bump.. I too want to see more of flea-powered OTL.
Here's something i simulated recently.. a pair of 6AS7/6080 triodes for each channel. The use of output coupling capacitor was intentional as i want to avoid using DC servo (which complicates the PS requirement, if not else). Overall, one heater power supply, one 300VDC HT and one -32VDC bias is all it needs for two channels.
The LTP perhaps can use 6SN7/6N8S if you want to run more plate current.. but considering 6AS7 is low mu, i think 1.5mA is enough to cope with the miller capacitance. It's a trade off between open loop gain and plate current.
It's not exactly 2W.. output is 5Vpk to 8ohm which translates to 1.5W. THD is 0.83%. I'm sure it can be improved.
Here's something i simulated recently.. a pair of 6AS7/6080 triodes for each channel. The use of output coupling capacitor was intentional as i want to avoid using DC servo (which complicates the PS requirement, if not else). Overall, one heater power supply, one 300VDC HT and one -32VDC bias is all it needs for two channels.
The LTP perhaps can use 6SN7/6N8S if you want to run more plate current.. but considering 6AS7 is low mu, i think 1.5mA is enough to cope with the miller capacitance. It's a trade off between open loop gain and plate current.
It's not exactly 2W.. output is 5Vpk to 8ohm which translates to 1.5W. THD is 0.83%. I'm sure it can be improved.
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There are several problems of designing a low power OTL like output impedance and effciency
Even when using the inverted Futterman configuration which give the lowest possible output impedance, smaller tubes will give quite high output impedance, here are some examples for various tubes that have been used in OTLs, this is for one pair.
6C33C 11ohm
6C41C 20ohm
6AS7G 20ohm both sections in parallel
6C19 50ohm
So more feedback is needed with the smaller tubes.
Another issue is that effciency will be even lower for a low power amp, our high power 80W amp has about 1.6 times higher efficiency than the 25W version.
Even when using the inverted Futterman configuration which give the lowest possible output impedance, smaller tubes will give quite high output impedance, here are some examples for various tubes that have been used in OTLs, this is for one pair.
6C33C 11ohm
6C41C 20ohm
6AS7G 20ohm both sections in parallel
6C19 50ohm
So more feedback is needed with the smaller tubes.
Another issue is that effciency will be even lower for a low power amp, our high power 80W amp has about 1.6 times higher efficiency than the 25W version.
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Is this correct ? I got just over 1 watt with SE OTL with 4 x 6S19 into an 8 ohm load
316a
That is correct according to sim, also compared with 25W model. This is SE-PP so it's more power than just SE.
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No problem, since I don't have one for scaled-down version, so I give you PSU for 25W model, it will give you some ideas. Please ask if any question. It has speaker protection unit, soft start included.
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