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In Reply to: RE: Here's what I found so far posted by airtime on April 18, 2014 at 08:25:55
Look at your output tubes and see if any are glowing and did you change them..The output tubes were pretty old and one or two may have failed.
Make sure the slide switch on the back is set in the normal position and not the test position..
Honest amplification is better than excessive 2nd order distortion anytime.
Edits: 04/18/14Follow Ups:
tubes are still the same and the switch is set to normal. I'll check tonight if any are glowing abnormally.
If the outputs went, wouldn't the sound have hum and not a low level distortion?
charles
If the outputs went, wouldn't the sound have hum and not a low level distortion?If one of the output tubes let go,you will get less than half of your power.Those tubes were quite old you had but they sounded good.I called and left you a message.
Honest amplification is better than excessive 2nd order distortion anytime.
Edits: 04/20/14
So they don't go with out a scream or anything nasty? Gee, I was expecting more from a blown output tube.
Can I visually see if one is dead?
charles
PS, I'll try and call tomorrow. I do appreciate your concern. Don't worry - old tubes do what old tubes do. Hey, they're ONLY 60 years old!
A tube can fail gracefully and just stop conducting. Better that than a short, by far. I have a pair of ASL Wave-8's that were playing happily one day. I heard a small, delicate "clink!" and one channel started doing exactly what you describe. The envelope had cracked, and it was obvious which tube it was because the getter was gone.You might have a set of 100-year tubes, but stats catch up to us all.
"LOL Those tubes should last a 100 years.Call me when you can."
ETA: 100 years x 365 days a year x 5 hours of operation a day is 182,500 hours. What tubes are those? Impressive life span, that.
Edits: 04/18/14 04/18/14 04/18/14 04/18/14 04/19/14 04/19/14 04/19/14
LOL
Those tubes should last a 100 years.Call me when you can.
Honest amplification is better than excessive 2nd order distortion anytime.
Since airtime seems to have solved the problem, I'll risk hijacking this thread with a question:
I'm measuring -18.5 volts at the grid of OP tubes, pin 2. This is after raising the value (resistance) of the bias supply R from 4.7Kohm to 5.6Kohm. Seems like I should be around -15V?
Also what current in mA should each 7189 draw? Can this be measured (the pair) at the 12ohm cathode resistors?
Peter
(as you can see, i made a little circuit board for the bias supply and phono filament circuit.)
Pete
You did a very nice job and I like your bias supply.I usually keep it at minus 16.5 to minis 18v..Minus 15 is way too low for this amp and the 7189s would be on fire.. Vg2 is higher due to the ultra linear transformer where as wired in pentode, Vg2 would be lowered to about 300v.
Leave it where you have it..This amp will produce about 22wpc with 7189s.
Honest amplification is better than excessive 2nd order distortion anytime.
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