Advice on 50C5 amp safety upgrades

[JMac52]

I needed a project, so appropriately I bought a little Sound Projects Co/Lectrolab/maybe Harmony also? R100B. I'll post before and after pics later; I have the tubes, 3 prong cable, knob and some other stuff on order, so for now I'm looking for some advice on a few power and safety related things.

I couldn't find a schematic for it, but someone did one for a Model 203 and the power section is exactly the same. Note it it does have a PT, but the heaters are run in series directly off the AC line. I will remove the death cap.

So my questions are:

1. Should I install a fuse? I ordered a fuse holder, but I only have 0.5A and 2A fuses. If I put a fuse in, what value should I use?

2. For the new power cable, should I wire line and neutral as shown with the (fuse) and switch on line, or wire like my 5F1 with the switch on neutral (and fuse on line)?

3. If my calculations are correct, given modern 120VAC line voltage that 130 Ohm resistor will be dissipating a little over 4 Watts. Should I beef it up in terms of power handling?
(It may be that this thing could be modified to do away with that resistor. There is an unused secondary tap, but of course until I get power to it, I won't know what the voltage on it is. However, since I'm not expecting this thing to sound good, I'm probably not going to want to go to that length to mod it).


Any and all help appreciated. Thanks!

 

[clydethecat]

A 1A slo-blo ought to be close to right, that's what's in a blackface Champ-Amp anyway.

 

[Jon Snell]

With the maximum heater - cathode voltage of 100volts, I would fit an isolating transformer for the heaters.
Very dangerous!

 

[BobbyZ]

The heaters are probably just grounded to the chassis at some point. So if you put a grounded cord on it, it might pop a ground fault outlet?
I'm not really sure of that, I avoid these amps like the plague.

 

[JMac52]

The heaters are tied to neutral. All of the chassis ground is on the secondary side of the PT (with death cap removed).

 

[printer2]

The heaters melting and shorting to the cathode is not a common event. If the chassis is grounded with a three wire cord in a fault situation the line would short to the cathode, through the cathode resistor to ground. A fuse on the heater circuit would not be a bad idea.

 

[Silverface]

So if you put a grounded cord on it, it might pop a ground fault outlet?
I'm not really sure of that, I avoid these amps like the plague.

I hate these amps. Damned widowmakers. The real cheapos made with masonite cabinets I disable and take to the hazardous waste dump. Nicer wood cabinet ones I strip, keep the cab and take the disabled chassis to the hazardous waste dump.

Always wire both the switch AND fuse on the hot side.

A grounded cord is absolutely useless.

The ONLY way to safely run it is with an isolation transformer.

If you don't want to do that, sell it, throw it away - or use it, but put a provision in your will that all your OTHER gear be donated to the annual TDPRI raffle.

I have no found a single one in 60 years of playing/working on amps that sounded good enough to bother with.

 

[corliss1]

Your 5F1 is wired incorrectly, even if it matches some of the more popular diagrams out there. I'd also cast my vote to either full isolation transformer for the whole amp, or one on the heater line, but as @Silverface said, these amps have never sounded good enough to me to be worth it. The sound and "worth" factor is completely up to you though.

 

[JMac52]

There is a power transformer as shown in the partial schematic, so not a true widow maker. The heaters are connected between line and neutral and with the line filter (death) cap removed, I don’t see how it’s any more dangerous than a light bulb in a lamp.

To be clear, I’m not expecting this thing to sound good.

 

[JMac52]

Your 5F1 is wired incorrectly, even if it matches some of the more popular diagrams out there. I'd also cast my vote to either full isolation transformer for the whole amp, or one on the heater line, but as @Silverface said, these amps have never sounded good enough to me to be worth it. The sound and "worth" factor is completely up to you though.

Yeah, my 5F1 is a Mojotone kit.

Adding a transformer is not going to be worth it for me, but I will explore the unused secondary tap for the heaters. That’s what Thomas Organ did in the Vox Student amp.

 

[Silverface]

There is a power transformer as shown in the partial schematic, so not a true widow maker. The heaters are connected between line and neutral and with the line filter (death) cap removed, I don’t see how it’s any more dangerous than a light bulb in a lamp.

Ah,I stand corrected. I see a 50C5 and I immediately think "widowmaker".

However, one correction to the above - yes, with the death cap removed the filament string is about as dangerous as a light bulb - but with the glass broken and the bare wires exposed.

What a weird design! I'd love to see the complete schematic - the preamp stage and the complete output stage.

To be clear, I’m not expecting this thing to sound good.

Neither am I. Although I WOULD like to see how it's designed, as far as a "restoration project" for playing purposes I think it's a waste of energy. It's strictly a technical oddity.

 

[JMac52]

Ah,I stand corrected. I see a 50C5 and I immediately think "widowmaker".

However, one correction to the above - yes, with the death cap removed the filament string is about as dangerous as a light bulb - but with the glass broken and the bare wires exposed.

What a weird design! I'd love to see the complete schematic - the preamp stage and the complete output stage.




Neither am I. Although I WOULD like to see how it's designed, as far as a "restoration project" for playing purposes I think it's a waste of energy. It's strictly a technical oddity.

I've marked up the 203 schematic to match what I have (2 inputs, no tone, plus some component value differences). If I survive, I'll post it and pics of the project.

 

[Silverface]

If I survive, I'll post it and pics of the project.

 

[Wally]

I've marked up the 203 schematic to match what I have (2 inputs, no tone, plus some component value differences). If I survive, I'll post it and pics of the project.

you have to love an optimist!

 

[BobbyZ]

Remember that schematics show how the electrons flow, not what they flow through. On the schmatic it looks like a wire goes from the "neutral" side of the heaters back through the resister to the power cord. In practice the chassis may be used for part of that connection. Since it originally had a two prong cord that wasn't polarized it'd plug in either way, making the neutral the hot. If one wire from the plug is connected to the chassis, you got a hot chassis.
If this amp is like that or not I don't know, but alot of old radios that run without a power transformer are. Those are relatively safe because the chassis is in side wood or plastic and the knobs are plastic. With a guitar amp the shield on the instrument cable hooks to the chassis and more often than not the strings on the guitar. That's where the shock hazard is.