Cathode Bias EL34 - Output Wattage?

[mikeboone]

Guys,

I have an amp that I put together that is a 2xEL34 PP with the volume/tone configuration of an 18W Marshall.

The amp sounds pretty good and is very stable, but nothing is ever good enough right?

I want to change this to cathode bias so I looked at the HotCat 30 and Matchless Chieftain to get an idea of how to reconfigure my output section. Seems straight forward.

My question is: Using this cathode biased configuration and a 50W OT from a JCM800, what will my estimated output power be? Shouldnt it be somewhat lower than the fixed biased config? How will the 50W OT affect that?

The reason I ask is I am looking different flavors of Alnico speakers and one of my favorites is the Em. Red Fang that is rated at 50W. Will I be cutting it too close with a single 50W speaker?

Thanks...
Mike

[fyl]

From the Mullard datasheet:

EL34 PP @ 375 plate, pentode mode, 3K5 OT
Cathode bias app. 30W / 270 Rk per tube
Fixed bias app. 48W / -33V bias

The OT is rated for max power/current, it'll be fine in both modes.

A single Red Fang is OK, but IMO an EL34 PP sounds better with two.

[mikeboone]

Thanks for the input...

The amp is in a 1x12 combo cab that currently has a Celestion G12K100. I gutted an old Blues Deluxe to build this. I use it on top of a 2x12 open back with a 1 Red Fang and 1 Wizard.

I want to change the G12K100 and wanted to ensure that the speaker I put in the 1x12 will be OK when i use it by itself.

FYI... The Red Fang and Wizard pretty much drown the Celestion out.

[Wilder Amplification]

Kind of impossible to calculate your output power if we don't know your B+ voltage or the rated plate-plate load of your OT. If we knew these figures, we could easily calculate it assuming the power transformer can source the current and has zero sag.

[Chuck H]

It's worth noting that in fyl's tube data example circuit all things are not created equal. You have to take the cathode voltage differential into account. The cathode voltage wasn't indicated but it has to be around 40 or 50, so that's a plate to cathode voltage of only 325 to 335. It's also worth noting that even if you upped the voltage to 420 (for a plate to cathode voltage of about 375) you probably still wouldn't get quite as much output, it depends of a few things , but it would be a lot closer and more of an apples to apples comparison as output power relative to bias method goes.

Chuck

[mikeboone]

Thanks again guys...

I found some info in another thread that lead me to another etc, etc... I eventually found some info on how to calculate it myself.

The main reason I asked is that I have read (but do not know) that operating the EL34's cathode biased would yield around 30 watts or so. Right now in fixed bias, they are somewhere around 50 according to the votages and bias settings I am currently running.

I wanted to be sure that it will be around 30 W so I dont blow my $265 50 W alnico speaker running it too close to its rated max power.

I played through a friends Hot Cat 30 (which is cathode biased EL34) on the clean channel and I really liked the feel of the cathode bias over the fixed. They sound similar until you start to crank them. I find it is "spongy" and lends itself nicely to blues. The fixed bais is a bit stiffer and breaks up later.

[mikeboone]

Hi again...

I am revisiting this amp project. I noticed that in the schematics for Matchless Chieftain and Hot Cat 30 (bothe cath. biased EL34) that the dopping resistors in the power supply are desginated as 10-12W resistors instead of 1-2W resistors that I am accustomed to seeing.

Does a cath biased amp need to dissipate more heat in these resistors than their fixed bias cousins? I have looked at 5e3 schematic and seen that they are not using this high W value resistors.

Any thoughts???

[Albert Kreuzer]

I noticed that in the schematics for Matchless Chieftain and Hot Cat 30 (bothe cath. biased EL34) that the dopping resistors in the power supply are desginated as 10-12W resistors instead of 1-2W resistors that I am accustomed to seeing.

Does a cath biased amp need to dissipate more heat in these resistors than their fixed bias cousins?

No. IMO it's just over-engineering to be on the safe side. Or to impress the customer
For the cathode R I'd use a 25W wirewound and keep it away from the bypass cap.

[mikeboone]

No. IMO it's just over-engineering to be on the safe side. Or to impress the customer
For the cathode R I'd use a 25W wirewound and keep it away from the bypass cap.

Are you suggesting separating the resistor and cap due to noise related issue or heat related issue? How far apart should i put them?

[Albert Kreuzer]

How far apart do they need to be? Is this for noise or to keep the cap from drying up from the heat from the resistor?

I'd keep them at least an inch apart, more if possible. It's to keep the cap from drying up. The resistor will dissipate 4 or 5W at idle, probably 8W or more at full bore. A 25W resistor will get less hot than a 10W. Noise isn't an issue here.

[mooreamps]

Hi again...



Does a cath biased amp need to dissipate more heat in these resistors than their fixed bias cousins? I have looked at 5e3 schematic and seen that they are not using this high W value resistors.

Any thoughts???

Well, it's not so much if you're using self bias or fixed bias. It's mostly the combination of cathode current squared times the resistance. So typically what I have seen, builders who go with fixed bias only put in a 1 ohm cathode resistor, for measuring and balancing the power tube current. This can be only a 1/4 watt since the I_squared_R is not very high.

But, the above comment is correct. A higher wattage part will not run as hot as a lower wattage will.



-g

[R.G.]

Hi again...

I am revisiting this amp project. I noticed that in the schematics for Matchless Chieftain and Hot Cat 30 (bothe cath. biased EL34) that the dopping resistors in the power supply are desginated as 10-12W resistors instead of 1-2W resistors that I am accustomed to seeing.

Does a cath biased amp need to dissipate more heat in these resistors than their fixed bias cousins? I have looked at 5e3 schematic and seen that they are not using this high W value resistors.

Cathode biased amps use their power supply less efficiently than fixed-bias amps.

For the same power supply (i.e. same available voltage and current out of the transformer/rectifier/capacitors) a cathode biased amp will put out less power. This is because the bias voltage is generated in series with the output stage. The bias voltage is significant, being 20-60V depending on which tubes and how much "on" bias is used. The voltage used in generating bias is not available for making voltage swing on the output tube plates, so cathode biased amps are, for the same power supply, able to put out less power.

The current which generates the bias voltage in cathode resistors also heats the cathode resistors. The power they generate is equal to the voltage across them squared divided by the value of the resistor. Doubling the cathode bias voltage quadruples the cathode resistor heat generated.

The current which the output tubes need to flow is dependent on the operating class. Most cathode biased amps are either deeply toward Class A, or completely to Class A. That means they let a lot of cathode current flow compared to Class AB biased amps. Note that cathode biased does not automatically mean Class A, nor does fixed bias mean Class AB. That's just the most common situation.

The actual power a resistor generates is easily calculable. It's equal to the voltage across it times the current through it - P= V*I. If you know the voltage across a resistor and the resistance, you know how much current flows in it, and therefore how much heat it generates.

Once you know the heat it generates, you get to pick how hot the resistor's surface gets by picking the wattage rating. Power dissipated and temperature are NOT the same thing. Temperature depends on how fast the heat can get out, not the amount of heat per se. A small grain of wheat bulb in an appliance may run on 12V at 50ma, or P = 12*0.05 = 0.6W. But the filament gets so hot it glows yellow-white. A 1W power resistor dissipating 0.6W will burn your finger, but will certainly not glow in the visible spectrum.

Resistors are often designed to run with a surface temp under about 200C. Some more, some less, but often, about 200C. This is the outcome of a number of factors. It is good engineering practice to de-rate them, specifying a resistor power rating twice the actual maximum power it will dissipate. This keeps the maximum surfact temp on the resistor down, and also gives it lower thermal stresses and longer life in general. Specifying a resistor derating of even greater amounts lowers the temperature, thermal stress and wear-out mechanisms even more. Some people go huge with power resistors. Some use 2:1.

Back at the question: Cathode biased amps *tend to be* more heavily toward Class A operation than fixed bias amps. This means they pull more idle current from the power supply, and get hotter inside from the power dissipated in the tubes and the cathode resistors. Dropping resistors from power amp stages to preamp stages don't necessarily need more current in a cathode biased amp - they're already class A - but a conservative designer might go way overboard with dropping resistor power ratings if the inside of his box is already hot.

So do cathode biased amps inherently need more wattage in the dropping string? No, not as a consequence of the cathode-biased-ness. But it's often very hot in there, and the designer may have gone way conservative on the power rating.

Look at the resistance and voltages dropped. You can calculate the power dissipated from that, and make your own judgements on how overboard 10W dropping resistors are.

[Enzo]

But isn't mikeboone's question about the resistors in the B+ string, rather than the cathode resistor?

If I read him right, he was asking about the B+ dropping resistors and wondering if they needed to be larger for some reason relating to the fact of cathode bias in the power tubes, not the cathode resistor itself.

Which leads me to agree with Albert - they want to imprss the customer.

Of course we also do want to keep hot cathode resistors away from bypass caps who prefer cooler environs.


AM I wrong here? A larger power resistor - of same resistance - wil itself run cooler than a small one, but the amount of heat dissipated into the chassis will remain the same. The larger resistor will make it less of an intense hot spot - lower temperature, but a larger surface radiating. Same net watts heating the space. Or am I missing something?

Point being the cooler resistor will indeed be more friendly to the cap next door, but for overall temperature under the chassis, not much difference. Or so I wants to think.

[R.G.]

But isn't mikeboone's question about the resistors in the B+ string, rather than the cathode resistor?

If I read him right, he was asking about the B+ dropping resistors and wondering if they needed to be larger for some reason relating to the fact of cathode bias in the power tubes, not the cathode resistor itself.

Yes, that's right. I probably answered more completely than he needed. And I also threw in an answer to the original poster about amp wattage.

I read it as about the B+ dropping resistors too. There's no inherent difference in the dropping resistors string in a cathode bias vs fixed bias amp. Amps is amps, volts is volts, and watts is watts.

Which leads me to agree with Albert - they want to imprss the customer.
...
AM I wrong here? A larger power resistor - of same resistance - wil itself run cooler than a small one, but the amount of heat dissipated into the chassis will remain the same. The larger resistor will make it less of an intense hot spot - lower temperature, but a larger surface radiating. Same net watts heating the space. Or am I missing something?

Nope, you're right.

Point being the cooler resistor will indeed be more friendly to the cap next door, but for overall temperature under the chassis, not much difference. Or so I wants to think.

Mother Nature thinks that too. It's right.

[mikeboone]

Well, guys...

I think I owe everyone a bit of thanks for the input. I put this guy to the test this weekend and it sounds really nice. The end design is the input (vol/tone) of a 18Watt coupled with the output section of a Chieftain... Oh, and I added the MV across the back of the PI.

I split the difference. I used 5W on my screens and the 2W dropping R on the supply that were already there from when it was fixed bias.

All in all it seems to be a success and another round of experience gained.

Thanks!!!