Have not been here lately, so you're making an amp eh? Is it a chip amp or is it a discrete amp where you can do things with stuff like negative feedback. If so there once was an amp called the Redmere which emulated a Fender, a Marshall and a Vox all by clever switchable routes within the amp circuitry itself. That would be cool but it all depends on what your approach is.

Here's my current design for the 500 watt bass amp I mentioned in my previous post. Like to get it all down in the computer before hacking things together.

https://picasaweb.google.com/106263582096865602887/SanAntonio?authkey=Gv1sRgCMPy5OnBx-WqMQ#5789297530033024290

Quote from: Roly on September 17, 2012, 10:03:40 AM
Link to project 27

Unless somebody pops up who has actually built and gigged one of these it's a bit hard to know what to tell you.  As a tech I think very highly of Rod's skills; I like the design concepts he has used, in particular current feedback to get a more valve/tube sound, passive Fender-style tonestack instead of the active Baxandal which is common in solid-state designs but is more suited to reproduction amps than guitar amps.

The design is very complete and you can even buy PCB's from him to make construction even easier.

Adding another preamp, or even two, alongside Rod's design would be very simple using a selector switch just ahead of the "Pre Out" socket.  The power supply would have ample reserve to run another preamp or two.

There are many designs for preamps on the net, and part of the interest in building your own is to try different designs and see how they work for you.  As for JFET's, there are many suppliers in Asia and Australia who will supply JFET's by mail order; in Australia there are Jaycar, Altronics, Oatley, and Wiltronics just to name a few.

As Rod himself says in the project;

"I suggest that you try it "as is" before making any changes."

An important influence on the final sound is the speaker selected, particularly the cabinet - open backed for rock and roll and sealed for cleaner jazz-like playing.

I have built and used Rods designs and think they are simply fantastic! I currently use a P27A/B with slight modifications:

http://www.youtube.com/watch?v=sq6Ea84t538

This amp blows away anything I have ever used before and amazes fellow musicians when I tell them it's solid state. I typically use it with a single 12" in an open iso-cabinet (demeter style) but also use it with some 4x10's and 4x12's. The sound and raw power are awesome.

I'm currently designing a nice 500 watt bass amp using his P68 and P27B boards, this one should raise the roof... at least that's what Rod tells me.

Oh yeah, I almost forgot. Changing the value of C17 will effect the biasing range as well and may bring you closer to the 5 volts you are looking for... all this stuff is interactive. If I got things confused with these reference designators, that's the same capacitor loudthud is talking about... he knows his stuff real good too and has helped me out of a tight spot on more than one occasion.

Hi, I'm new to this forum but not new to FET preamps. From what you have said I suspect that Q3 is flakey or C17 is too large or a combination of both. If C19 shows strong oscillation (I'm using the schematic designations because the veloboard layout don't tell me much) then the problem indicates something amiss with the Q3 stage. These are cheap FET's and quality control may not be what you would expect with them even from reputable sources. C17 can really cause havoc if not sympatico with the FET. I have had to modify the cap values (usually making them lower) on similar stage before and this seems to stamp out oscillation problems... then again, who really knows until you really dig into things and do some "what if's".