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Post by edoetsch on Aug 14, 2007 13:08:36 GMT -7
Can someone give a brief description of what to expect sonically when biasing tubes as hot as recommended and as cold as recommended?
I've been told it is unsafe to bias too hot, is there any danger in biasing too cold.
For cold and hot I am referring to lower amount of plate current and higher amount of plate current respectively.
(Also let me know if I am throwing around incorrect terms)
Thanks,
Doetsch
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Post by myles on Aug 15, 2007 9:49:40 GMT -7
If the tubes are overbiased and running too cool the amp will be grainy and not develop full power.
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Post by mudskipper on Aug 16, 2007 6:15:42 GMT -7
Can someone give a brief description of what to expect sonically when biasing tubes as hot as recommended and as cold as recommended? I've been told it is unsafe to bias too hot, is there any danger in biasing too cold. For cold and hot I am referring to lower amount of plate current and higher amount of plate current respectively. (Also let me know if I am throwing around incorrect terms) Thanks, Doetsch if you are referring to "plate voltage" when you say "plate current", it stays the same as you adjust your bias. when you adjust bias you are changing the amount of DC current at idle and not the plate voltage which stays the same. if i were you, i'd take a couple of min and read this page from Aiken amps' site: aikenamps.com/WhatIsBiasing.htm
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SG123
Full Member
Posts: 221
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Post by SG123 on Aug 16, 2007 13:23:31 GMT -7
I don't know if I like Aiken's use of the term "bias current" in place of plate or cathode current. Maybe it's just me, but that seems too close to the term "grid current". When a Class AB1 power section starts drawing grid current it is clipping.
The following is also from Aiken's site "The Last Word on Class A":
"The numerical suffix appended to the class designation indicates whether or not grid current flows in any appreciable portion of the cycle. A "1" suffix indicates no grid current flows, while a "2" suffix indicates grid current flows for some part of the cycle. Class A2/AB2/or B2 requires a very low impedance, transformer-coupled or DC-coupled driver stage. The standard AC-coupled phase inverter or single-ended driver stages used in nearly all guitar amplifiers will not allow grid current flow, so they are class A1/AB1/B1 amplifiers." ----------------------
And plate voltage WILL change as the output tubes are biased "hotter" or "colder".
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Post by mudskipper on Aug 17, 2007 8:20:35 GMT -7
And plate voltage WILL change as the output tubes are biased "hotter" or "colder". oh, it does? every time i biased my amps, they do shift by a bit in correspondence to the wall voltage shifting around but it appears quite consistent as i change the bias. what did i miss? if the plate voltage shifts so much, then the formula using a single one number to figure out the idle dissipation would be incorrect, too, no? thanks in advance
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SG123
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Posts: 221
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Post by SG123 on Aug 17, 2007 9:23:15 GMT -7
"if the plate voltage shifts so much, then the formula using a single one number to figure out the idle dissipation would be incorrect, too, no?"Correct. It's a moving target. Here's a quote from Lord Valve's FAQ on biasing: "Take another reading from pin THREE (remember to set your meter on the HIGHEST DCV scale before you do!) and write it down. This new reading should be LOWER than the first reading you took, because the tubes are drawing more current now and the plate voltage will drop somewhat." www.duncanamps.com/technical/lvbias.html
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Post by dixiechicken on Aug 17, 2007 12:40:45 GMT -7
If you have a resistor in series with the anode and another i series with the cathode of a tube. Then the voltages on the plate and cathode with respect to ground WILL CHANGE if the current flowing through the tube changes. (Ohms law)
A greater current flowing through the tube will cause a greater voltage drop across the resistors. Thus changing the voltages the tube feels.
For the sake of the argument we can assume the the voltage feeding the whole circuit is constant.
Cheers; Dixiechicken
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Post by mudskipper on Aug 17, 2007 13:07:33 GMT -7
"if the plate voltage shifts so much, then the formula using a single one number to figure out the idle dissipation would be incorrect, too, no?"Correct. It's a moving target. Here's a quote from Lord Valve's FAQ on biasing: "Take another reading from pin THREE (remember to set your meter on the HIGHEST DCV scale before you do!) and write it down. This new reading should be LOWER than the first reading you took, because the tubes are drawing more current now and the plate voltage will drop somewhat." www.duncanamps.com/technical/lvbias.html Mmmm.... the above page was a very good read. thanks for pointing me to that. i still don't recall seeing a large voltage swing (something like +/- 10v) on the plate, though. i'll have to pull one of my amps out this weekend and try it again.
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Post by iggs on Aug 17, 2007 20:32:12 GMT -7
If you have a resistor in series with the anode and another i series with the cathode of a tube. Then the voltages on the plate and cathode with respect to ground WILL CHANGE if the current flowing through the tube changes. (Ohms law) A greater current flowing through the tube will cause a greater voltage drop across the resistors. Thus changing the voltages the tube feels. For the sake of the argument we can assume the the voltage feeding the whole circuit is constant.Cheers; Dixiechicken So how does that work when measuring with a bias probe like Weber's Bias RIte ... because I don't see the voltage shift while biasing ...
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Post by dixiechicken on Aug 18, 2007 2:43:25 GMT -7
You'd probably have to tranform the output stage into simpler eqvivalent theoretical model to excatly see what kind of circuit is formed by the output stage.
Remember that the tubes are connected electrically/galvanically to the output transformer, the feeding powerleads, the filter caps, the decoupling caps, the PI-tube etc etc.
All these parts taken together will affect what you see and measure while biasing.
Or when it cames to Z-amps, take the simplest practical approach - ask the Dr himself. ;D
Cheers: Dixiechicken
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Post by mikekca on Aug 18, 2007 7:32:19 GMT -7
If you have a resistor in series with the anode and another i series with the cathode of a tube. Then the voltages on the plate and cathode with respect to ground WILL CHANGE if the current flowing through the tube changes. (Ohms law) A greater current flowing through the tube will cause a greater voltage drop across the resistors. Thus changing the voltages the tube feels. For the sake of the argument we can assume the the voltage feeding the whole circuit is constant.Cheers; Dixiechicken So how does that work when measuring with a bias probe like Weber's Bias RIte ... because I don't see the voltage shift while biasing ... In amps with wimpier power transformers, you get a voltage drop across the resistance of the windings as idle current increases. (see ohms law). Same goes for the output transformer. This translates into lower plate voltage. In most amps, you'd probably have to change bias current by 15 or 20mA to see a noticeable difference in plate current.
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Post by iggs on Aug 18, 2007 13:08:46 GMT -7
The tranny in Route 66 is anything but wimpy as far as I can tell ... ;D
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Post by nitehawk55 on Aug 18, 2007 13:37:57 GMT -7
The tranny in Route 66 is anything but wimpy as far as I can tell ... ;D If you will notice the stickers on the Trannys in Doc's amps most have " MADE IN CANADA " on them . They are not wimpy
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SG123
Full Member
Posts: 221
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Post by SG123 on Aug 18, 2007 13:39:50 GMT -7
Ta-da! That's the reason most folks here won't see much of a change. [ not the Canada part ;D ]
A. You're probably pretty close to where you need to be in the first place
B. You're working on good stuff to begin with
Imagine getting ready to fire up an amp you've built from scratch for the first time, measuring the plate voltage and the cathode current - which at that point could be anywhere. Or servicing a less well-built amp... there's a lot of them out there.
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Post by mudskipper on Aug 19, 2007 8:00:20 GMT -7
with my BF Deluxe, there isn't really noticeable difference in plate voltage, it certainly doesn't swing like a see-saw as i turn the bias pot for the idle dissipation of 50~70%. so i think, from a practical point of view, i can look at the plate voltage to be a static value with a mental note that if i change the bias by 15~20mA, PV needs to be checked as noted by Mike.
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Post by dixiechicken on Aug 19, 2007 10:57:01 GMT -7
Eminently practical "mudskipper"! Since you're already muckin' about with your bias-probe/voltmeter or whatever, it wouldn't take long to check Pv after a bias adjustment. ( remeber one hand in your pocket & don't answer the phone ;D) Cheers: Dixiechicken
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Post by Telemanic on Aug 19, 2007 11:30:45 GMT -7
I believe it's called IR drop. The inverse relationship bettween Volts and Amps. I get a definate change in center tap voltage as i adjust the bias on my Rt.66 and 65' Vibrolux Rvb, enough to change the dissapation calcs. when measured off the plates, but i suppose you would still be in the ball park if ya consider it static.
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Post by hdahs143 on Aug 19, 2007 14:11:27 GMT -7
I believe it's called IR drop. The inverse relationship bettween Volts and Amps. I get a definate change in center tap voltage as i adjust the bias on my Rt.66 and 65' Vibrolux Rvb, enough to change the dissapation calcs. when measured off the plates, but i suppose you would still be in the ball park if ya consider it static. I agree. The plate voltage doesn't change much, but there's not much point in doing the calculations if you're going to use approximate numbers.
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SG123
Full Member
Posts: 221
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Post by SG123 on Aug 19, 2007 14:26:34 GMT -7
There comes a point when you have to say "How close is close enough?". How do you know if the voltage change is caused by changing the bias, or by the line voltage changing - unless you're watching everything at once or have the luxury of a constant voltage supply? And do you take that fancy regulation to the club where the voltage dips from 124 to 108 when the ice maker kicks on?
Luckily there's a pretty good sized "window" where everything still sounds good. I'm gonna go play my guitar now.
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Post by mudskipper on Aug 19, 2007 17:44:40 GMT -7
Eminently practical "mudskipper"! Since you're already muckin' about with your bias-probe/voltmeter or whatever, it wouldn't take long to check Pv after a bias adjustment. ( remeber one hand in your pocket & don't answer the phone ;D) Cheers: Dixiechicken right. i actually do check both the both numbers back and forth while i bias. but this has been a good thread that's very informative, indeed.
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