UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post - Discussion of subcircuits for homebrew valve testers

Thatvalveguy · 8th Apr 2018, 3:01 pm

Thanks, i didnt think about that pair, they share pinouts with the Toshiba devices previously mentioned, i bought 100 pairs of the Toshiba's before they went unavailable at my supplier.
Current gain seems a bit low, but they will work.

Here's the changed circuit for the current measurement. Hand drawn of course.

The 47K/22n input RC filter largely rejects, the applied grid signal. From an AC standpoint the 47K is parallel to the 2K resistor because of the RC filtering.

The trim circuit for the LF 411 is not drawn, its the standard data sheet Trim circuit, but with a 50K pot. I was thinking of using a 2.5V shunt reference in the shunt regulator to supply the LF411 with -2.5V. Those IC's have much better temperature coefficient then zener diodes, any drift in the V- rail of the op amp is going to influence the offset voltage. LM336 or LM385 seems like a good candidate.

The oscillator section will have to drive about a 10K load, peanuts for the NE5332 that will buffer the oscillator.

The floating +5V supply for this circuit will have to supply about 20mA.

More to follow!

8th Apr 2018, 3:01 pm	#28
Thatvalveguy Triode Join Date: Apr 2018 Location: Nijmegen, Netherlands (You know from that bridge) Posts: 35	Re: Discussion of subcircuits for homebrew valve testers Thanks, i didnt think about that pair, they share pinouts with the Toshiba devices previously mentioned, i bought 100 pairs of the Toshiba's before they went unavailable at my supplier. Current gain seems a bit low, but they will work. Here's the changed circuit for the current measurement. Hand drawn of course. The 47K/22n input RC filter largely rejects, the applied grid signal. From an AC standpoint the 47K is parallel to the 2K resistor because of the RC filtering. The trim circuit for the LF 411 is not drawn, its the standard data sheet Trim circuit, but with a 50K pot. I was thinking of using a 2.5V shunt reference in the shunt regulator to supply the LF411 with -2.5V. Those IC's have much better temperature coefficient then zener diodes, any drift in the V- rail of the op amp is going to influence the offset voltage. LM336 or LM385 seems like a good candidate. The oscillator section will have to drive about a 10K load, peanuts for the NE5332 that will buffer the oscillator. The floating +5V supply for this circuit will have to supply about 20mA. More to follow! Attached Thumbnails Last edited by Thatvalveguy; 8th Apr 2018 at 3:09 pm.