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How to add tube current meter to 300B amp??

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Posted on March 20, 2014 at 09:47:08

Alpha Al
Industry Professional

Posts: 2958
Location: N. Carolina
Joined: February 16, 2004
Contributor
Since: December 3, 2015

I was given the task of repairing a pair of 300B DIY monoblocks. The original builder has passed away.

The design is by SJS Electroacoustics in the UK. It is a two-stage amp, with a 5842 triode driving the 300B through an IT. It has a complex power supply, with three different tube rectifiers and LC filter networks per amp (plate supply, driver supply and bias supply). Also a DC heater supply for the 300B.

The amps were working, but noisey. Floating a little DC on the driver's heater supply fixed the noise.

The amps were fitted with meters (which I assume are there to read tube current when setting the bias, which is variable). This part was never completed. The meters are 0-1mA. My question: how do I shunt the meters so a 0-1mA meter reads 0-100mA tube current? The filament (cathode) of the 300B returnd to ground through the usual hum balance pot.

The SJS Electroacoustics site shows this exact pair of amps that I have. (see link). They are interesting. They sound wonderful.

plumber's special monoblocks

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RE: How to add tube current meter to 300B amp??

SteveBrown

10:06:55 03/20/14

RE: How to add tube current meter to 300B amp??

Alpha Al

11:49:55 03/20/14

RE: How to add tube current meter to 300B amp??

SteveBrown

12:45:51 03/20/14

...

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RE: How to add tube current meter to 300B amp??

SteveBrown

10:06:55 03/20/14

RE: How to add tube current meter to 300B amp??

Alpha Al

11:49:55 03/20/14

RE: How to add tube current meter to 300B amp??

SteveBrown

12:45:51 03/20/14

RE: How to add tube current meter to 300B amp??

Paul Joppa

13:11:09 03/20/14

SteveBrown

13:43:30 03/20/14

RE: Thanks Paul..

Paul Joppa

16:38:11 03/20/14

Thanks Paul and Steve

Alpha Al

20:31:35 03/20/14

RE: How to add tube current meter to 300B amp??, posted on March 20, 2014 at 10:06:55

SteveBrown
Audiophile

Posts: 2454
Location: Portland, OR
Joined: November 14, 2002

I built that same design a long time ago, sourcing parts from SJS. I never liked the IT and later sold it (mine rolled off the highs). I also found it necessary to add grid stoppers to the 417a, which IIRC is not shown on the schematic. These use fixed bias via a seperate winding. All PS are choke loaded. Not sure if I can be of much help on the current meter, you'll need to know it's internal resistance so you can figure the shunt correctly. The 300b has DC heating, IIRC. Is the wiper of the hum pot grounded? If so, I'd put the meter there rather than the B+ side.

RE: How to add tube current meter to 300B amp??, posted on March 20, 2014 at 11:49:55
Alpha Al Industry Professional Posts: 2958 Location: N. Carolina Joined: February 16, 2004 Contributor Since: December 3, 2015	Yes, the wiper goes to ground. I planned on putting the meter there. I just need to know how to shunt the meter to make it read 0-100mA.

RE: How to add tube current meter to 300B amp??, posted on March 20, 2014 at 12:45:51
SteveBrown Audiophile Posts: 2454 Location: Portland, OR Joined: November 14, 2002	Use a variable voltage source and pass a current so that 1ma flows. Measure the voltage. Internal R = v/i. That R becomes one part of a parallel circuit. You know 1ma across, at a particular V, so then at that V what R do you need to pass the other 99ma. Solve for that (will be a tiny resistance), and there you go.

RE: How to add tube current meter to 300B amp??, posted on March 20, 2014 at 13:11:09
Paul Joppa Industry Professional Posts: 7295 Location: Seattle, WA Joined: April 23, 2001	The easy way to do this is to use a multimeter to measure the resistance of the meter. Shunt resistance should be 1/99th of the meter's resistance.

Thanks Paul.., posted on March 20, 2014 at 13:43:30
SteveBrown Audiophile Posts: 2454 Location: Portland, OR Joined: November 14, 2002	I thought about that but I didn't know if the voltage of the ohm meter might damage the 1ma meter, I guess not.

RE: Thanks Paul.., posted on March 20, 2014 at 16:38:11
Paul Joppa Industry Professional Posts: 7295 Location: Seattle, WA Joined: April 23, 2001	Start on a high resistance setting (the current will be very low). Switch to lower scales, watching the meter being measured. Stop before the needle goes off scale. That way you can be sure.

Thanks Paul and Steve, posted on March 20, 2014 at 20:31:35
Alpha Al Industry Professional Posts: 2958 Location: N. Carolina Joined: February 16, 2004 Contributor Since: December 3, 2015	I'll be sling'in solder this weekend. Thx, Al

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