100w valve head cutting out

[RionZion]

Hi all, i have a Wharfedale TCT100 100w all valve head running into a hand-made 2x12 celestion cab. The head is a bit of a rare beast, i dont think very many were made, they were later re-branded and now sell as Albion amps. I've been gigging the amp regularly for about 4 years now and had no problems at all until last night...
towards the end of a 2 and a half hour set, the amp started cutting out.. to be more accurate it lost volume considerably and the sound was weak and distorted (not in a desirable way, more crackly/broken sounding). I play through a distortion pedal and a wah as well as running a multi-effects through the send/return loop on the amp, so i bypassed the wah and distortion and plugged my guitar directly into the amp although i forgot to bypass the loop on the amp (gig pressure!). Still, the sound was the same... it seemed ok for the first few milliseconds of hitting a note on the guitar but would then immediately go quiet and broken-up. I'm fairly used to the sound of dodgy cables and the like and i'm as sure as i can be that this was an amp problem... besides which i ended up plugging the guitar into the p.a. to finish the gig and there were no signal problems there.
Have spent today testing the amp with my usual gig setup... played it for 2 hours at gig volume, left it on for several hours, came back and played some more... absolutely fine!
As i say, i've gigged this amp a lot over the last four years, it's been in some very tight spots, left on for hours, played at high volume and i've never had a problem. Now i'm paranoid it's going to happen again next time i have a gig... and i have no idea what might have gone wrong!
Actually come to think of it there is one slightly odd problem i've had in the past, but hasnt happened for quite some time now - the amp sometimes used to hum audibly through the speaker output on turning it on, kind of an interference sound with a certain amount of hiss as well although it remained constant while the guitar signal was fine and could be turned up to mask the sound. i usually found this went away after the valves had warmed up or after a judicious "tap" on the back of the amp. I should also mention that when i first got the amp, being a total newbie to head/cab configurations and having always played combos, i very stupidly was using two instrument cables for the speaker outputs!! Once i had burned out a few of these i realized my mistake and got proper speaker cables and all has been fine since.
Any thoughts, opinions or suggestions would be most welcome, cheers
PS have visually checked the valves and although i'm no expert they certainly seem fine, i'm not getting the characteristic crackle of dying valves anyway - been down that road before with my old laney vc30... the valve sockets seem sturdy enough, valves are all solid and look to be in pretty good shape although they all say made in china... except one sovtek in the input stage that i think previous owner must have put in, not sure how reliable chinese valves are considered to be?

[Chuck H]

Lately Chinese valves have been proving MORE reliable than Russian. At least that's the general consensus among many of the repair experts here and also my humble experience. But...

Not after they've been gigged regularly for four years!!! It might be time for some new tubes brother

I'll guess your problem was microphonic power tubes. When power tubes go grossly microphonic they sort of clamp and make a warbled distortion on hard notes, especially bass notes. Turn the amp on and ready to play. Then thump on the power tubes. If you hear anything more than a small thud or ping, that is, more like a PFFFFFTTTT! They might be bad. If you have indeed gigged them even semi regularly they're probably old enough to need replacement even if they're not microphonic and may have been the cause of the power fade and distortion otherwise.

Another possibility is that the amp is just well used and it's time to clean it up. Dirty or oxidized tube or socket pins, switches, pots, jacks (especially effects loop switching jacks) can all cause the problem you describe.

The good news is that these are the most likely causes with your symptoms. No guarantees. But check up on the age of the tubes and clean up all non soldered contact points first and see if that doesn't help.

[RionZion]

Thanks Chuck, great advice i'll look into the amp further. There's only a very short hum when the amp is first taken off standby, and even this seems to go away once fully warmed up. Just having a look around the internet at valves... plenty of chinese ones, with a fairly wide variation in price... would you recommend any particular make? cheapest ones seem to be Shuguang valves around $50 for a set of 4... too good to be true? or are they all the same ones just rebranded?! I think in my current situation i'm most interested in longevity rather than audiophile tone... i've been using fairly cheap chinese valves that were fitted as standard to the amp and to be honest have no complaints about my sound. I cant see the point in spending hundreds on a set of "premium tone" valves that might only last a couple of years, although i realize my tone could always be better, as a struggling musician i'm more interested in keep cost low at the moment!!

[Chuck H]

The lowest cost valves are the ones that work right and don't cause near future problems. Most of the repair guys here are recommending Ruby tubes. They are the same Shuguang tubes you'll get anywhere except that Ruby has tested and matched them and just basically put their own stamp of approval on them. Along with the best warranty offered for new tubes. Ruby culls and either tosses or returns tubes that don't make the cut. You don't want to try this yourself because if you do you WILL be tossing tubes and/or trying to negotiate returns. Many on line sources are very hard to negotiate returns with and even some vendors have given people problems. The guys that sell Ruby tubes generally accept returns without any problems. The premium Ruby is charging for their tubes is more than worth the extra cost IMHE because of saved trouble and time. If you choose a different route I can only wish you luck.

And, though you say it doesn't matter much to you, of four different brands of EL34's I tried recently the Ruby's sounded the best to me.

[pdf64]

A full set of tubes for a 100W amp may be unnecessary cost, though if you've got none at all, some good spares are a basic, eg a tested 12AX7 and a pair of EL34.
If you're in Europe, Banzai seems a good, cheap supplier for Ruby Ruby Tubes :: Tubes :: Banzai Music GmbH
However, my guess is that there may be a heat induced connection problem, rather than a bad tube.
Especially that the amp was likely built with a lead free process.
So it may be a good idea to investigate for local techs, as it will likely require a high level of competence to identify such an issue.

[Chuck H]

I've been gigging the amp regularly for about 4 years now and had no problems at all until last night.

And these were the tubes that were in the amp when HE got it. I'm not saying it isn't as you say. But I still think new tubes is a good start. And, as you said, having spares is good. If the old ones "work" then those would qualify. Unless they are microphonic. RionZion never reported on this.

[pdf64]

towards the end of a 2 and a half hour set, the amp .. lost volume considerably and the sound was weak and distorted (not in a desirable way, more crackly/broken sounding). I play through a distortion pedal and a wah as well as running a multi-effects through the send/return loop on the amp, so i bypassed the wah and distortion and plugged my guitar directly into the amp although i forgot to bypass the loop on the amp (gig pressure!). Still, the sound was the same... it seemed ok for the first few milliseconds of hitting a note on the guitar but would then immediately go quiet and broken-up.

Agree that it's possible the power tubes may be getting weak, but these symptoms seemed to point to an intermittent connection, possibly affecting the power tube operating conditions, eg screen grid supply.
As it's got 4 power tubes, it seems more likely that the issue is regarding a common supply rather than particular individual tubes, as one tube could be weak without the user necessarily noticing.
Of course one tube could already be weak, and the issue is that its buddy on the same side has an intermittent problem, in which case a whole new set of EL34 may be a good idea.

[Chuck H]

Well I do HOPE it's not an intermittent fault as the OP indicates not being interested in unnecessary expenditures But yes, there are symptoms pointing that way. It could still just be dirt and a little cleaning will do it. The effects loop get used so... (need crossed fingers emoticon).

[g1]

Agree with Chuck that anything in the FX loop is suspect. The first thing I would try when the problem strikes is to disconnect the cables at the send and return of the amp, and put a good patch cord between send and return.

[RionZion]

Hey all... did i bit of investigation today - i think i may have found the problem... one of my output tubes has a small but definite crack in the glass!! the tubes have the spring style retainers and i noticed on all the tubes there's a certain amount of wear on the glass where the edge of the spring is pulled tight against the glass - but on one of them it is so worn there is an actual crack in the glass. I guess i've had a pretty good innings with these valves considering i bought the amp second-hand originally anyway!
Also had a good look at the electronics, i'm no expert but everything looks to be in order, no signs of heat damage or corrosion, all solder looks bright and shiny, and generally very clean considering the age of the amp.
So have been browsing the web for tubes, Ruby EL34BHT seem the most suited at a reasonable price and sound very rugged. the originals were branded "Electron Tube:EL34-B" - i suppose i'll have 3 working spares in case of emergencies too, though highly unlikely i'll ever use them... maybe i should get into DIY hi-fi?
Also getting 4 replacement tube retainers of the wire type, which i think is far less likely to cause a weak point on the glass.
As an aside, i noticed also that the phase splitter valve is different to the other input valves, it's a sovtek 12ax7wc, the others are chinese 7025's. Apparently the 12ax7wc is a higher gain version of sovtek's previous 12ax7's... which might explain why i find it almost impossible to get a truly "clean" sound out of my amp. This must have been put in by previous owner maybe to boost up signal from a low level guitar, since the amp doesnt have hi/low inputs, just the one. I play a gibson SG so have the opposite problem... so last item on my shopping list is a 12AT7C also from ruby tubes, hopefully that will clean things up a bit

[RionZion]

A full set of tubes for a 100W amp may be unnecessary cost, though if you've got none at all, some good spares are a basic, eg a tested 12AX7 and a pair of EL34.
If you're in Europe, Banzai seems a good, cheap supplier for Ruby Ruby Tubes :: Tubes :: Banzai Music GmbH
However, my guess is that there may be a heat induced connection problem, rather than a bad tube.
Especially that the amp was likely built with a lead free process.
So it may be a good idea to investigate for local techs, as it will likely require a high level of competence to identify such an issue.

Thanks for the link, great site and very reasonable delivery

[pdf64]

An actual crack in a valve's glass envelope, ie such that its vacuum and so ability to function fails, will have a very obvious consequence on the appearance of the getter flash, ie it will turn white, see photo 4 www.thetubestore.com - Blue Glow in Tubes, Tube Getters, and other Info
From your description, what you've got may just be a scratch on the envelope surface.

If you're fitting different EL34, it may be beneficial to consider biasing What Is Biasing?
It may be possible to just replace them without making any adjustment and there not be any major problem. However, there's no guarantee of that and if the amp isn't adjusted to best suit the particular valves fitted then it's unlikely that optimum performance (in terms of tone, operational life) will be achieved.
The back panel of the amp may have bias adjustment / measurement points; unfortunately the manual for the current model doesn't provide any guidance for their use. Perhaps instigate what's on your amp. http://www.albionampsus.com/wp-conte...ser-Manual.pdf

The impact on tone / performance of an individual tube used in a regular (long tail pair) phase splitter circuit is limited, as it runs with local and global negative feedback. Whereas a 12AT7 is rather different to a 12AX7. It's very unlikely to cause a problem but may change the way the amp 'feels'.
Try it by all means, but it may be preferable to get the amp working right with the valve types it was designed around; then undertake such tweaks from that point.
7025 is just a different name for 12AX7 (as is ECC83); so feel free to swap your preamp valves around, eg to try a 7025 in the phase splitter.
In the days when valve production had to meet performance standards, 7025 indicated a particularly high spec 12AX7; these days it's just a marketing gimmick / label.

[RionZion]

Thanks pdf64, great info... have re-assembled the amp, really could find no obvious faults in the wiring or components. Have tested the amp again and all seems fine, real test will be a gig situation - will have to have another amp on standby this time! Regarding the tubes, they are behaving normally as per the article you linked... the cracks in glass are just like stone chips in a car windscreen. the most severe one clearly is an actual crack in the glass... will post a pic (not at right computer at moment). Anyway, i will definitely get some new tubes as it's about time... had the same thought regarding the input tubes so am also going to order a 12ax7 to compare to the 12at7. i swapped the 1st input valve with the phase splitter - no discernible difference - still cant get a clean sound that doesnt break up when i play hard, unless i back off gain and drive to the point that the signal level is crap. I think the 12ax7wc is just a bit over-powered for the circuit.
hmm.. biasing i did wonder about that, there are two adjustable resistors under a plate on the back of the amp... plus a three pin connection - will have to do some reading up

[pdf64]

To reduce the gain, consider using a 5751 somewhere earlier in the pre-amp, eg V1, V2, where the sections within the valve are likely to be arranged in series / cascade (they're effectively in parallel in the phase splitter).
A 5751 should give a similar gain reduction as a 12AT7 but should keep the tonal response of a 12AX7, as their output impedance is the same.
To really bring the gain down, consider a 12AY7. You may find that they sound nicer than 12AT7.

To check the EL34 bias, you'll need a multimeter, preferably one with a millivolt mV range that can measure 50 - 100mV with reasonable accuracy. For basic checks, something cheap should be fine.
It would be best to get details of the bias procedure from Albion, but it will probably involve putting the black probe on to the centre pin and measuring the outer pins mV with the red probe, and tweaking the left / right trimmers as required to get the left / right readings into the preferred range (likely about 60mV).
Whatever, you should be fine to just take measurements as indicated.

[RionZion]

...bit of a roundabout way of doing this but here's a pic of the so-called crack in the glass... opinions? http://music-electronics-forum.com/u...661-albums225/

hope it works, if not here it is anyway (didnt realize i could actually post pictures directly - duh!)