• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Red plate KT88

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bias ranges amoung exact same production runs can vary....commonly refered to as break up range (early, normal, late). Once you figure out what you amp needs, you can buy tubes in that range. example...a recent amp accuision needed 6550 tubes in the 28 to 31 ip range. if the tubes were outside of that range...no good. Ip's higher than 35 would blow fuses. So it turns out after hunting and testing...6550A tubes were more likly to be within 28 to 31 Ip than 6550WE and other varient tubes. So my answer to your question...you obviously need tubes that have a higher Ip range. Sell what you have and buy tubes that are a higher bias rating.
 
Were the cathode currents reasonably balanced? Were the sense resistors 'new' - eg. 5% tolerance? Was your meter a mV meter or did it have reasonable mV discrimination? Was the non-cherry-red related to a lower mV level?

Ciao, Tim

I believe Tim is getting to the heart of the problem. It seems that the tubes are getting higher current than the readings suggest. With a 1 ohm resistor and meter leads adding their own resistance the % of error could be plenty. Any other connections add complications. Try replacing the 1R with a 10R (measured and not just assumed 10R) and make sure your meter lead resistance is figured in. The % error will be smaller.
 
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I believe Tim is getting to the heart of the problem. It seems that the tubes are getting higher current than the readings suggest. With a 1 ohm resistor and meter leads adding their own resistance the % of error could be plenty. Any other connections add complications. Try replacing the 1R with a 10R (measured and not just assumed 10R) and make sure your meter lead resistance is figured in. The % error will be smaller.

Hmmmm? Yes! This could in fact be the reason. I will investigate this further.

Thanks Tim and 20to20

Cheers
 
Meter lead resistance won't have any impact for a voltage measurement across a sense resistor wired in to the cathode circuit :)

10R will be easier to read, but it all depends on the meter you use. You may have to dig out the specification for your meter and then attempt to interpret the accuracy (which is commonly a % of FS plus a number of least significant reading digits). And if it's not a new meter, then you have the issue of some kind of error coming in over time - at most you should get only a few mV for shorted meter probes.
 
Meter lead resistance won't have any impact for a voltage measurement across a sense resistor wired in to the cathode circuit :)
Ya, the voltage measurement is not where the error starts. It's in determining the resistance value. If you check the resistor and it "reads" 1R, the meter leads are adding anything from .1 -.3 R which means the real resistor value is .7 -.9. My Fluke 77 will read .1 -.2 R in the autoranging mode between leads, on a good day. So .05/.8 = .0625 instead of .05. Whereas .5/9.8 = .051.
 
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Yes, trying to read a 1 ohm resistor with a typical fluke is asking for question marks on accuracy. Even using a DC supply and a current meter and a voltage meter still needs meters with good resolution and accuracy. I take a backward step and use a new modern commercial resistor, with the view that anything from 1% to 5% tolerance is likely to give me more confidence than trying to make a 'measurement' of the resistors resistance value.
 
Yes, trying to read a 1 ohm resistor with a typical fluke is asking for question marks on accuracy.

Even using a DC supply and a current meter and a voltage meter still needs meters with good resolution and accuracy.

I think having the Fluke read my meter lead resistance is added accuracy. It's just that there is no "0" adjust possible as in many analog meters. I have three sets of meter leads from different quality meters I've purchased and they all have different resistances.
 
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