Express 5:25 very unequal bias current on the EL84! need help

[rolandibus]

Hi, I need your help,
this concerns my express 5-25 amp.
I can't get any further because there is a problem with the voltages on the two EL84s, and the bias current of
the two EL84s are very different, although both tubes are matched.
What remains to be said is that it is an export amp and is operated with 230 volts and all voltages are a little higher, as listed in the schematic!
Here are the voltages from A+-B+-C+
A+= 420V - B+= 360V - C+= 260V

And here are the measured voltages of the two EL84s once in "normal mode" and once in "SE (5 watt) mode":

IN "NORMAL MODE":
EL84 (first tube)
Anode (7) 420 V
Cathode (3) 1.6 mV !
Grid 1 (2) 17 V
Grid 2 (9) 370 V
BIAS CURRENT: 7.1mA !

EL84 (second tube)(the SE tube)
Anode (7) 420 V
Cathode (3) 14.7 mV !
Grid 1 (2) 17 V
Grid 2 (9) 370 V
BIAS CURRENT: 18mA !
-----------------------------
IN "SE MODE" (5Watt mode)
EL84 (first tube)
Anode (7) 370 V
Cathode (3) 1.2 mV
Grid 1 (2) 16.8V
Grid 2 (9) 160 V
BIAS CURRENT: 5.8mA

EL84 (second tube)(the SE tube)
Anode (7) 160 V
Cathode (3) 11.3 mV
Grid 1 (2) 0.5V
Grid 2 (9) 160 V
BIAS CURRENT: 99mA
--------------------
As you can see, the bias currents are very unequal in "normal mode" and probably in the "SE mode the SE tube bias current is to high "!
Both EL84s are matched and I tried again with 2 more matched Duet EL84 but always with the same result.
I also tested the resistors around the EL84s, but they are all OK!
as "watdog" says: - "those are some interesting values your seeing on the cathodes as well as some very different idle bias values as a result".
But why are the voltages at the cathode so different?
What remains to be said is that it is an export amp and is operated with 230 volts and all voltages are a little higher than with American amps!

Unfortunately I don't know what else I should check? Does somebody has any idea?
Thanks in advance for replying

 

[rolandibus]

Hi,
doesn't anyone have an idea what and where I should check?
Thanks in advance for replying

 

[ElectricTennisRacket]

Just guesses on my part as I'm not familiar with Express. I'm no expert. However, I have a LSC and if this circuit works similar to the Lonestar Special, then here's my 2 cents.

My initial thought is that although you may believe that you have matched tubes, the 'Normal mode' data says to me this isn't so.

If all the voltages and resistors check OK, this conjecture seems to be supported by the data. On that basis, the first tube appears to be running cold and not conducting properly. What's not clear to me is if the 2nd tube is running too hot.

Q: Does the Amp perform OK in SE mode? .....and probably not sound great in Normal mode?

You could try swapping the tubes between the two positions. If they are unmatched as I suspect, then one would expect the Tube 1 readings above to move with that tube to the Tube 2 position. The only other way of verifying this is with a Tube tester.

In SE mode, the active tube is put into class A biasing so the tube is always turned on, always conducting current so a higher mA reading would be expected, (not sure about 99mA though?) whereas the literature implies the other tube (I'm guessing the first) is shut down so it should not be doing much work. Normal mode is running in either class AB or Class B (I'm unsure which is it actually is) but in these modes each tube 'gets some rest' for a half-cycle so the average bias current draw will be lower than in class A, SE mode.

 

[MarkV User]

I agree with Tennis. I just made an account here because I had to make a repair to my Mark V amp and noticed some bias discrepancies like you did. I have an interest in tube amps and am repairing two now but am also just a beginner.

To understand what you are seeing, do multiple experiments. Swap the tubes and independently switch the positions of the two channels of the bias meter. For good measure I tried swapping the bias meter cables as well, so I tried measuring three different ways to see if the difference followed the tube, meter, cable, or amp channel. In my case it followed the amp channel. Then I realized that the channel modes on the Mark V90 work differently than I thought. For me the story is not complete just yet. I need to understand the amp power circuit next and also make a mod so I can adjust bias, and I will observe the differences between tube positions again in my “normal” mode, which is 90W, all tubes in class AB. Some people will add two bias pots, one for each side. The power stage driver / bias network has to be configured for that.

I only assume that on both our amps the open circuit bias voltage and tubes-installed bias currents should be equal if you swap tubes. The alternative is that one side of the driver could be intentionally a little hotter than the other. There are reasons people do that sometimes and I assume it is not the case here but haven’t verified it. I’ll spend some time with the schematics. Do you have schematics for your amp?

I hope the more knowledgeable folks here will tell us what they know about the output circuits for these amps.

By the way Roland, what is the problem you are trying to solve? In my case I have just put in new non-Mesa tubes so I’m going to implement a bias adjustment pot.

 

[DrKev]

I have no expertise but I do know that tubes that are sold as matched may not be, and expecting up to a 10% difference seems reasonable. And once they go into an amplifier and start working, they can drift further out of whack too. Honestly, I have tried some A-B recordings of pairs of tubes that I know are quite different and pairs that match quite well and I find it makes no meaningful audible difference.

While inside my 5:25 the other day, I did notice a remarkable lack of discoloration on the EL84 tube sockets and PCB, which is proof that no matter how much heat the amp gives off, in its 13 years of life it has never been biased too hot. I definitely cannot say the same for my old 15 watt Laney combo! So, as long as no tube is red plating I'm not gonna worry about it.