Need biasing advice, cathode biased amp.

[idebb]

Hi good people!

I'm wanting to bias the power tubes in my Dynacord MV17 amp,
after replacing a blown output tube.

I've never done this before so if someone feels an urge
to help by reading my calculations and thoughts and
tell me if I've got things right or wrong that would be so great!

This amp is cathode biased with one cathode resistor
for each of the two EL84 power tubes.
(Schematic attached below)



Bias Calculation for EL84 tube A

Cathode voltage: 12,8 V (measured from pin 3 to ground)

Cathode resistor: 303 Ohms (measured in circuit)

Cathode current: 12,8 V/303 Ohms = 0,04224 A

Plate voltage: 395 V (measured from pin 7 to ground)

Actual plate voltage: (395-12,8) V = 382,2 V

Dissipated power: 382,2 V x 0,04224 A = 16,1 W



Bias Calculation for EL84 tube B


Cathode voltage: 12,3 V (measured from pin 3 to ground)

Cathode resistor: 311 Ohms (measured in circuit)

Cathode current: 12,3 V/311 Ohms = 0,03955 A

Plate voltage: 395 V (measured from pin 7 to ground)

Actual plate voltage: (395-12,3) V = 382,7 V

Dissipated power: 382,7 V x 0,03955 A = 15,1 W


Am I doing this right? Is it OK to measure the resistors in circuit or should I take them out?

16 and 15 watts is above the maximum dissipation of 12 W for EL84 tubes. So if my calculations are right I should get a larger value cathode resistor, am I right? If so what value resistor should I put instead?

All of the voltages were measured at idle since I didn't have a guitar at hand for testing. Should I measure cathode voltage while playing through the tubes also?

Tube A is brand new from JJ tubes. Tube B is an old Telefunken. So they are not matched in any way. Could this be harmful to my output transformer? How do I balance these tubes so the core of the OT doesn't get saturated?

Any thoughts?

/Jonas

[Mickeystan]

Hello Jonas,

Before you change the cathode resistors to a new higher value, you may want to just try to change the tap on the primary of the power transformer. Thus, if you are now set to the 220vac tap as shown in the schematic, change it to the 240vac tap. This will effectively lower the B+ voltage and that will lower the disipation in the two output tubes. If this option is not tangible, you will have to increase the size of the cathode resistors until the current drops through the output tubes to something like 30 ma each.

Doug S. (aka Mickeystan)

[Eli Duttman]

Jonas,

If available, switching to the 240 V. mains position is good advice.

You are ignoring g2 contribution to cathode current. While high, plate dissipation is not as large as you've computed.

Proper matching of O/P tube pairs is IMPORTANT. Along with "standing" DC in the O/P trafo, distortion is implicated.

Use Russian 6П14П-ЕВ (6p14p-ev) stock, AKA EL84M, in the O/P tube sockets. The "Russky" is a genuine 7189 equivalent, which is very tough and also decent sounding.

Buy 8X 5% tolerance, 1 W. rated, 680 Ω Carbon film resistors. Use a paralleled pair, as the bias resistance, for each O/P tube.

[idebb]

Ok I´ve changed the PT tap to 240 V setting,
measured everything again and taken
screen current into account;

Is this right then?

Plate dissipation = (Actual Plate voltage x plate Current)

Actual plate voltage = (Plate voltage - Cathode voltage)

Plate current = (cathode current - screen current)

Cathode current = (cathode voltage / cathode resistor value)


Which means;

Tube A

Cathode current = 11,5 V / 303 Ω = 37,95 mA

Screen current = 0,14 V / 980 Ω = 0,14 mA

Actual plate voltage = (365 -11,5) V = 354

Plate dissipation = 354 V x (37,95 mA - 0,14 mA) = 13384 mW = 13,38 W

Tube B

Cathode current = 10,3 V / 311 Ω = 33,11 mA

Screen current = 0,64 V / 966 Ω = 0,66 mA

Actual plate voltage = (365 - 10,3) V = 354,7

Plate dissipation = 354,7 x (33,11 mA - 0,66 mA) = 11,5 W


I measured 220 on my mains voltage supply. Is it not bad/dangerous to mismatch the mains to the PT? maybe 20V doesn´t matter? Sorry if this question is stupid, but I'm in curious mode!

Switching to 240 volts made the plate voltage drop and at least one of the tubes is now within the 12 W rating.

Tube A is still to hot it seems...the screen voltage is lower on on this tube 0,14 V compared to 0,64 for the other tube. These numbers are the same even if I swap the tubes slot places. So it's not related to the tube it seems. Do you think the screen voltages are too wide apart? Maybe i need to replace the screen grid resistor? When the tube in the left slot died it kind of made some fireworks and smoke, so
I´m wondering if something went along with it.

By the way the screen resistor is the 1KΩ one that's connected directly to pin 2 right? Thats the one I've use for screen current calculation.

Should I replace the screen resistor for tube A or should I get a larger value cathode resistor or both?

Is there a way to balance the tubes DC against each other, so I don't have to buy matched tubes...I've avoided matching tubes because of reading stuff about canceling even order harmonics and because I can't really afford buying two tubes if only one breaks. There has got to be a better cheaper way...maybe installing a pot or something to balance the tubes. Or Maybe I could swap the tubes places every other month so the OT saturation evens out? Guessing wildly here!


Sorry for the long post, and thank you so much for helping me, I really appreciate it!

[Eli Duttman]

Jonas,

Proper matching of O/P tube pairs is very important. As your amp is self biased, you must rely on your vendor to do a good job. Buy from a source with proven reliability, who will back his goods up, if an "infant mortal" goes out the door. Don't be penny wise and pound foolish.

Even if the amp had individual bias adjustments, which it does not, you would still need pairs with well matched transconductance.

[SY]

The screen currents look suspiciously low.

[rotaspec]

Originally Posted by idebb

Is this right then?

By the way the screen resistor is the 1KΩ one that's connected directly to pin 2 right? Thats the one I've use for screen current calculation.


Sorry for the long post, and thank you so much for helping me, I really appreciate it!

Screen grid is pin 9 on EL84.
Pin 2 is control/signal grid which should have no current flowing at all.

Something is very wrong here..

[idebb]

Oh seems I had mistaken the control grid for the screen grid, thanks rotaspec, for pointing this out. I feel a bit embarrassed. Thats why the voltages were so low...

The control grids are 0,14 and 0,64 Volts. So there is some current flowing in my control grid, is this bad? Should it be zero Volts?

The screen voltages (pin 9) are 315 V. The screen resistor is 100 Ω. Then I get a screen current of 3,15 A. That can't be right either... I'm totally lost now.

[Eli Duttman]

Measure the drop across the 100 Ω resistor, then apply Ohm's Law.

[rotaspec]

To calculate the screen current, you need to measure the voltage across the 100 ohm resistor (or measure each end to ground and subtract to get the difference between the two readings), then the screen current will be (voltage across 100 ohms / 100).
You need to make sure that all your voltage measurements are taken with respect to the same reference point (usually power supply ground), or small variations will occur between readings which will confuse the issue even more.
As Eli pointed out, matched pairs of tubes is the best idea. Even if you manage to get the bias to the point where the idle currents are the same, variations in transconductance of mismatched tubes will make that exercise redundant.