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    New tubes too hot?

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    Pad_X

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    Post by Pad_X on Thu Feb 14, 2019 5:05 pm

    Would someone have a moment to comment on my newbie predicament? Thanks for your help!

    I just moved into a new house and am excited about setting up my restored St-70 and getting back to it's sweet sounds. One of the things that I had been waiting to do was to change out the power tubes with a matched quad new production Mullard EL34 tubes that I have been storing for the past year. Yesterday, I got to work but failed to complete the task when I simply could not bring them to an appropriate bias voltage.

    The previous tubes were set at .08v (1 ohm bias resister) and held steady. The Mullards held steady  at .10v at the lowest limit of the bias pot.

    The tubes I wish to replace are JJ E34L and have sounded great and lasted quite long without much degradation. I returned them to the amp sockets and reset their bias to the original setting for the time being until I figure out what to do.

    Did I receive a outlier set of tubes that I won't be able to use? Can this be remedied via a change in bias resistors? If so, what resistance would be appropriate that would allow the tubes to be biased?
    New tubes  too hot? Under_10New tubes  too hot? Dynaco10New tubes  too hot? Img_4814
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    Last edited by Pad_X on Fri Feb 15, 2019 5:39 pm; edited 1 time in total (Reason for editing : ADDED MORE PHOTOS)
    peterh
    peterh

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    Post by peterh on Thu Feb 14, 2019 5:19 pm

    As this seems to be a modified st-70 standard schematics and adjustments won't do.
    1ohm cathode resistors. Are they common for both tubes ?

    Anyway, the adjustment pot has resistors at both ends, some adjustment of these seems needed.
    A schematic is needed for more advice.
    Bob Latino
    Bob Latino
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    Post by Bob Latino on Thu Feb 14, 2019 6:37 pm

    In the photo I can see that you have replaced the selenium rectifier with a diode. A diode has more forward voltage than a selenium rectifier and will alter the bias point. To correct this on a stock type Dynaco ST-70 with the original driver board circuit, you need to drop the value of the two 10K resistors on the 7 lug terminal strip from 10K to about 5K. There is one 10K resistor across lugs 1 and 2 and another one across lugs 3 and 4. The most common way to do this is to parallel a second 10K 1 watt resistor with the two that are on there now to give an effective 5K across both lugs. This should allow you to bias those output tubes at the proper bias point.

    Bob
    WLT
    WLT

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    Post by WLT on Thu Feb 14, 2019 9:40 pm

    With .08 V across the 1 ohm resistor I am assuming you have both output tubes connected via the original design. That gives 40 mA per output tube. New tubes 50 mA per tube. The concern I have is the 1 ohm resistor. Is it really 1 ohm ohm? How accurate is your DMM? Most of these sensing resistors are higher- like 10 ohms. That is easier to measure accurately. Both for resistance and for
    Vdc. Enough of a rant.

    Lets assume you have a good 1 ohm ohm resistor. Changing the bias circuit is easy but it is best to keep overall resistance to around 30K ohms. Stock it goes diode, 10K resistor, 10K ohm adjustment pot and 10 K ohm resistor to ground. To give yourself more adjustment move the adjustment pot to a higher negative voltage. Therefore it should be diode, 5 K ohm , 10 K adjustment pot, 15 K ohm to ground. The circuit then presents the same load on the power transformer. Adjust for 40 mA each tube.

    Let us know how it comes out.
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    Wharfcreek

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    Post by Wharfcreek on Thu Feb 28, 2019 10:22 am

    I'm with WLT on the 1 ohm resistor. To me, use of a 1 ohm resistor just presents too much 'potential' for error. I'm believe as does WLT that replacement of those with a couple of 'precision' 10 ohm resistors would be a better set-up. Also, I note the 47uf / 160V caps in the bias supply. That's another area for consideration in addressing. Bumping those to 100uf @ 100V can be a bit of an improvement as well.

    As to the PS, that is one 'interesting' set-up. Rather a 'power supply cap board'...but without the board......lol. Looks good actually.....and seeming well done though I can't see the values used. I'd be interested in knowing what was they are, particularly the last two stages where they are 'not' in series. But, as both Bob and WLT have pointed out, you need to re-work the bias supply stage resistors.....and maybe even consider replacing the pots if they're old and dirty. Sometimes those things have been 'sprayed out' in the past with some rather 'concentrated' electrical cleaner. This, over time, can erode the internal media surface if not wash it away altogether. So, replacement of those pots can also add some reliability to the unit. Looks like one got replaced already as they don't look like a 'matched' set.
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    Pad_X

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    Post by Pad_X on Mon Mar 04, 2019 4:41 pm

    Per Bob's suggestion, I placed two 10k resistors across the extant ones in the bias supply terminal strip. After that, I was able to set the bias,  albeit, with the pot at the lowest limit on the right pot, and very near the lowest limit on the left pot.
    Wharfcreek and WLT, I will eventually switch out the 1ohm resistor, if you think this will provide more adjustability.
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    Wharfcreek

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    Post by Wharfcreek on Mon Mar 04, 2019 8:59 pm

    Just to clarify, the use of a 10 ohm resistor is for purposes of narrowing the margin of error in the process of bias adjustment. It does not affect adjustability in terms of ‘range’ as is done by changing the values of the resistors within the bias voltage supply circuit. But, what it DOES do is make it such that a slight variance in resistor tolerance or in voltage measurement across said resistor is less apt to result in an error in bias adjustment that can be adverse to performance or reliability, or both.

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