Contraption to read amps B+

[Guy77]

I rigged up this contraption to read my B+ voltage on my cathode biased amp (Liverpool) while I turn my VVR adjustment knob, without opening the amp from its cabinet. I did this because when I took the amp to different homes the wall voltage was different at each home.

I connected the gold/red pin with the blue wire (near top of pic labelled B+3) to where my B+ exits my vvr and then enters the 1K 25 watt resistor on Trainwreck amps.

To take a reading while my amp is still in its cabinet, I connect the red lead of my multi-meter to the gold/red pin outside the amp and the black lead to anywhere on the outside of the aluminium chassis.

Interesting thing I noticed. When my tubes dissipation rate was less than 95% my overdrive sound (while using my ppimv control) was not as tight as when it was at or above 95%. It was like there was some cross over noise in the overdrive sound. At 95% it was great!

Cheers
Guy

[M Fowler]

I don't know if it's just my aging or what but I seriously have been thinking about designing my home builds like old table top amps with meters built in the enclosure to read every thing, why not.

Mark

[Guy77]

I was actually thinking about doing the same thing. I was just to lazy to do it but it would be so cool to have meters mounted on the back or front. These are the ones I was thinking about.


http://www.ebay.com/itm/DC-500V-Red-LED ... xyLchRtH16

http://www.mableaudio.com/en/productview.asp?sid=5641


Cheers
Guy

[johnnyreece]

I did something similar in a JTM45, but I made a voltage divider because I was afraid of having 450+ volts so easily accessible. I haven't used it in a while, but I believe it shows as millivolts (for example, if my meter reads 480mV, it translates to 480 Volts).

[M Fowler]

Andy Fuchs put a meter on one of his new amp models.

Weber had a bass kit with the meter on the back.

Mark

[tictac]

An RCA jack doesn't seem like a safe choice to me. A test point jack that would accept DMM test probes would be better but either one would accept a paper clip or piece of wire....

I remember as a 3 or 4 year old sticking a paper clip into a 120V AC outlet to see what would happen... after I got buzzed I tried it again and the second time it didn't feel so good....

I don't think + 400 VDC will be so forgiving....

TT

[johnnyreece]

Here's what I used. Note, the actual values were 1M/1K (not 100). Less that half a volt is much safer than 400+. I also did it for both tubes (believe it or not, there was a slight difference), but one tube would really be sufficient, I'd think. I used very tight tolerance resistors (I want to say 1%).

[Guy77]

Thanks for the input everyone. Yes all that voltage is dangerous and I will definitely make some changes. I like your idea Johnnyreece! I think I will do something similar.

Cheers

Guy

[johnnyreece]

No problem! Just glad to help!

[Guy77]

I got my plate voltage test pins working now with the voltage divider !

Works like a charm 0.325 volts on my test points translates to 329 volts on my el84 plates! It is off by a tiny bit since my 1000 ohms resistor actually measure 996 ohms.

My amp is a Liverpool, (cathode biased el84) so I set it up a little different from what you would use on a JTM45. I wanted to attach a little drawing just in case anyone else wants to set one up on a cathode biased el84 amp

Cheers!

Guy

[johnnyreece]

Glad you're up and running! Yeah, it seems like you're close enough for rock and roll with what you've got in there.