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Old 15th January 2010, 10:41 PM   #1
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Default El Cheapo Grande

Eli mentioned the El Cheapo Grande in another thread I have up and I'm trying to find some more info on this amp; I can find tons on the original El Cheapo, but nothing on the Grande version. Can anyone shed some light on this design? Is it just the original with different tubes and beefier iron? Has anyone built one? I'd love to hear your thoughts if you have.
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Old 15th January 2010, 11:42 PM   #2
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Dude,

Your the 1st person to start an "El Cheapo Grande" thread, even though the idea has been bandied about more than once.

The 7591A is a nice tube that's as easy to drive as "12" W. types. Fisher and Scott took full advantage of that fact and used the "same" small signal circuitry with 7591As as they did with EL84s. What was done in the past can be done today. The 12AT7 based splitter/driver in EC will work with 7591s too. If a CDP is the signal source, a preamp with gain is not needed. Obviously, "iron" appropriate to a 30 WPC amp is necessary.

TungSols 7591 data sheet shows 400 V. on the plate, which means the B+ rail needs to be somewhat "taller" to compensate for O/P trafo primary losses. If "check valve" style decoupling is retained, "400" V. of B+ will be available to the 'T7s. The 'T7 sounds good with 200 to 220 V. on the plate and IB = 3 mA. Dividing 200 V. of drop in the load resistance by 0.003 A. yields a value of 66,667 Ω. Practical considerations say use 68 KOhms as the load and tweak the B+ rail voltage up enough to get the "magic" 200 V. on the plate. Only a minor adjustment of the 7591A operating conditions may be necessary, as 404 V. at the decoupling cap. is enough and O/P trafo losses may well result in a B+ rail that yields a bit more than minimum, at the decoupling capacitor.

Use 249 KOhm Vishay/Dale RN65 parts as the 7591 grid resistors, to make certain that the grid resistor value limit is not exceeded. Modern (EH) 7591As are intolerant of "liberties", in that value.

A Hammond 373BX power trafo will fill all needs. SS rectify the B+ using 2X 1200 PIV Schotky diodes or a "cockeyed bridge" and CLC filter the B+ with a 1st filter cap. just large enough to keep the rail voltage up. A small 1st cap. allows a few extra mA. of B+ to be safely drawn. A 10 H. Hammond 193M will compensate nicely for the small cap., in the 1st position. Pile energy storage up in the 2nd cap. "slot".

The 373BX's "5" VAC winding will be voltage multiplied to obtain the necessary B-. If B+ rectification is by "cockeyed bridge", the multiplier will be "tapped" to provide the 7591A bias rail (C-). If "classic" FWCT rectification is used, O/P tube bias can come from either the winding tap Hammond provides or tapping the B- multiplier. A "cockeyed bridge" is less costly to execute than FWCT is, when 2X expensive high PIV Schottkys are needed.
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Old 16th January 2010, 12:19 AM   #3
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Well I never thought I'd be the first person to do anything on this forum but I'm glad I could make some contribution!

Now, unfortunately most of what you wrote is Greek to me but I think with some careful study of the schematic I could probably get it. I really like Edcor iron so I'll probably look for something from them of similar spec.

Hopefully this thread will get some good action and we'll have some more people building this amp. It seems that the 7591 tubes can be had for a decent price making this a very nice project for those of us looking for something in the 30 WPC realm, but on a budget.

If its not asking too much, do you think you could drum up a schematic for the El Cheapo Grande? Maybe even work out a similar arrangement with Jim McShane for a parts package of the Grande version?
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Old 16th January 2010, 03:10 AM   #4
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I'd be more than happy to model the power supply in PSUDII and post the PSUD output and a PS schematic much like we just did in TubeMack's PP KT88 mono thread, since I could use a little more practice.

What volts and ma do you need out of each tap of the PS?
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Old 16th January 2010, 04:29 AM   #5
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Eli, this sounds interesting.
By "cockeyed bridge" I presume you mean half ss and half tube?

Gary
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Old 16th January 2010, 05:07 AM   #6
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Eli,
I thought I read in another thread that you wanted to do a Super El Cheapo with type 46 finals. What would that look like? Did you ever build it? Thanks for all your intelligent contributions.
JD
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Old 16th January 2010, 05:59 AM   #7
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Quote:
Originally Posted by rotaspec View Post
Eli, this sounds interesting.
By "cockeyed bridge" I presume you mean half ss and half tube?

Gary

Tube DIY Asylum: Revised "Cockeyed Bridge" Schematic by RioTubes
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Old 16th January 2010, 11:26 AM   #8
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Quote:
Originally Posted by jimmyd53 View Post
Eli,
I thought I read in another thread that you wanted to do a Super El Cheapo with type 46 finals. What would that look like? Did you ever build it? Thanks for all your intelligent contributions.
JD

No money, no build.

In matters of tubed circuitry, very little is new under the sun. It's more like selecting 1 from Column A and 2 from Column B, at a neighborhood Chinese resturant. All "El Cheapo" variants use the 12AT7 differential splitter/driver. The type 46 data sheet provides important info. The phase splitter trafo shown is step down. I see the drive voltage requirements of Class "B2" 46 "finals" as being within EC range. The remaining problem is coupling the splitter/driver to the 46s, without loss of gain. An IRFBC20 source follower DC coupled to the grids does that. Tubelab's Power Drive provides implementation details.
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Old 16th January 2010, 06:28 PM   #9
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hey-Hey!!!,
I'd expand things a bit further for the 'Grande' version. With 400V of B+ I'd go to 6L6GC with a 6k6 a-a load and 40% U-L taps. For the phase splitter try the 6H6Pi. Fixed bias from a FWB-rectified 25.2V trans from Radio Shack. It is ~$7 and can run to 450 mA.
The general topology is quite nice. Perhaps a variation with pentode LTP? E-Linear connection for FB instead of feeding the grounded grid of the diff amp/inverter.
cheers,
Douglas
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Old 18th January 2010, 12:50 AM   #10
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Well interesting subject. Based on the interest this weekend I transformed a couple of mono blocks in the closet to a kind of Grande El Cheapo. The driver is the 12AT7, 205v on the plates, 3ma per triode into a CCS. My B- supply is only about -60v so I cut the cathode R to 4700. The output tubes are 6L6 because I don't have any 7591's. The B+ for the output is about 415 with a tube rectified 5R4 supply. Output transformers are from a Fisher 500C receiver. The amps are running in as I type. Sound right out of the gate is okay, needs something more. By the way, I kept the 10k feedback resistor shown in the El Cheapo schematic I found. If someone has a better recommendation for this particular implementation, let me know. By the way, with the 6L6's in triode mode I'm getting right at 8 watts, maybe just a bit over.
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