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Old 13th September 2007, 05:09 PM   #1
Trout is offline Trout  United States
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Default Edcor In SE Guitar Amp

I have been tweaking / playing around with a simple Guitar amp design using the Edcor XSE 15-8-5K transformer trying to get a good sound using a cathode biased 6V6 and switchable for 6L6.

Right now, In the 6L6 switch position I get a really nice balanced sound. Plenty of clean headroom and excellent volume.

In the 6V6 position, Things are not exactly as I hoped.

The clean headroom is lower, which I expected, But it is rather Fizzy when distorting in the upper volume ranges.
I suspect it is not biased for correct dissapation, But it is common in guitar amps to hit 6V6's pretty hard.

I am using JJ 6V6S which can handle voltage extremes pretty well, but it still seems a bit hot. No red plating is occuring even after prolonged use, But tube life might be an issue.

Any input would be appreciated. I listed the voltages for the 6V6 on the schematic.
SE Amp W/Edcor

Thanks!!

Trout
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Old 13th September 2007, 07:48 PM   #2
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Hi,

I found this behavior too when designing my "Roctal" 6V6 SE amp. Try cutting the voltage on G2. I used 15K/2W, then bypassed with a 10uF cap.

Cheers!
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Old 13th September 2007, 08:25 PM   #3
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Quote:
Originally posted by Geek
Hi,

I found this behavior too when designing my "Roctal" 6V6 SE amp. Try cutting the voltage on G2. I used 15K/2W, then bypassed with a 10uF cap.

Cheers!

I can try that pretty easy.

What do you think about raising the Cathode resistor a bit more?

I had started at 470R and felt it was way to hot. The 680 seemes better.

Is there actually a limit how high I can go?

Trout
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Old 13th September 2007, 10:13 PM   #4
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Hi,

You'll pretty much start losing power with little change in sound, other than it's start getting raunchier earlier. I use 560 ohms on these, bypassed with either 330uF or 1000uF (depending on how much bottom you beed).

Cheers!

*edit*

To help you see what I did, here's Roctal's schematic:
Click the image to open in full size.
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Old 13th September 2007, 10:39 PM   #5
Trout is offline Trout  United States
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Quote:
Originally posted by Geek
Hi,

You'll pretty much start losing power with little change in sound, other than it's start getting raunchier earlier. I use 560 ohms on these, bypassed with either 330uF or 1000uF (depending on how much bottom you beed).

Cheers!

Very interesting design there!

I noticed your much lower voltages also.
300V anode
290V screen

I am about 100V over that in this amp. which is where my bias voltage caught my attention.

With the 6L6GC installed my voltages are lower, but the sound is outstanding. I hope I can find a compromise.

I hate to swap out the bypass cap, Silly me I used a single cap and just alternate the resistor Via the switch like on the schematic I posted above.

The 25UF is great on the 6L6GC.
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Old 14th September 2007, 03:05 AM   #6
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Quote:
Very interesting design there!

I noticed your much lower voltages also.
300V anode
290V screen

I am about 100V over that in this amp. which is where my bias voltage caught my attention.
Thanks!

Yeah, they are two different animals.

I've run 6V6 as high as 370V in PP for a true "18W" output amp. On the bench test, she lost darn near 10% emission in a few days when run at 380V or more at 12W plate dissipation (RCA, Sylvania and Reflektor tested).

You could SPDT a switch in there to:

1) Change cathode resistor + cap
2) Change B+

Another thought for you.... I found if a 6V6's supply is too tight, it won't "sag" well at high drive levels and fuzzes up, as well as too tight a screen.

Whereas.... a 6L6 can sag anything but SS diodes and regulator

Cheers!
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Old 14th September 2007, 03:49 AM   #7
Trout is offline Trout  United States
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I was looking at The Champ 12 schematic and they show 480V on the anode. But I suspect not all the listed voltages are correct.

The screen is set at 242V?? Looking at the Power supply I do not see how they could possibly be that low , 27V on the cathode is about where I would have expected.

http://www.ampwares.com/ffg/schem/champ_12_schem.gif


I guess it will be tough to find a happy spot between the 2.

Trout
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Old 14th September 2007, 06:10 AM   #8
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V3 in a Champ 12 is an EL34 though, isn't it?
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Old 14th September 2007, 12:00 PM   #9
Trout is offline Trout  United States
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Quote:
Originally posted by Geek
V3 in a Champ 12 is an EL34 though, isn't it?

Output:10-12 Watts
Preamp: 2 x 7025
Power: 6L6GC
Bias: Cathode bias
Rectifier: Solid State

My Bad, I thought it was 6V6 because I had one here to swap a speaker out and the guy had a JJ6V6S in it. I never thought to check the tube chart.

http://www.ampwares.com/ffg/champ_12.html


Trout
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Old 14th September 2007, 10:59 PM   #10
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6L6 - right!

Last few Champs I saw had EL34's in them - a common mod.

*makes note to snoop at the local L&M for stock amp notes*
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