Bias

Pundan · January 21, 2012, 06:20:35 PM

Hey, would this be good enough to use when changing tubes? I've never done a bias adjustment myself before, but I have a friend that knows what he's doing.

http://www.ebay.com/itm/Quad-Tube-Audio-Amp-Bias-Probe-Tester-6L6-EL34-KT88-6V6-/300602047839?pt=Vintage_Electronics_R2&hash=item45fd473d5f#ht_1532wt_1396

RAGER · January 21, 2012, 06:24:32 PM

Don't see why not

neighbor664 · January 21, 2012, 06:25:47 PM

Would you like a biased or non-biased opinion?

Hemisaurus · January 21, 2012, 07:00:22 PM

Long as it's wired up right. If the instructions are as engrish as the eBay post, it will help to know what you are doing.

Watch the eurotubes vid.

http://eurotubes.com/euro-Generic-Bias.htm

Mr. Foxen · January 21, 2012, 08:06:37 PM

Kept intending to buy one of these then being annoyed because half my amps run 6 EL34. Putting in shunt resistors and not putting them somewhere stupid enough that stuff arcs.

Hemisaurus · January 21, 2012, 10:00:55 PM

Looking, eurotubes sells one for $25 and for another $10 they throw in the multimeter.

VOLVO))) · January 21, 2012, 11:28:28 PM

I dig the multi-unit...

Ayek · January 22, 2012, 12:17:13 AM

Quote from: neighbor664 on January 21, 2012, 06:25:47 PM
Would you like a biased or non-biased opinion?

Ha!

Hemisaurus · January 22, 2012, 10:26:29 AM

Quote from: SunnO))) on January 21, 2012, 11:28:28 PM
I dig the multi-unit...

With meters being so cheap, it's almost worth running multiple meters at once rather than jumping back an forth. Course if you were doing a Mesa or a Peavey VB then it might start getting a bit expensive

I can see you with a little meter shelf, with a lip on it to hold them up, the four identical meters side by side, and the four tubes all jigged up as you tweak

Damn, I'm starting to convince myself now.

Yeah, especially on those units with one damn bias control for 4 tubes

dunwichamps · January 22, 2012, 11:42:37 AM

Quote from: Mr. Foxen on January 21, 2012, 08:06:37 PM
Kept intending to buy one of these then being annoyed because half my amps run 6 EL34. Putting in shunt resistors and not putting them somewhere stupid enough that stuff arcs.

1 ohm 1% 2W cathode resistors no good?

Mr. Foxen · January 22, 2012, 03:51:18 PM

That should have ended 'will do'.

dunwichamps · January 22, 2012, 04:44:30 PM

those SC run a high plate voltage?

Hemisaurus · January 22, 2012, 05:27:14 PM

Define high? I never saw one with more than 520 on the plate, MusicMan's on the other hand had 720

dunwichamps · January 22, 2012, 05:33:25 PM

Quote from: Hemisaurus on January 22, 2012, 05:27:14 PM
Define high? I never saw one with more than 520 on the plate, MusicMan's on the other hand had 720

i was thinking 600+ is high for a generic tube amp.

Hemisaurus · January 22, 2012, 05:38:15 PM

No, then

Mr. Foxen · January 22, 2012, 06:35:55 PM

Think the SC200 with KT88 has stacked HT giving 700 or so. Gonna have to be a bit more careful there, but that is 4 valves.

From memory I think I got 540v in the SC120 without valves loaded. Drops with valves so 520 sounds about right. Some of the Laneys and Carlsbros ran pretty high.

FullCustom · January 25, 2012, 10:40:47 AM

Quote from: Hemisaurus on January 22, 2012, 05:27:14 PM
Define high? I never saw one with more than 520 on the plate, MusicMan's on the other hand had 720

Yes, but the screen voltage is only 350. Screen current and voltage is one of the most overlooked things in tube amp design. Even with 700 Volts on the plates a Musicman amp is by no means running at it's limits.

Hemisaurus · January 25, 2012, 10:56:18 AM

Quote from: FullCustom on January 25, 2012, 10:40:47 AM
Quote from: Hemisaurus on January 22, 2012, 05:27:14 PM
Define high? I never saw one with more than 520 on the plate, MusicMan's on the other hand had 720
Yes, but the screen voltage is only 350. Screen current and voltage is one of the most overlooked things in tube amp design. Even with 700 Volts on the plates a Musicman amp is by no means running at it's limits.

Yep, the plate limits on an EL-34 are 800V, a pair at 800V running with I think (without checking the datasheet) a 11Kohm impedance can push 100W, I seem to recall the rating on the MM output is 5.5Kohm, so if it could handle the extra wattage you could use it to pull 200W out of the quad, big if though, I doubt it could handle the extra power long-term.

FullCustom · January 25, 2012, 11:39:53 AM

[/quote]
Yes, but the screen voltage is only 350. Screen current and voltage is one of the most overlooked things in tube amp design. Even with 700 Volts on the plates a Musicman amp is by no means running at it's limits.
[/quote]Yep, the plate limits on an EL-34 are 800V, a pair at 800V running with I think (without checking the datasheet) a 11Kohm impedance can push 100W, I seem to recall the rating on the MM output is 5.5Kohm, so if it could handle the extra wattage you could use it to pull 200W out of the quad, big if though, I doubt it could handle the extra power long-term.
[/quote]
Correct, I've worked on a few Bogner amps that were 185 Watts RMS with just four EL34's. My Traynor is 150 Watts and only useds four EL34's also.

Hemisaurus · January 25, 2012, 01:30:19 PM

Good to have my long term memory confirmed

I tend to check the datasheets anyway, before doing anything in the real world, better safe than sorry.

VOLVO))) · January 25, 2012, 01:35:12 PM

I wish my T was a little hotter sometimes. The 800 is TOO hot, I need something inbetween. A bassman or something. I really dig the "falling to pieces" pushed as fuck tone you get from Bassman/Twins.

Hemisaurus · January 25, 2012, 01:40:52 PM

You cut the wire on the master vol didn't you? Did you do the other suggested mods? Gaining up sections is a fairly easy thing, within limits.

VOLVO))) · January 25, 2012, 01:46:30 PM

Yup, Did that all that. It sounds great. I still haven't gotten around to recapping it, I'm sensing it needs it real bad. The 800 makes it sound like a toy in terms of volume.

This is Sim's old T, and mine sounds NOTHING like that. I need to get into some decent cabs, I think.

Hemisaurus · January 25, 2012, 01:51:10 PM

Why not use the T on the cabs you use the 800 with then?

VOLVO))) · January 25, 2012, 01:53:38 PM

I do, I switched them last week, and it was better, but still not even remotely like that. I need to come into some extra money, I'll build a couple and have decent cabs.