help calculating bias resistor

[iknowjohnny]

What i'd really like to understand is why not only the chieftain as i mentioned above, but now i see the bad cat also.....both have PA's almost exactly like mine except for the bypass caps. The chieftain is 40 watts and the hotcat is 30. Here are the specs...

PT V:
mine=300-300 with SS rectifier
Hotcat=321-321 w/tube rec.
Chieftain=350-350 w/tube rec.

First filter and choke:
ALL 3 have 33uf cans and a 20H choke except HC which has a 36H choke. But they are all the same basically

cathodes:
Hotcat=270R/220uf
chieftain=270R/250uf
Mine=WAS the same but i went to 450R after one tube was slightly red plating. No bypass caps

So in short the big diff here is the cathode resistors and the caps that mine doesn't have. Is it one of those 2 things that would make mine 10-15 watts and the BC and chieftain 30 and 40 respectively? I'm not saying this sarcastically, i really want to know because i don't understand why you guys are saying mine is probably 10 watts.

[iknowjohnny]

.............

[iknowjohnny]

Sorry about the multiple posts, but this board's "debug mode" glitch strikes again.

[skyboltone]

I don't have a scope or a 50 watt wire wound.

So use your VOM, set the gain so that you are just before breakup, and measure the voltage across the speaker while playing a chord.

[Jana]

Something else to keep in mind, just because an amp manufacturer claims a certain wattage doesn't mean that is what it is. The way wattage is supposed to be measured is the clean power of the amp, before the amp clips. Example, I built a 100 watt amp with 4 EL-34's. It put out 98 watts of clean power. Now if I really pushed it and went for max power, that amp actually put out close to 180 watts! But that is massive clipping.

A 50 watt Marshall does not put out 50 watts clean. I usually see them putting out somewhere around 35 to 40 watts.

You can't compare your amp to those you mentioned. First, you really don't know definitively what the parameters are for their power claims. Was it really clean or did they fudge it a little to boost the numbers. Do you have the same exact transformers that they have? You give specs for the power transformers but what about the output transformers? Is yours the same impedance? Same interleaves? Same DCR?

What about your phase inverter? Is it wired up exactly like those amps you quote with the same voltage?

The point is, if you really want to know what it puts out, take it to a tech and have it tested. You spent all this time on it getting it the way you want it, maybe this is the way to satisfy your curiosity.

[iknowjohnny]

Well, the thing is that i IS indeed curiosity as you say. After all, it's how it sounds and feels to you that matter, and it's loud enough. but i still can't help but wonder what it would sound like with more watts.
As far as whether the 2 amps i mention are putting out what they claim, i seriously doubt they could be putting out a mere 10-15 watts as was suggested mine is. thats why i want to know. And as with most amps i don't care for cranked tone like many do. I like a killer sounding pre with a clean output. So i am very curious as to how i can make my PA cleaner if it is as you guys suggest putting out very little.

[iknowjohnny]

Oh, and no, the PI on mine is marshalesque, plus i have NFB tho it's bypassable via a puh/pull knob. So my PI is 470R cathode, 1M grid leaks, and 10k tail. Thiers i think were both 47k tail with a 1.2 cathode and 1 M GL's. Which leads me to another question....are there better PI values for me if i want as clan and loud as possible from the PI on?

[Jana]

If you want to go for clean power you will have to go back to fixed bias. Bias for about 65 to 70% plate dissipation.

Change the PI to this: 1K cathode, 1M grid leaks, 15K tail, the usual 4k7 for the feedback point. I forget, what did you have for plate voltage? around 450, right? With that and these values you should get 45 watts clean, maybe a bit more. The numbers will vary depending on the OT but it should be in this range.

You will get the most power out of the cathode bias by tying the cathodes together and bypassing with a cap. 25uf should do it. I would guess, just a guess, that would get you to maybe 25 watts, maybe 30 if the planets are lined up right.

[Andy Le Blanc]

as for by passing the cathode resistor.... remember that the incoming signal
causes the plate current to fluctuate, so the voltage drop across this resistor
the bias voltage will also fluctuate.... if the signal makes the grid more positive
more plate current will flow..... and the voltage drop across the resistor becomes
greater and the negative charge it places on the grid also becomes greater
which works against the signal which is trying to make the grid more positive
this is not desirable because it reduces amplification (degeneration)
the bypass cap smooths out the fluctuations across the cathode resistor
and ensures full amplification over a broader range of signal

[iknowjohnny]

No way i'd go back to fixed. The amp sounds way better cathode biased. But i am interested in tying the cathodes together if you think thats a good idea for clean power. But are you saying it will work because there is now only one R, or because it's bypassed? because i gotta tell you, no one with a typical ear/feel for tone wouldn't easily see this thing sounds and feels way better w/o bypass caps.

that said, if i DO try it would i have to double the value of the single R 900 ohms? (it's now 1/2 that per tube, or 450 ohms per tube)

[Jana]

No. You have already found the ideal value for your cathode resistor to bias right at 100%. leave the resistors you have, just tie the cathodes together. This will give you 1/2 the total resistance that you now have, but that is what you need because two tubes will draw twice the total current, so, with half the resistance you will be right at the same voltage you are now. Make sense, lol.

Short answer. Tie cathodes together, bypass with cap, play guitar.

[Andy Le Blanc]

id go with common cathode too.... I really like the way two sounds at low volume
but the amp might suffer from the warm fuzzies when its pushed hard...
you can tweak the value of the cap to shape the tone you want....
a smaller value (s) will give more gain to the top....

[iknowjohnny]

I'm way ahead of you guys ! LOL seriously, i started messing with a calculator and i realized that i was cutting the resistance in 1/2 by using 2. So when it was red plating with 270R's it was the same as using a 135R single cathode R ! So i tried one 270R and took measurments then with the calculator i got 41 watts and the amp is WAY louder. in fact, it's stupid loud ! The only problem, and it's a biggie, is that i lost a good deal of tone at higher volumes. So i guess i was getting some good output compression as was said before. Anyways, what do you think of adding a second R in series that i would bypass with a switch when i want that less wattage tone? Thats probably how i'd always use it anyways, but there are some things li like about the higher power sound such as a better clean and more range in the presence and variable NFB controls.

[iknowjohnny]

Man, the more i try and read from you guys the more questions i come up with ! now i'm looking at the chieftain and hotcat and wondering how on earth do they get away with 270's like i had when it red plated? Thats 270/2 which is 135, much too little resistance for an amp like this, no?

[Jana]

you found your value for the resistors to bias right where you need to be, why mess with that? This isn't really a plug and guess part.

I suppose the other option would be to put a 25 watt rheostat in place of the resistors, put a stainless vent grill on the top of the amp and then when you get hungry, you have a toaster oven. Just turn it up for broil.

Who needs a blue plate special when you can have a red plate special!