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    Tubes4hifi driver boards

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    hogwylde09

    Posts : 6
    Join date : 2017-03-27

    Tubes4hifi driver boards

    Post by hogwylde09 on Thu Jun 08, 2017 11:59 am

    Just curious as to why a EF86 pentode driver board is not an option for sale here. I have an old ST-70 that I bought at a good price. I took a tube tester with me, my Sencore MU150. I pulled all of the Mullard xf4 EL34, tested them. All tested ANOS. I also noted the fact that the driver board, while old and dusty, was of the (yellowish/light green/gray) loaded with two Amperex EF86 and what I believe to be a 12AX7? PI. (I managed to break the glass on the US labeled 5U4G) so no great loss. It now has a new Valvo GZ34. It also had metal film resisters and Illinois Capacitors? Yellow like them anyway. The PT looked to be a taller 2 1/8" version and fiber (about the same color as the board) bias point meter probe jacks, in place of the front octals, and the bias ground jack mounted in place of the stereo/mono switch. The chassis was a little corroded, but with all the mods, I couldn't pass on it for the price. Now I was skeptical, because the seller knew nothing about it, not even that it was a Dynaco, no cage.

    I bought it brought it home, powered it up (full voltage) with a speaker load. All tubes lit, no smoke, no blown fuse. So it went into the second set of preamp outputs (McIntosh MC20). The first pair are running vintage MC75's. I could not believe what I was hearing, or expecting. The MC75's are driving Belle Klipsch, the ST-70 driving CF4 Klipsch. This system is so well balanced (front and rear stages), I am astounded. Had never heard a
    Dynaco before. I am sold on EF86 pentodes, as that is what is our main choice of tube is in guitar amp builds.

    So while my son builds custom guitar amps, we are always on the hunt for vintage tubes, old Hammond organ donor amps or anything with good iron and/or tubes. He gets a deal on a stash of NIB audio tubes, some huge
    homemade Karlson design speakers and a Dynaco PAS II and one of the cheaper Dyna tuners. So I begin the hunt for a second ST-70, to have a whole complete 2nd Dynaco system.

    I had a used Gibson Melody Maker, very little invested in it (for a 1964). And find a ST-70 on one of the better known musical instrument websites. Turned out the seller was in a 100 mile radius of me, he was interested in
    my guitar, myself in the ST-70. So a trade was made. (he told me it was working). It looked to be in almost mint condition. Pretty brown cage, perfect label and no corrosion on the chassis.

    But this one has issues, so a rebuild is obvious. For starters the stock 7199 board has been jumpered for 6GH8. This one has one weak Sylvania fat bottle welded plate 6CA7. The remaining 3 test good. It also has a lesser
    quality new production rectifier tube.

    But all of this leads me back to the original question. I am starting to buy parts for this project, but Tubes4hifi does not offer a EF86 driver board. And I know I have already made my first mistake. Ordered the Triode EF86
    board w/parts. Is this something that may be offered in the future? People tend to shy away from the EF86, but get good NOS ones, and microphonic and noise does not seem to be a problem.
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    PeterCapo

    Posts : 466
    Join date : 2008-12-05

    Re: Tubes4hifi driver boards

    Post by PeterCapo on Thu Jun 08, 2017 1:03 pm

    If you like the EF86 so much, are you saying you consider it a mistake that you just ordered for your second St-70 the Triode USA board that uses the EF86?  If so, why do you consider this a mistake?  It's [likely] the same board that you have in your first St-70 that you seem to like a great deal.

    The circuit the Triode USA board uses is essentially similar to the original Dynaco driver board except that it breaks-out the 7199 into separate gain and phase splitter tubes.  The tubes4hifi driver board is a different circuit topology for which Roy chose different tubes.
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    Bob Latino
    Admin

    Posts : 2583
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Tubes4hifi driver boards

    Post by Bob Latino on Thu Jun 08, 2017 2:59 pm

    hogwylde09 wrote:Just curious as to why a EF86 pentode driver board is not an option for sale here.  

    If you like the Triode board with the EF86 pentode input tube, then use it but consider that the VTA driver board (IMHO) is a more modern and a better design. Any EF86 driver circuit from Tubes4hifi would be a step backwards in design. The VTA driver board ...

    1. Has its on board power supply so that the board does not have to draw power from an external DC power source like the quad cap or a cap board. The Triode board has no on board power supply

    2. Has "CCS" which produces a constant current source on the phase splitter/inverter part of the circuit. The Triode board does not have CCS.

    3. Has its own "on board" bias system which allows the individual biasing of each output tube. The Triode board used the old Dynaco bias system.

    Also > The EF86 pentode driver tube IMHO is not as good a sounding tube as the 12AU7 dual triode input tube. In general, triode type driver tubes are simpler in construction than a pentode driver tube and are usually better sounding.

    Bob
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    hogwylde09

    Posts : 6
    Join date : 2017-03-27

    Re: Tubes4hifi driver boards

    Post by hogwylde09 on Thu Jun 08, 2017 8:43 pm

    I just have never had the pleasure of comparing the working one I have, to any other stock or modded 70. I know my favorite triode is 12AX7. And pentode either 6AK5 7 pin or the EF86.
    BUT thishe in a totally different environment. The guitar amp.
    I did see the benefits of the onboard bias trim pots. And was wondering about the PS caps under the board. I am beginning to realize, that is what the forum is about. Asking and getting real answers.
    And it is kind of overwhelming, the different companies and differences in products.
    I somehow thought it was either quad cap or the PS board that mounted under chassis with standoffs route.
    I will also say that I am more drawn to this forum because of the time and effort put forth by Mr. Bob Latino and members.
    With all of the competitors in the Dyna market, it'seems easy to look at pricing, and say to yourself "well I can get this part for xx dollars here vs. Xx dollars there". In today's economy, at least
    for me that is somewhat feasible.
    But rest assured I will be buying from tubes4hifi, in the future. Matter of fact, I may just do the 120 out of it.
    Thanks guys. I will get it right in my 61 year old brain eventually! Couldn't ask for a better system than I have now. Or maybe, YES. BTW I added pics of my 2 channel amps, when I first joined.

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    tubes4hifi
    Admin

    Posts : 1374
    Join date : 2008-11-30

    Re: Tubes4hifi driver boards

    Post by tubes4hifi on Thu Jun 15, 2017 7:13 pm

    the EF86 is a very good tube when you need ALOT of gain (microphones, phono circuits, guitar overload), but it won't compare to the quality results you can get using 12AU7s to drive an ST70 amplifier.

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