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Guitar Amp Prototype

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Posted on December 14, 2014 at 16:08:35
Triode_Kingdom
Audiophile

Posts: 10012
Location: Central Texas
Joined: September 24, 2006

I finally got around to reworking an old Stromberg console chassis that's been hanging around my garage for a few years. Most of the components that came on the chassis were less than hi-fi quality, so I decided to turn it into a guitar amp. It's only a prototype at this stage, but its performance is nice and clean, ready for the next phase. The pre/driver has plenty of headroom, easily pushes the EL34s to the limits of the power supply. Output is about 40W peak or 30W continuous. It still needs tone controls, but there's an unused 6SL7 section that might work for that. The gain of the amp is perfect for my guitar, and it's got excellent punch and tone. The power transformer is the original Stromberg. I replaced the original 6CA4 with silicon and added a bias supply and several RC networks for the preamp circuitry. The output transformer is NOS from a Peavey Classic 30. It was originally intended for a quad of EL84s, and it measures 3.9K/8 ohms and 3.8K/16. For anyone interested, here's the amplifier schematic and a photo...






 

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RE: Guitar Amp Prototype, posted on December 14, 2014 at 18:52:42
Very nice. Is that a concertina style phase inverter?

On a Champ and Vibro-Champ I redid for my own use, I bypassed the tone section. I use a Hotbox in front of the amp. And the tone/volume controls on the guitar. Blackface and Silverface Champs and VC's suffer from a midrange too scooped out, for a 6V6GT single-ended amp (compared to the tweed era Champs). Bypassing the tone control helps the mid's and give the amp a bit more gain.

I never messed with the OEM tone control caps or resistors, in these amps. I'd be interested in what TC circuit you come out with, for this DIY project.

 

RE: Guitar Amp Prototype, posted on December 15, 2014 at 05:11:28
Chip647
Audiophile

Posts: 2633
Location: The South
Joined: December 24, 2012
To what Fender said, you will likely end up with a guitar preamp on front of this as it is likely super clean and a bit dark. You have an extra unused 6SL7 section. Add a switch and use that section with a 300k plate load channel and a 0.001 coupling cap. This will leave your circuit stock until you throw the switch and it becomes more rock and roll.

 

RE: Guitar Amp Prototype, posted on December 15, 2014 at 11:52:17
Gordon Rankin
Manufacturer

Posts: 2928
Joined: June 9, 2000
TK,

The 6SL7 is not doing you justice. Having a 100K plate and 100K on the 6SH7 means you only have 50K effective impedance. In my experience putting a 500K pot instead of the 100K grid resistor on the 6SH7 would make a world of change and give you more sonic structure.

You have to watch out for too much gain unless that is the sound you are looking for.

Do the above and it will sound cleaner.

Thanks,
Gordon

J. Gordon Rankin

 

Tone Controls, posted on December 15, 2014 at 21:33:40
Triode_Kingdom
Audiophile

Posts: 10012
Location: Central Texas
Joined: September 24, 2006
"Blackface and Silverface Champs and VC's suffer from a midrange too scooped out"

Nearly all the circuits I've seen dip the midrange pretty severely. I have no idea what would be best at this point. Maybe I'll try something simple like the single-control Tweed (Princeton) design.

 

6SL7 Loading, posted on December 15, 2014 at 21:40:53
Triode_Kingdom
Audiophile

Posts: 10012
Location: Central Texas
Joined: September 24, 2006
Thanks for pointing that out Gordon. The 100K was left over from a previous test amp built onto this chassis. I had planned to change it before I posted the schematic, but I forgot. It should indeed be 500K. Those are 6SN7s by the way, guess I should have made the fonts a little bigger.

EDIT: I installed a 1M log this evening. That really kicked it up a notch. It's very dynamic, and my Peavey guitar can push the amp into mild distortion at full volume now. Nice improvement!

 

Try it with no adjustable tone circuit., posted on December 15, 2014 at 22:04:13
Think you may like this better. Use your OD pedal to taper the tone.

 

RE: Try it with no adjustable tone circuit., posted on December 16, 2014 at 08:16:19
Triode_Kingdom
Audiophile

Posts: 10012
Location: Central Texas
Joined: September 24, 2006
I'll look into that. Another option, I'm thinking about designing tone controls using one or two rotary switches. Ganged switches would resolve issues like pot tracking, midrange suckout, response ripple, etc. It's more work and a higher parts count, but it might be worth it if the end result is improved response. Guess I need to research this, just to be sure I'm not reinventing the wheel. :)




 

RE: Guitar Amp Prototype, posted on December 16, 2014 at 14:28:10
Gingertube
Audiophile

Posts: 545
Location: South Oz
Joined: October 8, 2004



Here is what I did - 2 channel Guitar Amp using 6SL7.

This is the London Power Standard preamp with minor component value changes to suit the 6SL7 instead of 12AX7 (mostly to cope with higher grid current in the 6SL7).

Replace the J201 JFETS with a simple switch. If you want to have footswitching of teh 2 channels the Yell and I will dig out the footswitching circuit I used to drive those JFET Switches.

Cheers,
Ian

 

RE: 6SL7 Loading, posted on December 17, 2014 at 09:10:51
Gordon Rankin
Manufacturer

Posts: 2928
Joined: June 9, 2000
TK,

Problem with 1M pots in the beginning of the circuit is you loose top end. My fav circuit now used by a bunch of top artists uses a 6072A and 500K pots at the start.

Thanks,
Gordon
J. Gordon Rankin

 

RE: 6SL7 Loading, posted on December 17, 2014 at 11:03:10
Triode_Kingdom
Audiophile

Posts: 10012
Location: Central Texas
Joined: September 24, 2006
"Problem with 1M pots in the beginning of the circuit is you loose top end."

I haven't swept this since I made the change, but I don't see how 1Meg on the grid of a 6SN7 will hurt response. This isn't a high gain (high Miller) triode. Am I missing something?


 

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