Sorado Rectifier/Transformer Question

[truth_hz]

So I ended up bringing in my old sorado to a tech here in portland, he was recommended to me personally from bob @ eurotubes. He cleaned up a few things for me, including a bias problem I had with the amp. Great, right!

well he ended up replacing my rectifier tube with a 5u4g.

I have heard many mixed messages about the ability of the transformers to handle the extra filament current(over the stock GZ34). I searched all over this forum, and others, but really haven't gotten a definitive answer, as to possible PT damage from this swap. Some say its fine, some say no.

I dug around a bit and found some (supposedly original) specs for the old P-782 Dynaco transformers:

http://www.the-planet.org/pdf/transformer.pdf
(near the bottom of page 7)


http://www.triodeelectronics.com/dytr.html
(these are new-made clone transformers)

Both of these sources indicate that the 5v winding for the rectifier is rated 3A. This seems to confirm that running a 5u4, or similar would be just peachy.

Would you guys know of any reason NOT to believe these specs...Ever heard of someone blowing a PT from using a higher current draw Recto?

any advice or help would be greatly appreciated.

[Soundmasterg]

The power transformer in the Dynaco Stereo 70 was at it's total current limit with that amp, and sometimes blow out, but I've never heard any issues about the Dynaco Mk. 3 transformer, which is essentially what the 200S/Sorado, etc., are. If it is indeed rated for a 3 amp filament current load, then it should handle the 5U4G fine.

I'm wondering who that tech was and why they put that tube in there? Was it Jeff Brown at Leighton Audio perhaps? He's usually very good and usually doesn't mod something from stock without explaining things first. If it wasn't Jeff, then you may consider taking your amp to him next time. PM me for his contact info if you need it.

Putting in the 5U4G would bring the plate voltages down quite a bit and probably will bring it down under the rated max voltage for the filter caps, so that could explain why that was done. It will have a bit more sag than stock too, and if thats a bass amp application that you're using it for, then that might not be what you want. If its a guitar amp, then that could be good. You could switch back to the GZ34 but the amp would have to be rebiased. You could also use a Weber Copper Cap rectifier which is a solid state plugin that emulates the sag and voltage drop of the tube rectifier. Ted Weber sells those in many types to emulate most of the rectifier tubes from yesteryear, and they don't wear out. Check them out at his site www.tedweber.com . Of course if you used one of those, the amp would have to be rebiased too. If you go back to the GZ34 or the Copper Cap equivilant, then the plate voltages would be up over what the can cap is supposed to handle due to today's higher wall voltages as compared to when that amp was made. It is possible to fix that situation by adding some 350v caps connected in series in place of the first two filter sections in the amp. This is what I did in my 200S, and it gives a 700v rating for those first two sections and even better, the small caps used are easy to find and cheap so when they require replacement in the future, it should be easy to do.

If you like the way the amp sounds now, then run with it. If you want it back to the stock rectifier, then that can be done too. I sometimes work on amps for locals, so if you would like to have some work done, or listen to my modded 200S or my stock 2000S to compare, then shoot me a pm. I'm in the portland area also.

Greg

[truth_hz]

thanks for your input greg. The fellow bob recommended was Bill @ Eclipse Audio. Bob had told me that he has his sunns serviced through through this dude.

I ran the sorado as is at practice tonight, for a solid 3-4 hours, and while the PT warmed up a bit, it didn't seem too hot(pretty normal actually), and seemed to be working fine. I will probably switch it back to the stock GZ34 though, to be on the safe side.

I'm using this amp for guitar, and man does it kill. I didn't have to turn it up past 3 on the volume dial, and I was drowning out the other guitar player, with his JCM900 through a 2x12.

I really love the fantastic string to string clarity it has, and still has quite a bit of low end punch. I have worked on a few amps before, and I redid the AC wiring myself: 3-prong conversion, replaced the fuse holder assembly, disconnected the polarity etc. Bill just took care of the bias circuit for me, as I really didn't have the time to do it myself. I was kinda surprised myself that he put in a non-stock recto, but I figured that he must know what he's doing...given the recommendation from bob.

The can cap is original, as far as I can tell, but I personally tested the power supply resistors, and a few other resistors in the amp for drift, and asked bill use a capacitance meter to verify that the cap can was still stable. I probably --should-- replace it anyways, but there was no abnormal hum in the amp, and the voltages were stable, as stated above.

I'll pm you my e-mail address, as I would love to see a schematic of the power supply mod you were talking about. you're right on the money...series caps would be a good idea to up the voltage rating. With a tube recto, you're limited to what, about 40uf or so for the first filter stages, so 2 80uf caps in series should work right...? I have a bias probe, and know how to bias myself. the kt-88's i dropped in sounded much nicer to my ears than the stock 6550's, I really dig the tone.

Thanks for you gracious advice and insight on the matter. Sunn amps weren't even on my radar until I moved out here to Portland(Beaverton) earlier this year. 

-Morgan

[Soundmasterg]

No problem Morgan! I've never heard of that Bill guy you mention, but for guitar it probably sounds better with the 5U4G than with the GZ34 since the voltages come down a bit and it browns out a bit. I'll send you an email a bit later tonight and we can go from there.

Greg