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Old 16th September 2003, 08:10 AM   #1
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Default Troubleshooting a PP EL34

Hi guys,

Yet another troubleshooting job from over at the AA. Diyaudio folk seem to be more helpfull when it comes to troubleshooting!

Tell me if this is bothering you.


The complaint :
About three years ago I build a valve amplifier, Canton Audio.

38 Watts RMS using EL34s push/pull and a passive pre amp, volume control.
The bais is set at 300 milli volts but can also increase up to 600 milli volts for Class A.
For some reason there is a drop off in the high frequency or lack off. The mid range is perfect, the bass is full and turn full.
Tubles: 12AX7, 12AT7, EL34s.

So what is the cause to my problem (s)?
Is the input tubes causeing this problem?
Can anyone advice?
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Old 16th September 2003, 08:11 AM   #2
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Default Schematic

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Old 16th September 2003, 08:12 AM   #3
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As this is a design from a manufacturer...could it indeed be the passive preamp? I.e. an impedance problem?
As one fellow posted at the AA?
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Old 16th September 2003, 09:37 AM   #4
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My suggestion would be to look at
1)input stage-cascode
2)feedback cap
3)output trans-Nothing can be done
Not to sure about ax7 working in a cascode style. normally every one roots for 6922. Should have tons of gains since it is a ax7 into a at7. Plus cascode is a very easy load to drive since miller capacitance is low. As i recall clearly when i used ax7 in a pre-amp high frequency was not that good. while at7 was rich sounding. Maybe too much of H2 but not enough H3 for the detail. Maybe to try to mod the upper cascode to 6922 for the H3.
Cascode do not make very good drivers as the top tube has high output impedance due tp the Rk is not bypass. Plus the loading resistor is 220k thus setting the output impedance to be very high.
Or wired up for triode mode, and remove the feedback or reduce the db and remove the frequency compensation cap in the feedback loop. Of course less power would be accomplish.
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Old 16th September 2003, 09:55 AM   #5
SHiFTY is offline SHiFTY  New Zealand
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If it has been used for 3 years, maybe the output tubes are getting weak?
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Old 16th September 2003, 10:23 AM   #6
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Default Measurements

It might be a good idea to check elsewhere if BOTH channels are behaving identically. As far as I know , when tubes deteriorate quickly , they don't do so at the same time.
So If both channels are identically down it must be something that is common to both channels and that leaves out the tubes and passive parts - except the power supply ! Then there are external sources , cables etc that will need to be checked. It is better to do a frequency sweep from the source and check all the way to see where the problem really is. Are the two tweeters blown in the speakers ? They usually die a soundless death !
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Old 16th September 2003, 12:11 PM   #7
SY is offline SY  United States
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Check frequency response of the amp with a scope and generator- the first thing you have to do is see if the thing that you THINK is a problem is the thing that actually IS a problem (remember SY's First Law?). If the frequency response is OK, look elsewhere. If it's not, it's almost surely the tubes causing the problem- if he is really running 600mV at the cathodes at times, he's banging those output tubes pretty hard.

(EDIT: Second thoughts- though I'm still guessing that the problem is elsewhere in the chain, Ashok's take on this is probably closer to correct than mine- check the rail voltages first. THEN check the frequency response.)
"You tell me whar a man gits his corn pone, en I'll tell you what his 'pinions is."
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