Help for a tube amp newb?

[Nadz1lla]

If the volume smoothly fades away until nothing comes out of the speakers over a matter of a few seconds, I'm guessing that means a tube(s) has gone?

To be fair, I was being a little silly, I had the master volume up to around 7 / 8 and my attenuator working at pretty much full tilt at bedroom levels, so I assume I just pushed the tubes too hard and they decided to call it a day.

[King Zog of Albania]

Almost impossible to say without looking at the amp! If you're lucky it'll just be a tube though. You didn't mention any burning smell so that's encouraging, if a capacitor fails - or worse still, the output transformer - then you'd notice a nasty electrical burning smell.

Does the preamp still work? If there's an effects loop then see if there's still a line level signal coming from the FX send. If not then you may have damaged a tube or the power supply could have died.

Do the tubes light up? If the mains fuse has gone then the amp will be totally dead, no power light and no heaters. If it does turn on and the tube heaters come on, is there any change in glow when you flick the standby, do any other elements inside the power tubes light up? If this happens or the plates start to glow red then switch off immediately.

Attenuators. Ugh.

[King Zog of Albania]

Forgot the most obvious test - take out the HT fuse and see if it's blown. Do this first. Check it with a multimeter if you have one, or hold it up to the light and check that the wire inside the glass is still there and not snapped or melted. If it's blown then replace with the same type, thank your lucky stars and maybe  consider not strapping your amp quite so hard in future!

[Nadz1lla]

When I turn on the main power switch, one tube flares bright then dies down to a glow, then the other tubes slowly heat up and glow the same as the first. When I turn the standby on, the two large tubes stay lit but both have small square "windows" that glow blue. I took that as possibly a bad sign and turned the whole lot off.

Yeah I know I pushed it, but my living conditions don't really allow for playing tube amps at 8pm. It's a JCA20h, so at 20w not the loudest of amps and the attenuator has a 50w limit, but alas there's no such thing as a "quiet" tube amp so I had to crank the attenuator. In a perfect world I'd live in a detached house with thick walls and have enough money to sound-proof a room, but being a pauper living in a terrace house with paper-thin walls... yeah.  :? *sigh*

[King Zog of Albania]

Ok, the good news is your HT supply is still working. The bad news is that something has suffered a failure, could well be a tube, might be the screen dropper resistors, outside chance it's the output transformer, really impossible to say without opening the amp up.

Not quite sure what you mean by "window" there but really, nothing should be glowing blue. Any chance of a pic?

First step would be changing the tubes and checking the bias and screen voltages.

The sudden bright glow in a preamp tube when the heaters come on is fairly common, happens with the phase splitter in one of my Marshalls, nothing to worry about.

[Dmoney]

I've seen plenty of valves have a blue glow. I think the 'window' he refers to is a hole through the plates of the of EL84's in the amp. Seeing blue glow from various output valves is pefectly normal and in a push pull amp normally flickers along with your playing. I believe it's cause by electrons striking materials like the glass or plates and becoming static charges. Something like that.

Seeing flashes of bright blue... arcing, miniature lightning, is bad. You'd probably hear that happened too. That is a bad blue glow!

[Nadz1lla]

Ah okies, Toe-Knee and others have all said in the past that Jet City output transformers were fairly unreliable, so it wouldn't surprise me at all if it was that.

Also, yeah, Dmoney I think hit the nail on the head here:

I think the 'window' he refers to is a hole through the plates of the of EL84's in the amp.

There's no arcing or anything like that, I'll try and get a video of it if you guys reckon it's okay to switch it on?

[Toe-Knee]

Ah okies, Toe-Knee and others have all said in the past that Jet City output transformers were fairly unreliable, so it wouldn't surprise me at all if it was that.

Also, yeah, Dmoney I think hit the nail on the head here:

I think the 'window' he refers to is a hole through the plates of the of EL84's in the amp.

There's no arcing or anything like that, I'll try and get a video of it if you guys reckon it's okay to switch it on?

I would check the valves before jumping to panic mode!

You mentioned that one of the power valves flaresup brighter initially. Could you do a quick video? it could be redplating but its hard to tell from the description.

This could potentially be a simple valve failure.

[Dmoney]

the flash is pretty common with certain preamp valves. Like King Zog says, it's caused when you apply voltage to the filaments in the valve. It's not uncommon. The construction of the valve is probably the main factor in whether it happens or not.

Redplating would appear as a red/orange glow on the large grey flat sections of plate material on power valves. If you can't SEE the valves when you are jamming hard into an attenuator, then they MAY have been redplating, overheating, and this leads to other issues. One issue with using an attenuator is like you say, cranking the amp and draining out the power. You can be driving the balls out of the amp and not really be aware of it, and then something gives out.

You can have some valves with poor vacuums that glow because gas in the envelope will catch electrons. That isn't always a big problem either.

[Nadz1lla]

Okie dokie, cheers for the info so far, guys. If I get enough time today (currently working my ar$e off) I'll take the amp out of the cabinet and take a video of turning it on etc so you can see what's happening.

Whilst I'm here, though: Toe-Knee, you said you've replaced some OTs on Jet City amps before. I know it changes the sound, so if it is the OT that's gone, now might be the time to experiment? Knowing nowt about OTs (or amp components in general), does it have to be "Soldano spec", or could you use, say, a Marshall OT instead? If so, how would it change the sound?

I'm thinking if the OT has gone, I'd like to replace it with something we all know is going to be more rugged, like a Marshall component etc.

[jpfamps]

I'm thinking if the OT has gone, I'd like to replace it with something we all know is going to be more rugged, like a Marshall component etc.

ie NOT a Marshall OT!

[Toe-Knee]

Okie dokie, cheers for the info so far, guys. If I get enough time today (currently working my ar$e off) I'll take the amp out of the cabinet and take a video of turning it on etc so you can see what's happening.

Whilst I'm here, though: Toe-Knee, you said you've replaced some OTs on Jet City amps before. I know it changes the sound, so if it is the OT that's gone, now might be the time to experiment? Knowing nowt about OTs (or amp components in general), does it have to be "Soldano spec", or could you use, say, a Marshall OT instead? If so, how would it change the sound?

I'm thinking if the OT has gone, I'd like to replace it with something we all know is going to be more rugged, like a Marshall component etc.

There are many options that you could go with. It really depends just how much you want to spend. InMadOut make great stuff that's well priced and I've heard great things about BLS (UK company) I have a OT but haven't finished building the amp yet so I can't say how it sounds.

I think Mercury magnetics make a drop in unit but this will be expensive and personally I think they're overhyped.

I second JPF on Marshall OT's not being rugged!

[Nadz1lla]

Oh dear, I thought Marshall were workhorses, heh. I have a lot to learn, methinks!  :lol:

[King Zog of Albania]

Very few modern amp manufacturers really fit what I'd call a "rugged" transformer. With more guitarists just playing to their bedroom walls, manufacturers are starting to think that making amps roadworthy and gig-worthy is an expensive overhead. If you're making a 50 Watt amp then why install an expensive and heavy transformer that can easily handle peaks of twice that when the player never turns it up beyond 3 when all the neighbours are out? Why not just install a component that may or may not last then replace the ones that fail?

Same with power transformers and power supplies, why build a choke filter when a CRC filter will the same job about 70% as well and at a tenth of the cost?

[Nadz1lla]

Just had a look at some of those companies, BLS is pretty reasonably priced, looked at Mercury... WOW! An OT for the JCA20H is almost as much as I paid for the amp, hah!

 :lol: