Hi. Running my TM36 with a pair of 1x12s carrying 8ohm Celestion Vintage 70s. So according to H&K I can use either a single one or the two in series giving 16ohm.
I wondered what the impedance was for the quoted 36W. If you don't know about these things it matters! You cannot get the same 36W output power into 8ohm and 16ohm without using a separate tap on the output transformer. From the same amp setup 8ohm should ideally give you twice the power out of the amp to 16ohm, (that's ideally but it will draw more, output stage allowing). So I asked H&K for advice, I asked if, considering that the H&K 2x12 is 16ohm, an 8ohm cab would affect output power. This is what I got from their technical department: "Hi Albert. If you have an 8 ohm cab it will indeed increase the volume. Kind Regards". We're all apparently too stupid to take in anything deeper than that, (despite in my case, a lifetime in the electronics industry designing stuff like this). I wasn't too impressed though I will grant them that I could have asked more specific and probing questions.
The subject of the "8-16ohm output impedance" needs a little clarification from H&K I think. My assumption is that they imply the output stage is beefy enough to work with a mismatch within that range, but I could be wrong! I would also assume that, if the output power is quoted at 16ohms, then 8ohms will push it much much harder, (more about the results of that in a few lines). If 36W is quoted at 8ohm load, then 16ohm loading will draw significantly less power, even though it still represents a mismatch and once again puts the output stage under more stress.
So to my reason for seeking other opinion. I have seen a horrid scary thing! I was running the amp into a single 1x12 8ohm cab with the power soak set on 1W. Great sound but you tend to push the amp harder of course. Turning it up to get enough power out of the 1W setting and tone from hard driving it means that one pair of output valves is actually running at full output power internally into the power soak load. It cut out completely. I stopped playing of course as you would and after 4 or 5 secs it kicked back in again. I've experienced this same effect 3 times now. I've taken to practicing at home on the 5W setting and not so high on the MV to keep internal dissipation down.
Even more scarily, once I was playing at 1W and high settings into 8ohm and the thing dropped out and the whole internal structure of the left EL34 started to glow red hot. Now this is a very bad thing for valves. It is known to happen usually due to low bias, but should not be accepted. I immediately kicked it into standby and it stopped trying to make the room look cosy and romantic. After that event the amp had dropped into diagnostic mode with that pair of valves turned off. I shut it down, waited a minute or so, then ran it up again convinced I had blown something. It was fine. I checked the bias settings, with my "bias setting" pick, LEDs flashed, I counted, and everything checked out ok.
I reported this to H&K asking for advice a few days ago now. The reply I have is "Hi Albert and thanks for your email. As the UK distributors for Hughes and Kettner your email has been forwarded to ourselves. I've copied our technical department into this email and they should be in touch shortly. If I can be of further assistance please do not hesitate to contact me. Best regards". It's only been a couple of days so they haven't replied yet. Has anyone else seen this sort of behaviour, or does any other tech savvy guy have any info on the output impedance question and whether it is linked in any way to what I'm describing?
And incidentally, is this site monitored by H&K themselves? Hi guys, are you out there? Is there anybody listening? Phone homeeeee........
I wondered what the impedance was for the quoted 36W. If you don't know about these things it matters! You cannot get the same 36W output power into 8ohm and 16ohm without using a separate tap on the output transformer. From the same amp setup 8ohm should ideally give you twice the power out of the amp to 16ohm, (that's ideally but it will draw more, output stage allowing). So I asked H&K for advice, I asked if, considering that the H&K 2x12 is 16ohm, an 8ohm cab would affect output power. This is what I got from their technical department: "Hi Albert. If you have an 8 ohm cab it will indeed increase the volume. Kind Regards". We're all apparently too stupid to take in anything deeper than that, (despite in my case, a lifetime in the electronics industry designing stuff like this). I wasn't too impressed though I will grant them that I could have asked more specific and probing questions.
The subject of the "8-16ohm output impedance" needs a little clarification from H&K I think. My assumption is that they imply the output stage is beefy enough to work with a mismatch within that range, but I could be wrong! I would also assume that, if the output power is quoted at 16ohms, then 8ohms will push it much much harder, (more about the results of that in a few lines). If 36W is quoted at 8ohm load, then 16ohm loading will draw significantly less power, even though it still represents a mismatch and once again puts the output stage under more stress.
So to my reason for seeking other opinion. I have seen a horrid scary thing! I was running the amp into a single 1x12 8ohm cab with the power soak set on 1W. Great sound but you tend to push the amp harder of course. Turning it up to get enough power out of the 1W setting and tone from hard driving it means that one pair of output valves is actually running at full output power internally into the power soak load. It cut out completely. I stopped playing of course as you would and after 4 or 5 secs it kicked back in again. I've experienced this same effect 3 times now. I've taken to practicing at home on the 5W setting and not so high on the MV to keep internal dissipation down.
Even more scarily, once I was playing at 1W and high settings into 8ohm and the thing dropped out and the whole internal structure of the left EL34 started to glow red hot. Now this is a very bad thing for valves. It is known to happen usually due to low bias, but should not be accepted. I immediately kicked it into standby and it stopped trying to make the room look cosy and romantic. After that event the amp had dropped into diagnostic mode with that pair of valves turned off. I shut it down, waited a minute or so, then ran it up again convinced I had blown something. It was fine. I checked the bias settings, with my "bias setting" pick, LEDs flashed, I counted, and everything checked out ok.
I reported this to H&K asking for advice a few days ago now. The reply I have is "Hi Albert and thanks for your email. As the UK distributors for Hughes and Kettner your email has been forwarded to ourselves. I've copied our technical department into this email and they should be in touch shortly. If I can be of further assistance please do not hesitate to contact me. Best regards". It's only been a couple of days so they haven't replied yet. Has anyone else seen this sort of behaviour, or does any other tech savvy guy have any info on the output impedance question and whether it is linked in any way to what I'm describing?
And incidentally, is this site monitored by H&K themselves? Hi guys, are you out there? Is there anybody listening? Phone homeeeee........