Push-Push OTL?

[StoneT]

Thanks all for the replies. I've done some further investigation.
Those PL519s sound interesting, I've not seen those before. This was intended to be for my living room though running high efficiency 8ohm drivers (single driver speakers) so 540W would a) destroy my speakers and b) disturb the police. While still in their station. On the other side of town! I was aiming for up to 5W into 8 Ohms.
And if I was worried about efficiency I'd be using sand.
Incidentally it seems odd to say

The correct approach is to use high impedance speakers...

I suppose all those people listening to Futterman variations or circlotrons on their rear loaded lowthers are way off base eh?
I think the arrangement does have it's merits. You can use virtually any tube with enough current to drive very low loads.
DF96- You are right about the load recieving only the partial signal that has been amplified, but that is the voltage that has been amplified. Considering two SRPPs at the output- as the voltage rises the current from the master will drive the slave CF more than its own drive signal due to the slave cathodes low impedance and lower tubes high impedance. Effectively it is still push pull regarding current as the slave side sees and inverted loadline (greater than infinate). The more asymmetric the input signal the lower the apparent load / the harder the output stages are pushed so in theory the tubes can be used as efficiently as any other topology.
Your last point about the load seeing common mode AC is also true but I don't know if that would affect anything, the speaker won't play anything common-mode, although you wouldn't be able to ground anything at the output, even with both output caps in place, so that may be an issue.

[toprepairman]

The first line of you original thread suggested you wanted something vuagely efficient.
The new circuit looks a bit better than the first, but...
The 5998's in the outputs are rated to run at 100mA with 110 volts across them. I don't have the full spec so not sure at what grid volts.
In this circuit the best you can get is about 400mA peak current, which translates to a peak voltage of 3.2V across your 8 ohm speaker. This is a massive 1.28 watts peak, or 640mW RMS. We're a bit short of the desired 5 Watts.
To get our 640mW, we'd need at least 100 volts across the valve delivering the current, which would mean something like a 400 volt supply rail. With a standing current of c.100mA for each valve, the supply would have to deliver 400 volts at 400mA, thats 160 watts in for .64 of a watt out, worse than 0.5% efficient.
Add to that each valve is dissipating 20 watts, when 11 is max then they will have a short life.
So back to the drawing board.

[toprepairman]

The people listening with Futtermans and the like, are all using PL518's, so have a peak current available of around 3 Amps. Across an 8 ohm load this is 24 Volts peak, and 72 Watts peak, 36W RMS.
Of course to achieve this at least 200 watts is drawn off the power supply, again vastly exceeding max permissable anode dissipation, which in practice means something like a practical limit of 10 to 15 watts output power, which is what we find in practice.
Here we are assuming using two valves in the output stage.

[DF96]

Bootstrapping is normally used when you want a component to 'disappear'.

"Efficient" is not a word I would use about two Class A outputs working against each other.

Maybe I have missed something, but to me it looks daft!

[toprepairman]

Have to agree with DF96, it's daft.

[StoneT]

OK, OK, I submit! Even LTspice is against me. It keeps locking up at 13us and saying 'Timestep too short' so i can't even test the circuit. I wasn't planning on using 5998s to make it. I was just trying to see if an unsuitable tube could be used to drive 8 Ohms. I would think that a few 6C33C would be a much better option. I honestly thought that most OTLs were based on 6C33C or 6AS7/6080 valves. As I said, I've never heard of these PL519 but they seem another good choice. My point about the earlier comment was that using high impedance loudspeakers, while obviously a good option, is no more 'correct' than finding ways for valves to drive low loads.
The whole idea sort of started half as a joke, like I said- I thought I had the worst efficiency amp, but then realised I could build a worse one...
And you can add 60W of heater power into that equation, and that's still only the output stage...

[DF96]

We all have daft ideas from time to time. Sometimes we notice before telling anyone else, sometimes we have to be told by them it is daft and eventually agree.

A few of us persist with daft ideas and manage to convince other people to spend their money on them. The big mystery then is: do the people selling daft ideas know they are daft (but hope we haven't noticed), or do they still believe them?

[atmasphere]

Originally Posted by toprepairman

The people listening with Futtermans and the like, are all using PL518's, so have a peak current available of around 3 Amps. Across an 8 ohm load this is 24 Volts peak, and 72 Watts peak, 36W RMS.
Of course to achieve this at least 200 watts is drawn off the power supply, again vastly exceeding max permissable anode dissipation, which in practice means something like a practical limit of 10 to 15 watts output power, which is what we find in practice.
Here we are assuming using two valves in the output stage.

I've been using OTLs for the last 35 years.

I've yet to use PL518s- too inefficient. That's a really high power tube but its hard to make power with them in an OTL. I use 6AS7Gs. Two of them with a much simpler circuit will get you more than 5 watts into 8 ohms. In a Circlotron.

If you used two 6C33s you would be good for 15 watts and if you used two 7241s you would be good for 50-60 watts in class A. I've done it. I use the 6AS7G as the socket requirements are easier and the tube is easier to find and usually inexpensive, yet has good linearity.

The project is quite feasable but the schematic rendered while functional, is otherwise very inefficient.

Checkout
What tubes for a tube amp?

[toprepairman]

The 6AS7G (6080) is rated to run at 125mA, that means it's happy with peaks up to 250mA. All well made valves have a fair bit of surplus built in to cope with ageing, so with the sails up and a good following wind you might just get 400mA peak. With both sections strapped together that gives you 800mA peak. Across an 8 ohm load that translates to 5 Watts peak. It's still only 2.5 Watts RMS.
Perhaps Atmasphere you'd like to share you circuit with the rest of us.
Regards
Henry

[atmasphere]

Originally Posted by toprepairman

The 6AS7G (6080) is rated to run at 125mA, that means it's happy with peaks up to 250mA. All well made valves have a fair bit of surplus built in to cope with ageing, so with the sails up and a good following wind you might just get 400mA peak. With both sections strapped together that gives you 800mA peak. Across an 8 ohm load that translates to 5 Watts peak. It's still only 2.5 Watts RMS.
Perhaps Atmasphere you'd like to share you circuit with the rest of us.
Regards
Henry

The circuit is in the link I provided. Its on the 3rd page. 5W into 8 ohms with a pair of 6AS7Gs is no worries. At that impedance though such an amplifier will not be class A.