Diy Preamp section of a guitar amp.

[wagnar]

Hello all,

I am about to start replicating a preamp section of Carvin Legacy or Friedman BE-100.
I had the schematics and have knowledge of dangers of working with high voltages. Know how to drain coupling caps, working with safety gloves...etc.

Bought a psu module from internet which takes 12 volts and converts to 110, 200, 220, 280 Volts respectively(depends on the selection on the circuit ). Probably I will be needing a good coupling after the rectified voltages.

What bothers me is, If a guitar amp's preamp section - let's say feeds with a B+ of 300 Volts, Do I have to pinpoint that exact voltage to make it work ? Will the sound change drastically ? Since this is my first build about valves, I would like to hear your suggestions. Any feedback, any idea is welcome and appreciated

Thanks

[Chuck H]

Getting the voltage on the tubes correct isn't critical, but closer is better. If you're too far off, like 20%, you could start to hear a difference.

That said... There will be other matters that will affect the sound. In a whole amp circuit the power supply voltages fluctuate quite a lot with power amp current. This affect on the operational characteristics as they relate to the tone is absent in a preamp only build. And the layout can affect the final sound A LOT too. Cross talk between circuits amounts to a bunch of invisible capacitors all interacting relative to each other and the outcome can be quite specific. With no power amp to interact with and likely differences in the preamp layout your circumstances in this regard will be different and that is going to sound different.

That isn't to say what you propose won't work. Just don't expect the final sound to nail the tone or feel of the target amp. It will almost surely sound different to you. For better or worse.

[wagnar]

Thanks for the info Chuck, that made much more clear for me why VLD1 (The exact preamp section of VLD100) was sounding really fizzy and brittle to me comparing the original amp. In order to achieve the closest and reliable tone; I will turn myself on cloning the whole amp circuit...

[Enzo]

Hi, write this on the wall so you can see it often:

"It is just a guitar amp"

And by that I mean there is nothing critical in them, they are not precision lab equipment. If you look at most preamp stages, they all look similar other than part values. And note they all run on anywhere from 200v to 400v. There is not an amp out there that will work with a 100k resistor but not work with a 110k resistor.

If your circuit was designed with 300v supply, if you dial it down to 250v, it might make a difference, but then you might LIKE or even PREFER that difference.

[J M Fahey]

Preamps all by themselves are in general buzzy fizzy, they NEED the waveshaping, EQ and compression provided by an overdriven power stage.

[Chuck H]

Preamps all by themselves are in general buzzy fizzy, they NEED the waveshaping, EQ and compression provided by an overdriven power stage.

This^^^^^^^^^^^^^^^^^^^^^^^^

Or... Other tone shaping circuitry. That isn't supposed to imply that there's a way to make a preamp sound like a "whole" amp. But there's no reason a preamp can't sound good in it's own way and on it's own terms. The sheer abundance of amps today that barely tilt their power amps and generate most of their tone from the preamp will certainly attest to that. In other words...

There's no need to plan on building the much more cumbersome and expensive whole circuit if that's not what you need. You can almost surely modify and dial in the preamp you've built to perform well, if not just like the full amp this preamp started out as.

So, I never said that a preamp can't sound good and only a whole amp can. What I said was that the working environment and operating conditions will make a difference for better or worse. I, personally, am a fan of refining the project I'm working on to it's potential before relegating it to a fail. You might be surprised what can be done if you ask different questions. Like "What can I do to make this preamp less buzzy? Then WE ask some questions and apply our experience in design and amps in general to the matter. You learn and your amp sounds better. Maybe even great, if still not like the amp it was modeled after. I think you get the idea.

[J M Fahey]

Yes, you *can* make a preamp sound good, and there´s lots of examples, but you´ll have to work harder, it´s not "automatic" just because you throw some tubes in there.

As just one example, for many years British "Guitar" magazine was accompanied by a CD, chock full of tasty demos, about al kinds of stuff: guitars, amps, pickups, pedals, whatever.

Amp demos of course used what they were demonstrating; but guitar and accessories *all* used the same, both for consistency and practical reasons : they plugged everything into a (then fresh News) Marshall JMP1 , using its built in cabinet simulator and then straight to recorder (whatever they used at the moment).

Sound was always KILLER.

[wagnar]

Thanks for all those info guys, I know what I am going to do. Gather the parts and build the frickin preamp. Then share the results with you all. My target is to get a good "saturated, compressed preamp tone". But first I need to build it and get it working, then I will come here again to ask you where I need to look for mojo. Since I won't be using a PT, things will be cheaper than building a clone.

[wagnar]

Ok, I have made my research, for sake of safety and my laziness, I have decided to build a jcm800 preamp pcb project. Checked the internet about it, how modifications can be done and listed my B.O.M. I got another question thou; normally when I build pedals, I try to use small and polyester caps. Does this applies to tube guitar preamps as well ? or should I go for the ceramic disks which can work in 1000-2000volts without problems ? Since this will be my first build with high voltages and etc. I do not want to feel like -oh crap, shoul've asked before- .


Ps: The links of the preamp if anyone is wondering. (I have eliminated the heaters and will feed the heaters from outside. the ground is only coming from 12VDC to 280VDC inverter.)

https://ibb.co/QkMkpx3
https://ibb.co/mH8sBmL

[Chuck H]

The most typical capacitors used in tube amp projects would be polyester film in the 400V to 600V spec. If you still want small, I used some smaller than typical size Panasonic's that I got through Mouser. Can't remember the model. If size is not of concern there are a bunch of boutique-y type products. The current standard that's considered mojo proper, but doesn't carry the designer prices would be the Mallory 150 series. If you're going to have tubes and components enclosed in a small space then heat may be of concern. Polypropylene dielectric capacitors are usually more temperature stable. I used to use the Orange Drop (Vishay) 715P series but the price got stupid and manufacturing started jumping around so I gave up on them.

[Leo_Gnardo]

The most typical capacitors used in tube amp projects would be polyester film in the 400V to 600V spec. If you still want small, I used some smaller than typical size Panasonic's that I got through Mouser. Can't remember the model. If size is not of concern there are a bunch of boutique-y type products. The current standard that's considered mojo proper, but doesn't carry the designer prices would be the Mallory 150 series. If you're going to have tubes and components enclosed in a small space then heat may be of concern. Polypropylene dielectric capacitors are usually more temperature stable. I used to use the Orange Drop (Vishay) 715P series but the price got stupid and manufacturing started jumping around so I gave up on them.

What Chuck said ^^^. Mouser carries the Panasonic "brown chiclets" in both polyester and polypropylene dielectric. Both are perfectly good caps, and at a value price. Choose lower voltage models & they're small enough to fit on most any pedal circuit board. You certainly don't need 1000V-2000V rated caps in a pedal, most are 9V powered, right? You wouldn't see 1000V unless you got hit by lightning, then you got other problems to worry about. I'm still a big fan of Mallory 150 polyester (mylar) caps though the prices have been rising lately. Mallory was bought years ago by Cornell-Dubilier so you have to search under that name if you're looking at Mouser, Newark, Digikey catalogs. Antique/CE still offers a selection at reasonable prices. Get 'em while you can!