Paul pepco model 201 isolation transformer

[pontiacpete]

Hi, I have a Paul amp here that I want to put a grounded cord on using an isolation transformer.
Found this schematic on the web with one wired in.
I have a couple of questions and was wondering if someone could please help me with.
The suggested iso tranny is 60VA, is this about right for this amp?
Do I need to have the bridge rectifier ? as well as the 35Z5?

Thanks
pete

[Jazz P Bass]

Why not simply plug the unmodified unit into an Iso Tranny?

The schematic shows using a bridge rectifier to obtain the B+, which may come out a tad high.
So they ran it through the 35Z5 to drop some of that voltage.

[g1]

The reason they added the bridge rectifier is to present a balanced load to the iso xfmr. The 35Z5 is a half wave rectifier, so it is only using half of the sinewave. An unbalanced load like a half wave rectifier can cause the transformer to overheat.
The other option is to use an iso xfmr that is twice the required size. If I recall correctly, Loudthud recommended derating the transformer by half if running into a half wave rectifier.

[Mick Bailey]

Increasing the transformer rating would depend on whether the originally intended transformer has enough core area not to saturate with the DC component of half-wave rectification and not overheat. Adding a bridge would be the better choice, as it decreases ripple and increases the efficiency and utilisation of the transformer.

[Enzo]

The original was half wave because, well, what else could you do? And it worked well enough.

Is the iso really under any threat? I mean the half wave bias supplies and low volt positive supplies in other amps don't stress their transformers. The transformer in the Fender reverb unit is half wave rectified.

Looking at this amp here, there is the parallel string of heaters on that secondary, so it isn't like the entire current is half wave rectified. I won;t argue the transformer might run warmer, but HOW MUCH?

As to the 35Z5, it will be responsible for both sag and voltage reduction for your B+, but more important, it is a series element in the heater string. If you take it out, the bridge will still make B+ DC, but what will you use to drop 35v in the heater string?

You have a ??? by the heater string resistor. Figure it out. Current is the same through a series circuit, so we should all know that the 12v wiring of a 12AX7 needs 150ma (0.15A), so the other tubes must need that as well, so I won't look them up. Now the three tubes add up to 97v. (50 + 35 + 12) So that resistor has to suck up the remaining 23v. 23v at 150ma tells me by Ohm's Law that it must be 153 ohms. 150 ohms is close enough standard value.

And with that we can go back to the transformer. You have 150ma draw for heaters. That 50L6 wants maybe 50ma including the 12AX7 triodes. (The 35Z5 won't use much current itself, any that goes through it will be flowing through the other tubes) SO it looks like only about 25% of the secondary current is half wave rectified. I don't see the threat.



Oh, and what WOULD we do without the 35Z5? That would leave the 50L6 and the 12AX7 - 62v heater. 58v left to drop. At 150ma, we need 58/.150 = 387 ohms. 390 is a close standard value. But it is also going to dissipate 8.6 watts, so a 15-20 watt resistor. Might as well leave the 35Z5 in there.

[pontiacpete]

That ??? string heater resistor is 150 ohms 10watts, and it looks like it's the original.

[Enzo]

Cool, validates my arithmetic.

[g1]

Just to clear something up, the OP's amp is a widow maker using the 35Z5 rectifier. The schematic he provided shows an iso-tranny and bridge rectifier added. So he's wondering what size iso-tranny is sufficient and whether the bridge rect. is really needed.

You have 150ma draw for heaters. That 50L6 wants maybe 50ma including the 12AX7 triodes. (The 35Z5 won't use much current itself, any that goes through it will be flowing through the other tubes) SO it looks like only about 25% of the secondary current is half wave rectified. I don't see the threat.

Thanks for pointing out the bigger picture here. It makes sense that the bulk of the load on the iso-tranny is the heaters (balanced).
I'm starting to think this whole issue of the iso-trans overheating may be more because of "cheaping out" and using too small or marginal isolation transformers such as the N68X.

[pontiacpete]

So is Enzo saying I don't need to use the bridge rectifier?

Is the 60VA large enough with or with out it?

[loudthud]

The reason they added the bridge rectifier is to present a balanced load to the iso xfmr. The 35Z5 is a half wave rectifier, so it is only using half of the sinewave. An unbalanced load like a half wave rectifier can cause the transformer to overheat.
The other option is to use an iso xfmr that is twice the required size. If I recall correctly, Loudthud recommended derating the transformer by half if running into a half wave rectifier.

When I wrote that it was just a wild guess. The problem is current imbalance is rarely specified on any transformer data sheet. If you were having a transformer actually designed by a competent engineer, he would probably cut it a little closer. For our purposes here, the tech needs to be aware of the problem and make adjustments if needed. But mostly you want to do it once and be done, not have to shoehorn a bigger transformer in later if it gets too hot. Don't use a torroid, they really don't like the imbalance.

I just did a quick sim on PSUD2 and 50mA load current with 50uF filter gave an RMS secondary current of 120mA. That's about 15VA and the 150mA heater string is 18VA. So a 50 or 60VA transformer is about right. Use a smaller one at your own peril.

I finally got the schematic to load. There are several problems.

1) Heater string is not connected for lowest hum. You want the 12AX7's heater to have minimum voltage swing, it needs to be on the ground side of the power supply.
1A) If using the bridge, connect the heater string between the bridge and the tube rectifier.
1B) If not using the bridge, it's ok as drawn, just remove the bridge.

2) Death cap not removed and "ground" not modified from original circuit.

3) Some resistor values are wrong or can't be read.

4) Tone control probably won't do much on the cathode of the power tube. Copy something from a tweed Fender.

5) AC wiring does not meet current safety standards. Fuse and switch should be on the same side. Don't forget the fuse, 0.5A or slightly higher.

[pontiacpete]

Thanks Loudthud,
I ended up ordering a hammond 169RS iso transformer, secondary current capacity at 435mA.

[pontiacpete]

Installed the isolation transformer and tried the schematic as is above with the bridge rectofier. It hummed pretty badly.
Now I'm trying Loudthuds "1A" approach without the bridge. And wired the hot line to fuse - light - switch- primary. Hum is reduce quite a bit.
I grounded the secondary of the isolation transformer and filter caps and other end of heat string, now very little hum.
I think this worked. Thanks for the help.

As far as the tone control goes, it doesn't work. I'd like to get i tworking better, but
I'm not sure what to do there. What tweed tone circuit could I change it to?

[loudthud]

The common Tweed tone control can be found here: http://www.el34world.com/charts/Sche...RVARD_5F10.pdf

[pontiacpete]

Thanks Loudthud!