Dynakit ST-70 and 5U4G or 5U4GB

Hi to everybody

I live in italy and changed stock st70 power trasformer with Triode PA521 power transformer. I changed also the quad filter cap with the new dynakitparts 30/20/20/20 uF @ 550/600 VDC FP (twist-lock) can style (1) 30 uF @ 550 VDC (600V) Surge (3) 20 uF @ 550 VDC (600V) Surge. High Quality "Low ESR" high voltage caps mounted in a twist-lock style can.
I mount also a stock PC-3 board with 6gh8 and a matched quad of NOS Telefunken.

I would like keep this rectifier instead of GZ34 Tungsram because i tried it (a GE 5U4GB) and i like how it sound (bass better controlled and scene with better defined contours).

Can i mount now for ever a 5U4GB rectifier or there may be some problem?

I read the transformer's specification and it support tube like this and with that capacitor the surge should be ok too... isn't it?

Thank you very much for your fast reply,

As you know my LS3/5a don't like very much powerful amplifier and the room isn't big, so i think the power with 5U4GB will be more than sufficient.

Know that i'm sure that nothing on my amplifier "will be smoke" i can try other tube without fail

Thank you again

All the best

Tiziano

Bob is absolutely correct. The 5U4GB will work and will not cause any damage to the ST-70, only reducing the output power a bit. Even Dynaco agreed. The manual for the Stereo 70 mentions the 5U4 (which was very common in the day as a rectifier tube in televisions) as a viable substitute.

I checked my ST-70 with several rectifiers, such as 5AR4 (modern chinese), JJ GZ34, Sylvania 5U4GB, Weber's WZ34 and WZ68; all these tubes were almost giving the same B+ (+/- 20VDC) and the same sound (to my poor ears)...
For three weeks, I use an 80' Winged C 5C4S (ciryllic 5ц4с), just for the look of it , with an NTC between pin8 and 1st filter cap (as this tube is given with a 4µF max 1st capacitor limit!); I must say that it runs flawlessly, with a B+ of 380VDC. Using a VTA board let me bias my amp correctly. I use soviet era's 5п3с (not the -E version) and I like them a lot. The basses are wonderful, after 20-30 hours using them.
Bob's kit power transformer doesn't runs too hot, as it was with KT88 or EL34. Different sounds with different tubes, but the ST-70 stays a gorgeous amp!!
Hope this can help,
BR,
Danskman

There have been several threads at various times on the suitability of subbing a 5U4G rectifier tube for either a 5V4G or a 5AR4 in a ST-70. I asked Will Vincent, a respected re-builder of ST-70s (among others) for his opinion and received the following email:

Harry: Never use a 5U4 in a Dynaco 70.....The power transformer is rated for 2 amps on the 5 volt winding....The 5U4 is rated at 3 amps......Not good......Will

So, I thought I should pass this along...

The "2A" rating of the filament winding in the Stereo 70 power transformer is probably calculated while taking into account heat buildup in the complete transformer when operating at capacity, at full continuous power. In more normal operation, things would be different.

Just for future reference, does this discussion apply to just the original Dynaco ST-70 or both the orig. and the current production ST-70 from Bob?
Thanks.

I think this has been covered before, how does the NOS Philips 5R4GYS compare to the GZ34 in the VTA ST-120? Recommended or not?