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Thread: Tube preamp PT secondary SSR switch

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    Tube preamp PT secondary SSR switch

    Hi,

    I wanted to incorporate a time delay switch for one of my tube preamps in order to prevent the CF from exceeding the heater to cathode limit so I decided to make a slow start switch for the HV secondary. My experience with SSRs is limited to using them as relay pop mute switches only so I was wondering if I did this right. The switch works so far but I'm not sure if I'm not missing something. The preamp has 5 12AX7s, current draw at full voltage is ~10mA. The SSR is rated at 600VAC or 600VDC, 50mA.
    One thing I noticed at the moment of SSR switching there's a short very quiet buzz in the PT.

    https://www.mouser.com/datasheet/2/3..._e-1299724.pdf

    Click image for larger version. 

Name:	PT Sec SSR Switch.JPG 
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ID:	56765

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    Lifetime Member Enzo's Avatar
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    I have never seen an amp with that feature.

    Just thinking, the cathode of a CF tube does not come up to voltage until the tube is heated enough to conduct. The plate will instantly come to B+, but not the cathode. So I think the tube itself protects this voltage issue.

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    Quote Originally Posted by GainFreak View Post
    Hi,

    I wanted to incorporate a time delay switch for one of my tube preamps in order to prevent the CF from exceeding the heater to cathode limit so I decided to make a slow start switch for the HV secondary. My experience with SSRs is limited to using them as relay pop mute switches only so I was wondering if I did this right. The switch works so far but I'm not sure if I'm not missing something. The preamp has 5 12AX7s, current draw at full voltage is ~10mA. The SSR is rated at 600VAC or 600VDC, 50mA.
    One thing I noticed at the moment of SSR switching there's a short very quiet buzz in the PT.

    https://www.mouser.com/datasheet/2/3..._e-1299724.pdf

    Click image for larger version. 

Name:	PT Sec SSR Switch.JPG 
Views:	34 
Size:	41.9 KB 
ID:	56765
    Yeah, it's not the heater cathode voltage you need to worry about at a cold start. It's the grid cathode voltage, where the grid is at the HT potential at start up (in a DC coupled circuit). The cathode voltage is grounded via the cathode load resistor, and only rises once the tube begins to conduct.
    (again, assuming your heater voltage is somewhere around DC ground)

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    I have never seen an amp with that feature.
    It's just a preamp.
    So I could be overreacting on this issue but my question was about any problems with the SSR switching as implemented.

    It's the grid cathode voltage, where the grid is at the HT potential at start up (in a DC coupled circuit).
    At startup the HV goes up instantly to 400V+. With this feature it goes gradually to no more than 150V.

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    Quote Originally Posted by GainFreak View Post
    At startup the HV goes up instantly to 400V+. With this feature it goes gradually to no more than 150V.
    Right. There’s certainly nothing wrong with a soft start circuit in this case. My point was simply that the grid/cathode voltage is the concern here, not the heater/cathode.

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    Quote Originally Posted by SoulFetish View Post
    Yeah, it's not the heater cathode voltage you need to worry about at a cold start. It's the grid cathode voltage, where the grid is at the HT potential at start up (in a DC coupled circuit).
    I suppose it depends on the cathode follower circuit. In a bootstrapped CF, the control grid voltage is still controlled by the cathode voltage, not the plate voltage. If the CF is biased by a voltage divider, the voltage will still be divided, not full B+.

    There is the chance that the plate voltage could exceed the preamp tube's maximum rating at start-up, before the tube conducts, but that's true of almost ALL tube preamps.

    Back in the days of the magazine Vacuum Tube Valley, I saw a circuit where someone used a slow warmup tube rectifier as a series pass device to delay B+. It didn't actually rectify. Don't kill the messenger. I'm just reporting what I saw. They think about this stuff a lot more in HiFi/audiophile circles than with guitar amps.

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    Quote Originally Posted by Rhodesplyr View Post
    I suppose it depends on the cathode follower circuit. In a bootstrapped CF, the control grid voltage is still controlled by the cathode voltage, not the plate voltage. If the CF is biased by a voltage divider, the voltage will still be divided, not full B+.

    There is the chance that the plate voltage could exceed the preamp tube's maximum rating at start-up, before the tube conducts, but that's true of almost ALL tube preamps.

    Back in the days of the magazine Vacuum Tube Valley, I saw a circuit where someone used a slow warmup tube rectifier as a series pass device to delay B+. It didn't actually rectify. Don't kill the messenger. I'm just reporting what I saw. They think about this stuff a lot more in HiFi/audiophile circles than with guitar amps.
    All fair point

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    I suppose it depends on the cathode follower circuit.
    It's a "regular" DC coupled "Marshall type" CF.

    I did some actual measurements with and without the delay circuit. Grid to cathode voltage doesn't exceed 50V at startup then becomes zero. Cathode to heater goes quickly up to 220V and then goes down to 180V which is it's normal operation voltage.
    With the delay circuit on the grid to cathode voltage is below 20V at startup. Cathode to heater goes up to 70V and after the 15 sec delay to 180V.
    I also have Merlin's CF protection circuit implemented (100Ohm/47k-1N4007).

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    Quote Originally Posted by GainFreak View Post
    Hi,

    I wanted to incorporate a time delay switch for one of my tube preamps in order to prevent the CF from exceeding the heater to cathode limit so I decided to make a slow start switch for the HV secondary. My experience with SSRs is limited to using them as relay pop mute switches only so I was wondering if I did this right. The switch works so far but I'm not sure if I'm not missing something. The preamp has 5 12AX7s, current draw at full voltage is ~10mA. The SSR is rated at 600VAC or 600VDC, 50mA.
    One thing I noticed at the moment of SSR switching there's a short very quiet buzz in the PT.

    https://www.mouser.com/datasheet/2/3..._e-1299724.pdf

    Click image for larger version. 

Name:	PT Sec SSR Switch.JPG 
Views:	34 
Size:	41.9 KB 
ID:	56765

    I really like this approach to solving the DC coupled CF issues in Marshall's where the standby does not removed the HT at all. I spent quite some time looking into this and concluded that heater to cathode insulation was not an issue. The problem was damage to the cathode coating caused by the positive grid voltage. The Merlin and other methods did not help at all. The easy solution was to avoid certain tubes and I now a stock of Chinese 12AX7s for this position as there were very reliable.

    My proposed solution involved a mod the the CF stage which monitored the current in the driver stage to turn the CF stage on only when warmed up. Works great but rather complex. Your idea will work too but it seems you need a longer turn on on delay than you currently have to prevent the positive grid voltage as you still get 50V if I read correctly. The critical thing you didn't show us is how you are driving the SSR.

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    Certain Mesa amps suffered the same failure, and their solution was to use Chinese 12AX7s in this position as well.
    So, were you able to determine what the voltage threshold was? Or why some of the clamping circuits were ineffective?
    Did it have something to do with the transition from a nonconducting state to conducting while the grid is still at a much higher potential?

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    Since it's a preamp, probably with low current demands, you could just use a slow warm-up tube rectifier for the power supply without too many sacrifices instead of silicon diodes.

    You can also reduce heater-cathode voltage by floating the heater supply at, say, +40 Volts. This is a time-tested trick, not only for reducing heater-cathode voltage, but for reducing hum with AC heaters (if your heaters are AC).

    Lots of ways to eliminate these problems.

    As others have said, some modern 12AX7s will not handle 180V heater-cathode voltages.

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    Quote Originally Posted by SoulFetish View Post
    Certain Mesa amps suffered the same failure, and their solution was to use Chinese 12AX7s in this position as well.
    So, were you able to determine what the voltage threshold was? Or why some of the clamping circuits were ineffective?
    Did it have something to do with the transition from a nonconducting state to conducting while the grid is still at a much higher potential?

    It was all here https://music-electronics-forum.com/...hlight=cathode

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    Quote Originally Posted by nickb View Post
    Nice

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    The easy solution was to avoid certain tubes and I now a stock of Chinese 12AX7s for this position as there were very reliable.
    Since the problem has been discussed several years ago in some threads here I also started using only Chinese 12AX7s as CF.
    I also started elevating the heaters at up to 80V but the preamp in question is an older one built on PCB and didn't want to cut too many traces. The heaters are 12VDC regulated, negative leg grounded.

    Below is thefull delay circuit schematic which is a modified version of one I found in the net.

    Click image for larger version. 

Name:	PT Sec SSR Switch full.JPG 
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    I suggest you change the cap to 470uf to give >45 sec delay.

    I'm not wild on the circuit as:
    1) It turns the SSR on quite slowly which might cause it to over heat and fail. I haven't checked the specs, just a maybe
    2) The SSR, if turned on near the crest of the HT waveform will experience a big current surge which might be destructive. Don't ask how I know this.
    3) The timing cap is not discharged particularly quickly so and so you won't get the intended operation if you turn off and then on again quickly.

    People will no doubt recoil in horror but I think I'd use a 50 cent micro controller that counted zero crossings of the low voltage AC waveform to do the job. You then get bragging rights for your digitally controlled CF turn on.

    PS: I missed this before. It seems you a have the SSR in just the preamp supply in which case I'm not concerned. I had thought you might be switching the main HT to everything.

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    Last edited by nickb; 01-27-2020 at 10:16 PM.
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