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    Bias question

    PeterCapo
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    Post by PeterCapo on Thu Oct 15, 2020 9:05 am

    As peterh indicated, other circuit parameters can come into play, which is almost certainly the case with your amp.

    The Van Alstine article was based upon the original Dynaco circuit design. While it's an impressive-looking article, my recollection is that it resulted in mixed listening impressions from those who tried it: http://www.enjoythemusic.com/magazine/manufacture/0902/index.html

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    Skelt

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    Post by Skelt on Thu Oct 15, 2020 9:50 am

    [/quote]
    (2 and 3)  
    The feedback circuits are altered. Unless one can verify it's effectiveness with scope
    and function generator this is suspect. Lacking these resources the best you can do is
    restore to original configuration.
    [/quote]

    I understand that is the prevailing opinion.  For now ill keep it as is unless I find something particularly dangerous or unsafe.

    The basic question is why is the bias so low and can't  be raised to what one expects it should be.
    I suspect the answer is on the driver board.
    My board has fewer resistors and more caps than oem.
    Is there a clear wiring diagram for a pc3?
    Bob Latino
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    Post by Bob Latino on Thu Oct 15, 2020 10:38 am

    PeterCapo wrote:As peterh indicated, other circuit parameters can come into play, which is almost certainly the case with your amp.

    The Van Alstine article was based upon the original Dynaco circuit design.  While it's an impressive-looking article, my recollection is that it resulted in mixed listening impressions from those who tried it: http://www.enjoythemusic.com/magazine/manufacture/0902/index.html

    Frank Van Alstine wrote that article 38 years ago and IMHO, the article is a bit outdated ... A couple of things I don't like ..

    1. The input filter to "restrict bandwidth" ... not really necessary. He is adding two capacitors and one extra resistor that the audio signal has to pass through before it even gets to the driver circuit. That can't help the transparency of the amp at all by adding two extra capacitors and one extra resistor before the audio signal even reaches the driver circuit.

    2. The use of a single 6GH8A pentode/triode driver tube per each channel to handle both voltage amplification and phase splitting/inverting. You really need separate voltage amplification tubes and splitter/inverter tubes to do the job right. Back in 2015 in the early days of the new (now defunct) Dynaco ST-70X they actually tried this circuit and then dropped it in favor of a circuit similar to what the VTA M-125's use. (a dual triode 12AU7 voltage amplifier and all triode phase splitter/inverter circuit)

    Bob


    Last edited by Bob Latino on Thu Oct 15, 2020 2:49 pm; edited 1 time in total

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    Skelt

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    Post by Skelt on Thu Oct 15, 2020 11:30 am

    Thank you Bob for responding to this thread. Ill keep your comments in mind when I get to reading that article. If I decide to rebuild I will definitely purchase one of your boards. Hopefully there's something written down somewhere on this one.
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    Skelt

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    Post by Skelt on Thu Oct 15, 2020 3:36 pm

    The driver schematic turned out not to as hard as I was expecting. I imagine staring at the amp for 5 days has helped.

    Much of the oem circuitry is gone.
    Pin 9 no longer has a cap and resistor to ground
    Feed back is only from the secondary of the OT.

    Missing resistors.  330k, 10ohm x2,  18k x2,  270k x 2

    Changed component values
    270k resistor changed to 110k at pin 2
    1.5M changed to a trim pot.  pin 3
    .05uF changed to 1.0
    620 ohm changed to 200
    47k resistors changed to 30k and the one at pin 8 has an additional trim pot.
    Coupling caps are .22
    The rca has a 75k resistor to ground in stead of 470k
    Bias question - Page 2 10152012
    peterh
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    Post by peterh on Thu Oct 15, 2020 4:19 pm

    Thanks to your work you now has most of the schematics for your amp. This will help
    diagnosing any future problems.
    Reverse engineering use to be difficult, you have now passed the task with good results. Congrats !

    Back to the 39 ohm cathode resistors and bias circuit that prevents you to set bias to 40mA

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    PeterCapo
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    Post by PeterCapo on Thu Oct 15, 2020 4:50 pm

    It seems like this amp was modified by someone with engineering knowledge, and this is his expression of what he liked.  Perhaps the bias is just set up as he intended it to be.  There may be nothing to do.
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    Skelt

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    Post by Skelt on Thu Oct 15, 2020 9:43 pm

    I agree the mods were done by someone with a plan.  

    Looking back at the PS I have an explanation for the low voltage at C measured 350 should be 415. The relay has one set of contacts that shorts the 2k resistor coming off the rectifier. If the relay activates then 430v is applied to the buss.
    The relay is a dpdt and has a latching circuit.
    The rectifier socket is what triggers the relay. A different tube must be required here for the pins to be functional.
    Any idea on how to figure out what tube is supposed to be there?
    Maybe this is the cause of the low bias.

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    peterh
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    Post by peterh on Fri Oct 16, 2020 2:33 am

    Skelt wrote:I agree the mods were done by someone with a plan.  

    Looking back at the PS I have an explanation for the low voltage at C measured 350 should be 415. The relay has one set of contacts that shorts the 2k resistor coming off the rectifier. If the relay activates then 430v is applied to the buss.
    The relay is a dpdt and has a latching circuit.
    The rectifier socket is what triggers the relay. A different tube must be required here for the pins to be functional.
    Any idea on how to figure out what tube is supposed to be there?
    Maybe this is the cause of the low bias.

    Bias question - Page 2 10152013

    Maybe a time-delay relay ? I have seen such in octal sockets
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    Skelt

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    Post by Skelt on Fri Oct 16, 2020 8:39 am

    This was an easy test. Energize the coil on the relay by shorting pins 5 and 7. Then check dc voltages. Now they are running a bit high.
    C 500
    B 460
    A 430
    I can now set the bias to 1.5volts.
    Which now leads to the next question.
    The 4 trim pots on the driver board how do I go about setting them?

    The one at pin 3.
    is it acting as a volume control or right to left channel ballance?

    The one at pin 8.
    I'm guessing the voltage drop at pin 8 to ground should be balanced with the voltage across the 30 k resistor at pin 1?
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    peterh
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    Post by peterh on Fri Oct 16, 2020 9:44 am

    the pot at pin 3 should be adjusted to get 25% of B+ at the anode of the pentode.
    The pot at pin 8 should be removed(shorted) as a concertine is inherently balanced and both
    the plate and cathode resistors are 30k.


    Last edited by peterh on Fri Oct 16, 2020 9:45 am; edited 1 time in total (Reason for editing : correcting)

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    Skelt

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    Post by Skelt on Fri Oct 16, 2020 1:26 pm

    The plate and cathode resistors must be slightly different values and that pot is for balancing the splitter.
    The pot at pin 3 did adjust the voltage at pin 2.
    I set it to 110v. being the voltage at B is a little over 400.
    I'm going to go ahead and hook it back up see how it sounds.
    peterh
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    Post by peterh on Fri Oct 16, 2020 1:33 pm

    Skelt wrote:The plate and cathode resistors must be slightly different values and that pot is for balancing the splitter.
    The pot at pin 3 did adjust the voltage at pin 2.
    I set it to 110v.  being the voltage at B is a little over 400.
    I'm going to go ahead and hook it back up see how it sounds.

    Wrong. The plate and cathode resistors should be equal.
    Current through the tube and resistors are equal, thus the voltage across the
    resistors is equal.
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    Post by Skelt on Fri Oct 16, 2020 1:51 pm

    Weather or not if its the right way to do it the pot at the cathode definitely adjusts the balance. I had two volt meters hooked across those resistors.
    peterh
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    Post by peterh on Fri Oct 16, 2020 2:15 pm

    Skelt wrote:Weather or not if its the right way to do it the pot at the cathode definitely adjusts the balance. I had two volt meters hooked across those resistors.

    If the resistors are equal and the power tube is healthy and matched then there
    is no place for adjustment.


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    Skelt

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    Post by Skelt on Fri Oct 16, 2020 3:34 pm

    A bias resistor let go after 10 min with a signal.
    I see what is meant by red plating.
    Luckly I had a way to make 35 ohms so I'm back in business.
    With higher buss voltages the lows have more authority,
    The highs are as sweet as ever. And the break up when I lean on the volume is gone.
    Let me put some hours on it and ill report back

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    Skelt

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    Post by Skelt on Fri Oct 16, 2020 3:39 pm

    peterh wrote:
    Skelt wrote:Weather or not if its the right way to do it the pot at the cathode definitely adjusts the balance. I had two volt meters hooked across those resistors.

    If the resistors are equal and the power tube is healthy and matched then there
    is no place for adjustment.


    I understand what you are saying and it makes perfect sense but I'm not going to change anything in the circuit for now.

    Next up Emerson Lake and Powell
    fredeb
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    Post by fredeb Yesterday at 3:44 am

    If tubes are running a little hot now , I would install 100e 10W resistor at each power tubes cathode and bias to 4V .

    4V/100e = 40mA ; 40mA x 500V = 20W dissipation ( that's excluding power dissipated by G2 )

    So can , in this scenario be biased to 3.8V .

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    Solder Slinger

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    Bias question - Page 2 Empty Regarding the Pentode or Triode configuration question in the first post...

    Post by Solder Slinger Yesterday at 2:46 pm

    First of all, the ST-70 was never built in a pentode configuration, it was ultralinear. It could be converted to pentode or triode operation, but pentode was rarely used as ultralinear gave better performance at a very slight decrease in power output from pentode mode. That said, if the green or green / white leads from the transformer are connected to pin 4 of the EL-34 output tube socket, then your amp is in ultralinear mode. I would advise adding a resistor on these leads if one is not present, 100 ohm to 1200 ohm 5 watt (non-inductive) are values to consider. 100 Ohm is the traditional value but others suggest a higher value that will decrease power output slightly but improve the sound quality. I use a 680 ohm in my ST-70.  If there is a jumper or a resistor from Pin 3 to Pin 4 (again, 100 to 1200 ohm, non-inductive), and the green and green / white leads are not connected to anything, then your amp is in triode mode.  

    Personally, with your speakers ( I believe I saw they were Klipsch somewhere in the postings ), I would try the triode mode. You should still have plenty of power.

    Regarding biasing, I would recommend 35 ma to 40 ma, not only will this improve tube life, but it will give a richer sound.

    -Ed


    Last edited by Solder Slinger on Tue Oct 20, 2020 3:29 pm; edited 1 time in total

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    Solder Slinger

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    Bias question - Page 2 Empty Two other things, for what they are worth...

    Post by Solder Slinger Yesterday at 3:26 pm

    One, add an NTC to one of the power cord leads in the amp. A NTC is a Negative Temperature Coefficient resistor that starts as a high resistance and drops to almost zero ohms as it warms up.

    Why ?

    One, your amp was built for 115 to 117 VAC, today it's commonly 122 or higher. The NTC will knock a couple of volts off that number and bring you tube heaters closer to 6.3 vac, which will extend their life. I believe a CL-80 is the proper value for an ST-70 ( 3 amp ), but some of my friends have added an CL80 to both the hot and the return leads to lower the voltage further or paralleled a pair of 2 amp CL-90s to get even a higher initial resistance. Measure the AC across EL-34 pins 2 & 7 to see what your allegedly 6.3 VAC actually is.

    More importantly, the slow turn on will help protect you tube heaters from an inrush of current when you "flip the switch". Robert Tomer, a tube expert from the 1950s indicated in his book that an NTC will reduce the probability of tube heater failure by a factor of 4.

    Second, if you have an accurate voltmeter with a 200 mv scale, consider the following; use a 1 ohm bias resistor between pins 1 & 8 to ground. Using the 200 mv scale on your meter, you read the bias current in equivalent milliamps, i.e. 100 mv measured = 100 ma. An additional benefit is lower overall amp impedance, your dynamics will probably be better.

    I buy a 1 ohm, 1/2 % resistor from Mouser which gives me a precise measurement with my accurate meter.  

    -Ed

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    Skelt

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    Post by Skelt Yesterday at 3:38 pm

    I read the Van Alstine article and I agree with Bob. I believe adding a band pass filter on the input would not help to th transparency. I don't see any thing on Van Alstine schematics that matches with what I have.


    I've put over 20 hours on it now since the resistor was replaced. Its not getting any hotter than the unmodded one. I'm using one amp per channel and a 10ohm resistor across the unused OTs.
    The bias is holding steady at 1.5v.  I'm satisfied its holding together so it can come back to the bench for more analysis.

    I can see now courtesy of my drawings that this  amp isn't in triode mode. I might try that later after further circuit discussion.

    Here is an updated PS schematic. I added in the relay and fixed an error on the 1200uF caps. They are in series not parallel yielding 600uF and 500 volt ratings.
    The voltages on the sketch are before and after the relay kicks in.
    They are higher than what the dyna manual says they should be.
    Are they too high for the rest of the circuitry?
    A 420
    B 460
    C 500

    Is there any benefit or harm with A and B in parallel? Both are hooked to C and aren't in series.

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    Last edited by Skelt on Tue Oct 20, 2020 3:53 pm; edited 1 time in total
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    Post by Skelt Yesterday at 3:52 pm

    Solder Slinger wrote:One, add an NTC to one of the power cord leads in the amp. A NTC is a Negative Temperature Coefficient resistor that starts as a high resistance and drops to almost zero ohms as it warms up.

    Good idea, Ill look onto putting one in especially since it had a SS rectifier.


    Second, if you have an accurate voltmeter with a 200 mv scale


    I do

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