Isolation 1:1 for widowmaker

[Smokebreak]

I've got this Supro Bantam 1611 here that runs off the wall line voltage. The tubes are an odd configuration, 12ax7, 50L6, 35z5 that I can't find a schematic for, so I'll have to trace one out to post when I get a chance. The 6611 is the closest I can find.
I need an isolation transformer. Is this one suitable?http://www.mouser.com/ProductDetail/Tri ... NUfEiAY%3d

[M Fowler]

That should work fine its 1:1 115v to 115v 50va same as the Allied Electronics isolation transformer I used.

[johnnyreece]

I used that Triad in my little throwback build. Seems to work fine to me! Here's the thread on that:

https://tubeamparchive.com/viewtopic.ph ... light=50l6

[Smokebreak]

Awesome. Thanks for the confirmation guys.

What a cool lil amp Johnny! Same output tube as I'm using. Nice name too

[Stevem]

Those tubes add up to only 97 volts so there a resistor in there to eat up the rest of the line voltage for the heaters.
Be sure to check the condition of that resistor as many times there near spent!

[johnnyreece]

Awesome. Thanks for the confirmation guys.

What a cool lil amp Johnny! Same output tube as I'm using. Nice name too

Thanks. I still have parts to build more of those...maybe I'll do that one of these days...I like using tubes that aren't mainstream. Good luck on your build!

[Smokebreak]

I've begun work on this amp, converting it over to a safe one.
I'm currently tracing out my amp, and will get into the isolation implementation in a bit, but preliminarily, I have a question about the grounding. Please reference the posted schematic, as the circuit is appearing close enough for the sake of this question.

Let's look at the first stage. The input jacks are connected to the metal chassis, providing both circuit and chassis ground, as is the first stage cathode resistor. Familiar enough!
Now the second stage cathode and volume pot(amongst other things) connects to the "neg" of the filter caps, then one side of the line voltage, which makes it dangerous(?). However, none of this business is connected to chassis ground.
It appears that all of this is referenced to chassis ground via a 150R/1W resistor in series with a .005, which goes to ground. All of THAT is in parallel with the 270K//.005 to ground at the bottom of the schematic.

What am I missing here? Why are all of these elements I'm used seeing go to ground going to 117V, and what determines what goes to chassis ground, and what goes to 117V?

[Smokebreak]

Here's the amp I have. I can't find a input reference to ground for the lo channel. The high channel gets it through the cathode of v1?
Check out the clever bypass switching for the High, or maybe bright channel.
I'll post my proposed schematic for the conversion tomorrow.

[Smokebreak]

Here is my proposed schematic for the rebuild of this Supro Bantam 1611 with the isolation transformer. I sure could use some extra sets of eyes before I start to go at it.

I'd like to continue to use the 35Z5 rectifier, but it's half-wave, so I've got a solid state bridge rectifier before it. Still running the heater string off the isolated 115VAC.

I've attached the scheme I traced from the original amp also.

I still don't understand why some of the "usual ground points" are straight to the chassis, and some are returned to the neutral side of the 115V line. For example, why is the volume pot connected to the line, and the preceding cathode to the chassis? Why the differentiation?

[martin manning]

I don't know what to make of the original schematic as drawn. I've seen the "floating ground" with the parallel cap and resistor between circuit ground and the chassis before, but I don't see how the first stage could operate with that between the bottom of the cathode resistor and circuit ground. Does the amp work now? I believe you could just put the isolation transformer between the line and the original line-in connections, and earth the chassis.

[Smokebreak]

I don't know what to make of the original schematic as drawn. I've seen the "floating ground" with the parallel cap and resistor between circuit ground and the chassis before, but I don't see how the first stage could operate with that between the bottom of the cathode resistor and circuit ground. Does the amp work now? I believe you could just put the isolation transformer between the line and the original line-in connections, and earth the chassis.

Exactly. Honestly, I didn't even try and turn it on.

I was hoping to just insert the iso after the line and be done with it, then came across this : http://www.instructables.com/id/Isolati ... r-problem/

My Iso is 50VA and basically that article says : As the name implies, half-wave rectification only uses one half of the AC waveform, and blocks the other half. Power transformers are really designed to be symmetrically loaded. The flux field collapses as one peak falls, and the transformer expects an equal load--and an equal amount of magnetic force from the complementary peak. Without a load on half the cycle, the collapse of the field causes the transformer core to become saturated much more quickly than normal. That puts a "standing" DC voltage on the transformer. The N-68X, being a small transformer, isn't designed to handle this.

That's where the bridge rectifier addition comes in, but things go over my head quickly, at this point.

Thoughts on whether his musings are accurate?

[martin manning]

It's true that a half-wave rectifier is less than half as efficient with respect to DC current available from a given transformer. I'd be inclined to hook it up as simply as possible and see how hot it gets, then decide if you need to go further. If you have a Kill A Watt device you could measure how many VA it draws in that configuration.

[JazzGuitarGimp]

I think I understand what's going on with the two grounds in the original design: the 270K resistor gives the user a current-limited path to the hot side of the line should the AC plug be plugged in backwards - the resistor potentially keeps the "lethal" out of "lethal shock hazard". The 50nF cap in parallel with 270K resiator provides a lower-impedance "ground" path for the AC signal between the first stage the rest of the amp. At first thought, deleting the these two components makes sense; simply tie all grounds to chassis once the isolation transformer is grafted in - but I'm thinking that "isolation circuit" might have an effect on the amps' sonic character. I would be inclined to keep it in the circuit, then, you can just put a clip lead across it temporarily to take it out of the circuit so that you can listen with it both in and out of the circuit.

As for the PT and the half-wave bridge concern: the only thing that will destroy the transformer is heat. Keeping that in mind, I would build it without the FWB first, and keep an eye (read: a finger) on the PT until you know whether or not you've got a problem. My guess is, the heater string is going to draw as much, if not more current than the B+ string, and the heater string, being unrectified AC, is not going to contribute to the overheating phenomena.

[Smokebreak]

Thanks for the input Lou! I'll adress that when I get to it, but I'm starting to think that network may have some other effect, lending to my current confusion, now that I just finished wiring, and ready to crack a beer...

I'm throroughly confused at this point. I got everything wired up, and am not getting any DC out of the rectifier that came with the amp, or another rectifier NOS that I bought for the amp.

I am getting AC coming in from the wall fine, and am reading 133VAC across the secondaries. As for my heater voltages, I'm not so sure how to interpret them. I'm looking for some help here. I've attached a drawing of exactly how I've hooked the amp up. Also, I grounded it like a "normal" amp, and removed the 270K to ground network coming off the old neutral line.

Even though I've got 133VAC(which I'm assuming is acceptable given a non fully loaded 115:115) across the secondaries, my plate of the rectifier only has 34VAC on it!!

Also, I've put a NOS 50L6 and a new 12AX7 in there but nothing changes.

The other possible clue is that with one of the rectifiers(not the one I have readings for here), the plate voltage jumps up to 45VAC or something.

Surely I don't have 2 bad rectifiers eh? Or a bad socket...

[Smokebreak]

Ok so looking at it, all my grounds are entirely floating and have no reference to any voltage, it looks like
Do I need to ground that bottom leg of the secondary?
Or prop up the heater string/secondary with that damn 270K/5n to ground?