more powerful class A guitar amp

[centervolume]

Question -

is class A power determined by the max output of a given output tube, even in parallel single ended?

If so, I'm thinking that puts a pretty low ceiling on class A power for tube guitar amps at least.

I built a parallel SE design that can hit just under 25 watts with 5881s and a SS rectifier plug.

Maybe properly fed 6550s could get that up around 30-35 watts? Is it possible to get to 40-45? poss with an 807? (even though that tube choice doesn't seem too practical for guitar amp design).

Maybe I'm way off or oblivious to some basic knowledge, but I'm loving the sound of my parallel SE build and looking for a way to get that tone closer to the 40 watt territory.

[pdf64]

No need to stop at 2 parallel tubes. Laney had a 50 watt SE model, before they trimmed their product range.
But class A can be push pull too; with the benefit that its OT can be a lot smaller.

[centervolume]

I found the Laney L50H. 5 X EL34 tubes apparently gets you to 50 watts well on the other side of anything that resembles efficiency.

So I guess I'm looking for a way to get close to 40 with just a couple tubes max... KT120 mwahaha

[DummyLoad]

the attached makes just over 40W. not cheap to build and will be HEAVY: power supply complexity and B+ level are not for novice builders. killer tone.

ditch the preamp and graft in your own. keep from the 6V6 onward.

not shown are a 100K 3W bleeder on the 370V G2 supply rail and a 330K 3W on the KT120 anode supply rail. 

--pete

[centervolume]

Pete
I think this may be what I am after. Let me study this a little bit. I'm working with a tweed Vibrolux chassis, so if I'm keeping the 6V6 then it's good to have the silicon rectifier setup. The 6V6 is working with VR2 as the driver?

The build I want to graft this onto comes in at 27 pounds with C10Q, weber 025130 PT and musical power supply OT 20 SE. I installed a toggle to go from standard SE to parallel. I'll also look to shift VR3 into neg feedback control... A recent draft of my schematic is below (I'm using 2nd triode of v2 as gain recovery after the tone stack). I see the 2nd triode of V2 on the wooly bully is not used, and then there's the 6V6... is the 9 pin ecc*** style tube insufficient to drive the KT 120s? The OT is only 30 watts what actual wattage? Also, Guessing the 2 hammond transformers, 2 chokes and a 40/50-watt speaker will account for the weight. Maybe 35 pounds total?

I have 4 or 5 builds under my belt so I'm still a little new. I built a 3x12 5F8 high power twin which is a charming appliance. Now Im looking at 40 watts as a sweet spot but in a deceptively small package like a tweed deluxe or poss even harvard/vibrolux cab. Will have to check for physical space clearances. Have not built with KT 120 before, could be interesting.

I'm interested to learn more about the particular challenges of the power supply. Is that 2 chokes on the power supply I see?

[1blueheron]

Centervolume,

Just curious what the application is, that you are striving for so much power in a little box. 

[jjasilli]

Question - is class A power determined by the max output of a given output tube, even in parallel single ended?
A class A amplifier is defined as one which is biased to a point where plate current in all the output devices flows for the entire 360 degrees of an input cycle, at the full, unclipped output of the amplifier.  Or, as stated in the RCA receiving tube manuals:  "The classification depends primarily upon the fraction of input cycle during which plate current is expected to flow under rated full-load conditions".  The key phrase being "under rated full-load conditions", which is a requirement for amplifier classifications to be meaningful..   This is typically done by biasing the output stage halfway between cutoff and saturation, with the plate load impedance to an appropriate value that gives maximum undistorted output power.  This is the least efficient method of amplification, because the output devices are dissipating maximum power with no input signal. . . For audio amplification, a class A amplifier can be either single-ended or push-pull. https://www.aikenamps.com/index.php/the-last-word-on-class-a


Biasing for Class A requires a relatively low B+ voltage, so that the tube(s) do(es) not draw too much current.  In AB operation, at least one pair of tubes with inverted signal phase are required.  A high B+ voltage can be used.  Ea power tube draws "too much current", but that's OK because it has a cooling-off period for 1/2 the cycle.  So, AB operation enables the same tubes to operate at a higher voltage & to draw more current, compared to Class A.  Power (Watts) = Voltage X Current.  Hence, Class AB operation allows for much more Watts than Class A. 

[centervolume]

Centervolume,

Just curious what the application is, that you are striving for so much power in a little box.

Blue Heron - I've been getting more and more satisfaction from building and have just started to transition from replicating schematics to innovating a little. In terms of application, I got into amps as a harmonica player (barf bags available at the exit). I ended up going through about 75 vintage amps over a couple of decades and through that got a more and more specific sense of what I either thought would work or was just plain curious about (insert 3x12 high powered tweed twin build here). Some of my builds have made it into hands of pro players and this particular one is in response to a conversation with one particular player. Overall, I have noticed that players seem to be moving away from larger, heavier, bulkier amps. So that's sort of where my mind has been on this. 25-40 watts with toggle between single and parallel single ended. I admit I like the idea of walking on stage with a cute little thing that then ends up peeling wall paper as needed.

Also jjasilli thanks so much for posting the definition, very helpful

[1blueheron]

Centervolume,

I'm still a little bit at a loss here, as it seems if you want a light, cute little box that peels wall paper, going SE is the opposite direction of your stated goals.  What do you have against PP amps and why are you trying to go SE "class A" vs PP class A or even PP AB?    If its just to be different I get it, but I am having trouble understanding why one would intentionally choose to hobble the horse before it even get out of the gate.  A phase inverter tube is certainly a light lighter than the extra iron and power required for the SE to match the output of a PP.  What am I missing?

[DummyLoad]

SE sounds better.


--pete

[shooter]

cute little box that peels wall paper

they are a myth, enhanced by digital junk and too many switch's n knobs.

build DL's SE, hire a roadie 

[centervolume]

Centervolume,

I'm still a little bit at a loss here, as it seems if you want a light, cute little box that peels wall paper, going SE is the opposite direction of your stated goals.  What do you have against PP amps and why are you trying to go SE "class A" vs PP class A or even PP AB?    If its just to be different I get it, but I am having trouble understanding why one would intentionally choose to hobble the horse before it even get out of the gate.  A phase inverter tube is certainly a light lighter than the extra iron and power required for the SE to match the output of a PP.  What am I missing?

That’s where the harmonica part comes in. It’s 100% a tonal considèration. I am getting better results for this instrument in particular. My basic approach has been to dial in the tone first to the best of my ability and now I want to see if I can preserve that and notch the power up. I’m sure there’s likely better ways to go about it but I like how building is a personal style kind of thing and if I make some dumb mistakes along the way those are mine as well and I think they’ll help me in the long run.

[1blueheron]

OK, I am starting to learn/understand now.  You want SE because you feel it has better tone characteristics.  I can go with that, subjective as it may be.

I am a novice and admittedly way out of my league here but I like to brainstorm with people like you who want to push the envelope and do unusual things.  I find I learn the most when there is smoke in the room 

If what I have read on the interwebs is accurate....  SE is supposed to sound better due to the even order harmonic distortion it produces in the output stage.  So if even order harmonic distortion is your Holy Grail in tone, and SE amps are somewhat self limiting in output power when weight and size is a factor (you could build something like this 250W SE amp if weight isn't a factor http://tubelab.com/prototypes/833-se/  ) then we might need to think outside the box.

Traditionally, I would say this is where a mic outside the small box and house amplification comes in, but I assume you are ruling that out for one reason or another. 

The next option would be to figure out a way to amplify the tone of the SE amp without a house system.  What if rather than using bigger output tubes, you went the other direction and used smaller output tubes that would give you your even order SE harmonic distortion at very low output levels and then ran that into a very low distortion solid state final stage within the amp?   One of the organs I recently disassembled was a reed organ. (think giant harmonica)  It used a small mic in an acoustically dampened enclosure to amplify the reeds through a SE tube amp.  Perhaps this same principle cold be applied to mic the SE amp inside the box, capture its magical tone, and then amplify that signal using SS or a PP tube amp. 

I guess this is where they round up a posse and burn me at the stake with hot tubes for blasphemy....

[1blueheron]

Just had another idea.  What if you built a 2X12 combo with dual 30W SE amps and 2 speakers contained in it.  Could be run as dual mono for 60W output or stereo 30Wper ch.   Both amps could run off a single PT to save space and weight but use dual rectifiers and a pair of OPT's.  With a pair of very efficient speakers you would likely have a true wall paper peeler that one man could still carry.

[shooter]

Just had another idea.

getting warmer; 

more than one here use single KT88's mono running about 15-20W voiced to taste, small venue, bring 1 n a cab, bigger, bring 2 and 2

[kagliostro]

Gu50

is a cheap military russian tube developed starting from the military german LS50 (a very famous tube)

(russian got LS50 projects when they arrived in Germany during WWII)

https://tubes-store.com/product_info.php?products_id=82

the sochets

Aircraft
https://tubes-store.com/product_info.php?products_id=317

Tank
https://tubes-store.com/product_info.php?products_id=530

China cheap socket (in China they developed the FU50 that is the same tube, but russian tube are claimed to be better)
https://valvole-audio.com/product_info.php?cPath=27_28&products_id=214

The tube was used on aircrafts and tank, so it is a very very strong tube

(consider it a ruggerized KT88)

can give 18W in SE with 300V B+ -- 250V on G2 -- 2K Load

a pair in PSE and you have 36W (a pair in PP with 800V B+ will give 120W)

There is a german civil version of this tube, the EL152, but it isn't so easy to find and cheap like the military russian version (many people say that russian improved the original LS50 german tube)



Franco

[centervolume]

very cool - thanks for all the input. I like learning about new tubes and thinking outside of the box for sure. I'll reflect on all this input and make some physical measurements to see what works :)

[Willabe]

SE sounds better.

But does SE have as much bloom/sag/sustain as PP?

[Willabe]

What if you built a 2X12 combo with dual 30W SE amps and 2 speakers contained in it.  Could be run as dual mono for 60W output or stereo 30Wper ch. Both amps could run off a single PT to save space and weight, ......  that one man could still carry.

The iron, PT/OT/choke for 2x 30w SE amps, even using a single PT for both, would still be very large/heavy and cost big $$.

The PT would still have to supply all the current needed for the B+ feeding the output tubes. SE 30w will suck a lot of current, so more copper winding and more/bigger iron core lams.

This is why you don't see higher wattage SE guitar amps, cost and weight.