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SE 6L6 Instrument Amp Build / Introduction

daytonampnerd

Member
Messages
11
Hi guys! First post. I've been building and working on tube amps for a few years now and wanted to document and share a build that I'll be beginning tomorrow. I'll be using some parts recycled from a Webcor Reel to Reel and some other spare bits and pieces that I have hanging around to build a general purpose amplifier. It's mostly for guitar but will second as a bass practice amp that won't wake the kids.

The amplifier itself is a simple design. It sports a 6SJ7 pentode gain stage similar to that in a DC30 mated to Spitefire Volume + Tone controls with a bypassable bright cap. The power amplifier is pretty much lifted right out of a Fender champ with intention to swap 6V6 and 6L6. The power supply is intended to be pretty stiff. I don't like too much squish until it's balls-out.

Schematic:



Bits and pieces:



The aluminum box I plan to use will arrive tomorrow. Things might be tight, it's dimensions are only 8" L x 6" W x 3-1/2" H. The small size could make for a pretty cool suitcase amp if I choose to go that route with it.

Here's a few examples of my work to substitute as eye candy until parts arrive.

"The Scrapper": A Matchless Spirtfire circuit built out of a Hammond AO44 Reverb Amp. I took advantage of the 6GW8's high mu triode sections and added a 12AX7 to keep the footprint small. It is literally a lunchbox sized amp. Controls are Volume, Tone, and Master.





The "B-30" is a take on the venerable B-15 bass amp using a recycled organ chassis and PT. It's a single '68 B-15 preamp married to a doubled up '64 cathode biased power section. Controls are Volume, Bass, and Treble with Ultra Hi and Ultra Lo switches.





B-15 in a box: A 'B-15N preamplifier stuffed into a pedal sized project box. The transformer was salvaged from a badly damaged Allen T44 organ power supply. It has 6.3VAC and 200VAC secondaries, perfect for my purposes. Controls are Volume, Bass, Treble, and Master.



I'm a big fan of Jess Oliver's Portaflex amps and have been known to restore a few electrically and mechanically.



I also recently purchased a Rickenbacker (Rickenbacher) Electro amp and will be posting a separate thread for its restoration once it arrives.
 

Jeff Gehring

Gold Supporting Member
Messages
6,800
Welcome! Looks like a fun little build. Couple notions for you:
  • you might add a grid to ground resistor for the 6V6/6L6. I assume you'll be doing a tip shunt to ground on the input jack ?
  • For less SE hum, you might give thought to establishing an additional RC filter node between the rectifier and your existing A/B/C filter nodes, still feeding the power tube plate off the A node. The extra RC ahead of the existing first stage really gives you good bang for the buck in reducing ripple hum (since you won't have the hum cancellation that a push-pull final would give you...
Cool Ampegs!!!
 

daytonampnerd

Member
Messages
11
Thanks!

Yes, there will be a tip shunt to ground on the input. Perhaps I should consider a 220k like that found in the champ. This was a rather hasty drawing as things have been stressful with a new baby and the eldest back in the hospital. I build amps as a sort of catharsis when things get crazy.

I also agree that an extra filter node may be beneficial. I built a champ type amp before using a similar power supply setup that does have some hum present.

Aside: I may not bother with the bright cap/bypass setup I mentioned.
 
Messages
2,918
Welcome! Looks like a fun little build. Couple notions for you:
  • you might add a grid to ground resistor for the 6V6/6L6. I assume you'll be doing a tip shunt to ground on the input jack ?
  • For less SE hum, you might give thought to establishing an additional RC filter node between the rectifier and your existing A/B/C filter nodes, still feeding the power tube plate off the A node. The extra RC ahead of the existing first stage really gives you good bang for the buck in reducing ripple hum (since you won't have the hum cancellation that a push-pull final would give you...
Cool Ampegs!!!
I recently built the AX84 Single Ended Lead Tube Guitar Amplifier Project. I added the Cap/Resistor setup between the rectifier and standby switch from the Plexi SE PCB version 1.0 power supply . No issues with hum.
 

daytonampnerd

Member
Messages
11
The "chassis" arrived and provides more than enough space to work with. It's a little taller than I'd like but the thinner box I wanted to use was sold out. I'll begin the drilling and placement of major components this evening. I may even do some wiring later on tonight.
 

bob-i

Member
Messages
8,763
Love the portaflex wall. I had a B15 in the 60s, never should've sold it. When I had bigger gigs, I'd borrow a friends B18 and stack them. I'd put a board over the opening in the B18, put the B15 on top and put the B18 head on the floor. I thought it was more power than I'd ever need and it was probably about 60-70 watts total.

I met Jess Oliver a few years ago, he was about 90 but sharp as a tack. He talked about designing those amps and the challenge of tweaking the frequency response for maximum punch and volume without blowing speakers. He sure had the formula down cause they sound amazing.
 

daytonampnerd

Member
Messages
11
Man, I wish I got to meet Jess and pick his brain. I really dig the combination of B-15 and B-18 together. 80 watts of bassy bliss!

I did some rush building in between feeding the baby, changing diapers, and such. After about four hours I had a functioning amp. Most everything was eyeballed since I was in a hurry to see how it sounded. This was my first time using a pentode preamp and I couldn't wait to try it out.

A couple pictures mid-build:





Post-Testing:





The thing just sounds so clean and pure. The lows are fat and the highs are crisp. It is also dead freakin' silent. The additional filter node went a long way in squelching hum.

The amp only begins to overdrive on the bridge pickup of my hollowbody when I thrash on it. (The pickups are pretty low output FWIW.) I plugged my B-15 preamp box in and cranked it up to get some dirt on. It delivered delicious rock and roll overdrive. Boosting the treble on the pedal brought forth one of the best tones I've ever heard.

I may try running a 6SL7 with a James tone stack into a 6SJ7 for the front end of a push pull amp at some later date after having that experience.

Next up is testing it with the bass when everyone is awake tomorrow.

Yes, gut pics are on the way. It's a little messy at the moment and I want to tidy it up a bit before I post pics.
 






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