Guitar Amp from Old Record Player

[furrnornos]

I have an old record player with a 1 tube amp.
I put a photo of the schematic that is (helpfully) on the bottom of the case.

It uses straight 120V from the wall for the DC power supply, which seems dangerous. I'd like to put an isolation transformer in there, but I have a few questions.

I modeled the power block in PSpice and it looks like the output transformer is tied to a 50V p-p signal thats been offset +20V.

Does that make any sense?

Thanks
-Jordan

[img:800:600]http://i28.photobucket.com/albums/c235/ ... 7a3b14.jpg[/img]

[shoggoth]

If this is the free pspice and you're using their diode models - they have a low breakdown voltage (50v or 100v or something). So you have to use a few in series.

[Structo]

Just a guess but I think that is just a preamp for the phonograph.

And yes, those are known as Death Amps because of the direct house current to the amp or transformerless amp.

So you need to at least incorporate a 1:1 isolation transformer to make it safe.

What are your plans for this?

[furrnornos]

If this is the free pspice and you're using their diode models - they have a low breakdown voltage (50v or 100v or something). So you have to use a few in series.

Genius. Now both the output transformer node and the plate node show ~83V DC. I guess the output transformer needs more juice and less regulation?

Just a guess but I think that is just a preamp for the phonograph.

Nope - thats all of it.

So you need to at least incorporate a 1:1 isolation transformer to make it safe.

What are your plans for this?

I'm thinking something like this; I can use the center tap on the secondary to get 24V for the heater.

That seems like enough power, right? This is going to be like a 2W amp or something.

[ToneMerc]

Unless I'm wrong, all this is a one tube power amp for a crystal phono cartridge and of course there's a 90v motor for spinning the platter. My concern would not be be power supply, but the lack a preamp amplification stage to accept a guitar pickup.


TM

[Invertiguy]

As noted by everyone above, that amp was intended to be driven directly by a crystal phonograph cartridge and doesn't have nearly enough gain to be used with a guitar signal, so you'll need to build a preamp. Also, you definitely want to replace the rectifier with normal silicon diodes if it's a selenium type- they produce toxic smoke when they fail, and they WILL fail.

[ToneMerc]

By the time one chases the input stage and then upgrades the power supply to be inline modern safety standards or practices, one could have built a Champ style circuit.

TM

[furrnornos]

Guitar pickups have lower output than a phonograph cartridge?

The idea was to drop in an isolator, a 1N4007 for 1/2 wave rectification, and leave everything else as-is (except the caps). I can reuse the chassis, mounting, output transformer + speaker (theyre riveted together).

Oh and I have 3 of them, so something simple/cheap I can replicate easy three times.

I figured I'd need to adjust the filter on the input to sound right and leave it alone otherwise.

I didn't realize guitar pickups were so much quieter than a phonograph transducer.

[furrnornos]

What if I add a single ended op-amp in a can-type package so I can still point-to-point wire it? What would get me past the input level problem.

Besides that, does the transformer I picked out look ok? Is there a cheaper option around?

[Structo]

TM is right, most likely a crystal phono cartridge with high enough output to feed the power tube.

I would guess it has more output than a guitar pickup.

If you have a good meter, put a record on it and measure the output of the cartridge.

[eniam rognab]

run it with a clean booster pedal, or fuzz/overdrive to see if it'll work

[furrnornos]

Is there any harm in making it a full wave rectifier instead of 1/2 wave? It just gives me better regulation but no voltage increase, right?

[shoggoth]

No harm, just less ripple. The 25L6 has an example plate voltage of 110v in the data sheet I looked at so you're probably going to be fine. The isolation transformer output voltage may be higher than wall voltage with the light load of a single tube though, so it may be more than the 85v you'd expect.

EDIT - I also looked at that transformer you mention, if you power the heater from the 60v AC you'll need a 10W minimum (preferably 20W) 116 ohm resistor to eat up the excess voltage, which is pretty grim. You might want a second 25.2 transformer for the heater instead, you can get them from Radio Shack pretty cheap.

EDIT #2 - the example grid voltage is -7.5v, so you're going to need a decent amount of gain from that op amp. I think you'll want a gain of at least 50 from the op amp for clean, and more if you want to overdrive the power tube.

[furrnornos]

I went with a 80VA P115V/230V, S115V/230V... I got 5 of them used on eBay for $75 shipped.

EDIT #2 - the example grid voltage is -7.5v, so you're going to need a decent amount of gain from that op amp. I think you'll want a gain of at least 50 from the op amp for clean, and more if you want to overdrive the power tube.

I'm considering the LME49710 since its in a can and will look cool with the other components - but I have a few 6AV6's, and since they have the same heater current I could pretty easily use them for the preamp. I dont know if I can get the gain up to 50 with one of them, though.

Do you mean 50dB or 50V/V?

[shoggoth]

It's parallel 115v output, so you'll have to run them in parallel and use a bridge rectifier, which will give you 160v - possibly closer to 180v given the low current draw. Dropping over a hundred volts to power your heater is really impractical given its current draw, so a second transformer for the heater is recommended. It's going to be quite a big resistor to drop voltage w/ the 50mA the power tube draws too, but not as bad as a 300mA heater.

I mean 50v/v gain. The op amp you selected will give you 40v/v gain I think, so you'll be able to play clean. String up two in a row if you want to push the tube.

I'd get the cheapo Radio Shack 25.2v transformer, power the tube heater w/ the AC, and then half-wave rectify both a +17v and -17v from it as well to power the op amp(s).

Of course it's late, I'm tired, and I'm just some shmoe on the internet, so double-check all this math.

EDIT - and you could also use the power transformer backwards, putting 120v across the two secondaries in series, and getting 60v out of the other side, which will give you 85v dc (well probably more w/ your light load) once its put through a bridge rectifier.

EDIT 2 - I dunno about using 6AV6's, it's a steel chassis right? Drilling holes in steel stinks. Or do you have existing cap can holes you can stick a tube socket into? They also want 300v on the plate, so you'd have to use the secondaries in series to get above 300v, and then drop 200v for the power tube plate. That's a giant 4k power resistor, 10w minimum, 20w recommended.