Another Egnater issue

[The Dude]

It's odd that B+ drops substantially with tubes in, yet bias measurements show that tubes are not conducting much. These ideas seem to contradict each other. Could you have a meter problem- low battery or something? Otherwise, since you tried new tubes already, could you have a socket problem? Do you see any signs of socket arcing?

[Randall]

I don't believe it is a meter problem. I put in a fresh battery, and verified measurements with a second Fluke.

The sockets look pristine, I see no evidence of arcing.

[Helmholtz]

You could measure total HT current consumption by connecting an Ammeter across the standby switch.

[The Dude]

I'm also wondering if there is some resistance somewhere in the path of B+ causing B+ to drop so substantially with very little load (like a bad standby switch, fuse connection or something else). It doesn't seem to be caused by tubes overconducting, so maybe the circuit is current starved. Check B+ right at the diodes (like D101 cathode). Then check at tubes. Is the voltage the same, or is there a large voltage drop there?

[Randall]

With the standby switch off, probing my ammeter across it measures 54.7mA dc.

With the standby switch on, I measure 328v at D101 cathode, and 325v on 6V6 pin 3.

[Helmholtz]

With the standby switch off, probing my ammeter across it measures 54.7mA dc.

With the standby switch on, I measure 328v at D101 cathode, and 325v on 6V6 pin 3.

Nothing suspicious here.

Seems to leave only the PT. Maybe a bad internal connection, which should get rather hot locally. Or just poor regulation.
But none of that is likely to cause increased bias voltage.

[Randall]

I am really at a loss here.

[g1]

With unit running in 'play' mode, and bias set for whatever hottest idle you can get, what is your AC voltage at HV winding? (D101 anode to D102 anode)
According to specs in post #9, should be 590VAC.

[Dave H]

I measure 55.8 vac on the cathode of D102 ar 120 Hz.

How did you measure this? On the AC range of a DMM or peak to peak on a scope? Can you post a scope screen shot?

Either way with a total DC current of 55mA and a 100u capacitor the ripple should be way less than this unless the cap isn't 100u or there is a high resistance connection to the cap. Check the resistance between C127 +ve terminal and D101, D102 cathodes and C127 -ve terminal to PT secondary center tap (QC107)

From looking at the transformer spec the bias voltages are correct but B+ should be as in post #6 (370V with tubes in)

[Randall]

With standby in play mode I measure 604vac between the anodes of D101 and D102, with my Fluke 77 DMM. I will remind you that as I mentioned, the PT spec sheet is not the PT that is in this amp. I do believe it is original to the amp, as it does have a Rebel-20 Power Transformer label on it, as does the OT. It is Taiwanese made, not Chinese like the spec sheet, if that makes any difference.


I measured the 55.8vac ripple on the AC range of my Fluke DMM. I am posting a scope shot as requested. After seeing this scope shot, maybe now we are getting somewhere. Why would the bottom half of the wave be clipped like this? Settings are 2v/div, with the probe on x10.



I have not checked the resistances on C127 just yet, since it means pulling the main board.

[Helmholtz]

Settings are 2v/div, with the probe on x100.

Would mean 200V/div ?? I think you used a 10:1 ("x10") probe.

What's the time base setting?

Certainly looks fishy. E.g. only one diode operative?

[Randall]

Yes, you are right, probe is x10, my mistake. Time setting is 1mS/div.

[Helmholtz]

Yes, you are right, probe is x10, my mistake. Time setting is 1mS/div.

So one rectified half-wave seems to be missing. Check both diodes and their connections.

[Randall]

The diodes check good in circuit with the diode tester on my fluke. I know that doesn't neccesarily mean one isn't failing under load. Also, the cathodes of both D101 and D102 look the same on the scope with the chopped off bottom. I replaced C127, but how can I tell if it is failing, even though it was new? Would it cause a problem such as this? Also, I just realized I installed a 100uF/400v, not the 450v called for, would this be an issue here?

[The Dude]

The cathodes of both diodes are the same electrical point in the circuit, so of course the scope will show the same thing at either cathode. It doesn't mean one diode isn't working. If the voltage unloaded at that point is less than 400VDC unloaded, then a 400V cap would be fine, but the rectifier circuit doesn't yet seem to be working correctly. When you get it working right, the unloaded DCV will probably go up.