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and measured the plate voltage on my '67 Super Reverb - 500 volts. Tried some different 5ar4 tubes and saw similar numbers. I'm a bit concerned as I see the reading should be in 460 VDC range. Any thoughts?
In addition, the clincher is that the power supplyB+ filter capacitors should be rated for at least 500V.Originally posted by BWilliamson
Could be a number of things:
PT stock? What's the datecode on the PT?
Early silverfaces went to a 5UG4 rectifier and the PT's had a bit higher B+. Knowing Fender they could have switched design specs/suppliers at any time and not made mention of it.
What the voltage coming out of your outlets, if that's abnormally high that could also be causing the higher voltage readings.
I would take a guess that a tolerance of 10% was quite common between transformers at the the time and still today. Taking the reading from the schem of 460v means a swing of 46 volts either way.
The voltages on the schem are from one amp and one point in time most likely, lots of variables could have contribulted to those readings. Don't take to much stock in it, more a target and they were throwing hand grenades.
As long as your biasing your tubes in a safe range you should be alright. Using the bias calculator on the WeberVST BBS and 500v it show 26.6ma for 70% and 34.2 for 90%(Very HOT!). Setting it even lower than 70% wouldn't hurt if the tone is to your liking as there is no hard answer for bias settings. Use your ears and your eyes and make sure the plates are red-plating.
I have no illusions of real world tube data, imagine that most of these online calcs are set for "Safest" generic scenario's to cover ones butt. Just used it because it was handy, didn't notice that the post previous was yours or I wouldn't have even brought it up and defered to you.500V X 40mA = 20 watts which is 67% dissipation for REAL 6L6GCs which have a max dissipation of 30 watts.