Is 500v too high for 6L6gc?

[Dreams]

Hey everyone.

I've got this Gibson amp on the bench. 2x6L6gc PP.

I'm trying to confirm a few things. I've never seen 6L6s running this hot before, but it looks like they're running more or less how the schematic wants them.

The plates read 502 to ground, 475 between plate and cathode. Cathode resistor (shared) is now 220 ohms, with 27.8 volts across it.

This gives me about 28w dissipation, about 60ma per tube.

I've seen class A 6L6s before, but nowhere near this plate voltage. Am I gonna smoke 'em? The original cathode resistor was 180 ohms, and with that in the circuit, I was getting around 35w dissipation, 70ma per tube. Since 6L6s are 30w max dissipation, I jacked up the cathode resistor.

Some additional info:
380v on the screens
5k output primary
No significant DC on the grids

It's an old amp, so the voltages run a tad high, but REALLY close to the schematic values.

Any thoughts welcome! Thanks guys.

[Willabe]

I've got this Gibson amp on the bench. 2x6L6gc PP.

Which Gibson amp?

Got a schematic?


             Brad      

[phsyconoodler]

It will use up tubes faster but that's about right for cathode biased amps.That's one reason to switch over to fixed bias.That and more output power.

[Dreams]

Oh yeah, whoops. GA-200

Schematic isn't on the internet, but I'm gonna try to attach it. Then it will be. It's just a (bad) photo of the one glued to the side of the chassis.

Similar to the GA-100. Check out the GA-100 schemo on the official Gibson site:

www.gibson.com/Files/schematics/GA-100%20BASS%20AMP.pdf

I only mention this because I can't find anything anywhere about a GA-200 that uses the tube compliment that this one uses. The GA-100 is very close in that regard.

Check out the weirdo compression circuit.

I've thought about fixed bias, but I'm just trying to get it up to factory(ish?) spec right now. There's no bias tap on the PT, and it runs kinda hot right now... Got a little more work to do. Not yet.

I'm specifically interested in people's thoughts on the screen voltage, and generally interested in general thoughts.

[Dreams]

Schematic:

[tubeswell]

For a current production 6L6GC, 500V on the plate is getting to the comfortable upper limit methinks

[Dreams]

Yeah exactly. It all just makes me a little uncomfortable. I don't wanna run them so hot they're on the edge of meltdown the whole time, and I don't want to have to replace tubes every month either.

SHOULD I be thinking about these things, or am I just being paranoid?

[phsyconoodler]

Pop a ener in series with the PT center tap or a resistor and cap and lower the voltage.Then you have nothing to worry about.

[SILVERGUN]

Here's another option for you:
http://www.geofex.com/article_folders/vintvolt/vintvolt.htm

That amp was designed and built when the voltages coming out of the wall were lower...
A 10 VAC difference in input power can wind up to be an ~60-70VDC difference in B+...
Check the filament voltages as well,,and if they are high, consider this option ...

I recently did one and just built it into the head cabinet  

[phsyconoodler]

10v ac won't increase B+ by 70v.

[SILVERGUN]

10v ac won't increase B+ by 70v.

Sorry...I was estimating,,,but I was high  ....I had used a 12V xfmr and it actually wound up "bucking" 14V
I went out and took some exact measurements so you'd have a better guide to what to actually expect....
Used a Variac to alter input and got these readings:

Input 121 VAC= 457 VDC at first filter cap-----6.7 VAC filaments
        111 VAC= 414 " "   ---------------------6.1 VAC  "      "                      
(SO a 10 VAC drop actually netted 43 VDC drop)

Here's more:
        107 VAC= 402 VDC B+ ----------------5.9 VAC filaments
 **This was my personal experience (dropped 55 VDC with a 14 VAC input reduction)

The higher the secondary V rating of your PT the more effect the input reduction will have (proportionate to the windings)....so your results will be slightly higher, based on your 502V B+,,,meaning if you used the same xfmr as me, and dropped 14VAC input, you could expect to drop ~ 60V B+

Thanks for keeping me honest.....phsyc  
        

[Dreams]

Thanks for the replies, but I feel as if I may have been misleading with the title of this post.

Maybe it should read more like:

Is 500v to high for cathode biased 6L6GCs?

I'm quite comfortable putting 500+ volts on 6L6s. I do it in Ampegs, but they're usually fixed bias, AB circuits conservatively biased. Dissipation isn't as close to max.

My concern isn't so much about plate voltage. The schematic claims 490 on the plates, and I'm at about 500 here. I've never seen a class A 6L6 amp running so close to max spec before WRT plate voltage/dissipation. I upped the cathode resistor (because with the old one, tubes were running at a calculated 35w dissipation). Now it's 220R, up from 180R.

I'm not comfortable enough with the way the math works out to not ask for a second opinion.

The screens are keeping this thing in check, yeah? I got 380 volts on em. Should I go lower? Maybe some 1k-ish limiting resistors? Am I just worried about nothing???

[Dreams]

Are you guys saying that dropping the B+ is something that SHOULD be done, or something that COULD be done?

[PRR]

> am I just being paranoid?

Perhaps.

It IS working the tubes hard. Use good tubes and carry spares.

[alerich]

Thanks for the replies, but I feel as if I may have been misleading with the title of this post.

Maybe it should read more like:

Is 500v to high for cathode biased 6L6GCs?

I'm quite comfortable putting 500+ volts on 6L6s. I do it in Ampegs, but they're usually fixed bias, AB circuits conservatively biased. Dissipation isn't as close to max.

My concern isn't so much about plate voltage. The schematic claims 490 on the plates, and I'm at about 500 here. I've never seen a class A 6L6 amp running so close to max spec before WRT plate voltage/dissipation. I upped the cathode resistor (because with the old one, tubes were running at a calculated 35w dissipation). Now it's 220R, up from 180R.

I'm not comfortable enough with the way the math works out to not ask for a second opinion.

The screens are keeping this thing in check, yeah? I got 380 volts on em. Should I go lower? Maybe some 1k-ish limiting resistors? Am I just worried about nothing???

The tubes are running on the hot side to be sure even with the larger 220 ohm cathode resistor. That 180R was really gettin' it.  I don't know what the amp sounds like but I personally would probably raise the cathode resistor value a little more to ease up on the tubes a little more. Just personal preference. I might also decide that is sounds better with the 220R and put it back. Season to taste so long as you don't burn the joint down.

475VDC cathode to anode shouldn't be a problem. I ran inexpensive current production Shuguang 6L6GC tubes in my Super Reverb at that voltage with no problems. Happy as clams.

I really don't understand the question that ties plate voltage and plate dissipation together. I always thought they were two separate entities. The tube has a maximum amount of DC voltage you can apply across the cathode/anode and it has a maximum amount of current that you can draw through it. You can exceed one without exceeding the other. I didn't think that it mattered whether the tubes are cathode biased or fixed bias as far as how high the plate voltage can be set.

Your plate voltage seems fine. Cathode bias probably helps in that regard since more current draw means the cathode resistor drops more voltage lessening what the tube sees. The current is what is putting the boots to those tubes. If it were my amp I'd try a little higher cathode R and see if you still dig the tone.

[SILVERGUN]

Are you guys saying that dropping the B+ is something that SHOULD be done, or something that COULD be done?

I was saying "it could be done" as a solution to your situation...
If your not comfortable with the tubes being run that hot, then the Vintage Voltage idea is one possible option to cool things down a bit 

[DummyLoad]

if you drop the current through the output stage, then B+ rises, you'll be like a dog chasing it's tail. if you want less B+ with that PS, then burn it off.
IMO; run old stock tubes and leave it be.

--pete