Converting mono AM radio/phono amp to turntable amp

[beasleybodyshop]

A friend of mine dropped off this old AM radio/phonograph console to me a few weeks ago. It's a Stewart Warner! I just thought they made old hot rod gauges. Learn something new every day...

Originally this thing was just an AM radio with a built in record player. I would like to just convert the tube amp inside it to work with a modern style record player. I know its going to be a mono amp, but that's ok. Attached is the schematic i found for it. Not knowing anything about radios, i have a few questions:

If i want to just make this into a phono amp, can i get rid of the 6J5GT oscillator tube, 6BA6 mixer tube, and the 6SK7GT I.F. Amp tube from the circuit and work from there?

The connecting wires leading to the record player and power cord have been removed, so im not sure where they connected originally. I see from the schematic that the pickup for the record player connects via resistor 37 to an item labeled "34", which i am guessing is some kind of switch? I dont see a switch inside this amp. If it is a switch, looks like it can connect to the input of the 6SQ7GT via some kind of transformer...transformer coupled? Can i omit the transformer and substitute a coupling cap?

Sorry, no gut shots right now, my phone is MIA. Does this seem like a feasible project? We are going to refinish the console, and install a new speaker and a modern style record player inside it, and i would love to make it work so they can at least play mono records on it.

[DonMoose]

You could definitely delete the radio-specific circuits, but it looks like you'll need a phono preamp stage (with RIAA equalization) for your preferred cartridge type (Moving Coil or Moving Magnet) and a stereo->mono mixer stage. I think doing it in that order would be slightly better, but can't articulate why.

Hope this helps!

[beasleybodyshop]

You could definitely delete the radio-specific circuits, but it looks like you'll need a phono preamp stage (with RIAA equalization) for your preferred cartridge type (Moving Coil or Moving Magnet) and a stereo->mono mixer stage. I think doing it in that order would be slightly better, but can't articulate why.

Hope this helps!

Thanks for the direction Moose. I know next to nothing about different types of record players, cartridge types, etc.

With a Stereo-mono stage, that would take my stereo records and correctly play them into a mono amp? Where might i find a proper schematic for one of those?

[DonMoose]

With a Stereo-mono stage, that would take my stereo records and correctly play them into a mono amp? Where might i find a proper schematic for one of those?

I'm pretty sure that's just a 2x1 mixer stage. A pair of triodes with a common plate resistor would be the deluxe way to do it. A single triode with a series R from each channel to the grid and a high-value grid-to-gnd resistor, say (68k+68k)/1M would work just as well.

Google the term RIAA phono preamp (include tube in there to eliminate the op-smp onesif you like).

[beasleybodyshop]

With a Stereo-mono stage, that would take my stereo records and correctly play them into a mono amp? Where might i find a proper schematic for one of those?

I'm pretty sure that's just a 2x1 mixer stage. A pair of triodes with a common plate resistor would be the deluxe way to do it. A single triode with a series R from each channel to the grid and a high-value grid-to-gnd resistor, say (68k+68k)/1M would work just as well.

Google the term RIAA phono preamp (include tube in there to eliminate the op-smp onesif you like).

Thanks.

Say for the sake of argument, i used this Phono pre here (attached at bottom)

instead of separate R/L phono, couldnt i connect both signal outputs together to mix them and connect that to the grid of the preamp tube? Would that solve both mixing the stereo to phono problem and the RIAA compensation?

[rp]

Can't help technically and not sure what year your radio's from, but I do know that early on there was no standard phono cart eq and cart and phono stage were often dedicated, and many old designs were for ceramic carts that had very high outputs w/ respect to modern. So if this is from that time you would pretty much wind up building a phono stage from scratch to use it with a modern MM cart. I think it all sort of came together in the late '50s so if that's the date you might be ok, but google some more or check on DIYA before you spend too much time on it.

[beasleybodyshop]

RP, all research indicates that this console was built in 1956.

[rp]

The problem will be if the pre is for a ceramic cart as is most likely as cheap stuff like radios and consoles used those till the '70s. Moving magnets are about 3-5mv output, ceramics 1V or more. If you have a strong MM you might be ok, I'd worry more about the eq. The little I understand here is that ceramics are have trouble with the RIAA eq used to cut the records, also picked up and made odd noises that to be correct somewhere, and there's the issue of their peculiar loading and capacitance that are beyond me, but I would guess phono pres in cheap radios and consoles using ceramics would try and correct for the crappy cart it came with rather than being universal like the better expensive stuff that starts to appear in the stereo LP age.

Easy way to me is to get it working without to much time invested - kluge it best you can keep your expectations low and plug the modern TT into it and listen, if the eq sounds terrible and/or the volume is very weak there's not much you can do other than build/get a modern phono stage. If it sounds ok then finish it up nice. Post a pict of the old TT it came with.

[rp]

I just noticed the original radio schematic you posted, I thought the phono schematic was a part of that. Yes, the radio was for a ceramic that pretty much went straight in, tells you how much gain they had, and the radio had so little bandwidth they didn't need to eq it. I can't help you with were it would be best to inject the signal but I would definitely not bother building a phono pre, even the lamest modern circuit you follow it will be way too good for this, and if you are following a vintage schematic you avoid picking the wrong one.

Plus, someone else was asking about TTs and hifi here and if you haven't bought a TT yet you may not need a phono stage, in the digital age cheap $150 type TTs come with some sort of adequate phono preamp built-in, since '90s onward, receivers, stereos, and of course computers no longer have requisite phono inputs. Otherwise look up Audio Technica phono preamp - it's like $50 and works well enough. Cheaper than that you'll have to go on ebay and find something from radio shack or shure thing - just make sure it's for a modern cart.

[DonMoose]

Plus, someone else was asking about TTs and hifi here and if you haven't bought a TT yet you may not need a phono stage, in the digital age cheap $150 type TTs come with some sort of adequate phono preamp built-in, since in the digital age, '90s onward, receivers, stereos, and of course computers no longer have requisite phono inputs. Otherwise look up Audio Technica phono preamp - it's like $50 and works well enough. Cheaper than that you'll have to go on ebay and find something from radio shack or shure thing - just make sure it's for a modern cart.

Ooh - nice point.

As to that schematic, sure - you could parallel the outputs, though I'd use a bigger series R on the output and maybe AC couple into the grid to protect the op-amps.

[rp]

yes you can just parallel to outputs to mono on the pre or TT leads I did this some times as a kid for some reason, you can build a little break out box w/ 3 RCAs. If you do it out of phase and play a mono lp you can use that to check how unbalanced or off-axis (azimuth?) your cart is, for whatever that's worth to you

As an aside: not sure if paralleling the leads is audiophile approved and I wouldn't do this if I had a $50K hifi but then you'd likely have a dedicated mono cart and phono preamp for your large collection of mono LPs. Yes, there are back to mono guys out there. Supposedly, you haven't lived till your hear an all tube triode system, w/ one large horn speaker, dedicated mono cart, playing a '50s mono In Living Stereo LP. The reasoning isn't that oddball, supposedly the golden age of recording was mid 50s to early 60s and the most diligent stuff was still done in mono with simple straight forward micing. The Beatles oversaw their final mono mixes then went home, the stereo mixes where just quickly thrown together by assistants. After the mid 60s recording got too complicated, too many mics, too many channels, phase issues, and too much processing. That's the logic of it, anyway.

So you're on a noble path - All Tube All Analog Mono - go for it