HK Triamp MK II Amp 1 is blowing preamp tubes

Hello HK players

I have a problem with my Triamp Mark II the Amp 1 is blowing preamp tubes usually in the preamp 2 position the tube with out the Can around it.... I have been and use JJ tubes from EuroTubes All other Amps 22 and 3 work as they should- I sometimes think it is in the switching region from amp to amp utilizing the stage board----I had it in the Shop at Savage Audio the builder of Savage Amps in Minnesota--they were unable to find fault --- it worked took the amp home jammed some seemed ok the set up for practice hooked up the stage board amp warmed up about 15 minutes and the amp 1 was DOA ---- I reaching out to you guys to get some thoughts on this matter before I return it to the shop again. Thanks

You say the amp is "blowing tubes" Dr.T. What do you mean by that? Is it that the valve, (forgive me, I'm British ), actually has to be replaced after this problem occurs as it has gone to the big jam session in the electronic sky in one fell swoop? Or is it that the DOA problem happens and then can clear itself for a while allowing the amp to run successfully with the same valve in place?

Also, you mention the "Stage Board". I'm not sure which board you mean. H&K don't use that terminology in their documentation anywhere.

Can you confirm which valve it is that you mention generally has the problem? I'm not a Triamp user, (never seen one in real life), so it is important that we get the problem clear in my head. If you are looking at the amp from the front, that's directly at at the controls, is it the second from right towards the input socket? The Service Manual doesn't give any info other than the PCB layout for identification, not even a picture of the amp itself. That shows what is labelled as "Tube2" in that position.

Looking at the circuit schematic it strikes me that, if you have symptoms of the amp just not working then coming back to life without permanent damage to the valves, I would suspect the switching relay which swaps between Clean1 and Clean2. V1a/b triodes are used as a pair of separate input stages, one each for the Clean/Crunch and the Lead channels. The Clean channel signal is then fed either direct to V3 for Clean1 or through V2 to V3 for Clean2. The relay controls that bypassing of V2. If you have a dodgy relay contact you could be left with no connection to V3 in the Clean2 state.

Are you able to do any testing/measurement inside the amp yourself? You have to be blunt and honest with yourself on this and not just say "yeah, sure" as the voltages you are dealing with will kill you stone dead if you get something badly wrong! It honestly requires experience. You will also need a decent meter to get any meaningful results with a pair of good safe probes. You would be reading line voltages of about 300V upwards at times.

Hello brodonbert

Ok here is what I have experienced and determined since the last post--- I have now determined that the supposed blown tube/valve was not blown and in fact is working in the Triamp after installed...I did note that when the Amp 1 when failed I could faintly hear the amp -- so I shut down and restarted the next day and the amp been working since  ---  so I have let the amp run 5 to 6 hours and the amp still works while selecting channels-=- I concur with your thoughts questioning the switching relay --- anytime I noted failure it was coming from Amp 2 operation then going into the clean Amp1 settings with the pedal board or stage board --- The tube/valve I was instructed was a failure is working  -- I was also thinking the relay or possible cold solder connection-- I even took apart the channel selecting pedal board/ stage board for visual inspection and I have backup to compare with-- both pedals used produced same results  --I noted this failure originally using the  pedal amp selector ---  


I have a great deal of confidence in your diagnosis involving the channel switching. The evidence and rational support your working hypothesis ---

I do have a Gardner Bender Digital Multimeter GDT-311


If I need a Fluke or better meter let me know

The repair tech did say he guaranteed his work so it failed again bring it back--- in that regard  If you could assist me in a specific manner to relay the info I would have him locate that issue and check while I am there at the shop-- I totally understand safety is a must.  


I appreciate your knowledge.

Brad

No problem with the meter Dr T.  As long as it is safe to read high voltages around 500V or so at a reasonable accuracy it will be fine.  I didn't mean to imply that there was any special need, I was only thinking of your own safety while working.  If you dig around you should find that the Triamp schematic is available online.  We can't post them here as we always honour H&K's intellectual rights but not everywhere is as rigorous as we try to be.  I'm sure you can find it on other forums or schematic sites.

I would say, now that we know the valve is fine there will most likely not be a problem with the power line.  Your tech will check out the voltages down the line from cap to cap as a matter of course.  We can expect them to drop at each stage as is usual of course.  As I said, these start at just under 500V on the output valve anodes so care is needed when doing it.

I've had another look at the info I found on the amp and I have spotted another potential cause.  Do you have a noisy or even loose Clean1 Gain control?  If it switches back to Clean1 from Clean2 it is connecting the signal chain directly to the wiper of that pot.  If the wiper was not connecting as it should you would get the dropout you describe.  You would also hear a very faint thin version of what you are playing too as there is a small HF bypass cap on the pot which allows some signal to pass even with the wiper not connecting.  That is something else to think about and test.  If you can get the amp to dropout again then just rotate or waggle the Clean1 Gain control knob it would probably kick back in again if that is the problem.

If you can rule out the Gain control I strongly lean towards that relay and if it were me I would replace it as a matter of course as it is cheap and fairly easy.  But I have had a working career of doing this sort of task and I have tools which would make it easy and reliable.  Whether you should go ahead with it would depend on how good you are at soldering on PCBs of course.  If you are not experienced in that way then get someone to do it for you.  Removing a component like that relay, a large stiff body with multiple terminals which cannot all be heated at once, is much more difficult than refitting the new one and you can make a real mess of the PCB if you botch the job.  It is a 24V DPDT type with a pretty standard pinout so you should be able to get a match pretty easily.  They go for around £1.50 over here in the UK and are available at most electronic outlets.  It should be in a format something like this:  Relay

You will find the manufacturer and the code number written on the top of the relay body and from there Google is your friend.  It doesn't have to be the same manufacturer at all as long as the DC coil voltage is the same, that's 24V in our case, and the maximum switching voltage and current are not exceeded which would be difficult to do as we are using it in a low voltage high impedance context where there is really no voltage or current to consider!  It is possible that the relay could have worn or scorched terminals though there should be absolutely no voltage on either of the points it is switching between to cause arcing.  I would think a replacement would best be done by your tech guy unless you are very confident you have the experience, it shouldn't cost much in terms of his time if you source the relay yourself and just get him to fit it.

Either sourcing and fitting the part yourself if you have the skill set or sourcing the part and getting him to fit it would at least remove that from the equation at only a little cost, though everything seems to point at it as a likely cause.  Then the only other likely cause would seem to be that Gain pot.