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Old 28th August 2011, 02:04 PM   #1
dzseki is offline dzseki  Hungary
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Default Restoring an old tube amp

Hi,

I am on restoring an old tube amplifier which I get for free.
This is a hungarian made PA line amp using two ECC83s and two EL34s (BEAG EA080). The amplifier came without tubes, and probably sat for decades.
Generally I know the theories of the tube amps, but I don't have much experience in practice.
First I want to make it work in the original state, however with it's 100V output it is not very useful, later I might want it to convert it to a guitar amp with new OT.
So far I reformed the HV filter capacitors, and got some NOS russian 6N2P tubes instead the ECC83s, and bought two lightly used EL34s (not pair, not even the same brand(a Mullard, and an RFT), but I thought they will be good for testing purposes -as you can see the bias can be set independently)
The preamplifier section is seemed to be working fine, but after placing in the output tubes the amplifier looks to be very unstable, the Mullard EL34's anode glowing up red (not immediately, but sooner or later always), I tryed to swap the tubes, if something wrong with the sockets, but always the mullard glowing up. The G1 is about -40V.
I have an other problem too, the amp is always oscillate if the volume knob is not turned all the way down. The OT has a monitor coil which gives 6V on 15ohm, now there is a 6 ohm speaker attached to it as load for the transformer.
Every help will be appreciated!
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Old 29th August 2011, 12:50 PM   #2
drewl is offline drewl  United States
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Guitar guys would kill for the Mullard and RFT EL34's.

You should stick with the 12AX7's (ECC83) it's a much more common tube used in 99% of tube guitar amps.
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Old 29th August 2011, 01:00 PM   #3
SY is offline SY  United States
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Replace the power supply electrolytics. Also all of the decoupling caps. If there are any paper caps in there, they may also have deteriorated. The switch should be thoroughly cleaned- same with the controls. Your input potentiometer could be dodgy, which would definitely make the amp unhappy.

Do you have any photos, especially of the innards?
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Old 29th August 2011, 07:48 PM   #4
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Quote:
Originally Posted by dzseki View Post
the Mullard EL34's anode glowing up red (not immediately, but sooner or later always),
This is your decoupling capacitors failing (leaking).
Replace every single small value capacitor with nice polypropylene ones, then it will be fine.

Then think about replacing all of the electrolytic ones. You can leave the old cans in - new ones are much much smaller!
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Old 29th August 2011, 08:22 PM   #5
dzseki is offline dzseki  Hungary
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Thanks guys for replying. It was the decoupling cap indeed.
Quote:
Guitar guys would kill for the Mullard and RFT EL34's.
probably but both are single tubes, fortunately this amp do not requires matced tubes.
Quote:
You should stick with the 12AX7's (ECC83) it's a much more common tube used in 99% of tube guitar amps.
Here in Hungary it is easier to get those old russian tubes than good ECC83s
Quote:
Replace every single small value capacitor with nice polypropylene ones, then it will be fine.
I did not have acces to shops in the weekend so I installed some NOS foil cap in ceramic envelope (probably oil cap?) so far it is working fine.
I am planning to leave as much original parts as I can, until it works safely.
Look at the pics, the amplifier was made in 1966, and please note the new (white) coupling caps
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File Type: jpg DSC00843.JPG (801.3 KB, 129 views)
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Old 31st August 2011, 06:13 PM   #6
dzseki is offline dzseki  Hungary
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As you can see the 840V B+ is produced by the serial connection of the two secondary coils of the power transformer. Should I use it by switching the coils parallell as reduced output power mode? what else should I modify to work?
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