new champ build aa764

[scole]

it makes a crackling sound when turned all up. 406 volts pin 3, 424 volts 1rst cap node, 26.8 volts across the cathode resistor wich is actually 460 ohms. i think i have 20.2 watts dissipation. i found this by 26.8/460=.058. 406-58=348. 348x.058=20.18. question, should i bring the plate voltage down to 350 as on the schematic?

[MWJB]

"should i bring the plate voltage down to 350 as on the schematic?" you can if you want to...I seriously doubt that the high plate dissipation is causing your crackling, otherwise every SF champ in the world would crackle ... and they don't.

You should only be deducting the 26.8v at the cathode from your 406v at the plate, not 58v.

406 - 26.8 = 379v

379V * .058mA = 22W, subtract 1W for screen current = 21W/55mA.

Check integrity of solder joints/connections. What other voltages do you have? Any pics of the chassis/layout?

[scole]

heres some pics, im not sure if the board shot is close enough. what other voltages would i need?
http://www.flickr.com/photos/1045148...7601675424588/

[scole]

i was reading angela instruments article on building a champ. he says the best sounding champ hes ever heard was one wich had a superchamp power transformer, and had 407 volts on the plate. im at 406 volts. so do i rebias to get the dissipation down to 14 watts? what should it be for this amp?

[Bruce / Mission Amps]

The guy at Angela is such a nut case salesman that I would be hard pressed to find serious credibility in too much of what he says about the "best" of anything.
However, some of the coolest sounding screamin' monkey Champs are running the power tube at +15 to 18 watts.
They just won't sound like that for months on end.

At +406 volts DC plate voltage, you'd want about 32ma-34ma on that 6V6.
Since you have to subtract the cathode voltage from the plate voltage to determine actual idle dissipation rating the, you have to start somewhere else first.
Try a 510 ohm, two watt resistor and see what the cathode voltage is.
Be prepared to use something a little higher in resistance, but before changing the biasing resistor to far, I'd drop the B+ with a couple (in series) 5 watt, 9v to 15v zener diodes .

[MWJB]

The pics don't seem to show final layout/wiring?

You may as well check screen voltage (6V6) pin 4 and preamp plate & cathode voltages, while you have the meter out.

510 ohms is only going to marginally reduce plate dissipation, I'd go in at 680ohms, this will probably get you in the 15-16W range. As Bruce says, if you want to drop the plate dissipation stick with the 470ohm and zener down the B+.

[scole]

i always panic when i fire up a new amp. when i first put my bias meter on it read almost 60ma. so i started putting resistors in series with the 460 ohm cathode resistor untill i got to my largest one, 250 ohm 5w cement job. then i saw on the meter it says for class a 55 to 60 ma, so i took out the 250. now it looks like the 250 is going back in.

[MWJB]

How does the amp sound its best, with the 460ohm or 710ohms? That's the best way to decide.

[Bruce / Mission Amps]

Scott... of course you do not want to exceed tube specs for very long but, it isn't so much the max plate current as it is the total dissipation of the tube.
A 6V6 is a 12-14 watt tube.
Running them at 17-21 watts, in the long term, is really hard on them.

For best results you really want to keep the tube closer to that 14 watt rating.
Cooling the tube down with a higher bias voltage on the cathode will make it so the tube will clip unevenly if driven hard enough because the positive going grid signal will top out and draw excessive current and saturate... (it really can't go very much further anyhow because it already is nearly saturated in DC current) but the negative one will never bottom out because the cathode positive voltage is so high, it might be able to be driven right into class B cut off.
IMHO,
Lowering the plate and screen voltage, to keep the tube cooler in idle current, will give you better balance and a cleaner tone.
Cleaner tone is critical with a small SE class A amp like this because it is low power and you'll be able to overdrive the power tube anyhow but if it is over-biased with respect to the high plate voltage, it is actually easier to get it to clip with less drive signal, making the amp sound fizzy and distorted at a lower volume level.
Now, I still do not know exactly what your cathode voltage is... so,
710 ohms and 57.5ma of current means you have 41v on the cathode!!
... could that be right?
406v-41v=365v.
365v x 57.5ma = 21 watts!! YIKES!
Get yourself a few 9.1v to 15v, 5 watt zener diodes in there (reverse biased to ground of the center tap in the high voltage secondary) and get this under control and I think you'll have a better sounding amp.
I've run as much as 45v worth of zener diodes in some small SE 6V6 amps and I think they always sound better idling at 12-14 watts with a lower B+.

[MWJB]

"Now, I still do not know exactly what your cathode voltage is... so,
710 ohms and 57.5ma of current means you have 41v on the cathode!!
... could that be right?" Well the cathode voltage will rise some (maybe up to 30v-ish? so will the plate voltage, but the plate current will come down. Let's say scole ends up with 395-400v on the plate once cathode voltage is subtracted, 31/710 = 43.6mA * 400 = 17.4, subtract 1W for screen current = 16.4W +/- 1W...as a rough guesstimate, erring on the high side.

Dropping the plate voltage to 380-385v (before subtracting cathode voltage) with 460ohms & zeners will get you around the 14-15W mark.

[scole]

i think my cathode voltage was 26.8v. or you mean with the 710 ohms of cathode resistor? also would changing to a 6l6 help? by the way i added some more board pics

[Bruce / Mission Amps]

Hmmm I just spotted something that has my attention.
What rectifier is that? I think it says Fender but it looks like a GZ34... or GZ30 which is 5V4GA type tube in a small bottle... both of which should not be in this amp.
If it is a Russian 5Y3GT then that is where all the extra B+ voltage is coming from because the Russian 5Y3GT is not a real 5Y3GT.
Do you have an old or NOS 5Y3GT laying around to try in there?

[scole]

your right bruce, it is a sovtec 5y3, its making a jigly sound so i took it out and put in an old rca 5y3. it sounds a little better. i ended up with a 1k cathode resistor, ill check voltages again with the rca 5y3. with the sovtec i have 434 v pin 3, 438 pin 4, 34.5 pin 8, 34.4 across the cathode resistor, i think im right at 14 watts dissipation. the amp is very loud not much breakup with single coils, not bad with humbuckers. i was expecting a little screamer, this thing will hang with a deluxe no problem.

[scole]

ok heres with the rca 5y3, 412v pin 3, 416 pin 4, 32.5 pin 8, the voltage across the cathode resistor is 32.4. now the cathode resistor measures 990 ohms.

[Bruce / Mission Amps]

OK I think the cathode resistor value is way too high.
I really don't think you should be over 510... instead get the plate and screen voltage down.
So, you lowered the plate a little but it's time to get your screen voltage down to around 405 to 410vdc...
What is the value of that first dropping resistor between the first to sections on your filter cap?
I think it needs to be a lot larger value then what you have now.
If it is a 10K make it 15K.
Plus you could run a 2k7 to 5K resistor from the filter cap lead to the screen lug
#4 to drop it down a little.
Many novice builders are under the assumption the screen in a 6V6 draws enough current to put a load on that resistor... typically, at idle it will be one or two ma.
That means 1v to 2v drop for every 1000 ohms of screen resistor value.
Although not critical to get your screen voltage below the plate voltage, you can see it shouldn't be hard to get it down a little lower then the plate voltage for less idle current draw.