DIY Fisher 800b into a mic pre - diyAudio
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Old 5th January 2013, 08:01 PM   #1
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Default DIY Fisher 800b into a mic pre

Hey guys, total noob here. I have an almost mint condition Fisher 800b hi-fi amplifier that I want to convert into a mic pre. Beyond recapping it, I really don't know what else I'll need to do--though I imagine it is quite a lot.

Anyone have any ideas on how to help make this conversion happen?
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Old 5th January 2013, 09:25 PM   #2
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Why on earth would you want to vandalize a fine vintage piece, which can play music far, far, better than current mass market guano? All of the power supply SS diodes need to be replaced with modern, low noise parts. All electrolytic caps. need replacement. The signal caps. may still be good. Carbon composition resistors need to be checked for drift out of tolerance. Watch your step around the tuner sections, lest you create an alignment issue. Talk to Jim McShane about tubes and parts. Jim can provide what you need to make the power supply BETTER than it originally was.

Left side of 800B schematic. Right side of 800B schematic.
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Old 5th January 2013, 11:24 PM   #3
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Originally Posted by Eli Duttman View Post
Why on earth would you want to vandalize a fine vintage piece, which can play music far, far, better than current mass market guano? All of the power supply SS diodes need to be replaced with modern, low noise parts. All electrolytic caps. need replacement. The signal caps. may still be good. Carbon composition resistors need to be checked for drift out of tolerance. Watch your step around the tuner sections, lest you create an alignment issue. Talk to Jim McShane about tubes and parts. Jim can provide what you need to make the power supply BETTER than it originally was.

Left side of 800B schematic. Right side of 800B schematic.
Damn, thanks man! That's a huge help. And yes, this thing sounds amazing. I was tempted to use it for mixing, but it makes everything sounds amazing, so that's no help for dailing in a mix, haha. I just love the sound so much, I want to be able to have that in my vocals.

A couple clarification questions, because I'm not good at this:

1. How do I specify between electrolytic vs. signal capacitors?

2. By checking for "drift out of tolerance" do you mean making sure their actual impedence still matches up with what the stripes and schematics read?

3. When you say "the tuner section" are you referring to the am/fm tuner? I had imagined I would want to remove all of the potentially noise-producing components that won't be a part of the pre-amp circuit. Is this not the case?

4. Do you know if I'll be able to convert the phonograph input into a mic in without altering the impedance or anything?
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Old 6th January 2013, 12:45 AM   #4
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Quote:
Originally Posted by Klopfenpop View Post
Damn, thanks man! That's a huge help. And yes, this thing sounds amazing. I was tempted to use it for mixing, but it makes everything sounds amazing, so that's no help for dailing in a mix, haha. I just love the sound so much, I want to be able to have that in my vocals.

A couple clarification questions, because I'm not good at this:

1. How do I specify between electrolytic vs. signal capacitors?

2. By checking for "drift out of tolerance" do you mean making sure their actual impedence still matches up with what the stripes and schematics read?

3. When you say "the tuner section" are you referring to the am/fm tuner? I had imagined I would want to remove all of the potentially noise-producing components that won't be a part of the pre-amp circuit. Is this not the case?

4. Do you know if I'll be able to convert the phonograph input into a mic in without altering the impedance or anything?
Agree with Eli, it seems a bit of a waste to strip out a perfectly working receiver just to get a mic preamp out of it. There are much easier way to do it imo - there are many fine tube mic preamp diy projects available. Just saying... If you must, I would not touch/rip out anything in the receiver, but just focus on modifying the input section and take the output from either one of the three outputs - reverb, line or tape monitor on the back of the unit.

Jaz
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Old 6th January 2013, 12:54 AM   #5
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Originally Posted by jazbo8 View Post
Agree with Eli, it seems a bit of a waste to strip out a perfectly working receiver just to get a mic preamp out of it. There are much easier way to do it imo - there are many fine tube mic preamp diy projects available. Just saying... If you must, I would not touch/rip out anything in the receiver, but just focus on modifying the input section and take the output from either one of the three outputs - reverb, line or tape monitor on the back of the unit.

Jaz
I don't really have much use for it as a receiver. These days I almost only ever listen to music on the go. Not very much time to sit at home and listen. When I do have time to listen to music at home, I'm usually working on a mix. And in that case, I want as uncolored a listening experience as possible so I can dial in the mix.

Is there any modification I'll need to do beyond making the inputs and outputs fit? Do I need to alter anything in the input circuitry or anything else between in and out? I imagine the phonograph input is going to be the same as a microphone input as far as the signal level it takes, right?
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Old 6th January 2013, 01:38 AM   #6
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Quote:
Originally Posted by Klopfenpop View Post
I don't really have much use for it as a receiver. These days I almost only ever listen to music on the go. Not very much time to sit at home and listen. When I do have time to listen to music at home, I'm usually working on a mix. And in that case, I want as uncolored a listening experience as possible so I can dial in the mix.

Is there any modification I'll need to do beyond making the inputs and outputs fit? Do I need to alter anything in the input circuitry or anything else between in and out? I imagine the phonograph input is going to be the same as a microphone input as far as the signal level it takes, right?
I see, so you are a producer! Cool... I think there are a couple of ways to do it: 1) rip out the existing RIAA network for the phono input and adjust the parts for mic input; or 2) take out V5 (which is used for the AM only) and put in a 6AU6 and configure it for mic input like: Vacuum Tube Stereo Microphone Preamp

I think 2 might be easier, just swap the tube and clip/change a few parts around the tube, clip/jumper some wires and you are done. Vinyl is coming back big time, so you may want to keep the phono input just in case...

Jaz
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Old 6th January 2013, 03:10 AM   #7
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That's 1 hell of an AM tuner. Notice the TRF front end. Not surprising, considering that when the 800B was new, lots of music (of all kinds) was to be found on the AM band. Given the utter garbage on AM these days, sacrificing the AM tuner, as long as it can be reactivated, is probably best.

Let's talk about getting the most out of the jack set available. A picture of the unit's back will help, in terms of making choices. The tape head I/P is a total waste, in this day and age. A single pair of RCA females should be hard wired to the low level mag. preamp. Switching mV. level signals is a bad idea. Replacing the factory source selector switch with Mouser stock # 105-14572 (inexpensive) allows for the simplification. Make certain to save the OEM part, just in case.

I'm not a fan of Fisher phono sections, as they, among other things, have notoriously poor bass extension. It's easy enough to upgrade to a tweaked RCA circuit (schematic provided), which will yield good bass and also be able to truly drive the recording O/Ps.

I suspect space for the mic trafo inside the 800B's cabinet will be hard to find. A small aluminum "bud" box and short cabling will deal with that detail. Cinemag is a good place to get the mic trafo from.
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Old 6th January 2013, 02:44 PM   #8
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Quote:
Originally Posted by Klopfenpop View Post
Hey guys, total noob here. I have an almost mint condition Fisher 800b hi-fi amplifier that I want to convert into a mic pre. Beyond recapping it, I really don't know what else I'll need to do--though I imagine it is quite a lot.

Anyone have any ideas on how to help make this conversion happen?
Could I just mention that I totally agree that it would be a shame to tear up this receiver. You could sell it and use the funds to buy a suitable kit or amp to rebuild. A near mint 800B is of real interest to a lot of vintage gear enthusiasts and should sell quickly once people know it's for sale.

I suspect that getting the noise level down with the Fisher would be difficult too, there is so much circuitry in there and there's not much space for extra added items as some have said already.

I have to confess - when I saw this thread it struck me as identical to saying "How can I convert my near perfect 1966 GTO into a front end loader?"
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Old 24th April 2013, 07:58 PM   #9
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An update!

What with several moves, I never got around to starting any of this. And now my wife and I bought our first house. I'm a bit of a record collector, so I'm starting a vinyl and cocktails night (calling it Bin & Tonic) for friends to come over and share our records with each other.

I now have a very good reason to keep this as a receiver.
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Old 24th April 2013, 09:48 PM   #10
rmyauck is offline rmyauck  Canada
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Congrats on the new house! Glad to hear your not going to alter the Fisher!

It's very wise to recap it though as you Risk the Transformers as caps can fail suddenly! Check the resistors also along with cleaning tube pins & sockets, switches, & pots.

Get a kit of parts from Jim McShane and just change One or Two Caps at a time to avoid mistakes. Lots of detailed pics will help!
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