On 17/01/2016 7:02 AM, John Heath wrote:
On Saturday, January 16, 2016 at 2:05:19 PM UTC-5, Trevor Wilson
wrote:
On 15/01/2016 1:52 PM, John Heath wrote:
On Tuesday, January 12, 2016 at 9:51:45 PM UTC-5, Trevor Wilson
wrote:
I have one of the above-mentioned amps on the bench. Pretty
straightforward - 3 X EL84 output valves have air inside the
valve. The plate on one valve appears to have suffered
somewhat, but I don't expect any serious problems apart from
that. However, my main questions a
* Should I match the output valves (more accurately, buy a
matched quad from Evatco)? Cost = AUS$100.00/set. Tung-Sol,
Genalex (I have an aversion to Chinese valves, even though they
are cheaper), Electro-Harmonix, JJ, etc. * My other supplier
has no matched sets (and I don't want to buy a crap-load so I
can match them), but sells two brands: Sovtek $13.75 each or
Electro-Harmonix $17.05.
Although cost is not an over-riding concern, the time to ship
the Evatco valves could be. My other supplier can ship today.
Thoughts appreciated. Particularly from PA.
-- Trevor Wilson www.rageaudio.com.au
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The plate on one valve appears to have suffered somewhat !!
Here is the schematic
http://www.blueguitar.org/new/schem/peavey/c30schem.gif
You could have a leaky grid capacitor C32 or C35 .0022 uf at 400
volts if the plate appears to be over heated. They are cheap so
just change them both. Also tube amps are still popular so a
music sort with a repair shop will usually have a tube tester. He
can match up the output tubes for gain , emission and check if
gassy. If you feel uncomfortable repairing this amp a local tech
in a music store can do all this for you. Do not go to the TV
repair man. He is qualified but it is unlikely he will have a
tube tester. You want the repair tech in a music store as he will
have a tube tester and has been there done that many times and
will know how to match output tubes and change coupling
condensers if necessary.
**Thanks for the tips, but the job was completed a couple of days
ago. A few points:
* I have a valve tester (AVO VCM163). * The initial fault (AFAICT)
was due to the owner transporting the foot pedal inside the
enclosure. At some time, the foot pedal made contact with the
nipple on one EL84, thus allowing the ingress of air. Amp was
switched on and two other valves failed soon after. * I am quite
comfortable servicing valve amps, but, due to the output stage
arrangement, I was curious about the importance of matched output
valves. * Control grid Voltages were within spec, as were all other
Voltages (though plates were a little higher than the schematic
indicated, due to higher local mains supply (240VAC). * The local
Peavey agent supplied a schematic and board layout. * The screen
supply had also failed.
-- Trevor Wilson www.rageaudio.com.au
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I do not see feedback from output tubes to split phase tube as can be
seen on the diagram. With solid state amps there is feedback from
output to input so matching gain is a non issue. However in your case
of a tube amp without the benefit of feedback to correct for this
leaves you vulnerable to the gain of the output tubes. Then again
soft distortion from the limits of tube amplifiers is the charm of
having tube amps and the reason they are still sold today. Play and
enjoy as perfection is not the goal of music.
Just check now and then that the output plated are not glowing red
hot. If this is the case then coupling condensers C32 and C35 are
suspect and could smoke those expensive output tubes. Do not mean to
be pessimistic or rain on your parade but when you are in the service
business all electronic equipment is junk as I only see it when it is
broken not working. Your own experience in service will confirm this.
Eye ball the tubes now and then to make sure the plates are not red
hot in the same way you check the oil now and then in your car.
Speaking of this I need to get off the net and buy some antifreeze
.
**Since the amp is used professionally, I ran it under a test condition
which was somewhat more rigorous and lengthy than normal. Following on
from PA's suggestions, I fitted some 1 Ohm cathode resistors to each
output valve and monitors the cathode current. It was within 10% for all
valves. I consider that to be fine. The major problem with this amp (as
with most of it's type) will be the owner. If she transports the foot
pedal inside the amp again, then there is a high degree of risk of
further damage.
--
Trevor Wilson
www.rageaudio.com.au
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