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Old 1st June 2008, 12:29 AM   #1
diyAudio Member
 
Join Date: Dec 2003
Location: Missouri
Default I wanna build a CHEAP mono tube amp, need advice....

Well, me being the broke, and incredibly cheap, “student” that I am......here's what I've been thinking of....

I want a mono PP tube amp for the horn section(active crossover system) of my "mono rig" which can be referenced in other threads of mine linked here

Seperating Preamp from Amp in Integrated Amps

Looking for schematic/info for a Knight KN 3050 PA Amp

Right now the horn is crossed over at around 1200hz(would like to do 800hz, but those modules for the Peavey xover seem to be getting kinda pricey on fleabay...might have to look in modding the “cans”(I have more than a few) if possible, or even just ditching the Peavey and building my own tube crossover.....but that’s ANOTHER thread). The horn is currently being amplified by half of a Crown D-150(yeah I can hear the screeching and gasps from here....that’s what I’m here for...the D-150 will be for subwoofer duties *only*, after I build the "horn amp" obviously).

Now comes the fun part…what I have been “thinking”.....here’s what I have in my "Parts bin" that might work……

I have a knight KN530 (mono EL34PP, good OP trans.....I think....probably should test that), but no power transformer. Finding a *cheap* direct replacement PT has yielded no results. From the spec sheet I “need” around a 350 swing.....which are relatively available on ebay......but there is another cheaper option that I have been considering......

I also have a Thomas Model A-1 “tone control” unit, that is an EL84 based mono unit also PP I think(don’t have a schematic….yet). Pictured here
Click the image to open in full size.
And here
Click the image to open in full size.
I haven’t taken any readings on the power trans for specs, but would assume that it has a typical EL84 swing of around 250v to 300v. The unit DOESN’T have filament power though, so I suppose that I would have to use a stand alone low power trans to supplement. I have a old "tube tv tuner" that might have one to "donate" to the cause.....

I was thinking that I could pull the power trans from the Thomas and use the output trans from the Knight and put together an EL34 mono PP amp(I have more than a few EL34s from my Dynaco ST-70 days) with a “decent” amount of power for a horn, even if not using the EL34s to their "full potential". I’m not going for ear bleeding noise, just good sound quality…..with an occasional cranked up song or rif…..

I still haven’t decided on a driver stage, though I do have a ton of 12au7 tubes.

I was also thinking of using the knight schematic as a starting point, as designing my own amp is still a bit “out of my league”. I realize that with a different PT, many of the component values will be different, but I feel fairly confident that I can “compensate”.


So….what should I do……stick with my above “ideal” plan and use the EL34s(much preferred), or step down to the EL84s, and basically just use the guts of the Thomas.

Should I copy the knight topography, or is their a better option?

Additionally, I am going to custom build my own chassis(actually two…one for the power, one for the output) so that it will fit behind(around...two triangle bases) the horn itself, as I am running out of room on my “platform”. I have a basic layout drawn up and all the components mentioned should fit just fine.

Any and all ideas or suggestions would be greatly appreciated…..

Thanks for your time…..

Nate
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Old 1st June 2008, 01:54 AM   #2
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Join Date: Dec 2003
Location: Missouri
Default took some basic measurements

So after feeling a little lazy, I got motivated and tore into the A-1. Clipped all the stuff going to the power trans(including cap), set the multimeter to AC, hooked up power and got this

(2)Blue wires - 56vac
(2)Green wires - 7.68vac
(2)Red wires - 633vac....(woohoo)
(1)Green/yellow wire that was attached to chassis......didnt measure that at all.......not sure exactly what it does(center tap of some sort maybe???).

So yeah....no amperage readings, but I was apparently wrong about not having any filament power. Its been a few years since I got the unit.....I must of been thinking of another "project"......

anyway here is a pic of the carnage......

Click the image to open in full size.

Obviously I was testing the cloth cover wires......

so what is the best way to get amperage specs on a tranformer such as this?

Thanks again.....

Nate
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Old 1st June 2008, 03:14 AM   #3
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Location: Monroe Township, NJ
Nate,

Use the Knight O/P trafo in a Mullard style circuit. A 6AB4 voltage amplifier and a 6J6 LTP splitter/driver will yield plenty of gain. An AnTek AN-4T360 power trafo will provide all you need. SS rectify the B+. Voltage multiply 1 of the paired 5 A./6.3 VAC windings to obtain the negative bias rail. The 2nd filament winding has current to spare for the 4 signal tubes.

If the Knight O/P trafo lacks UL taps, Maida Regulate g2 B+.
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Old 1st June 2008, 04:40 AM   #4
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Eli,

I really do appreciate the help and all the info, but I really wasnt kidding when I said I was cheap(I ain't broke.....but I'm badly bent....lol).

Ive got a long sad story(bad back....no work).....not to mention a wedding(that we're paying for ourselves) coming up, so seriously; I'm trying to do this project with parts on hand, or MAYBE a $20(shipped) ebay transformer.

Granted, the transformer you suggested is only $50, by the time you add shipping.....its just not in the budget.

Thanks again though....I'll use what info of yours I can.....

Nate
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Old 1st June 2008, 10:56 AM   #5
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Nate,

Insufficient coin of the realm bedevils us throughout life. Time for "Plan B".

You have mail! Build a 1 channel "El Cheapo" variant on the Thomas chassis. Contact Jim McShane, to obtain the EL84/6BQ5 cathode bias network values. Get the additional power "iron" from Aliied Electronics. $12.41 buys the B- trafo and $7.74 buys a Triad C-24X filter choke. SS rectify the B+, as you need the Volts and UF4007s cost about 15 cents each.
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Old 1st June 2008, 12:03 PM   #6
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Default Re: took some basic measurements

Quote:
Originally posted by camusmuse
So after feeling a little lazy, I got motivated and tore into the A-1. Clipped all the stuff going to the power trans(including cap), set the multimeter to AC, hooked up power and got this

(2)Blue wires - 56vac
(2)Green wires - 7.68vac
(2)Red wires - 633vac....(woohoo)
(1)Green/yellow wire that was attached to chassis......didnt measure that at all.......not sure exactly what it does(center tap of some sort maybe???).

so what is the best way to get amperage specs on a tranformer such as this?
So, what's wrong with using this guy as your power transformer? If the only hangup is not knowing the current limits, you might be able to guess by looking at what it used to drive. I've also found this article to be extremely helpful:

http://www.diytube.com/phpBB2/viewto...t=1465&start=7
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Old 2nd June 2008, 03:28 PM   #7
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thanks again for the schematics eli.....i definitely think I'll give that one a spin.....

though I still may try to make my original plan work too......I just ornery like that.......

time to start making up a parts list I suppose.......
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Old 3rd June 2008, 02:07 AM   #8
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Join Date: Dec 2003
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Eli.

Yeah......I think im pretty much sold on El Cheapo.....

I found the "bargain basement" thread on another forum....got to page six, then decided to skip to the end to make sure it was still alive.....I WILL read the whole thing though.......pretty fascinating, and very encouraging too.....great stuff.

Whats the best way of "getting a hold of" Mr. McShane?

Is the ElCheapo "bridgable"? It would be nice to be able to make a "real one", or at least have the extra power...if just for the "head room".

When you suggest that I use the Thomas chassis, do you mean use the output trans as well, or do I need the specific one that the amp was designed for. FWIW, I am actually going to custom build a chassis for the amp anyway. But I do plan on reusing the sockets and anything else I can.....which brings me to the next question....

Are there still "kits" available? If so from whom?

Thanks again.......

Nate
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Old 3rd June 2008, 02:54 AM   #9
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Join Date: Apr 2004
Location: Monroe Township, NJ
Nate,

Jim McShane's web site is here. AFAIK, Jim still has small parts kits for "El Cheapo".

For a mono amp, recycle the power and O/P trafos on the Thomas chassis. If the Thomas O/P trafo has UL taps, use 'em and get an honest 12 W. "El Cheapo" works well with mediocre O/P "iron", but better stuff, like DynaClone Z565s, is taken advantage of.

Bridging tubed power amps can be problematic. You have to connect 16 Ohm speakers to the 8 Ohm taps and 8 Ohm speakers to the 4 Ohm taps. Unless the O/P trafos are most unusual, 4 Ohm speakers are out, when bridged mode is employed.
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Old 3rd June 2008, 03:19 AM   #10
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I was thinking more of "bridging" before the output trans, kinda like "stacking" the tubes(not sure if that is the correct way to describe it. or if it is even possible with the elcheapo design). The output trans that is on the thomas is pretty small, so I was considering using the output trans from the knight, as it was used with a pp el34 setup, which from what I understand is very similar to the dynaclones(or at least the original dynacos.

I use to have two st-70s that were each run in mono.......boy I miss them....lol. They were running a pair of basically stock Infinity Quantum 5s which surely ran lower than 4 ohms at times....the dynacos never seemed to mind too much......

Thanks again for your time....

Nate
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