how dangerous are floating ground amps (old Supro)

[HTH]

I've got an old Supro Bantam amp here with no PT, just rectified directly from the mains. It uses an 50L6 output tube and a 12AX7 preamp.

Its a 115vac model, so its currently being used here in the UK on 240v via a 1:1 isolation transformer with the twin 115vac primaries in series.

The mains is wired with B+ and B-, the chassis being referenced via a floating ground system via a 270k resistor and a cap who's value I can't read. This system appears very dodgy as you could have leakage current on the chassis is the cap becomes leaky. also, as it's still got a two-prong mains lead (currently using an adapter plug), the chassis could be hot.

This all seems VERY dodgy to me and I wouldn't be comfortable using it. Its not my amp and I'm just doing some maintenance work on it as the owner couldn't find anyone willing to look at it. I'm always up for a new challenge, so I'm taking a look, but I'm really worried about him using it the more I read about these type of amps. Should I be worried?

What are the typical safety steps to take to make the amp safer? - I'm thinking that a 3-prong mains lead with the chassis referenced to mains earth would be a start. also fusing the primary on the 1:1 isolation transformer. I'm gonna replace the electrolytic caps and also the resistor+cap being used to float the B- connection.

Anything else?

[Steve Conner]

Since you already have an isolation transformer, you can just get rid of that resistor and cap, and ground the chassis directly to the ground wire of the mains lead. Make sure this grounding goes solidly through to the sleeve of the input jack too.

Even if you didn't ground this amp, it would still be safer than plugging it directly into the mains, because the isolation transformer breaks the circuit that would electrocute you. But direct grounding may reduce hum, and will protect you if the isolation transformer ever fails.

The primary should certainly be fused, to prevent a fire if anything shorts out.

[HTH]

Since you already have an isolation transformer, you can just get rid of that resistor and cap, and ground the chassis directly to the ground wire of the mains lead. Make sure this grounding goes solidly through to the sleeve of the input jack too.

Even if you didn't ground this amp, it would still be safer than plugging it directly into the mains, because the isolation transformer breaks the circuit that would electrocute you. But direct grounding may reduce hum, and will protect you if the isolation transformer ever fails.

The primary should certainly be fused, to prevent a fire if anything shorts out.

this is the schematic of the amp Steve. there are some things such as the volume control and cathodes that would normally be grounded that are connected to B-, would you still get rid of the floating ground and just earth things as per normal in a modern amp?

[R.G.]

I've got an old Supro Bantam amp here with no PT, just rectified directly from the mains. It uses an 50L6 output tube and a 12AX7 preamp.

Its a 115vac model, so its currently being used here in the UK on 240v via a 1:1 isolation transformer with the twin 115vac primaries in series.

The mains is wired with B+ and B-, the chassis being referenced via a floating ground system via a 270k resistor and a cap who's value I can't read. This system appears very dodgy as you could have leakage current on the chassis is the cap becomes leaky. also, as it's still got a two-prong mains lead (currently using an adapter plug), the chassis could be hot.

This all seems VERY dodgy to me and I wouldn't be comfortable using it. Its not my amp and I'm just doing some maintenance work on it as the owner couldn't find anyone willing to look at it. I'm always up for a new challenge, so I'm taking a look, but I'm really worried about him using it the more I read about these type of amps. Should I be worried?

If I understand you correctly - YES! BE WORRIED!

The part that concerns me is the "1:1 isolation transformer with the twin 115vac primaries in series." If that means a 120:120:120 isolation transformer, it's fine. If it means a two-winding 120:120 transformer with both sides in series to stand up to 240Vac and the 120 for the amp taken off one side of the line and the center of the two windings, it's dangerous as a rattlesnake.

What are the typical safety steps to take to make the amp safer? - I'm thinking that a 3-prong mains lead with the chassis referenced to mains earth would be a start. also fusing the primary on the 1:1 isolation transformer. I'm gonna replace the electrolytic caps and also the resistor+cap being used to float the B- connection.

Anything else?

If it were me, and the isolation transformer is as I feared, I would personally not touch it until the owner let me put in a proper isolation transformer that isolated it from the AC line, much like the first connection I mentioned (either 240:120 isolation or 120+120:120 isolation) so there was no copper connection between primary and secondary. I would deliver the full lecture about why I would not work on it, too, and the possibility of death from using it.

If the owner wanted to go ahead with fixing, I'd convert it to a normal 3-wire setup, which is likely to be quieter as well as safer.

Since you already have an isolation transformer, you can just get rid of that resistor and cap, and ground the chassis directly to the ground wire of the mains lead. Make sure this grounding goes solidly through to the sleeve of the input jack too.

Even if you didn't ground this amp, it would still be safer than plugging it directly into the mains, because the isolation transformer breaks the circuit that would electrocute you. But direct grounding may reduce hum, and will protect you if the isolation transformer ever fails.

I read his description of "120 to 120 isolation transformer" differently, that being a 120 to 120 isolator that had been cross wired into a 240:120 autotransformer, non-isolating. I'm prepared to feel foolish reading it that way if by chance I can save the guy's life...

If your reading was correct, yes, I'm in complete agreement.

The primary should certainly be fused, to prevent a fire if anything shorts out.

OOOH, yeah! Goes without saying!

[Steve Conner]

No need to panic! (hopefully!)
We're in the UK, so a 1:1 isolation transformer bought here would be 230:230. Most I've seen actually have two 115v windings on each side, so you can have a free conversion between 115 and 230 if you like. I assume this is the case here.

Either way, ground the chassis and all that stuff that was floating. B- is ground now. You need to do it anyway, and if there is an isolation problem, it'll convert that to an obvious short across the mains.

[R.G.]

No need to panic! (hopefully!)
We're in the UK, so a 1:1 isolation transformer bought here would be 230:230. Most I've seen actually have two 115v windings on each side, so you can have a free conversion between 115 and 230 if you like. I assume this is the case here.

I'm not familiar with the UK style of isolators. If they have 120 isolated somewhere, that's great. The only ones I know of here in the USA with 120 and 120 would be two-winding only, and the stacked-winding is the only way I could see to make that 240-120.

If it's as you guessed, yep, it's still isolated and safe. I just overreacted on the safe side...

[HTH]

thanks for all the help on this one and also putting my mind at ease.

the transformer that the owner supplied and has been using previously has two 115v windings on the primary side and the same on the secondary side. it's got the primaries in series so we can feed 240v into it, then the two 115v secondaries are connected in parallel.

I've just rewired it like every other amp I've ever seen - cathodes, pots etc... referenced to the chassis. The weird rectifier and heater hookup is completely isolated from ground now. The heaters are in series and somehow it all seems to work.

I don't feel that the on/off switch built into the volume pot is such a great idea either. Would prefer a mains on/off toggle switch wired in there, but I'll have to ask before drilling this guy's 50+ year old amp.

The tone is pretty flat when I tried it earlier on - I'm trying to convince him to let me tweak it a little. its such a simple circuit now I've got my head round it - much like a tweed champ.

[HTH]

ok, so I've powered it up and I'm getting no sound at all.

here's the modified amp schematic...

theres probably something insanely simple I'm overlooking, but I can't see it.

the heaters are fine, I'm getting 13vac and 53vac on the 12AX7 and 50L6 respectively. no B+ though - its obviously the weird rectifier arrangement that I've changed.

if someone can spot the mistake and point me in the right direction I'd be grateful. I'm pretty sure its just the rectifier hookup.

oh, and here's the original schematic... http://www.schematicheaven.com/barga...upro_s6611.pdf

[R.G.]

theres probably something insanely simple I'm overlooking, but I can't see it.

the heaters are fine, I'm getting 13vac and 53vac on the 12AX7 and 50L6 respectively. no B+ though - its obviously the weird rectifier arrangement that I've changed.

if someone can spot the mistake and point me in the right direction I'd be grateful. I'm pretty sure its just the rectifier hookup.

It is. If you have it wired to the schematic, the negative side of the B+ filter caps are open compared to the incoming 117Vac (which is no longer the AC power line). The filament supply is connected across the 117Vac, so it gets power.

For proper operation
(1) bolt that transformer down inside and make it not-easily-removable.
(2) put the power switch and fuse in the primary of the 240Vac on the line side and hardwire the 117Vac secondary.
(3) properly three-wire the incoming AC, with green-wire ground to the chassis ground.

[Prairie Dawg]

It is. If you have it wired to the schematic, the negative side of the B+ filter caps are open compared to the incoming 117Vac (which is no longer the AC power line). The filament supply is connected across the 117Vac, so it gets power.

For proper operation
(1) bolt that transformer down inside and make it not-easily-removable.
(2) put the power switch and fuse in the primary of the 240Vac on the line side and hardwire the 117Vac secondary.
(3) properly three-wire the incoming AC, with green-wire ground to the chassis ground.

This setup "Transformers? We can't buy any-accounting shot us down! Think of something quick" was quite common in AA5 radios which were designed for low cost. It may well have been RCA's doing-they were pretty well known for helpfully developing applications for stuff they'd cooked up.

At least you didn't have a resistance cord-a/k/a rug igniter, and at least the amp case is not metal.

The Arvin people (who are still around by the way see http://www.arvinmeritor.com/about/history.asp) well, their stock in trade was metal stampings, so they came out with a line of stamped metal case radios of 3 and 4 tube variations-I have a number of them. All well and good as long as you kept them away from a path to ground, and the heavy coat of paint helped even though you could still feel a little tingle if you touched the case and not the plastic knob.

So when I see that some bright fellow has stripped and chrome plated an Arvin I get nervous.

[HTH]

It is. If you have it wired to the schematic, the negative side of the B+ filter caps are open compared to the incoming 117Vac (which is no longer the AC power line). The filament supply is connected across the 117Vac, so it gets power.

For proper operation
(1) bolt that transformer down inside and make it not-easily-removable.
(2) put the power switch and fuse in the primary of the 240Vac on the line side and hardwire the 117Vac secondary.
(3) properly three-wire the incoming AC, with green-wire ground to the chassis ground.

Thanks for the help R.G., but just to clarify since I'm being especially thick here, all that I need to do is connect the filter cap ground (i.e. mains ground) to the 117vac negative side ???

Like in this schematic....

[52 Bill]

The Arvin people (who are still around by the way see http://www.arvinmeritor.com/about/history.asp) well, their stock in trade was metal stampings, so they came out with a line of stamped metal case radios of 3 and 4 tube variations-I have a number of them. All well and good as long as you kept them away from a path to ground, and the heavy coat of paint helped even though you could still feel a little tingle if you touched the case and not the plastic knob.

I used to have one of those three tube Arvins that I would listen to in bed at night under the covers. The tingle was part of the fun! My dad worked for Arvin back in the day.

[R.G.]

Thanks for the help R.G., but just to clarify since I'm being especially thick here, all that I need to do is connect the filter cap ground (i.e. mains ground) to the 117vac negative side ???

Like in this schematic....

Yep. I think that will make the B+ work. If it doesn't, yell.

[HTH]

Yep. I think that will make the B+ work. If it doesn't, yell.

ok, I've got B+ now, but very little volume and suspect the wacky plate and cathode values to be the cause.

EDIT: I've took a fairly drastic approach and instead of massaging the amp back into life, I've just rewired the preamp how I feel it should be going by instinct of what I KNOW works. I'm glad to report that something approaching a Champ preamp has done the job. Also rewired the B+ line so that the preamp has its own filter node separate from the screens and plate.

EDIT2: big love to R.G., Steve and everyone else who helped with this. I've learned a lot working on this amp and the owner has a kickass amp thats not gonna kill him.

[HTH]

Just an update on this one for future reference if anyone comes across one of these amps.

The stock preamp sucks, not just a little, but 100%. I rewired to something approaching tweed Champ specs but the tone was just not quite groovy enough.

I found an old Magnatone 107 schematic thats quite similar to the Supro Bantam and sounds REALLY cool... http://www.freeinfosociety.com/media/pdf/4908.pdf

Its got some nice twang and snap to it that my Tele is loving it - I'll be sorry to see this one go back to the owner.