Inexpensive balanced push-pull amp with DHT driver.

[Cressy Snr]

Aye it's a right royal pain.

The filament is common to each half of the valve, on pins 1 and 7 with a centre tap on pin 4,
which means you can't wire the cathodes as a push-pull driver like you would a pair of,
for example, 300B in a push-pull output stage, so there's no hum cancellation,
and that of course is the stumbling block, because the noise gets amplified.

[Cressy Snr]

Well I got it working with DC heating, but to be frank, the sound is not a patch on what I was getting with 13D1 drivers.
There is a liquidity to the top end that is delightful on the right programme but elsewhere the sound is too thin. It represents everything I don't like about hi-fi sound.
Liquid treble is fine but if there is no support from underneath then there is no point listening.

With the 13D1 in the driving seat the sound was refined, well fleshed out with a nice top end; not quite as airy as the DCC90,
but oh so good everywhere else. The bass in particular is fantastic with the 13D1

So that's that for me.
The amp will now be properly rebuilt as a parallel-push-pull, class A/B, triode strapped, 15 watter with 13D1 diff pairs driving the outputs.

Just need a decent chassis to put it in now. It will not be red