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The Audiophool Page

This page will have my audio related projects and information on my audio system(s)

Here's a stereo Amplifier I built for computer audio - about 2 Watts per channel from a pair of 6AQ5s - could be more, but these tubes are a bit weak.  Since the power transformer runs a bit hot, I'll need to rebias it if I put in stronger ones - but it sounds great the way it is. Schematic   I have the STANDBY switch since the B+ voltage is a bit high until the tubes warm up.

Infrared photo - that power transformer gets a little warm, though nothing compared to the output tubes.

5/30/05 update: After a few years of running, the tubes all measured weak (though it still sounded fine at the low power needed for computer speakers).  The 6AQ5s measured about 60% and 30% emission - so I replaced the output tubes with a pair of these:

(Appropriate, since this sits on top of my IBM computer at work...)

A later amp took care of two problems with the previous one - the transformer is a bit larger, so it runs cool, and it's a toroid, so there's no 60 Hz field to make my monitor jitter.  I made a circuit board for this one, and when Poptronics gets around to publishing my article, I'll make these available for anyone who wants to duplicate it.  But if you can't wait, here's the schematic:  6BQ5 Amp.  (1/12/03) OK, I guess I can stop waiting for Poptronics to publish it, since they have stopped magazine publishing entiely...

Bottom side shows there wasn't much to wiring it:

(but see the rework near the center of the PCB?  Oh well, we'll fix it in revision B...)

Below is a Heathkit A-7E Amplifier I found at a flea market.  The blonde finish plywood cabinet is just  SO 1950s!  The can electrolytic was warm after a short period of operation and there was +5V on one of the output tube grids - time for some new capacitors!  At least the transformers are OK.  This uses a strange set of tubes - 12SL7 phono preamp, 12SQ7 tone control driver, 12SN7 preamp and phase inverter, and 12A6 outputs for seven big monophonic watts.  Even stranger - I had spares for all of 'em!  Schematics for most of the versions:    A-7B   A7-C   A7-D  A-7E

Here's the chassis:

I sold the Heathkit mono amp via eBay, and then, of course, another one turned up.  This one has 12V6 output tubes, but is otherwise the same.  Most these you see will have one of the transformers replaced, as there's no line fuse.

1955 and 1956 ads:

I found this Fisher 800C on the AudioKarma message board.  The seller (also the AK webmaster, Tim Polniak) lives in Buffalo, so we met halfway in Batavia.

It needs an AM antenna,  power switch, which will be be a challenge to find, and a 6HR6 tube, which I have here...someplace...

Found the schematic in Sam's Index, and bought a used copy from Michelle Troutman.

Schematic is copyright Sams Publications, reproduced by permission.  Page 1   23 4

2/17/03 update:  IT LIVES!  I stuck a 6BA6 in the empty tube socket and replaced an open resistor in the power supply and got AM and FM reception - two output tubes are a bit weak, so I'll probably get some new Russian 7591s eventually (sold all my 7591s on eBay before this showed up, dammit!).  I have made a few minor modifications so far, adding an inrush limiter (Keystone CL-70) and a grounded power cord, and I added 10 Ohm / 1/4W resistors in each output tube cathode as a way to check current (they act as fuses, too...).  It sounds real nice with a CD player.

This little Bogen E-8 PA amp from the 40's will be my workbench amp.  Needs caps... d'ya think?  Found it at the AWA spring swap meet.  Uses an odd set of tubes - push-pull 6F6 and 6AD7.  Schematic

My next DIY project will be a stereo 6V6 amp.  Built a guitar amp using 6V6s about 35 years ago, and want to hear them again.  I'll use a couple of output transformers from an old console which have a extra feedback winding.  Here's a preliminary schematic.  The combination of cathode and adjustable bias is to accomodate the feedback winding and to let me use the tubes I have.  May have an AC balance control too, before I'm done.  I'll leave room for better transformers, but that may wait for the next amp... I have a BUNCH of cool looking 6BG6Gs that I'm itching to put to use...

6/12/03  And wouldn't you know it... I found ANOTHER receiver at the hamfest before I finished fixing up the Fisher... this one is a Lafayette LR-800.  The 6BQ5s in this don't belong - can't be happy at 460V... I will have to look for some 7189s.  I did get a schematic for it... page 1   page 2

1/3/04 I put in some Russian 7189s and cleaned the controls - it sounds pretty good, but I think it will need some new caps - and there are a bunch...
 
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