500v on 6L6GC? Should I look for a replacement?

[apehead]

Hello,

I have a Peavey Butcher. The plate voltage is about 500v, give or take 10v or so. The amp is designed for 6L6GC's. Why would the plate voltage be so ridiculously high? This limits my plate current to less than 50mA, due to plate dissipation restraints. Should I use another tube type instead of the 6L6GC? Perhaps 6550's or EL34's? I also have a buttload of 6146B's laying around.

[Enzo]

What is so ridiculously high about 500v? 460-500 volts is what I expect to find in a 100w amp. Limits it to 50ma? So? I don't recall ever setting a mainstream amp to 50ma. These amps have been around working just fine the last 20 some years, why should we feel the need to re-engineer them now?

If you WANT to try different tubes, then sure, why not? PV power transformers are usually robust and probably have the capacity for the higher heater currents. The bias supply could be easily modified for adjustability. Scren resistors are 100 ohm, you might want to change that for other tubes. But all doable.

[apehead]

I bought JJ tubes for the amp, and after mentioning it to a co-worker, he commented on what a high plate voltage that was for a 6L6GC. So I took a look at the JJ datasheet for their 6L6GC's, and the maximum plate voltage listed for pentode operation is 500V! Is just seems to be cutting it a bit close. Believe me, after dropping the cash for a new quad of power tubes, I'd really rather not replace them again.

JJ 6L6GC Datasheet

[Enzo]

They are in no danger.

Go look at some old Fender amps where Leo ran 6V6s 100 volts over their "maximum."

[bob p]

to make sense of the plate voltages, it would help to list the cathode voltage as a reference.

[apehead]

The cathode was grounded until I added 1 Ohm resistors, there is now about a 42mV drop across them

[bbartilson]

Believe me, after dropping the cash for a new quad of power tubes, I'd really rather not replace them again.

I have another area of expertise, and that happens to be motorcycles. Your comment about 'running so close to the limit' reminds me much of when guys actually go and read their owner's manual and find out the recommended oil change interval is 6k-8k miles. "How would anyone be so stupid as to wait so long to change their oil?"

Fact of the matter is that like a guitar amp, the parameters for maintaining a motorcycle are not hard lines, but depend entirely on the service life of the unit in question.

In your case, you measure something around 500V on the plates, and it concerns you. Someone (whose qualifications are unknown) suggests to you that this is 'awfully high', and you're further concerned.

Now there *was* a time in the middle 90's where you simply COULD NOT buy a decent set of 6550's. During that time, I would go through a dozen tubes before I could find a sextet that would even power up in an SVT without at least one providing a fireworks show. The spec of the tube was 800 volts on the anode for Pentode operation, but they couldn't stand the 550 volts nominal in an SVT.

The point being:

1. Spec sheets are wonderful, but real world experience counts for more.
2. The biggest thing you're missing here is the real world status of the amp while it's in service (i.e., hopefully it's on a gig most of the time) Take your variac and crank it down to 95VAC input to the amp. Now measure your B+. This is what you're seeing at the beginning of second set, the coolers and air-conditioning are all running full blast, and your amp is struggling to provide anything near what it's output power is.
3. On the same topic, a true measure of B+ and/or Plate voltage (which aren't necessarily the same) should also include voltage at the outlet, because unless your supply is regulated like mine, voltage at the wall outlet is a big variable from one day to the next. So what you measure as 'on the line' one day might be 10% lower the next day, simply because of the time you chose to take your measurement.
4. As Enzo suggested, it's not really a problem in the first place. Hence his short note suggesting just that. Years of experience translate that 500 volts of B+ is nothing to be scared of in a 6L6 amp. One might be concerned if you were using Chinese tubes. JJ's? It's a non-issue.
5. How long your tubes last has much more to do with how you set the idle current than it does what the idle B+ or Anode voltage is. Just like running your motorcycle engine at or near red line requires more frequent oil changes. How you set it is a whole 'nother can of worms.
6. If you truly wish to lower the B+ voltage to within what you consider to be a 'safe' margin, there's no need to change output tube types. A simple string of diodes or a high wattage zener will take care of that for you with less hassle and much less expense. This is a reasonable goal, and a personal choice. Any number of folks around here would be happy to help explain how to do this.
7. Seems if you're concerned about saving money, the last thing you would suggest is throwing out a perfectly good set of output tubes without having at least tried them. Any decent supplier (including me) will warrant tubes against infantile failure. If they don't stand up to the B+ on initial power up, you probably don't want them in your amp in the first place.


Best,
-Bill

[Jester]

6. If you truly wish to lower the B+ voltage to within what you consider to be a 'safe' margin, there's no need to change output tube types. A simple string of diodes or a high wattage zener will take care of that for you with less hassle and much less expense. This is a reasonable goal, and a personal choice. Any number of folks around here would be happy to help explain how to do this.

Hi Bill,

Would you be so kind as to show me how to do this with a Zener diode. I've got a couple of mains transformers that are putting out more than they should, and so would like to drop about 80 volts for a '59 Bassman (5F6-a).
Thanks. Jester

[Jester]

6. If you truly wish to lower the B+ voltage to within what you consider to be a 'safe' margin, there's no need to change output tube types. A simple string of diodes or a high wattage zener will take care of that for you with less hassle and much less expense. This is a reasonable goal, and a personal choice. Any number of folks around here would be happy to help explain how to do this.

Hi Bill,

Would you be so kind as to show me how to do this with a Zener diode. I've got a couple of mains transformers that are putting out more than they should, and so would like to drop about 80 volts for a '59 Bassman (5F6-a).
Thanks. Jester

[Rob Mercure]

Hey Enzo,

I don't have my Peavey factory schemos handy - not doing warranty anymore - but isn't this amp essentially like the Music Manns with hollow state output and SS input where the output stage was essentially "Class B?" While the plate voltages were very high on both designs the actual dissipation was quite small - less than 20 W on a 6CA7 for the MM - so the tubes tended to last forever.

So while I completely agree with you this, unfortunately, some of the other analogies no longer hold true as this amp's design philosophy uses the tubes in "different' manner.

Rob

[Enzo]

No the Butcher is utterly conventional. A balls to the wall tube amp. Has the plexi style front end - the high gain input jack goes through one more stage of amplification.

This amp has lasted and worked over 20 years without modification. I don't know why all of a sudden it needs to be changed.

[Gtr_tech]

Its pretty much a JCM800 knock off, not much in common with a plexi.

[Gtr_tech]

You need to be concerned with plate *current*, not plate voltage. Good quality 6L's will laugh at 500v. But get the current idle too high on any tube at any working voltage and thats asking for trouble.