Any way to cross-reference PT and OT part numbers?

[chrislathan]

The old Magnavox console amp I scored has a huge PT and medium-sized OT that are in very nice condition. The amplifier was rated at 20 watts, and I was wondering if there was a way to reference these part numbers to see if they are comparable to other uses.

A cursory glance at Google only lends itself to MFG specific things, and little is to be had for 1958 Magnavox parts

[shooter]

1958 Magnavox parts

If google can't find it, time for deductive logic, If you know what they powered, and what output tubes they were connected to, you have a pretty good Idea of what you can build.   try finding a schematic if possible, it took me awhile, but I finally found a schematic that matched a 1939 Zenith 12 tube'r.  from that I knew the output was a single 6V6, champ it became!

[HotBluePlates]

The old Magnavox console amp I scored has a huge PT and medium-sized OT that are in very nice condition. The amplifier was rated at 20 watts, and I was wondering if there was a way to reference these part numbers to see if they are comparable to other uses. ...

IMO, the most-often repeated mistake newer builders make is to attempt to rob parts from an old chassis to build something different.

What tubes were in that Magnavox?  Most likely, the PT and OT are best-used by keeping them in that Magnavox chassis, with the power supply, output section and maybe phase inverter intact.  Fiddle the preamp as needed to be guitar friendly.

You know the parts in the chassis work appropriately together. You know they work as-configured in that chassis.  They may not work well if removed, unless configured in the exact same way in whatever new item you build.

This has often come up when someone gets a really old amp with a field coil speaker or a choke-input power supply.  In these cases, the PT has too-high voltage to be used with a standard cap-input power supply without something like a field coil to drop a LOT of voltage.  Or you get an odd OT primary impedance which works best for the output tubes in the original chassis, when used at the power supply voltage in the original chassis, and with the bias method/voltage of the original chassis.

So find a schematic for the console amp and figure out if you need to add/modify a preamp to make it suitable for guitar use.

[chrislathan]

This one is 4-6v6, 2-5y3's and 2-12ax7's. As it sits, the entire preamp/input section is remotely mounted on about 3' of cord and a phono plug.

I ordered the SAM too soon, and only got the one for the preamp section, so I'm going to order the proper one for the amplifier section now.

[silverfox]

Is it this one?

silverfox.

[chrislathan]

No sir. Mine has no volume, tone or any controls without the outboard amp171aa unit. Have the schematic, but it's in PDF

[chrislathan]

I scanned the SAMS for the preamp. It's a doozy. Will attach tomorrow

[HotBluePlates]

This one is 4-6v6, 2-5y3's and 2-12ax7's. As it sits, the entire preamp/input section is remotely mounted on about 3' of cord and a phono plug. ...

Excellent!  So you have all the bits on that one chassis which you'd ordinarily keep intact.  Everything from phase inverter to output is there.

At this point, you could build any preamp you like which can deliver 1-2v output when given a guitar-level input.  You could even power the new preamp from your existing amp chassis, as the original did.

For convenience, you may choose to swap the original's octal speaker output for a 1/4" jack.  You could also use a 4-pin XLR Socket & cable to connect the power from the power amp chassis to the new preamp.  I did exactly that in my 25L15 build.  Powering from the power amp chassis while also including a ground connection in that cable (pin 8 & 9 of the original power socket) would also nix ground-loop hum you might get if the two were powered independently.

Signal from Preamp to Power amp could be delivered by a cable with any connector style you like (RCA, 1/4", XLR, etc).

You may find the feedback is more than you need for a guitar amp.  I'd at least try it first, then maybe adjust R14 higher.  You might find the amp is a good pedal platform given a suitable preamp with a good set of tone controls.

[chrislathan]

i should remember that scanning at 600DPI makes for a HUGE file... here's the schematic for the preamp section

[PRR]

> a way to reference these part numbers to see if they are comparable to other uses.

?? The data and cross-ref are IN there.

The OT is 3.7K CT to 4.3 and 0.4 Ohms. This is essentially the 6V6 data-sheet's 14 Watt condition, doubled-up. So we know this is a 28 Watt part. Also we see the B+ was 305V, with 18.5V dropped in bias and 10V dropped in OT winding, which is again in line with 6V6 data-sheet's 14 Watt condition.

We see places for "replacements" from five major iron mongers. All blank. SAMS did not identify any part-number they were sure would work. (They do give two PT replacements and five choke replacements.)

_I_ would stick with a quad of 6V6 at 300V. i.e. keep the PT and the dual 5Y3. Call it over 30W guitar-rated. A quad 6V6 is a very nice amp.

5U4 can replace dual 5Y3, save a little space and 6W total heat.

A pair of 6L6 *might* work. I'd have to figure the loadline. Offhand I think the idle dissipation is a bit much for the 6L6GCs I have used/abused. Frankly, I think quad 6V6 is cheaper than pair 6L6 plus heavy thinking.

The "radical" (not very) alternative is to solid-state the rectifier for 400V B+, figure some form of fixed-bias, and run 6L6GC in a 5F6a-like condition. Gets you over 40 Watts. Not a big bump from 30W.

[drgonzonm]

I am not sure magnavox made their own pt's and ot's, if not, there may be manufacturer's reference numbers, and part numbers on them.  You might be able to go back to the original manufacturer and determine some specs, and then cross reference from that manufacturer's data.