How hard is my quad of EL84s working...too hard?

[dehughes]

Well, here are the specs:

EH EL84s in an AC30 amp with a Mercury Magnetics Woden PT

Mullard recto (5AR4) Pins 4 and 6: 303v AC

Recto Pin 8: 359v

OT center tap: 359v

After the choke: 340v

EL84 plates: 349v

EL84cathodes: 13.00v

Now, I just swapped in a 62 ohm 25w resistor for the 62 ohm 10w resistor that was in there previously, and I seem to be noticing a bit more power section hummm than before. It goes away when I double the 1st filter cap value from 16 to 32 uf, so I'm inclined to say that all is well...but it seems that the hummm is louder than before. It could just be me, but...oddly enough, these numbers are slightly higher than with the 10w 62 ohm resistor in there...and both measure out within .8 ohms of each other.

Do these numbers indicate that the power tubes are working too hard? They are not red-plating, and when I drop the voltage down a bit (inserting a 100ohm resistor between the recto and the 1st filter cap) it is less hummy...but then the voltages are dropping to about 311v on the plate and 11.54v on the cathodes, so that makes sense...

Anyway, lemme know if this all seems normal. It is my first AC30...so I'm still learning...

Thanks.

[David Root]

According to the ratings you're running a bit hot, but hey, that's an AC30 design "flaw"!

13V/62 ohms=210mA total cathode current. (349-13)*.0525=17.6W dissipation per tube vs. 14W by the book. Resistor rating 10W is fine (.210 squared*62=2.7W).

OTOH an original AC30 ran a 50 ohm cathode resistor at 340V plate so you're in the ballpark! Does it sound good? I don't recall the plate/screen filter cap "classic" values, but these old circuits generally don't respond well to over-filtering tonewise, although it may well reduce ripple hum.

Lower current tubes might help. They would certainly last longer.

What OT are you using? What's the Zpri?

[dehughes]

According to the ratings you're running a bit hot, but hey, that's an AC30 design "flaw"!

13V/62 ohms=210mA total cathode current. (349-13)*.0525=17.6W dissipation per tube vs. 14W by the book. Resistor rating 10W is fine (.210 squared*62=2.7W).

OTOH an original AC30 ran a 50 ohm cathode resistor at 340V plate so you're in the ballpark! Does it sound good? I don't recall the plate/screen filter cap "classic" values, but these old circuits generally don't respond well to over-filtering tonewise, although it may well reduce ripple hum.

Lower current tubes might help. They would certainly last longer.

What OT are you using? What's the Zpri?

Oh man, this forum rocks so hard....thanks so much, David! I figured I was in the ballpark with the values but just wanted to make sure I wasn't overlooking something completely obvious. Thanks.

The amp sounds VERY good...save for the slightly-louder-than-before hum....but then I'm chalking that up to some power section oddity, as it does go away when I double the filtering value from 16 to 32uf right after the recto. I checked all the solder joints touched by my resistor-swapping and they seem clean and solid, so who knows... The amp has an ability to actually start to sustain into really interesting harmonics, if the right chord is played with the right touch. I've not experienced that in another amp...so this is very cool. With the power reduced and the filtering doubled it is very quiet, so I'm happy there...especially considering how hard the EL84s are being hit.

When I asked for these EL84s I had Jim pick out a lower current set, and they do sound very nice....

The OT is a Mercury Magentics Woden clone...dunno the primary impedence, but I'm sure someone else here does....

One other question....I have the 100 ohm 25w resistor connected directly from pin 8 to the fuse (and on to the first filter cap, etc...). If I'm using a switch to bypass a the resistor, connecting pin 8 directly to the fuse and 1st filter cap, would there still be current running through the resistor even when the switch is on? I'm wondering if my possibly clever idea is maybe causing the voltages to be slightly higher than they ought to be...

Also, what type/rating of Zener diode would I use to drop the voltage to closer to AC30 specs, given the voltages I just measured?

Thanks!

[dehughes]

Okay, get this. My wall voltage is 120v....man...that's higher than before...this is causing my amp's internal voltages to be even higher than when I measured them a few hours ago!

Then, I'm thinking, well, I should probably swap out the power tubes just in case this odd hum is a result of bad tubes. So I swapped out the recto with two different ones (no change...) and then swapped out my relatively new EH EL84s with a quad of relatively new JJ EL84s...and whaddya know....the amp is nearly dead quiet again. All I did was swap out a cathode resistor, and the next thing you know, I turn the amp on and the power tubes are hummy? It makes no sense. Tubes...eesh. For now, I'll go with the theory that the EH power tubes were on the verge of going bad....which sucks as they aren't that old. BUT, these types of amps do really work the tubes, huh...

So, here are the new voltages as of 5 minutes ago, with the JJs in and the wall voltage up to 120v AC:

Recto pin 8: 367v

OT CT: 367v

After the choke: 354v

JJ EL84 screens: 354v

JJ EL84 plate: 359v

JJ EL84 cathodes: 11.7v

SO, this give me about 16.15w plate dissipation, yes? That's a little better than the 17+ watts that the EH's were kicking out...still high, but do-able, and close to what I've guestimated the various AC30-type amps are running.

Also, when I switch in the 100 ohm, 25w resistor after the recto, I end up with 323v on the plates, 318v on the screens, and 10.4v on the cathodes for about 12.13w on each tube. That's a bit better...and at this setting, with the filtering bumped up, the power section is SUPER quiet.

What a bummer that tubes sound so good before they melt, huh....

[David Root]

That's all very consistent with what an AC30 should be like, especially the touch response.

Wall voltage in my shop varies from 121 to 126VAC. This is NOT normal!At home it is just above 120. Most places in the US and Canada these days will be 120VAC minimum, so if you have a "close" circuit design on plate voltage, don't be buying any 115 or 117 V PTs, or you'll have to put in a Zener.

I'm sure the Mercury Magnetics iron makes a difference as well. Probably made a difference in your bank account too, I'll bet!

IIRC, the Zener should be rated approx. for 115% of the voltage drop you want at the max. current draw. I only used one once and I think I had to get a 60V diode to get a 50V drop. Maybe someone with a better memory could fill in the details for you.

[dehughes]

That's all very consistent with what an AC30 should be like, especially the touch response.

Wall voltage in my shop varies from 121 to 126VAC. This is NOT normal!At home it is just above 120. Most places in the US and Canada these days will be 120VAC minimum, so if you have a "close" circuit design on plate voltage, don't be buying any 115 or 117 V PTs, or you'll have to put in a Zener.

I'm sure the Mercury Magnetics iron makes a difference as well. Probably made a difference in your bank account too, I'll bet!

IIRC, the Zener should be rated approx. for 115% of the voltage drop you want at the max. current draw. I only used one once and I think I had to get a 60V diode to get a 50V drop. Maybe someone with a better memory could fill in the details for you.

Yeah, the tap on the MM tranny is 117v, so I should probably drop things a bit, yes? I'd like to keep the amp working at "standard" voltages close to what it would have been operating at back in the day. What type/rating of zener should I look at, then, given the above voltage measurements?

Thanks!

[David Root]

I'd look at the '60s AC 30 schematics, best I remember is 340 VDC plate with a 50 ohm cathode resistor. You're a bit high on that plate voltage but the low current selected EL84s should lay that off a bit, although it might change the odd/even harmonic distortion slightly. ~17W dissipation is OK if the EL84s are good. The AC30 set up right is basically Class A to about 22W output, then it's forced into hot AB1. It's designed to run hot.

If it sounds good, don't sweat it, lay back and enjoy, it ain't rocket science!

[dehughes]

I'd look at the '60s AC 30 schematics, best I remember is 340 VDC plate with a 50 ohm cathode resistor. You're a bit high on that plate voltage but the low current selected EL84s should lay that off a bit, although it might change the odd/even harmonic distortion slightly. ~17W dissipation is OK if the EL84s are good. The AC30 set up right is basically Class A to about 22W output, then it's forced into hot AB1. It's designed to run hot.

If it sounds good, don't sweat it, lay back and enjoy, it ain't rocket science!

Right on...thanks. I was thinking about doing just that. I must say though that I don't like the tonality of these JJs....too dark and muddy. Strong low mids...but not enough articulation and air on top.

Any recommendations other than the EHs? Can one still find Ei EL84s anywhere?

[tubetek]

E-I seems to be way too inconsistent; a real shame...
I'm probably gonna get flamed for this, but have you tried the
Sovtek EL84M/7189? Matchless used 'em; rated for 400v on the plate!!
The drawback seems to be that they sound "HI-FI" or too clinical in some guitar amps. They do work great in old HI-FI equipment; I'm running a quad in a Scott 222C Integrated...the plate was over 400v at 115vac in and using a GZ34 recto.
Summary- They may be too clean for some but they can certainly handle the voltages without any changes to your circuit; maybe you'd want to try them before making any major changes...just My .02
tubetek

[dehughes]

Right on....thanks. I was reading an older EL84 comparison of Sovtek, Sovtek EL84M, JJ, Ei, and Raytheon tubes...

http://www.rru.com/~meo/Guitar/Tubes/Miles/el84-1.html

http://www.rru.com/~meo/Guitar/Tubes/Mo ... -el84.html

...and was surprised that the Sovtek tubes didn't get panned. I actually liked the tone better of my EH set, BEFORE then started humming away, that is...so maybe the Russian stuff really isn't as bad as it is often made out to be.

[David Root]

How about NOS Russian, er, Sovietski Unionski, 6P14P-EV? This tube is USSR military, possibly the same design as the Sovtek EL84M, however its spec sheet rates it for 500 VDC!! And 25W dissipation.

I have a pair I got from Lord Valve, dated 1982, but I've not tried them in an AC30 (obviously!). They can handle anything an AC30 or Matchless can serve up.

LV sez they are actually 7189As, and he thinks they sound great, but then he's selling them ($44.00 a matched quad). Worth a try I'd say.

[dehughes]

How about NOS Russian, er, Sovietski Unionski, 6P14P-EV? This tube is USSR military, possibly the same design as the Sovtek EL84M, however its spec sheet rates it for 500 VDC!! And 25W dissipation.

I have a pair I got from Lord Valve, dated 1982, but I've not tried them in an AC30 (obviously!). They can handle anything an AC30 or Matchless can serve up.

LV sez they are actually 7189As, and he thinks they sound great, but then he's selling them ($44.00 a matched quad). Worth a try I'd say.

OOOH! Good idea....I'll have to see if I can get a hold of him...I might have his contact info somewhere...

Thanks!

[drz400]

Okay, get this. My wall voltage is 120v....man...that's higher than before...this is causing my amp's internal voltages to be even higher than when I measured them a few hours ago!

Then, I'm thinking, well, I should probably swap out the power tubes just in case this odd hum is a result of bad tubes. So I swapped out the recto with two different ones (no change...) and then swapped out my relatively new EH EL84s with a quad of relatively new JJ EL84s...and whaddya know....the amp is nearly dead quiet again. All I did was swap out a cathode resistor, and the next thing you know, I turn the amp on and the power tubes are hummy? It makes no sense. Tubes...eesh. For now, I'll go with the theory that the EH power tubes were on the verge of going bad....which sucks as they aren't that old. BUT, these types of amps do really work the tubes, huh...

So, here are the new voltages as of 5 minutes ago, with the JJs in and the wall voltage up to 120v AC:

Recto pin 8: 367v

OT CT: 367v

After the choke: 354v

JJ EL84 screens: 354v

JJ EL84 plate: 359v

JJ EL84 cathodes: 11.7v

SO, this give me about 16.15w plate dissipation, yes? That's a little better than the 17+ watts that the EH's were kicking out...still high, but do-able, and close to what I've guestimated the various AC30-type amps are running.

Also, when I switch in the 100 ohm, 25w resistor after the recto, I end up with 323v on the plates, 318v on the screens, and 10.4v on the cathodes for about 12.13w on each tube. That's a bit better...and at this setting, with the filtering bumped up, the power section is SUPER quiet.

What a bummer that tubes sound so good before they melt, huh....

IF you could drop the Screen voltage to 300V you would be running much safer, I think the JJ's sound great and they are reliable

[dehughes]

Yeah, if I switch in the 100 ohm 25w resistor after the recto, the voltages drop down to about 303 on the screens and about 311 on the plates....which is cool, but I still like the sound/vibe/feel of the amp with the EHs running at full power. Still though, if I destroy a set of tubes every 6 months running at bedroom levels, then that might be worth changing....but then that's just the AC30 thing, isn't it?

Does anyone know the total current draw of an average AC30 under average playing conditions? How would I go about testing this, to see if I can indeed run a 5V4 in this amp without risking its life as well? The 5V4 brings the voltages down to almost 300 on the screens, so that's another logical option for tube life extension without sacrificing too much feel.

[Tdale]

According to the ratings you're running a bit hot, but hey, that's an AC30 design "flaw"!

13V/62 ohms=210mA total cathode current. (349-13)*.0525=17.6W dissipation per tube vs. 14W by the book. Resistor rating 10W is fine (.210 squared*62=2.7W).

Let me see if I understand this:

Plate voltage is measured on one of the plates.
The cathode voltage is measured across the cathode resistor.
But when you calculate the output power, you have to divide the cathode current (210) by 4, since there are 4 tubes?

I just went to measure my DC30, and discovered that the tube I dropped on the floor yesterday didn't make it after all... a grey substance is now covering the inside of the tube...

Tommy