My ignorance runs deep...

Chris Scott wrote:...so I'm hoping someone can explain this to me.

After posting my last query, I decided to just set the bias current to the -37v figure called for on the voltage check points page from Dynaco, and now the B+ voltages are good, and the power tubes appear to be biased within range as the plates show no overheating when viewed in a dark room...

...yet they're drawing around 70 milliamps of current when I interface a bias probe (same as taking a reading across a 1ohm resistor connected from pin 8 to ground) which I use when setting the bias current on a guitar amp.

In a guitar amp with +/- 440v on the plates, I'm pretty sure that an EL34 is going to be already starting to redplate pretty severely at this setting, yet they appear to be happy in this amp...

Yeah, I'm missing something...

Chris Scott wrote:...so I'm hoping someone can explain this to me. 8)

After posting my last query, I decided to just set the bias current to the -37v figure called for on the voltage check points page from Dynaco, and now the B+ voltages are good, and the power tubes appear to be biased within range as the plates show no overheating when viewed in a dark room...

...yet they're drawing around 70 milliamps of current when I interface a bias probe (same as taking a reading across a 1ohm resistor connected from pin 8 to ground) which I use when setting the bias current on a guitar amp.

In a guitar amp with +/- 440v on the plates, I'm pretty sure that an EL34 is going to be already starting to redplate pretty severely at this setting, yet they appear to be happy in this amp...

Yeah, I'm missing something...

PeterCapo wrote:Around 70mA (actually, 65mA) through each EL34 is normal for the Mark II.  If you are using JJ E34L, do I remember something about them being more rugged than the average EL34?

corndog71 wrote:To clarify what peter said, the VTA70 uses 10 ohm cathode resistors for each tube and each should measure .4 - .5 vdc.

In the classic dynaco amps the cathodes of both output tubes were tied together so the measured number would be for both tubes.

Here's how I worked it out.

1 ohm cathode resistor : 0.050V which is equal to 50mA of idle current per tube or 0.100V / 100mA for 2 tubes.

corndog71 wrote:To clarify what peter said, the VTA70 uses 10 ohm cathode resistors for each tube and each should measure .4 - .5 vdc.

In the classic dynaco amps the cathodes of both output tubes were tied together so the measured number would be for both tubes.

Here's how I worked it out.

1 ohm cathode resistor : 0.050V which is equal to 50mA of idle current per tube or 0.100V / 100mA for 2 tubes.

corndog71 wrote:I wonder if it really matters what the value is of those cathode resistors.  Some guys use 1 Ohm, some use 10 Ohms, and vintage guys prefer the original value of the circuit.  

peterh wrote:

corndog71 wrote:I wonder if it really matters what the value is of those cathode resistors.  Some guys use 1 Ohm, some use 10 Ohms, and vintage guys prefer the original value of the circuit.  

No, it does not matter what value of measurment resistor as long as one is aware
of it's value when measuring the current. The resistor should be small enough not
to affect the tube's working point. Anything < 25ohm is ok here.  

1 ohm is convenient as it converts to 1mV = 1mA
10 ohm "flameproof" 1/4w is convenient as they will blow when a tube flashes over,
this save output transformer and other stuff ( a fuse won't open as fast !)
Dynaco choose an odd value in order to have the biaset value 1.56V , same voltage as
a drycell. Instrument of unknown accuracy could be used since the cell and biaset
should give the same reading.

Peter W. wrote:"When a fuse interrupts in a DC circuit the interrupting causes the element to turn to plasma which continues to conduct long after it should. This extended time causes damage to the circuit. Ceramic fuses are often filled with sand which helps prevent the plasma from forming."