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bikerhifinut · 5th Feb 2016, 6:12 pm

I have a strange phenomenon on this amp.
A "hot spot" on one EL34 on each amp.
swapping valves round shows the hot spot stays with the socket.
its the same corresponding socket on each monobloc.
All voltages, anode, screen grid, cathode check out within spec. And no DC leakage from coupling capacitors.
The only thing I can think is that the grid leaks are quite high at 680k which is the value specified by HJ Leak for their TL25+ as these are Clones of that circuit as you all know.
I could go down to 470k or 330k i reckon without any noticeable loss of performance.
The other factor is the Heater voltages are high due to our high mains voltages at around 7volts. I wondered if excessive emission from the hot cathodes might be having an effect?
Bit of a mystery.
I have used KT88 and 6550 in the amp, the circuit has enough flexibility to handle these as the values I chose will keep a KT88 running well under its rated power and in permanent ClassA as far as I can ascertain. The mains transformers are well over specified so any extra current draw is catered for.
Using the higher rated valves I dont get this phenomenon.
Am I simply running the EL34's too hard? And would larger screen grid resistors make any difference? As i understand it they are really only in circuit to protect the output transformer if the valve goes S/C? Leak never even put them in their designs.
Ht to CT of output transformer 450V, anodes at 443V there or thereabout and cathode volts at 34V using 470R cathode resistors and 50uF bypass caps.
Puzzler this one as I can not see any reason why one valve would run hotter than the other as the circuit conditions are the same on each phase.
It's put paid to my conclusion that I had a bad quad of Sovtek EL34's, although I suspect they might all be a bit borderline and run, as these are, right on their limits, this could be why.
I have no qualms about reworking the circuit to use KT88's, the output transformers were designed to be suitable for either a pair of EL34 at 25-30W with 8 ohm output, or a pair of KT88 at up to 50W but 6 ohm output impedance, which is fine with my nominal 8 ohm speakers, or the spare pair of monitor audio 6 ohm speakers I have.
I wonder if the uprated mains transformers are to blame?

A.

5th Feb 2016, 6:12 pm	#135
bikerhifinut Octode Join Date: Nov 2013 Location: Penrith, Cumbria, UK Posts: 1,993	Re: EL34 power amp project I have a strange phenomenon on this amp. A "hot spot" on one EL34 on each amp. swapping valves round shows the hot spot stays with the socket. its the same corresponding socket on each monobloc. All voltages, anode, screen grid, cathode check out within spec. And no DC leakage from coupling capacitors. The only thing I can think is that the grid leaks are quite high at 680k which is the value specified by HJ Leak for their TL25+ as these are Clones of that circuit as you all know. I could go down to 470k or 330k i reckon without any noticeable loss of performance. The other factor is the Heater voltages are high due to our high mains voltages at around 7volts. I wondered if excessive emission from the hot cathodes might be having an effect? Bit of a mystery. I have used KT88 and 6550 in the amp, the circuit has enough flexibility to handle these as the values I chose will keep a KT88 running well under its rated power and in permanent ClassA as far as I can ascertain. The mains transformers are well over specified so any extra current draw is catered for. Using the higher rated valves I dont get this phenomenon. Am I simply running the EL34's too hard? And would larger screen grid resistors make any difference? As i understand it they are really only in circuit to protect the output transformer if the valve goes S/C? Leak never even put them in their designs. Ht to CT of output transformer 450V, anodes at 443V there or thereabout and cathode volts at 34V using 470R cathode resistors and 50uF bypass caps. Puzzler this one as I can not see any reason why one valve would run hotter than the other as the circuit conditions are the same on each phase. It's put paid to my conclusion that I had a bad quad of Sovtek EL34's, although I suspect they might all be a bit borderline and run, as these are, right on their limits, this could be why. I have no qualms about reworking the circuit to use KT88's, the output transformers were designed to be suitable for either a pair of EL34 at 25-30W with 8 ohm output, or a pair of KT88 at up to 50W but 6 ohm output impedance, which is fine with my nominal 8 ohm speakers, or the spare pair of monitor audio 6 ohm speakers I have. I wonder if the uprated mains transformers are to blame? A.