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In Reply to: Re: Problem solved! posted by RAYDOG on July 26, 2002 at 08:12:35:
RD,Mark the tube which you think is the culprit. Keep all the original tubes in reserve. This will help you, if you should need to isolate a tube gone South, in the future.
The JJ EL84 are pretty good. Tell me how those EH 12AU7, AT7 sound.
Good listening. (PS: you may need to re-bias the amp with those new JJ's installed).
Follow Ups:
The amp is self biasing. The owners manual just says to make sure you use "matched quads". Might there be something else I will need to do when installing the tubes? The manual gives no values for tube settings.
"Honey, that's the same amp we have always had, I swear!!"
RD,If you have access to the owner's manual, see if you can find how the amp idle bias is set.
You should find terms like FIXED BIAS, CATHODE BIASED, or AUTO-BIAS.
FIXED-BIAS is usually adjustable via a potentiometer (nutty, isn't it?). This is the amp which would need to be measured and possibly adjusted after you've placed the new JJ tubes into the unit.
CATHODE-BIASED usually has one or more resistors tied from the power tube's cathode to ground (a schematic would help). For an EL84 amp, the resistors I've seen are about 100-150 ohms per push-pull pair. The rating is normally around 5 watts, for this resistor. Plus a 25-50 mfd bypass cap(s), in parallel to the resistor(s).This amp is somewhat self-leveling and as long as the tubes you replace into the amp have somewhat similar characteristics (read: true EL84)---you won't need to change the cathode tied resistor and thus, the idle. For the JJ EL84---you shouldn't need to worry.
AUTO-BIAS means there is additional circuitry to adjust the idle bias. You should't need to change out anything or adjust anything.
Thanks to all of you guys for putting up with my dumb questions. I have Triangle Titus speakers on order and Mapleshade interconnects & speaker cable on order to go with the Anthem. Can't wait to hear them all together.
"Honey, that's the same amp we have always had, I swear!!"
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