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AB2 Guitar Amp?

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Posted on July 23, 2016 at 12:00:47
Triode_Kingdom
Audiophile

Posts: 10011
Location: Central Texas
Joined: September 24, 2006
I have several sets of identical OPTs/Power Xfmrs that I'd like to use for DIY guitar amps. Although the amplifiers were originally rated at 50W output, the OPTs were designed for a wider BW. I'm certain they'll output 100W or more peak power over the narrower range of guitar, even at the bottom end. Similarly, the OPTs originally powered quite a bit more circuitry than a single amp, so there's considerable excess current capability.

In a "normal" push-pull configuration with 6L6s, the audio output power using these components is limited essentially by the gain of the two output tubes and available anode voltage (about +450V). However, the use of an AB2 driver with an IT to the grids would change all that. A pair of 6L6GCs or 6P3S-Es might be capable of 80-100W peak/intermittent output power in that configuration. This would make much better use of the components, and would put the amps into a power range that I consider more worthwhile for the work involved.

The one unknown in all this is whether AB2 sounds significantly different than the typical AB1 amp when it crunches. Maybe the commercial manufacturers have already tried this and discarded it for that reason. Or, maybe they decided it was more cost effective just to increase the voltage and/or parallel more output tubes for higher power levels.

So, does anyone know if there are 6L6GC datasheets that show AB2 operation at more than +360V? Anyone here have any hands-on experience with this sort of thing?


 

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Just one question, posted on July 23, 2016 at 12:36:17
Chip647
Audiophile

Posts: 2631
Location: The South
Joined: December 24, 2012
Why are you shooting for more than 50 watts?

SPLs with guitar rigs are primarily a function of the speakers and cabs. Good 12 or 15 inch guitar speakers with close to 100db per watt efficiency put out a massive amount of sound with very little input. Using a front port or sound hole you can make a 2 x12 or 4 X 12 absolutely deafening with 10 watts. For live performance with a PA you will have to dial it way back. Counting on the overdrive of the output tubes for tone with a 100 watt amp will ensure you will never get to use it that way unless you install a speaker attenuator (which kind of defeats the purpose of 100 watts)

My thought is that AB2 is not used because you have to have an expensive, complicated AB2 driver to make it work and it will not be reliable. AB2 with one bad feedback loop issue will melt the amp. 4 EL34 or 4 6550 tubes will put out more than you can use without touching AB2. I have built a Fender PS300 clone (300 W output).

That being said, I have a 304TL staring at me that is going to make a 50 watt A2 triode guitar amp, just because.

 

RE: Just one question, posted on July 23, 2016 at 14:55:52
Triode_Kingdom
Audiophile

Posts: 10011
Location: Central Texas
Joined: September 24, 2006
My reasoning is simply that the system should be capable of roughly 100W, so that's what I want to build. I understand the issue of too much power, but there are various ways to reduce headroom. That's another topic for another day. Anyway, let's say I want to do this "just because," same as your 304TL. :)

About the AB2 driver, I don't believe there's any more risk or general complexity than with any other design. These power transformers have a separate B+ winding that was originally used with a second tube rectifier to power a second amplifier chain. That chain used a pair of 6GW8s (ECL86) in push-pull, so the power supply is perfect for a SE driver capable of a few watts output. That would be a very simple circuit, and the IT would provide DC isolation between the driver and output stage. No chance of a cascading failure. Really, I think this will be a no-brainer, but there's no substitute for actual experience. I can't be the first person to do this, right?






 

RE: Just one question, posted on July 23, 2016 at 15:49:13
Chip647
Audiophile

Posts: 2631
Location: The South
Joined: December 24, 2012

"a SE driver capable of a few watts output.
IT coupled OPT would provide DC isolation between the driver and output stage.

I can't be the first person to do this, right?"

The reason this is uncommon is that usually only a 12AX7 is the "chunky" phase inverter and output tube grid driver for guitar amps. :-). This approach is tons cheaper than yours. No, you are not the first person to do this. TubeLab above sets his Strat to "Kill"

 

RE: AB2 Guitar Amp?, posted on July 24, 2016 at 07:27:34
Never DIY'ed an amp, as you described. For guitar use, I've found that a four 6L6GC amp fairly stable (for 85-100 watt range).

 

RE: AB2 Guitar Amp?, posted on July 24, 2016 at 16:14:35
hennfarm
Audiophile

Posts: 535
Location: Oregon
Joined: October 8, 2008
"Different than AB1 when it crunches"?
Way different. A great many pp guitar amps circuitry hovers around the fender bassman with the 100k/82k long tail pair. Which in my opinion what one hears when it crunches, its the LTP pooping out. Look up data sheets for 6bg6, 807, 1625? For AB2 data. JH

 

OK, that Killed it!, posted on July 24, 2016 at 20:51:58
Triode_Kingdom
Audiophile

Posts: 10011
Location: Central Texas
Joined: September 24, 2006
Great suggestion to use the 807 sheets as an example of AB2! I totally forgot how significantly AB2 reduces the plate-to-plate Z. That's the fatal flaw in this concept. Thanks hennfarm, better to realize this now before I start buying parts!






 

RE: OK, that Killed it!, posted on July 24, 2016 at 22:19:07
hennfarm
Audiophile

Posts: 535
Location: Oregon
Joined: October 8, 2008
100w for geetar is easy with a pair of 35w sweep tubes in AB1. Go with a full wave supply to the plates and screens get fed from the center tap. -75 to -100 vg1. Hell, pushing the grids positive through an interstage you could prob hit 200w peaks! It depends on whom the amp is built for. Guitar players are looking for a certain sound and to achieve it it seems required to stick pretty close to that ole bassmann circuit, a bouncy squishy compressing supply, a tone generator with just the right recovery time. Building a guitar amp that a player will like has proved more challenging than building a hifi amp with little to no sonic signature. My2

 

Last DIY gutiar amp, posted on July 25, 2016 at 08:21:38
Made a head unit of a Fender Bandmaster Reverb type amp, for a guy. 40-50 "clean" watts worked pretty well. He has 2 or 3 pedal boards loaded with various setups depending on whether he is in studio or doing gigs. What music is being covered, etc.

The nature of the big trannies and PSU makes the unit pretty tough and reliable. Suitable for a lot of various speaker cabs.

Also made a 4 X 12 cab. Two Weber 12A125 and two Celestion Vintage 30 (last of the British made). Nice bass with the midrange of the Webers mixed in.

Good for medium sized gigs (hundreds). Anything larger get miked and mixed to house systems.

 

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