Another Egnater issue

[Randall]

It's the same amp, Rebel 20, different issue. Background is it came in with a shorted rectifier diode (D102), two blown fuses and a dead 6V6. I corrected these, and now am trying to bias it per the directions printed on the board. This amp runs pairs of 6V6 and EL84 tubes simulationiously. There is a control to mix between them represented by the four ganged pot VR1. This control does not seem to have any effect on bias values. There are two test points with 1R resistors to ground with 40mVDC for the EL84s, and 45mVDC for the 6V6s printed next to them. The most I can get with them both maxxed out is 20mV for the 6V6s and 29.5mV for the original Egnator branded GTs. A fresh set of Sovteks yield 20mV. I also used a bias probe on the 6V6s, and got only 8 - 10 mA max at 334v.

I have tried three different combinations of tubes, fresh JJ 6V6s with Sovtek EL84s, and a second fresh set of JJ 6V6s with the same Sovtek EL84s. These two combos yield approx. the same results, 18.7mV on the 6V6s, and 20mV on the EL84s for the first set, and 20mV on the 6V6s, and 19mV on the EL84s. Then, as mentioned 17mV on the second set of JJ 6V6s, and 29.5mV on the Egantor GT EL84s. For bias voltages I am getting -33.5VAC on both pin 5 of the 6V6s, and -16.5VAC on both pins 2 of the EL84s. For plate voltages I am getting 329vdc to 335VDC on all four tubes, depending on time of day.

I went to the Egnater site, as I have heard Bruce is pretty easy to work with. It says you must fill out the online form and someone will get back to you. So I did, and waited over a week, but got no response. So I filled it in again, this time with more detail, but still nothing. So I then called the service number they have posted, but it is no longer in service. Then I called the main number, but got a recording from Boutique Amp Dists. Finally, I called the 877 number, and got that same company's accounting dept. SO I guess I am not getting any help from them.

It would be nice to know what voltages I should be seeing, but the print I have does not include any. I am stymied because if any of the caps around the bias pots were leaky, I would expect the neg. bias supply to be dragged down, cause more current, not less. D203 tests good in circuit, and R221 measures 4.4K in circuit. Those are the only parts common to both sides I believe.

Does anyone have an idea of what to do here?

Egnator REBEL20sch 8-09 with Values.pdf

[Helmholtz]

What is the screen supply voltage (B+3)?

[The Dude]

Have you checked that all of your output tubes are conducting? Pull one tube in each set, then try the other. See what changes are made to your bias current. In other words (using arbitrary numbers), if you measure 20mA total, then pull a tube and still measure 20mA, the tube you pulled isn't conducting. If you pull a tube and measure no bias current, the tube that's still installed obviously isn't conducting. Are screen resistors all good and screen voltage is there on all tubes/sockets?

[Randall]

Tube voltages are:

6V6

2. 3.4 vac
3 329 vdc
4. 324 vdc
5. -33.5 vdc
7. 3.4 vac
8. 0.017 vdc


EL84

1.
2. -16.3 vdc
3. 0.028 vdc
4. 3.3 vac
5. 3.3 vac
6.
7. 330 vdc
8
9. 324 vdc

All tubes are confirmed as conducting. Pulling either EL84 results in dropping from 29mV to 18mV. I know the 6V6s are conducting because I put both pairs I tried on a bias probe, recording 8-10 mA between to four.

Screen resistors look good visually, but are inaccessable without board removal. I have proper screen voltages as shown in my above measurements.

[g1]

Are all resistors in the bias circuit stock values (no mods) ?
What is the DC voltage at each end of R221 ?

[g1]

Plate voltage seems low compared to what has been reported elsewhere. Check what it rises to with power tubes removed.
Others have mentioned 400V unloaded and 370V with tubes in.

[Randall]

R221 has -70v on one side and -59v on the other. I believe the bias resistors are original, the ones I can get to are correct values, a couple I can't get to, but the solder looks untouched. The whole amp shows no sign of servicing.

Plate voltage with tubes in is 329v. With tubes out is 401v. I wondered if the plate voltage was low right from the start, and thought it would be nice to know what it should be.

[g1]

Somebody elsewhere mentioned having bias set for where you are at (20mV per pair) and having 400V on the plates.
What is your heater voltage? (never mind, I see it's around 6.6V) So primary side of PT should be good.
Have you checked ripple on B+ since changing those diodes?

[Jazz P Bass]

Here is the power transformer file with secondary ac voltages listed.

[SoulFetish]

Here is the power transformer file with secondary ac voltages listed.

Randall, your bias voltage seems like it's more negative than it should be from looking at the schematic and transformer data. can you measure the AC voltage from ground to each leg of the HT winding?
Judging by the ratio of increased negative voltage measurements and drop in measured HT voltage, my working suspicion is that maybe at some point someone reversed the center tap and bias winding?
This could be why some components in the HT supply were starting to fail. (might also explain the strange large drop in DC under light load.)

[Dave H]

This could be why some components in the HT supply were starting to fail. (might also explain the strange large drop in DC under light load.)

If both ends of the winding measure OK I'd check D101 and D102 again and make sure the ripple on C127 is 120Hz. One of the diodes could be open circuit

[Randall]

The transformer in this amp does not match the lead colors exactly to the spec sheet provided. Most of them are correct, but for instance the secondary wires connected to QC 104 and 105 are both red, the bias wire connected to QC 208 is white as per the chart, but the center tap wire connected to QC 107 is green/yellow which is not indicated any on the chart.

The diodes D101 and D102 were checked before I installed them, and check good in circuit.

Secondary voltages measured to ground at QC104 and QC105 are 305 vac at each side.

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

[Dave H]

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

With the low idle current you have I'd expect less ripple voltage than that. Try replacing C127

[g1]

With the low idle current you have I'd expect less ripple voltage than that. Try replacing C127

Also, I'm hoping 305VAC rectified should give a lot more than 330VDC.

[Randall]

I did replace C127 as a precaution when I replaced D102 and D101.