volt meter on cathode

[John]

This is something I've often thought about doing just for fun - installing a DC volt meter on an amp face to watch the needle swing when playing power chords. Question is, would it actually do that? Would it somehow put an extra load on the tube? Draw more (too much) current? Or is this one of those Very Bad Ideas? This would be strictly a cosmetic "oh isn't that funny" thing, not an actual useful thing.

[Willabe]

Hi John,

Weber has a few different analog meters that look pretty good and I think would work? I have a few laying around for a bench PSU I want to build. $$ not bad ($8.50) and there's pics of them on the site. They have both mA and volt meters in different ranges.

I don't know if you can leave them in the cathode circuit while playing?

https://taweber.powweb.com/store/meterord.htm


                        Brad      

[John]

I don't know if you can leave them in the cathode circuit while playing?

Exactly. I don't know either 

[jjasilli]

Could be done.  As on the Weber site, read across a 1Ω resistor, or knock-down B+ with a voltage divider (using large values so as not to draw much current). 

Because this is for show, I think the issue will be the meter's scale vs. dramatic effect.  I.e., a 30V swing will not look like much on a 1000  or even a 100 unit scale.  But a 30V swing will be eye-catching on, say, a 50V scale.  So, you need to determine your voltage swing and pick a meter with a scale only a little bigger than that.   

[kagliostro]

What about an EM84 tube like in Selmer Zodiac Twin 30 ?



or an EM800




here a video sample
http://vimeo.com/32222683#

K

[jazbo8]

The magic eyes are very cool! For a retro look, just hook up an old ampere meter and watch the needle bounce while playing and it can be used for making bias adjustment when installing new tubes or tweaking.

Jaz

[John]

You're right, I think the milliameter would be simpler and give more bounce?

And Kaglio, those magic eyes are cool too!

Upon further review, those magic eyes start to freak me out after a bit. 

[PRR]

Meter in the cathode will not deflect much on single-ended amps nor on cathode-biased push-pull.

With fixed bias push-pull there will be deflection. Think 5F6-A Bassman. 80mA idle to 220mA full power sine.

There are some scaling issues to suit available meters. You want an amp-meter, with max current much less than idle current (so the tubes won't notice). 1mA is fine and often cheapest. You must split current between a shunt resistor and the meter. But the meter resistance is low and uncertain. You would probably add a series resistance to the meter to make it about 1 Volt full scale. So 1V/1mA= 1K total, probably 800 or 900 ohm resistor and the rest in the meter. Now bring your cathodes together in a resistor which drops 1V at somewhat over the amp's full-roar current. Say 250mA, at 1V, is 4 ohms.

The magic-eyes are cute but need many-many volts of signal. And the polarity is awkward.

In either case, the dynamic motion of the metering isn't telling you much different than hanging a VU-like meter across the output. And a LOUD-speaker output can easily deflect the eye-tube. Need a signal rectifier but today 1N4007s are really cheap.

[stratele52]

I don't know if you can leave them in the cathode circuit while playing?

Exactly. I don't know either 

Yes

[John]

In either case, the dynamic motion of the metering isn't telling you much different than hanging a VU-like meter across the output. And a LOUD-speaker output can easily deflect the eye-tube. Need a signal rectifier but today 1N4007s are really cheap.

That sounds like the much simpler way to go. Thanks.