Amp gurus help me bias

[VanR]

I built an ax84.com High Octane about 6 years ago. Since I only had a P/P output transformer on hand I built it 2-EL84's P/P. Cathode bias, ~250 volts on plates. 130 ohm common Cathode bias resistor.

Last week I retubed it with some new JJ EL84's (31 mA rated on box) and when I did the amp lost all it's balls. Much cleaner amp now. I was thinking I needed to bias it a little hotter to get the grit back. Does this make sense? Or am I mistaken?
So tonight I got a reading of 8.15 volts across the 130 ohm resistor which gives me ~62 mV or ~31mV each tube correct? I was thinking of bumping the cathode bias resistor down to 100 ohm/10 watt I have in my box but I'm not really sure what figures I should be shooting for as far as bias. How hot should I be shooting for? And is biasing hotter going to give me more dirt from the amp or less? Thanks for your help guys.

[rockon1]

250V X .031 = 7.75 watts dissapation. From my understanding thats pretty cold for a cathode biased amp using 12 watt tubes. I believe you should be closer to max dissapation at idle-around 10.5 to 11.5 volts.

FWIW-My Crate palomino V32 is at 13.5 and a bit high but I left it like that because I didnt want to mess with the resistor. It will go thru tubes faster. Bob

[phsyconoodler]

Most 18 watt marshalls have about 300+ volts and use a 91 ohm cathode resistor.At 250v you are not using the tubes at all with a 130 ohm resistor.
You may need a 90 ohm or less to get this amp singing nicely.
Yes,it will grind harder with a smaller resistor value.
I'm not sure what that 31ma rating is but a cathode biased amp will be biased close to 90% of plate disipation.You are way cold.

[VanR]

Thanks the old tubes (also JJ's) were marked 43 I think when I bought them from tubestore back in 2001 so I believe they were drawing more current, hence the better sound with the 130 ohm resistor.
I think I'll change the bias resistor to 100R/10 watt which I know is overkill, but it's what I have and see what happens. I also have some 100R/2 watt. Is 2 watt enough?

[John Phillips]

I built an ax84.com High Octane about 5 years ago. Since I only had a P/P output transformer on hand I built it 2-EL84's P/P. Cathode bias, ~250 volts on plates. 130 ohm common Cathode bias resistor.

Last week I retubed it with some new JJ EL84's (31 mA rated on box) and when I did the amp lost all it's balls. Much cleaner amp now. I was thinking I needed to bias it a little hotter to get the grit back. Does this make sense?Why not just put the old tubes back if you liked it better? :confused:

At least measure the amp with them in and work out what current they're drawing - it might be nothing to do with that. Hotter bias does not necessarily give more gain and distortion - if the plate voltage is fairly low (which it is), the operating point may be low enough that the ideal bias is far below maximum dissipation.

With a cathode-biased amp the current it idles at should be largely independent of the tube characteristics anyway - those current figures for the tubes are for a fixed bias setting. It may just be that you prefer the sound of the other tubes.

[rockon1]

Thanks the old tubes (also JJ's) were marked 43 I think when I bought them from tubestore back in 2001 so I believe they were drawing more current, hence the better sound with the 130 ohm resistor.
I think I'll change the bias resistor to 100R/10 watt which I know is overkill, but it's what I have and see what happens. I also have some 100R/2 watt. Is 2 watt enough?


I would think higher wattage would be better than low. My V32 (quad of EL84's running at 300v) uses a 10 watt. Im sure someone more knowlegable will have a definative answer. Bob

[Pearly Gator]

Here is the Weber bias calculator. It will calculate the bias and resistor value for you. http://www.webervst.com/tubes/calcbias.htm

Gary

[John Phillips]

I think I'll change the bias resistor to 100R/10 watt which I know is overkill, but it's what I have and see what happens. I also have some 100R/2 watt. Is 2 watt enough?
Easy to work out...

Power = voltage-squared divided by resistance. (Or, current-squared times resistance if you prefer.)

So if you know the voltage across the resistor (assume 10V as a good guide figure for EL84s), and the resistance (100 ohms), that very simply works out to 1W dissipation. (Equally if you're going for a maximum of 50mA per tube for 12W dissipation at 240V, 100mA total for two tubes, it works out the same.)

So a 2W resistor would be the bare minimum to use to allow a safety margin and for the fact that the inside of a tube amp gets hot, which reduces effective cooling. In practice I would use a 5W wirewound, which would much reduce the chance of the resistor getting damaged if a tube blows. There's no harm at all in using the 10W though.

[VanR]

Why not just put the old tubes back if you liked it better? :confused:

At least measure the amp with them in and work out what current they're drawing - it might be nothing to do with that. Hotter bias does not necessarily give more gain and distortion - if the plate voltage is fairly low (which it is), the operating point may be low enough that the ideal bias is far below maximum dissipation.

With a cathode-biased amp the current it idles at should be largely independent of the tube characteristics anyway - those current figures for the tubes are for a fixed bias setting. It may just be that you prefer the sound of the other tubes.

Well I guess I could put the old tubes back in, but they have been in the amp for 6 years now. Of course obviously I haven't been frying them with 400 volts either so they may last forever. And the amp was in storage for about 3 of those 6 years. I just started using the amp hard about a year ago. I just thought it was time for some new tubes as I'm going to be gigging again starting after the first of the year and this little amp and it's twin brother are going to be my live rig.
Tonight I think I'll put the old tubes back in and do my measurements again and see what I come up with.

[VanR]

Well I have to say that John was right. I installed the old tubes in the amp tonight and read the voltage drop across the 130 ohm cathode resistor and got almost the same reading I got last night with the new tubes. Read 7.95V with the old tubes as opposed to 8.15V with the new tubes. Not much difference huh? Anyway I guess the old tubes just sound better and I'm leaving them in there for now.

[rockon1]

Well I have to say that John was right. I installed the old tubes in the amp tonight and read the voltage drop across the 130 ohm cathode resistor and got almost the same reading I got last night with the new tubes. Read 7.95V with the old tubes as opposed to 8.15V with the new tubes. Not much difference huh? Anyway I guess the old tubes just sound better and I'm leaving them in there for now.

Im surprized you arent going to try the 100ohm resistor. I would think the thing would get a lot more grind with the tubes running hotter. Just a thought. Bob

[VanR]

Im surprized you arent going to try the 100ohm resistor. I would think the thing would get a lot more grind with the tubes running hotter. Just a thought. Bob

I still may do that in a few days. Just not at the moment. As I'm sick with this flu/cold thing. I'd be interested to see if I notice any difference in the tone of the amp. From what John has said I might not gain anything at all. Still considering it.

[VanR]

Well in spite of the flu/cold I have I went ahead and installed the 100R (actually 99ohm) cathode resistor and did my measurements. Got 7.11volts across the resistor so it looks like it only bumped it up to 35 ma each tube. Don't know how the amp sounds yet as it is too late to play it here. I'll find out tomorrow.

[rockon1]

Well in spite of the flu/cold I have I went ahead and installed the 100R (actually 99ohm) cathode resistor and did my measurements. Got 7.11volts across the resistor so it looks like it only bumped it up to 35 ma each tube. Don't know how the amp sounds yet as it is too late to play it here. I'll find out tomorrow.


8.75 watts per tube. Too bad you didnt have a 60 ohm or so to try. John knows best but I love to tinker(or read about it)! A little knowledge can be a dangerous thing! Bob

[John Phillips]

Ha, I certainly don't always know best! :)

(Or a huge amount of the proper theory, actually...)

What I have discovered from my own tinkering is that it's surprisingly hard to alter the bias point in a cathode-biased amp with either different tubes (which usually make almost no difference), or even with the same tubes and a different resistor value. You can, but you need a much larger change than you may expect. The circuit is extremely self-compensating and tries to fight any change you make.

And it doesn't have that much effect on the tone, either - much less than in a fixed-bias amp. That's why I thought it was just a tube issue... ;) Colder tubes just sound colder, even though they bias about the same in dissipation.

If the old ones have been running fairly cool, there's no reason to think they're worn out yet either. EL84s don't have an inherently shorter life than other tube types, it's just that most amps that use them thrash them at 15W (or more!) in some cases... it's no wonder they only last a couple of hundred hours.

[VanR]

I should have stated that these latest numbers are with the old tubes installed not the new ones. Haven't tried it with the new tubes.

[rockon1]

I should have stated that these latest numbers are with the old tubes installed not the new ones. Haven't tried it with the new tubes.


Its a tad hotter...any more grind too it?:drool Bob

[VanR]

Its a tad hotter...any more grind too it?:drool Bob

Have to wait till the wife goes to work tonight. But even then I won't really be able to crank a whole lot. (new neighbor)

[rockon1]

Ha, I certainly don't always know best! :)

(Or a huge amount of the proper theory, actually...)

What I have discovered from my own tinkering is that it's surprisingly hard to alter the bias point in a cathode-biased amp with either different tubes (which usually make almost no difference), or even with the same tubes and a different resistor value. You can, but you need a much larger change than you may expect. The circuit is extremely self-compensating and tries to fight any change you make.

And it doesn't have that much effect on the tone, either - much less than in a fixed-bias amp. That's why I thought it was just a tube issue... ;) Colder tubes just sound colder, even though they bias about the same in dissipation.

If the old ones have been running fairly cool, there's no reason to think they're worn out yet either. EL84s don't have an inherently shorter life than other tube types, it's just that most amps that use them thrash them at 15W (or more!) in some cases... it's no wonder they only last a couple of hundred hours.

Ive been troubleshooting/building automatic transmissions for 30 years now. While there is a hugh amount of technical theory behind it all these days nothings compliments the theory behind it better than good old practical application! I listen intently when you post. :)

On a side note-My Palomino V32 is running hot 13.5 watts dissapation. Ive left it that way since changing the resistor seems difficult (for me at least) since its PCB board mounted. From what your saying I'd need a pretty big jump in value to bring it down....(300 volts with a 60 ohm resistor) One on these days im going to practice soldering on a junk board-Im centainly not going to attempt it with my meager solding skills at the moment! Bob

[VanR]

Got a chance to play it for a minute and I think it sounds better. Seems to have more midrange and more bite to it. More snap to it. Thinking of bumping the resistor down to about 80 or 90 if I can find one locally.

[rockon1]

Got a chance to play it for a minute and I think it sounds better. Seems to have more midrange and more bite to it. More snap to it. Thinking of bumping the resistor down to about 80 or 90 if I can find one locally.

From what John Phillips said it takes a good bit of change to get a significant change in settings. Might want to try a 60 ohm. Cheap enough. Bob