I was wrong. I misremembered. I definitely have seen a design (may have been 807s) where, in the article, the author suggested the use of 1 ohm plate loads to prevent current hogging. He was using parallel push-pull sets.
Atma-Sphere uses cathode resistors, no plate resistors, in the Circlotron design. He does advise 5 ohm resistors if using American 6AS7 tubes. Somehow, I equated that with current hogging due to mismatched sections.
I'll have to look back at some articles and find the small value plate resistor reference.
The cathode resistors are definitely not used as fuses, LOL!
From Ralph in a diyaudio post:
Quote:
FWIW there are some fundamental misconceptions here, the first being that all tubes are equal which they are not. From the very beginning of OTL technology in the early 50s, OTLs have used low-impedance tubes. That may sound oxymoronic to some, but we are talking about tubes that handle a lot of current and don't take much plate voltage to do it. Hence: the 6BQ6 above is a poor choice to make power, and the 6AS7G is an easy choice. According to the post above, that's 8 power tubes for 40 watts into 8 ohms. Eight 6AS7Gs can do 60 watts into 8 ohms, and that is running class A.
The 'room heater' thing is a common misconception, based on the idea that lots of tubes make lots of heat, which they don't (they make a little, with the exception being the 6C33 that has a very hot filament). What makes the heat, at least in our amps, is the class A operation. You can run the amps all day in Standby and at the end of the day, grab the tubes and hang on to them without getting burned. But from stone cold, 30 second warmup, and then turn them on- 30 seconds later they are pretty hot! Its the class A that makes heat- so **thats** what makes for a room heater, tube or transistor.
If you run feedback, THD can be in the 0.00?? range at full power. I like to run them without feedback and depending on setup the THD can be between 0.5 to 3% at full power. It always amazes me how many people have the misconceptions seen in the above post- after 30 years I'm still responding to comments like this.
BTW, American 6AS7s can do more power (eight of them will do 80 watts into 8 ohms) but they are the devil to keep running. If you look in the RCA receiving tube manual: 'fixed bias not recommended'... -well that is precisely what we have to do, so you have to use either the Russian 6H13C or the Chinese 6N13 which is their equivalent type. They seem to hold up fine with fixed bias service, as does the Sylvania 6AS7, which is rare.
If you are dead set on using American tubes, it is possible. The cathode resistance must be increased to a minimum of 5 ohms 5 watts (which is a good idea anyway). If the tubes are NOS they must be preconditioned for 72 hours to minimize arcing.
A note on the B+ power transformer! We are not running a bipolar supply. Circlotrons aren't set up that way. They are the only circuit I know of that allows true push-pull without an output transformer and without complementary devices. So you need two equal floating power supplies, so either 2 identical power transformers per monoblock or a unit with dual secondaries.
Each supply will have about 140V output. You'll want each one to be rated at double the idle current at the minimum. In this case our idle current is about 60mA per section, so each supply will idle at 240 mA if 4 power tubes total are used. The supply should be capable of at least twice that under continuous duty.
The 6AS7G has a 6.3V filament that needs 2.5 Amps. You will need a sizable transformer to run them, but I have found them off-the-shelf. For a monoblock just using 4 power tubes, we need 10 Amps continuous. Its nice to have it in a dual secondary, so each phase has its own winding, but its not essential.
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