recommended bias level , tubes

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Xplorer
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recommended bias level , tubes

Post by Xplorer » Thu Jun 09, 2011 12:37 pm

hi !
i'm starting to need some informations about the GEC KT66 tubes that are into the jtm45/100 ...

i don't know what's the recommended bias level for these, and the mA and mA/V needed for each tube of a quad, and more informations perhaps, if i'll buy some. i don't know how it's calculated, and which version of the GEC kt66 to look for .
i know that it's 560 v from the transformer, like in the originals, but i hope that it'll be ok, with these old tubes, even if they're said to be the only tubes that can handle that power, not the modern kt66.
and if i lower the voltage, what happens to the sound ?

do you have any informations, recommendations for a nice build please ?
thanks !

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Re: recommended bias level , tubes

Post by neikeel » Sat Jun 11, 2011 7:09 am

Bias calculation is no different than for any other amp.

You have your plate voltage (?540vdc)

You will measure your bias current at idle (are you using 1ohm on the ground or a dedicated probe?).

Kt66 are 24/25watt valves so using 70% of max diss at idle is 17.5watts

17.5/540 = 32mA

If you are using the 1ohm method you will be measuring the screen draw as well so will be measuring 5-10% higher than you are really biased at so could go to 35mA depending on what your screens are drawing (can be quite high on GEC KT66s).

Also depends if you are after maximum tone, maximum valve life or a compromise between the two :wink:
Neil

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Re: recommended bias level , tubes

Post by Xplorer » Sat Jun 11, 2011 8:24 am

thanks a lot !
well, i built a metroamp once, but i really forgot the methods, what it means, etc...
so thanks.
i don't know about the 1 ohm or dedicated probe method.
the transformer will deliver 560 volts , so yeah, then the result should be arround 31 - 32 mA , but does it mean that the gec tubes i'll have to look for will have to be only this value, all four ? not 50 mA , not 71 mA ? .... is it the method ? the strange thing is : why do some kt66 have such a different value ? why aren't they all the same value, stock ?
if i lower the voltage, could i still find the same result perhaps, if the mA of the kt66 is different ? is it comparable ?
like : 500 volts , and some 35 mA kt66 ? or 450 volts and some 38,8 mA ?
do the kt66 have their current measure lower, when they're close to die ? like 1 mA ?
i'm looking for tone, at 560 volts, but then it'll mean that the tubes will last just a week , right ?
it'll be a clone of this one :

http://www.youtube.com/watch?v=4dwyrs5pRaI" onclick="window.open(this.href);return false;

http://marshallvintagemodern.com/viewto ... f=5&t=5992" onclick="window.open(this.href);return false;

this one is a clone of the previous one : http://marshallvintagemodern.com/viewto ... f=5&t=6740" onclick="window.open(this.href);return false;

thanks !

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Re: recommended bias level , tubes

Post by neikeel » Sat Jun 11, 2011 8:41 am

Xplorer wrote:thanks a lot !
well, i built a metroamp once, but i really forgot the methods, what it means, etc...
so thanks.
i don't know about the 1 ohm or dedicated probe method.
the transformer will deliver 560 volts , so yeah, then the result should be arround 31 - 32 mA , but does it mean that the gec tubes i'll have to look for will have to be only this value, all four ? not 50 mA , not 71 mA ? .... is it the method ? the strange thing is : why do some kt66 have such a different value ? why aren't they all the same value, stock ?
if i lower the voltage, could i still find the same result perhaps, if the mA of the kt66 is different ? is it comparable ?
like : 500 volts , and some 35 mA kt66 ? or 450 volts and some 38,8 mA ?
do the kt66 have their current measure lower, when they're close to die ? like 1 mA ?
i'm looking for tone, at 560 volts, but then it'll mean that the tubes will last just a week , right ?
it'll be a clone of this one :
http://www.youtube.com/watch?v=4dwyrs5pRaI" onclick="window.open(this.href);return false;
http://marshallvintagemodern.com/viewto ... f=5&t=5992" onclick="window.open(this.href);return false;
this one is a clone of the previous one : http://marshallvintagemodern.com/viewto ... f=5&t=6740" onclick="window.open(this.href);return false;
thanks !
I think that you may have to do some background reading to understand what is happening when you bias. The voltage I refer to is the measured plate voltage on your respsctive KT66 (pin 3) with the amp warmed up to normal operating temperature with al valves in and hooked up to an appropriate output load (a cab or attenuator on load).
If you have a matched set of original GEC KT66s from the factory they will all measure within a few mA of each other, in fact if you went back in a time machine and bought from different sources they would probably be close enough for guitar amp use (I would aim for a range of 5-6mA across a quad and match the OT load per pair (ie high and low in V5 and V6, middle two in 7 and 8, or a similar combination).
Valves wear at different rates and will measure different current at idle and will of course perform differently (hence matching for hifi at different levels of output,)
Neil

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Re: recommended bias level , tubes

Post by Xplorer » Sat Jun 11, 2011 9:12 am

ok, i think i understand that it's when the bias enters in the calculation : giving for example a 50 mA kt66, the bias will enter in the course to use them at 32 mA, is that the right thought ? so it doesn't matter if all four kt66 are 70 mA or 50 mA , but not lower than 32 mA, is that right ? cause if at least they're matched, the bias will be used to optimise the equation, with the voltage, using : 17,5/560 = the ideal value. am i right ?
so if it's good, i'm supposed to prefer a quad of 70 mA gec kt66, rather than a quad of 40 mA kt66, as they might last longer, as the bias tuning will take less from them in proportion ?
and yes, i should read some nice explanation somewhere.

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Re: recommended bias level , tubes

Post by Xplorer » Sat Jun 11, 2011 9:16 am

and how important is this "6,5 mA/V" or "1 mA/V" reading ? i don't know too about the A/V.
i've seen some kt66 measured at 90 mA. is that good ? some at 55mA , some at 1mA , but it's strange.
it's easy for me to be lost in the area. i should really understand all these stuffs before i buy some NOS glass.
Last edited by Xplorer on Sat Jun 11, 2011 9:19 am, edited 1 time in total.

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Re: recommended bias level , tubes

Post by Xplorer » Sat Jun 11, 2011 9:18 am

... and some have a grey glass, some have a clear glass ... damn, :) i'm in need of knowledge lol

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Re: recommended bias level , tubes

Post by neikeel » Sat Jun 11, 2011 9:48 am

Ah I see you are talking about the readings quoted in sales pitch for GEC KT66, not bias readings :D

Look here:http://www.vintagehiwattrestorations.co ... vt0iof5th0

This is the correct data sheet for them.

You look at the settings for Class AB in push-pull as your reference points.

Valve testers are different and the seller should quote their measured values vs the correct spec for their machine.

Read here:http://en.wikipedia.org/wiki/Vacuum_tub ... d_pentodes and http://en.wikipedia.org/wiki/Vacuum_tube#Failure_modes

Here is good too (sorry cannot have y copy of book :wink: )http://books.google.co.uk/books?id=YjaS ... &q&f=false

Finally:http://www.diyaudio.com/forums/tubes-va ... -mean.html
Neil

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Re: recommended bias level , tubes

Post by Xplorer » Sat Jun 11, 2011 10:20 am

wow ! thanks a lot ! there will be a tempest into my brain now ! lol .... better than nothing ;)
i'll come back on this.
the first link shows an ARS cap, is this right ?

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Re: recommended bias level , tubes

Post by neikeel » Sat Jun 11, 2011 1:02 pm

Xplorer wrote:wow ! thanks a lot ! there will be a tempest into my brain now ! lol .... better than nothing ;)
i'll come back on this.
the first link shows an ARS cap, is this right ?
Er no! :oops:
That was me lookiing for a 16/32 can!

This is the one:
http://www.drtube.com/tubedata.htm#KTetrodes
Scroll down to kinkless tetrodes GEC/Osram
Neil

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Re: recommended bias level , tubes

Post by Xplorer » Sat Jun 11, 2011 2:03 pm

great infos ! thanks !
http://www.drtube.com/datasheets/kt66-mov77.pdf" onclick="window.open(this.href);return false;

the strange thing, to me, is that the kt66 has an absolute max of 550 volts, and i wonder why the original jtm45/100 were designed with a transformer that delivered 560 volts !

it seems that i should run these kt66 between 500 and 530 volts , right ? would they last at least 5 - 10 years or only a few months ? i have no idea ...

on these links, very interesting stuffs, though i'll try to look for the basci things i'd have to know to buy a nice set, and how to use them and tune the amp. after that ... i won't remember lol.

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Re: recommended bias level , tubes

Post by Xplorer » Sat Jun 11, 2011 7:49 pm

............. yeah, i tried to read the wilkipedia stuffs about the electrons and triode and pentode, and anode etc , and ..... i don't understand anything ! lol it's too difficult for me :shrug: :D

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