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Old 3rd December 2006, 01:27 PM   #1
adason is offline adason  United States
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Default vintage tube amp repair help

Hi everyone,

I have obtained this little vintage amp on ebay and would like to fix it.
As you can see its very old amp, which by the way uses no power trafo, its hooked strait to mains. Not very safe, I know, I will use isolating trafo in the future, for now its hooked to variac.

Anyway, I replaced power supply capacitors and put connectors on input. One chanel is loud and clear, the other is quiet and distorted. I swapped tubes between chanels and measured all the pasive parts, do not see any difference between the chanels.
I need your suggestions, please help me fix this amp. It seems simple task, when one chanel is working and other not, but I got no idea what to do and been stuck like this.

It uses 12AX7 as preamp and splitter tubes and 25L6 on output.

Here is picture.
Thanks,
ed
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Old 3rd December 2006, 02:38 PM   #2
Zibi is offline Zibi  Poland
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Check all solder joints.
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Old 3rd December 2006, 03:01 PM   #3
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You might check the output transformers to see if a winding is shorted or open, and if possible, wire the working side's OPT to the non-working channel to see if that works.

Taking voltage measurements from 0 to plates and cathodes on all tubes, as well as other voltages, might help people here to diagnose.

You probably know this already, but most variacs are autoformers and do NOT isolate. Be very careful and drain all your caps as well after you unplug.

--Jeff
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Old 3rd December 2006, 03:27 PM   #4
adason is offline adason  United States
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hi Zibi, I have inspected all the solder joints and wires visualy, maybe I should resolder bad looking ones.

Jeff, I believe output trafnsformers are good, both measure the same resistance on primary and secondary for both chanels. I compared resistances on both chanes, I see no difference.

Yes, I know, my variac is autoformer too.

I am going to measure voltages.

ed
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Old 3rd December 2006, 04:08 PM   #5
Zibi is offline Zibi  Poland
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Quote:
Originally posted by adason
hi Zibi, I have inspected all the solder joints and wires visualy, maybe I should resolder bad looking ones.
ed
It is a good idea. Check also volume potentiometer and coupling capacitors.
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Old 3rd December 2006, 04:56 PM   #6
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It doesn't look like that amp has a power transformer. If it doesn't, I suggest slapping a cheap 1:1 isolation transformer to greatly reduce your risk of frying yourself in many unpleasant ways.
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Old 3rd December 2006, 05:28 PM   #7
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Don't plug anything into it except for a battery operated CD player or similar until you have an isolation transformer. The circuit is probably isolated from the chassis... but only by a 50-year-old capacitor, which is probably leaky... as are the remaining capacitors.

If there's a model number, I'll check the Sams index and see if I can turn up a schematic.
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Old 3rd December 2006, 05:47 PM   #8
adason is offline adason  United States
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success! sweet success!

yes, yes, it was a capacitor, not in the power but the one from splitter to output tube

first I measured all the voltages and there were significant differences between the chanels

than I noticed that one big mp capacitor has a crack in its cover.
I unsoldered it and while its capacity was still ok, 68 nF, it was partly shorted, 150k ohm...

so I replaced it, and the chanel works!

Thanks all for your help!

I am sooner or later going to replace all the capacitors, even they are nonelectrolyts.

Than I am going to draw the schematics, maybe we can tweek it. Its playing both chanels happily now. Thanks,
ed
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Old 3rd December 2006, 05:51 PM   #9
adason is offline adason  United States
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Hi Tom,

the only information on the chasis is number: 3985-01A, 173946.
It does not take much to draw the schematics, I will do it.

ed
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Old 3rd December 2006, 09:52 PM   #10
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Adason,

The 25L6 draws 300 mA. of heater current. In a series heater string, like your amp uses, all heaters must draw the same current. That means the 12AX7 heaters are set up for 6.3 V./300 mA. So, the total requirement of the series string is 112.6 V. The higher "average" line voltages encountered today can be putting a stress on the tubes. A 24 Ohm/5 W. metal oxide resistor, that costs about $0.50, added to the series string will make the total voltage requirement 119.8, which (IMO) is better.

I'm assuming the amp's B+ is 1/2 wave Selenium rectified. Selenium rectifiers are ticking TOXIC time bombs, that must be replaced by modern Silicon parts. Adding an isolation trafo to the amp makes things MUCH safer and it gives you the option to full wave bridge rectify the B+. A "typical" operating point for PP 25L6s shows 25 mA. idle current per tube. A Triad N-68X isolation trafo, which costs about $11.25, will easily provide the B+ current 2X 12AX7s and 4X 25L6s draw. You are safe enough with the series heater string connected directly to the AC mains, as long as a DPST power switch (1 leg for heaters and 1 leg for B+) is used. Separate fuses for each switch leg are required.

Following Tom Bavis' recommendation of using only a battery powered portable CDP as the source until an isolation tafo is installed is MANDATORY. Until the iso trafo is installed, connecting to an AC mains powered source is DEADLY DANGEROUS.
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