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  • 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?
    Last edited by boroman; 03-23-2020, 12:30 PM.

  • #2
    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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    • #3
      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?
      Last edited by Helmholtz; 03-23-2020, 09:26 PM.
      - Own Opinions Only -

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      • #4
        what about filament voltage?
        some amps have a reduction in standby

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        • #5
          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.
          "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

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          • #6
            "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
            It's weird, because it WAS working fine.....

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            • #7
              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.
              - Own Opinions Only -

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              • #8
                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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                • #9
                  Is there an actual problem?
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                  • #10
                    Could it be the 5AR4 tube causing problems?
                    As you have 2 amps, just exchange rectifiers.
                    - Own Opinions Only -

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                    • #11
                      Originally posted by pdf64 View Post
                      Is there an actual problem?
                      ^^^^^^ That.
                      "Yeah, well, you know, that's just, like, your opinion, man."

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                      • #12
                        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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                        • #13
                          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).
                          "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

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                          • #14
                            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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                            • #15
                              I'll monitor those voltages today.
                              Results?
                              - Own Opinions Only -

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