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  1. Replies
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    The R4/diode/Q1 is basically a crude current...

    The R4/diode/Q1 is basically a crude current mirror. That is the current through R4 is mirrored to the collector of Q1. The current is not mirrored exactly due to the presence of R3 and the fact that...
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    Enough! Let the OP try it and report back.

    Enough! Let the OP try it and report back.
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    Yes, but that is the problem. At the higher...

    Yes, but that is the problem. At the higher supply voltage the current thru R4 is higher and therefore the drop across the diode increases. That causes Q1 to start to turn on. Increase R4, lower the...
  4. Thread: Banner ads

    by nickb
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    Mine is "whoisitreally"...

    Mine is "whoisitreally"...
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    Increase R4 to 22k or so.

    Increase R4 to 22k or so.
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    I second this. I just had an AC30 here with new...

    I second this. I just had an AC30 here with new JJ EL84's that with the 50 ohm cathode resistor, new coupling caps, new cathode bypass cap and grid leaks that would slowly go into thermal runaway. ...
  7. I didn't mean to hit send on that one as it was...

    I didn't mean to hit send on that one as it was half baked but I see that I did!
  8. In a preamp stage, as we are talking about here,...

    In a preamp stage, as we are talking about here, there will no such (switch on ) full voltage B+ transient. The rise time will be very slow due to the filtering action of the preamp supply. A...
  9. I don't see this for realistic applications...

    I don't see this for realistic applications unless you are talking about fault conditions. This application would be in the preamp. The supplies will be filtered by something like a 22uf and 10k...
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    Very excessive voltage (unusual), gas or broken...

    Very excessive voltage (unusual), gas or broken electrodes that short out are more common IMHO. Over dissipation can lead to both the last two failures.
  11. I doubt it as the current is limited by the 100K...

    I doubt it as the current is limited by the 100K resistor. In addition the voltage falls slowly and the capacitor will greatly suppress that ( i = C * d(v)/d(t) ).

    Of greater concern is that FETs...
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    Look at page 7. C3 is on the FAT cct. The version...

    Look at page 7. C3 is on the FAT cct. The version on page 11 shows that cap as C4. Your readings show R9 is open.
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    My first suspect would be that either C3 is...

    My first suspect would be that either C3 is leaking or R9 is open (or bad connection to R9). Also note there is an error in the schematic in the the bottom end or R9 is connected to ground. So with...
  14. You can lose two of the wires to the probe by...

    You can lose two of the wires to the probe by using a battery instead of the heater supply. I see there is a transformer in the control box to provide isolation and possible a step up to power the...
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    It is a sort of tripler as the clamp diode of the...

    It is a sort of tripler as the clamp diode of the "doubler" is referenced to the negative rail of about 19V. This makes the peak voltage at the junction of the 47uF and the clamp diode 60V or so....
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    I suggest you change the cap to 470uf to give >45...

    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...
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    It was all here...

    It was all here https://music-electronics-forum.com/showthread.php?t=46108&highlight=cathode
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    I really like this approach to solving the DC...

    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...
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    dmeek & g1: hugely appreciated. Thx.:thumbsup: ...

    dmeek & g1: hugely appreciated. Thx.:thumbsup:

    My first ever multilayer recording was done on one of these!
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    G1: Are you able to post or pm me the schematics?...

    G1: Are you able to post or pm me the schematics? Did you ever get a SM?
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    I have one...pm me if you want it.

    I have one...pm me if you want it.
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    I'd cascade the mic input through a tone stack...

    I'd cascade the mic input through a tone stack and then to the phono input. Maybe add a cathode bypass cap to to mic input for less hum and more gain. Also add a further RC filter stage in the power...
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    Could make a guitar amp with some mods. ...

    Could make a guitar amp with some mods.

    Schematic:

    56739
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    The ticket says "Buzzing and grinding". Two...

    The ticket says "Buzzing and grinding". Two pots, the range switch and several electrolytics bad. I replaced all the electrolytics, which is something I hestiate to do, but it was warranted.

    Here...
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    A Rare one: Wallace AC5075-XT

    On my bench today...

    56669
    56670
    56671

    56672
    PS: Even with the ally chassis it weighs a ton!
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    If you remove the NFB by connecting the right...

    If you remove the NFB by connecting the right hand end of the 1.5K to ground instead of the output, do you still get ringing? If so, then the cause is not the feedback. Note the frequency of the...
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    I bet it doesn't cost them $60. Maybe $3.

    I bet it doesn't cost them $60. Maybe $3.
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    Guitar amps!...

    Guitar amps! https://voxamps.com/en-gb/product/mv50-boutique/
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