Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => PC Express and JSchem - Schematics and Layout diagrams => Topic started by: tubenit on November 12, 2008, 06:20:07 am
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One of the forum members was wanting a 6L6 cathode biased amp 15-30 watts with one tone control that had a very clean Chet Atkins type tone with reverb.
Given those parameters...... I drew this up which I have not built. Some of my thinking behind it is this:
- only two gain stages using 12AY7, or 5751, or 12AU7 & LTPI using either 12AT7 or 12AU7
- cathode cap and resistor values that allow cleaner tone on preamp gain stages
- James tone stack built on a 500k dual ganged pot. The JTS has a cleaner tone, IMO and this can
be built with the dual ganged pot for one knob control. The JTS shapes from a fairly flat frequency line
to the Fender mid scooped as you turn up. (plug in the values in the Duncan Tone Stack calculator)
I have used the James Tone Stack on numerous amps and have experimented with lots of value options.
- I chose the reverb circuitry that is used on the D'Lite type build. The reason for that is that it has a
warmer tone with more mids reverbed and uses a smaller value mixing resistor instead of a 3.3.M
which I thought would be problematic with only two gain stages. I use this reverb on all three of my
own amps and strongly prefer it over the Fender reverb which reverbs the high frequencies. I was
concerned that with a Fender reverb circuitry in this amp design that it would sound brittle instead of
a warmer tone (like Chet Atkins used)
- the first gain stage has a bright (.68), full (5uf) and fat (10uf) switch with center off spdt
- the dwell pot would be on the back chassis panel & the presence pot could be an internal trim pot
or on the back panel or simply a resistor paralleled with .1 cap.
- passive effects loop works great with digital delay or tremolo and I use this on all three of my amps also
The goal is a warm but clean/clear tone amp which has a simple design and only vol, tone & reverb knob. I listed a schematic with a range of values to experiment with and where. And a schematic of what values I would start with prior to tweaking.
With respect, Tubenit
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In the same thread about trying to capture a Chet Atkins Standel type clone was this schematic and comment that was posted by Sluckey.
Schematic attached. Here's what the designer had to say about it...
I can't say that the schematic exactly matches the original. During my hours of digging up facts I found out where the tone control notch was, how similar the circuit was to certain Gretsch amplifiers, what class the amp ran in, the speaker used, the physical size, tube setup, etc. From all of the tidbits I was able to piece together the schematic. I have to say that I am pleased with the results.
I used a Hammond 272FX for the power transformer mostly because I had one on hand. A better choice would be the 273BX this gives a higher B+ voltage and would allow using a 5U4GB rectifier as the original used. I used the GZ34 rectifier because of the lower internal voltage drop, thus maintaining the same output power from the amp.
My output transformer came from an old Scott or Fisher stereo. It happened to have the correct impedences and physical size I needed.
I changed a few things on that posted original schematic. I added a passive effects loop so digital delay or reverb could be added. I drew it with 6L6/5881 power tube. The original schematic listed 807 power tubes with a 175 ohm/10W cathode resistor and a 6600 ohm OT secondary. Other than those it should be identical. There is a schematic & layout here for this amp with reverb also
(which was not part of the original schematic I copied).
I find the plate values on the preamp usual? Note that the treble and bass are off seperate plates. Apparently this is modeled after some Gretch preamps:
http://www.schematicheaven.com/bargainbin/gretsch_6160.pdf
With respect, Tubenit
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OK, reportedly this has some of the features of an original Standel type amp. I changed the "original" input, I added either an effects loop or reverb and I changed the 807 power tubes for 6L6 power tubes. I have NOT built this. I personally would lean towards building one with a passive effects loop and maybe use a digital delay or reverb pedal in it.
CHECK for errors! You may want to get some more input from the forum prior to setting out on this build.
EDIT: OK, apparently the true original Standel did not have a paralleled V1 (which adds about 30% gain). I think you could simply substitute a 5751 or a 12AY7 in V1 position. OR just use half of one triode and leave the other one (of the 12A_7) unwired.
With respect, Tubenit
Please PM me if you see any errors.
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This is an example of the Williamson Power amp that the Standel is fashioned after. I don't know if Standel used these same 6SN7 tubes however? Reportedly, the Standel used 12A_7 tubes.
Tubenit
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This is an example of the Williamson Power amp that the Standel is fashioned after. I don't know if Standel used these same 6SN7 tubes however? Reportedly, the Standel used 12A_7 tubes.
Standel used 12AU7's in the PI and driver sections.