I feel like an asshole -
the bias on my amp is getting 10 volts, somehow, and obviously the 50K bias pot doesn't have the scope to chop that down. The user manual says something about setting the bias to .6 volts for an EL34 tube. That's a whole lot of difference, and I'm guaranteeing is the source of what I've thought was a grounding issue for the last month; preamp issues for years prior to that.
Granted, all of this only started making sense recently.
I'm learning for the sole purpose of loving this amp forever.
-Having cut the bias PCB resistor (R28 schematic B) and testing it, it's fine.
+replaced R29 since it was cut
-bias diode tested ok
-bias pot tested ok
--BD3 was unresponsive, ruined. along withe three spares residing on the spare board. replacements ordered. F.
After staring at the bias board all day, just looking at it and making faces, I realized that after the brief space of time where the bias test points registered correctly (before the tube selector switch separated,) and I rebuilt said switch, I hard-shorted all three positions on the switch horizontally (where the switch contacts work vertically.) This, I admit, makes me feel like a total doofus.
+cut shorts on switch between EL34 contacts. Left one short due to ruined traces.
-accidenatlly soldered bias test points backward
+fixed bias test points
&I really want to try a set of 6L6 power tubes in this amp, but I've had so much sonic success with the EL34. I'll switch if AMT makes a solid state analogue.
test after this day of debacle sent me like 24 volts across the bias. I went to bed feeling defeated.
Sunrise
What I am now doing is drawing up a technical routing diagram for myself to figure out where each of the connector wires leads to, so I stop forgetting. Also just now again rebuilding the switch again.
Also found a forum post on the 60/30 watt triode/pentode version, where someone was experiencing a similar issue with the bias being out of scope. Amp bias measured 0.0 v with the power tubes removed. The final suggestion was to replace the 10 ohm resistors, even if they were testing properly, due to component execution via over-volts.
+Tore out the four 10 ohm resistors, though they tested fine for resistance (doesn't mean under load, as we learned from the posts) and swapped them.
*F1 on the bottom of the power PCB is a test point derived from the bias multi cable connector.
&got distracted and tested a bunch of points, including preamp socket pins with tubes in. Each preamp tube is routed differently but i'm getting significantly less resistance over combinations of pins 7, 8, and 9 on V1, as compared to V2 and V3. Visually brightens less quickly than the other two, and measurements are consistent when shuffling tubes.
Reassembled, bias tests with 0.00 volts. Unfortunately, never took measurements before surgery began. It measures correctly for the moment, amen.
tubes in, measures 14v. at least we're getting closer.
next step is shuffle tubes. still haven't received the test set.
next step is investigating the cathode resistor bypass capacitor.
shuffle and whatever I said before complete, still getting 14v across bias, basic assumption now is that it's the power valves.
Only game to play is wait for the test set to come in.
<3
the bias on my amp is getting 10 volts, somehow, and obviously the 50K bias pot doesn't have the scope to chop that down. The user manual says something about setting the bias to .6 volts for an EL34 tube. That's a whole lot of difference, and I'm guaranteeing is the source of what I've thought was a grounding issue for the last month; preamp issues for years prior to that.
Granted, all of this only started making sense recently.
I'm learning for the sole purpose of loving this amp forever.
-Having cut the bias PCB resistor (R28 schematic B) and testing it, it's fine.
+replaced R29 since it was cut
-bias diode tested ok
-bias pot tested ok
--BD3 was unresponsive, ruined. along withe three spares residing on the spare board. replacements ordered. F.
After staring at the bias board all day, just looking at it and making faces, I realized that after the brief space of time where the bias test points registered correctly (before the tube selector switch separated,) and I rebuilt said switch, I hard-shorted all three positions on the switch horizontally (where the switch contacts work vertically.) This, I admit, makes me feel like a total doofus.
+cut shorts on switch between EL34 contacts. Left one short due to ruined traces.
-accidenatlly soldered bias test points backward
+fixed bias test points
&I really want to try a set of 6L6 power tubes in this amp, but I've had so much sonic success with the EL34. I'll switch if AMT makes a solid state analogue.
test after this day of debacle sent me like 24 volts across the bias. I went to bed feeling defeated.
Sunrise
What I am now doing is drawing up a technical routing diagram for myself to figure out where each of the connector wires leads to, so I stop forgetting. Also just now again rebuilding the switch again.
Also found a forum post on the 60/30 watt triode/pentode version, where someone was experiencing a similar issue with the bias being out of scope. Amp bias measured 0.0 v with the power tubes removed. The final suggestion was to replace the 10 ohm resistors, even if they were testing properly, due to component execution via over-volts.
+Tore out the four 10 ohm resistors, though they tested fine for resistance (doesn't mean under load, as we learned from the posts) and swapped them.
*F1 on the bottom of the power PCB is a test point derived from the bias multi cable connector.
&got distracted and tested a bunch of points, including preamp socket pins with tubes in. Each preamp tube is routed differently but i'm getting significantly less resistance over combinations of pins 7, 8, and 9 on V1, as compared to V2 and V3. Visually brightens less quickly than the other two, and measurements are consistent when shuffling tubes.
Reassembled, bias tests with 0.00 volts. Unfortunately, never took measurements before surgery began. It measures correctly for the moment, amen.
tubes in, measures 14v. at least we're getting closer.
next step is shuffle tubes. still haven't received the test set.
next step is investigating the cathode resistor bypass capacitor.
shuffle and whatever I said before complete, still getting 14v across bias, basic assumption now is that it's the power valves.
Only game to play is wait for the test set to come in.
<3
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