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Old 10th April 2011, 02:27 PM   #1
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Default troubleshooting a Mingda MC34AB

New to me amp. I am used to repairing ss gear, not familiar with tubes.
When I first turn on the amp, one of the 12AT7 tubes flashes like a light bulb, then dims down to about normal. Once the amp has been on and warmed up, I get a popping sound thru one of my speakers, looked at the tubes and the 4 EL34's that are on the same side as the 12AT7 that lights up are arcing.

I replaced the 12AT7 with a different one and it still does the exact same thing.

I swapped out the EL34 tubes to the other side - it still does the same thing.

Curious if anyone had any ideas on what I should be checking.
Thanks
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Old 10th April 2011, 03:35 PM   #2
M Gregg is offline M Gregg  United Kingdom
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Quote:
Originally Posted by donberry View Post
New to me amp. I am used to repairing ss gear, not familiar with tubes.
When I first turn on the amp, one of the 12AT7 tubes flashes like a light bulb, then dims down to about normal. Once the amp has been on and warmed up, I get a popping sound thru one of my speakers, looked at the tubes and the 4 EL34's that are on the same side as the 12AT7 that lights up are arcing.

I replaced the 12AT7 with a different one and it still does the exact same thing.

I swapped out the EL34 tubes to the other side - it still does the same thing.

Curious if anyone had any ideas on what I should be checking.
Thanks
Just a thought,

The first thing I would check is the voltage set to the correct voltage for your mains? It says it is switchable!

If its under waranty send it back before you loose it!

The EL34's arcing is very dangerous it can stuff the output Tx if it hasn't already! Also it can damage the EL34's.

You need to remember that you are talking high voltages and your amp may hold charge after power off. Test before you touch!

The first thing you need is a schematic, do you have one?

Then check the bias on the tubes. Do any of the tubes plates glow red? This is not the heater. the black metal plates inside the tubes.

The pre-amp / driver tube is unlikly to be the cause!
Could be B+ is to high.
It could be the connections on the tube grids are wrong.
It could be a bad coupling cap.

Is this a new amp or second hand?

The thing to do is compare the two sides of the amp (connections to the tubes) if the fault does not follow the tubes then its in the circuit on that side!

Hope this helps!

Regards
M. Gregg
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Last edited by M Gregg; 10th April 2011 at 03:53 PM.
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Old 10th April 2011, 09:18 PM   #3
DF96 is offline DF96  England
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Initial brightness from a 12AT7 (or other types) is not unusual and does no harm. It is common in valves based on Philips designs. However, worth checking that the heater voltage is correct.

I would concentrate on the other problems, which sound more serious.
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Old 10th April 2011, 11:58 PM   #4
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thanks for the information - is appreciated. Just kind of bummed. I tried to save a few pennies and bought a used one - I knew I should have bought new

Going to open her up tomorrow and check the bias and see if any caps have failed.
I do not have schematics, can't seem to find any. Will compare readings with the good side.
I have been told how to set the bias
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Old 11th April 2011, 10:43 PM   #5
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hmmmmm, On a whim, I tried it out as an integrated amp - there was still a feint "pop" sound in both speakers, but I played it for a good 15 minutes at fairly high volume and the EL34's did not arc........
so - as a power amp , one side arcs after about 5 minutes, as an integrated it does not......
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Old 12th April 2011, 10:07 AM   #6
DF96 is offline DF96  England
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Excuse my ignorance, but what exactly do you do to it to convert it between 'integrated' and 'power' modes?
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Old 12th April 2011, 10:42 AM   #7
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This amp apparently has different input. Presumably, the power amp input couples the signal directly to the phase inverter, while the integrated inputs run the signal through the preamp as well. I don't have a schematic though, so I'm going off what I could find using Google.
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Old 12th April 2011, 10:55 AM   #8
DF96 is offline DF96  England
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Difficult to see how that could affect the output stage, unless it also changes the feedback.
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Old 12th April 2011, 12:49 PM   #9
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Quote:
Excuse my ignorance, but what exactly do you do to it to convert it between 'integrated' and 'power' modes?
flip a switch - it is a power amp and an integrated amp
Click the image to open in full size.
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Old 12th April 2011, 03:43 PM   #10
M Gregg is offline M Gregg  United Kingdom
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Does it make any difference if you switch from UL to Triode?

Can you take a photo of the inside at a resonable size so we can have a close look? (Keeping to limits)

Or maybe a few covering each part.


Regards
M. Gregg
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