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    ST70, Gold Lion KT-77's low bias

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    mmmalmberg

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    ST70, Gold Lion KT-77's low bias

    Post by mmmalmberg on Fri Mar 02, 2018 5:13 pm

    One of my four winged-C Svetlana's was flaring and my ST-70 was blowing fuses. I had first tried swapping rectifier tubes having blown a fuse before seeing the tube flaring.

    So ordered Gold Lion Kt-77's, can't bring the bias up past about 1.38VDC. Is this a risk of going to KT-77's? Bias was fine with the Svet's and whatever tubes I had before those.

    Thanks for any tips. I did a few searches but there are so many variety of low bias threads, I hope this exact question hasn't been discussed before (probably has...) if so just let me know.

    Any thoughts appreciated.
    -Mark M.

    p.s. No it's not stock, different driver board and upgraded p.s. and power xformer, but has been running fine for years 'til now...
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    Bob Latino
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    Re: ST70, Gold Lion KT-77's low bias

    Post by Bob Latino on Fri Mar 02, 2018 5:49 pm

    mmmalmberg wrote:One of my four winged-C Svetlana's was flaring and my ST-70 was blowing fuses. I had first tried swapping rectifier tubes having blown a fuse before seeing the tube flaring.

    So ordered Gold Lion Kt-77's, can't bring the bias up past about 1.38VDC.  Is this a risk of going to KT-77's? Bias was fine with the Svet's and whatever tubes I had before those.

    Thanks for any tips. I did a few searches but there are so many variety of low bias threads, I hope this exact question hasn't been discussed before (probably has...) if so just let me know.

    Any thoughts appreciated.
    -Mark M.

    p.s. No it's not stock, different driver board and upgraded p.s. and power xformer, but has been running fine for years 'til now...

    If your amp still has the original Dynaco bias system, try paralleling each of the two 10K resistors on the 7 lug terminal strip with another 10K resistor. This will alter the bias range to the point where you should now be able bias your tubes properly. This has come up before from people who have replaced the selenium rectifier with a diode. A diode has more forward voltage than the selenium rectifier and requires somewhat lower value resistors on the 7 lug terminal strip.

    Bob

    BNR_1

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    Re: ST70, Gold Lion KT-77's low bias

    Post by BNR_1 on Fri Mar 02, 2018 5:57 pm

    In case you wanted a bit more info to follow on to Bob's suggestion. I just read it yesterday so the timing was perfect.

    http://dynacotubeaudio.forumotion.com/t43-selenium-rectifier?highlight=selenium+rectifier+resistor+mod

    mmmalmberg

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    Re: ST70, Gold Lion KT-77's low bias

    Post by mmmalmberg on Fri Mar 02, 2018 6:20 pm

    Thanks Bob and BNR. So, if I haven't replaced the rectifier with a diode, this is still a solution, yes?

    If I go back to EL34's am I likely to need to undo this mod?

    Thx again...

    BNR_1

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    Re: ST70, Gold Lion KT-77's low bias

    Post by BNR_1 on Fri Mar 02, 2018 6:48 pm

    mmmalmberg wrote:Thanks Bob and BNR. So, if I haven't replaced the rectifier with a diode, this is still a solution, yes?

    If I go back to EL34's am I likely to need to undo this mod?

    Thx again...

    Just to be clear we are speaking of the selenium rectifier. If the existing is still wired it should be replaced with a diode. 1N4007 silicon diode is adequate.

    Regardless of the tube you choose to use (EL34, KT77, etc.) the selenium rectifier should be removed and replaced with a modern diode.

    Separate of those questions, I would assume your selenium rectifier was replaced given the modifications you described on this amp.

    mmmalmberg

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    Re: ST70, Gold Lion KT-77's low bias

    Post by mmmalmberg on Fri Mar 02, 2018 11:30 pm

    BNR_1 wrote:

    Just to be clear we are speaking of the selenium rectifier.  If the existing is still wired it should be replaced with a diode.  1N4007 silicon diode is adequate.

    Regardless of the tube you choose to use (EL34, KT77, etc.) the selenium rectifier should be removed and replaced with a modern diode.

    Separate of those questions, I would assume your selenium rectifier was replaced given the modifications you described on this amp.

    I don't remember replacing it but it doesn't mean I didn't, that was probably 12-14 years ago. I'll check - thanks:)

    Just to get further to the heart of things, is it typical or at least logical for some reason that the switch to KT-77's from EL-34's resulted in the inability to reach the correct voltage? (they were said to be "drop-in replacements")


    mmmalmberg

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    Re: ST70, Gold Lion KT-77's low bias

    Post by mmmalmberg on Sun Mar 04, 2018 5:45 pm

    Checked both my st70's and selenium rectifiers are long gone. I have a CAE PC-S7UB power supply board in each.  There's no terminal strip as described, so I assume those parts may be on the p.s. board. Looks like maybe R15 and R16 per the docs http://www.curcioaudio.com/ST7U-B-DOC.pdf

    OK working as I write this, piggybacked 10K's on top of the 10K's (didn't have any 1W resistors of range to just replace). Happy Gold Lions.


    mmmalmberg

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    Re: ST70, Gold Lion KT-77's low bias

    Post by mmmalmberg on Sun Mar 04, 2018 5:45 pm

    (thanks all:)

    mmmalmberg

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    Re: ST70, Gold Lion KT-77's low bias

    Post by mmmalmberg on Sun Mar 04, 2018 7:59 pm

    Quick follow-up, is 1.56 (or measured resistor value / 10) still to be considered correct bias voltage with the kt-77's?

    mmmalmberg

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    Re: ST70, Gold Lion KT-77's low bias

    Post by mmmalmberg on Mon Mar 12, 2018 1:06 pm

    Well in any case it's working and sounds as good as my ears can hear:) Now on to the intermittent hum/noise in my CJ PV-5...
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    Bob Latino
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    Re: ST70, Gold Lion KT-77's low bias

    Post by Bob Latino on Mon Mar 12, 2018 1:21 pm

    mmmalmberg wrote:Quick follow-up, is 1.56 (or measured resistor value / 10) still to be considered correct bias voltage with the kt-77's?

    If the board that is being used in your amp uses the original bias pots, then you would normally use the 1.56 bias point BUT > because of the state of today's output tubes, you should probably lower the bias point on your amp to about 1.25 VDC (actually 1.248 VDC) per each channel. This lower bias setting will give a bias current of 40 milliamps per each tube of the pair on each channel. Your tubes will last longer and the amp will sound the same.

    Bob

    mmmalmberg

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    Re: ST70, Gold Lion KT-77's low bias

    Post by mmmalmberg on Wed Mar 14, 2018 11:00 am

    Thanks Bob - I had them at 1.5 and will drop to 1.25; much obliged.

    DC92

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    Re: ST70, Gold Lion KT-77's low bias

    Post by DC92 on Sun Jul 29, 2018 10:03 pm

    Hi Folks, I'm going to hop on this thread with my own issue. You've all helped in the past tremendously. I've got an st-70. I replaced the Quad Cap 3 or 4 years ago to the one Bob recommended. I am using JJ KT77s. Recently, I had a tube flame out. I replaced it with a matching set. Rebiased and everything was fine. I recently checked the bias, out of principle more than any audible issues. I found the bias to be running really cool, like at .80 per side, instead of the usual 1.56.

    Maybe the amp is running a touch quiet but other wise sounds pretty normal.

    I'll add that if I remove the Rectifier and the 4 output tubes, and I check the output sockets from the 5th hole, the reading begins at around -1.03 and drops .001 per second till stopping at .890 for all pin 5s across all 4 sockets at the lowest bias turn and -.871 at full throttle bias turn.



    Any ideas what might be causing this problem? Thanks!
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    peterh

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    Re: ST70, Gold Lion KT-77's low bias

    Post by peterh on Mon Jul 30, 2018 1:46 am

    DC92 wrote:Hi Folks, I'm going to hop on this thread with my own issue. You've all helped in the past tremendously. I've got an st-70. I replaced the Quad Cap 3 or 4 years ago to the one Bob recommended. I am using JJ KT77s. Recently, I had a tube flame out. I replaced it with a matching set. Rebiased and everything was fine. I recently checked the bias, out of principle more than any audible issues. I found the bias to be running really cool, like at .80 per side, instead of the usual 1.56.

    Maybe the amp is running a touch quiet but other wise sounds pretty normal.

    I'll add that if I remove the Rectifier and the 4 output tubes, and I check the output sockets from the 5th hole, the reading begins at around -1.03 and drops .001 per second till stopping at .890 for all pin 5s across all 4 sockets at the lowest bias turn and -.871 at full throttle bias turn.



    Any ideas what might be causing this problem? Thanks!
    Seems to be a measurment problem.
    Pin5 on the output sockets is the G1, and it should read -30 to -40V in your case.
    Before doing anything more drastic, do check the voltmeter . Then remount tubes and
    adjust bias as per manual.
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    Bob Latino
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    Re: ST70, Gold Lion KT-77's low bias

    Post by Bob Latino on Mon Jul 30, 2018 7:36 am

    DC92 wrote:

    I'll add that if I remove the Rectifier and the 4 output tubes, and I check the output sockets from the 5th hole, the reading begins at around -1.03 and drops .001 per second till stopping at .890 for all pin 5s across all 4 sockets at the lowest bias turn and -.871 at full throttle bias turn.

    Any ideas what might be causing this problem? Thanks!

    If you still have the original Dynaco selenium rectifier, the selenium rectifier may be bad ? Replace the selenium rectifier. If the selenium rectifier has already been replaced with a diode, it could be the two electrolytic capacitors on the 7 lug terminal strip that are bad ? You did say in your first post that the amp had been modified."p.s. No it's not stock, different driver board and upgraded p.s. and power xformer, but has been running fine for years 'til now.."You should probably post or show what the modifications are ? There may be something wrong with the bias system on your amp ? On an original type Dynaco ST-70 you should get from about -48 VDC to about -24 VDC from pin #5 to chassis ground on each output tube socket as you turn the bias pot from one end to the other and that voltage should hold steady at all points along the way.

    Why don't you post a fairly large photo of the inside wiring here ? Maybe one of us can pick up on something in the wiring to help you out ?

    Bob

    DC92

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    Re: ST70, Gold Lion KT-77's low bias

    Post by DC92 on Wed Aug 01, 2018 10:58 am

    Bob Latino wrote:
    DC92 wrote:

    I'll add that if I remove the Rectifier and the 4 output tubes, and I check the output sockets from the 5th hole, the reading begins at around -1.03 and drops .001 per second till stopping at .890 for all pin 5s across all 4 sockets at the lowest bias turn and -.871 at full throttle bias turn.

    Any ideas what might be causing this problem? Thanks!

    If you still have the original Dynaco selenium rectifier, the selenium rectifier may be bad ? Replace the selenium rectifier. If the selenium rectifier has already been replaced with a diode, it could be the two electrolytic capacitors on the 7 lug terminal strip that are bad ? You did say in your first post that the amp had been modified."p.s. No it's not stock, different driver board and upgraded p.s. and power xformer, but has been running fine for years 'til now.."You should probably post or show what the modifications are ? There may be something wrong with the bias system on your amp ? On an original type Dynaco ST-70 you should get from about -48 VDC to about -24 VDC from pin #5 to chassis ground on each output tube socket as you turn the bias pot from one end to the other and that voltage should hold steady at all points along the way.

    Why don't you post a fairly large photo of the inside wiring here ? Maybe one of us can pick up on something in the wiring to help you out ?

    Bob

    Thanks for the response. The selenium rectifier has been changed. I changed the quad cap to your recommendation about three, maybe four years ago. I've always gotten 1.56, since I made those changes, but now I'm getting half of that, max. I've tested my voltometer and it reads 1.56 on batteries. My ST-70 has not been modified other than those mentioned: upgraded quad cap. KT-77s, selenium rectifier replaced.

    What pictures would be most helpful?
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    Bob Latino
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    Re: ST70, Gold Lion KT-77's low bias

    Post by Bob Latino on Wed Aug 01, 2018 11:05 am

    Post a fairly large photo of the inside wiring of your amp.

    Also, try this > With all the tubes out of the amp and with the bias pots centered, measure the DC voltage to chassis ground on pin 5 of all output tube sockets. Let me know what voltage you get at all four measuring points ?

    Bob

    DC92

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    Re: ST70, Gold Lion KT-77's low bias

    Post by DC92 on Thu Aug 02, 2018 11:03 am

    Thanks so much for the response and the help.


    When I tested pin 5 on all four output sockets, I got an initial reading of .723 VDC with a drop of .001 VDC / sec until I stopped measuring with a reading a .356 and still dropping at that steady rate across all four tube sockets. As I dailed up and down the bias pot, it didn't affect the bias readings from pin 5 at all. However, the bias does change when the tubes are plugged in, and I measure the bias from the front sockets. They read up to .80 but don't get higher than that.

    When I posted here several years ago, I had a hum across both channels and a bias issue. You recommended I change the Quad Cap, which I did and it fixed the hum and the bias.

    I don't really have an audible symptom yet other than a slightly noticeable drop in volume, maybe. Otherwise, it seems to be working ok at the moment.

    I don't guess it would be the quad cap?

    But I know something must be happening or this bias issue wouldn't be happening.  





    Last edited by DC92 on Thu Aug 02, 2018 11:57 am; edited 2 times in total (Reason for editing : Added info.)
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    Bob Latino
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    Re: ST70, Gold Lion KT-77's low bias

    Post by Bob Latino on Thu Aug 02, 2018 3:17 pm

    Either that selenium rectifier is bad OR the two capacitors on the 7 lug terminal strip are bad.

    Bob

    DC92

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    Re: ST70, Gold Lion KT-77's low bias

    Post by DC92 on Thu Aug 02, 2018 3:33 pm

    Bob Latino wrote:Either that selenium rectifier is bad OR the two capacitors on the 7 lug terminal strip are bad.

    Bob

    Thanks so much, Bob.

    I'm not sure if it shows, but the selenium rectifier has been replaced. You can see the replacement diode connected to the lug. However, that replacement work preceded me.

    Can you confirm that it has been replaced? And, if so, should I try to replace that diode?

    And is there anything special I need to worry about in replacing the two the caps on the 7-lug?

    Lastly, what is the danger of running this amp in the current state?

    Thanks so much!

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