Mismatched valves and biasing

[SwamiRob]

Recently bought a guitar amp off eBay (sorry on both counts...) and after getting in the practise room I noticed one of the power valves had been replaced with a Fender one, something not mentioned on the auction. I've heard alot of opposing views on how much tube brands and consistency matter, but at the least it'd be nice to hear that if everything's biased then it's not gonna knacker my amp if that's hopefully true.

The rig seems a little bit on the quiet side for a 100W guitar amp with a 4x12 which is a bit worrying, I know 4x12s aren't an exactly ideal design for projecting guitar or most things for that matter, but I'd kinda expect it to at least be fairly deafening when I have the thing on half volume.

Which is another issue cos the guy said there were celestion GT75s in the cab, something which has turned out to not be true looking in the back, just has some very generic looking drivers in with nothing but "8ohms" stamped on the back, although the cab itself seems very well made and has acoustic treatment in it and everything which is pretty cool cos I hate all those bassier then bass guitar cabs you get. The cab apparently has a 102dB sensitivity rating with whatever weird generic drivers are in it though, so I'd expect it to maybe be a tad louder then it seems to be at the moment especially since I'm running the texture knob thing on the power amp pretty much in full A/B mode, dunno how sensitive a really sensitive guitar driver is compared to a bass one though.

Anyway, all that and I bought the thing from a policeman...

[T-Bird]

Hi.

If You can't trust a policeman, who can you trust?




As for the tubes, and them being different kinds and obviously unmatched, whenever an (ab)used tube amp is bought, one usually counts a cost of new tubes and biasing when figuring the final figure.

Unless the tubes that are in there are vintage, and test good, just keep them as spares for a SE amp. If You have one.

Regards
Sam

[SwamiRob]

Well it's a Peavey Valveking, and it has a "texture" knob which you can use to change the output from class A/B to class A to change the phase of the power tubes or something along those lines. So you go from a very clean sound with alot of head room (although not so much in my case it seems...) to a very saturated compressed sound. I dunno what proper eBay etiquette with tubes is, but I don't even know if it has been biased and there certainly wasn't mention of a missing tube. To be honest I really know very very little about power amp types and even less when valves are factored into it. The only thing I know about valves is to be very bloody careful to get the impedance matched correctly.

Here's the eBay auction: http://cgi.ebay.co.uk/ws/eBayISAPI.d...#ht_613wt_1143

[SwamiRob]

Bump!

[LongHairFreak]

I was told by a tech from Savage Audio that output measurements (biasing?) is what needs to match up. Manufacturer matters not a wit but for personal taste and flavor.

For the record, my West has one tube that is of a different manufacturer (though obviously of the same make) from the others and it's running great.

[SwamiRob]

Cheers, that was what I was most worried about, didn't really think it'd be the case that having different valves in the poweramp would be disastrous if they were biased properly but just thought I'd check. Do feel a bit scammed that he didn't mention that either, but obviously a little more scammed that I haven't got the £200 odd worth of speakers that made me buy that rig instead of waiting for another valveking.

Also, the amp seems to be running pretty hot, I dunno how hot a guitar amp, or any valve amp that hasn't got a fan, is meant to get when they're pushed a little. All I know is my guitarists Fender Deluxe in my other band doesn't seem to get that hot, and he's using it for heavy rock/hardcore/etc with it, does very well for itself been 40W with only 1 speaker, but he does have to rag it a bit.

[T-Bird]

Hi.

Originally Posted by SwamiRob

Cheers, that was what I was most worried about, didn't really think it'd be the case that having different valves in the poweramp would be disastrous if they were biased properly but just thought I'd check.

You're kidding, right?

While it's remotely possible, biasing different valves/tubes without having individual bias adjustment trimmers is next to impossible if a reliable operation is the requirement.

OTOH if the amp does have individual bias adjustment for every valve, it's possible to use "mismatched" valves even though the performance does suffer.

Originally Posted by SwamiRob

Also, the amp seems to be running pretty hot

A sure sign of trouble, at least from over the 'net troubleshooting POW.

Tech time.

Regards
Sam

[SwamiRob]

Cheers for the help, what made me worry less is before I bought it I saw this site (http://ultimate-guitar-valveking.wik...+-+Electronics) cos I'd come across a mention of a bias pot mod and wanted to check it out. On that section I posted there seems to be a bias for each valve. Will the texture control not have anything to do with bias then and is something different all together? Some details about how it works are here http://www.peavey.com/products/insta...mps/valveking/ , but like I said I haven't a clue when it comes to valve stuff.

Anyway, this guy probably deserves a bollocking for not mentioning the non-standard tube on top of the fact 200 pounds worth of speakers seem to be missing...

[LongHairFreak]

By no means am I a tech, so perhaps I'm missing something? From what I understand, the norm is to replace all power tubes at one time so that they all may age consiststantly.

When I had to replace one though, my amp's bias was set, current draw tolerances were measured and a fourth tube was found that matched (within 5%) the remaining three. Manufacturer/bottle shape was not even considered.

Not the first choice sure, but this got me up and running and I saved a few bucks. Though I'll likely have to replace more tubes sometime, it runs great right now.

Is this wrong?