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Thread: bias raising AFTER standby...

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    Question bias raising AFTER standby...

    I have an odd case that I could not understand at all (still learning)
    It's '69 Ampeg B15NF (fixed bias) 2x6L6GC + 5AR4.

    Now - amp is warming up, and when I hit standby I see on my meter that bias is *just starting* to raise up from 0 to the +/- 40mA per tube - it takes around 30 seconds. On my other Ampegs from that era, after warmup, when I hit standby I have that reading after 1-2 secs. So, what I'm trying to tell, it looks like amp behaves like its' super cold and not warmed up - the same like you would be hit power and standby at the same time from the beginning. I have recapped it and also re-did bias section (new cap, new resistor). It plays great overall.

    What could be the reason of that behavior?

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    Last edited by boroman; 03-23-2020 at 12:30 PM.

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    A lot of output valves take a while longer than others to reach full temperature. If the 6L6s are well worn, that can cause that phenomenon.

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    The most probable reason is the slow HT/B+ ramp-up caused by the long warm-up time of the GZ34/5AR4 of around 20s. Idle current depends on B+ and especially on screen voltage.
    A directly heated 5U4 would be faster but will reduce B+.

    No other time constant in a properly working tube is long enough to explain a delay of 30s.

    Could you post a schematic?

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    Last edited by Helmholtz; 03-23-2020 at 09:26 PM.
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    what about filament voltage?
    some amps have a reduction in standby

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    If you want to compare to other amps you need to also look at the rectifier type, and the standby switch arrangement.
    In this case, the schematic I saw shows the standby switch connecting the HT winding center tap to ground.

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    "when I hit standby I see on my meter that bias is *just starting* to raise up from 0 to the +/- 40mA per tube"

    Let's first get our terminology right. What you are describing is not your bias, it is your tube current draw. Your bias is the neg voltage at the letter B at the lower left and upper right of the schematic. That is applied to pin 5 of your 6L6s, and should be there when in standby.

    Also don't say +/- 40ma. It is always going to be positive, so just say 40mA.

    https://www.thetubestore.com/lib/the...-Schematic.pdf

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    It's weird, because it WAS working fine.....

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    I recommend to monitor ramp up of B+ and screen voltage. There is a slight chance that the delay is caused by a leaky filter cap that re-forms when voltage is applied.

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    Quote Originally Posted by Randall View Post
    "when I hit standby I see on my meter that bias is *just starting* to raise up from 0 to the +/- 40mA per tube"

    Let's first get our terminology right. What you are describing is not your bias, it is your tube current draw. Your bias is the neg voltage at the letter B at the lower left and upper right of the schematic. That is applied to pin 5 of your 6L6s, and should be there when in standby.

    Also don't say +/- 40ma. It is always going to be positive, so just say 40mA.

    https://www.thetubestore.com/lib/the...-Schematic.pdf
    Yes, this is that schematic. When I said +/- 40mA I meant 36-41mA range when I engage it.
    I have two the same amps on the bench. One is like that, one is normal...

    I recommend to monitor ramp up of B+ and screen voltage. There is a slight chance that the delay is caused by a leaky filter cap that re-forms when voltage is applied.
    I just recapped both amps. I'll monitor those voltages today. Could it be the 5AR4 tube causing problems?

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    Is there an actual problem?

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    Could it be the 5AR4 tube causing problems?
    As you have 2 amps, just exchange rectifiers.

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    Quote Originally Posted by pdf64 View Post
    Is there an actual problem?
    ^^^^^^ That.

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    Quote Originally Posted by boroman View Post
    ...When I said +/- 40mA I meant 36-41mA range when I engage it...
    It's a handy, common (technical) shorthand to use the tilde symbol to indicate an approximate value, eg ~40mA.

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    Quote Originally Posted by pdf64 View Post
    It's a handy, common (technical) shorthand to use the tilde symbol to indicate an approximate value, eg ~40mA.
    I'm with you, but it's also used to indicate AC, so it can sometimes get a bit problematic. In this case, it's obviously DC, but I do recall cautioning someone against using it once (in a more confusing context).

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    Yes... I have never seen the amp that behaves like that. And when you hit standby you must wait 30 seconds to get full bias/warmup on the tubes, so you can't start playing right away.
    I have swapped rectifier tube (power tubes also) and nothing is changed. Where should I look for a mistake? I have not changed original wiring, just replaced 3x40uF multicap with 3 separate 47uf, and the alone 30uf by 33uf (+recapped bias section). Bias resistor is 1W (and wired in series with a 20k pot) instead of original 1/4W - is wattage of the bias resistor a key here?

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    I'll monitor those voltages today.
    Results?

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    Quote Originally Posted by Helmholtz View Post
    Results?
    They do not differ from the other B15N head I have here, nothing suspicious but maybe I'm missing something.

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    Quote Originally Posted by boroman View Post
    They do not differ from the other B15N head I have here, nothing suspicious but maybe I'm missing something.
    THe question is if they (especially screen voltage) ramp up slowly as well.

    Also monitor how the bias voltage at point B changes in the first 30 seconds.

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    Last edited by Helmholtz; 03-30-2020 at 03:35 PM.
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    Quote Originally Posted by Helmholtz View Post
    THe question is if they (especially screen voltage) ramp up slowly as well.
    Also monitor how the bias voltage at point B changes in the first 30 seconds.
    After powering screen and plate goes up to around
    - 185v screens pin4
    - 190v plate pin3,

    After I hit standby, they go to around 489v (screen) and 496 (plate) volts. It takes around 30 seconds to get to those readings (the same for bias)


    What is "point B" if I may ask?

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    It's your negative bias supply. Left side of posted schematic, 2/3 down from top.

    Justin

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    What is "point B" if I may ask?
    Look at the schematic.

    After I hit standby, they go to around 489v (screen) and 496 (plate) volts. It takes around 30 seconds to get to those readings (the same for bias)
    Ok, here is your real problem. Full idle current can only develop when supply voltages are fully up. I guess the voltages in your other amp ramp-up faster? Are standby switches wired identical?

    I know you recapped the amp but still suspect filter caps. New caps are not necessarily good caps.

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    Quote Originally Posted by Helmholtz View Post
    Ok, here is your real problem. Full idle current can only develop when supply voltages are fully up. I guess the voltages in your other amp ramp-up faster? Are standby switches wired identical?
    I know you recapped the amp but still suspect filter caps. New caps are not necessarily good caps.
    I have tested those caps before I put them in the amp, but ok, they could went bad somewhere during installation.
    Question is: when the amp is warmed up and all the voltages are up, it sounds great - no hint of hum, full power.

    Should I still blame those caps then?

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    Please answer my questions.

    And yes, new ecaps can have excessive leakage current at higher voltages which might not show with low voltage testing. After some operating time leakage reduces and voltage increases.

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    Last edited by Helmholtz; 03-30-2020 at 11:24 PM.
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    Answering that questions, both amps are recapped and both amps have identical circuit and standby switch wired the same way.

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    It takes around 30 seconds to get to those readings (the same for bias)
    Same with other amp (I already asked)?

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    Double check that you have the bias cap wired with positive to ground. Also, check your voltages are stable at the PSU caps rather than at the other end at the tube.

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    Double check that you have the bias cap wired with positive to ground.
    I also considered that possibilty but a reverse wired bias cap would develop high leakage current. That would cause less negative bias voltage and high idle currents.

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    Last edited by Helmholtz; 03-31-2020 at 06:08 PM.
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    Quote Originally Posted by Helmholtz View Post
    I also considered that possibilty but a reverse wired bias cap would develop high leakage current. That would cause less negative bias voltage and high idle currents.
    It's in correct position (positive to ground). I'll post pic later.

    By the way. I have changed value of this cap to lower and bias is rising way quicker now. How low could I get here and be stable? the bias cap is 100uf/100v. On previous years it was 50uf, and later years - 2x10 uf (quite different approach).

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    By the way. I have changed value of this cap to lower and bias is rising way quicker now.
    A lower value bias filter cap will accelerate the buildup of bias VOLTAGE. But it might even somewhat slow down the ramp-up of tube idle CURRENT.

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    Your other Ampegs from that era, do they have + HV rectifier with tube or solid state.
    Recommendation for + HV rectifier with tube
    https://ampeg.com/pdf/R12R212R.pdf
    16. Power switch turns the main power on and off. Always turn this switch on first, off last.
    Turn the Standby switch (#15) on at least 30 seconds after turning on the Power switch.
    The assumption is that in your '69 Ampeg B15NF output tubes (2x6L6GC) and rectifier (5AR4) are new, not "as new"
    Old power tubes, dried electrolytic capacitors cause longer warming up time.
    Question
    When you turn on the amp, for which time (sec), voltages stabilize and reach the rated value (+430 and -50V).

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    https://www.vintage-blue.com/assets/b15nf_service_manual.pdf

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    Quote Originally Posted by Helmholtz View Post
    A lower value bias filter cap will accelerate the buildup of bias VOLTAGE. But it might even somewhat slow down the ramp-up of tube idle CURRENT.
    I think I have found out the issue.
    As I have two identical amps on my bench, one was working fine few years with me on the road and in the studio. And it's EXACTLY wired the same, recapped with the same components and connections - that lead me to overall confusion. BUT I have found that they are not quite wired the same way! But it's not my fault, it's a factory issue.

    It all comes to wiring the rectifier socket. The "GOOD" amp has PT red wires connected to pins 4 and 7 (factory solder joints). The "BAD" has them on 4 and 6 (also factory solder joints, red-pen marked). NOW - the schematic calls for 4+6, the rectifier tube does not have even any pin on 7th slot and the 7th pin on the socket is not wired to anything.

    So what I've done, I've resoleded the 6th pin -> 7th in the "bad" amp and VIOLA! It works great. No more 200+DC voltages on caps and anodes when powering the amp (now close to zero), and after I hit standby it catches bias voltage right away within seconds. So performing excellent. Now, WHY??? Can someone explain it to me?

    Picture proof: Here's a shot of "GOOD" amp on my bench that was main amp for years. It's FACTORY soldered to pin7. No signs of solder or any other work on pin6. It even has factory red markings on solder joints there. When powered ON, I have 0VAC on red pin4, and 720VAC on red (unused) pin7. When amp is in playing mode, both pins have equal 360VAC. What's ever more odd, I looked at photos I have of my older (cathode biased) and younger (fixed bias but different PCBs) B15Ns and they all have connection on pins 4+6 and they all have absolutely no issues and problems. WTF is going on

    Click image for larger version. 

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    Well done spotting that! I guess your amp was half wave rectified all these years. The HT should have been more saggy under load. The weird HT winding Vac in standby are probably due to the bias supply rectifier being half wave and connected to the pin 6 winding leg, as standby lift the CT 0V connection.

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    Last edited by pdf64; 04-15-2020 at 10:42 AM.

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    Quote Originally Posted by pdf64 View Post
    Well done spotting that! I guess your amp was half wave rectified all these years. The HT should have been more saggy under load. The weird HT winding Vac in standby are probably due to the bias supply rectifier being half wave and connected to the pin 6 winding leg, as standby lift the CT 0V connection.
    I get that, but what's up with 200V on the plates/ps_caps when not in playing mode (4+6pin) ---> and when I resolder 6pin to 7pin I have no voltages on plates/ps_caps and amp have correct (quick) bias ramp buildup then. So with that type of connection I have more sag but proper voltages everyhwere?

    Let me tell you what I see it. Should there be 720V on pin6? I think it could kill a rectifier tube quick? If the amp would have 0V on pins 4 and 6, and then after standby - 360V on each one, then it's a perfect connection? Do I think right?

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    Confused - is the amp now working right? The HT should have been saggy before, when it was half wave rectified, stiffer now that it’s full wave.
    It would be best not to use this hot switching type standby with a tube rectifier http://www.valvewizard.co.uk/standby.html
    The 720Vac is immaterial; as there’s no current flow via the tube it’s equivalent to a spurious voltage, similar to static electricity.

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    Quote Originally Posted by pdf64 View Post
    Confused - is the amp now working right? The HT should have been saggy before, when it was half wave rectified, stiffer now that it’s full wave.
    It would be best not to use this hot switching type standby with a tube rectifier http://www.valvewizard.co.uk/standby.html
    The 720Vac is immaterial; as there’s no current flow via the tube it’s equivalent to a spurious voltage, similar to static electricity.
    Maybe I got it wrong, sorry, once again... I understood that connecting of of those wires to "dummy" PIN7 makes the amp more saggy? Or the other way? (when it's connected to PIN6 to one of the half of the recitfier)?

    The case is, I've got four B15Ns here.
    1st - c. 1962 with factory pins connected to 4+6. No voltages on PS caps/anodes when powered on. Full voltages when in playing mode, bias ramp up very quickly. -> ALL OK
    2nd - c. 1969 with factory pins connected to 4+7(dummy). No voltages on PS caps/anodes when powered on. Full voltages when in playing mode, bias ramp up very quickly. -> ALL OK
    3rd - c. 1969 with pins connected to 4+6. around 200V on caps and anodes when powered on. Full voltages when in playing mode, bias ramp up very slowly (gets like above when pin 6 is unsoldered or soldered to pin7)
    4th - 1972 with pins connected to 4+6. No voltages on PS caps/anodes when powered on. Full voltages when in playing mode, bias ramp up very quickly. -> ALL OK

    So, no circuit changes, no schematic changes in this section.

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    No doubt that connecting to pins 4+6 is correct. Using pin 7 makes no sense. It results in utilizing only one half of the HT winding and only one of the rectifier diodes, resulting in increased sag and low frequency (60Hz) ripple. It's a wiring error.

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