The Dynaco Tube Audio Forum

Dedicated to the restoration and preservation of all original Dynaco tube audio equipment - Customer support for Dynaco VTA tube amp kits, all Tubes4hifi.com products and all Dynakitparts.com products


    Dynakit ST-70 and 5U4G or 5U4GB

    Share

    Tiziano73

    Posts : 29
    Join date : 2012-09-11
    Location : Italy

    Dynakit ST-70 and 5U4G or 5U4GB

    Post by Tiziano73 on Wed Oct 10, 2012 8:09 am

    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?


    Last edited by Tiziano73 on Wed Oct 10, 2012 2:29 pm; edited 1 time in total

    Bob Latino
    Admin

    Posts : 2411
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Dynakit ST-70 and 5U4G or 5U4GB

    Post by Bob Latino on Wed Oct 10, 2012 10:50 am

    Hi,

    Yes - You can use a 5U4 rectifier in place of a GZ34 tube rectifier in your ST-70. A 5U4 rectifier has a higher voltage drop than a GZ34 so the B+ will be a little lower. Two things will happen. One - Overall amp power will drop a little because the B+ is a little lower. Now, You may not even notice a slight drop in power unless you play the amp really loud. Two - The amp will have a fairly different bias point so you will have to rebias the amp.

    Bob

    Tiziano73

    Posts : 29
    Join date : 2012-09-11
    Location : Italy

    Re: Dynakit ST-70 and 5U4G or 5U4GB

    Post by Tiziano73 on Wed Oct 10, 2012 11:15 am

    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" Smile i can try other tube without fail Smile

    Thank you again

    All the best

    Tiziano


    GP49

    Posts : 732
    Join date : 2009-04-30
    Location : East of the sun and west of the moon

    Re: Dynakit ST-70 and 5U4G or 5U4GB

    Post by GP49 on Thu Oct 11, 2012 5:38 pm

    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.

    danskman

    Posts : 17
    Join date : 2012-01-14
    Age : 57
    Location : Bière, Switzerland

    Re: Dynakit ST-70 and 5U4G or 5U4GB

    Post by danskman on Fri Oct 12, 2012 2:04 am

    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 Cool , 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

    hapinoregon

    Posts : 1
    Join date : 2013-07-23

    5U4G for ST-70

    Post by hapinoregon on Tue Jul 23, 2013 4:08 pm

    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...

    Bob Latino
    Admin

    Posts : 2411
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Dynakit ST-70 and 5U4G or 5U4GB

    Post by Bob Latino on Tue Jul 23, 2013 4:29 pm

    hapinoregon wrote: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...

    I don't agree with Will ... MANY times over the years I have used a 5U4 in place of a GZ34 and never had a problem ... Dynaco themselves in the ST-70 manual mentioned that a 5U4 could be used in place of a GZ34 .. See below the page directly from the Dynaco ST-70 manual ..

    Bob


    GP49

    Posts : 732
    Join date : 2009-04-30
    Location : East of the sun and west of the moon

    Re: Dynakit ST-70 and 5U4G or 5U4GB

    Post by GP49 on Wed Jul 24, 2013 11:14 am

    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.

    Dave_in_Va

    Posts : 142
    Join date : 2013-04-02

    Re: Dynakit ST-70 and 5U4G or 5U4GB

    Post by Dave_in_Va on Wed Jul 24, 2013 2:18 pm

    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.

    Bob Latino
    Admin

    Posts : 2411
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Dynakit ST-70 and 5U4G or 5U4GB

    Post by Bob Latino on Wed Jul 24, 2013 2:32 pm

    Hi Dave,

    The VTA ST-70 has a 4 amp filament winding line. (the VTA ST-120 has a 5 amp filament winding by the way) From personal experience over 40+ years of dealing with these amps, I would say that that the suitability of using a 5U4 as an alternate rectifier applies to BOTH the original Dynaco ST-70 and the VTA ST-70.  In both cases the use of a 5U4 as a rectifier in these amps will cause a small loss of power due to a somewhat lowered B+. You will be hard pressed to notice any power loss in either case ..

    Bob

    jdm

    Posts : 45
    Join date : 2011-04-04

    Re: Dynakit ST-70 and 5U4G or 5U4GB

    Post by jdm on Mon Jul 29, 2013 11:24 am

    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?

    Sponsored content

    Re: Dynakit ST-70 and 5U4G or 5U4GB

    Post by Sponsored content Today at 11:51 pm


      Current date/time is Sat Dec 03, 2016 11:51 pm