JCM900 50w dual reverb low on power

[Alex R]

Yes Marshall had a spell of making amp with 5881s didn't they - not sure if it's this period but it could be an explanation. Suspect though that the low primary impedances mean it's an EL34 amp - but will do the backwards low-AC-volts test on the OT later today and find out all about it for sure & report back.

[Alex R]

Here is how the OT test went. Everything was disconnected from the transformer.

on the 8 ohm secondary: 4.86vac
across both legs of the primary: 66.7vac
CT to left: 37.3vac
CT to right: 28.9vac

primary volts squared 4448.89
secondary ditto 23.62
thus impedance ratio 188.36
thus primary impedance 1507

v low even for EL34s (and definitely low for 6L6s!)

Fused turns in the OT then?

[black68]

I Have Been Following This Post With Alot Of Interest In Finding Out What The Cause Of The Problem Is.not Too Long Ago I Had A 50w Jcm900 With Very Low Volume.after A Long Time Of Checking Everything I Could Think Of,i Removed Every Pot And Found The Treble Pots Carbon Board Was Cracked In Half. I Replaced The Control And The Problem Was Solved.i Hope This Helps

[Amp Kat]

Here is how the OT test went. Everything was disconnected from the transformer.

on the 8 ohm secondary: 4.86vac
across both legs of the primary: 66.7vac
CT to left: 37.3vac
CT to right: 28.9vac

primary volts squared 4448.89
secondary ditto 23.62
thus impedance ratio 188.36
thus primary impedance 1507

v low even for EL34s (and definitely low for 6L6s!)

Fused turns in the OT then?

Boy this is a tough one Alex but I'd have to think exactly what you do that the turns are fused. Main reason is that the numbers you gave reflect exactly the ratios that you first posted but IME these trannies are usually very close side to side. I checked my spare JCM 900 OT yesterday and it was dead on 16.1/16.1/32.2. Now a 5150 OT is about 10 ohms from side to side however the numbers are much higher and as you also pointed out that primary impedance is very low. As for the 5881 thing just check and see if pin 1 & 8 are tied together which I think they are and your fine there. Many of the Marshalls that use 5881 also have a metal grill that screws to the chassis to protect the tubes and that cover will not fit if EL-34's or 6L6's(most unless NOS) are used.

[Alex R]

Yeah I just seem to be having a run of tough ones...

What about the 60v B+ sag between idle and 'full' power? - seems quite a lot to me. Maybe because the OT starts to short a bit... but I'm remembering that I subbed the OT and still had low power. Maybe I'll rig up an external B+ supply and see what happens. Also I need to look again at the board for something letting the plates communicate like Enzo suggested.

[Enzo]

I am falling into the bad OT camp at this point.

[Alex R]

Calling Marshall today for a new one... will post outcomes.

[Alex R]

OK well I fitted the new OT. Much better... but one of the power valves wouldn't bias right - most I could get it to draw was 15mA. So I fitted a nice new pair of 6CA7s and now it's FINE! With 36mA per side it puts exactly 50w into my 8ohm dummy load. A bit low I guess but I do not propose to worry about this because it sounds ok... well, it sounds like a JCM900 & is loud with loads of bottom so enough already.

Goes to show the multiform variety of OT failures. And the trickiness of two-fold problems (the valve was also performing wrongly - it seemed to be drawing good current through the old OT and looked ok on my tester but I'm wondering whether the OT itself was drawing some current to earth. Maybe I'll unwind it and take a look. Once the new OT was in, the bias was extremely cold, which suggests to me that not everything that was going into the centre tap was flowing down through the valves. How's that for a theory?

Well it was an annoying but very instructive fix. Many thanks to all who helped me learn.

[Amp Kat]

Nice job Alex and were glad you got your amp going. Sometimes amps do their own things and it's all for a reason but it's kinda like figuring out a massive jigsaw puzzle. It doesn't all come together until all the right pieces fit and I'm sure there was a reason it all happened in the first place and my guess is a bad tube. Many OT problems are caused by this and tube testers don't show when they are getting ready to create a proble just whether they have allready created a problem. Opens due to bad cables blown speakers and other various disconnects when the amp is pumping all it's worth are another problem that happens unknowingly until the aftermath but I'm just stoked you got it going. Cheers !

[Alex R]

Thanks Kat, nice of you to say all that. The guy whose amp it is said it's been sounding poor and eating power valves one or two a year for a while now. So some long-discarded valve caused it all I guess. He's been in the US with you guys for a week, back tomorrow; I'll be expecting a nice big smile when he hears it esp since before he went he called to hurry me up cos I'd had it for 3 weeks. Hey I'm busy, I'm busy! Repairs take time!