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Need biasing advice, cathode biased amp.

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idebb

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Joined 2010
#1
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
 

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#2
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)
 
#3
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.
 
#4
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!
 
#5
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.
 
#7
idebb said:
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!
Click to expand...

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..
 
#8
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.
 
#10
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.
 
#12
I've measured the voltage with one clip to one side of the 100 Ω screen resistor and one clip to ground. I get 315 V. Is this not the right way to measure the voltage drop?

If it is, I get 3,15 A with ohms law...that number is way to high! It should be milliamps, so what am I doing wrong?

Thanks for bearing with me!
 
#15
Eli, you say I should get matched set of tubes. I've read from several places (forums and tube amp websites) that matching tubes is not good. Some of the people saying this are quite well respected in the tube amp building world.

This is cutout from Kevin O Conners site
Tube Amp FAQ, Tube Amp Info, Tube Amp Lore

Q: What's the deal with matched tubes? Some experts say they're very important, and these days it doesn't seem to be too expensive to get matched tubes. Then they talk about matched preamp tubes, and I don't know where to get those.

A: You don't need matched tubes of any kind in your guitar amp. If you are trying to achieve vintage Fender, Marshall, Vox, Silvertone, Gibson et al. tones, then you simply plug in the tubes you have, check the bias and play. No manufacturer of musical instrument amps uses matched tubes, with the possible exception of Groove Tubes.

As discussed in the TUT-series, matched tubes will drift out of balance over time due to electrical imbalances in the circuit and the different response of each individual tube to mechanical stimulus. Drop the amp and one tube may break while the other survives, even though they were electrically "matched" when you bought them.

As discussed in this FAQ and in our books, asymmetries in the push-pull output stage, and in the handling of the signal throughout the signal path, contribute to the harmonic balance and thus the warmth of the tube amp's sound. You can build in specific asymmetries, or use unmatched tubes or even different tube types to play with asymmetry.

Matched triode sections are often cited as "beneficial" in the Schmitt splitter used in most guitar power amps. The circuit is inherently out of balance and has skewed values to restore some semblance of output signal balance. Perfectly matched triodes would offer no actual benefit and would contribute to higher levels of odd-order harmonic distortion. These sound "crisp" in small quantities but are "harsh" in large levels.

Again, just as with separate output tubes, the two sections of a dual triode will be quite close in their performance since they are made on the same production line, likely one after the other. There is still the possibility that mechanically jarring the tube will upset one side more than the other, so the expensive matching you think you paid for is lost forever.

Save your money!
Click to expand...
 
#16
OK, this is a musical instrument amp. "Perfect" matching of O/P tubes may well be unnecessary. However, proper operation still requires tubes that are reasonably close. An old used TFK in 1 "hole" and a nearly new JJ in the 2nd "hole" is not a good idea. Buy a couple of Russian 6П14П-EV tubes.

BTW, put a CCS in the tail of a Schmitt phase splitter and it is inherently balanced. 😉
 
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#17
The power dissipated in the cathode resistor is the voltage across IT times the current, not times the plate voltage... if theres a 12v drop across the cathode resistor with 40 ma flowing through it, the power dissipated in the resistor is only a half watt. (12 x .040 = .48 w) A 1-watt resistor should be adequate; I'd use a 2w.

Bud
 
#18
According to one of the experts over at the 18 watt forum a 10 mA difference is just about right if you don't want a sterile tone. I´m only 5 mA apart so maybe I´m a bit too matched, at least for his taste! 😉... Maybe you like the sound of matched tubes best, Eli...me I really don't know cause I can't really afford to experiment too much so I figure as long as it sounds decent and there isn't too much noise and the tubes aren't redplating, then I'll be fine. If I had more money I'd buy a matched set just to hear the difference.

This thread at the 18watt forum is good reading;
18 Watt Community Center - Access Denied

You have to register to read it though.
 
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