I still prefer US, European or Japanese vintage production over the new stuff!
Is a 1980s RM-9 in good working condition vintage enough? :) They just going like the Energizer Bunny
Does anyone care to ask an amplifier designer a technical question? My door is open.
Ralph, did you perhaps mis-read this. He said the idle goes down not up as the amp warms. This is the opposite of thermal runaway.Yes- you are correct- totally misread that one. It is pretty weird. I’m wondering how the thermal feedback is accomplished. If its done with an active device, that device might be seeing its hfe drift up as it heats up- thus causing the outputs to shut down more than expected. A schematic would be interesting. Nelson Pass has designed an interesting amplifier based on the old VFET/SIT devices. As solid state goes one of the most musical I’ve heard. https://www.diyaudio.com/forums/pass-labs/276711-sony-vfet-amplifier-2-a.html It also employs low voltage rails, about 25-28 volts. |
@roberjerman
So Roger: new production tubes from our old Cold War rivals. Can the Russians really make and sell quality tubes at such low prices? A quartet of 12AX7's for $18 (shipping free!). We did that ourselves back 50-60 years ago. But that was when US tube production was in the millions for the radio/television market! Do Russian mfgr's have to compromise on material quality and less vacuum- pumping to sell at a low cost? They are doing better and better. Sometimes I send back a whole batch but not so much lately. Low noise 12AX7 are hard to make. Where do you get 4 for $18? Im sure they are not tested for noise or microphonics. Luck of the draw like poker. Sadly the USA makers never got the noise as low as the Europeans. However USA makers were very good at power tubes. Vacuum pumping is not a problem, cathode coating is. They told me when I visited the EI factory. "All we know is we mix up the cathode coating in a tin pail with whatever water and let it sit overnight, its better that way. However we don't know if thats because it sits overnight or perhaps the night watchman takes a leak in the bucket." Im not kidding and that came via Dragona, our translator, right from the head of production. I spent 10 days at that factory. Sad its gone, they were on to something and I wanted to help by offering noise testing which they could not do. I passed on their offer to make me $100,000 worth of tubes, prepaid. they were broke, Good luck seeing those. |
@terry9 I see you have jumped on the bandwagon of expensive parts. I have not yet found them to be superior and they are costly and often fragile. Those resistors especially. I dont see the benefit. As far as dialectric, i understand it well. Let me ask your a question. How much AC voltage (changing voltage) is across a coupling cap in any amplifier? I am still curious about your falling idle current with rising temp. How do you drive the capacitance of all those QUADS. Have you done some measurements on what the combined impedance of the whole setup is? How do you arrange them? |
There are several Russian sellers on EBay that claim to have military (OTK) grade 12AX7's made circa '80 at Saratov. The prices are astonishingly low! I am tempted to buy a quartet. If I remember right, I might have bought a quartet of 6L6's from one of these sellers a few years ago. I will have to search through my tube stockpile to verify this! I recall the boxes had the proper Cyrillic printing. Thought I'd put them in my FenderTwin Reverb! |