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    Barn find ST70

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    PeterCapo

    Posts : 380
    Join date : 2008-12-05

    Re: Barn find ST70

    Post by PeterCapo on Mon Oct 05, 2015 9:57 pm

    I’ll try but cannot promise no errors.

    1. The 470KΩ resistors are wired from the input signal connector to ground and establish the input impedance for each channel of the Stereo 70.  I believe they might also function as grid leak resistors for the input to the gain stage (first section) of the 7199.  From the Aiken Amplification glossary:
    "Grid leak resistor - a very large resistor from the grid of a tube to ground, which is used to generate the bias voltage for the tube.  See "grid leak biasing" for an explanation of how this works. This term is sometimes incorrectly used when referring to the grid-to-ground resistor in a cathode biased configuration, which is used to provide a DC ground reference for the grid circuit."

    2. "So any voltage change with the larger caps can be offset by the bias pots."  I would not expect any significant change in voltage due to larger value bias capacitors.  Difficulties in adjusting the bias pots sometimes happen when a silicon diode is substituted for the original selenium stack possibly in conjunction with the AC mains being too high or too low.

    3. Biasing the 7199s occurs with the associated resistors.  As long as the resistor values are in-tolerance, and the power supply voltages are correct, and the 7199s are good samples, they will bias up properly.  The 82pF does not block DC to the phase inverter grid, because it is not in series with the connection.  There should be no appreciable DC current flowing into the grid of the phase inverter (at pin 9), as the input impedance of the grid is extremely high.  I believe the 82pF is there to siphon off any ultrasonic oscillations, or something to that effect.

    4. Pins 2 and 9 are connected together to couple the audio signal from the first section of the tube (gain) to the second section (phase splitter/inverter).  I assume there is no coupling capacitor because the DC voltage at pin 2 helps to bias the phase inverter triode.  The voltages on the two plates are different because they have different source voltages from the power supply and different value plate resistors.

    5. Testing the choke out-of-circuit should result in about 1.75 Henries and 62 ohms of DC resistance.  Actually, looking at the schematic, you should also be able to take these measurements with the rectifier tube removed, assuming the circuit is in the original configuration.

    6. For the 270KΩ resistors, notice the asterisk.  This means they need to be a 1% matched pair.  I believe this is so that the bias voltage (injected at eyelets 6 and 21) is evenly divided to each pair of EL34 tubes - there's only a single bias adjustment pot per channel.

    7. The 1KΩ resistors at the input grids of the EL34s are "grid stopper" resistors, and they are not there to set the level of the signals going into the EL34s.  In terms of the audio signal level, their value is not critical because 1KΩ is negligible compared with the very high Z-In of the EL34 input grids.  The grid stoppers are used to prevent "parasitic oscillations," which gets kind of technical: http://www.aikenamps.com/index.php/grid-resistors-why-are-they-used

    8. The return path is through the EL34s.

    If I have anything wrong here, please speak up.  Thanks.

    GP49

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    Re: Barn find ST70

    Post by GP49 on Mon Oct 05, 2015 10:41 pm

    Reads OK to me, Peter.  

    In #2, the electrolytic capacitors in the bias section appear to be "flipped around" because the bias supply is negative.   This HAS tripped up even knowledgeable builders since almost all the time in the heyday of tubes, we dealt with positive supplies.

    In #8, the return path is through the EL34s, then through the common cathode resistor to ground.  The DC current through the tubes and then the resistor raises the high end of the resistor above ground, according to Ohm's Law, and that is what we measure when setting bias...we are not actually measuring the bias, which is the negative voltage on the grids of the EL34s.  As the bias varies, the cathode current and the voltage on the cathode resistor varies, a numerically higher negative bias on the grids causing lower quiescent current and a lower voltage on the cathode resistor.  Consider also that any signal through the amplifier also flows through the transformer secondary, the EL34s, and the cathode resistor, so that you can't set the bias while the amp is passing a signal.


    Last edited by GP49 on Tue Oct 06, 2015 6:57 pm; edited 1 time in total

    jrubin

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    Join date : 2015-08-28

    Re: Barn find ST70

    Post by jrubin on Tue Oct 06, 2015 6:36 pm

    I removed my dumb question.....

    Incidentally pins 4 and 6 on the GZ34 should be read with respect to ground and not each other.     reading across 4 and 6 doubles the voltage reading where 4 and 6 to ground shows 360vac
    however, if someone could beasure 4-6 with the rectifier removed for resistance (with the power off) id like to know if they match my reading of 61ohms

    Jordan


    Last edited by jrubin on Tue Oct 06, 2015 6:42 pm; edited 1 time in total

    PeterCapo

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    Join date : 2008-12-05

    Re: Barn find ST70

    Post by PeterCapo on Tue Oct 06, 2015 6:41 pm

    360VAC to ground from each of the red leads individually, 720VAC or so measured across both red leads is inevitable.  Please recheck each red lead to ground.

    PeterCapo

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    Re: Barn find ST70

    Post by PeterCapo on Tue Oct 06, 2015 6:46 pm

    jrubin wrote:I removed my dumb question.....

    Incidentally pins 4 and 6 on the GZ34 should be read with respect to ground and not each other.     reading across 4 and 6 doubles the voltage reading where 4 and 6 to ground shows 360vac
    however, if someone could beasure 4-6 with the rectifier removed for resistance (with the power off) id like to know if they match my reading of 61ohms

    Jordan

    What is your AC mains reading with 360VAC at each of pins 4 and 6 with the rectifier in place?  If you have the older transformer, it should be 117VAC.  Not sure, but later Stereo 70s might possibly have had a 120VAC primary.  So, if your mains read 117VAC or 120VAC with 360VAC at each of pins 4 and 6, your HV secondary is just fine.

    To check the DCR of the choke, remove the rectifier tube and just measure across the choke itself.

    Hope you are observing safe work practices.

    jrubin

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    Join date : 2015-08-28

    Re: Barn find ST70

    Post by jrubin on Tue Oct 06, 2015 6:53 pm

    you were quick..... your reply was quicker than my redaction..



    anyway..... Id flowcharted the AC and the little DC I have to work with took rd-bl all the way to pin 5 with the associated voltage drops


    RD-BL to Bottom of selenium rectifier 50VAC
    Top of selenium rectifier to first 10K resistor -65VDC
    After 1st 10K resistor -37VDC
    10K POT WIPER (both set to same OHMS) -33VDC
    END of both 10K POT wipers -25VDC

    Appearances 1,2,22,23 to ground -33VDC
    After 1K resistor (all 4 el34's) -33VDC (end of EL34 GRID BIAS CIRCUIT)

    jrubin

    Posts : 47
    Join date : 2015-08-28

    Re: Barn find ST70

    Post by jrubin on Tue Oct 06, 2015 6:55 pm

    PeterCapo wrote:
    jrubin wrote:I removed my dumb question.....

    Incidentally pins 4 and 6 on the GZ34 should be read with respect to ground and not each other.     reading across 4 and 6 doubles the voltage reading where 4 and 6 to ground shows 360vac
    however, if someone could beasure 4-6 with the rectifier removed for resistance (with the power off) id like to know if they match my reading of 61ohms

    Jordan

    What is your AC mains reading with 360VAC at each of pins 4 and 6 with the rectifier in place?  If you have the older transformer, it should be 117VAC.  Not sure, but later Stereo 70s might possibly have had a 120VAC primary.  So, if your mains read 117VAC or 120VAC with 360VAC at each of pins 4 and 6, your HV secondary is just fine.

    To check the DCR of the choke, remove the rectifier tube and just measure across the choke itself.

    Hope you are observing safe work practices.


    No rectifier and 4EL34's in place and 1 7199
    With 360V at pins 4 to ground and pins 6 to ground      primary on power transformer shows 115V exactly


    I dont want to go higher than 115V input till the rectifier is in place in the hopes that it might load down the circuit when installed.  I cant install the rectifier till my capacitors arrive

    jrubin

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    Join date : 2015-08-28

    Re: Barn find ST70

    Post by jrubin on Tue Oct 06, 2015 7:29 pm

    Look what arrived today


    4x 6GH8A
    1X OY3


    jrubin

    Posts : 47
    Join date : 2015-08-28

    Re: Barn find ST70

    Post by jrubin on Tue Oct 06, 2015 9:02 pm

    Should I be concerned that the RF choke is .75 henries, or is that a good value?

    PeterCapo

    Posts : 380
    Join date : 2008-12-05

    Re: Barn find ST70

    Post by PeterCapo on Tue Oct 06, 2015 9:11 pm

    Try lifting one of its leads and remeasure.  Perhaps it does need to be out of the circuit to get a good reading.  In-circuit, even with the rectifier removed, it is connected to the quad cap, which might skew the reading. While you're at it, with the lead lifted, confirm the DC resistance, too.

    DynakitParts
    Admin

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    C-354 Choke

    Post by DynakitParts on Tue Oct 06, 2015 9:25 pm

    A new original Dynaco C-354 choke measures around 1.75 Henries & 62 ohms. Measured out of circuit...

    Kevin


    Last edited by DynakitParts on Tue Oct 06, 2015 9:27 pm; edited 1 time in total (Reason for editing : Bad speller...or is it spellar?)

    jrubin

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    Join date : 2015-08-28

    Re: Barn find ST70

    Post by jrubin on Tue Oct 06, 2015 9:36 pm

    when i replace the quad cap ill measure the choke and report back

    PeterCapo

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    Join date : 2008-12-05

    Re: Barn find ST70

    Post by PeterCapo on Tue Oct 06, 2015 9:38 pm

    Sounds good.  My error previously telling you to measure it in-circuit.

    Bob Latino
    Admin

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    Re: Barn find ST70

    Post by Bob Latino on Tue Oct 06, 2015 10:25 pm

    You can measure the resistance of a choke in circuit and it will be much less than an ohm off its actual value. The choke is between the first and second sections of the quad cap. Both of those sections will have a very high resistance (if the quad cap is in good condition) that will have virtually no affect on the resistance measure across the choke. If a choke has a resistance of 62 ohms out of circuit, it will probably measure 61.9 ohms or something like that in circuit ..

    Bob

    jrubin

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    Join date : 2015-08-28

    Re: Barn find ST70

    Post by jrubin on Tue Oct 06, 2015 10:34 pm

    The quad cap is all but destroyed with 1 reading at 10uf two open and one shorted. I dont recall which points had what, but have a new one on the way

    jrubin

    Posts : 47
    Join date : 2015-08-28

    Re: Barn find ST70

    Post by jrubin on Wed Oct 07, 2015 9:41 pm

    The quad cap has arrived and was installed....    RF Choke measured about 1.55 H out of circuit
    Posting movie part 3 tomorrow.     360V on the rectifier showed 117VAC IN.
    Further DC circuit testing will happen tomorrow.   The EL34's were able to BIAS properly.
    117VAC showed exactly 6.4VAC on the heaters.  I would doubt the 1 missing 7199 would make any considerable difference on circuit loading
    Still waiting on the 7199 adapters and the two new filter caps on the bias circuit
    Forgot to take a DC reading of the rectifier tube , ill put that in the next movie


    Once I have the replacement tubes for the driver board installed ill start doing some noise testing with shorting plugs and a few 8OHM 100W dummy loads on the output

    After that its just signal tracing step by step to ensure everything is in working condition


    Then it will be completely disassembled and restored

    jrubin

    Posts : 47
    Join date : 2015-08-28

    Re: Barn find ST70

    Post by jrubin on Thu Oct 08, 2015 11:50 am

    Video 3 posted of the epic saga.....



    jrubin

    Posts : 47
    Join date : 2015-08-28

    Re: Barn find ST70

    Post by jrubin on Thu Oct 08, 2015 9:40 pm

    Somebody tell me what im missing here.

    Two 7199 heaters draw .9 (.45x2) amps at 6.4VAC on the heater circuit

    Four EL34's draw 6.4 (1.6x4) amps at 6.4VAC on the heater circuit

    GZ34 draws 1.9 amps at 5V on the heater circuit


    Thats a total heater draw of 9.2 AMPS












    MontanaWay

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    Re: Barn find ST70

    Post by MontanaWay on Thu Oct 08, 2015 9:50 pm

    jrubin wrote:Somebody tell me what im missing here.

    Two 7199 heaters draw .9 (.45x2) amps at 6.4VAC on the heater circuit

    Four EL34's draw 6.4 (1.6x4) amps at 6.4VAC on the heater circuit

    GZ34 draws 1.9 amps at 5V on the heater circuit


    Thats a total heater draw of 9.2 AMPS












    yes, but it is divided over two separate 6.3V filament windings and a separate 5V rectifier filament winding. So you can 'half' the filament current for the EL34 and 7199's, giving you say 3.65A or so for two EL34's and one 7199. I think your transformers filament winding are rated at 4A each, so plenty to spare. So the filament wiring will need to be split up like that, 2x EL34 and 1x 7166 per 6.3V winding.
    The 5V filament for the rectifier I think is rated at 2 or 2.5A on the original transformer, not sure about that.

    jrubin

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    Join date : 2015-08-28

    Re: Barn find ST70

    Post by jrubin on Fri Oct 09, 2015 10:43 am

    I was looking at this from the wrong angle.... Thats why I shouldnt post before bed

    The correct idea was that if tube X's heater uses 1 AMP at 12V. The current burden on the Power transformer on the input given 120V would be 1AMP/10 or 100ma


    Using that

    7199 heater 6.3VAC .45A @ 117V by [18.57] = 0.024A or 24mA X 2 tubes = 48mA

    EL34 heater 6.3VAC 1.6A @117V by [18.57] = 0.086A or 86mA X 4 tubes = 344mA

    GZ34 heater 5V 1.9A @ 117V by [23.4] = 0.081A or 81mA X2 tube = 81mA


    Total heater current draw at 117VAC = 473mA

    jrubin

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    Join date : 2015-08-28

    Re: Barn find ST70

    Post by jrubin on Fri Oct 09, 2015 11:05 am

    I should have the new CAPS 100uF tonite. What id like to do is take some measurents of the bias circuit before and after replacement and document the change.


    In the old circuit before the changes im going to bring down the bias output reading to about 1.4V at the test point on both sides. just to be sure that any changes make dont underbias the EL34's. Ill also look at the quality of the DC from the selenium rectifier at the capacitors on the oscope.

    with this well get a better understanding of what happens if the CAPS in the bias circuit are doubled in value..... I dont predict any significant change (perhaps a slight,slight negative voltage drop) (LESS NEGATIVE VOLTAGE)


    Also,


    The GZ34 Max current at 350V is 250MA as per the diagram

    If the current for each EL34 biased at 1.56V readout is 50ma then the total draw is 200mA

    The 7199 tube pulls 9 1.1 and and 12.5 / .35 and 3.5 for all plate and grid currents totaling 26.45mA X2 52.9

    This totals 252.9mA

    Am I wrong or does the current for this circuit exceed the capability of the GZ34 to provide power ?






    corndog71

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    Re: Barn find ST70

    Post by corndog71 on Fri Oct 09, 2015 11:39 am

    Maybe this will help.

    http://www.dynakitparts.com/image/schem/PA-060.pdf

    zx

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    Re: Barn find ST70

    Post by zx on Fri Oct 09, 2015 1:18 pm

    Thanks for your time ..... an posting the info...youtube....vary nice






    thanks for the site Bob....................

    jrubin

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    Join date : 2015-08-28

    Re: Barn find ST70

    Post by jrubin on Fri Oct 09, 2015 3:02 pm

    corndog71 wrote:Maybe this will help.

    http://www.dynakitparts.com/image/schem/PA-060.pdf


    I dont believe the question was fully understood. As shown below, my math shows that the current draw on the rectifier is close to or exceeds its limit for 350V


    The GZ34 Max current at 350V is 250MA as per the diagram

    If the current for each EL34 biased at 1.56V readout is 50ma then the total draw is 200mA

    The 7199 tube pulls 9 1.1 and and 12.5 / .35 and 3.5 for all plate and grid currents totaling 26.45mA X2 52.9

    This totals 252.9mA

    Am I wrong or does the current for this circuit exceed the capability of the GZ34 to provide power ?

    Bob Latino
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    Re: Barn find ST70

    Post by Bob Latino on Fri Oct 09, 2015 3:22 pm

    You are probably correct about the current draw being at the limit of a 5AR4 rectifier BUT > Dynaco used the excellent Mullard 5AR4's which would probably take the 250 milliamp or so current draw in stride. On the VTA ST-70 amps we recommend a 40 milliamp bias setting for each output tube. The 40 milliamp setting is probably easier on today's 5AR4's which are not as "sturdy" as Mullard's 5AR4.

    Bob

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