Last update 10/8/07

tube mic power

 

 

 

 

HJFP2/HVTP2 Mods, Upgrades ...

Below are tables for changing the pad value of the HJFP2/HVTP2 if you desire.  It is simply a matter of changing R3 and R4.  The nominal value is -14dB.  Use the closest resistance value to those listed in the chart, preferable 1% metal film or better.  Basically you are changing the ratio of R3/R4, while keeping the sum the same.  R3 and R4 form a voltage divider before the first gain stage.

Theoretical

HJFP2

dB attn

H

R4, Kohms

R3, Kohms

-14

0.199526

31.6

127.0

-15

0.177828

28.2

130.4

-16

0.158489

25.1

133.5

-17

0.141254

22.4

136.2

-18

0.125893

20.0

138.6

-19

0.112202

17.8

140.8

-20

0.1

15.9

142.7

-21

0.089125

14.1

144.5

-22

0.079433

12.6

146.0

-23

0.070795

11.2

147.4

-24

0.063096

10.0

148.6

 

THEORECTICAL VALUES: HVTP2

dB ATTN

H

R4, Kohms

R3, Kohms

-14

0.19953

50

200

-15

0.17783

44

206

-16

0.15849

40

210

-17

0.14125

35

215

-18

0.12589

31

219

-19

0.1122

28

222

-20

0.1

25

225

-21

0.08913

22

228

-22

0.07943

20

230

-23

0.07079

18

232

-24

0.0631

16

234

 HJFP2: ADDING A SWITCH FOR CHANGING THE INPUT TRANSFORMER TURNS RATIO:

One person asked for this, so here it is.  I have not done this, so I can't promise it will not induce noise extending the wires, because this is the most critical point in the circuit, the mic input.  The circuit below is a basic series/parallel switch on the input transformer primaries, changing the ratio from 4:1 to 8:1.