Sammas
Well-known member
I have a matchless lightning clone that is giving me some trouble.
Right in the middle of the most blistering rendition of stairway to heaven the output volume dropped off almost complete - During the rehearsal, I couldn't actually hear anything coming out of the amp, but when I brought it home and tried it again there is a very faint signal still present when the volumes are cranked.
The preamp gain and tone still work. I am almost positive this is a problem related to the output stage (2x EL84 for 15watt...).
I am measuring 370v (+/- 4 volts) out of the GZ34 rectifier. I am getting about the same on the primary center tap of the transformer, with a few volts lower on the plates of the EL84's. The screens are really low though. About 215v with a set of Electro Harmonix valves, and about 235v with a set of JJ's.
The EH tubes were purchased new to figure out what the heck is going on.
I replaced all the 22k resistors in the power section. Both of the 100ohm resistors that split the B+ voltage to each EL84 screen measure spot on....
I am baffled. Without the tubes in, all the voltages jump... with the screens sitting about 30v below the plates.
What the heck is going on? Is there a chance the new tubes and the old tubes are both duds?
Or has something more sinister happened? Like perhaps the old tubes were on their way out, caused some parasitic oscillation outside the audible range and cooked the output transformer (it doesn't look or smell cooked)... or perhaps a shorted turn on the secondary? Would alterations to the load on the plates of the output valves adversely affect the screen voltages?
Right in the middle of the most blistering rendition of stairway to heaven the output volume dropped off almost complete - During the rehearsal, I couldn't actually hear anything coming out of the amp, but when I brought it home and tried it again there is a very faint signal still present when the volumes are cranked.
The preamp gain and tone still work. I am almost positive this is a problem related to the output stage (2x EL84 for 15watt...).
I am measuring 370v (+/- 4 volts) out of the GZ34 rectifier. I am getting about the same on the primary center tap of the transformer, with a few volts lower on the plates of the EL84's. The screens are really low though. About 215v with a set of Electro Harmonix valves, and about 235v with a set of JJ's.
The EH tubes were purchased new to figure out what the heck is going on.
I replaced all the 22k resistors in the power section. Both of the 100ohm resistors that split the B+ voltage to each EL84 screen measure spot on....
I am baffled. Without the tubes in, all the voltages jump... with the screens sitting about 30v below the plates.
What the heck is going on? Is there a chance the new tubes and the old tubes are both duds?
Or has something more sinister happened? Like perhaps the old tubes were on their way out, caused some parasitic oscillation outside the audible range and cooked the output transformer (it doesn't look or smell cooked)... or perhaps a shorted turn on the secondary? Would alterations to the load on the plates of the output valves adversely affect the screen voltages?
