Jubilee: 50w hum... 100w no hum

[Arrested Dev]

I have an original 1987 Jubilee 2555 head. Haven't played it too much in last number of years. And I am about to take it in for service - general maintenance / check up here in Austin at Tone I/O.
Borrowing a buddy's 2x12 (1936 with GT75). Sounds great but I'm definitely going to get Vintage 30 type 2x12 VERY soon.

Only issue I've noticed is that amp delivers a 'hum' when switched in 50w mode. Hum immediately goes away when I switch to 100w. Yes, I know the solution is to play in 100w .

So, I'm just curious if you all have any thoughts on what this might be, since I just want to bounce that against what the local tech thinks as well. Appreciate your thoughts.

 

[Pete Farrington]

It may just be that the amp’s been unpowered for too long, and the ecaps have degraded.
HT hum in the power amp should get almost completely cancelled out in either mode, though the triode 50W mode can be expected to have more ghosting intermodulation at high power output.
If there’s a noticeable degree of idle hum, it may be that the switch is bad, which might be expected, given the massive voltage between its poles at high power output. A bad switch migh have the effect that in triode mode, it’s only running on 3 output valves.
Or it may be that due to failed HT ecaps, there’s just a massive amount of HT hum at the reservoir, and any slight imbalance is making it audible.
Are you able to check idle anode or cathode current of each output valve, eg with a bias probe?
How does the amp sound when it’s cranked, master vol up full, in both modes?

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[Gene Ballzz]

@Pete Farrington

I'm not sure I see how the amp is "only running on 3 output valves" in triode mode? Please enlighten me, unless that was just a typo?

Also, might not one tube being not up to snuff (compared to the others) be causing a problem? A quick/easy test may be to take the two outer tubes and swap them with the two inner tubes?

Simply Askin'?
Gene

 

[Pete Farrington]

I'm not sure I see how the amp is "only running on 3 output valves" in triode mode?

Given the symptom, the 50/100W switch may be faulty such that one of the outputs valves isn't getting its screen grid connected when in 50W triode mode (but is connected when in 100W pentode mode). without screen grid voltage, there's no anode current, so the valve would effectively not be there.
I've rephrased my earlier post a little to try and make that clearer, sorry

might not one tube being not up to snuff (compared to the others) be causing a problem? A quick/easy test may be to take the two outer tubes and swap them with the two inner tubes?

I was thinking that a bad valve should affect both modes similarly.

 

[Jon Snell]

I would fit a balanced quad or output valves and the hum should go away.

The schematic for the Triode Pentode mode, is not the correct schematic as it only reduces the gain, not the power of the output stage by switching from Pentode mode, to Triode mode ... unless you are mistaken and in which case a set of valves that are balanced, should sort out your problem.

 

[Pete Farrington]

The schematic for the Triode Pentode mode, is not the correct schematic as it only reduces the gain, not the power of the output stage by switching from Pentode mode, to Triode mode ...

I think it’s common for Marshalls to implement what they describe as a half power mode by switching the output valves to triode operation, eg https://el34world.com/charts/Schematics/files/Marshall/Marshall_jcm900_dualrev_50w_4500.pdf

Have you checked that for typical valve guitar amps, triode mode doesn’t reduce the max achievable power output? I think it does, though not perhaps as much as to halve it.