12SQ7->50L6 SE design...request for comments

[Zipslack]

I had an old 40's or 50's phonograph that was given to me (seized motor, dry-rotted rubber, no needle). I decided to try to do something with the cheesy little amp. It was a typical 35z5, 12sq7, 50l6 amp similar to the 3-Tube Audio Amplifier from Alfred Morgan's Second Book of Radio & Electronics http://www.bignick.net/images/RadioPix/MorganAmp.pdf. I pulled inspiration from the Sonic MA1 amp KIRTLAND electronics Hand Wired Guitar Amplifiers ? and the Gibson EH-100 Prowess Amplifiers - Gibson - Schematics - Eh 100 2. Components/values are based on what I have in my junk collection (like the 2:1 step-down power transformer, which inspired this design).

This is what I have come up with (DISCLAIMER - this is a "seat of the pants" design...no curves or calculators were used...no algebra was harmed in the making of this amp). I've got a sloppy prototype put together and it works okay with an MP3 input. I still have to test/tweak with guitar and then do a "real" assembly. I'm expecting mostly clean output with maybe just a hint of overdrive at max volume.

I'm interested in having some other sets of eyeballs check this and see if there's anything that needs to be changed/added/corrected, or if you have any suggestions to make the circuit better. Thanks in advance for any replies.

[loudthud]

The Morgan amp was the first one I ever built. I think I was in 6th grade. If you are using that schematic, note that the 150 ohm resisitor in the heater string needs to be 5 or 10 watt. I remember Morgan left it off the bill of materials ha ha. Are you using the 60V transformer idea with the voltage doubler? That's actually a neat idea. The 220uF caps are overkill, 100uF would get the job done.

[Zipslack]

Ooops - forgot the schematic

Oops...just realized I forgot to attach the schematic.

I used the Morgan schematic for the general concept of the two tubes that I have available (60V series string, without the dropping resistor), the voltage-doubler from the Sonic MA-1, and the biasing of the 12SQ7 from the Gibson EH-100 with a typical Fender hi-input. I'll probably go ahead and add a 25uF cathode bypass to the 50L6 output...depends on how much volume that adds. The general idea is a low-watt, mostly clean amp for bedroom/practice/recording to use with pedals that allows me to use some otherwise "junk" tubes. If it works well, maybe I'll do a few for sale/trade/gifts.

If some of the values seem "wonky" (500K volume, 120R cathode resistor, power supply values), it's because I'm working completely out of junkbox stuff before I commit to buying/sourcing "good" parts.

BTW, good point on the dropping resistor - the old phono chassis actually had a 150R ceramic 10W riveted to the chassis - I'm not using it in my design since I'm using the doubler (driving the string directly from the 63VAC secondary).

[loudthud]

R1 should be 68 ohms, not 680 (from the kirtland schematic). You could go with a lower value, just something to act like a fuse. Prehaps 10 ohms 1 watt.

[Zipslack]

Thanks for catching that...it's a typo on my schematic. I actually have a 68R in the circuit. I think I'll be able to hook up a cheap guitar to it tomorrow for testing/tweaking. I'm hoping it will be worthwhile; otherwise, I'll replace the 12sq7 with a 12ax7 like the Sonic MA1 and experiment some more.

[printer2]

12SQ7, basically treat as a 12AX7 right? I think the 3k on the cathode is too large, maybe cut it in half. Oh wait, running at a low voltage. Maybe double the plate resistor instead. Cathode bypass on the power tube for sure, you need all the help you can get.

[Zipslack]

Yeah, 12SQ7 is supposed to be about 100 mu, so basically half of a 12AX7. I still have to play around with plate and cathode resistors - like I said, I haven't bothered plotting curves or calculating anything (no fun in that!). Actually, I decided to make the power tube bypass cap switchable - it makes a nice volume boost. Overall, it stay mostly clean with a cheap Squier strat that I'm using for testing...may get a little breakup with humbuckers. The volume pushing a cheap 12" console speaker is usable - comparable to a 386-based amp, but clean all the way (I'm guessing 1/2 to 1 Watt output). I may try playing around with the plate resistance to see if I can get it to distort/overdrive naturally; otherwise, I have my Valvecaster to push it with.

Mainly just something different to play with, so I'm not looking for "boutique" sound...just usable. While I have the power-supply setup, I may try an AA4 type of configuration http://www.kirtland.com/guitar1/AA4_..._schematic.gif to use up some old radio tubes, but stick with the 50L6 output, or maybe just go with a 12AX7...but this is such a simple little circuit that I would hate to clutter it up with 2 gain stages.

[printer2]

12AU6?

[Zipslack]

12AU6?

Thought about that...I know I have a couple of 6au6, but not sure if I have a 12au6...have to check the stash, but I'm fairly happy with the way it's coming. I actually prefer to have usable clean.