• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Seeking help about my 2A3 to 45 conversion

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I've been happy with my JE Labs SRPP 2A3 since i made it. A recent opportunity allowed me to secure a pair of Cunnigham CX-345. Since the SRPP 45 of Esmilla is very similar to its 2A3 version, I converted my amp earlier (1.5K cathode resistor, 1K grid resistor added), but no modifications were made to the power supply circuit.

I plugged it in and measured B+ but using 2A3, just to make sure i don't blow up my CX-345. It sounds good on this setup but I have a problem to fix -

the B+ goes to 380V before settling down to 324V which is about right.

this will definitely kill my tubes in the long term, and I need to know how to fix it.

Can I just put a bleeder resistor on the cap after the choke? Please advice. Any thoughts will be greatly appreciated.
 
Ex-Moderator
Joined 2003
I've not seen a diagram for you amplifier, but I'm assuming that it has a choke input supply. Does it use silicon or thermionic rectifiers?

Damper/efficiency diodes will slow down the HT so that the output valves are ready for it.

If the rectifiers are silicon, the problem can be solved by a zero-voltage switching relay (which can switch the 240V needed by your valves). The relay switches the AC to the rectifier on command from switched low-voltage DC, which you can easily delay.
 
Ex-Moderator
Joined 2003
Your problem is that the rectifier warms up faster than the output valves. Changing your (double diode) rectifier for a pair of (single diode) efficiency diodes will solve the problem. You only draw 110mA, and your reservoir capacitor is tiny, so almost any efficency diode will do. You might well choose to use a larger diode simply because the socket will fit the (already punched) hole.
 
The use of a 'grid stopper' resistor before the input grid of the output valve is to help reduce the dangers of parasitic oscillation - ideally the resistor should be connected as close to the grid connection on the valveholder as possible. The effect on such a resistor on overall gain is negligible.

Is it necessary on your new circuit? If your 45 amp has a greater gain then it might be more susceptible to parasitic oscillations I suppose... You may find that it is not strictly necessary although many designers like to include these resistors to help ensure stability, as the cost is so small, both in physical cost and in loss of gain.
 
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