Jcm800 Downscale

[soundguruman]

If you carefully analyze pretty much "all" popular tube amps....
(especially Mesa)
You will see that the ratings of the tubes are constantly exceeded, deliberately.

If you run the tubes in the recommended perimeters, the amp sounds pretty bland.

What we are usually trying for is to make the amp scream, by torturing the tubes. That's normal.

If you ran the tubes as "recommended" they probably last 20 years...

[exclamationmark]

Couldn't an inductor with 40v make nearly 80v ac and be a way to cheat?

I'm fairly sure when plotting the load line of anything with an output transformer (the output transformer IS the inductor!) you can potentially set the end of the load line at double your B+ supply. I think some pass regulator/sweep tubes might sound cool at low voltages.

[printer2]

If you carefully analyze pretty much "all" popular tube amps....
(especially Mesa)
You will see that the ratings of the tubes are constantly exceeded, deliberately.

If you run the tubes in the recommended perimeters, the amp sounds pretty bland.

What we are usually trying for is to make the amp scream, by torturing the tubes. That's normal.

If you ran the tubes as "recommended" they probably last 20 years...

Yeah but you can only pass so much current. I would think there is a limit to mow many electrons boil off with a given heater current. At low voltages it is hard to get close to the plate dissipation of the tube, much easier at high voltages where we limit the current by the load impedance.


Nothing wrong with the OP to reexamine the scope of the project. It may take more in time than is prudent to invest in (Mine was challenging as outlined and then to throw a monkey wrench into things the software I was given by my instructor would not do what we thought and I had to write my own, a project in itself). As far as us spinning our wheels here I am game to give it a go. I plan on doing a P-P amp with the 6AU6 in all the preamp positions and it is looking like sweep tubes for power. Might have to wait until the snow flies, I have much to do on the house before then and I have a couple of amp projects to complete first (and possibly a guitar). Even a couple of watts would be enough, a bedroom amp would be just fine.

[Austin]

PS: I refuse to consider these for class AB2 designs unless plate curves are supplied for grid voltages higher than "0". Sorry.

Before throwing datasheets on the table, please check yourself how much current they can handle between 40 and 10 V DC.
Remembering that *max* current must be achieved at *min* voltage, because the remainder must be transmitted to a load.

Here is a possible candidate, dirt cheap, has same pinout as 6L6 and lists positive grid curves for a2 and low voltage operation: 6W6

http://www.mif.pg.gda.pl/homepages/frank/sheets/093/6/6W6GT.pdf and the 6y6 is also a possible candidate with same pinout, cheap price, low volts however no positive grid curves which may be a good thing if using tube drivers and cap coupling

http://www.mif.pg.gda.pl/homepages/f...93/6/6Y6GT.pdf

Not really sure if either of these would outperform the 6080 but they might be easier to actually use because of the lesser drive reqs and heater, they would probably also sound better for guitar since they are pentodes.. Looks like about 50 ma possible with 40v screens and neg grid bias, a2 would be the way to go still for any power. But I have learned alot having thought about this so much the last couple days, thanks to you all

[printer2]

Here is a possible candidate, dirt cheap, has same pinout as 6L6 and lists positive grid curves for a2 and low voltage operation: 6W6

http://www.mif.pg.gda.pl/homepages/frank/sheets/093/6/6W6GT.pdf and the 6y6 is also a possible candidate with same pinout, cheap price, low volts however no positive grid curves which may be a good thing if using tube drivers and cap coupling

http://www.mif.pg.gda.pl/homepages/f...93/6/6Y6GT.pdf

Not really sure if either of these would outperform the 6080 but they might be easier to actually use because of the lesser drive reqs and heater, they would probably also sound better for guitar since they are pentodes.. Looks like about 50 ma possible with 40v screens and neg grid bias, a2 would be the way to go still for any power. But I have learned alot having thought about this so much the last couple days, thanks to you all

Have some 6W6's, might give them a try after getting the current amp off the bench.

[Austin]

Have some 6W6's, might give them a try after getting the current amp off the bench.

not sure if this is useful but 6w6 and 6y6 in the THD Univalve amp I have are listed among the acceptable tubes, I have both and the 6w6 sounds VERY live on the low volt setting, and is pretty much glowing a dull redish on the plate. It is the "go to" for heavy metal tone... I like it as it gives the Jimi feedback (think "Foxy Lady" intro) without being too loud or too close to the speaker. The 6y6 also is also at the verge of glowing plates but has a distinctly different tone, much fatter high cholesterol kind of sound, a bit like a vintage Tung-Sol 5881 on high volts (which is also on the verge of red plating) and very good for the Albert King kind of tone.. It kinda seems that when you really push a tube they give the sweetest tone. The 6w6 is just a crazy tube in the Uni, it sounds like no other and I keep coming back to it even though I am not a metal player because it sounds so very sensitive.

[jazbo8]

Seems like this thread is not going anywhere... Perhaps the OP can contact yosimitesam144 for some scope shots if he still wants to continue with the project. There is one shot on the following thread:

http://music-electronics-forum.com/t20403/

Jaz

[J M Fahey]

Yes, this post deviated into a "Find the weirdest tube" contest.
Funny, sort of, but absolutely useless to the OP who, let's remind it, wants to build a "JCM800 preamp" clone.
The added limitation of never going over 40V does not help much either, sort of trying to swim with a V8 engine block hanging from the swimmer's neck.
Not that it's *impossible*, but certainly easy it ain't.

[Chuck H]

Indeed, Austins proclivity for "what if?" or "technically, this wierd thing is possible." hasn't really advanced the OP's goal much. If Austin were to try even half of the circuits he mentally masturbates about he'd have no time to type!

[printer2]

Yes we did go off on a 'what if' route but we already concluded that there is little chance of just subbing in tubes into the stock circuit run at 40V and have it sound remotely like the original amp. Heck, more chance of getting something Marshally sounding if the OP subbed in Fets in the tube positions running at the given voltage limit. Would not be a bad idea as models for his sims are readily available. And no, I am not going to visit Fets for now even though I want to try them out also, I have to catch up on my building.

[Chuck H]

Thanks for having a sense of humor and rolling with it Austin.

[Austin]

Thanks for having a sense of humor and rolling with it Austin.

Thanks to you guys too for keeping me on my toes ..
I have wanted to build a low voltage tube amp for a while now, and is probably why I have been so enthusiastic about it.. I have all the stuff to try it already, it is why I bought those big tubes to begin with. I can think of some ways that might work, hopefully I can make some time to breadboard something up here in a few days and report back. One problem I see for all tube is since it will have to be AB2 to get any power, possibly the easiest way is to use a phase splitter interstage transformer for a power tube driver but I don't have one(yet). The IC for a driver would be another easy way, even though it feels kinda like cheating..

[Enzo]

SOmehow I am reminded of the old gag where a couple people stand on a busy sidewalk looking up and pointing, soon other people stop and look up too, trying to see whatever it is those first two guys must be looking at. People start talking to one another, someone decides they must be looking at X, and on it goes. Meanwhile the first to guys have stepped away just watching to see the crowd mentality in operation as they all strain to see.... nothing.

100 plus posts, and I have to admit I still am not sure what the poor guy wanted to build.

Did he want to design a real 2204 and build it, but cannot use more than 40v, assuming it might run on real voltage once done? Or does he want to run the 2204 circuit on 40v? Or does he want to emulate the 2204 with a circuit using 40v? etc etc etc. There is a difference between looking for low voltage tubes and running real tubes at low voltage.

[Alan0354]

This might be relevant: Fet version of the JCM800

Best effort I've seen in the public domain, but it needs 180V. I don't think life would be worth living if I could only use voltages below 40V.

Come to think of it, even a 40kV limit would be harsh.

Last year, I designed a clean booster using diodes at the emitter of the BJT to increase the 2nd harmonic like this schematic. But the result is not impressive, it give a little more of the bell sound, but not the most natural. I decided to let it go.

[jazbo8]

Since this thread is already getting to the top 10 in terms of replies and with more than a thousand views, I guess there must be strong interest in the JCM800 from many on this forum. It is just unfortunate that nothing came out of it, because the prescribed conditions were simply ridiculous. May be we can move the thread towards a more productive direction by re-setting the design conditions, since I did not google it, perhaps such threads already exist on this forum or other forums, anyway, it might be worth a try given the current "popularity" of this thread.

Jaz