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Fisher 440A project - need help

intotubes

intotubes

Well-Known Member
I have traced out the circuit for a Fisher 440-A console amp. After scouring the internet I have found that this amp's power supply was designed to be connected to the preamp portion of the unit. Apparently the 6BQ5 bias is dependent on it. One tact was to put a 350 ohm 10 watt resistor to ground at the point where the preamp plug connects to one side of the bias pot. This would create a voltage divider (I think) between the 47K resistor on one side of the pot and the 350 ohm resistor to ground. The data sheet for 6BQ5 says to use 130 ohms for a cathode bias resistor. What gives?
Also this is the strangest power supply I have seen yet in my limited experience.
No center tap and the legs are tied together with a cap. Please look at it and explain to me what's going on with it.
There is also a large section of the PS that presumably can be removed since there is no preamp connected to the plug anymore. I plan on using a separate preamp with this amp.
No voltages yet, I am still pondering this schematic and want to understand how to bias this correctly.

Thanks!
 

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The power supply is a voltage doubler, as used in most Fisher amps. They ran the tuner from 1/2 voltage of the output stage. Transformer is probably about 130 VAC.

The 6BQ5s are running on a higher voltage than normal, and drop some of the excess using the preamp tube heaters. Obviously the output stage is biased to about 150 mA total, 37.5 mA per tube (12AX7 heaters draw 150 mA...). The 6BQ5s are actually biased POSITIVE with repect to ground, unusual, but OK, as the cathode is even MORE positive, and it's cathode-to-grid voltage that matters. 350 Ohms DOES seem a bit high - though it would be about right if there are 4 tube heaters in series with the cathodes.
 
Thank you for the fast reply Tom. It must get old telling the noobs the same things over and over.
When I hear about biasing it is "cathode bias resistor" and "cathode bias" and such. But what I am really interested in is grid bias? The grid must be negative with respect to the cathode. This is done with a cathode bias resistor? When you say the tube is at higher than normal voltages are you referring to plate voltage or cathode (or grid)?
I take it this is an unusual circuit?
When I turn that bias pot, where am I measuring the voltage and what do I want to see? Am I adjusting it so the grid is negative with respect to the cathode and am I shooting for a specific amount?
Can I remove the part of the PS in red? It shouldn't affect the rest of the PS should it?
I have inserted the suggested placement of the 300-350 ohm resistor as suggested by an old Yahoo Groups post. Is this correct? From what I understand there were three 12AX7 heaters in the preamp circuit.

Sorry for all the questions. I am really trying to learn this stuff.
 

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Well cathode voltage is higher than normal, and so is plate (it's the difference plate-to-cathode, that matters). I've seen this bias scheme in a number of vintage amps. One less power resistor, and DC heaters for free!

Yes, you can delete those parts in the power supply - only needed for tuner. A resistor of about 240 Ohms would drop 36V at 150 mA, same as three 12AX7 heaters. You'd adjust the bias control for 36V (maybe a little less, to make it easier on the output tubes) across that resistor. The resistor should be at least 10W, 15 or 20W would be better. Matched output tubes are required.
 
Thanks Tom. I have been reading your two posts several times and looking at the 6BQ5 data sheet to try and get a handle on what's going on in this circuit. I now understand both your posts. Thank you for making it easy to understand.

I am trying to set a bias current of 37.5 ma per tube (4 tubes) to equal the original design of 150 ma, the same as a 12AX7 heater draws. I am ASSUMING the tuner/preamp used 12AX7s and used three of them. Ohms Law says three tubes at 12.6V and .15A equals 252 ohms (the 240 you suggested). Using a resistor of this value and adjusting the bias pot to obtain the required 36 or 37 voltage drop insures that the bias is set to design specs.
Although I don't understand what bias is and how to determine what it should be, I understand this much and I can get the amp working now.
Thanks again. I really appreciate the help.
-Mark

*edit*
OK, I found a great write up in simple terms on what tube bias is, what it's for and how to adjust it. http://www.geofex.com/tubeampfaq/TUBEFAQ.htm#bias
I now see the need for a matched quad of output tubes. There is no way in this circuit to measure individual tube bias since all the cathodes are tied together. It would take considerable modification to be able to balance unmatched tubes. I suppose I could add another bias adjustment pot and isolate the two channels. Then I would only need matched pairs.
There is a dim glimmer of understanding now...
 
I am back to working on this amp. Took a long break to work on other projects.

In the power supply schematic (updated and corrected from 1st post) what is the purpose of the .01/600 cap in parallel with one of the diodes? It is the same as the one used for the "death cap" on the 120V line.
 

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Last edited:
Tom Bavis said:
I've seen this bias scheme in a number of vintage amps. One less power resistor, and DC heaters for free!

A resistor of about 240 Ohms would drop 36V at 150 mA, same as three 12AX7 heaters. You'd adjust the bias control for 36V (maybe a little less, to make it easier on the output tubes) across that resistor. The resistor should be at least 10W, 15 or 20W would be better. Matched output tubes are required.
Click to expand...

I am going to take advantage of the DC heaters for free. I disconnected the 2 7247 filaments and will route the old 12AX7 filament wires through them instead. In place of the old pigtail to the tuner, I put an old tube socket I had and will put a weak 12AX7, 12AU7, or something with a 12.6V filament in there and use it for the 3rd filament to get a total of 37.8V in the bias circuit. This way I will be using the old circuit w/o the added high power resistor and will get DC filaments to the driver tubes. Brilliant! (an idea that was suggested from another board - can't remember where)
 
Have any desire for a 440-T?

I have a spare 440-T, which I believe is the front end for that unit. PM me if interested....
 
analog addict said:
I have a spare 440-T, which I believe is the front end for that unit. PM me if interested....
Click to expand...

Thanks for the offer but it has already been modified beyond the point of mating to the original tuner/preamp. My intent is to make it a stand alone amp - fed by a tube preamp, specifically my PAS-3.

I'm actually looking for advice from the sages on a couple of things I'm not understanding yet. It's amazing how much one can learn from rebuilding these things, web searches, and the friends on this board. I am so appreciative of the people here that willingly share their knowledge and experience.
 
intotubes said:
I am back to working on this amp. Took a long break to work on other projects.

In the power supply schematic (updated and corrected from 1st post) what is the purpose of the .01/600 cap in parallel with one of the diodes? It is the same as the one used for the "death cap" on the 120V line.
Click to expand...

OK, nobody knows.

I'm leaving it out.
 
intotubes said:
Thanks for the offer but it has already been modified beyond the point of mating to the original tuner/preamp. My intent is to make it a stand alone amp - fed by a tube preamp, specifically my PAS-3.

I'm actually looking for advice from the sages on a couple of things I'm not understanding yet. It's amazing how much one can learn from rebuilding these things, web searches, and the friends on this board. I am so appreciative of the people here that willingly share their knowledge and experience.
Click to expand...


Hi --- I have recently acquired a Fisher 440A Amplifier from a Custom Electra IV E44 Console and am looking into how to make it a standalone amplifier.

Did you complete your Standalone 440A Project and was it successful?

Would you be willing to share details of your Modifications?

I followed what you did in removing unnecessary parts that were there to drive the Tuner Preamp on the 440T thru the 9 pin connector, but am unclear as to what your added Resistors Values are that make up for the missing 12AX7's in the Tuner and where did you put them.

According to the 440T schematic in the Electra IV Service Manual there are 4 12AX7's in the Tuner/Preamp that must be taken into account.

Thanks in advance.

Stew
 
A single 12ax7 wired in 12v mode is basically an 84 ohm resistor (12.6v / 0.15a). In 6.3v mode, its a 21 ohm resistor. Depending on the setup basically you multiply that value by however many tubes and sub in a resistor of that value (or the nearest available value) where it fits. It will likely get fairly toasty, so attaching it to the chassis may not be a bad idea.
 
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