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So I have several 7027a's laying around. I have a bias probe from Eurotubes that tells the current of each tube when off standby. My range of several US made 7027a's in ma is 21.4-34.1 while in the amp. I was able to find four in the 31-34 range that I have in my V4B. I stuck two in the 25 range in my VT-22.
My question is: what is considered new, and what is considered the minimum good threshold?
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Going off the bias current alone will not tell you if the tubes are good or bad.
Ampeg has a non adjustable fixed bias in the v-series, which is very conservative and biases most tubes very cold. If your 7027 tubes are vintage, then they have a 35 watt max plate dissipation. If you have some running in the 31-34ma range in a v4 they are running at 50% max plate dissipation at idle. That is considered by most to be very cold. In these amps I think it is wise to keep them running somewhat cold. Not only for the long term reliability of tubes (especially new tubes), but because the v4 tends to start sounding pretty mushy and undefined when biased warm or hot.
You can always change the bias for those tubes that measure 25ma since that is getting a little too low.
I tend to feel that 50-60% max plate dissipation at idle is perfect for the v4/vt-22
Which means roughly 32-39ma per 7027a
Or 28-34ma per tube for 6L6gc
