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Pair of Fisher CA-40 Oddballs

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I seem to be a magnet for attracting strange classic amps...consider this week`s offerings

A pair of Fisher CA-40 ``Master Control-Amplifiers`circa 1958 Mono PP

Tube complement:

4XEL84 (output)
2X6BW4 (Rectifiers)
1x12AU7 Cathode Follower
1X12AX7 AF Amplifier
1X12AX7 AF Amp-Phase Inverter
1x12AX7 Preamplifier

Questions:

Anyone familiar with these puppies...any observations both subjective and objective.

I haven`t come across dual rectifiers before...what was the reason for this

I brought these up slowly on my variac over the space of a couple of hours and everything is holding...all caps resistors electrolytics are original and will obviously need replacing but any suggestions re: the schematic where circuit improvement could be made...or put another way do any significant flaws in its topology jump out at anyone.

(Sorry my keyboard won`t let me type question marks tonight...might be telling me something;))


These strange puppies also sport a feature called the `TONESCOPE`` and I quote: This unique device indicates in graphic form the settings of the BASS and TREBLE Controls. You can see at a glance how the Tone Controls effect the output of the Master Control Amplifier. These Controls operate in the usual manner: clockwise rotation produces boost: counterclockwise rotation results in attenuation.As you adjust the Bass and Treble Controls a white line will move on the ToneScope panel. When this line is horizontal, the Tone Controls are in mid-position and produce a flat ouput response. When Bass or Treble tones are boosted the line will turn upwards...(and visa versa, of course) Although the ToneScope graph is not calibrated in db, it is a simple matter to approximate the decibel values... Pretty sophisticated tone-control technology is it not:rolleyes:

Anyways some pics and scans for your helpful input, amusement and-or interest,


Leon
 

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There are some nice things in the schematic. For instance, buffering the tone controls with a CF.

The 6BW4 is a good tube limited to 100 mA. A 6CA4/EZ81 is good for 150 mA., which is not enough to support 4X 6BQ5s plus small signal tubes. A pair of 6BW4s was an economical way to get the current handling capability needed. Avery was known to pinch pennies.

The 12AX7 triode makes a poor "concertina" phase splitter, given its low gm and high RP. The only reason I can come up for Fisher's repeated use of the setup is "bean counting". Fittingly, an early Fisher stereo amp provides the fix. Use a 12DW7/6247 as the power section voltage amplifier/phase splitter. I've uploaded the necessary schematic fragment.

The source selection capability was fine in 1956. It needs to be changed, given current realities. The same issue was dealt with in this recent thread. You would select a non-shorting Lorlin model from Mouser with 2-12 selectable positions. Obviously, 1 new rotary switch goes in each monoblock.

Fisher phono sections have good mid-range and wretched bass extension. Unlike the other fellow, you definitely have what it takes to switch to a passively equalized, tweaked, RCA setup. Install a DC heater supply for the phono preamp or, at the very least, use a Sovtek 12AX7LPS. The 'LPS is a genuine 7025 equivalent that contains a spiral wound, hum bucking, heater.
 

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Thanks Eli...boy Avery must have been cheap...was it really less expensive to slap in two 6BW4's instead of say one 5Y3/5AR4 or something along those lines?

I expected your comments regarding the use of the ubiquitous 12AX7 as a phase splitter in this case (and in nearly all cases given the Rp/and gm limitations of this tube)...the substitution using the 12DW7/6247 makes sense. thanks for schem. :)

And thanks for the link to the other thread on source selection switch

Having built one of the passive eq'd and tweaked 7025 RCA phono sections based on your revisions to the classic cheapo RCA..I'm certain this will provide a significant improvement over the existing phono stage.

Regards. Leo
 
Thanks Eli...boy Avery must have been cheap...was it really less expensive to slap in two 6BW4's instead of say one 5Y3/5AR4 or something along those lines?

Darned tootin Avery was tight. His estate paid for 2 complete rebuilds of the Lincoln Center hall (which now bears Fisher's name) the NYPO calls home. A man does not accumulate that kind of cash by accident.

A couple a 6BW4s might not have saved much, if anything, over a 5AR4, but not having to deal with a 5 VAC filament winding in the power trafo did save some cash. Avery and Co. were very effective "bean counters".

Fortunately, Fisher tended to use decent "iron", which leaves room to maneuver.

Leo, I wish you every success in your refurb. job.
 
Leo,

I took another quick look at the schematic. The voltages they ran the "finals" at beat the living guano out of them. Back when the amps were made, tubes were cheap and, with some frequency, bottles labeled 6BQ5 were actually 7189s. The only current production tube I would use for replacement "finals" is the Russian 6П14П-EB (6p14p-ev), AKA EL84M. The 6П14П-EB is a true 7189 equivalent, which is tough as the proverbial brick outhouse, and its sonics are quite decent too.
 
I'm using the Russian equiv in an HH Scott 299B. They sound good and they can take the voltage.

Nice to hear Frank...I'm not usually a Sino-Russian tube fan given my experience with guitar amps where I find they just don't have a lot of life in them...but as far as audio applications I defer to Eli's experience with them...he seems to be a fan of many new issue Ruskie tubes and your experience only confirms his opinion...so good to know I have an option instead of pricey NOS.
 
Leo,

A good clue to quality in Russian tubes is the EB (ev) suffix. They're milspec. The Soviet regime did not neglect the military. The 6П14П-EB is a fine tube. OTOH, the 6П14П is a POS. The 6Н23П-EB is a good 6922. The 6Н23П is a very mediocre 6DJ8.

Another thing to look for in Russian tubes is the OTK marking. That stuff was part of the military inventory.
 
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