Tuesday, June 7, 2016

Loose thoughts by Bjorn Landfeldt

Hughes & Kettner Tubemeister 36 repair

A couple of weeks ago I got a new 2x12 inch Celestial Vintage 30 cabinet which I was eager to plug in and test with my H&K Tubemeister 36. After some 10 minutes of playing on the clean channel, the amp started to distort notably and then there was a flash and sizzling sound before the amp went silent. At the back, all four TSC lights were on indicating no output current.

The manual states that this is most likely due to a blown anode fuse which can happen when the amp is turned on with a faulty tube and the TSC does not have time to shut things down.

After searching far and wide on the web I found little information that could help me with this problem, as it happened after ten minutes of playing it didn't sound like the TSC would not have had the time to shut tubes off.

Anyway, the problem was indeed a blown anode fuse and perhaps also a faulty tube causing this, which as far as I can tell would be very unusual as I have played the amp perhaps 100 hours and have the anniversary edition with military grade tubes which should last a LONG time. The amp has not been carted around but stayed on top of my speaker at home so there should be very little risk of mechanical damage done to the tubes. In any case I would imagine that the TSC should have shut the tubes off before the anode fuse fried, but perhaps this circuitry is faulty (amp one month out of warranty of course).

I thought, I'd just post a couple of photos showing where the fuse is located for others who might experience a similar problem, it is really easy to get access to but the amp has to be opened up from the bottom. Just turn it upside down and unscrew all the screws at the bottom and remove the plate. The fuses are located on the circuit board lower down near the power supply connector. You have to remove the red and green wires to get to them and make sure to remember which color sits on which pin when you install them back.

There are two fuses, 400 mA and 800 mA. In my case it was the 400 mA fuse that blew. I would recommend to use a wooden or plastic tool to help loosen the fuse as there are some fairly sizable capacitors in there which may hold residual charge. As there is no circuit diagram available as far as I can tell, it is difficult to know where there may be voltage left on the circuit board.

It is easy to check if the fuses are OK. Test with a multimeter if the resistance is close to zero or infinite. If infinite, the fuse is blown (if you are holding the probes with your fingers you may get some MOhm reading since  a little current goes through the body at this point but the fuse should be close to zero if it is OK).

Closeup of fuses and were they are located.