Fisher 460A 440A 490A Console Tube Amplifier Conversion 

At long last! After 15 years in the audio hobby, I found my first Fisher tube amp in the form of a 440-A pulled from a local Custom Electra IV. I'm an engineer. I tend to trust maximum ratings. I don't understand why it's considered OK to run the tubes above maximum voltages. (341V Vp is 41V higher than the tube data sheet maximum; 320V at the screen is 70V higher than maximum) I used a 750ohm 10w (should be 12w, didn't have one) voltage dropping resistor right after the rectifier because without it, I was measuring close to 420V going to the anodes. This, along with a 3K R126, got my my plate and screen voltages within 10% of the 6BQ5 maximums and still sounds great. I used a 1K for R123 to keep the plate voltages of the 7247 within their maximum parallel rating (250V) Compared to my JE-Labs/Kegger/Blueglow EL-34 SE, this amp seems to me to have better separation and extremely good clarity and realness. For years, I had ignored PP amps and focused on SE (easier to build), but I find that the good-ol' 6BQ5 is just a pleasure to my ears. I like that crisp, detailed, musical sound they have. These OPT are even smaller than the ones on the Pilot 402 that I had. But they're fine. Bigger doesn't mean better in terms of OPT size in my experience. I've had 2 fantastic PP amps with miniature OPT that sound superb. I use a sub, but even without it, I get some of the best bass I've ever gotten on my high efficiency PA style 12" woofers. I need to compare this to my EL34SE amp to really determine the differences. I'll get to that very soon, now that winter is approaching. Someone also recommended to me that I remove the NFB loop & listen. I want to get familiar with the stock sound so I can compare. Thanks for this video! There's not really any info on the net on how to do this conversion.

Sounds very nice love to see it running in the dark, the blue glow is mesmerizing. I’m seriously looking to take my 680A with 7591 and place it in a new chassis with a bit different layout of the transformers. Back and center the PT with all necessary caps and resistors center in a copper box 1/4” off the top yet under the chassis with copper tubes and plastic for any wires from one out put transformer to other side, tubes 2ea 7591 back a bit 1 1/2” near the output transformer, 1ea 12AX7 centered between the 7581 up front with as little cross wiring as possible. Dreams.

This little Fisher amp would be marvellous in my setup

That's what I'm talking about! Iv'e got a couple of console amps that I have been meaning to sort through. One without available schematic which is similar to this Fisher in interconnection to the preamp. Now I have a bit more info to unravel it with. You are too humble! This is good sh...t , I mean stuff. There are a lot of these beauties out there that need some love! Iv'e been listening to a Stromberg Carlson ASR 433 stereo 24 with El84's I recently acquired. Quad Tube match a must but those little OPT's surprised me with amazing bass. Still looking for the exact schematic. Thanks for keeping the filaments glowing!

I used the EL84/6BQ5 for over 50 years; it keeps astonishing me till today..... The best choice of any brand/type of EL84 I use ( in my experience and personal opinion) is the E84L or 7320 from Siemens ....and often labelled in the Philips group but made by Siemens . Even when they look totally used they often still perform within specification. This tube can make dance the dead...........

You could put a 'juwel' pilot light in the hole in the front where the power cord used to be.

I completed the rebuild on my 440-A and it is dead quiet, as in "ear up to the horn, quiet". I was able to get all voltages within 2% of the factory schematic in this stand-alone build. My other amp is a Kegger/Bluglow KT88/el34SE and this 440-A sounds very similar and better in some ways. The 440-A has some extreme detail and rich tones, but the soundstage isn't as large. The SE amp has a much larger soundstage, but a smoother and softer presentation that is more sensitive to recording quality. BTW, leave the feedback loop in. I removed it and it did not sound as good.

love the fisher amp, i have an old solid state one and it has been the death of many good speaker, pure power :)

I would go a step further with this conversion. Separate adjustable bias for each output tube. Matched sets operating on a common cathode resistor do not always wear evenly and with time become mismatched. Bias checking and resetting in such a setup is futile. Take each cathode to ground through a one ohm cathode current setting resistor. Bias is drawn from a capacitive voltage divider and rectifier-filter off of the B+ winding, not much current required. Performance benefit is a bit more power output, slightly lower distortion, and longer tube life.

Great video. So informative and easy to follow. My Pilot 402 taught me that pipsqueak OPT can sound plenty good.

you're 100% right re: being interesting that the series wired filaments were tied to ground, that was a no-no!

It's a good idea to bypass that cl80 with a relay. I never leave them in circuit longer than necessary.

Although unusual, using the heaters of the preamp valves as the cathode bias resistor for the four output valves in parallel is a clever design which has the following advantages: (1) Normal cathode bias, as opposed to fixed grid bias, has negative feedback which keeps the bias current more stable. But, as valve heaters have a positive temperature coefficient (the resistance increases with temperature) this forms additional negative feedback which further stabilizes the cathode current of the output valves. (2) With this circuit, the heater current is DC (provided that there is a decoupling capacitor from the output valves cathodes to ground) so mains hum around the preamp valves is greatly reduced. (3) The power, that otherwise would be wasted with conventional cathode bias, is used for the preamp valve heaters, so the load on the power transformer is less and the amplifier runs cooler. (4) If any of the preamp valve heaters go open circuit, or if any of the preamp valves are removed, the output valve's bias current will be reduced, so the output transformers will not be damaged. There is a danger with this circuit arrangement though: parasitic oscillation whereby the signal from the output valves gets back to the preamp valves via the heaters. This could damage the output transformers. For this reason it may be wise to connect each of the preamp valve heaters to ground with 100nF ceramic capacitors. Note that the above is all theory: I have no actual experience of the amplifiers in question.

I wonder if they used the tube filaments as cathode bias resistors because the resistance would be higher when first switched on then resistance would drop as the heaters heated up and would allow the normal set amount of current to flow for the bias at that point. Maybe would have been easier on the output tubes.

You can drill holes on edge of other holes, What you need is a thick piece of aluminium with a hole the size of your drill and some strong clamps. Clamp on the aluminium guide on the work piece and drill 😉

One of my tubes is red plating, soft red plating. How does one bias this amp?

But those poly caps on the mains isn't a safety cap is it? And better still why not put three safety caps in a triad for extra filtering. R10 resistor why didn't you put 2 or 3 in series and feed them to ground to make up the difference of not having the tubes. Wouldn't it be better to have the thermistor after the regulator not before? Make the amp more punchy

Regarding your mic issues, how about a vintage mic and tube pre-amp project. Other than affording a large diaphragm tube condenser mic, for great performance bang for buck for your application, check out the Nakamichi CM300 mics. Get ya some! Ask me about batteries as this used to be an issue but is no longer. I am just diving into the classic recording mic pre-amps. I believe there is a lot of interest among musicians and aspiring RU-vid users . Could be an interesting look at tone stack voicing, dynamic range etc... Keep em glowing....

The 0.47uF coupling caps are film. I thought it was only 'lytics that needed to be replaced.

Why don’t you just individually bias the output tubes?

Love your work..

Hey do you have any 2 mono 440 Ampex converted as line preamps

Do you recall what the plate current was using the cathode resistor you used ? I'm guessing you were running the output tubes pretty cold ?

Add the 480a and 481a to the mix. '64 CE VII .

Should that external feed "umbilical cord" jack be blocked with epoxy or something so that someone one down the line doesn't plug a preamp into it?

Fale sobre o Fisher 600 .obrigado

Why not bias outputs separately? Four pots can't cost that much.

That filament-as-bias-resistor design strikes me as so shifty and un-Fisher-like that it seems like an afterthought, cobbled together at the last minute to satisfy some criterion that wasn't really in the original design.

Panasonic ECQ caps, correct?

Power light???