Identifying valve / replacing? - UK Vintage Radio Repair and Restoration Discussion Forum

bigfathairyvika · 27th Jul 2021, 7:29 pm

Hi All,

I have a valve amplifer, very likely Chinese which works fine, but when it needs some new valves I may have a challenge.

It uses two el34's in push pull ( standard mullard configuration) with a preamp valve and then a valve driving the two el34's.
The EL34's are marked, the preamp valves are marked 6N2 and are dual triodes wired in parallel.
The valves driving the EL34's are not marked but are dual triodes with the heaters in parallel. Pins 4 and 5 joined and pin 9 return.

I don't mind rewiring heaters and modifying components to suit replacement valves but plug and play would be preferred.

My knowledge of valve circuits isn't great but I am able to work things out!

From datasheets and pin outs I've deduced ( maybe incorrectly ) that the unknown valves are not 6n6p, 6n1p or ecc88.

Could anyone offer advice?

Oh, I haven't yet tried it, but I do have a valve curve tracer (dos4ever jobby) I can plug them into.

Edit: forgot... unknown valves are 9pin.

Thanks,

Mark

GrimJosef · 27th Jul 2021, 7:49 pm

It sounds like it's got a 12.6V heater (pins 4 and 5) with a centre-tap (pin 9). So it'll probably be equivalent to an ECC81, 82 or 83. The way to find out which is to measure the DC electrode voltages, relative to signal ground, with the amp powered up and to note the value of the cathode resistor(s). You can then work out the current that's flowing through the valve and the bias voltage (Vgrid-Vcath) and knowing the anode voltage check against the characteristic curves in the 81, 82 and 83 datasheets.

Cheers,

GJ

cathoderay57 · 27th Jul 2021, 9:10 pm

According to Wikipedia it is probably close to an ECC83/12AX7 https://en.wikipedia.org/wiki/6N2P but I haven't verified this with a datasheet. Note it says the 2 types aren't pin compatible so don't try plug and play unless you've rewired the valve socket. Jerry

bigfathairyvika · 27th Jul 2021, 10:00 pm

Thank you both.

I'll measure the voltages and resistances tomorrow and check out some datasheets.
Knowing the voltages I believe I should beable to put the valve in the tester and sweep a curve and compare it to a few.

Mark

27th Jul 2021, 7:29 pm	#1
bigfathairyvika Hexode Join Date: Nov 2009 Location: Cambridge, Cambridgeshire, UK. Posts: 492	Identifying valve / replacing? Hi All, I have a valve amplifer, very likely Chinese which works fine, but when it needs some new valves I may have a challenge. It uses two el34's in push pull ( standard mullard configuration) with a preamp valve and then a valve driving the two el34's. The EL34's are marked, the preamp valves are marked 6N2 and are dual triodes wired in parallel. The valves driving the EL34's are not marked but are dual triodes with the heaters in parallel. Pins 4 and 5 joined and pin 9 return. I don't mind rewiring heaters and modifying components to suit replacement valves but plug and play would be preferred. My knowledge of valve circuits isn't great but I am able to work things out! From datasheets and pin outs I've deduced ( maybe incorrectly ) that the unknown valves are not 6n6p, 6n1p or ecc88. Could anyone offer advice? Oh, I haven't yet tried it, but I do have a valve curve tracer (dos4ever jobby) I can plug them into. Edit: forgot... unknown valves are 9pin. Thanks, Mark

27th Jul 2021, 7:49 pm	#2
GrimJosef Dekatron Join Date: Sep 2007 Location: Oxfordshire, UK. Posts: 4,311	Re: Identifying valve / replacing? It sounds like it's got a 12.6V heater (pins 4 and 5) with a centre-tap (pin 9). So it'll probably be equivalent to an ECC81, 82 or 83. The way to find out which is to measure the DC electrode voltages, relative to signal ground, with the amp powered up and to note the value of the cathode resistor(s). You can then work out the current that's flowing through the valve and the bias voltage (Vgrid-Vcath) and knowing the anode voltage check against the characteristic curves in the 81, 82 and 83 datasheets. Cheers, GJ __________________ http://www.ampregen.com

27th Jul 2021, 9:10 pm	#3
cathoderay57 Nonode Join Date: Oct 2017 Location: Bristol, UK. Posts: 2,382	Re: Identifying valve / replacing? According to Wikipedia it is probably close to an ECC83/12AX7 https://en.wikipedia.org/wiki/6N2P but I haven't verified this with a datasheet. Note it says the 2 types aren't pin compatible so don't try plug and play unless you've rewired the valve socket. Jerry

27th Jul 2021, 10:00 pm	#4
bigfathairyvika Hexode Join Date: Nov 2009 Location: Cambridge, Cambridgeshire, UK. Posts: 492	Re: Identifying valve / replacing? Thank you both. I'll measure the voltages and resistances tomorrow and check out some datasheets. Knowing the voltages I believe I should beable to put the valve in the tester and sweep a curve and compare it to a few. Mark