Head Amp of the glowy kind.

[Firebottle]

Can you guess what it is yet?




Built yesterday into an old analogue satellite receiver case, which actually turned out to be about the right size.
I wanted to make a valve based head amp so I can test sound differences with various output caps, but with more than a couple of volts across the caps.

Experimental valve head amp:




Based around a 9V transformer (bottom left in the photo) to feed the heater regulator, plus a 17V/230V transformer (top left) to give approx 110V AC for the HT rectifier.

Ignore the transistors, links and resistors on the PCB, they were just part of the satellite power supply. The heater regulator is on the heatsink, rather OTT for the dissipation needed.
The third piece of iron is another transformer used as a choke, about 33 Henrys, then followed by a MOSFET capacitance multiplier, essential to reduce hum to acceptable levels.

The business end, simplicity in itself:




How does it sound I hear you ask, pretty bl**y fabulous
There is some tube rush but not bad at all, about the same as surface noise you might get on some vinyl. I'm very happy with the choice of valve, I had to swap one that was a bit noisier.

Valves are from my 'stock box' so have cost next to nothing, they are miniature 7 pin single triodes, EC900:




Gain is a useful x21, is it microphonic? Of course it is , though within acceptable limits.

Very pleased with the outcome.

[Jimbo]

Wow didn't expect you to go down this route Alan.

[Firebottle]

I won't be offering any for sale, hence the post is under 'The Drawing Board'.

I suppose it is/was a bit of an itch I had to scratch. I'll bring it over next time we are in the UK.

[Jimbo]

I won't be offering any for sale, hence the post is under 'The Drawing Board'.

I suppose it is/was a bit of an itch I had to scratch. I'll bring it over next time we are in the UK.

Great, look forward to hearing that. How do you think it compares to the tranny head amp?

[Gazjam]

Looks great Alan!

[awkwardbydesign]

Not much room in there for "proper" caps!

[Firebottle]

Compared to the FET amp it is definitely noisier, but I would say they are equal in their transparency.

Plenty of room for 'proper' caps. That is part of the reason for the build, to try with a selection of output caps.

To be continued............

[Barry]

You could always fit anti-microphonic valve bases, if you can find some B7G type. What is the output impedance?

[Firebottle]

What is the output impedance?

Just measured it Barry and it is 3K ohm

[Ali Tait]

How about a hybrid to get the noise floor down?