Bias meter for st70

[kheper]

An interesting bias meter, for up to 100ma.

http://cgi.ebay.com/100mA-mA-meter-for- ... 93QQrdZ1QQ

So, how to hook it up?

Forget about individual tube biasing. Use
one meter per PP pair and connect the 8s
and 1s of a pair of EL34s, as in the original
dynaco schematic, but leave out the
resistor. Put the meter 'between the 8s'.
(We are measuring combined cathode
current here, correct?) Hook up the other
PP pair identically. Shoot for a hair less
than 100ma on the meter with a bias
pot per pair. Leave the meter in circuit
constantly or switch it in and out with a
SPDT switch? Won't the switch make
hideous popping noises, if moved when
the amp is running?

Or, stay with individual tube bias. Use 1
meter per PP pair, and put the meter
'between 1 and 8' of each tube. With a
rotary switch shunt the current thru the
meter or have it bypass the meter,
altogether. Hook up the other pair
identically. Shoot for 50ma on each
tube with the bias pots.

Or, just use 1 meter per amp and a rotary
switch?

Where am I going wrong?

[TomMcNally]

You'd put the meter is series with the cathode resistor to ground. You'd leave the resistor in the circuit. You could defintely do it with a rotary switch, although I'd have to think about that a little. Bias for adjustment purposes really should be read with silence, so any pops wouldn't matter.

I have a pair similar to those that I haven't used yet. My big 300B amp has a switchable meter I can read all of the voltages and currents with, kinda nice looking, but functionally worthless, as nothing is adjustable except the B+ via the Variac.

[kheper]

Thanks Tom.

[EWBrown]

Another procedure, is to use a 10 ohm, 1% 1 or 2 watt cathode resistor for each EL34, and use a small 3 or 4 digit DVM module and a 4 position seitch, simply read the voltage at each tube's cathode, and set for 500 mV, which equates out to 50 mA for each tube. A 1volt DC analog meter wil also work fine, for the "retro" look. Yellow_Light_Colorz_PDT_11

I like that hefty 4700 uF 450V Hitachi electrolytic.... definitely NOT rectifier tube-compatible :o

/ed B in NH

[TomMcNally]

That's how my RCA meter on the 300B works Ed ... the full scale for voltage is 750V, so with a 1ma movement, a 750K resistor will read 750 volts ... they sample B+ in about 6 spots, and for current, I picked the appropriate resistors for the current scale on the meter, which are sampling cathode current on the 300B's and 6SN7's ...

Like I said, nothing to adjust, but it looks cool and is functional anyway.

[mesherm]

If you use one of those digital panel meters you will need to supply it with 9-12 VDC. So a small bridge rectifier and a cap or two from the filament supply should do the trick. I have had trouble with some cheaper modules if the power supply ground is common with what your trying to measure so an isolated supply off of the filament winding should work fine.
It amazing to me that some manufacturer hasn't marketed a nice analog 150ma meter that would fit right into the front Dynaco chassis tube holes.

[EWBrown]

If the DVM module has an LCD display, I'd SWAG that a 9V battery would keep it fed for a LONG time... Else, a small "utility" trannie (autopsied from a spare wall-wart) snuck under the chassis somewhere and any supporting ciorcuitry. I know, LEDs look cooler, but they gobble current.

Just how many pounds of, er, stuff can one cram onto and into an ST70 chassis, anyway ? Yellow_Light_Colorz_PDT_01 I just see it as another challenge...

/ed B in NH

[TomMcNally]

I have a bunch of nice LED panel meters (Datak ?) with self contained 115 VAC power supplies. I just don't like the thought of combining tubes with solid state stuff. If I WAS going to do anything ... I was thinking about a Microcontroller, probably a BASIC Stamp 2 (since that's the only one I know how to program well enough) that would sample the bias reading on each tube, and adjust it via some motorized pots. It would power the amp on, wait about a minute for it to stabilize, keep the inputs muted, then adjust. If it realized there was something wrong, it could beep, or call your cell phone and let you know, or just say something through the speakers.

[kheper]

TomMcNally -
'I was thinking about a Microcontroller,
probably a BASIC Stamp 2 [] that would
sample the bias reading on each tube, and
adjust it via some motorized pots. It would
power the amp on,' etc. etc.

Now that is some really fancy DIY, If
not DEY (Do Everything Yourself).

[mesherm]

How about a 9 volt battery and a 6 volt reed relay so the battery is only energized when the amp is on?

[EWBrown]

That would work, too... Yellow_Light_Colorz_PDT_02

Let's see, how about an "active" cathode resistor for the 300Bs...

The PIC controller, which in turns operates a thyratron-controlled servo motor, which controls the bias pot... Oops, Doctor Rube Goldberg, please call your office... Yellow_Light_Colorz_PDT_04 Yellow_Light_Colorz_PDT_08 Yellow_Light_Colorz_PDT_01

Going back to more 1930s appropriate "bear skins and stone knives" technology: (Ya gotta be a Trekkie of the Old School to get that one)

Perhaps a 15-25 watt 120V light bulb, its "almost hot" filament resistance should be about 880 ohms with approx 80 mA (70V) flowing through it... Of course, cold resistance is going to be MUCH lower... If the current rises, the filament gets hotter, the resistance increases, and the current drops back down to some "balance" point. Best to experiment with the bulb, a variac and an AC ammeter to get the initial bulb wattage thingie dialled in first, before sacrificing those WE300Bs onto the altar of science...

Might make for an interesting experiment. I've seen a variation of this in some tube integrated amps, the common cathode "resistor" for the four EL84s is the filament of a 12AX7 (12.6 VDC, 150 mA), which is usually used in the phono stage, which is quieter with DC on its filament. And a potentially really nasty unintended feedback loop. :o

/ed B in NH

[SteveH]

Figured I'd resurrect this :D

So were you able to implement this successfully ? Im thinking of a 5 position rotary switch (one for each tube; and an 'off' position); but I am trying to figure out how the circuit would look....

[kheper]

Look at the schematics for the Citation II and
the Marantz 8.

http://pages.prodigy.net/jimmcshane/CitIIsch.jpg

http://www.triodeel.com/marantz8.gif

[dhuebert]

MTCW: Forget about bias. Tweak it a few times during the first hours of operation and then forget it. You will find the tubes will change at first and then settle down and all your fancy-dancy circuitry will serve no purpose other than to look cool. But if you're absolutely convinced bias is critical, try this approach:

http://www.normankoren.com/Audio/TENA.html#Bias

Personally, I think it can be done more simply but the priciple is good.

Don

[burnedfingers]

Personally, I think it can be done more simply but the priciple is good.

Why go to that bother?

I just add additional bias pots and a slight modification to the driver board and I am able to adjust each tube's bias.