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    12BH7 in VTA70 with stock power supply transformer questions

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    Mossback

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    Join date : 2009-04-21

    12BH7 in VTA70 with stock power supply transformer questions

    Post by Mossback on Wed May 11, 2011 4:59 am

    My first post hope you all can help me out.
    Starting my ST-70 rebuild with the low gain VTA70 board and stock power supply transformer, I like the board very much very good quality.
    This amp will use a vintage Mullard GZ34 and OTK 6p3S-e tubes.
    My question is if anyone has used 12BH7 tubes in place of 12AU7's? If so what are your thoughts?
    I have a concern the stock power supply transformer would suffer from the additional currant needed for three 12BH7’s. Is it necessary to use three 12BH7’s or a mix with 12AU7’s.
    Thank you.... Dan

    Bob Latino
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    Re: 12BH7 in VTA70 with stock power supply transformer questions

    Post by Bob Latino on Wed May 11, 2011 7:49 am

    Hi Dan,

    12BH7 tubes are physically "taller" than 12AU7 tubes and are probably more durable in their ability to handle voltages. I have used both types on the VTA lower gain board and I don't really have a preference for the center (initial voltage amplifier for both channels) position. For the two side phase splitter/inverter tubes (IMHO) a 12BH7 may be a better choice because of its ability to swing voltages better than a 12AU7. If you want to mix the two tubes on the lower gain board, the best mix is probably a 12AU7 in the center and two 12BH7 tubes in the two side positions. Depending on your hearing, your speakers and your associated audio gear you may find one combination better than the other.

    On either the VTA standard gain 12AT7 board or the VTA lower gain 12AU7/12BH7 board, the center tube has the most influence on the sound of the amp. On the lower gain board either a Mullard CV4003 or an RCA clear top are two of the better choices for the center voltage amplifier.

    Bob

    Mossback

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    Join date : 2009-04-21

    Re: 12BH7 in VTA70 with stock power supply transformer questions

    Post by Mossback on Wed May 11, 2011 9:01 am

    Thank you very much Bob that is exactly the information I was looking for. Once I get it going in about three weeks I will report back my findings.
    I’ll also comment on the OTK 6p3S-e tubes plate voltage and stuff.

    Bugs

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    Re: 12BH7 in VTA70 with stock power supply transformer questions

    Post by Bugs on Wed Sep 21, 2011 9:47 pm

    Hi Bob,

    I was emailing one of the web tube vendors and asked for their opinion on which 12bh7 would be a good substitute for a 12au7. Their response was that a 12bh7 was not a substitute for a 12au7, end of story. I've used the EH 12bh7s in my VTA so I know they work. Do you know what this vendor was talking about?

    Thanks

    Bob Latino
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    Re: 12BH7 in VTA70 with stock power supply transformer questions

    Post by Bob Latino on Thu Sep 22, 2011 7:03 am

    A 12BH7 is not an exact substitute for a 12AU7 but it can be used in place of a 12AU7 in the VTA lower gain board for the ST-70. A 12BH7 has slightly lower gain than a 12AU7 and has some other characteristics that are different than a 12AU7 BUT > at least in the VTA driver boards, it works OK as a substitute.

    Bob

    jhoak

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    Re: 12BH7 in VTA70 with stock power supply transformer questions

    Post by jhoak on Sat Sep 24, 2011 6:56 pm

    I can tell you with absolute certainty that the 12BH7 tubes work VERY well in the low gain VTA board. I'm listening to a pair of Electro Harmonix (gold pin) 12BH7 tubes in the outboard sockets in my ST-120 as I write this.

    I bought a 'matched" set of 3 Psvane 12AU7s thinking that they would be better than the recommended (and somewhat expensive) CV4003 tube in the middle socket and outboard sockets. As it turned out the Psvane 12AU7 bested the CV4003 in the middle spot but the EH 12BH7s were MUCH prefered by me in the outboard sockets.

    Of course it's your amp your speakers and your ears. Swap tubes at will and have fun. That would be the nature of this at times silly hobby.

    Enjoy and always remember... It's not about the hardware it's about the music that comes out.

    tubes4hifi
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    Re: 12BH7 in VTA70 with stock power supply transformer questions

    Post by tubes4hifi on Mon Sep 26, 2011 11:40 am

    if you have a STOCK Dynaco ST70 then you are likely to overheat the power transformer by usin 12BH7s, however if you have our VTA ST70 the power transformer has enough extra current capability to handle the 12BH7s, which have double the normal current draw of 12AU7 tubes

    jhoak

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    Re: 12BH7 in VTA70 with stock power supply transformer questions

    Post by jhoak on Mon Sep 26, 2011 11:50 am

    HMMM... I had always assumed that there was some "reserve" capacity available.

    This assumption was based on the fact that the ST-70 was designed to power a preamp by way of the front octal sockets. Is this incorrect? How much current would such a preamp draw? With no preamp hooked up wouldn't that represent a bit of additional current available?

    Mossback

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    Re: 12BH7 in VTA70 with stock power supply transformer questions

    Post by Mossback on Tue Apr 17, 2012 11:44 am

    Well it's taken me nearly a year to finish the amp I had to set it aside for other things.
    A little history. Over the past few years I've rebuilt two other ST70's and a MKIV with more or less stock driver boards but much better powersupplys. They were OK but I've never heard a Dynaco ST70 that sounded better than average.
    After rebuilding this ST70 with the VTA board I have changed my mind. This ST70 has a detailed extended musical sound I like very much. I only re-used the old case and transfomrers. The VTA board is sweet!
    I ended up using new 12BH7's and OTK 6p3S-e from Jim McShane. Like Jhoak, I tried a new Psvane and some vintage 12AU7's but have not made up my mind what one is the best choice. I kinda like a Telefunken but others may like something else.
    To my ears the 12BH7's in the outer two positions betters a number of high quality vintage 12AU7's. The powersupply stock transfomer after 24 hours only becomes slightly warm.
    Bob, thank you for your helpful answers.

    Mossback

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    Join date : 2009-04-21

    Re: 12BH7 in VTA70 with stock power supply transformer questions

    Post by Mossback on Tue Apr 17, 2012 11:49 am

    tubes4hifi wrote:if you have a STOCK Dynaco ST70 then you are likely to overheat the power transformer by usin 12BH7s, however if you have our VTA ST70 the power transformer has enough extra current capability to handle the 12BH7s, which have double the normal current draw of 12AU7 tubes

    In my case this is very true after 24 hours the PS transformer becomes a little warm not at all hot.
    The VTA board is great. I used PIO USSR signal caps. I found the board simple to install the directions were clear and detailed.

    sKiZo

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    Re: 12BH7 in VTA70 with stock power supply transformer questions

    Post by sKiZo on Sat Oct 26, 2013 9:17 pm

    Yeah, yeah ... I know ... old thread ... What a Face 

    Just scored a used pair of Sylvania 12BH7a's off that auction place. Was $5 too much?

    Both test very strong at 130-130 and 105-105 with 70-70 being good and 130-130 being best. Also tested for shorts and leaks with perfect results.

    Question being ... how close should they match? Any slight difference in output shouldn't be an issue with dual volume controls to adjust the balance? I was figuring to try them as channel drivers with an RCA clear top in the middle.

    skriefal

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    Re: 12BH7 in VTA70 with stock power supply transformer questions

    Post by skriefal on Sun Oct 27, 2013 12:05 am

    $5 would be a great price for even one 12BH7 in good condition. But I suspect that you knew that. Smile

    Can't really help re: the matching question. But I thought that the ST-120's CCS circuit would help with that?

    sKiZo

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    Re: 12BH7 in VTA70 with stock power supply transformer questions

    Post by sKiZo on Sun Oct 27, 2013 12:34 am

    Ah. Good point. I think ... here's a good thread on CCS that helped me out ... some ... scratch 

    http://dynacotubeaudio.forumotion.com/t1419-the-new-ccs-vta-st-70-st-120-driver-board-photos

    I also followed the link to the developer's website and thought this was the important part.

    With the values shown, however, it will still have plenty of adjustment range to accomodate reasonably matched LTP triode pairs and output tube pairs.

    There of course he was talking about a point to point circuit that the TI replicated in the transistor ... and what exactly would "reasonably matched" in this tube family be?

    However ... if I'm reading it right, the CCS balances the outputs from the center driver/multiplexer, which would still mean that the channel drivers would need to be fairly evenly matched for good balance? In that case, the individual volume controls would be handy to tweak a tube set that's not perfectly matched.

    Should be fun to add to the collection. I've heard a lot of good as to what the 12BH7 can do in an amp that has the reserve to load them properly. If I remember right, the draw is about twice what you'd see with a 12AU7.

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