1978 Fender Champ repair & bias

[mort]

I searched the site but didn't see much that directly relates to my question. Maybe it's in there somewhere but I couldn't find an answer.



With a SE 6v6 amp like the Champ, can I still use a 1ohm resistor between V2P8 and ground to read/calculate current draw? I've done this on a Bandmaster and a JTM45, but never a class A.



A friend of a friend brought me this Champ with a vacuum leaked 6v6 and a burned up tube socket. Replaced the tube and socket and this is what I have on V2:

P2- 6.66vac
P3- 389.1vdc
P4- 398.3vdc
P8- 25.5vdc

and 365vdc from P3 to P8

It's been a while since I pulled out the iron and meter.. hopefully I'm not asking too obvious a question.

I did a quick play test and it has full volume but the sound gets a bit harsh and rattles a bit at higher volumes. It may be something in the speaker, but I want to make sure everything else looks good before replacing it.


As always, thanks

[potatofarmer]

In a cathode-biased amp the idle current is set by the cathode resistor, so adding a 1 ohm resistor to ground from the cathode (in parallel with Rk) would result in some pretty hot bias.

Just measure the value of the existing cathode resistor, turn the amp on, measure the voltage across it and get out the calculator.

[mort]

edit: oops. I see where the difference is. On some other amps the cathode goes to ground. I'll just do what you said

[mort]

So with .49K on the cathode resistor at 25.33vdc, that should be 0.05169a, or ~52mA.

And do I multiply the the current by 365vdc, according to my readings above, to show 20+Watts?

[Enzo]

The 1 ohm sensing resistor goes in series with the cathode resistor. Actually, in series with the cathode, regardless of what the cathode connects to. If for example you have a 470 ohm cathode resistor, the difference between that and 471 ohms will not be noticed. Since current is the same through a series circuit, it doesn't matter whether the 1 ohm sensor resistor is on the cathode end or the ground end of the cathode resistor.

Power dissipation in the tube is the product of current THROUGH the tube time voltage ACROSS the tube. Across the tube means end to end - cathode to plate. The reading to ground is not involved. Sounds like you are doing that.


And yes of course, you can just use the existing resistor and the voltage drop across it. In amps lacking a sense resistor, we like to use a 1 ohm added in because it doesn't interfere with the amp operation and the math is trivially easy. But Ohm's Law is Ohm's Law, and ANY series resistance can do the job, just get out the pocket calculator. So that cathode resistor works fine.

[mort]

I'm reading of a couple different ways to lower the current draw. Says it should be at around 35mA. What is the preferred method, change the cathode resistor or lower B+? And if lowering the B+, what is the most practical method?

[Jazz P Bass]

The cathode voltage divided by the cathode resistance =current (ma)
The plate voltage minus the cathode voltage multiplied times current (ma)= power dissipated.
If you want to change it, change the resistor.

[mort]

right, so 25.33 / 490 = 0.05169


and


(365 - 25.33) x 0.05169 = 17.557



so I guess not more than 20 watts, but still a bit high for a single 6v6. The data sheet for the 6v6gta lists it around 4 watts, 5 watts max for class A. I'm reading that Fender pushed 'em alot more than that, but should I still be trying to lower the dissipation. based on my data?

[eschertron]

Mort,
I picked up a 79 champ recently, so I'm following this thread with interest.
Your B+ voltages seem really high, but I read that the US mains voltage has been steadily increasing, so no real wonder why. I'll have to poke around at my champ and read the voltages. Haven't yet done anything to it. Time will tell.

Unless the B+ is a real reliability issue, the simplest solution in my book is to increase the cathode resistor. Some posters here have gone as high as 680R, but I think in that case they were stuffing a 6L6 in the socket. I'd have to look at the data sheets to see how that changes the load.

[eschertron]

so I guess not more than 20 watts, but still a bit high for a single 6v6. The data sheet for the 6v6gta lists it around 4 watts, 5 watts max for class A. I'm reading that Fender pushed 'em alot more than that, but should I still be trying to lower the dissipation. based on my data?

Tube dissipation and tube audio power are not the same thing. While a 6V6 is (conservatively?) rated at 5W audio power, Fender typically has them biased to dissipate 14W or more. It's not easy math to get audio power, but the thumbrule I use for class A is about 1/3 of the dissipated power gets through the OT to the other side. Someone here can confirm or deny that, of course...

[g1]

What he said ^^^^^
Tubes are biased with reference to max. dissipation, not audio power output. Single ended amps are Class A. Class A single ended amps are generally biased near 100% of max. dissipation, but sometimes even more. A 6V6 is rated as a 14watt tube. Some stock Champs are biased as high as around 18watts.
I would suggest sticking with 14 watts, and make sure it doesn't redplate in use.

[mort]

All good observations, thanks.


I still have a sound issue to work out, and I would like to rule out the over-bias as the cause. What's the safest way to go from 17.5w to ~14w dissipation?

[Jazz P Bass]

Increase the resistance of the cathode resistor.
Have you tried a different speaker yet?

[mort]

not yet. I only have on hand a couple of 12" 8 ohm and a 5 1/2" 4 ohm. Not exactly an ideal comparison with any of them.

[mort]

I did just try the 5.5" speaker (Jensen MOD), and knowing its characteristics, I do believe the speaker in the Champ is probably mostly to blame for the foul sound.


So it's off to speaker shopping and getting a few resistors in to play with the bias. Will report back with results.