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    DC Voltages look suspicious

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    Yvon Massicotte

    Posts : 13
    Join date : 2009-07-28
    Age : 74
    Location : Montréal, QC, Canada

    DC Voltages look suspicious

    Post by Yvon Massicotte on Wed Feb 20, 2013 5:25 pm

    Hi!

    I just finished building my ST-70 out of an old unbuilt kit and the low-gain VTA board. Bias adjustments went fine as well as the AC balance. I fed the amp with a sine wave generator and, on my scope, everything looked fine.

    I gave the amp a music test and the sound seems very good. There is, however a problem that bugs me: the voltages do not come at all to the same values as shown on the schematic:

    • B+ to the center-tap of the output transformers' primaries is 450V which is fine.


    • The B+ going to the VTA board is 412V which is 20 v above the specified 390V on the schematic but still acceptable.


    • Between R13 and R11, I measure 320V against the 310V; again very satisfactory.


    • It is when I measure the voltages on V1 plates that I see a huge difference: 227V (pin1) and 224V(pin 6) when the schematic specifies 150V !


    • Also the schematic shows 100V DC at the junction of R27 and R17 and I measure 68V !


    • The phase splitter tubes show 272V on their plates against 200V on the schematic !


    • Bias voltages are normal at -38V.


    Is it possible that the tubes supplied with the VTA board are out of spec? They are ECC81 from JJ Electronic? Those measures are identical on both channels. Could someone help me here with a few suggestions?

    Yvon

    Bob Latino
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    Re: DC Voltages look suspicious

    Post by Bob Latino on Wed Feb 20, 2013 7:30 pm

    Hi Yvon,

    The thing to do on your amp is to change out the 2200 ohm (2.2K) resistor on the quad cap for a resistor of a somewhat larger value like a 3.3K or a 3.6K. That resistor on the quad cap is responsible for dropping the B+ voltage to the driver board. In some cases to get the driver board B+ below 400 VDC you have to up the value of that resistor. As soon as the driver board B+ voltage is below 400 VDC, most of the other driver board voltages will come into line.

    The other thing you mentioned was that this was "an old unbuilt kit." If it was a kit from the 1960's that power transformer was probably rated for 115 volts instead of 120 volts. Operated at 120 volts, your voltages will be higher than normal. The 450 VDC that you get at the B+ to the output transformer primary center tap is a little high also.

    Another thing you could do is operate the amp using a 5U4 rectifier tube in place of a GZ34. This would drop the B+ to the whole amp and maybe you wouldn't have to change out the 2200 ohm resistor on the quad cap.

    Bob

    hawaii.ken

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    Join date : 2012-01-31

    Re: DC Voltages look suspicious

    Post by hawaii.ken on Wed Feb 20, 2013 9:22 pm

    Kudos for finding an "unbuilt" ST-70

    Yvon Massicotte

    Posts : 13
    Join date : 2009-07-28
    Age : 74
    Location : Montréal, QC, Canada

    Re: DC Voltages look suspicious

    Post by Yvon Massicotte on Wed Feb 20, 2013 9:47 pm

    Hi Bob,

    Thank you for your suggestions.

    My Dynaco is indeed a 1967-68 unit. I will try with a 5U4 tube and, if necessary after that, I will raise the value of the voltage dropping resistor on the Quad cap.

    I well remember the 5U4. It was the rectifier tube in our old Admiral TV set in the fifties. It used to burn quite often! A quick look on the Net shows a fair choice from quite inexpensive ones to 130$ ! Is there a brand you would recommend?

    Finally, you mentioned that 450V on the primary center tap is also a bit high. What would be the ideal value?

    Yvon

    Bob Latino
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    Re: DC Voltages look suspicious

    Post by Bob Latino on Thu Feb 21, 2013 8:27 am

    Hi Yvon,

    Having 450 VDC on the output transformer center tap (which is after the choke) means that you have an even higher 465 VDC or so off pin 8 of the rectifier. Ideally, the output transformers should look at 425 - 440 VDC on an ST-70. Your 450 VDC to the output transformers is just a little high and would probably be OK. The real problem is that the tubes that you buy today do not stand up to high voltages as well as the tubes made in the 1950's and 1960's. Unfortunately, higher than normal voltages usually translate into shorter tube life.

    Bob

    Yvon Massicotte

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    Join date : 2009-07-28
    Age : 74
    Location : Montréal, QC, Canada

    Solution at hand

    Post by Yvon Massicotte on Sat Feb 23, 2013 12:24 am

    Bob,

    I have a very powerful variac. I decided to use it to check what you're saying about old Dynaco power transformers. I plugged my ST-70 through the variac set at 110V AC.

    Indeed, as you predicted, the GZ34 pin 8 came out at 434V, the B+ at the primary center tap at 422V and the B+ to the PCB at 391V. Wonderful!

    Alas! The voltage at V1 plates was still 204V. This is way higher that the schematic's 150V.

    Then, I noticed that the schematic puts R13 at 15K whereas the part supplied by Roy in my VTA kit was 33K which is double the specified value. As the voltage drop that I measure across R13 is 94V, the current value is around 2.8mA. The schematic puts that value at 5mA.
    The same 2.8mA flows across the load resistor R11 which brings the voltage at V1's plate at 203V, (actual measured value 204V).

    So my measured voltages seem right. Is there a specific reason why Roy changed the value of R13 to 33K or should I put back the schematic's 15K? I will try to contact him on this matter. A static plate current of 3mA is quite different from one of 5 mA, so I will wait for his opinion on this.

    In any case, I feel it is important that PROPER VOLTAGES be given on the schematic. It is an essential debugging tool.

    Best regards,

    Yvon

    tubes4hifi
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    Re: DC Voltages look suspicious

    Post by tubes4hifi on Sun Feb 24, 2013 3:46 pm

    two things about this . . . 1- we found out that somehow I had accidently supplied Yvon with the wrong tubes, he got the hi-gain tubes with the lo-gain kit. 2- after testing dozens of these boards that I've built, the voltages seem to be all over the place (which doesn't make sense)
    but they have all sounded fine. We know that +/-10% nominal voltage variations are normal and acceptable, most of this variation is due to variations in the tubes and different brands etc.
    Maybe we can get more of a consensus here on plate voltages (pin 1 and 6) of the 3 tubes for the low-gain version (non-CCS).
    My various versions of schematics thru the years have listed 150-200v on the center tube, and 200-300 volts on the outer tubes.
    My current ST70 is in between boards again (lo-gain 9 pin, CCS, and octal versions) so until it's back together let's hear from some other users.

    Yvon Massicotte

    Posts : 13
    Join date : 2009-07-28
    Age : 74
    Location : Montréal, QC, Canada

    Variations between tubes have important effects

    Post by Yvon Massicotte on Thu Feb 28, 2013 5:27 pm

    Roy is right here:

    Normally, there should be around + or - 10% variations on measured voltages, especially since the passive components on the VTA all have tight tolerances and those voltages are all measured under static DC conditions where the only effect of the signal capacitors is to block any DC path!

    Thus the reason for all these variations can only be attributed to diverging DC plate resistances between tubes. Tolerances between units even of the same type and brand seem to be important and it would not surprise me that what is identified as 12AU7, when measured, has little to do with the published data specs.

    I'm wondering if the voltages found on the schematic diagram are actual measured voltages or calculated ones based on the data sheets. Roy could help us on this.

    It is possible to actually calculate the voltages by using the data sheets and curves for the 12AU7 and see how close one gets to those numbers with real tubes.

    I'm tempted to do it!


    tubes4hifi
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    Re: DC Voltages look suspicious

    Post by tubes4hifi on Thu Feb 28, 2013 7:20 pm

    it's probably been 5 years since I measured actual voltages on the ST70 board. After building thousands of them, it got to the point of why bother? But in the interest of keeping things accurate, let's get some input from customers with the latest version of the board,
    feel free to post plate voltages here on this thread (pin 1 and pin 6 on each tube, center, left, and right).
    Meanwhile I'll try to get my own amp back in one piece, it's constantly changing from the old version to the new version to the octal version,
    to help customers with boards that aren't quite right due to wrong placement of parts or poor soldering.

    ixe13

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    Join date : 2012-07-11

    Re: DC Voltages look suspicious

    Post by ixe13 on Fri Mar 01, 2013 5:42 pm

    Hi,

    I did my homework... Wink

    A original ST-70 that I restored with Roy's VTA board with the 12AU7. The new version.

    The amp has the original ST-70 power transformer, and the main here is 117 volts today. This amp sounds great, rock solid Bias.

    3 JJ 12AU7
    4 JJ 6l6GC
    Ruby GZ-34

    So here goes:

    Center :
    Pin 1 :122V
    Pin 6 :129V

    Left :
    Pin 1 :216V
    Pin 6 :211V

    Right :
    Pin 1 :213V
    Pin 6 :211V

    But I completly forgot to mesure the B+ Ooops!

    Anyway, that's what I have on this amp

    Cheers

    tubes4hifi
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    Re: DC Voltages look suspicious

    Post by tubes4hifi on Fri Mar 01, 2013 6:05 pm

    thanks ixe13
    is that the latest version with the CCS or is that the older version?

    ixe13

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    Join date : 2012-07-11

    Re: DC Voltages look suspicious

    Post by ixe13 on Fri Mar 01, 2013 6:26 pm

    Hi Roy,

    CCS, I believe, since it has the 2 LM334. Right?

    tubes4hifi
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    Re: DC Voltages look suspicious

    Post by tubes4hifi on Fri Mar 01, 2013 10:34 pm

    yes, that's right, so instead of me installing that in my ST70 this weekend, I'll plug in the octal version, which should have identical voltages, but I'll find out for sure!

    1973shovel

    Posts : 23
    Join date : 2009-12-13

    Re: DC Voltages look suspicious

    Post by 1973shovel on Sat Mar 02, 2013 10:29 am

    Hello Yvon,

    I have always had similar issue with my low gain (non-CCS) VTA board, in that the voltages didn't match several schematics I was sent, or found here. The B+ was within the 410 to 430 volts going to the board, which one schematic indicates, but the board voltages didn't match. I re-checked all resistor values, and they were within tolerance.

    Several inquires regarding the "correct" voltages didn't give me an answer I felt comfortable with. Hopefully bringing the topic up here will lead us to the optimum working voltages for this circuit.

    I will dig through my notes, or hook up the amp and re-measure the voltages and post.

    Thanks for a great topic!


    tubes4hifi
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    Re: DC Voltages look suspicious

    Post by tubes4hifi on Thu Mar 07, 2013 5:59 pm

    I took some measurements on an octal version VTA70 the other day, and they should be about the same as the latest "classic" version with CCS
    that I've been selling now for about a year.
    With 350vdc B+ on the board (it was 400v to the OPTs) I got 120vdc on the plates of V1 (center tube) and 200vdc on the plates of V2 and V3 (the outer tubes). All about 5-10% less than what I show on the schematic that comes with the board kit. That was with 120vac input.
    Pretty close to what IXE13 was getting.

    Yvon Massicotte

    Posts : 13
    Join date : 2009-07-28
    Age : 74
    Location : Montréal, QC, Canada

    Re: DC Voltages look suspicious

    Post by Yvon Massicotte on Tue Mar 19, 2013 12:35 am

    I finally received the JJ ECC82s from Roy. (Let me simply mention here that I had been sent the wrong tubes for my VTA board and this mistake was certainly the main reason for the huge discrepancies in measured versus published voltages that I had encountered).

    Waiting for the JJ's ECC82s to arrive, I bought locally 3 NOS 12AU7s branded "Canadian Marconi Radiotron" made in the 1950s. I decided then to measure voltages with the JJ tubes first and then the Marconi ones and to compare the numbers with calculated values using the tube data sheets. Here are the results:



    - AC voltage input was set at 115VAC by a Variac to take into account that my amp is using an old Dynaco power transformer, designed for 115VAC instead of today's 120VAC.

    Point of Measurement Marconi JJ Calculated
    V-pin 8 rect. 442V 442V -
    V-choke out 431V 431V -
    V- VTA board 380V 383V -
    Point of measurement Marconi-Left Channel Marconi-Right Channel JJ-Left Channel JJ-Right Channel Calculated
    V - junction R11-13 and R12-R14 258V 256V 256V 258V 260V
    V1(input tube) plate 130V 132V 130V 136V 128V
    V1(input tube) cathode 4.2V 4.24V 4.26V 4.16V 4.4V
    V2(PS) plate 210V 211V 206V 208V -
    V3(PS) plate 204V 205V 218V 204V -
    V2(PS)-cathode 94V 93V 91.5V 94.5V -
    V3(PS) cathode 94V 93V 91.5V 94.5V -
    R17-R18 tail resistor 90.4V 89.7V 91V 88V -
    As can be seen here, the Marconis and JJs give essentially the same voltages and for the input tube V1 the values come very close to the calculated ones. In fact, the agreement is so close that I decided not to go into the calculations (I don’t use computer programs for these, so they are quite long to make) for the phase splitter tube and take the values as the proper ones.

    Comparing these measurements with the values given by Roy in his latest schematic sent to me, the agreement is acceptable and the outstanding differences may stem from the 400V that he puts for the B+ to the board against the 380V that I measured.

    Yvon

    tubes4hifi
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    Re: DC Voltages look suspicious

    Post by tubes4hifi on Tue Mar 19, 2013 1:04 am

    alright, nice work, now there is very definitive voltage checks on this board, and glad to see they match what I got a year or so ago.
    I tested another amp recently, and I'll probably wind up changing that 2.2K 3W dropping resistor on the quad cap to a 1.8K 3w to give slightly more voltage (closer to 400v) the the board, but it works fine even at 350v.

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