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Old 30th August 2013, 10:41 PM   #11
kevinkr is offline kevinkr  United States
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My one foray into OTL land resulted in a circlotron based design using six 6C33 per monoblock, scorcher, speaker destroyer, and heart breakingly good sounding during the short few hours when things sort of worked. Never achieved the target output power due to the stupidest driver design I have ever devised. Unreliable, and untrusted, and while I was doing this I was also tinkering with my first 45 SE design which ultimately sent me in another direction and to another world.
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Old 31st August 2013, 02:32 AM   #12
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Originally Posted by ubza1234 View Post
so put very simply, if I had enough tubes in parallel, (to increase the output current) could the output transformer be eliminated? or does it still need Impedance matching? the number of tubes may be impractical, but it was just a thought I had.

cheers
Yes, you can do that. However, there aren't very many tube types where this is practical. You have power triodes:

6AS7 (power dual -- series pass device originally)
6C33C (high gm, series pass device)

That's about it for power triodes.

You could also use the TV HD pents with screen drive to get the massive current peaks that 8R speeks will require. Back in "the day" OTLs used special speeks with unusually high voice coil resistances (up to 1K0) or you could series connect a lot of 8R drivers to get that load resistance up.

In any case, your two main topologies are the SEPP (seen with SS designs, as complimentary NPN/PNP or N-Channel/P-Channel devices are available) or the Circlotron bridged amp topology.

Since there are no "P-Channel" VTs, the SEPP will require some phase balancing tricks like the Futterman designs, or some other means of balancing the two phases.
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Old 31st August 2013, 06:02 AM   #13
cnpope is offline cnpope  United States
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Quote:
Originally Posted by Miles Prower View Post
Yes, you can do that. However, there aren't very many tube types where this is practical. You have power triodes:

6AS7 (power dual -- series pass device originally)
6C33C (high gm, series pass device)

That's about it for power triodes.

You could also use the TV HD pents with screen drive to get the massive current peaks that 8R speeks will require. Back in "the day" OTLs used special speeks with unusually high voice coil resistances (up to 1K0) or you could series connect a lot of 8R drivers to get that load resistance up.

In any case, your two main topologies are the SEPP (seen with SS designs, as complimentary NPN/PNP or N-Channel/P-Channel devices are available) or the Circlotron bridged amp topology.

Since there are no "P-Channel" VTs, the SEPP will require some phase balancing tricks like the Futterman designs, or some other means of balancing the two phases.
I've built four OTLs so far; two using a pair of 6C33Cs in a Futteman style totem-pole configuration. One using four EL609s, in a circlotron. And the most recent is a Futterman-style using four 6082s, which are equivalent to 6AS7s. I've never had anything but a coupke of minor failures of tubes, with no dramatic effects. My speakers are 4 ohm (nominal) impedance, and all the OTLs drive them nicely. They are really all simple and trouble-free. I don't really see the point of messing with output transformers, when it is so easy to make tube amplifiers without them.

Chris
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Old 31st August 2013, 12:35 PM   #14
Defiant is offline Defiant  United States
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The most novel OTL's I have seen are those designed for directly driving electrostats. They usually use HV shunt regulator triodes and run at quite high voltages.

I dabbled with the OTL thing, but for regular PP topologies I find a good output transformer to be superior. If I want/need an amplifier without an output transformer, I use silicon.
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Old 31st August 2013, 12:39 PM   #15
AJT is offline AJT  Philippines
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same here, to me a tube without the OPT is no tube amplifier...
if i want output transformerless i'd go solid state...
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