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rbone100
12-17-2004, 10:36 AM
What is the correct bias setting for each model?

VHT CLX?
VHT UL?

Any others?

Giga
12-17-2004, 11:34 AM
What is the correct bias setting for each model?

VHT CLX?
VHT UL?

Any others?


I only know for the UL: VHT recommends the 70% rule and I also talked to Greazygeo who liked 60% better.
With the 6550's I ended up with 45 mA sounding best for me and with the Svetlana KT88's I liked around 55 mA better. Both are within the 60-70% region.
Right now I'm trying to get a hold of Sovtek and EH KT88's to try. I'll let you know how that turns out.

Giga.

Sonar
12-17-2004, 12:25 PM
What is the correct bias setting for each model?

VHT CLX?
VHT UL?

Any others?

Ralphie,

What method are you using to determine bias readings? (OR what bias TOOL are you using -- ie: Alesandro Bias Meter, Weber BiasRite, etc.).

I think we would want to be a step further than setting ALL tubes (ie: EL34, EL84, KT88) for a certain predetermined "number". Each amp is different and these general rounded suggestions will get you in the ballpark, but shouldn't be considered a method to get your ass in the correct seat.

Lord Valve had a nice article at: http://www.duncanamps.com/technical/lvbias.html

"Unless you take the plate voltage into consideration, a current specification is meaningless. For instance, 40 mA at 250 volts is 10 watts; the same 40 mA at 500 volts is 20 watts... TWICE as much. In both cases, the current is the same. Amps vary; two identical amps can have plate voltages which differ by as much as 20%. Just because you have a schematic that specifies the plate voltage in your amp as being at 450VDC, don't expect to see that voltage when you take a measurement. TAKE the reading, don't assume the voltage will be as specified. Trust your meter."

Especially considering that I see you with quite a few very nice amplifiers (and tubes ain't cheap), you should work with a tool that gives you the actual plate voltage also.

If you don't have a Weber BiasRite meter (WITH the PV or Plate Voltage option) yet, I'd certainly consider getting one.

These are an ESPECIALLY great tool for the guys with EL84 powered VHT amps (except the new cathode biased 30-watt, I'd imagine). This is because Weber VST will make the units with 9-pin adapters for these EL84 powered amps. They are the only folks I know making this.

The Weber BiaRite makes bias reading and adjustment a breeze. Damn fine tool from a great company. Weber even has a quickie "Bias Calculator" link on their Tube Bulletin Board.

rbone100
12-17-2004, 04:35 PM
I have a Weber BiaRite but I haven't used it yet. I want to learn how to use it before I need to learn how to use it! :roll:

Sonar
12-17-2004, 06:52 PM
I have a Weber BiaRite but I haven't used it yet. I want to learn how to use it before I need to learn how to use it! :roll:

Well.....no time like the present!

You could start off by simply "reading" and writing down what the BiasProbe tells you. I mean, don't open the amp or plan anything yet....just a simple reading.

So plug those adapters in and stick the tubes in the other end.

Then fire up your amp as usual and get it to operating temperatures.

Then write down those two numbers for each tube: bias & plate voltage.

Shut the amp off and we'll meet back here and talk about your results!

You game? :wink:

rbone100
12-17-2004, 08:05 PM
Ok, I'm game... But I won't be able to do this until tomorrow afternoon (Saturday).

I guess this gives me a good reference point of where to start if I use the same tubes!

Sonar
12-18-2004, 12:18 AM
Ok, I'm game... But I won't be able to do this until tomorrow afternoon (Saturday).

I guess this gives me a good reference point of where to start if I use the same tubes!

Yup! See where the original tubes are. See how close they are still "matched". See how they are currently biased compared to whatever might turn out to be the "optimal" suggested settings.

All very interesting and valid in the search for ultimate tone.

rbone100
12-18-2004, 03:07 PM
Hey Sonar - Do I need to connect the speaker cab to the head when I do this--Have a load on the head?

Ralphie

NoGodsNoMasters
12-18-2004, 05:08 PM
Hey Sonar - Do I need to connect the speaker cab to the head when I do this--Have a load on the head?

Ralphie

Yup

Holger

rbone100
12-18-2004, 05:18 PM
Update:

I tried to use the BiasRite but the tubes won't fit in the sockets while in the BiasRite sockets. I am going to have to take the amp out of the chassis in order to do this. I don't have that much time this afternoon to take it out of the chassis so I will do this tomorrow. I should have gotten the right angle attachments!

Ralphie

Sonar
12-18-2004, 11:44 PM
Update:

I tried to use the BiasRite but the tubes won't fit in the sockets while in the BiasRite sockets. I am going to have to take the amp out of the chassis in order to do this. I don't have that much time this afternoon to take it out of the chassis so I will do this tomorrow. I should have gotten the right angle attachments!

Ralphie

Holger is correct that the speaker output MUST be attached to a load (the speaker).

I should ask what configuration of cabinet are you trying this on. I mean. If it's that top mount combo, the chassis will need to be completely removed from the cabinet AND then disassembled.

The front mount combos and heads are easier because you can remove the four mounting screws from the top and slide the chassis out just enough for the tube adapters to be attached AND give access to the 'lil blue bias adjustment pot inside your amp.

Several considerations about sliding that chassis "out just a bit":

#1: If you have reverb, you'll have to remove the upper reverb wire retainers that affix the wires to the chassis (This is mainly a consideration with the combo). On the head version with reverb, the reverb wires are often suspended by nylon tie wraps to the holes in the reverb pan. Cut the ties and have replacements available for reinstallation. OR it may be some other fastener (if you have reverb AT ALL).....my point is to look for ANY other fasteners and NOT just tug on the chassis being hung up by the reverb leads.

The BEST idea is to have a small flashlight handy before you start and look around inide that cabinet before you do anything else. Get to know that sucker literally inside AND out. I use a creepy new LED flashlight that has replaced my drawerful of Mini Maglights. Emits a strange white/blue light that is WAY bright and great for this exact stuff. Another tech I know bought a cool LED flashlight rig that he wears on his head like a hat (he got it at a bicycle specialty shop and YES.....it looks stupid). But it's really cool because it's pointed wherever he looks and it's "hands-free" in operation. (Am I sounding OLD, or what?) :lol:

#2: Consider that the shielding on the inner top of your chassis is done with thin metalized foil that is stuck to the roof of the cabinet like sticky shelf paper in your granny's house. If you catch that stuff on the front lip of the chassis as you slide the chassis out, it will tear and curl and basically get f**ked up. Be mindful of this stuff being there and STOP if the chassis offers any resistance when you start pulling it out. Use finesse and a bit 'o english......never force.