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    Another Stereo 70 Resto-Mod Completed

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    tomlang

    Posts : 34
    Join date : 2009-08-12

    Another Stereo 70 Resto-Mod Completed

    Post by tomlang on Tue Nov 03, 2009 9:24 pm

    Here is my ST70 I just completed. Many probably would not choose solid state rectification (there is 600 uF on-board and NO hum) but most will probably agree the amphenol jacks in the front do a good job of providing modern physical separation of the input jacks while covering up (just barely) the old holes for the original jacks and especially the unnecessary stereo-mono switch hole.

    I did provide a thermister to limit turn-on in-rush and I am absolutely NOT convinced that "cathode stripping" of electrodes is anything to worry about (I have yet to see ANY data that says it is a problem).

    I get 465 volts (see below for more data) on the plates of the output EL34's and am using 12AU7's on the driver board with even lower gain parts than is specified. I cannot imagine how much gain there is to contend with (as in big loud with little preamp volume turn-up) with the 12AT7's and glad I did the 12AU7's.

    Transformer is BUGS old one (thanks BUGS), outputs A470's. Paint is rust-oleum hammertone on the chassis. Sockets are Omron PL08's but holes are enlarged some via sanding cylinders to fit.

    FYI, the EL34's shown had some weird distortion going on with loud but not too loud listening. Bias adjustments did not seem to help. These EL34's are not new though. Put in some Westinghouse 6L6WGB's biased at 40 mA and huge difference sounds better. Disclaimer...I did not have the OPT's grounded, see below.





    Last edited by tomlang on Tue Nov 17, 2009 11:35 am; edited 2 times in total

    Bob Latino
    Admin

    Posts : 2378
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Another Stereo 70 Resto-Mod Completed

    Post by Bob Latino on Thu Nov 05, 2009 2:27 pm

    Hi Tom,

    Nice work, the amp looks great ... Very Happy I love seeing custom ST-70's in a variety of different setups. I am also not convinced of so called "cathode stripping" of output tubes by the use of a "non delayed" solid state rectifier. The Dynaco ST-35 and SCA-35 amps both used solid state rectifiers with no time delay and thousands of these amps are still working perfectly to this day after 40+ years of use.

    Any chance we could see a photo of the interior wiring of your custom ST-70?

    Bob

    Bugs

    Posts : 91
    Join date : 2009-04-17

    Re: Another Stereo 70 Resto-Mod Completed

    Post by Bugs on Thu Nov 05, 2009 4:59 pm

    Hey Tom,

    Nice job, that is a very nice looking ST 70. I'm glad the transformer worked out for you, I'd be stubbing my toe on it if it was still sitting around the apartment. What tube sockets did you wind up using?

    Best,
    Doug

    tomlang

    Posts : 34
    Join date : 2009-08-12

    Re: Another Stereo 70 Resto-Mod Completed

    Post by tomlang on Thu Nov 05, 2009 5:12 pm

    I'm using Omron 8 pin sockets, #PL08. They're advertised as being for relays but I thought I'd try for tubes. But like I said, I had to enlarge the holes on the chassis (since I was doing other cutting of the chassis for one of the caps and the front jacks I went ahead and ground/sanded the chassis holes bigger) as I felt this was easier than modifying the sockets. The sockets are cheap, a buck-seventy each at Mouser.

    I like to maintain a vintage look, that's why black (no pink, green, etc.) transformers for me and I thought the hammertone looked the part as well.


    Last edited by tomlang on Mon Nov 23, 2009 11:15 am; edited 1 time in total

    tomlang

    Posts : 34
    Join date : 2009-08-12

    Re: Another Stereo 70 Resto-Mod Completed

    Post by tomlang on Sat Nov 14, 2009 6:17 pm

    This is a revised pic of underneath showing the proper grounding of the output transformers to the star ground. At first I did not have this ground and had distorted sound and bad waveforms as shown below.

    Never mind all the heat shrink. The transformer prev owner thought it was bad and left just enough wire length for me to solder onto.



    Last edited by tomlang on Mon Nov 23, 2009 11:15 am; edited 1 time in total

    Bob Latino
    Admin

    Posts : 2378
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Another Stereo 70 Resto-Mod Completed

    Post by Bob Latino on Sat Nov 14, 2009 7:24 pm

    Hi Tom,

    It looks good ... As for your issue with amp. Sometimes on those older cloth lead output transformers with the BLUE and BLUE/WHITE and the GREEN and GREEN/WHITE wires, you can make a mistake with the colors because the color on the cloth leads fades and they look almost the same color. Just make sure that the BLUE and BLUE/WHITE leads go to pin #3 (the plate) and the GREEN and GREEN/WHITE leads go to pin #4 (the ultralinear screen tap)

    One last thing.. What size thermistor are you using on the fuse post? I know most of them are CL-?? (30, 50, 80 etc.) and have you ever checked how long is the delay time before the high voltage comes up ? I was just curious as to whether it was 15 seconds, 30 seconds or a minute or what ?

    Bob

    tomlang

    Posts : 34
    Join date : 2009-08-12

    Re: Another Stereo 70 Resto-Mod Completed

    Post by tomlang on Sat Nov 14, 2009 8:05 pm

    Bob, the thermister is a CL60, 5 amp, 10 ohm and I picked it as an educated guess. I got it thinking with all the capacitance I might blow fuses on a cold start. Any delay of B+ is very short, few seconds.


    Last edited by tomlang on Mon Nov 23, 2009 11:16 am; edited 1 time in total

    tomlang

    Posts : 34
    Join date : 2009-08-12

    Re: Another Stereo 70 Resto-Mod Completed

    Post by tomlang on Sun Nov 15, 2009 2:38 pm

    Transformers were checked very carefully and were hooked up correctly. But I did reverse the wires from the phase splitters to the EL34 grids and no change (this was a suggestion from diytube board).

    I also moved neg feedback wires to other outputs and changed the nfb resistor significantly.

    The following pics show what I have on each channel. No nfb in first pic (I get rated output or nearly so) and then nfb hooked up. No other changes were made to scope settings, etc. as I disconnected and reconnected nfb wire.




    tubes4hifi
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    Re: Another Stereo 70 Resto-Mod Completed

    Post by tubes4hifi on Sun Nov 15, 2009 8:38 pm

    Hi Tom,
    without seeing or hearing the amp my first guess is that possibly the transformer outputs are wired wrong or out of phase, and it SHOULD have been EXTREMELY obvious when you swapped the output wires from the PCB to the tubes (a much faster and easier way to change phase than rewiring all the OPT leads). I don't know how small your NFB resistor is, but I usually try to keep it above 4.7K as the amp really was designed for around 6.8K to 7.5K for the NFB resistor.
    I'd also guess maybe you had the phase correct before, and then after you swapped the output leads from the PCB it was worse, but with so little NF maybe it wasn't an obvious difference (it should have made a HUGE difference).
    With the tubes out and the amp off, you can make sure you have the OPT leads correct, with an ohmmeter the plate lead will measure around 170 ohms and the grid lead (for UL mode) will measure around 130 ohms, both compared to the common B+ lead. Check BOTH halfs of the OPTs and make sure they are wired to the correct output tube and in the correct phase.
    I built a similar amp a dozen or so years ago with 1000uF on the B+, I used both a CL70 and a 50 ohm 10 watt resistor (in series) to slow the current surge into the large capacitors and even with those you'll get full B+ in around 5 seconds.
    Also make sure the PCB is only getting 400vdc of B+, you may need to increase the 2.2K 1 watt resistor I usually spec to much more than than, maybe 10K at 3w, the PCB draws around 7ma, you've got 475v, so 75v/7ma = 10K
    Roy www.tubes4hifi.com/VTA70wiringdiagram3.GIF

    tomlang

    Posts : 34
    Join date : 2009-08-12

    Re: Another Stereo 70 Resto-Mod Completed

    Post by tomlang on Sun Nov 15, 2009 10:17 pm

    I've got 15k as a fb resistor and have connected it to all outputs, i.e. 4, 8, 16 ohms. I will do the swap leads from the VTA board to the EL34's experiment again and look more carefully this time for any changes but it seemed like there was none before...

    For giggles, I also added another 10k in series with the cap/resistor feedback circuit and again, not good.

    I'll check the dcr's tomorrow...

    tomlang

    Posts : 34
    Join date : 2009-08-12

    Re: Another Stereo 70 Resto-Mod Completed

    Post by tomlang on Mon Nov 16, 2009 12:52 pm

    I changed more resistors and caps around no change. Here are my resistance reading and they are NOT what you say above.

    One side of ohmmeter connected to B+ lead, which is also a common point for both transformers.

    Trans #1, tube 1:

    plate to B+ 103 ohms
    screen to B+ 34 ohms

    Trans #1, tube 2:

    plate to B+ 90 ohms
    screen to B+ 30 ohms

    Trans #2, tube 1:

    plate to B+ 103 ohms
    screen to B+ 35 ohms

    Trans #2, tube 2:

    plate to B+ 91 ohms
    screen to B+ 30 ohms

    I'm certainly guessing both output transformers are bad, but am perplexed as to why they are both so consistent, one would think failures would not present themselves so exact in each case.

    FYI this unit had a shorted power trans when I got it. Who knows what it saw before, maybe ran for a period of time with no speakers connected, bias way high, etc. Thoughts?


    Last edited by tomlang on Mon Nov 23, 2009 11:17 am; edited 1 time in total

    Kegger
    Guest

    Re: Another Stereo 70 Resto-Mod Completed

    Post by Kegger on Mon Nov 16, 2009 3:21 pm

    Looking at your underside pic's I noticed something..

    Your output transformers ground is not grounded, that could defiantly cause strange things happening there.

    Kegger

    Posts : 5
    Join date : 2009-11-16

    Re: Another Stereo 70 Resto-Mod Completed

    Post by Kegger on Mon Nov 16, 2009 3:46 pm

    Alright I've registered now, Smile


    I'll be curious as to what you find out, I doubt highly you have 2 bad output transformers.

    The ohm readings you took look fine to me, some of these outputs measure a bit lower then others.
    (but the fact they are consistent with each other and track the taps properly I believe they are fine)

    tomlang

    Posts : 34
    Join date : 2009-08-12

    Re: Another Stereo 70 Resto-Mod Completed

    Post by tomlang on Mon Nov 16, 2009 4:11 pm

    At nearly exactly the same moment I was running out of clip leads (to run a OPT from another amp) I read your post and wa-la...

    DING, DING, DING, WE HAVE A WINNER. Guest aka Kegger wins! Great catch! You know I just wired the thing up without thinking about grounding the OPT's. Was probably thinking of car stereos, and gee, this is a trans should be floating, WRONG!

    Listening to it now, works great.

    And yes, was doing lowest gain (yours keggers) version from the get-go, although now I have a mix of lowest-gain-version and lower-gain-version resistors...at least same on both channels, I'll leave alone for now. Notice good waveform in background.


    tomlang

    Posts : 34
    Join date : 2009-08-12

    Re: Another Stereo 70 Resto-Mod Completed

    Post by tomlang on Tue Nov 17, 2009 12:09 pm

    OK, now for some data the working amp above:

    Line voltage: 118 volts

    Output tube biases set at 40 mA.

    CL60 thermister in series with transformer primary drops 1 volt at load.

    B+ delay 2-3 seconds.

    B+ rise, maximum is 498 VDC just before tubes start to load it.

    B+ with EL34's at 40 mA bias, no input signal is 465 VDC.

    B+ with EL34's both channels driven to just below clipping at full output is 435 VDC.

    Amp       Tubes                                  Vp-p   Vout rms     Power rms Watts

    ST70      6l6wgb Westinghouse          45           16                34

    ST70      6l6gc Svetlana                     48           17                38

    ST70      EL34 Sovtek black base      50           18                42

    ST70      EL34 Svelana brown base   50           18                42

    This was measured on an oscilloscope until observed clipping at 1000 Hz input, driving both channels into 7.5 ohm resistive loads. I have no distortion meter so that is unknown. Data was consistent channel to channel, repeatable, and believable. YMMV.

    Brinkman

    Posts : 14
    Join date : 2009-01-01

    Re: Another Stereo 70 Resto-Mod Completed

    Post by Brinkman on Wed Nov 18, 2009 4:11 pm

    tomlang wrote:
    DING, DING, DING, WE HAVE A WINNER. Guest aka Kegger wins! Great catch!

    It's great to see Kegger here. I seldom visit AK anymore, but when I did, he was a guiding light.

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