Magnatone tube amp identification and safety.

[MLW9110]

I bought a non-functioning Magnatone tube amp from a friend. The power cord was very bad, insulation falling off of it. Previous owner told me it hadn't worked in years (probably decades). It came with an Oahu Diana lap steel guitar that he bought second hand. The only identification on the amp is a metal plate on the bottom with Magnatone No. 4538 on it.
I'm pretty sure it's a Magnatone Starlet amp. Model 107. The speaker is an 8 inch Jensen made in 1967. I think the amp is older than that though. I think the power/volume control knob isn't original either.
My amp has a silver capacitor on the chassis that I don't see in other versions of the amplifier.
The cabinet has the "mother of toilet seat" finish. An ACOUSTIC sticker was put on top by the previous owner.

On to the technical part of the discussion!
I replaced the power cord with a modern one with a 3 prong plug. I thought it would be smart to put a ground on the chassis. The amp powered up and worked good before I attached the ground wire. After that it hummed loudly and didn't amplify the guitar input. I disconnected the ground wire and the amp worked normally. I then discovered that there was 120vac from chassis to ground with the amp turned on. No voltage to the ground with the amp turned off.

I figured that if I swapped the hot and neutral wires from the power cord, the chassis would be cold when on. I did that and felt that it would be safer that way. Someone more knowledgeable than me informed me that this is a "hot chassis" amp and that I should connect the power so the chassis was cold with power OFF. So I put it back that way.

The amp is working with very little hum and has a nice clean tone. I played my National single-cone resonator with a Highlander pickup through it and it sounds pretty good.

Now I'm not sure what is safer, hot chassis with switch on or off. I've read about putting an isolation transformer in but I'm not sure that's necessary.
I'm attaching pictures of the amp and the schematic. The amp has a 12SL7GT tube instead of the 12AX7 on the diagram. The silver capacitor isn't shown on the tube layout drawing.
I hope someone can help me with advice on this amp.
Mike

 

[musipol]

If it was mine, I'd use an iso xfmr.

 

[MLW9110]

If it was mine, I'd use an iso xfmr.

You're probably right. I found this detailed discussion today. It even addresses the schematic I attached to this post. I need to decide whether to add one and how it should be wired. The link describes how to do it but not specifically with my amp.
I think I would add a separate power switch and fuse somehow.

https://robrobinette.com/Widowmakers.htm

 

[CJ]

Nice amp!
That celluloid covering can be brittle, and it is highly flammable. You can redo the amp without all the shock hazards if it is a player. If it us going on Reverb then keeping it stock might get you more $$.
Got a pic of the lap steel?

 

[MLW9110]

If I added an isolation transformer, I would leave the amp as original as possible.

This guitar on Reverb is very similar to the one my friend has.
https://reverb.com/item/6288207-oahu-diana-lap-steel-guitar-1949

 

[MLW9110]

After more research I know quite a bit about this amp now. The closest thing to it I've been able to find is a Magnatone Starlet.
I don't think the speaker was made in 1967 now. What I thought is a date code might be a model description. The date code is the 220734. Jensen (220), year (7), 34 (week). The 7 could be 1947, 1957 or 1967. If the numbers on the tube bases are date codes, it would suggest the amp is newer than 1948. So I'm assuming 1957 for now.
MFP-8-67 could be a model ID. I haven't been able to decode it yet. F could be "Field" and P could be "Alnico". -8 could be 8". 8-67 also could mean mfg date of Aug 1967 which happens to coincide with the 34th week of 1967.
No idea yet what MC8R.C7492-2 means yet. I also found 308Y03HI printed on the speaker cone. The second 3 could be a 5 or 8 though.

 

[Philbo King]

Those old hot-chassis amps will kill you if things go wrong in just the right way. Be careful.

 

[CJ]

More dangerous to drive a car or shoot crack or have unprotected sex with multiple partners while shooting crack and driving a car.

But easy to convert into Champ circuit and get away from the funky tubes and series string.

50 percent of the sound of a guitar amp is the speaker and the cabinet, so if you use a 6v6 you will not be able to tell much difference between the stock amp.

 

[Hubbub]

1) Leave the amp wiring basically as is for preservation purposes, then
2) Add an isolation transformer between power cord and the amp so you don't kill yourself, then
3) Connect the ground wire of the power cord solidly to the amp chassis.

Get in the habit of unplugging the power cord when the amp is not in use, so that the isolation transformer doesn't run idly. But don't be overly concerned if it is left connected now and then.

The amp may work better with the .05 cap and 170K resistor (seen paralleled to "ground") shorted out. With the isolation transformer they are no longer needed for "safety".

Don't be surprised if the B+ filter capacitor needs replacement. it looks to be a 30+50 uF at 150V, which was quite common back then. In the past I have gutted an old filter cap and packed new caps in the shell, even using the existing wire leads from the old cap. This keeps the appearance vintage while the performance is up to date.

 

[CJ]

You can wire the iso xfmr to the power switch.

Or use a wireless for your guitar.

What this amp really needs is a 1/2 amp fuse.

Try to find an iso xfmr with a tap. Sometimes they run hot on the sec side. This would not be good on an amp designed for 110.