You are loosing your time with the 6BQ5 IMHO... Every OTL I tried with this tube was poor performance, when not oscillating and/or distorting. Use a 12AU7 for headphone amp OTL output stage : it will work better, but I think that it would be best to forget OTL circuit for headphone amp... And go for a solid-state device.
A+!
You would need a lot of 12au7 to drive 33r headphones in OTL. ECC88 and ECC99 are much better suited.
For a decent output level into 33 ohms you would need an idle current of around 60 mA. This implies a peak of 120 mA at 0V on the grid, which requires 185V anode to cathode for the 6BQ5. Dissipation would be around 11W for an output of less than 60 mW. Other tube choices are better (6AS7, ecc99, 6EM7 etc) because they can achieve the required current at much lower voltages.
(This is for triode connection of the 6BQ5)
(This is for triode connection of the 6BQ5)
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Just for fun... the attached schematic could get you at 0.7Vrms @33r with your 150V B+. It's enough for many low impedance cans. See : Headphones sensitivity, impedance, required V/I/P, amplifier gain
But you need a lot of feedback (hence the ecc83 at the input) and solid state help to get you there.
But you need a lot of feedback (hence the ecc83 at the input) and solid state help to get you there.
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Heater current is 2.5a on the 6AS7 and we were trying to reduce the size of the amp. Is there a smaller tube then the 6AS7 that would provide some current at low voltages and has a lower heater current?
Yes because we're using two 12vac transformers as a power transformer but I could add a voltage multiplier if needed.
The goal is to keep it small and as simple as possible. This is his first amp and he plans to use it at work on a daily basis.
Maybe I'm a horrible person for suggesting, but, why not the 50C5? It is designed for low voltage and would eliminate the need for a seperate heater supply by using the B+ in a series string. Although most do not consider this a "good" tube, I have no issues with the way any 50C5 amp I've heard sounds myself.
We may end up building a PP amp with 6V6s just in case there's a need to power a set of small speakers.
I've been experimenting with one already using a 12AX7 with the first 1/2 as a gain stage and the second 1/2 as a concertina PI.
I had a question concerning one of the concertina schematics I found online. It's shown below. What is the purpose of R8 and C5?
I've been experimenting with one already using a 12AX7 with the first 1/2 as a gain stage and the second 1/2 as a concertina PI.
I had a question concerning one of the concertina schematics I found online. It's shown below. What is the purpose of R8 and C5?
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