room to cram in two channel switching relays and control logic, without introducing loads of noise.

It's probably better to build an amp from the ground up for this sort of thing.

You would need one pot for the clean volume, one for gain, and one for master volume. To switch between crunch and lead the one gain setting would probably work (depends on how you tweak the values) and then you might need to attenuate the lead setting from the crunch just a little bit to use the master volume. This would take some experiment, but I think it should be relatively easy. You may find that no attenuation would work for the lead channel if all you use it for is solo work. It may make for just the right amount of boost. All this is assuming that knob real estate is an issue.

[Icarus]

As far as real estate (Holes)
Im modding a Plexi Reissue . Im not too concerned about adding holes
If so it might be only one.

Another point for those who use FX
Im going to change the Existing FX Loop
From series to a Parallel Loop .

Im starting to think series loops are not that great for
REV/Delay & FX Processors.

And sure any Schematics towards the Mods would be cool

[paulster]

anybody got this mod in a Layout form??

There isn't a schematic, let alone a layout that's been shared for the Caswell mod.

Those who have paid to have it done clearly like keeping it close to their chests.

A JCM800 with an extra gain stage is definitely the right ballpark though, and ties in with the block diagram Caswell published.

If the lower Xtra Gain toggle is towards the right, that gives the original‘Stage 1” mod that I did before #39. An amp with that mod was used on many of the 80's hits by the band Power Station, as well as many others. If the center footsw (#39) red led is lit, the extra tube stage is engaged. #39 didn't have the 2 extra gains, so for that sound, push both toggles to the left. The upper toggle engages the Xtra Gain for the extra tube stage.

From this text we can extract that the lower toggle adds additional gain, without an additional tube stage. This is likely to be switching in a cathode bypass cap or possibly changing the voltage divider ratio.

We can also extract that the upper toggle adds additional gain to the additional tube stage. Again, likely to be switching in a cathode bypass cap or possibly changing the voltage divider ratio.

For #39 neither of these would be on, so it's basically just another tube stage that's been added.

[erigm]

As far as real estate (Holes)
Im modding a Plexi Reissue . Im not too concerned about adding holes
If so it might be only one.

Another point for those who use FX
Im going to change the Existing FX Loop
From series to a Parallel Loop .

Im starting to think series loops are not that great for
REV/Delay & FX Processors.

And sure any Schematics towards the Mods would be cool

Icarus,

PM Larry (novobirsi or whatever) with you email address, and he'll send you a parallel FX loop schematic he developed for Marshall type amps. It is getting great reviews from members of this board.

As far as a schematic for the mod, just use the schematic I posted. Make the third stage just like the second stage (100k plate/10k cathode followed by 470k/470k voltage divider bypassed with 470p). Tweak from there to suit your taste.

[Icarus]

erigm & others thanks for the Info
I have a copy of the FX Loop but haven't followed the thread for a while.

I used the Caswell name basically to describe the Idea of Marshall mods
The few sound clips I have heard seem a little warmer & less "Metalic"
Than others.
Im not really concerned about which mods I will choose.
If there are better mods around or suggestions let me know

I just need 3 good sound
Decent Clean
Good Crunch Rhythm ... AC/DC "Highway to Hell" to Old school Scopions
And High Gain Lead...... (thick & not too bright )
Santana , Sykes ,Saraceno, Lukather (Live in Paris)

[valvehead]

hmmmm...

i just tried this circuit , used a 33k for the mystery cath resistor.

Can you guys post your plate voltages on the pre tubes?

mine are very high--over 300

the amp sounds awful--like a bad fuzz style pedal.


wondering where your plates are as far as voltage ?