| Chris Pope |
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Reply with quote | #1 |
Hi,
I'm gearing up to build the Transcendent OTL design in Audio Reality (25W+25W).
In the schematic, all tube heaters run from the same power supply. But the maximum heater/cathode voltage for 12AX7 and 12AU7 is 180V, and for JJ EL509-S it is 150V. Since the lower set of output tubes have cathodes at -170V, this would seem to mean the heater supply needs to be at below -20V, while one of the 12AU7 tubes has its cathode at +375V, which would seem to mean the heater supply needs to be at above +195V.
I know the heater supply is floating, but whatever it floats at, it can't satisfy the conditions of being below -20V and yet also above +195V at the same time! This seems to mean that at least some of the tubes are getting well above their maximum allowed heater/cathode voltage. Are those heater/cathode max. voltages in the datasheets very conservative, so that they can be safely ignored, or is there something to worry about here?
I was thinking about using a separate heater supply for the output tubes (could be AC, I suppose?), held at about -85V, and then a regulated DC supply for the input and drive tubes. But even for the input and drivers, it seems to be stretching the limits to run them all on the same supply. (Cathode voltages range from a few volts on the 12AX7 up to +375V on one of the 12AU7 drivers.) It almost seems like V1 and V2 need a heater supply held at about +75V, and V3 needs yet another heater supply, held at about +370V. Again, am I worrying too much about the maximum heater/cathode voltages given in the tube datasheets?
One other question concerns delayed B+ on startup. In many tube amp designs the turn on of the B+ is delayed by a minute or two, to allow the cathodes to warm up. There is no provision for this in the OTL design in Audio Reality. Is this because experience suggests delayed B+ is unnecesary?
Thanks for any replies!
Chris
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| C.B. |
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Reply with quote | #2 |
The heater (filament) on all tubes is an entirely separate circuit from the plate/grid/cathode system. In the T8, the two are not connected (with one exception--see below), at least not the way I built mine.
By "maximum heater/cathode voltage", I assume the data sheets are talking about the maximum voltage potential between the two, not any actual connection. As far as I understand it, this is just a guideline to avoid 'filament stress'. There is a small shunt capacitor between floating filament ground and circuit ground in the T8, and this is BR's 'remedy' against filament stress. Very important--don't leave it out.
For the 12AX7 and 12AU7, the filament supply can either be 12.6V (series) or 6.3V (parallel)--the T8 uses a series/parallel configuration, with a load resistor 'standing in' for the fourth (missing) tube. Most of the output tubes that can be used in this OTL design require 6.3V, which means they can be series wired in groups of 4 and supplied with 25V, as shown in Audio Reality.
The filament supply can be either AC or DC, but I would recommend using the supply shown in the T8 schematic, which is unregulated DC. Don't play around with AC or a regulated suply, unless you REALLY know what you're doing. The T8 schematic is fine the way it is.
Also, do NOT delay the B+ supply--bad idea. This topic has been convered before on this forum--do a search.
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| Chris Pope |
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Reply with quote | #3 |
Hi C.B.,
Thanks, I'll look for past threads on delaying B+.
Yes, my other question was entirely concerned with the possibility that heater/cathode breakdown voltages might be exceeded. The cathodes of the lower EL609 tubes are at -170V, while the cathodes of some of the driver 12AU7 tubes are at +375V. That's a 545V potential difference. If all the filaments are at essentially the same voltage (give or take about 18V or so, depending on where in a chain of series-connected filaments a particular one lies), then no matter what voltage the floating filament supply floats to, then somewhere there are going to be heater/cathode potential differences considerably in excess of the maximum values stated in the tube datasheets. I guess you are saying that in practice one can get away with exceeding the stated maxima by quite a large margin?
Chris
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| Bruce Rozenblit |
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Reply with quote | #4 |
Guys, its a proven design. There are T8 amps out there that have been in operation for over 12 years with no problems. The heater floats with respect to the circuit and is coupled to ground through a small capacitor. This relieves the voltage stress.
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| Chris Pope |
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Reply with quote | #5 |
Thanks Bruce. That's good enough for me! I'll stop worrying and go ahead!
Chris
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