First time putting new tubes into Peavey VB-2

[Billythebass1]

Hi Guys, after a lifetime of transistor and hybrid amps I’ve finally got an all-valve head. The VB-2. Sadly it sounds crap and so I’ve got 6 new JJ EL34 Mark 2 tubes that came today. Can anyone help with two questions please? The tubes have values written on the boxes. PC and TC. What’s that? The PC values range from 78 1 to 78 7 in the second box. The TC values range from 10 7 to 11 with no value in the second box. In ordering a matched sextet are these values close enough? Secondly, having never swapped the tubes in a head before I believe the current tubes are the original set from 2008 and therefore from watching YouTube I don’t intend to do any more than swap them straight out and not mess with anything else. Any tips on this at all? Sorry, that’s a big ask but any help gratefully received. Thanks.

 

[Downunderwonder]

No idea but @BassmanPaul might.

 

[BassmanPaul]

Nope, sorry but I’m coming up blank. They could be from some kind of rig that the vendor uses to match his tubes. I’d give them a call for an explanation.

I feel that going out and buying a set of tubes without first verifying what the problem is was premature.

With a new set of output tubes the bias needs to be checked and reset as required. There again, it’s possible that the bias has drifted and that’s why the amp sounds bad.

Honestly I think that a look see by a tube amp tech might well be worth it.

Just for interest, in what way does the amp sound like crap?

 

[agedhorse]

I would want to verify that the tubes that are in the amp are in fact bad before just replacing tubes. There could be other damage that under some conditions ruin a new set of tubes. I would really recommend having a tech go through it to be sure everything is good (and to identify WHY the amp sounds like "crap"... have you played another one that sounds better side by side?) before installing the new tubes and biasing the output stage properly. Very important.

 

[Billythebass1]

Thanks guys. It’s always difficult to describe a sound but it’s like a metallic distortion right from the start, and not in a good way. The preamp section sounds clean when taking a feed from the tuner socket into the power amp section of another amp. So I figured it’s the output tubes. I didn’t say, for the sake of a short story, that I took it to an authorised Peavey service centre near me. They had it for a week and after a few phone calls they said it seemed ok but only changing the tubes would show if this is the problem. They said I could save money doing it myself. So figuring that they aren’t really interested I took it away and have ordered new tubes. To do it myself.

 

[coreyfyfe]

The tuner output is tapped from the input signal so you’re missing most of the preamp in your assumption that it’s power tubes.

I’d say find a better tech, the one you used doesn’t seem interested in actually servicing your amp.

 

[agedhorse]

The tuner output is tapped from the input signal so you’re missing most of the preamp in your assumption that it’s power tubes.

I’d say find a better tech, the one you used doesn’t seem interested in actually servicing your amp.

Agreed.

 

[bb5000]

PC = plate current, TC = tube transconductance....as measured from the tube tester that the vendor/distributor used. Gives a hint about the tube characteristics. Similar values indicates a good match (in the tube tester scenario that was used at that time, not necessarily in your amp (but very likely)). And as previous posters have stated, bias needs to be set for new tubes, and it's very possible that the problem could be something else.. power tube failure most often lead to blown fuses etc. It's possible that the tubes may have drifted over time, sometimes a bias setup is all that's needed instead of replacing the tubes. If the preamp sounds crappy (the VB2 has a DI out from where you can tap the preamp signal to another power amp), the troubleshooting should start with the preamp..
That being said, I've serviced amps that have had power tube issues , not blowing fuses. Partly working with blown screen resistors etc...sounds bad, but no fuses blow. The whole circuit needs to be checked at such times to find the issue.

 

[BassmanPaul]

Agreed.

Yep Very much agreed.

 

[Billythebass1]

Thanks very much for these useful thoughts guys. I can check the DI out no problem; and if that works out I’m left with how to bias the new tubes. There’s no video on YouTube that is specific to the VB2 as far as I can find. So next question has to be about that. Any ideas where I can go for that? Really what I’m hoping for is that I’ll learn over the years to look after it myself if at all possible. Cheers

 

[agedhorse]

Interesting, creative and unproductive way to proceed with your amp. I would not put a new set of output tubes into an amp without verifying that there are no problems. It's a quick way to ruin a perfectly good set of new tubes.

 

[Billythebass1]

Ha! You’re absolutely right of course! I’ll keep you posted thanks all so far.

 

[Downunderwonder]

Good amp techs!
Good amp techs!

 

[Billythebass1]

I’ll check that out, maybe there’s someone here in the UK. Thank you.

 

[Billythebass1]

Thanks to bb5000, I must acknowledge the idea to try the DI socket. Did so and both pre and post DI options are working good through an external power amp. I can see why the overdrive channel divides opinion on the VB2; it seems full on buzzsaw distortion right from the start. I may have to play around with it. So I genuinely believe it’s just in need of new power tubes so far. Can anyone help with biasing the tubes with a multi meter? Thanks again.

 

[Paulabass]

When you own a tube amp, you should also know a trusted tech. It's like a Dr./patient thing. Not saying tube heads new a lot of service, but you should know where to go when the need arises. Most music store 'techs' are not experts (with exceptions- apologies to the stores with great techs.)

 

[Billythebass1]

Hi guys, well I’ve got as far as testing the current bias which is 48.1. Beleive me when I say this a voyage of discovery. I guess the next thing is to work out what it should be! What do I need to look at? Are there tube tables or something out there which will reference the specs or the range of values for the new tubes? Thanks, Will

 

[agedhorse]

What are you measuring, voltage or current?

What is the current of each tube? You may have to calculate this.

In order to know the range so acceptable plate currents, you also need to know that plate voltage that the amp operates at.

You might need to contact the manufacturer to see if that information is available. A good tech would do all these things.

 

[Billythebass1]

The multi meter is set to read DC volts and is reading 48.1. The tubes have a plate current of 78 according to bb5000 earlier. This is 61% then of 78. Should be about right do you think? I’d have to ask Peavey what voltage the amp operates at but I guess most VB2s would be the same?

 

[S-Bigbottom]

It is great that you want to learn, but continuing to power on a sick amp risk you causing more damage every time you turn it on to do more trial and guess work. This will cause your current problem to have to be fixed plus whatever just went. More cost and less likely to be repairable.

Using an amp you do not mind losing forever is one you practice to learn on, not one you need to work.
Take it to another Tech now before something else goes.