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My SE 6V6 with tweaks
My SE 6V6 with tweaks
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Old 17th February 2019, 01:39 PM   #1
snaarman is offline snaarman  United Kingdom
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Default My SE 6V6 with tweaks

Hi there..

While I'm waiting for my "one valve amp" experiment to get going I thought I would put a couple of my amps up for comment.

This here is a variation on the 5F2A 6V6 Princeton, so the design starts as an early Fender Champ but with a tone control and some extras.

A bit of background first:

I bought a Fender Champ 600 second hand last year. It sounded pretty good except the six inch speaker is too small, the hum was awful, and there was no tone control. So I started the valve amp mods addiction: I fixed the hum and pulled the bias back (what were you thinking, Fender?) and added a decent tone pot. Now it started to sound really good.

I couldn't get that crunch sound without thrashing the amp - and it was much louder than I expected - so I created an asymmetric diode overdrive tweak to the preamp stage, and a tweak to the power amp feedback. Finally I had the thing sounding very nearly how I wanted it. We plugged it into a 4x12 cabinet at a recording studio and the result was so nice, that I gave it to them on long term loan. Apparently it has already been used on a couple of recordings...

Anyway - I decided to build my own amp from zero rather than modify another Fender - and most of this circuit uses ideas from that Champ experience. I started with a 5F2 style steel chassis and fitted dual 6V6s in parallel and the usual 12AX7. I gave myself some ground rules. All valve as far as possible. No transistors or op amps in the signal path. Just two control knobs. A basic amp, no tremolo or reverb. No hum please: It should be as quiet as a HiFi amp.

The tone stack is a variation on the Blackface. I included the asymmetric diode clipper in the preamp to give low level breakup and also a LED pair in the feedback loop to sweeten the high level breakup. Yes I know there's a transistor there, but it's wired as a diode and not really in the direct signal path I swapped the usual red power light for a blue LED push button switch, and this brings on the diode and LED clippers for that late night sweet crunch.

There's also a power resistor L pad switch available at the back of the amp to drop the output from about 8W to 2W. The tone and volume pots are CTS pull switch types so I have mid and bass lift available. So despite the simple looking panel, there are a range of clean and light overdrive sounds available. However you have to thrash it very hard before you get proper output valve breakup - at which point it is much too loud for my type of music (finger style blues noodling).

Finally - the box. I took an empty 1990s transistor Princeton, re-covered it in tweed and chrome corners, routed the rear for the new chassis and fitted a Greenback 12 inch speaker.

Now its pretty much finished, I am very pleased with the tone. The PSU section gained an inductor, the OPT got moved slightly, and now this is one quiet amp, particularly in comparison with the two Fender amps I have owned. Unfortunately it weighs a ton and I kinda needed something smaller.

But that's another story...




5F2 mods.jpg

Big Princeton.jpg

Last edited by snaarman; 17th February 2019 at 01:58 PM.
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Old 17th February 2019, 08:29 PM   #2
djgibson51 is offline djgibson51  New Zealand
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Nice. What was the B+?
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Old 17th February 2019, 08:39 PM   #3
snaarman is offline snaarman  United Kingdom
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Quote:
Originally Posted by djgibson51 View Post
Nice. What was the B+?
Hi

The anode B+ is about 330v. The screens are at 245v before the screen stopper resistor. I wanted to run the valves fairly cool so only about 30mA per valve.

Pete
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Old 17th February 2019, 10:36 PM   #4
Enzo is offline Enzo  United States
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SIngle ended amps tend to "run hot" because they are class A amps, and so 90% is common.
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Old 17th February 2019, 11:41 PM   #5
Gnobuddy is offline Gnobuddy  Canada
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Quote:
Originally Posted by snaarman View Post
Yes I know there's a transistor there, but it's wired as a diode and not really in the direct signal path
Uh huh. Sure thing. Yessirree!

It's your amp, and if you enjoy the solid-state distortion you built in, that's all that matters!

Blackstar has already gone down this road, using diode clipping distortion-generators in the middle of what they too claim are all-valve amps.
Quote:
Originally Posted by snaarman View Post
I took an empty 1990s transistor Princeton, re-covered it in tweed and chrome corners, routed the rear for the new chassis and fitted a Greenback 12 inch speaker.
Nice work! Congratulations! That is a nice build in all sorts of ways.
Quote:
Originally Posted by snaarman View Post
I kinda needed something smaller.
I know that feeling very well. I still haven't found my sweet spot, and don't know if it exists. 2W is nice for clean tones but way too loud for overdrive. 100 - 200 mW is about right for overdrive, but too quiet for good clean tones.

To get around this problem, one route I am exploring now is discussed in the "mini amp" thread I mentioned in your single-valve thread. The idea is to use a few-milliwatt valve amp, followed by a guitar speaker / cab emulation filter, followed by a (solid state) flat-response active speaker system. Clean and overdriven SPL will no longer inextricably linked to the valve amp's output power.

I look forward to reading more about your future experiments and builds!


-Gnobuddy
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Old 18th February 2019, 08:27 AM   #6
snaarman is offline snaarman  United Kingdom
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Thanks :-)

I put up a thread on my smaller amp which is based on this same circuit, with a couple of subtle tweaks.

I like both amps. This big 8W one solves the drum kit and Sax problem I get in the church band, it is plenty loud enough for that. (However I do note that my Greenback is very directional. The kick stand I built into the amp is essential, I need to be face on to the centre of the speaker to hear the correct tone.) However this is one heavy amp for an old guy like me. So if it looks like a small band, I take the little one and that is just about right when maxed out.

Pete
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