mk60
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Hi all,
I've been able to get my hands on a second used UFO25, and it has arrived today. I've set both up with the same selections of tubes from the stock complements that came with them (OA3 input regulator and 7DJ8 input tube). I bridged both into mono using the instructions in the manual, hooked everything up on both sides, set all four gain knobs across both amps to 14 clicks, and sat down to enjoy the fruits of my labors.
Upsettingly, I found the right channel was noticeably louder than the left, despite the gains being set equally.
In order to verify, I hooked up my left speaker, alone, up to both amps separately and played the same 125 Hz test tone, with a dB meter set up, and found that indeed, that one amp was playing much louder than the left at the same gain settings (in clicks): 68 dB vs 65 dB. Hrmph.
Thinking about it, there are of course many factors that could lead one amp to perform differently from the other: any of the 14 tubes, overall age of components (amps were built 4 years apart) etc etc.
So, I decided to do some systematic tests swapping tubes between them and re-measuring.
I made the following individual tube swaps (and undid each swap before trying the next):
-Single input 7DJ8 tubes (the one from the older amp is a Philips, the newer says Matsushita Japan) -Pair of 6P15P output tubes -Single input OA3 regulator tubes -Single recifier tubes -Pair of output OA3 regulator tubes
In each case the older amp played the tone between 64.4 and 65 dB whereas the newer amp played the tone between 68 and 69 dB. I feel like this rules out tube quality/age as being the main issue.
I also swapped the power cables between the two amps (one is an audioquest, the other came from a rice cooker), no change.
The only other somewhat major difference between the two amps is that the older amp has Jupiter capacitors whereas the newer one has Miflex. Given the caps have the same values, though, could this be the cause of a 3 dB difference?
I emphasize that both amps were set to the exact same gain on both knobs, 14 clicks. If I turn the quieter amp up to 15 clicks, they'll actually match closer than they did at equal clicks, but now the 15 click amp will be ~1 dB louder than the 14 click amp, so that's no good either.
Anyone have any ideas as to what's going on or what I should be trying? Is it wrong for me to expect that I should be able to gain match these amps so that I can use them as monoblocks?
Thanks for any advice!
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