Anyone out there try a SEPP OTL guitar amp?? - diyAudio
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Old 12th October 2005, 11:11 PM   #1
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Default Anyone out there try a SEPP OTL guitar amp??

I was kicking the idea around, and I was wondering if anyone had tried it... seems pretty simple to impliment.
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Old 13th October 2005, 05:59 PM   #2
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I once built an SEPP output stage using 2 6AS7's. Top and bottom tubes were 6AS7's with both halves paralleled. Then I used a large collection of big resistors for a load, trying values from 8 ohms to 600 ohms as the load resistance. I measured the power output and distortion for each value to determine the optimum load. The best compromise for this combination was about 500 ohms. I don't remember the exact power output (I am at work and I don't have my notes handy) but I believe it was about 25 watts. This circuit (with 6AS7's) has a bad tendency toward thermal drift and runaway. It may be more stable with better tubes. The point here is that you would need a lot of tubes in parallel to be able to drive an 8 ohm speaker directly. If you were willing to use an OPT it might work. For a guitar amp the OPT could be a simple toroidal power transformer, since the frequency response would be adequate. The other option would be to wire all of the speakers in a cabinet in series, but that would preclude using the cabinet with other amps.

At one time I built a guitar cabinet using 8 6X9 inch car stereo speakers. The local K-mart was going out of business and I bought the speakers cheap. They could handle 60 watts each, times 8 speakers. This thing had a wicked lead guitar scream that penetrated well, but didn't work too well with a bass. I think the cabinet was too small.
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Old 13th October 2005, 07:34 PM   #3
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That's the problem that I figure I'd run into. Headphone amps are ont thing, but 8 ohm guitar speakers... grrr... that's pretty low.

Any other OTL sedign that you think might work better?
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Old 13th October 2005, 08:33 PM   #4
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^^^^^^

Wiggins Circlotron
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Old 13th October 2005, 09:46 PM   #5
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A circlotron or any other OTL design is going to have a tough time driving an 8 ohm speaker directly. Most comercial OTL amplifiers use 6 to 10 tubes in parallel to generate the required current, and tons of negative feedback to linerize the circuit and reduce the output impedance. None of this is particularly rugged. Not the best thing to use if you are a gigging musician. Live on stage.... an amplifier meltdown....

A possible alternative is a hybrid configuration with a tube directly coupled to a power transistor or mosfet. I briefly experimented with this about 2 years ago, with some success. I have not had time for further experimentation. See my web page for further info (limited). http://www.tubelab.com/SuperTubes.htm

If you want to build a really crazy guitar amp, see the 833A page. I have already decided that I will build one. The prototype sounded absolutely wicked! I am currently in the parts gathering stage. I will post further info as the amp progresses.
http://www.tubelab.com/833SE.htm
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Old 13th October 2005, 10:55 PM   #6
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Quote:
Originally posted by tubelab.com


If you want to build a really crazy guitar amp, see the 833A page. I have already decided that I will build one. The prototype sounded absolutely wicked! I am currently in the parts gathering stage. I will post further info as the amp progresses.
http://www.tubelab.com/833SE.htm
An 833 guitar amp! Now we're talking. Awesome. I can't wait to hear sound clips.
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Old 13th October 2005, 11:01 PM   #7
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Quote:
I have already decided that I will build one
Great. Not many would dare to.
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Old 13th October 2005, 11:13 PM   #8
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Quote:
Originally posted by tubelab.com
A circlotron or any other OTL design is going to have a tough time driving an 8 ohm speaker directly. Most comercial OTL amplifiers use 6 to 10 tubes in parallel to generate the required current, and tons of negative feedback to linerize the circuit and reduce the output impedance. None of this is particularly rugged. Not the best thing to use if you are a gigging musician. Live on stage.... an amplifier meltdown....

A possible alternative is a hybrid configuration with a tube directly coupled to a power transistor or mosfet. I briefly experimented with this about 2 years ago, with some success. I have not had time for further experimentation. See my web page for further info (limited). http://www.tubelab.com/SuperTubes.htm

If you want to build a really crazy guitar amp, see the 833A page. I have already decided that I will build one. The prototype sounded absolutely wicked! I am currently in the parts gathering stage. I will post further info as the amp progresses.
http://www.tubelab.com/833SE.htm
Nice '73 Challanger too!! A friend of mine just finished his '69 Charger 500. It was blue, but he got it painted Hemi orange. Sad story though, when he got the original 440 redone and delivered, the delivery guy drove the fork truck right into the hood! I'd have died. I think it's all together now. Someone had offered him an absurd amount of money for it, but it's one of those "not for sale at any price" kind of things.
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Old 14th October 2005, 03:40 AM   #9
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The 833A tests could be heard a block away, in a noisy neighborhood. I didn't think to capture the sound, but I am sure that my sound card could not do it justice. Imagine a Vox Tone Bender (or Arbitter fuzz face) playing through a black face Champ, a VERY BIG Champ!

The Challenger has been a 6 year project. It is drivable now, but not quite street legal. No lights, no seats, no dashboard, etc. I drive it around the neighborhood on Sunday mornings when the old people that call the cops are not home. Funny, the cop that lives down the street thinks it is cool. Even the burnout. Some day it will be finished. The only place that I can work on it is outside. Here it is 90 degrees, raining, or both most of the year, soon we will have car fixing weather, while you guys are freezing.
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Old 14th October 2005, 03:55 AM   #10
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"...soon we will have car fixing weather, while you guys are freezing."

Not to mention 833A guitar playing weather. That has got to really pump out the BTU's.
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