Tube Dilemma......

[madhuskey]

Hi Everyone,

Okay, could'nt afford the Svets for my VHT UL, so i rang back my VHT Dealer and asked him to send up some VHT badged Sovteks......after waiting 2 weeks a pair of Electro-Harmonix KT88s turn up in the post.....my Dealer says " no problem, these should be fine, just repace your knackered power tube and its opposite matching tube with these two, everything will be fine " [img]graemlins/scratchhead.gif[/img]

Erm......

1) They dont fit too well, they are bigger than the Sovteks and the spring clips keep popping off when i fit them ( but they do go in )

2) What about biasing? I'm reluctant to even turn the UL on in case i'm damaging something, i mean, these amps are'nt cheap [img]images/icons/frown.gif[/img]

3) EH's wouldnt be my first choice tube by a long stretch but its hard to find a dealer round here to re-bias your amp [img]images/icons/frown.gif[/img]

Help [img]images/icons/confused.gif[/img]

Steve

[Cleveland Metal]

Re: Tube Dilemma......

Well, I would have to recommend having someone bias the amp.

Are you saying out of 4 tubes, you are only replacing the the bad one and it partner? I'd recommend replacing all four at the same time.

Basically, if it's out of bias, you shouldn't "imediately' damage the tube or the amp. BUT, depending on whether it's biased hot or not, you will either sound bad (cold), or shorten the life of the tubes (too hot), along with placing some undue stress on the amp's power supply. I really say to get it biased at a tube change.

[madhuskey]

Re: Tube Dilemma......

Thanks CM, will go with your advice [img]images/icons/smile.gif[/img]

Cheers,

Steve

[Wizard of Ozz]

Re: Tube Dilemma......

If your dealer told you to use 2 different brands of tubes, without any mention of them being matched (or not) and not to worry about biasing, and just swap them in, he should be beaten with a lead pipe. Morons like this have no right giving amp advice. I definitely wouldn't run my UL with 2 different brands of power tubes in the amp. Normally you would buy a matched quartet of tubes to ensure they draw almost the same amount if current. The industry norm for matched power tubes is +/- 1 mA (or less) difference between the tubes. You have no way of knowing how far off the current draw will be for these new EH tubes let alone transconductance etc. If money is an issue and you have no other option but to use the 2 EH tubes mismatched with the 2 Sovtkes, biasing is a must. You don't have to bias preamp tubes (because they are self-biasing), but after installing new power tubes your amp should always be re-biased.

If you can't find a tech nearby, get a decent Digital Multi-Meter and do some research on the net. There are plenty of great sites with how to tube biasing info.

Good Luck. [img]graemlins/headbang.gif[/img]

[Mordor]

Re: Tube Dilemma......

Be very carefull,also with the plug not connected you can receive an electroshock!

[madhuskey]

Re: Tube Dilemma......

Thanks again for the replies everyone [img]graemlins/headbang.gif[/img]

I'm not gonna try any DIY amps stuff, i wanna live... [img]images/icons/wink.gif[/img]

I'm getting it done by a Professional but i have to wait 3 weeks before he can do it ( he's very busy )

Gives me time to save up for some Svets [img]images/icons/grin.gif[/img]

Thanks again,

Steve

[Cleveland Metal]

Re: Tube Dilemma......

Svets, and done by a tech = best course of action.