Push-pull EL84 amplifier

 01.05.2014

 

* Why I chose the EL84?

– because I have a Siemens matched quad and a dozen soviet military 6P14P-EV tubes.
– I have 95 dB sensitivity speakers, so the low-ish 5 watts of power produced by a triode-strapped PP EL84 amp would be enough.
– I will build a bigger KT88 push-pull someday, now it’s time for EL84 sweetness.

* What configuration?

I have settled on a simple, classic design: a common cathode voltage amplifier followed by a split-load phase splitter, driving a p-p output stage, with no negative feedback.
– the voltage amplifier is CCS loaded for maximum gain and lowest distortion. The high-quality CCS is easily adjustable to set the quiescent current (about 7.5 – 8 mA).
– the cathode of V1 is LED biased.
– the power stage uses “fixed” bias, not the usual cathode bias, avoiding the electrolytic capacitor bypass. This allows for individual fine bias tuning.

* Which output transformer?

I found some cheap Italian transformers, made by Novarria, that proved to be of decent quality.

* What preamplifier/splitter tubes?

A low noise, high quality, low to medium-mu tube will amplify the input voltage. 110 – 115v is the maximum acceptable plate voltage, because of the phase splitter. Candidates:  6N23P-EV, ECC88, 6211, 5814, 6CG7, 12BH7. Do yourself a favor and avoid the 12AU7/ECC82 – I have never heard one that I liked.
These tubes need *at least* 5 mA of current through them to sound good.
My choice went to the 6211, one of the best sounding triodes of this class.

* Why the CCS load?

Because it reduces distortion and maximizes the tube’s gain. Thanks to the constant current, we can use LEDs to bias the cathodes – the LED will be very linear, and it saves us the trouble of decoupling the cathode with an electrolytic cap. The CCS is made with cascoded DN2540 depletion mode MosFETs.

* The power supplies

The amplifier is fed from a C-L-C supply followed by separate adjustable stabilizers (not regulators!), one per channel, with delayed soft start. A timer made with a 555 turns on the HV after 15 seconds, giving the tubes’ filaments time to warm up.

The preamp stages should be decoupled with R-C cells. Because of the CCS’ extremely high impedance, a local big reservoir cap is not needed.

 

The final schematics:

Photos:

 

It sounds very good. On my small bookshelf speakers it sounds better in pentode mode; on my big 95dB speakers, the triode mode is more refined and pleasant.

The amp has an output bypass circuit, made with relays: it allows me to easily choose between this and another amp, both of them wired to one pair of speakers.