sunn first gen model t problem..any advise?

[pombagira]

hi okay, so my first gen sunn model t has lately started playing up. Seemingly the output has dropped to around 162 watts whereas before it was coming in at 190watts. This has been recorded using the correct meter. The strange thing is that the tone has disappeared. It doesn't seem to have any brightness to it unless you turn the treble up to 10 and the brightness up to 10 and presence up to 10 something I have never needed to do in the past.  Even then on a low volume it is overdriving like it has never done before. So what is it? Of course my fear is that it is the holy grail of the amp the output transformer...which as far as I am aware cannot be replaced by an equivalent. But is that heading in the right direction. It is currently in with my amp tech guy in denmark street, london. I have a second gen model t as well and I plan to bring that in so he can compare and contrast between the two. I have to say the second gen has always been a brighter and louder amp than he first gen.     So please contact me if you  have advise based on similar experiences or perhaps you have technical knowledge beyond my experiences...any advise at all will be more than gratefully appreciated. Cheers, pete

I have subsequently noticed that someone else has already posted an almost identical message. Of course the difference here is that with everything on ten it is still pushing out 162 watts. When new valves were fitted just over a month ago though the amp was producing more like 190 watts. So what has degraded. And before you ask about the valves, other valves were also placed into the amp yesterday and they produced about the same output. The main thing is it doesn't sound as lively as it did and isn't as responsive as it once was. So there is a problem but what the hell is it??

[mckinnon audio]

  Hi Pete,sounds to me like the preamp isn't driving the power amp enough.Check all your pre tubes,the volts going to them,and the filter caps supplying the B+ to them.If it was a tranny prob.,you'd be blowing fuses or seeing smoke by this time I'd think.Hope this helps,Mel.

[loudthud]

The Model T uses what is called an Ultra Linear output configuration which uses a special transformer. The Marshall Major used the same configuration. Mercury Magnetics in the US can rewind or build a replacement. Chances are you just need a new set of output tubes. KT88's are a good choice.

[pombagira]

okay so if it is the valves then why would they have degraded so quickly..it would mean this year alone I have put in three sets of valves. That surely can't be right. Of course the second gen is working fine and therefore the tech should be able to figure by comparing the two amps  whether the problems lies in the pre tubes or the main output valves. It is interesting to note my tech also had envisaged that the problem might lie with the pre tubes. However, he went over everything today and found everything to be working fine. Of course there is a problem...but in some ways if the amp just stopped working it would be easier to isolate the problem than like it is at the moment.
So I know that mercury magnetics could rewind but are they now manufacturing off the shelf replacements for the model t because the last time I was in contact with them they said they hadn't started to produce a replacement. Okay so this was six months ago so has something changed. I sure hope so because I would like to have a standby just in case especially as i live in the UK.     

[mckinnon audio]

  Hi Pete,just a thought,check your plate resistors,47 ohm/5 watt and your screen resistors,1.5 Kohm/5 watt,these ones go to your ultra linear winding.All 4 of each type should be matched as close as possible.Where you've gone through sets of tubes,I'd check these.Hope this is of some help,Mel.

[pombagira]

thanks mel. I will let you know how he gets on comparing the two model ts. I did speak with him today and he said he had looked at at everything you suggested..but now he has the two amps he will do this again and see if anything comes up. I do remember that four 6550 electro-harmonix valves were put in at the beginning of the year and they degraded super quick and the amp started to sound like it does now.ie lifeless with little tone.

[pombagira]

hi mel a slight delay with regards to getting back to you. After managing to source a scope that worked, because our amp tech discovered his didn't want to play ball when it came to discerning where the problem on the model t, he found the following that the pre-amp was producing a clean signal, however the presence was causing it to distort. Now I am no technician but he found there was an issue with the push/pull of the preamp, and after reconfiguring this aspect of the amp...I have what can only be described as a totally transformed amp. Unlike my second gen Model T, every dial on the amp now works to full effect, that means if you wanted to you could increase the volume up to ten. Now I know that may sound like a really obvious thing to say but on the second gen the volume is really only to be found up to five, after which the sound thickens without really getting much louder. It may be the case it is so loud at five that I don't need to turn it up anymore than that. however, with the first gen I have an amp that is totally malleable, since it can produce any sound I want it to. It really is like a different amp and it is now pushing out at full volume something close to 175 watts. It has become a total beast. I also ordered an output transformer for my sunn's from Mercury Magnetics just in case something does happen to one of them in the future. It's made me sleep alot easier because of it. I should add I don't really drive either of my sunns particularly hard since I actually use four amps as my backline. Of course the model ts take pride of place.
Anyway, thanks for your help mel. 

[mckinnon audio]

  Hi there,glad things worked out well for you.I always prefered the 1st gen T's over the 2nd's.Two different amps really.I had to sell mine and deeply regret it because I doubt I'll see another one in the same near mint shape as the one I had.Great amp,use it as much as you can,good luck,Mel.