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 Post subject: King size homebrew amp with pix. What output tubes?
PostPosted: Oct Sat 31, 2009 8:01 am 
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I lucked into this homebrew oldy recently. Nice workmanship on the part of the builder. I do not know what output tubes were used.


The preamp and phase inverter are EF-86 and 6CG7. Since the 6CG7 is a 9 pin version of the 6SN7GTB according to my RCA tube manual, the front end appears to be cloned from an Eico HF-35 Cathode resistor for self bias is 250 ohm. Has a 2.25 Hy Merit choke rated at 250 mA. The power trans is a big Magnavox that looks like it could handle 300 mA all day. Filtering is CLC so it is not choke input. The two large caps are not electrolytic, but the two smaller ones are. All of the filter caps are rated at 600 VDC. Was surprised by that as I don't recall ever seeing electrolytics rated that high before.



The output transformer is what attracted me to this piece. It's an Acrosound TO-350 which is their biggest as listed in my Acrosound catalog. It was designed for ultralinear connection even if the screen grids were to be run at a lower voltage. The example shown in the Acrosound catalog is a 100 watt push-pull amp using 6146 tubes. "Hi-Fi sound for an entire stadium".

Obviously the builder did not use 6146 tubes. He tied the separate windings via that barrier strip such that it is connected as in a normal ultra-linear.

The impedance match is 6600 ohms, the same as their smaller TO-300 for which KT-66, 807, 5881 and 6L6 are listed as possible outputs in the catalog. I suppose a pair of 6L6GC would be right at home and provide lots of power especially if I change the bias to fixed. Your ideas? Possibly EL-34 as in the the rest of the Eico HF-35 circuitry?

Also the power transformer at no load starts drawing power as in saturation at about 105 volts and takes over a third of an amp at 120. Is that a normal draw for hysteresis and eddy or whatever? I found no obvious faults in the power transformer.

Thanks for info,

-- Rich


Last edited by tubeR on Nov Sat 07, 2009 6:29 pm, edited 1 time in total.

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 Post subject:
PostPosted: Oct Sat 31, 2009 3:13 pm 
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A little off topic but you would be amazed how much you can sell that Acrosound audio transformer for.

Sal

PS: Please edit your photos so it fits on the page without scrolling over.

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 Post subject:
PostPosted: Oct Sat 31, 2009 3:47 pm 
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1/3A isn't really too extreme - it'll probably draw 3A at full tilt...

What is the supply voltage? 6550 seems most likely, but 6600 Ohms could be used at lower voltages with EL34, 6L6. Pins 1 & 8 will be jumpered together if it was EL34s


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 Post subject:
PostPosted: Oct Sat 31, 2009 6:44 pm 
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With no tubes .3 or .4 A draw should be ok. Nice find.


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 Post subject:
PostPosted: Oct Sat 31, 2009 7:42 pm 
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Tom Bavis wrote:
What is the supply voltage? ......... Pins 1 & 8 will be jumpered together if it was EL34s


You nailed it Tom. Pins 1 and 8 are indeed tied together. So therefore this is likely an Eico HF-35 clone with some variations. The TO-350 seems a bit of overkill in this circuit. However, if it can handle 100 watts, up to 175 mA per tube and within 1 dB from 7 Hz to 70KHz, 35 watts should be a breeze and even better quality.

A no load test of the power transformer showed only 385 VAC per side, so voltage is not extreme. At the same swap, picked up a Peerless Altec-Lansing R-800-A power transformer that is rated at 300 mA at 400-0-400. Unloaded, it did not draw current until it hit 120 VAC input. That gives me an option if I want to change power transformer. The nice thing about a homebrew is being able to modify it without feeling like you've destroyed an antique.

Sorry about the image size Sal. Still getting the hang of taming Picasa to be a source for my ARF pics. And yes, am somewhat aware of audiophile demand for Acrosound.

--- Rich


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 Post subject:
PostPosted: Nov Sun 01, 2009 6:06 am 
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Rich,
No problem. BTW I use irfanview, great free graphics program to resize, modify, crop etc.

Regards,
Sal

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 Post subject:
PostPosted: Nov Sun 01, 2009 4:02 pm 
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Interesting amp, and a nice find.

As Sal points out, the TO-350 is a very unique and desirable transformer. It has separate tertiary/screen windings, and was ideal with the 6146 which isn't a good tube for U/L use. The published schematic in the ACRO catalog used the somewhat massive TP-550 power transformer. I have one of those and finally used it in another project as I never thought I'd locate a pair of TO-350's.

Dynaco had an equivalent transformer - the A-440/441, although I've never seen a published Dynaco schematic for this transformer. A copy of the Dynaco transformer specs (as well as the Acro catalog) are at:

http://www.the-planet.org/data.html

If it were mine, I'd be very tempted to rebuild the original Acro 6146 schematic.


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 Post subject:
PostPosted: Nov Sat 07, 2009 6:40 pm 
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Have repaired the amp. Sounds great :D

Here is more info as my "latest on the bench".

http://oak.cats.ohiou.edu/~postr/bapix/MullrdHB.htm

--- Rich


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 Post subject:
PostPosted: Nov Sat 07, 2009 10:09 pm 
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Congratulations, tubeR. Keep up the good work!

Best regards, :)

Larry

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