200S Diode Mod

[Greg_M]

As all fans of the 200S Sunn amp know it is essentially a Dynaco Mk III. I use Dynacos for HiFi and so I frequent a Dynaco Forum. This topic came up recently and I thought I would pass it on. I plan to do the mod, as according to the thread it extends the life of the 5AR4/GZ34 tube.

Here are some excerpts from the thread......

Unfortunately today's new manufactured 5AR4 tubes are not built like the good old USA or Mullard 5AR4 of years gone by.
What your friend can do is a easy mod that most likely help the life of the 5AR4 tubes, he should remove the wires from pin 4 and 6 on the 5AR4 tube socket, move them to pins 5 and 7 and solder a 1N4007 (UF4007) diode from pin 5 to pin 4 and a diode from pin 7 to pin 6. The anode of the diode (stripe on diode) must face pin 4 and pin 6.

Regards,
Sal
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After I did this mod the voltage readings on pins 4 and 6 decreased quite a bit...to about 203VAC. Pins 5 and 7, the pins with the PT leads, still test normal at about 379.5VAC (albeit a little high.)
I assume this is normal?

Ed
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After the diodes on pins 4 and 6 you have to set your meter to DC. You will measure pulsating DC before the filter capacitor, set your meter to DC and measure at pin 8 of the 5AR4 tube.

Sal
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Call me crazy, but I think the PT's in both of my 70's are running a tad cooler after this mod?
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Here is the included diagram




 

[Oli]

Thanks for the info!

[Johnny Guitar]

Could someone explain what this does. And what is the difference in a GZ34 and a 5AR4?
Thanks,
Johnny G.

[Greg_M]

Apparently changes the AC to pulsing DC
GZ34 is the old/another name for the 5AR4



Diodes only allow 'lectricity to travel in one direction through them. So if you put AC through one it it would not allow the 'lectricity to flow in the reverse direction, giving only pulses of (essentially) DC. Add a capacitor to store up and smooth the pulses and you have a DC power supply.

I guess it makes the 5AR4 tube happier to only have to deal with the pulsed DC rather than full AC

[Soundmasterg]

This mod essentially allows the solid state diodes to do the rectification (changing AC to DC) and the tube is in place for it's voltage drop and sag effects and that is it. So yes, it is stressed a lot less than it might be otherwise, but the power transformer shouldn't run any cooler as it is still supplying the same current levels as before. This mod is recommended to get around poor quality 5AR4/GZ34's mainly. You can also use a solid state replacement module such as a Weber Copper Cap which actually emulates the voltage drop and sag of the tube rectifier to a certain extent. I've A/B'd one in my AC30 with a NOS Amperex GZ34 and modern GZ34's, and it sounded just about as good as the others. I carry one as a backup since they don't go bad if you run them within their ratings. If you put one of these in instead of the tube rectifier, and didn't add the diodes, then you would still get the voltage drop and sag effect, and your transformer would run cooler since it wouldn't have to supply the 5v filament current and the plate current for the tube rectifier.

Greg

[Greg_M]

.......but the power transformer shouldn't run any cooler as it is still supplying the same current levels as before.
Greg

Just to clarify....
Things in italics are quotes from a thread on another forum, not me. I have not tried this mod yet

[EdBass]

For those who really want the "factory" performance out of their restored/refurbished tube rectified Sunns, I have a line on a bunch of incoming NOS Mullard GZ34's.
I believe they are currently in transit "across the pond", the selling price here (USA) hasn't been determined yet.

Not the metal base F32 codes, either.
These are the "real deal"; actual NIB, unused fat base "F31" code Mullard branded Mullard GZ34's, which are getting extremely scarce, and are pretty much considered the "cream of the crop" as far as rectifier tubes by most tube-o-philes, and as such won't be cheap.

If you are familiar with the above nomenclature, you probably already know that, though... 

However, they unquestionably should "rectify" any rectifier tube issues with an old Sunn without needing this clever "rectifier in a rectifier" mod. 

PM me if anyone is interested, I'm mostly a SS rectifier kinda guy and won't need many of them for myself.

[Soundmasterg]

.......but the power transformer shouldn't run any cooler as it is still supplying the same current levels as before.
Greg

Just to clarify....
Things in italics are quotes from a thread on another forum, not me. I have not tried this mod yet

Yah, I know that...I just wanted to point out for anyone who was considering doing this mod what the affects would be.

Greg

[EdBass]

Well, the GZ34's have survived the trip;



Three boxes full;



Early 60's production, just the thing to "rectify" one's old Sunn's power supply situations...

[Greg_M]

Yowza!

What did you want ea?

[Soundmasterg]

That must have cost you a healthy amount of money Ed!

How much you want each for them?

Greg

[EdBass]

Well, not my personal money, but a boat load of it for sure! $149 each, they don't appear to be the "fat" based late 50's as I assumed; probably early 60's production, but for F31 code (not F32 or F33) Mullards that's a deal based on my research.
If anyone is interested, drop me a PM. I don't really want to turn this in to a classified ad and detract from the original thread, and they bulk of them are already spoken for anyway.
It just so happened that I heard about the GZ's incoming right before I read Greg_M's thread, and I mentioned them so maybe some fellow Sunn-o-philes could snag a few.
The GZ34's are just a fraction of the total tube types "passing through", if anyone is looking for pristine NOS glass drop me a PM, I might be able to help.

Now, back to your regularly scheduled programing...