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Old 5th August 2012, 06:19 PM   #1
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Default Very Strange Preamp Issue

My All-In-One Octal Aikido preamp was acting up today... the right channel lost gain. I removed the preamp to confirm that was the problem, it was.

I checked all the input wiring, selector switch and volume controls, and bypassed them by connecting the RCA jack directly to the input on the PCB to be sure they were not the issue. No problem with the wiring.

I moved both input and output tubes right to left. The problem did not change channels so I do not believe the problem is in the tubes.

Here is the strange part. The difference in gain between channels only occurs when B+ is on. I noticed this when turning the preamp off, I have separate heater and B+ toggle switches, and I only turned the B+ off leaving the heaters running. When B+ was switched off the gain between channels evened out and the soundstage centered like it should. Then the PS caps discharged and noise increased, so I flipped B+ on again, and sure enough the gain in the right channel reduced again, collapsing the soundstage off to the left.

So I have repeated this a few times now to confirm... when I flip off B+ I get about 60 seconds of perfect sound, flip on B+ and gain on the right channel becomes less than the left.

Unfortunately, my meter can only read DC volts and ohms, so I cannot check capacitance, though I'd guess a PS capacitor has possibly failed. Any advise would be very welcome!
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Old 5th August 2012, 06:40 PM   #2
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Quote:
Originally Posted by Davec113 View Post
My All-In-One Octal Aikido preamp was acting up today... the right channel lost gain. I removed the preamp to confirm that was the problem, it was.

I checked all the input wiring, selector switch and volume controls, and bypassed them by connecting the RCA jack directly to the input on the PCB to be sure they were not the issue. No problem with the wiring.

I moved both input and output tubes right to left. The problem did not change channels so I do not believe the problem is in the tubes.

Here is the strange part. The difference in gain between channels only occurs when B+ is on. I noticed this when turning the preamp off, I have separate heater and B+ toggle switches, and I only turned the B+ off leaving the heaters running. When B+ was switched off the gain between channels evened out and the soundstage centered like it should. Then the PS caps discharged and noise increased, so I flipped B+ on again, and sure enough the gain in the right channel reduced again, collapsing the soundstage off to the left.

So I have repeated this a few times now to confirm... when I flip off B+ I get about 60 seconds of perfect sound, flip on B+ and gain on the right channel becomes less than the left.

Unfortunately, my meter can only read DC volts and ohms, so I cannot check capacitance, though I'd guess a PS capacitor has possibly failed. Any advise would be very welcome!
There's nothing strange.
bad plate resistor, bad cathode resistor, etc...
get in there w/ your ohm meter.
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Old 5th August 2012, 06:40 PM   #3
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The first thing I would do is to use your DC voltmeter to check voltages on both channels and compare them. The bad channel should show itself this way. And since one channel is good, and the PS is common to both, it's not the PS.
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Last edited by HollowState; 5th August 2012 at 06:48 PM.
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Old 5th August 2012, 06:59 PM   #4
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Originally Posted by HollowState View Post
The first thing I would do is to use your DC voltmeter to check voltages on both channels and compare them. The bad channel should show itself this way. And since one channel is good, and the PS is common to both, it's not the PS.
There are separate RC filters for the L and R channels in the last stage of the PS in an Aikido. I am thinking maybe it's the final PS cap in the R channel?

What seems strange to me is that the difference in gain goes away when the B+ is switched off.

I will measure the plate and cathode resistors and check voltages across them...
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Old 5th August 2012, 07:04 PM   #5
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Originally Posted by Davec113 View Post
There are separate RC filters for the L and R channels in the last stage of the PS in an Aikido. I am thinking maybe it's the final PS cap in the R channel?

What seems strange to me is that the difference in gain goes away when the B+ is switched off.

I will measure the plate and cathode resistors and check voltages across them...
The tube will pass signal from cathodic emission only.
The heater is on, and the grid will pass signal to the plate. No B+ required.
However as you mentioned there is no gain.
This indicates a bad plate resistor.
If the plate resistor blew, the tube may be drawing too much current, but still working.
(I always use 1W metal film or larger for this)
OR the plate resistor may have just failed from constant on / off cycles...
YES a tube passes signal, with just the heater turned on.
Many people do not realize this, and it seem weird, but nothing weird about it.

Last edited by soundguruman; 5th August 2012 at 07:11 PM.
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Old 5th August 2012, 07:34 PM   #6
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Measurements below. All resistances check out ok. I don't understand what is causing the huge voltage drop on the R channel. Still thinking the 200uF cap in the final RC filter, but I don't think this would cause a voltage drop?

The plate resistors are 12W Mills wirewound. Yes, huge overkill and they still measure 2k.

B+ Before final RC 341V
B+ After RC Left 317V
B+ After RC R 312V

R12 (2k) plate resistor L: 22.5V R: 27.5V
R3+4 (680r) input tube cathode L: 3.37V R: 1.7V
R6 (330r) output tube cathode L: 3.6V R: 1.9V
R7 (330r) output tube cathode L: 3.0V R: 1.5V

Last edited by Davec113; 5th August 2012 at 07:42 PM.
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Old 5th August 2012, 07:48 PM   #7
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Bad contact in the valve socket? I had exactly this issue once with an aikido (it was a full aikido octal kit bought from broskie that I bought from another member here). The sockets looked shiny and new but needed tweaking with a pliers to close the contacts.


Might be worth trying?


Fran
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Old 5th August 2012, 07:51 PM   #8
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Originally Posted by woodturner-fran View Post
Bad contact in the valve socket? I had exactly this issue once with an aikido (it was a full aikido octal kit bought from broskie that I bought from another member here). The sockets looked shiny and new but needed tweaking with a pliers to close the contacts.


Might be worth trying?


Fran
It has worked fine for a few weeks without the tubes being changed out... when the problem first happened the tubes hadn't been touched in at least a week.
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Old 5th August 2012, 08:13 PM   #9
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2k seems small for a plate resistor, but that's beside the point at the moment. Your measurements do show reduced current through one tube but how significant that is I don't know.

Best of luck...
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Old 5th August 2012, 08:44 PM   #10
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I also took measurements at the tube sockets, it isn't good.

Somehow the B+ isn't getting to the R tube's plates, the left side's plate voltage is 315 while the right side is getting about half that.

OH... I think I just figured out that the last PS cap must be shorting B+ to ground in the R channel. Going to check resistance across these caps right now...
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