How to tell if the Valves need changing in a 2nd hand amp

adampeter

I have a 2nd hand Marshall jubilee 2525c currently winging its way to me, was a very good deal and i have wanted one for a long while.
Is there an easy way to tell if all is ok on the Valve front? should you change them just to be sure?
Never bought a 2nd hand valve amp and i've spent a long time playing through a helix and FRFR system, so my ears need to get tuned back into a real amp sound.

guitartango

i think you can gently tap the tube with a pencil, if it makes a strange noise then it needs to be replaced, but then again i am no expert.... 

munckee

I'm no expert on the subject but have bought 3 second hand valve amps.  I've only changed a valve once when there was clearly an issue and it needed a new power valve.  I wouldn't change unless you think there is an issue.

ICBM

Turn the amp on and play through it. If it sounds like you expect/want a Jubilee to sound, it's fine.

Modulus_Amps

ICBM said:

Turn the amp on and play through it. If it sounds like you expect/want a Jubilee to sound, it's fine.

Also make sure the power valves are not red plating (red glow on the large plates)

Also, if it is being shipped to you make sure the valves are all seated correctly before turning on for the first time.

ICBM

Modulus_Amps said:

Also make sure the power valves are not red plating (red glow on the large plates)

Also, if it is being shipped to you make sure the valves are all seated correctly before turning on for the first time.

Sorry, I was being deliberately flippant .

But basically the point is that unless there's anything wrong, don't change the valves.

adampeter

But basically the point is that unless there's anything wrong, don't change the valves.ICBM said:

Modulus_Amps said:

Also make sure the power valves are not red plating (red glow on the large plates)

Also, if it is being shipped to you make sure the valves are all seated correctly before turning on for the first time.

Sorry, I was being deliberately flippant .

That's cool, i'm flippant all the time, 


May seem a silly question, but how do you know when the valves are on their way out? is it gradual or sudden?
TBH i flip gear so often i never had an issue with valves before

adampeter

Modulus_Amps said:

ICBM said:

Turn the amp on and play through it. If it sounds like you expect/want a Jubilee to sound, it's fine.

Also make sure the power valves are not red plating (red glow on the large plates)

Also, if it is being shipped to you make sure the valves are all seated correctly before turning on for the first time.

Thanks for the tip mate

JohnCordy

With my Mesa Lonestar I found that it kept blowing fuses when I turned it off. I borrowed a mate's Orange tube tester which is super handy if you can get hold of one? I had one dodgy tube that turned out to be the culprit...

MichaelAT

Both things can happen: they can go south immediately (if something shorts) and they also lose sparkle over time. The 2525 is new enough on the market that I would not worry about preamp tubes. They can last quite a long time. Personally I'd buy some new EL34 & bias them properly... after a while, if you decide to keep it.

gringopig

.

fretfinder

If you turn the amp on and when it’s warmed up it starts making a noise like frying eggs that doesn’t go away, it could be a valve issue. Replace the preamp valves one by one with a correct replacement that you know to be working, and you’ll find the dodgy valve (if it was a valve problem in the first place).

Moloch

This seems a good place to ask my own valve question (sorry if this comes across as a hijack, @adampeter. Just seems we have similar questions and the answers I get may give you some clarification too).

So, like Adam I tend to go through gear quite quickly. However, a real stayer (as in 18 months) has been my EVH 5150 III 50w combo. Last night though, something came up and I'm pretty sure it's a tube problem.

So, for anyone who doesn't know, the 50w version of the 5150 is a pseudo three channel amp. By which I mean there are three options, but the first two (Green and Blue) share the same EQ, gain and volume controls. In practice switching from the Green to the Blue channel is really like hitting a very powerful boost pedal. In short, they seem to share everything. Meanwhile the Red channel, where the absolute insanity happens, seems to stand alone.

Last night, while playing my usual terrible metal chops, I switched from the usual channel to the green and found that it was silent. or at least appeared so. After a lot of tinkering, taking all my pedals and switchers out of the mix and going straight into the amp, I found that it's actually just really quiet. The Blue is too. The Red though roars as usual.

I've never really had valve issues before, even on my Peavey Classic 30 that I use for cleans and have had for almost ten years now. My suspicion here is that a power valve that is presumably specifically for the Green/Blue part is on its way out and that it's a relatively simple change. However, when speaking to the guy who does my set-ups (and who techs for various metal bands), he reckons it could still be a preamp tube. I have my doubts, but as I say, he's dealt with stuff for guys like Carcass and Angel Witch, so with my limited experience I can't really discount his opinion, even if he's mostly a guitar guy.

I just had a go with my Telecaster and the Green and Blue channels both sounded pretty suturated when I pushed the gain, even though I was using single coils.

Naturally I want to keep costs to a minimum, especially given I've just bought a Fractal AX8 and so the amp might be sold soon. As such, I'd like to avoid a complete revalve. However, I'll completely switch out the preamp or power amp valves (whichever is home to the problem valve) to be fair to any future owner.

Where do your suspicions lie, guys?

ICBM

If it's a valve at all it's a preamp valve. Certainly not a power valve, they are not specific to any of the channels.

Have a look in the back and see if all the preamp valves are glowing properly - there should be two spots of orange light at both the top and bottom of each valve. If not, that valve is either faulty or not making a good contact with the socket - wiggle it and see if it comes on. But even if all of them appear to be lit correctly it still could be one, since that's only the filament glow not the signal voltage. To definitely confirm or eliminate a vale problem, you need one spare 12AX7 - replace each of them in turn with it and see if/when the problem stops.

If it's not a valve it could be a sticking or faulty channel relay, which is going to be a much bigger pain to fix even for a tech. Less likely but still possible is a preamp plate resistor, faulty volume pot, or a bad solder joint.

Moloch

Okay, so I've taken a look inside and removed the metal protectors from the pre-amp valves. However, I can't really see anything there. That doesn't alarm me much though, given that both channels are saturated (especially the Red): 

Where I can see a contrast though is in the power amp values. The one on the right is far brighter at the top than the one on the left:

olafgarten

Darryl_Morris said:

Okay, so I've taken a look inside and removed the metal protectors from the pre-amp valves. However, I can't really see anything there. That doesn't alarm me much though, given that both channels are saturated (especially the Red): 

Where I can see a contrast though is in the power amp values. The one on the right is far brighter at the top than the one on the left:

Looking at the tube layouts it's most likely V1, V5 or V6. 

12AX7 Tubes are sort of like 2 tubes in one package, Half of V1 is used for Channel 1/2, and V5 and V6 are used exclusively for Channel 1 and 2.

Both channels share a power amp. 

V1 is the one furthest to the right, behind the Output Transformer and V5 and V6 are the two in front that are in line with the Output Transformer. 

ICBM

*It is nothing to do with the power valves.*

Difference in brightness of filament glow and the blue glow when the valve is conducting are normal and not a sign of incorrect operation.

If it's a valve problem at all, it's a preamp valve.

Moloch

I should have an EHX 12AX7 knocking around that I used when trying to fix an old Blackstar Metal pedal, However, I can't find the damned thing. Should really just bite the bullet and buy a couple from the sound of it.

ICBM

If you can't find your spare valve, you can use one of the three valves from the red channel - V2, V3 or V4 - to test the green/blue channel - V5 and V6 - since you know they're all working fine. V1 is shared between both channels, but it won't be that if you're hearing plenty of gain but no volume. If it's either of the others it's more likely to be V6 than V5 for the same reason.

Moloch

Progress report:

I found my spare 12AX7 and gave it a go in V1, V5 and V6. I didn't play any guitar on it and just went by the 'Thumb on cable jack' buzz. No improvement. because my EHX valve has been around a while, I followed @ICBM's advice and took one from V4, trying it in V5. No change. Just to be sure, I also tried the red channel with my EHX valve in it. Still loud as hell.

I'm no pro, so I can't say for certain that I installed the valves perfectly, but there seemed to be no change at all (good or bad), so the V1, V5 and V6 valves don't seem to be the problem.

One thing that I did notice is that there's a random Groove Tube in there among the others (which are unmarked, but I believe are supposed to be JJs). Clearly the previous owner (or GAK where he got it from) switched one out at some point.

ICBM

It sounds like it’s not a valve problem unfortunately.