Cool old amp. I have never been able to get any info on it...

[jbow]

The amp is a Stewart with a double TV front and two speakers. I've never seen anything like it, or another one.
It uses 2 50L6 power tubes. It looks really cool to me. When I got it someone had crammed 2x8s into it but they had a piece of tape between one of the speakers and a tube because they didn't fit. I made a new baffle and installed 2x6 Webers. Going to wire them up tomorrow too... I thought someone might like seeing it. Yeah, it needs a cleaning too!
... have you ever seen one?






I hope it sounds as cool as it looks!

Julien

[nashvillebill]

Looks cool!

Bear in mind: this is almost certainly a hot chassis design. Make certain you do not get electrocuted!

[jbow]

Like an All American 5? I'll be careful. I didn't think about that, THANK YOU!!!! I have a cord from a curling iron with a built in circuit breaker for AM5s. I will use it.

One hand!

J

[olddawg]

Like an All American 5? I'll be careful. I didn't think about that, THANK YOU!!!! I have a cord from a curling iron with a built in circuit breaker for AM5s. I will use it.

One hand!

J

Use the appropriate isolation transformer. You can even mount it inside the amp.

[jbow]

I looked again. It has two transformers. One smaller, obviously the OT and a slightly larger one that looks about the same only bigger. I don't know enough to know if it might be a choke but I have not powered it up in a long time. I'll open it up and see how the larger one is wired, if I can't figure it out I'll post a pic of the wiring in the chassis. Thanks for the warning, I never thought about it. I am careful with AM5 radios but this never crossed my mind on a guitar amp.

J

[Tom Phillips]

It has two transformers. One smaller, obviously the OT and a slightly larger one that looks about the same only bigger. I don't know enough to know if it might be a choke but I have not powered it up in a long time. I'll open it up and see how the larger one is wired...

A tip off is the 50L6 power tubes. They have 50 Volt heaters and are therefore designed to be used in circuits without a power transformer. What are the other tube types? If the "larger transformer" has only two wire leads that will confirm that it is a choke.

Cheers,
Tom

[jbow]

Thanks Tom.
Both transformers have four wires. I opened it and took some pictures but I think it is an odd design. The chassis is just ne piece of metal, folded. The pots and inputs are right on the chassis, it is just painted. So I don't see how it could be hot since you can't help touching it, but it still confuses me. I need to swap out the power cord too as it is a two prong... but at this point I want to be sure what I am doing so I will post some pictures in a bit.

Julien

[olddawg]

Thanks Tom.
Both transformers have four wires. I opened it and took some pictures but I think it is an odd design. The chassis is just ne piece of metal, folded. The pots and inputs are right on the chassis, it is just painted. So I don't see how it could be hot since you can't help touching it, but it still confuses me. I need to swap out the power cord too as it is a two prong... but at this point I want to be sure what I am doing so I will post some pictures in a bit.

Julien

Is it a pp amp or 2 single ended power amps driving one speaker each? One transformer may be just for the HV and the the filaments are series with the AC line. I would draw my own schematic, at least for the power supply and the filanents.

[jbow]

Is it a pp amp or 2 single ended power amps driving one speaker each? One transformer may be just for the HV and the the filaments are series with the AC line. I would draw my own schematic, at least for the power supply and the filaments.

I am not that good... to draw my own schematic, but it would be a good learning project! Drawing a schematic would be a huge learning help to me, I started tube electronics a little late in life. Rather than describe it or wait I have some pictures. Here they are:

Thanks for the help!! You guys may have kept me from hurting myself!
I will add a three prong cord to it before I power it up again.

Julien

[Jazz P Bass]

Rather than drawing the schematic right away, why not start with drawing the layout.

From that you can develop the schematic.

[olddawg]

There are many on this board more knowledgeable than me. But that looks like a selenium rectifier connected to one transformer. That would be your B+ supply. The other transformer is your OT. So... I would suspect it is a series filament push pull amp with those 50L6 tubes. I wouldn't put a chassis ground on it. I would use an isolation transformer. And ....I would replace that selenium rectifier with some diodes to start. Be careful, they can be quite toxic. Interesting. I have never seen a setup like this.

[jbow]

There are many on this board more knowledgeable than me. But that looks like a selenium rectifier connected to one transformer. That would be your B+ supply. The other transformer is your OT. So... I would suspect it is a series filament push pull amp with those 50L6 tubes. I wouldn't put a chassis ground on it. I would use an isolation transformer. And ....I would replace that selenium rectifier with some diodes to start. Be careful, they can be quite toxic. Interesting. I have never seen a setup like this.

OK... I have do a little searching and don't understand where or how to install an isolation transformer. A Google search turns up all kinds of results, it is something I have never dealt with. Is there a resource you know of online, perhaps a YT video or any instructional info on this particular IT application? I saw some mounted on the speaker frame and some small round isolation transformers. I am ignorant about isolation transformers but I can learn. I have several tube audio books I can check too, and I will.
Thanks for the help. It does seem to be a rare amp, I've looked and have never seen another one, not a picture, not a mention. I may take it by ACK Electronics in Atlanta. There a boutique builders who come in there and there is one guy who works on amps, on the side, there.

Julien

[Jazz P Bass]

The isolation transformer is simply a transformer that goes between the power outlet and the amps power plug.
110Vac to 110Vac.
The only requirement is that the transformer must be sized (the VA rating) to meet or exceed the amplifier draw.

[Tom Phillips]

We don't have the benefit of reviewing the complete schematic of this amp. However, if it is just the tube heaters that are in series and connected directly to the AC line AND the B+ supply is derived from the secondary of a transformer AND a grounded power cord is properly fitted to the amp...Then I don't see the need to use an isolation transformer with the amp. After all, we don't feel the need to add an isolation transformer to a table lamp or our Christmas tree lights. Right?
Comments invited.
Tom

[nashvillebill]

Yep, I had a Sound Projects amp that was exactly like that: heaters fed from the AC line, but a power transformer for B+, and I did not see a need for an isolation transformer.

It would be great if the OP could sketch us a layout showing the wire connections, then we can figure it out.