EL84 Bias Values, crackling, etc....

[stoneattic]

So I have a Mesa Studio Caliber (DC-2) that I have been using for years as my main gigging amp and at practice the other day it started making a crackling noise. The crackling is intermittent, random and with about 10-20 seconds between. It seems to happen regardless of if I'm playing or holding the strings, etc.

I have a few new spare matched pairs of (EI) tubes that I had picked up a while ago so figured that the Mesa EL84s were old anyway, so I was going to put a pair of EIs in. But since Mesa wants you to buy their "pre-selected" tubes I measured the bias on the Mesa tubes first using my Amp-Head bias probe and got the following results with the Mesa and EI tubes:

Mesa
V7 I=38mA V=390V
V8 I=29mA V=391V

EI (set 78)
V7 I=51mA V=365V
V8 I=46mA V=365V

EI (set 104)
V7 I=44mA V=374V
V8 I=40mA V=374V

All pairs are supposedly matched. It looks to me that the Mesas are not matched, at least not anymore. But the large differences in current and voltage readings between the Mesas and EIs surprised me. Any thoughts about the suitability of the EIs without a re-bias? The EI current number are higher than I expected based on various references regarding EL84s.

My next question is where and how do I get replacement Mesa tubes? I hear there are different "colors". I can't find any colors or markings on the tubes besides "Mesa" and "el84".

I have no idea if this is even related to the crackling issue but I want to get this sorted out first before tackling the crackling. (poetry unintended)

I have a gig tomorrow night and would prefer to use the Mesa, but I have spare amps so it's not the end of the world to use another.

Thanks!

 

[FXR]

I did the same thing basically. But I wound up with three sets of Mesa EL84 tubes. All labeled gry. I had white originally.

Put the first set in, sounded like junk in about 4 hours use. Second set, same thing. Third set, same thing, but I got real curious the third time and looked to see the right tube from the rear red plating.

I retried them a couple of time since to see if it was my ears getting tired. But they sound terrible and the one tube I labeled as red plating does so in either socket.

I tried to call mesa Thursday about this to see if it was the amp or the tubes and which to RMA and how, but they called back at 6pm my time and I was too involved with the kids by that time to have a coherent conversations and said I'd call back.

I'm back to using the original Mesa EL-84 tubes the amp came with.

Anyway, where I was going is that I'm gonna order a bias testing device and will read all 4 sets and make a report FWIW. It was originally for my ability to choose tubes that bias in the area I like the sound from. But with your post, we can collect some reference date.

Oh yeah, one more thing. Mesa 12AX7, they look like JJs currently. Got 3 a couple of weeks ago. Two the top glass nipples broke right off. I've changed so many tubes, too many to count, with those shields on them and NEVER BROKE ONE before! I'm not impressed with Mesa tubes based on my experience. Another reason I want to take measurements. I want to prove it's a real problem and not me being whiny. That may go either way though.

 

[stoneattic]

I only had the crackling happen at practice on Wed. I didn't play it at all on Thurs and on Fri I just did the bias measurements without playing. Then I put the EIs with the 40/44 bias numbers in and played for a bit and it sounded wonderful. It shimmered with great harmonics and, at least for me, sounded better than with the Mesas. There was no sign of the crackling, but then again I only played it for half an hour, not four.

I worry that the higher bias current will be really rough on the tubes and perhaps the amp. Any thoughts on that?

I have plenty of 12AX7s so I may throw some new ones in, but I tapped each one last night with a chop stick and they were all silent, so I don't think they are the cause of the crackling.

Either way I will bring a spare amp tonight, as usual.

 

[FXR]

My journey started with a noise too. Long story, but it's gone and I'm not 100% sure what caused it or fixed it. It was an intermittent crackle.

One noise I did hear and track down was with the EL-84 tube keepers. They rattle and sometimes when things are right. In fact, I took mine out yesterday and folded them into an X to try and snug them up to the tubes. So far it's worked.

As for your current draw, I'm not up to speed 100% on design to be honest, although I have studied a bunch of math and different design scenarios, schematics, we may encounter regarding voltage and current on output tubes. But, the gut always worries about high current. I don't really know your amp to be honest as to what might be normal though.

This is the exact thing I joined the board to discuss and learn more about as well. That's why I jumped in. I wish I could add more then just my experience, but I'm hoping combined we come up with answers or maybe get an explanation.

 

[O==00==O]

EDIT: nevermind.

The JJ EL84 is supposedly the best current production EL84, but things change all the time. The 2003 JJ EL84 spec sheet (not sure how much they've changed production since then) shows a max plate voltage of 300v and typically 48mA current, but like most new productions 12AX7's, their actual performance is too far out of spec. Also, here's some specs on various kinds of tubes: http://www.guitaramplifierblueprinting.com/datasheets.html

I'm thinking about looking further into mod'ing my Studio Caliber to run on a Class A KT88 or 6L6's in AB. I think the Studio Cal power transformer can handle it.

 

[MJ Slaughter]

I can't speak to the more technical aspect of your questions but will say I have been using JJ EL84s in my DC-3, and previous amps, with great success. Good tone and dependability. For a quick answer you may be able to get something from Eurotubes.com. They deal with the JJ's and seem to be knowledgeable on the Mesas. My last matched quad are 27ma.

This may not be causing your cracking problem but it was for me. My DC-3 would crackle with nothing plugged in and when I played something it got worst. The severity was a bit random but there was always a little noise there. After troubleshooting all I was able to do I remembered a member on this board monstatone said about he always replaces the return or send, don't remember which one, jack in the FX loop. It's the jack with the switch. I just sprayed it with contact cleaner, worked a jack in several times and it's been fine since. He said it happens many times when an FX loop doesn't get used. Even if it's not your problem it's a handy think to store in the back of your mind.

 

[O==00==O]

Hey, I've been looking into this issue on my studio caliber / dc-2. I've talked to Patrick @ Mercury Magnetics, Myles Rose, read Aiken's site, and have been simulating circuits with some programs. The main issue is that the current production EL84's (even the JJ's which are the best current production tubes) can't take the voltages that this DC-2 was designed to give. I am adding a choke after the rectifier bridge, before the 220uf filter caps, to lower the voltage a bit. It will also filter the power which will greatly reducing noise and supposedly make it richer sounding, too.

 

[ja22y]

Mesa
V7 I=38mA V=390V
V8 I=29mA V=391V

EI (set 78)
V7 I=51mA V=365V
V8 I=46mA V=365V

EI (set 104)
V7 I=44mA V=374V
V8 I=40mA V=374V

Thanks!

Came across this while searching for something. EL84 operates at 12W. What you really need to watch for is the plate dissipation. If the voltage here is the plate voltage (taken between pin7 and 3) and I is the cathode current, assuming a typical 4mA screen current ,plate dissipation for the mesa v7 is (38mA-4mA) 390V= 13.2W, and V8=9.7W. In this case the value closer to V8 (for cool) and closer to V7 (for hot) is what you want when looking for a replacement. Those EI are way too hot, they might redplate and smoke the screen resistor.

 

[BobR]

Came across this while searching for something. EL84 operates at 12W. What you really need to watch for is the plate dissipation. If the voltage here is the plate voltage (taken between pin7 and 3) and I is the cathode current, assuming a typical 4mA screen current ,plate dissipation for the mesa v7 is (38mA-4mA) 390V= 13.2W, and V8=9.7W. In this case the value closer to V8 (for cool) and closer to V7 (for hot) is what you want when looking for a replacement. Those EI are way too hot, they might redplate and smoke the screen resistor.

If the DC2 is fixed bias(as I believe it to be) you want preferably no more than 70% of max dissapation at idle.
12 / 390 = 31 ma X 70% = 22 ma The V8 in this case is still too hot. Not surprising as Mesa has these amps burning way too hot. Bob

 

[stoneattic]

It's been a while since I chimed in on this so here's the latest:

The EIs got rid of the crackling and have been working fine for gigs and practices for the last few months. I am still worried about the high currents, as everyone else who posted seems to be as well. I'm going to order a few sets of other brands and see what that gets me.

It does sound really good this way it is though.

 

[BobR]

It's been a while since I chimed in on this so here's the latest:

The EIs got rid of the crackling and have been working fine for gigs and practices for the last few months. I am still worried about the high currents, as everyone else who posted seems to be as well. I'm going to order a few sets of other brands and see what that gets me.

It does sound really good this way it is though.

The dc-2 appears to have the same exact bias set up as the DC-3. A 470K resistor piggy backed on the 120 should get the bias down. Bob

 

[stoneattic]

It's been a while since I chimed in on this so here's the latest:

The EIs got rid of the crackling and have been working fine for gigs and practices for the last few months. I am still worried about the high currents, as everyone else who posted seems to be as well. I'm going to order a few sets of other brands and see what that gets me.

It does sound really good this way it is though.

The dc-2 appears to have the same exact bias set up as the DC-3. A 470K resistor piggy backed on the 120 should get the bias down. Bob

Any suggestion on wattage rating? I doubt I have any thing higher than 1/2watt laying around so I will have to order. The schematic I have on it doesn't show wattage ratings.

 

[BobR]

It's been a while since I chimed in on this so here's the latest:

The EIs got rid of the crackling and have been working fine for gigs and practices for the last few months. I am still worried about the high currents, as everyone else who posted seems to be as well. I'm going to order a few sets of other brands and see what that gets me.

It does sound really good this way it is though.

The dc-2 appears to have the same exact bias set up as the DC-3. A 470K resistor piggy backed on the 120 should get the bias down. Bob

Any suggestion on wattage rating? I doubt I have any thing higher than 1/2watt laying around so I will have to order. The schematic I have on it doesn't show wattage ratings.

1/2 watt is fine.

Disclaimer- Lethal voltages are stored inside the amp even when its unplugged. Work inside at your own risk!

Measure the neg bias voltage first and then afterward. It should read about -14 afterward . Bob

 

[stoneattic]

1/2 watt is fine.

Disclaimer- Lethal voltages are stored inside the amp even when its unplugged. Work inside at your own risk!

Measure the neg bias voltage first and then afterward. It should read about -14 afterward . Bob

I added the 470K in parallel with the 120K (got 100K when I measure across the combo). I then retested the EIs and got the expected ~-14mA:

EI (set 78)
was I=51mA V=365V now I=36mA V=389V
was I=46mA V=365V now I=32mA V=390V

EI (set 104)
was I=44mA V=374V now I=30mA V=399V
was I=40mA V=374V now I=25mA V=399V

I guess I didn't think about the voltages going up (V=IR duh).

My questions now are:
Am I better off with the higher currents/lower voltage with more closely matched tubes, or low currents slightly less matched tubes?
How close should "matched" tubes really be?

Thanks for the help!

 

[BobR]

1/2 watt is fine.

Disclaimer- Lethal voltages are stored inside the amp even when its unplugged. Work inside at your own risk!

Measure the neg bias voltage first and then afterward. It should read about -14 afterward . Bob

I added the 470K in parallel with the 120K (got 100K when I measure across the combo). I then retested the EIs and got the expected ~-14mA:

EI (set 78)
was I=51mA V=365V now I=36mA V=389V
was I=46mA V=365V now I=32mA V=390V

EI (set 104)
was I=44mA V=374V now I=30mA V=399V
was I=40mA V=374V now I=25mA V=399V

I guess I didn't think about the voltages going up (V=IR duh).

My questions now are:
Am I better off with the higher currents/lower voltage with more closely matched tubes, or low currents slightly less matched tubes?
How close should "matched" tubes really be?

Thanks for the help!

IMO your far better off with lower current draw. Im running 22ma at 425 volts on the plates in my DC-3. Mine would slightly redplate almost any tube at -11 at 400 volts. You would have to be in a darkened room but it was there. Why because they were idling at 140%!!

Even your coolest running tube(40ma @ 374) was idling at almost 120% before the mod! (Bear in mind 70% is the target for fixed bias amps.) Now the same tube is running at roughly 80% idle-much better! Congrats! Bob

 

[BobR]

Oh and your hottest running tube (51ma @ 365 ) was running over 155% before the mod(slight redplate zone). It still runs at 110% but thats a lot better. Bob

 

[stoneattic]

Oh and your hottest running tube (51ma @ 365 ) was running over 155% before the mod(slight redplate zone). It still runs at 110% but thats a lot better. Bob

Thanks for the help. I've got the 25/30 pair in now and band practice tomorrow. We'll see (hear?) how it goes. I was real worried about those high currents too. I feel much better now.

 

[BobR]

Oh and your hottest running tube (51ma @ 365 ) was running over 155% before the mod(slight redplate zone). It still runs at 110% but thats a lot better. Bob

Thanks for the help. I've got the 25/30 pair in now and band practice tomorrow. We'll see (hear?) how it goes. I was real worried about those high currents too. I feel much better now.

In the words of Hannibal from the A team- "I love it when a plan comes together!" Bob

 

[moxy]

I have a DC-3 with the same crackling..
removing the 5532 op amp for the record output fixed my problem
now to figure out why ...