Mains transformer overheating with higher attenuation

[sickman82]

Just wanted to ask if anybody could offer an explanation as to why my Marshall Super Lead is overheating with higher attenuation levels.. -16db, -14db and -12db. I bought a TAD Silencer a while back, mainly used it at -6db or -8db in the past, but in preparation for upcoming gigs I've been taming it with higher attenuation at rehearsal.

The higher attenuation levels overheat the mains transformer to the point that the smell of varnish coating on the outside of the transformer and internal winds fills the room. The transformer is way too hot to touch and remains that way for hours after turning it off. Nothing else in the amp is overheating, the OT and choke are both ok. The attenuator itself is not hot either.

It's worth noting that I replaced the mains transformer in this amp back in May/June with a custom build from Majestic Transformers here in the UK. I recorded with the amp in June without the attenuator and using the new mains transformer, it was on for 8 hours and did not overheat.. it actually remained surprisingly cool. I babysat it the whole day because of the transformer being new and untested and no issues whatsoever.

It's also worth noting that the overheating doesn't seem to occur with less attenuation.. -8db or -6db. Or at least, it doesn't get as hot.. not hot enough for me to notice the smell of varnish.

All voltages inside the amp seem normal, everything actually seems to be ok. I bias the amp rather cool, the HT is 485v and I bias around 30mA. I like it biased around 60%.

I originally thought it could be the warmer weather we've been having, ambient temps around 70° or so. But the past rehearsal we had on Tuesday was way cooler and still the same issue.

I emailed TAD, and their help is about as useful as a chocolate teapot. They just said, and I quote..

"Dear Danny,
is it a Marshall from the early seventies? Is the Amp working correctly? If both is yes, than I presume that this will also occur with all other attenuators on the market".

I'd rather not go back to the Lar/Mar MV.. I have nothing against that setup, but I prefer the sound of the amp with NFB running as it should be.

Any ideas? Cheers for any help you can offer. It's appreciated greatly.

[Littlewyan]

Hmm the TAD Silencer could be a Reactive Load so it could be making the amp work a lot harder than usual.

The first obvious question is, do you run your amp the same way through the reactive load as you do unattenuated? As in, do you crank the volume either way?

Also do you know what load the Silencer is meant to work on? I.e. 4Ohms, 8Ohms etc

I personally prefer resistive loads as they're nicer to the amp.

[sickman82]

I run the amp the same way yes, but it's not usually cranked.. usually around 5-6 as I put reverb and delay in front so I need that little bit of leeway.

I think the Silencer is a combination of resistive and reactive.. I asked TAD that question before I bought it. It's the 16ohm version, they make an 8 and 4 ohm version as well but my cab is 16 ohm so I got that one..

[Littlewyan]

Do you notice the EL34s redplating at all when you play with the attenuator?

[sickman82]

I'll take a look tonight with the lights out and see if it does, but in the past when I have looked they didn't seem to be redplating..

Could it be an impedance mismatch on those higher attenuation settings?

[Littlewyan]

Its possible but I would have thought you'd have felt the OT getting hot. Have you emailed Majestic about this? As they might state its normal for the PT to get hot. My PT has been too hot to touch before and Classictone assured me that it can be normal.

I am very wary about Reactive loads though.

[sickman82]

I spent about an hour playing through the amp last night with the lights dimmed on the -16db setting, then again this morning taking a line out from the attenuator to my interface and the attenuator set as a load box. The mains transformer got super hot, the OT and choke remained cool. There was no redplating of the valves.. all seemed normal.

I'm guessing it's not as simple as measuring across the output of the attenuator to check the impedance? I measured anyway, on the Load box setting I get 2 ohms, up to the 'full'setting (no attenuation) it read 1k. Does that sound normal?

[d95err]

Do you keep the same setting on the amp when comparing different attenuation levels, or do you crank it more when using more attenuation?

I.e. does your amp get equally hot when cranking it without the attenuator?

[sickman82]

Same settings always yeah, fairly conservative settings really because I use reverb and delay up front.. no fx loop.

The only time I've used it without the attenuator was for a recording session this year, it ran for 8 hours without getting anywhere near this hot. It was very warm, but not hot to touch. I did have a small fan pointed at the back of the amp with the cover removed though.. I did that because I'd never had the amp on for that length of time and the mains transformer was brand new so I was being over cautious and babysitting the amp. I doubt very much a small household fan pointed at the back of the amp would negate this kind of overheating though.

[d95err]

I did have a small fan pointed at the back of the amp with the cover removed though..

The fan and open cover probably made a huge difference for cooling. I think the amp has heat problems at full bore, regardless of if it goes through an attenuator or straight to the speaker.

I suggest installing a small fan to blow cold air onto the transformer and tubes. You could use the trick from the Marshall Powerbreak and power the fan from the speaker signal.

[Littlewyan]

A fan won't help cool that transformer. You have to remember its a thick iron core. Anyway you shouldn't have to resort to that.

Those resistance measurements are off, with no attenuation it should go straight through to the speaker cabinet. The Load Box resistance seems really low but its probably a reactive load so I don't know how they will measure up resistance wise.

Did you have the speaker connected when you measured the resistance?

[sickman82]

Yeah.. there is no way a fan could cool this transformer dude.. it's hot, I mean could cook your eggs on it hot. It's a huge piece of iron too.

No, resistance was measured with nothing connected to the attenuator. Just straight across the output. I did wonder if that could be off because of it being a reactive load. Why such high resistance for the 'full' setting setting though? Bad rheostat perhaps?

[Littlewyan]

It will be because you had no speaker connected. That load reading won't change though as thats meant to be for when you don't use a speaker. It is odd that its so low though.

Measure the attenuation levels again but this time with a speaker connected.

Have you contacted Majestic about it?

[sickman82]

I did this quick before I left for work, so I'm going from memory.. measuring with the cab hooked up to the attenuator and using the lead that connects to the speaker out on the amp to measure the resistance varied from 42 ohms up to about 55ohms..

I was planning on calling Majestic on Monday.. I'll report back when I have spoken with them. They coated the varnish pretty thick on the outside of the transformer.. but still, the core is clearly getting overheated.

[Littlewyan]

That does seem very high resistance wise. I'm wondering if something is wrong with that attenuator. Have you emailed TAD with those measurements to see if that is normal?