HK Tube-sheister 36

[drewl]

Wow, what a piece of shyster.

Looks like the bias control to the two outside sockets is f-d.

Someone here posted the service manual for the 18. Looks close enough.

Some nice ideas here but implemented just very, very poorly with very bad construction.

[Chuck H]

How about some pics so we can all scoff at it

[drewl]

Okay, I will take some.
I think a couple mosfets that control the cathode on the EL84's are bad.
Looks like a royal pain to swap them out.

[Jazz P Bass]

I had two of these show up within one week.

The first one had microphonic Chinese EL84's.
And really bad soldering on the load resistor bank.

The second one had 'NO' solder on the cathode mosfets.
(Well, very little, anyway.)

[drewl]

On two of the EL84's the Mosfets are passing some nasty ripple. The mosfets are good.
The noise is either coming from the Bias module or something in the path.

Let you know what I find.

[Swamp]

I repaired a 18, that burned the bias tracks.

The amp had 2 "L" type connector and were too near the tracks.
This connector overheated and burn the PCB.
I remember to change the mosfets just in case.

Just repair the tracks and subtitute the mosfets and you should be good. It worked for me.

[drewl]

Great, thanks.
I will check those out when I pull the main board.

[Chuck H]

From a design perspective, why use a MOSFET to fix the cathode bias? Is it a dynamic bias or something? I do fixed cathode bias with the typical resistor and a zener valued just lower than the resistor voltage will ever sag. I tested the zener in this circuit and it was drawing about a half a watt so the resistor still carries most of the load and the zener doesn't need to be very high rated. It only carries whatever voltage would rise on the resistor if the zener wasn't there. I use 5W DO package diodes and, of course, high rated resistors are easy to get. Never had a failure. Unless the MOSFET is being used in some dynamic way, perhaps this could be a more reliable alternative.

[Jazz P Bass]

On two of the EL84's the Mosfets are passing some nasty ripple. The mosfets are good.
The noise is either coming from the Bias module or something in the path.

Let you know what I find.

The mosfets are in the cathode path.
So first, rule out the power tubes.

Rant: I am o/k with Chinese preamp tubes.
But their EL84's are a different story.

[Chuck H]

Rant: I am o/k with Chinese preamp tubes.
But their EL84's are a different story.

Ruby, a company known their excellent warranty, service and product reliability, and usually selling selected and matched Shuguang tubes, stopped selling the Chinese el84 in favor the JJ product. That tells me all I need to know.

[drewl]

It was the Mosfets.

Replaced them and it's fine.
Here's some pix of the boards and little control board

[teemuk]

From a design perspective, why use a MOSFET to fix the cathode bias?

At least TubeMeister 18 is fixed bias. The MOSFETs are there for plain switching purposes: Either on (very low resistance) or off (very high resistance). This either keeps the stage operational as usual fixed bias or puts it in "standby" by interrupting the cathode circuit with high resistance. Little leak current will prevent issues with tube reliability.

If you look at it closer, the MOSFETS are actually controlled by a microprocessor. The same microprocessor samples cathode current via those 1R5 resistors, generates a PWM signal to automatically adjust bias of each tube individually (and if they were clever they programmed it to adjust only when "idling"), and - if things fail - also shuts off the MOSFETs (and power tubes), which at least ideally should happen a LOT faster than waiting for fuses to blow up.

I would think there are also some sort of programmed-in muting sequences and also the MOSFET gate drive circuit can powered off via usual standby switch. ...But now the switch won't be subject to usual arcing issues due to high voltage + current surges, and the tube cathodes won't get "poisoned" due to lack of plate voltage.

In essence it looks like a very clever system and, IMO, is how biasing, fail safes, standby switching and all should be implemented to tube amps in this modern world. (Especially when they live outside the comfort zone of someone's living room and are subject to frequent tube swaps and all kinds of experimentations with the aforementioned).

Yes, poor implementation is always a concern but personally I give a big nod of respect to guys who think outside the box and design this sort of stuff.