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Old 8th November 2003, 07:18 PM   #1
JDeV is offline JDeV  South Africa
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Default 6072A Line-Amp

(Follow up on this - http://www.diyaudio.com/forums/showt...559#post242559 )

I just completed this one and got some final measurements and some questions. I used a nice solid Rhodesian Rose Wood for chassis with stainless steel top and copper cladding on inside. Alps Blue pot and Holco resistors. Input and output wires are some left overs from "Van Der Hull" and "Nordost" interconnects. Main trafo and separate heater PSU trafo are in separate box , connected with 8 core umbilical to pre-amp box. Still using mixed brand 6072's , but are very pleased with sound.


1) Except for lefthand 20H choke - still old one - all other chokes and trafos are new, but I still get low frequency oscillation on speakers when using 6072 with Mullard 5-20 - why? But no problem with SS amp
2) How can I fix this problem ?

Here is my voltages and other notes and some pics:
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Old 8th November 2003, 07:20 PM   #2
JDeV is offline JDeV  South Africa
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Top:
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Old 8th November 2003, 07:23 PM   #3
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Inside:
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Old 8th November 2003, 07:24 PM   #4
JDeV is offline JDeV  South Africa
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More inside:
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Old 8th November 2003, 07:25 PM   #5
JDeV is offline JDeV  South Africa
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Rear - with 8 pin connector:
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Old 8th November 2003, 09:57 PM   #6
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Hi,

Quote:
but I still get low frequency oscillation on speakers when using 6072 with Mullard 5-20 - why? But no problem with SS amp
Do I read this correctly?
The preamp + Mullard amps goes into LF oscillation but the same preamp and SS amps works fine?

To my logic I'd then conclude something's wrong with the Mullard amp but obviusly you don't seems to think so...

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Old 8th November 2003, 11:47 PM   #7
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Default Motorboating?

You'd be surprised, but at low levels many valve amplifiers have lower f-3dB points than their transistor counterparts. The problem you describe sounds like motorboating, but that seems unlikely in your pre-amplifier circuit, so what frequency (estimated) are we talking about? 1Hz-2Hz?. The only thing I can presently see that looks odd is that you are using raw HT to set the elevation of your heater voltage, rather than filtered HT. I can't see how this could cause motorboating, but it's definitely not a good idea...

Edit: I've just taken a closer look at your photographs, and although I'm delighted to see a variac lurking in the background for a gentle start-up, I'm not so happy with your layout. Why are those two signal valves so far apart, and why are their associated components so far away? Audio components belong on the valve bases. Power supply stuff belongs further away. I know you have DC heaters, but the twist and layout of your heater wiring is not good.
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Old 9th November 2003, 08:57 AM   #8
JDeV is offline JDeV  South Africa
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Quote:
Do I read this correctly? The preamp + Mullard amps goes into LF oscillation but the same preamp and SS amps works fine? To my logic I'd then conclude something's wrong with the Mullard amp but obviusly you don't seems to think so...
No, not that I don't think so, it is quit possible. I just don't understand why it happens and how to solve it. Because if I use SS preamp with Mullard, all behaves very well.


Quote:
Edit: I've just taken a closer look at your photographs, and although I'm delighted to see a variac lurking in the background for a gentle start-up, I'm not so happy with your layout. Why are those two signal valves so far apart, and why are their associated components so far away? Audio components belong on the valve bases. Power supply stuff belongs further away. I know you have DC heaters, but the twist and layout of your heater wiring is not good.
All audio related components are on valve bases (pic.2) , other stuff down in box (pic.1) is for DC heater supply. It is difficult to measure, but oscillation is between 1Hz and 5Hz. It varies all the time. The 2 signal valves are for left and right channel. I used both sides of 6072A / channel . Is it importand that L and R must also be very close together? The twist in wire is only to keep them together, not really for any other reason. I can untwist and use cable ties instead if it will be better.

(2 pics in 1)
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File Type: jpg comps.jpg (45.5 KB, 787 views)
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Old 9th November 2003, 11:37 AM   #9
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Well, you should do the obvious first - have PS decoupling close to the valves; keep input and output wiring well separated; follow a sensible star-earthing scheme - have separate earths for PS and amp and join them in one point. Sorry if you've already done this - i really hate staring at photos.
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Old 9th November 2003, 11:43 AM   #10
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It's not important to keep channels close together per se, but it keeps the earth wiring to the common power supply under control.

Assuming you have access to an oscilloscope:

Is the oscillation leaving the pre-amplifier? (DC couple the oscilloscope)
Is the heater supply stable?
Is the HT supply stable? (AC couple the oscilloscope)

As analog_sa says power supply decoupling close to the valves and star earthing usually avoids most problems.
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