VM showing 0 ma with bias probe on 64 Fender Concert Amp

[concertamp64]

I tried multiple VMs set a DC 200ma and got same 0 current readings.
Bias probe tested good per EL34's continuity test.
I biased voltage (old way I guess) on pin 5 as low as possible 36.4 volts. Bias pot range is 36.4 to 50 with a 24k bias pot resistor. 15k resistor was replace previously by an amp tech.
Plate voltage pin 3 is 468 and 471 on old GT 6l6s with low hours....is this considered a little high?
I would like to try new JJ 6l6s but would like to get the 0 milliamps issue sorted out first...
Any comments are welcomed...

[shooter]

install 1 ohm resistors cathode to ground, no guess no mess

[concertamp64]

will adding the resistor make the bias probe show current?

[shooter]

ohm will
1volt /1 ohm = 1 amp
so the DCvolts you measure across the resistor will equal the current the tube is drawing.  simple, no extra pieces that can be misleading or worse detrimental

[sluckey]

Most bias probes read mV across a 1Ω resistor built into the socket. Hoffman's bias probe actually puts the DVM in series with the tube cathode and reads actual current in mA. So, who's bias probe are you using?

[concertamp64]

DOUK AUDIO Nobosound

[PRR]

The mA range on your meter is burned out.

[concertamp64]

Dang I must have 3
VMs with burned out ma ranges

[PRR]

> Dang I must have 3 VMs with burned out ma ranges

Really? Sorry to hear that.

Note that a VOLTmeter plus a 1 Ohm resistor is harder to blow up.

[concertamp64]

You are right RRR....my VMs fuses (2) were blown I will fix and try the bias probe again. The cathode resistor fix....I'm not sure I want change / add modifications to this amp and mess up the vintage stuff...etc. What about my voltage 468 and 471 I mentioned earlier...does that seem high?

[2deaf]

Bias probe tested good per EL34's continuity test.

What is EL34's continuity test?

Does the Douk bias probe interrupt the pin 8 circuit, i.e. is there continuity between the female pin 8 and the male pin 8 with the 5.5mm power plug disconnected? 

[concertamp64]

EL34's bias probe continuity check....
You can use a multimeter set to continuity to check the red and black wire connections.
My multimeter beeps and this is very handy.
These two points should have continuity
1 - The tip of the black wire and pin 8 on the tube base = continuity
2 - The tip of the red wire and pin 8 on the tube socket = continuity

[HotBluePlates]

... What about my voltage 468 and 471 I mentioned earlier...does that seem high?

Yes, and your outlet voltage may be higher than 120vac.  I used a variac to lower the voltage feeding my vintage amps until I measured 6.3vac on the tube heaters.  The schematics aren't always 100% right, but they show 440vdc for the output tubes.  You have 6% and change higher, which could be due to outlet voltage up around 126-128vac.  You'd know for sure if you measure more than 6.3vac on the tube heaters when 6L6s are in the amp.

Speaking of heaters, EL34s may be pushing it on a 64 Concert (though I don't know for certain).  EL34s draw ~67% more heater current than 6L6s.  Read that too literally, and didn't realize you were talking about Doug.

[concertamp64]

I'm using 6L6s on this 64 Concert

[concertamp64]

my outlets are at 124

[2deaf]

I'm using 6L6s on this 64 Concert

I knew that.

Was pin 8's continuity interrupted when you measured the plate voltage?

[concertamp64]

no

[concertamp64]

comment made earlier 'install 1 ohm resistors cathode to ground, no guess no mess'....trying to understand ..If that is done what happens to the current Fixed Bias circuit with bias pot etc...is it left in place...is it still used some way?

[2deaf]

Yes

[concertamp64]

what wattage should the 1 ohm resistor be?

[shooter]

1/2w should be fine.  you're looking for milli-VOLTs

[concertamp64]

hmmm..... thought we are looking for milliamps

[shooter]

1volt /1 ohm = 1 amp

they are equivalent

[sluckey]

hmmm..... thought we are looking for milliamps

YOUR bias probe is looking for milliamps. But if you install the 1Ω resistors as suggested, then you meter will safely measure millivolts.