I'm building an 807 amp at present using some OPT's a kind friend gave me. The OPT's were designed for 4 x PP UL EL34's at 50w 3471 ohms pri /8r load. I've hand wound a mains toroid to give me 400/450v @ 1A.
I've knocked up a "chassis" out of wood for the OP stage and a variable fixed bias supply 0 to -100v. I can drive the OP stage using an Advance Type 81A sig gen, this gives 0 - 20v RMS balanced out of phase. I can't apply NFB, but it does give a good indication and allows me to play about with different settings etc.
If you have a few tfmr's in your stash, try and knock up a PSU and do the same, it gives you a good idea of what's possible better than paper calculations.
Andy.
I've knocked up a "chassis" out of wood for the OP stage and a variable fixed bias supply 0 to -100v. I can drive the OP stage using an Advance Type 81A sig gen, this gives 0 - 20v RMS balanced out of phase. I can't apply NFB, but it does give a good indication and allows me to play about with different settings etc.
If you have a few tfmr's in your stash, try and knock up a PSU and do the same, it gives you a good idea of what's possible better than paper calculations.
Andy.
So Im expecting that at least 20-25W is possible from a pair in Class A running as tetrodes?
Ive got all the gear from a Transformer winding business that shut down and plan to wind my own transformers.
I will go with ultra linear configuration, but what is the typical impedance for a pair? From memory its about 4K
The problem with the 807 is it's stated screen grid Vmax of 300v, I've run them at 400v in a UL config and there are a couple of references online about running G2 at higher voltages as long as G2 dissapation max is not exceeded.
They are going to get hot in Class A, a cooling fan would be a must.
The STC datasheet states 14.5W for two 807's PP tetrode connection with a load of 5000 ohms, this would make the load 2500 ohms for four. I would think 3k might be nearer the mark. If you have the luxury of winding gear you could wind primary and secondary with several taps.
"6L6 - oids sound the best in AB2 P-P. " Yes, but a bugger to drive which would call for direct coupling from the driver stage meaning a separate beefy negative supply. Not impossible but adds complexity.
Andy.
They are going to get hot in Class A, a cooling fan would be a must.
The STC datasheet states 14.5W for two 807's PP tetrode connection with a load of 5000 ohms, this would make the load 2500 ohms for four. I would think 3k might be nearer the mark. If you have the luxury of winding gear you could wind primary and secondary with several taps.
"6L6 - oids sound the best in AB2 P-P. " Yes, but a bugger to drive which would call for direct coupling from the driver stage meaning a separate beefy negative supply. Not impossible but adds complexity.
Andy.
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I would take the 300V on the grid with a grain of salt, as it appears many have pushed the voltages way past this limit, and I dont think the datasheet was ever revised.
All i know is that the 807 is one of the most tried and tested tubes out there, many HAM operators would run these things for many hours in transmitters with glowing red plates and they seemed to handle it.
I have built a guitar amp using a quad in class AB1 with 400V running on the grid and plate and its never given any issues, obviously class A will draw more current, but obviously running these tubes ultra linear could be an issue if you were trying to keep to 300V, as you really need to push these tubes to 400V I feel to run properly.
Thats not far off what im running my quad in with my guitar amp at 2K primary for a quad. So for a pair, you think 5K primary?
I always had 4K in mind from what I calculated some time back, but as you say i could add some taps if i wish.
There are commercial companies VTL that run 807's triode connected at 560 volts 36 ma.
I built my amps at 500 volts triode connected and the 807 lasted for years. best sounding 807 amps. STC recommends 3K triode connected for 15 watts, you need a good driver circuit.
Maybe Chinese made ones would blow up.
I built my amps at 500 volts triode connected and the 807 lasted for years. best sounding 807 amps. STC recommends 3K triode connected for 15 watts, you need a good driver circuit.
Maybe Chinese made ones would blow up.
Im reading other threads that say 807's even sound better at much higher voltages.
Someone experimented and increased the plate voltage to 800V and stopped in fear of it going up in smoke but said it really livened up at 700V or more.
Not sure what screen voltage was used though.
"I would take the 300V on the grid with a grain of salt" To a point, no one makes these anymore, they should be treated with some respect.
If you look at some datasheet's for 807 valves they have two tables of values, - CCS and ICAS which essentially are heavy duty full on use as in a transmitter on a battleship say and less taxing occasional use, with the former down rated as a result. Most amps we build for AF use will run 807's with only occasional peaks of OP. Still, I've seen several 807's with glowing screen grids which ain't good. A screen grid is delicate in comparison to an anode.
A.
If you look at some datasheet's for 807 valves they have two tables of values, - CCS and ICAS which essentially are heavy duty full on use as in a transmitter on a battleship say and less taxing occasional use, with the former down rated as a result. Most amps we build for AF use will run 807's with only occasional peaks of OP. Still, I've seen several 807's with glowing screen grids which ain't good. A screen grid is delicate in comparison to an anode.
A.
Actually CCS covers Broadcast Transmitters, say WABC or CFRB or the BBC, which run full power most or all day every day; and may have significant loss/harm when off the air. (This might indeed cover a battleship's RADAR, but not its communications which are short and occasional.) (Military usually has other derating factors and used to work closely with tube makers.)
Oh for sure, but I really dont know if running a pair of 807s in ultra linear at 400V would matter. Problem is 807 tubes need to run at a high Plate voltage to get them to sound nice, which means as ultra linear you are stuck with your plate voltage and cant do much about it.
I really dont know what voltage to run these tubes at now, i had about 380-400V in mind, but going by what others are saying, you really need to run these at 600-700V to sound good. Dont know what truth there is to that, but you may be surprised to learn that 807 tubes are in fact still being made in Russia and China. But of course the older ones are better quality. Ive got plenty of used ones that still test good and a fair few more boxed tubes, seems to be the most abundant NOS power tube out there.
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