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
.