Power tranny output voltages.

[jmaf]

Well, after putting in those screen resistors, and changing the resistors around the bias pot, I now have 412V on the plates, and it's biased right at 40mV. And it doesn't sound anywhere near as sweet as it did!

The sound of frying tubes is always sweeter, but the tubes were frying.... You are now much closer to vintage specs IIRC, except for the screen resistors I recommended you add, those did not exist in the earlier models I think. They will preserve your tubes and insert a bit of negative feedback on the screen, which may the the effect you disliked....

Remember you are reading the 40mv at the cathode, so you can go up to about 45 mA per tube because your screen is pulling 4 or 5mA. Perhaps that'll liven it up a bit.

Edit: If you are running some of the newer 5881's or 6L6GC's you can consider the tube wattage to be 30 Watts per tube. 30/412 = 73 mA per tube.... 70% of that is 51 mA. So at 40mA you are running cold for a modern 6L6GC.

[pinkphiloyd]

Okay. I've set the bias closer to 51 (I'm running brand new JJ's), but the gf is asleep so I won't be able to really try it out until tomorrow. I'd like to thank you all again. This is one of the better boards I've found. Great advice and no BS. You've all been great.

[pinkphiloyd]

Also, I remembered earlier that I'm using a choke that's rated significantly higher than what's called for originally, I'm thinking that's probably responsible for the weirdness.

[jmaf]

Also, I remembered earlier that I'm using a choke that's rated significantly higher than what's called for originally, I'm thinking that's probably responsible for the weirdness.

The bigger choke could cause some sag, but I wouldn't make it the main culprit for changes in your tone. I say the new tubes + bias = your culprit. The Super was made for 6L6G which is a lower voltage 6L6GC, so to run 6L6GC's on it and expect it to sound similar to the vintages, you need to run it a bit hotter than usual. I hope the 51 mA bias helps out, otherwise we'll have to evoke some other magic to make that amp sound as you like it - Let us know how it sounds tomorrow!

[MWJB]

If you like the sound of higher currents and are happy to take a slight hit on outright power, consider a fixed bias to cathode bias switch. Cathode bias typically runs higher idle currents (signal current doesn't rise as greatly as fixed bias, so idle currents can be higher), sounds sweeter & has some built in compression, at your B+ 300-330ohms at 10W for the cathode resistor would work well, use a 100v rated bypass cap.

I wouldn't call 40mA "cold", even at ~400vdc, that's still >16W per tube idle dissipation. You shouldn't be suffering the effects of excessive crossover distortion at anything much over late 20's to 30mA...but as Jmaf says, tubes do sound sweet when being punished. I know of harp players who idle 6L6s at 4W idle dissipation...that's cold! ;-)

[jmaf]

I know of harp players who idle 6L6s at 4W idle dissipation...that's cold! ;-)

I'm gaining more and more interest in harp amps, because I'd like to build my old man a harp amp some time. Knowing they bias them really cold is news to me, and it's helped in my research, thanks.

[MWJB]

It doesn't work so well with some speaker arrays, 2x10" for instance still like regular/hot plate currents (all else being equal), likewise if you are looking for a specific tone like a cathode biased 50's style tube PA tone, they still like/need hot currents. Cold/cooler currents work well in fixed bias 1x12", 1x15" & speaker arrays with more than 2x10...like 3x10, 4x10 (the amps I specifically had in mind are all Fender style, fixed bias, 4x10"), or a mix of speakers of differing sizes.

[jmaf]

It doesn't work so well with some speaker arrays, 2x10" for instance still like regular/hot plate currents (all else being equal), likewise if you are looking for a specific tone like a cathode biased 50's style tube PA tone, they still like/need hot currents. Cold/cooler currents work well in fixed bias 1x12", 1x15" & speaker arrays with more than 2x10...like 3x10, 4x10 (the amps I specifically had in mind are all Fender style, fixed bias, 4x10"), or a mix of speakers of differing sizes.

Thanks for sharing, MWJB, appreciate it. Apologies to to the OP for the partial thread hijack.

[pinkphiloyd]

Hey guys, sorry, life is crazy. It sounds a lot better now, biased at 51. Doesn't sound as good as it did up around 70, ha ha, but it's definitely sweet. I'd say since this was my first amp build, ever, it'd have to be called a resounding success. I didn't even really have to trouble shoot or have hum issues. Just fired it up and it worked. Thanks again for the help, everyone. Thinking about trying a Spitfire or Princeton so I'll probably be back soon!

[jmaf]

Hey guys, sorry, life is crazy. It sounds a lot better now, biased at 51. Doesn't sound as good as it did up around 70, ha ha, but it's definitely sweet. I'd say since this was my first amp build, ever, it'd have to be called a resounding success. I didn't even really have to trouble shoot or have hum issues. Just fired it up and it worked. Thanks again for the help, everyone. Thinking about trying a Spitfire or Princeton so I'll probably be back soon!

Nice, pinkphiloyd. Here's hoping to see your next successful build! It's addictive, isn't it....

[pinkphiloyd]

It is. Gonna try to source everything myself for this one, and punch/drill my own chassis, so I'll probably be back soon. I don't expect it to go as well.

[pinkphiloyd]

One more quick round of questions about this 5F4. I've been reading and studying, and I realized I had some wiring and stuff not quite right, and also I've come to understand tubes and bias a little better, I think.
So, I realized what wiring mistakes I had made, fired it up, and checked the voltages. And they were all out of whack. They didn't match up between the tubes, and I had only been checking one tube. So I drained the caps, fixed the areas, fired it up, and biased, and now this is what I have. With JJ 6L6GCs. Data here. http://lib.store.yahoo.net/lib/thetu...e/JJ-6L6GC.pdf

Plate voltage = 400V
Cathode voltage = 50.8 mV
Screen voltage = 404V
Grid voltage = -37V

So, my dissipation is

.0508(400)= 20.32 Watts. This tube can handle 30 Watts, 70% of which is 21 Watts, so I'm well within spec. And both tubes match up now. So am I good? Do I have this right finally?

[MWJB]

Looks like it.

Negative grid voltage is not really relevant, the cathode voltage/current is what you aim for, the negative grid voltage really just ends up as a byproduct of that.

Did you add 470ohm screen grid resistors to pin 4 of the 6L6s? (Recommended).

[pinkphiloyd]

I did.

[pinkphiloyd]

Getting a bit of fizz at higher volumes or at lower volumes with the treble set high. Doesn't last long. It's noticeable on attack and dies almost instantly. Initially thought it was a speaker but my DC-30 sounds great through the same cab. Thoughts? Bad tube?