How do you calculate the power output of your SE amp?

[tubeswell]

Or to put it another way how do I calculate the power output of my 5F2A SE amp? ;-)

It is running a 6V6GT.

The B+ is 371VDC

The Plate is 364VDC

The Screen is 305VDC

Cathode to Plate is 288VDC

I have got a 470 Ohm cathode resistor. The amp sounds louder than I expected.

I gather I have to make some measurement of current somewhere and then calculate something. Can somebody please edjamacate me?

Thanks in advance.

[Jag]

If your numbers are correct, you've got a problem!

I assume you are wanting to calculate the idle dissipation as opposed to output power.

To calculate the idle dissipation you need to know the plate current and plate to cathode voltage.

You already know the plate to cathode voltage.

You can get the plate current by measuring the voltage across the cathode resistor and divide it by the value of the cathode resistor. Since you say the plate voltage is 364V (I assume this is to ground) and the plate to cathode voltage is 288V, I assume the voltage across the cathode is 76V (364 - 288).

The idle current is found by dividing the voltage across the cathode resistor by the value of the cathode resistor (simple Ohm's law). 74V/470ohms = 0.157A (157ma !!!). That includes the screen current as well, but screen current is so small (a couple or three ma) many people ignore it.

To figure out the idle dissipation, you multiply the plate to cathode voltage by the current draw. 288V*0.157A = 45.22W. Considering max. dissipation for a 6V6 is 12-14watts, I hope I have either misinterpreted your data or you have made an error in your measurements.

[Jag]

If we assume that what you have listed as Cathode to Plate voltage is actually Cathode to Screen voltage, then voltage across the cathode resistor is 17V (more reasonable). That would make your plate to cathode voltage 364V-17V=347V.

Solving for this we get

17V/470Ohms= .036A (36ma)
347V*.036A=12.46W

I think this is the more likely scenario. That dissipation is bang on and I bet that amp sounds great.

BTW, I'm always amazed at how loud a little Champ style amp can actually be. The amp I use for most gigs is an SE 6L6 amp with a Plexi-ish preamp that I built a couple of years ago and I find it plenty loud.

[tubeswell]

Thanks Jag

I took the measurements again.

Plate to cathode is 344V (I must've been measuring the cathode to screen before)

The Cathode to ground is 19V, so with the 470Ohm resistor that makes 40ma plate current.

so the dissipation is 13.76W. (Is that still okay?)

The amp doesn't sound too bad, but I 've never owned a champ before so I'm still getting used to it.

I measured the screen voltage again after its been on for a half a minute or so and it settles down to 302v to ground at idle. The amp sounds a bit squishy when I crank it - Do you think it might sound better if I can get the screen voltage up a bit, and if so - How high should I try and go? (The plate-to-ground is definitely 364V at idle, so there is 62V to play with). At the moment I have 11k as the first supply resistor (2 x 22k 1W in parallel).

[Jag]

In one of my own Champs, using a current production 6V6, I'd be comfortable with 13.76W as long as the tube was not red plating. However, on an amp that is not mine or if I had an NOS 6V6 installed I'd be trying to get it to 12W or less. Having said all of that, I have several BF and SF Champs with original tubes that are happliy running at around 14W dissipation. I think it's pretty common in the lower powered Fenders.

Generally, the screen voltage should be lower than the plate voltage. I believe someone on this forum (Bruce?) once said he likes the tone of an amp with screen voltage about 20V below plate voltage but that wouldn't make it a rule. I'm not really sure how low is too low for the screen. I'd probably be trying to get the screen voltage higher. Once again, many of my "stock" champs have the screen voltage equal to or a couple volts higher than the plate. This seems to be common and, as far as I've understood, it's not a huge problem. Personally, I have found I like my Champs best when the screen is within +/- a couple volts of the plate.

Depending on what you mean by "squishy" I think what you are describing is sag. The harder you push that amp the more it will sag. Most of my Champs start to get squishy at around 8 on the volume. Generally I like the sound of them best just before that point. If I want more drive, I put a pedal in front.

Keep in mind, these are my own opinions/understandings. While I do have EE cred in my distant past, there are plenty of others on this site who have way more experience and knowledge than I do. I build amps in the same way I cook which is, if it tastes good and I didn't burn anything up it's a good recipe. I know what rules not to break but, beyond that, I'm more interested in how the amp sounds than I am in having it be theoretically sublime.

[tubeswell]

Thanks Jag

When I first built it, I only used 20uF in the first filter stage and the plate (355) and screen (298) voltages were lower, and I recall the cathode voltage was 18V, this would give me 12.8W dissipation.

The way it sounds with 40uF filtering at the first stage is a bit more phasey on the notes when it gets warmed up. The plates aren't red-plating and are surrounded by a distinct bluey-purple glow from head to toe in the dark, which isn't a danger signal to me.

The 6V6GT is a new sensor 'tung-sol' reissue in there, so I guess the voltages are okay.

I guess I could make the 2nd 20uF filter cap in the first stage switchable on the ground side of the cap, but would I have to put anything else in that switch to stop switch-popping noise?