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One output transformer hotter than the other [message #9162]

Wed, 17 August 2005 20:38

hurdy_gurdyman
Messages: 416
Registered: May 2009

Illuminati (1st Degree)

I have an old Scott LK-48-B integrated amp. I've had it in a cabnet since I bought it a few years back and had low speed fans cooling it. I moved it to the top of the cabnet with no fans a few days ago. Now I notice that the right channel output transformer runs hotter than the left, quite noticably. I checked with a VOM all the voltages. Each output tube plate has 420 VDC, as the schematic says it should (supposed to have 7189 tubes. I am running JJ EL 84's.) I have no way to accurately check bis current, but measure -31 VDC at the grids. both sides measure the same. I adjusted for 0 volts DC balance between the pair of tubes on each channel. Right channel still runs hotter. After running the amp for 16 hours, I can place my hand on the right tranny and count to 10 without burning, but it's very uncomfortable. The left side is not very uncomfortable, just warm.
My question would be if this sounds like it's within normal range for tube amp channel-to-channel variation, or do I have a problem? If I have a problem, what should I do next?

Dave

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Re: One output transformer hotter than the other [message #9163 is a reply to message #9162]

Wed, 17 August 2005 22:28

Wayne Parham
Messages: 18793
Registered: January 2001

Illuminati (33rd Degree)

Can you measure plate currents? Alternatively, you might measure the voltage across cathode resistors, and then calculate cathode currents. That might shed some light on things.

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Re: One output transformer hotter than the other [message #9164 is a reply to message #9163]

Thu, 18 August 2005 01:35

Thermionic
Messages: 208
Registered: May 2009

Master

Sounds almost like you have some ultrasonic oscillations. They'll make an OPT get super hot. But, I guess that's really unlikely, as that amp has grid stopper resistors thicker than flies everywhere, and if was an ultrasonic oscillation that bad the plates would probably be glowing red.

Mant times, an ultrasonic oscillation will modulate other frequencies, and you'll have a weird audible hum along with it, or even other oscillations at audible frequencies.

Thermionic

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Re: One output transformer hotter than the other [message #9165 is a reply to message #9164]

Thu, 18 August 2005 06:12

hurdy_gurdyman
Messages: 416
Registered: May 2009

Illuminati (1st Degree)

There's no hot glowing tubes or funny noises. Runs quiet, and has been using these tubes without even a hiccup for almost a year now.

Dave

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Re: One output transformer hotter than the other [message #9166 is a reply to message #9163]

Thu, 18 August 2005 06:13

hurdy_gurdyman
Messages: 416
Registered: May 2009

Illuminati (1st Degree)

Wayne,

I just have an inexpensive VOM that adds resistance when making a current measurement.

Dave

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Re: One output transformer hotter than the other [message #9167 is a reply to message #9164]

Thu, 18 August 2005 06:15

Damir
Messages: 1005
Registered: May 2009

Illuminati (2nd Degree)

In "normal" work, power dissipated in winding resistances is low, and possible imbalance between the channels is probably fraction of W.
It can be oscillation in one channel (large current through the output tubes), or just layout issue - one OPT closer to some hot components (tubes).

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Re: One output transformer hotter than the other [message #9169 is a reply to message #9166]

Thu, 18 August 2005 08:16

Wayne Parham
Messages: 18793
Registered: January 2001

Illuminati (33rd Degree)

Do you have a schematic? If there is a cathode resistor, you can measure the voltage across it and thereby know the current.

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Re: One output transformer hotter than the other [message #9170 is a reply to message #9162]

Thu, 18 August 2005 16:53

Poindexter
Messages: 108
Registered: May 2009

Viscount

Great Googly Moogly, they run those EL84s hard! Plate max.

Dave, since the 'bias take' resistor in the common cathode tail is 10Ω, your tubes will be at dissipation max at a common cathode voltage of 0.6v (60mA for the two tubes). Measure the DC voltage at either cathode of the pair, and see what you get. Compare to the other pair. I would say that you'd like to see 0.5 Vdc or so on each side.

If they're pretty close, turn the amp off and use your ohmmeter to measure the DC resistance of each half of the primary on each side, and compare likewise. You may have some corrosion or something in one transformer that is raising its DCR and causing the same amount of current to heat it up. Another way to check for this is to test the DC voltage on the plates, as a high R winding will cause more voltage drop across the primary. The plates are pin 7.

An odd thing; there's a DC balance pot after the bias adjust so you can set the OP of each tube in the pair, but I didn't see any way to test for this; or a bias take jack either. AFAICS, you'll just have to take off the bottom plate and stick your hot probe on the right spot. The cathodes are pin 3.

Questions?

Poinz

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Re: One output transformer hotter than the other [message #9171 is a reply to message #9170]

Fri, 19 August 2005 09:34

hurdy_gurdyman
Messages: 416
Registered: May 2009

Illuminati (1st Degree)

Poinz,

Just before I read your post this morning I did some more tinkering. Pin three has been adjusted on each tube to read 0.5 VDC (was a bit higher.) All plates read the same at about 420 VDC (my meter isn't very fine at this high of a voltage.) I'm getting - 16 VDC at the grids (pin 2.) I believe somewhere I stated -32, but it seems I was looking at the wrong scale (my eyes aren't as fine as they used to be.) No DC off-set between pins 2 of a pair of tubes. I'm going to let it run awhile and see what happens. If the right channel is still hotter, I'm going to assume that it's because it is located between the other output tranny and the 5AR4 and power tranny. If the transformer has something inside it shorting a bit, there's not much I can do about it, so might as well run it. It's been going for about a year on this set of JJ/Tesla EL-84's with no problem, so perhaps I'm just being paranoid.

Dave

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