TYPE 36 DIY Valve Pre Amplifier


#1

Built by forum member ‘Disarmament’ of Art of Sound.

This hand built pre-amplifier proved highly capable during a local bake-off against the likes of Modwright, Music First and Ayre, enough for the host to commission one. Below is a brief description from the engineering wizard responsible regarding the valves used etc.;

The first gain stage valves are RCA 37 single triodes from the 1930’s, the cathode follower stage is a 6SN7GT. The design is based on a design by Bruce Berman first published in the Sound Practices magazine. Although Bruce Bermans design used RCA 76’s in the first gain stage. The 37 and the 76 are very similar, but the 37 is a lower Mu valve than the 76 so the gain is lower.

The preamp has a voltage gain of around 4.

Both the 76 and the 37 triodes are very linear, but in my opinion the 37 sounds the nicest out of the two.

The preamp can use either the 37 or the 76 without changing anything. However both these valves are becoming scarce, especially the 37. Replacing the valves would need some care as these can be microphonic so it may need a selection of valves to find the best pair.

Photographs show how well made this thing is, the case has one or two minor blemishes, but nothing significant. Collection/AA taxi/Lopwell or meet up preferred, not a fan of couriers. I am not in any rush to sell, so may just bring it along to Lopwell for anyone to try out.

For sale at a price well below the price of the parts used - £450.


#2

Very tempted I have to admit. Need to do some reading after supper by which time I suspect it will have been grabbed.


#3

A bit more info kindly supplied by the maker following some questions on AoS;

The stated voltage gain is correct at about four times, or 12db. That is with the 37’s currently fitted.

The balanced output is provided by phase splitting the unbalanced output through a pair of Sowter 10K to 10K 1:1 ratio isolator/ balancer transformers.

The secondary of these transformers have a centre tap about which the phase is split, so in this case the transformers are wired as 1:0.5+0.5 ratio.

There is one other point which needs to be made clear. The output of the preamp is designed to drive a high impedance load of 50K ohms minimum, preferably 100K ohms. If any prospective buyer is considering driving an amplifier with a lower input impedance the output capacitors will need to be changed, otherwise there will be significant loss in the bass frequencies.


#4

SSTP