Hey Guys, been a while since I've posted. I've had a Bernie amp sitting collecting dust for a 'no rush' job and I want to finally get it moving. The problem is there is no schematic for this amp and the wiring is completely confusing - neatly done, yes, but all the wiring is the same white colour and tracing any lines have been really infuriating.
Basically, this is a dual amp chassis built in the 80's - one PT, two OT's, two standby switches. Each side has 2x 12AX7 and 4x EL84. I think the basis is a Vox or Hiwatt design, but none of the schematics i've dug up have come close.
There is no output right now. Power On, standbys On, I measure about 500VDC on the output of the ss rectifier, BUT it doesn't jump up right away, it climbs up over about 30 seconds after switching on AC. Then, when I switch off either of the standbys the voltage falls to about 100+VDC. The EL84 plates read about 100+VDC volts too.
I've never had this problem with another amp so I'm stumped. Any ideas what to check out next?
Much appreciated!
Basically, this is a dual amp chassis built in the 80's - one PT, two OT's, two standby switches. Each side has 2x 12AX7 and 4x EL84. I think the basis is a Vox or Hiwatt design, but none of the schematics i've dug up have come close.
There is no output right now. Power On, standbys On, I measure about 500VDC on the output of the ss rectifier, BUT it doesn't jump up right away, it climbs up over about 30 seconds after switching on AC. Then, when I switch off either of the standbys the voltage falls to about 100+VDC. The EL84 plates read about 100+VDC volts too.
I've never had this problem with another amp so I'm stumped. Any ideas what to check out next?
Much appreciated!
Pull the output tubes, now what do the voltages do? And what kind of current is being drawn, and does pulling the tubes materially affect it?
Also, look from your rectifiers to the filter cap, is there a series resistor, and might it be really high in value.
This sounds like a stereo power amp, so the EL84 power tubes have a 12AX7s are an input buffer and a phase inverter. I can't say some schematic will be exactly your amp, but EL84 power amps are all darn similar. You need good B+ on screens and plates, and a reasonable bias voltage on the grid, unless it is cathode biased. All these can be checked without a schematic.
Also, look from your rectifiers to the filter cap, is there a series resistor, and might it be really high in value.
This sounds like a stereo power amp, so the EL84 power tubes have a 12AX7s are an input buffer and a phase inverter. I can't say some schematic will be exactly your amp, but EL84 power amps are all darn similar. You need good B+ on screens and plates, and a reasonable bias voltage on the grid, unless it is cathode biased. All these can be checked without a schematic.
Hi again Enzo!
I've pulled the output tubes and the voltage test is the same. Slow climb to 500V, then pulling either of the standbys brings it down to about 180V, or both together, down to 115V.
I removed the 12Ax7s as well and the voltage falls to 260V with both one or two standbys flipped off.
The whole amp is drawing 450mAac without output tubes, 420mA without any tubes. With all tubes installed the current settles at 600mA regardless of standbys.
I'm thinking this must be a PS issue because how can both sides of the amp be exhibiting the exact same problem...this should be an easy amp to diagnose because there are two of everything and if both sides are exhibiting the same issue, the power supply is the only thing left?
I've pulled the output tubes and the voltage test is the same. Slow climb to 500V, then pulling either of the standbys brings it down to about 180V, or both together, down to 115V.
I removed the 12Ax7s as well and the voltage falls to 260V with both one or two standbys flipped off.
The whole amp is drawing 450mAac without output tubes, 420mA without any tubes. With all tubes installed the current settles at 600mA regardless of standbys.
I'm thinking this must be a PS issue because how can both sides of the amp be exhibiting the exact same problem...this should be an easy amp to diagnose because there are two of everything and if both sides are exhibiting the same issue, the power supply is the only thing left?
OK, so the tubes do not substantially spike the current. From 450ma to 600ma equals about 18 watts more from the mains, which is probably mostly heaters.
Your high voltage crashes, so start at the start. What does the HV AC do before the rectifier? Does it stay up or crash as well? Is your rectifier a four diode bridge or a center tapped winding and two diodes? In any case is there an open CT connection or a bad ground at the bridge? When no current is demanded, a high series resistance won't drop much voltage, so the B+ can climb, but as soon as you load it it crashes.
Your high voltage crashes, so start at the start. What does the HV AC do before the rectifier? Does it stay up or crash as well? Is your rectifier a four diode bridge or a center tapped winding and two diodes? In any case is there an open CT connection or a bad ground at the bridge? When no current is demanded, a high series resistance won't drop much voltage, so the B+ can climb, but as soon as you load it it crashes.
Pre-rectifier AC does not crash. Standbys ON I have 370vac on each tap, switching off standbys bumps it up to around 380vac.
There are 2 diodes used for a full wave rectifier...meaning there should be a CT ground...
Here is the PT layout...I know its not 100% certain based on color alone, but here goes...
Primary has two black (120V) and one gn/yl stripped (ground). The secondary side has two green (6.3v), two red (380/380), yellow (ground via a 1.5A fuse), and brown is unused and taped up under the chassis and hidden by the bell cover. The filament circuit doesn't seem to have an artificial center tap... multimeter time:
Fuse out:
Red wires have a 15ohm resistance to the yellow.
Green wires have a 1M resistance to the yellow...
could there be another wire under the chassis and both center taps tied together?
There are 2 diodes used for a full wave rectifier...meaning there should be a CT ground...
Here is the PT layout...I know its not 100% certain based on color alone, but here goes...
Primary has two black (120V) and one gn/yl stripped (ground). The secondary side has two green (6.3v), two red (380/380), yellow (ground via a 1.5A fuse), and brown is unused and taped up under the chassis and hidden by the bell cover. The filament circuit doesn't seem to have an artificial center tap... multimeter time:
Fuse out:
Red wires have a 15ohm resistance to the yellow.
Green wires have a 1M resistance to the yellow...
could there be another wire under the chassis and both center taps tied together?
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