Fender Twin Amp High Mains Current draw in Low Power mode

[nevetslab]

I had received a Fender Twin Amp from a steady client about a week and a half ago, which had been red-plating. I didn't find that happening while I had it on the bench, though there was evidence of it having cooked two of the four tubes, and only two of the four were still workable tubes, from a reasonable plate current match standpoint. Not having a matched quad set of 6L6GC tubes on hand, I ordered that set, which came in last Friday. When I installed them Monday before the client came to pick it and an Ampeg SVT-AV up, it was suddenly pulling loads of AC mains current in the Low Power mode, so I wasn't able to let it go until I got to the bottom of that.

Before I opened it up yesterday, I had pulled up the Service manual for it, and in their Tech Notes in the manual, Tech Note TN96-3, one of the known issues with it is from the failure of that Output Power Switch arcing internally as a result of continual switching power levels when the amp is in Operate mode (not in Standby). DPST toggle switch. One set of switch contacts changes the HT supply to the O/T Center Tap, while the other set changes the bias voltage from -52V to -20V (approximately). The fun begins when the HT contacts arc, and no longer reduce the HT voltage from 455VDC to 230VDC in the Low Power mode.

That seemed to be what I was getting, while it was fine in the High Power mode. After cutting off the fixed leads of the Cutler-Hammer DPST switch, and re-terminating the leads for the replacement DPST switch, I ground off the two rivets holding the switch together, having already left enough lead length to strip and test with the ohmmeter. My Fluke 8060A DMM has a Conductance Mode, that being the reciprocal of ohms, allowing VERY HIGH resistances to be measured (though not as high as an Electrometer). My 'low current' measurements indicated it was reading in excess of 10,000 Mohms, though measuring between the two sets of contacts, there was a reading, which there wasn't in the normal Ohms Mode (which will read up to 300Mohms). After I opened the switch for inspection, sure enough there was evidence of arcing between the terminals used on the HT side.

I didn't find any of the parts in the power supply circuit that appeared damaged, and all sounded/measured fine again with the new switch. I will confess having switched the Twin Amp and other amps having an Output Power switch while in Operate mode, though normally, I'd only change it while in Standby mode. Odd there's no warning posted around the switch regarding that distinction.

Tech Note TN96-3.pdf
Twin-Amp-1995-2001-Schematic-PCB Layout.pdf

[Helmholtz]

An arc provides a low resistance current path. Other than an AC arc a DC arc once ignited will persist until the contacts (electrodes) have burnt back wide enough or the current supply is interrupted.

[g1]

I'd like to know what the secret is of the new type switch to get around the problem.

[Helmholtz]

I'd be interested in knowing the mains currents at idle in high and low power modes.

[nevetslab]

I'd be interested in knowing the mains currents at idle in high and low power modes.

@ 120VAC, the power consumption for Hi Power Mode @ idle was 140W & Lo Power mode @ idle was 100W. I was on the Valhalla 2101 Power Analyzer, and didn't stop to check the AC Mains current (have to switch between Line Voltage and Current Consumption on one of the readouts), while my Magtrol 1400B always shows all three.

[nevetslab]

I'd like to know what the secret is of the new type switch to get around the problem.

So would I. This amp WAS the new type.....Cutler-Hammer. I didn't look up the part number. The two sets of leads were welded to the contacts, and the actuator moved an insulated contactor-roller between making or breaking contacts. The parts are now in my trash box, but still in there, amidst the rest of the accumulating deposits. I might be able to find them all and do a re-construction. I'll have to look and see.

[Helmholtz]

@ 120VAC, the power consumption for Hi Power Mode @ idle was 140W & Lo Power mode @ idle was 100W. I was on the Valhalla 2101 Power Analyzer, and didn't stop to check the AC Mains current (have to switch between Line Voltage and Current Consumption on one of the readouts), while my Magtrol 1400B always shows all three.

Thanks, but it's actually mains currents I am interested in.
Reason is that I think that in low power mode the amp uses half-wave rectification. This should increase the magnetizing/exciting component of the primary current. As this part of current is essentially reactive (90° out-of-phase) wrt the mains voltage, it won't show in real power.

[g1]

Reason is that I think that in low power mode the amp uses half-wave rectification.

I'm confused. The schematic is in the first post. From what I see, the top of C38/C39 stays the same regardless of the half power switch (as evidenced at C44 above CR5). In low power, the B+ just comes from the C38/C39 junction, through CR15.

[Helmholtz]

From what I see, the top of C38/C39 stays the same regardless of the half power switch (as evidenced at C44 above CR5). In low power, the B+ just comes from the C38/C39 junction, through CR15.

Maybe I am missing something, but here is how I see it:

In low power mode the power amp supply current is taken from the junction between C38/C39. As caps can't carry net DC current, the load current must now be supplied via the CT of the HV winding. That essentially leaves the CR8 for rectification.
Hence half-way rectification and net DC in the PT.

Edit: Seems I was wrong. I missed the second current path through CR7 which makes it a two-phase (non bridge) rectifier.

[nevetslab]

Maybe I am missing something, but here is how I see it:

In low power mode the power amp supply current is taken from the junction between C38/C39. As caps can't carry net DC current, the load current must now be supplied via the CT of the HV winding. That essentially leaves the CR8 for rectification.
Hence half-way rectification and net DC in the PT.

Edit: Seems I was wrong. I missed the second current path through CR7 which makes it a two-phase (non bridge) rectifier.

.....with the load current to the CT of the O/T thru CR15. The client already has the Twin Amp, so I'm not presently able to get the AC Mains Current reading in the two modes, until I get one from our rental inventory over here to check.

As you have a better understanding of the power supply circuit in this amp.....what is the purpose and nature of the diode network CR33 thru CR38, sitting between the junctions of C38/C39 & C40/C41, with C51 across CR15?

[Helmholtz]

what is the purpose and nature of the diode network CR33 thru CR38, sitting between the junctions of C38/C39 & C40/C41, with C51 across CR15?

Good question!

I also wondered about the purpose. The diode bridge with the zeners seems to be a bidirectional clamping network that prevents the voltage difference between its left and right connections from exceeding +/- 95.4V. Not sure why it's necessary. It might be (part of) a snubber for L1 to limit flyback voltage when switching between high and low power modes but seems quite elaborate. Maybe someone has a better explanation for the need to clamp the voltages.

[g1]

They call it out as 'crowbar protection', for the screen node caps C40 & C41? pg.12 of attached service manual.

[nevetslab]

They call it out as 'crowbar protection', for the screen node caps C40 & C41? pg.12 of attached service manual.

Good eyeballs! This Service manual, unlike so many from Fender, DOES have Tech Notes covering a variety of things, and, there it was. Just one of those many pages I breezed past without reading.

[g1]

Since we're in circuit analysis mode, one more question.
The 3rd iteration of this amp (red knob "The Twin" > '94 Twin-amp > Pro-tube Twin-amp) adds a diode (reverse biased) from node marked B+ to ground. Labelled D204 on schematic attached. Is this again having to do with inductive spikes from the choke if switched live?

[Helmholtz]

I think that D204 is an additional measure to snubber the voltage spike (and maybe ringing) across S203 (hi/lo power switch) caused by a sudden reduction/inversal of the current through the choke when switching from high to low power. It's always some circuit inductance which strikes an arc between switching contacts by back EMF/ flyback voltage.
To be sure I would need to observe switching transients with a DSO.

(And don't expect too much from someone who has a Rep Power of 5 )