Use for VT1399 Interstage

[DaveP]

Last year I bought a VT1399 interstage from Livinginthepast in the UK.
http://livinginthepast-audioweb.co.uk/xfrmrvt1399.php

I intended to use it in my BA-6A from scratch project, but it did not have enough bass response in that circuit.  EMRR asked me to test it for him in a new application and suggested I post the results, so here goes:-

The RCA 86 compressor uses an interstage for the negative CV and it appears to have a use here.

I set up a 6SR7, which is a single triode (12AU7 spec) plus double diode metal tube with 250V through the 15K primary.  The cathode resistor was 1.8k drawing 5.5mA at 10V.  The interstage is very sensitive to the cathode capacitor value which can be chosen to lift the bass response at various points.  With a 2v signal you can get 27V on the plate which will give you 20Vdc from the secondary.  See attached schematic.

You can see from the table that they used several types of TX for this duty and they all needed various load resistors and zobels to damp the response, this interstage is no exception.
Doug asked me to use the CT points 1A/3A to give a 1.5:1 step-down.  The results are in the following post.

best
DaveP

 

[DaveP]

Here are the results of the test with various load resistors and caps.

So with 2Vrms input the tube gets 27.6V rms on the plate.  I wired the secondary  using the 1A/3A taps.  If you look at the rca86 setup they had a table of values of components used to correct the output including a zobel.  This is probably required to stop HF ringing.  The data shows negative DC against frequency, this avoids having to use the scope all the time, and any case, its the dc you are interested in.

I tried it with 2x47k to a 0.22uF cap/1M resistor first, this is the first row and it also had a 50uF cap on the cathode.
The second row is with 2x33k and 22uF.

Results: the higher the load resistors, the higher the top end and the tendency to ring, the bass is also better.
I did try it with 2x15k but it was crap so I didn't record it.  With 2x33k it flattens the top end ok.  There is definitely scope for playing around with the values, particularly with a zobel in circuit.  The cathode cap can boost the 50Hz at the expense of the 20Hz and vice versa.

If you care to boost the bass a little I think it will do the job ok.
best
DaveP