Effects of Bias /Ammeters....

[Jeffrey Davison]

Does having a bias meter, or rather an ammeter in the cathode to ground connection of a DHT to measure current, have a detrimental effect?
During operation, the meter has an inductance and back EMF that can effect the signal. I was reading a reply from a post of a few years back, it was suggested that a big electrolytic (bypass?)cap be put across (parallel) to the ammeter to rid the circuit of the nasties of back emf from the natural movement of the meter during play.
Does not the introduction of a capacitor also introduce it's own set of nasties? Is this a trade of of the lesser of two evils?
Would a better solution be to just have a spst switch to have the meters operate only while setting bias and then switched out during listenting?
I'll admit, I'd like to have the meters on all the time, but, I would not want to pay the price of sonic degradation just to see those little pointers doing their happy thing.

Comments?

Jeff Davison

[Ty_Bower]

Originally Posted by Jeffrey Davison

Would a better solution be to just have a SPST switch to have the meters operate only while setting bias and then switched out during listening?

I'd be careful with any kind of switch, and be absolutely certain it makes the connection through the meter before it breaks the connection to ground. Remember there is current flowing through the tube. If you suddenly open up the switch at the cathode, you're going to get a nasty inductive kick from the output transformer attached to the anode.

[aardvarkash10]

alternatively, don't use a moving coil meter. Use a solid state device instead.

I think you are a bit paranoid here - remember the ammeter is reading across a comparatively minute resistance in shunt. The voltage difference across the shunt is 3/5 of 5/8 of b*gger-all, and any back-EMF will be sucked into the black hole of this low resistance.

[HollowState]

Personally I would never put an ammeter directly in the cathode (filament) circuit of an output tube. Partly for the reason you mention, however tiny, and partly for practical reasons. Were a tube to short, there goes your meter. And shunting the movement with a capacitor would act as a shock absorber giving the meter a spungy delayed action. Perhaps for the better. At normal listening levels you won't see a lot of movement anyway.

I always use a small carbon resistor as a current sampler and measure with an analog milliammeter or even a microammeter through a suitable series divider. I much prefer the isolation by doing it that way. And I don't leave it in the circuit all the time. A small value resistor will produce a very small amount of degeneration in the output stage lowering distortion in the process. One of these days I'm going to try one of those chicom digital readout panel meters.

[refference]

Originally Posted by Jeffrey Davison

Does having a bias meter, or rather an ammeter in the cathode to ground connection of a DHT to measure current, have a detrimental effect?

I can not see any problem here . I often do that .

Connect the ampmeter , in the usual way , in series between cathode and ground , then connect paralleled to the meter , a good-quality spring-action
NORMALLY-CLOSED push-button .

In normal operation the current will flow thru the push-button , if you press the button , the current will be deviated and will flow thru the meter , so you you can read the value .

The big advantage of this method is that the cathode is ALWAYS connected to the ground , by the push-button , or by the meter . ALWAYS !!
There is no danger to get nasty inductive kick from the output transformer .

Carlos

[Michael Koster]

Originally Posted by Jeffrey Davison

Would a better solution be to just have a spst switch to have the meters operate only while setting bias and then switched out during listenting?
I'll admit, I'd like to have the meters on all the time, but, I would not want to pay the price of sonic degradation just to see those little pointers doing their happy thing.

Comments?

Jeff Davison

If you use an SPST switch across the ammeter to bypass it there will still be a pop when you switch it in and out. I would expect on the order of 1/10 to 1/2 volt depending on where you are in the ammeter's range. You would have to turn the amp off before flipping the switch or mute the output or maybe try a big cap with the switch to muffle the pop.

If it makes you feel any better, ammeters in the 100mA and 200mA range as you are using already have an internal shunt resistor on the order of a few ohms. The meter coil itself is probably ~1000 ohms. This would reduce any inductive effects by about 40db...

Could also use 1 ohm resistors and cheap 200mV digital meters (~$30). Turn the meters off when not used if you're worried about the little oscillator in them. I think the whole meter can float if powered by a 9V or a small split bobbin supply.

Make sure anything you do for the 304tl cathode can handle 500VDC wrt ground on it.

Cheers,

Michael

[Jeffrey Davison]

The meter would always be in circuit, but there would be a short between the inputs. The actuated switch would just lift the short so the meter by itself would still be in circuit. When the swich is disengaged, the meter would again be shorted....... there is always a "made" circuit.

JD

Originally Posted by Ty_Bower

I'd be careful with any kind of switch, and be absolutely certain it makes the connection through the meter before it breaks the connection to ground. Remember there is current flowing through the tube. If you suddenly open up the switch at the cathode, you're going to get a nasty inductive kick from the output transformer attached to the anode.

[Jeffrey Davison]

these are the cheap meters I was looking at...

Panel meter 200ma 300B KT88 WE 6L6 5881 KT90 EL34 amp - eBay (item 300375604811 end time Feb-08-10 17:42:59 PST)


JD

[Michael Koster]

Originally Posted by Jeffrey Davison

these are the cheap meters I was looking at...

Panel meter 200ma 300B KT88 WE 6L6 5881 KT90 EL34 amp - eBay (item 300375604811 end time Feb-08-10 17:42:59 PST)


JD

OK, 0.6 ohms and 150mA is only 90 millivolts. That will give you about 500mV at the plate and order of 15mV at the speaker. Probably will still be an audible pop but unlikely blow up any speakers.

With that low an internal resistance, I wouldn't worry abou tthe coil at all.

Just mind the 500V to chassis when you mount it...

Michael

[makaiyumtony]

Hi Jeffrey
This is the way I am using, the DC power supply socket and plug, socket in amp side plug connect to a multi meter, it connect and cut almost at the same time, I don't worry this matter of voltage in tube's plate because I am using transmitting tube, they can last very high voltage and I only use in 1/3 of them
regard tony ma

Originally Posted by Jeffrey Davison

Does having a bias meter, or rather an ammeter in the cathode to ground connection of a DHT to measure current, have a detrimental effect?
During operation, the meter has an inductance and back EMF that can effect the signal. I was reading a reply from a post of a few years back, it was suggested that a big electrolytic (bypass?)cap be put across (parallel) to the ammeter to rid the circuit of the nasties of back emf from the natural movement of the meter during play.
Does not the introduction of a capacitor also introduce it's own set of nasties? Is this a trade of of the lesser of two evils?
Would a better solution be to just have a spst switch to have the meters operate only while setting bias and then switched out during listenting?
I'll admit, I'd like to have the meters on all the time, but, I would not want to pay the price of sonic degradation just to see those little pointers doing their happy thing.

Comments?

Jeff Davison