matchless Chieftain

[JC@]

Hello. Long time since my last visit...
Well, I have a Chieftain on the bench that blew a couple of things right after a tube change.
Now repaired, I see a terryfying 69mA per tube at 367 volts on the plates, when the rule says I should see not more than 41mA per tube.
Could we call this situation "normal" in a Chieftain?
Thanks in advance.
jc@

[imaradiostar]

how does it sound?

That's right at 25 watts per tube which is, in theory, 100% dissipation for an EL34. I've heard rumors of Matchless amps running EL34's as hot as they could for a certain sound. I couldn't say with any certainty if they'd still sound that way if they were biased colder.

What brand tubes are they? Are they red plating at idle?

Jamie

[johnk_10]

Could we call this situation "normal" in a Chieftain?
Thanks in advance.
jc@

nope. not that i'm aware of. they definitely won't last very long like that.

[JC@]

how does it sound?

That's right at 25 watts per tube which is, in theory, 100% dissipation for an EL34. I've heard rumors of Matchless amps running EL34's as hot as they could for a certain sound. I couldn't say with any certainty if they'd still sound that way if they were biased colder.

What brand tubes are they? Are they red plating at idle?

Jamie

Hi Jamie, the tubes are a pair of new russian Tung-Sol, and they look ok for the moment, no red plating, no internal sparks... no jumps on the mA meter
This amp according to the felt pen writings inside the chassis is from 2007. It has 300 ohms power resistors in the bias circuit. I read some posts in other forums where they said that previous units came with a hotter 270 ohms resistors.

[JC@]

nope. not that i'm aware of. they definitely won't last very long like that.

That was what I thought. I read in a post somewhere that tubes in the Chieftain, as is, that is, original circuit, would last two to six months.

[imaradiostar]

I was just looking at the schematic for the Clubman (which I think has the same output section) on schematic heaven and if you believe the voltages on the various terminals it's idling at over 30 watts per EL34!!

They have an excellent rep in Nashville for sounding fabulous. I wonder if they also have a rep for torching El34's?

How long has it been running that way? Are they red plating?

Where did you find "the rule" about 41 ma? The only rule I know would be the 70% rule which is really pretty bogus anyway. Bias the amp so the tubes don't self destruct and the amp sounds good, right?

jamie

[imaradiostar]

Hi Jamie, the tubes are a pair of new russian Tung-Sol, and they look ok for the moment, no red plating, no internal sparks... no jumps on the mA meter
This amp according to the felt pen writings inside the chassis is from 2007. It has 300 ohms power resistors in the bias circuit. I read some posts in other forums where they said that previous units came with a hotter 270 ohms resistors.

ooops, sorry, missed this post before man.

I've had a pair of EH 7591's idling right at 100% dissipation for about three years, gigged an average of once a week for an hour and they still sound good and haven't blown up yet. If it works, enjoy it!

jamie

[imaradiostar]

Wow, too many posts in too short a time here! I should think before posting.

What is the voltage at each cathode? The plate voltage seems a little low compared to what I'd expect from a Matchless EL34 amp. Does it have a 5u4 in it instead of a 5ar4/gz34?

JT

[johnk_10]

I was just looking at the schematic for the Clubman (which I think has the same output section) on schematic heaven and if you believe the voltages on the various terminals it's idling at over 30 watts per EL34!!

IME, you can't tell what current a tube's drawing from a schematic diagram since all tubes, even from the same batch have a different current demand and transconductance. it's not until a tube is in the amp and measured that you can really see what it's actually drawing.

2 cents.................

[imaradiostar]

You can on a cathode biased amp because you know the cathode current (volts dropped across cathode r/resistor value) and the voltage between the cathode and the plates (more or less- drop across the PT should be minimal).

jamie

[JC@]

jamie, johnk_10, thanks for jumping in.

I use the 70% rule, expecting 23 watts for a modern production EL34 to give it a little more safe margin.
With certain amps I try to hook meters here and there just to double check the numbers match the formulas. I guess I should use the scope to watch the output more than I actually do, but that's more work, and as said before, I prefer to adjust bias on the cool side.