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phase shift vs OPT saturation

The cutoff of the coupling cap with that grid bias resistor is 19.2Hz.

Frequency in Hz = 1,000,000 / 2Pi x C (in uf) x R (in ohms)

2Pi being 6.28
R = 330,000

This will cause phase shift up to about 200Hz; that can sound like a loss of impact to the ear.

If your OPT is rated to go lower without saturation, I would increase the value of the coupling cap. Look at what the LF spec of the transformer is and don't exceed it.

Keep in mind that the design might have been limited to 20Hz so as to prevent saturation in the OPT; this might be part of why the original amp performed the way it did. But if you have a better OPT that you're working with you might be able to get around this limitation.


(But I would give this some thought. The EL84 is meant to be easy to drive so you could build a higher performing circuit that uses less tubes. You could use a 12AT7 arranged as a differential amplifier.

One grid could be the input and the other where the feedback is received. The advantage of this is that the circuit could then be fully differential; resulting in considerably less distortion- smoother and more detailed. If you had a negative supply (which could be derived from the HV windings of the power transformer) then you could set up a nice CCS for the 12AT7.

To increase the differential effect (and reduce distortion) you could use a CCS in the cathode circuit of the power tubes as well.

Just a thought...)

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