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Old 10th June 2003, 11:07 PM   #1
arnoldc is offline arnoldc  Philippines
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Default Seeking help about my 2A3 to 45 conversion

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.
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Old 11th June 2003, 12:01 AM   #2
EC8010 is offline EC8010  United Kingdom
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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.
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Old 11th June 2003, 12:31 AM   #3
arnoldc is offline arnoldc  Philippines
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The SRPP 2A3

Click the image to open in full size.

The SRPP 45

Click the image to open in full size.

My PSU is 5U4G - 20uF - Choke - 200uF
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Old 11th June 2003, 01:57 AM   #4
EC8010 is offline EC8010  United Kingdom
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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.
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Old 11th June 2003, 05:20 AM   #5
arnoldc is offline arnoldc  Philippines
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thanks
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Old 11th June 2003, 05:26 AM   #6
EC8010 is offline EC8010  United Kingdom
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You're welcome.
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Old 11th June 2003, 09:56 AM   #7
SHiFTY is offline SHiFTY  New Zealand
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Use a slow warm up rectifier- GZ32/5V4 is my favourite, cheap and available. They will take longer than the output tubes to warm up.

Or, just use 450V capacitors and don't worry about it. Many tube amplifiers do this, and have survived...
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Old 11th June 2003, 04:32 PM   #8
Joel is offline Joel  United States
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A 60v over-voltage at startup is not going to "kill" your tubes.

Keep in mind that with large signal swings, the plate voltage could climb to that level anyway. Much too much is made of startup currents/voltages.
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Old 12th June 2003, 02:25 AM   #9
arnoldc is offline arnoldc  Philippines
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thank you guys for calming me down
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Old 12th June 2003, 07:52 AM   #10
mig-ru is offline mig-ru  Russian Federation
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By the way, you might want to get rid of the grid resistor on the 45, just cuts out power being made by the preamp stage.
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