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Click below for pics, etc. My Friend Rnolan's been given this amp, and his post at the bottom of that thread below, provides details of the valve complement in his.Three 5Z3s?
So it's using cathode bias for the KT88s? Yes, or isn't it that simple?
Does my friend Richard need / must he get - a KT88 that matches one of the others in the amp?
OR a matched quad?
I used to believe that matched pairs of EL84s were not required for LEAK Stereo 20s. HJ LEAK wrote this in the owners doc'n. I thought that was due to cathode bias? I do have four matched pairs in the LEAKs.
TIA
Warmest
Tim Bailey
Skeptical Measurer & Audio Scrounger
Edits: 08/31/16 08/31/16 08/31/16Follow Ups:
Doesn't matter. Unless you can adjust the idle current of individual tubes, matched pairs will have lower distortion. Matched quads will make the channels more similar. If you can adjust idle current you can get away with slightly mismatched tubes. It is best to match tubes not just for idle current, but also for transconductance (GM) (aka Transfer, Mutual, Dynamic etc.) Change in plate current over the change in the grid voltage.
See above?!
Warmest
Tim Bailey
Skeptical Measurer & Audio Scrounger
they probably paralleled the 5z3 to get enough current capability for supplying the amp. Can you not tell your friend to trace the circuit from p3 of the cathode to what should be a 5 or 10 watt resistor? GEC tube specs call for 2 x 525 ohm 1 per tube for the kt88. So, if it 1 resistor that would put the value at around 260 ohms and matched pairs of output tubes.
Frankly, i don't trust those odd named chinese amps that seemingly pop out of nowhere and disappear just as quickly. My worst fears are that they take rejected components from a a name brand that failed quality control and change tags and use these parts for a somewhat cobbled together design with a price tag that is too good to resist. A recipe for disaster if you ask me.
Might be nice to get the three 5Z3 matched, too. With that kind of current need, wonder why the manufacturer didn't go to a FW UF5408 bridge?
A lot more reliable, IMHO.
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