Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: neil.dolan on June 27, 2013, 12:36:23 pm
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Hi I am new to this forum and I will be changing my el34's I have purchased new old stock rivera el34's and the test value is 10. What does this refer to? I am currently researching how to bias them and the mathematics includes the voltage check. Do I need a special tool to test this or can I use the number on the tubes, they are a matched pair and produced in germany checked in sun valley california. Any help greatly appreciated. Should I just put them in as a quick check?
Neil. :BangHead: :BangHead: :BangHead:
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... I have purchased new old stock rivera el34's and the test value is 10. What does this refer to? ... :BangHead: :BangHead: :BangHead:
Who knows what that means? Its an arbitrary number that the 'seller' must've come up with. Usually one tests tubes by measuring their transconductance (in mSv)
... I am currently researching how to bias them and the mathematics includes the voltage check. Do I need a special tool to test this or can I use the number on the tubes,...
You bias the tubes so that they will reliably operate within their rated plate and screen dissipation ratings. For output tubes operated in a push-pull output stage, anywhere between 60-85% of the maximum rated plate dissipation is usually sufficient. The maximum rated plate dissipation can be obtained from a relevant tube datasheet . (e.g.: see Franks pages: http://scottbecker.net/tube/sheets/164/e/EL34.pdf (http://scottbecker.net/tube/sheets/164/e/EL34.pdf)). You will see that in the case of an EL34, the maximum plate dissipation rating is 25W. Therefore the tube needs to be biased so that it is idling at somewhere between about 15W and 21W.
You need to measure (or assess) the actual plate current and plate voltage in your amp in order to do this properly. (Plate dissipation (Wa) is a factor of plate voltage (Va) x plate current (Ia), i.e.; the same thing as Watts = Volts x Amps). The plate voltage will be order the order of 400 volts or so, so you have to be careful making this measurement. Plate current will be in the order of a few 10s of milliamps. You can measure this in a proxy way by measuring the voltage drop (in mV) across a 1% tolerant 1R resistor in series with the cathode and the ground return. The reading in mV across a 1R resistor will be equivalent to the reading in mA. This will be a measure of the total tube current (plate current and screen current). As the screen current is only a fraction of what the plate current will be, then measuring the tube current in this way will give a reasonable conservative estimate of the plate current.
Should I just put them in as a quick check?
If the last lot of tubes in the amp were EL34s, that should be okay. If they were 6L6s or some other type of 'pin-compatible'* octal output tube, you should re-bias. If the plates start to glow a cherry red colour, you'll know that they are running too hot.
* EL34s are only 'pin compatible' with 6L6s, if the suppressor grid pin and the cathode grid pin on the tube socket are wired together.
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Really appreciate the help, I don't want to bias if not necessary but I do understand the importance, if I do have to bias the adjuster pot is so hard to get at in my marshall jtm 60 that I have considered drilling a small hole in the chasis to gain access without removal, what is your advice? I could then bias without a lot of the danger. Other Marshalls seem to have this facility as standard. :help:
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A lot of mass-produced factory amps don't have bias adjustment. They conservatively set them up at the factory to enable you to just plug in the right sort of tubes and play. Your amp might be one of those.
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right here goes :w2: I will let you know what happens.
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Played beautifully then high pitch squeals and microphonics. any ideas, I think the tubes may be knackered.
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Bias for EL34, you need to consider a few things. You will need to subtract the screen from the plate. There are examples all over the place, but you will need to get voltage and current readings of the tubes.
Some people simply set EL34 to -35v, but I hear lots of people with problems with these tubes as well.
http://www.aikenamps.com/Biasing.html (http://www.aikenamps.com/Biasing.html)
Another
http://aga.rru.com/TechTips/disp.cgi?file=LV/BiasAdj.tt (http://aga.rru.com/TechTips/disp.cgi?file=LV/BiasAdj.tt)
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Played beautifully then high pitch squeals and microphonics. any ideas, I think the tubes may be knackered.
No way to know for sure. Could be your tubes simply redplated. If it got worse the longer the amp was on, the hotter the tubes got.
You need to get voltage readings from your tubes.
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high pitch squeals and microphonics.
Are usually a sign of microphonic pre-amp tubes (Not output tubes - bad output tubes usually sound crackly, staticky or whitenoise-hissy)
Pull each pre-amp tube out and flick it with your fingernail while holding the bottle up to your ear. If it makes a single soft 'dink' type sound it should be okay. If it jangles like a bell, its haddit.
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A little more help attached.
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Thanks everyone I really appreciate the help, I have never changed the preamp tubes and it is 17 yrs old, this may be the problem. Not got lots of money in fact the output tubes were off the kids for fathers day, do i have to replace all pre amp tubes? or is there an easy way to detect the problem? should i buy one and try it one at a time? Or should i bite the bullet. Did sound really sweet best it has ever sounded with the rivera's. :worthy1:
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they are a matched pair and produced in germany checked in sun valley california.
There are no companies in Germany at this time making EL34's that I know of. Most likely a tube made in China (or Russia) and re-labeled in Germany then matched in Cali.
What amp are you re-tubing and can you post a schematic of it?
Brad :icon_biggrin:
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the dreaded jtm 60 a notoriously difficult amp to bias and has overheating probs, no I got a schematic. when i tapped the valves with a pecil al of them made the same sound just a tapping sound through the speaker. :sad2:
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took all of the preamp tubes out changed them around except for the 12ax7 sounds sweeeeeeeeeeeet. Thanks to everyone hope this is the cure, and the kids may buy me a new set of preamp tubes next year. Thanks a lot amp heads. :icon_biggrin:
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took all of the preamp tubes out changed them around except for the 12ax7 sounds sweeeeeeeeeeeet. Thanks to everyone hope this is the cure, and the kids may buy me a new set of preamp tubes next year. Thanks a lot amp heads. :icon_biggrin:
Did you try the fingernail flick test like I told ya?
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Yes and thanks to that I think they may be fine.The amp has not been used for two years and because I only use it at home it has neither been over used or driven hard, :icon a few hours a month, as I use my tranny amp most of the time , as good as it is I have missed that valve tone and you forget over time just how damn good that feeling is. Great forum. I am considering making a fender bassman at some stage any advice. Thanks again see you all soon. :icon_biggrin:
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I am considering making a fender bassman at some stage any advice. Thanks again see you all soon. :icon_biggrin:
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You are :hijack1: hijacking your own thread. Leave that to us.
Seriously, is the amp playing OK. If not, and you do plan to build amps you should remove the chassis from this one and begin here. I have attached schematics for a JTM 60. Not sure if this is your amp or not. Do you have a multimeter? You should start by getting voltages from your power tubes. It was said squeals come from preamp tubes, but I have had power tubes do this too. Not often as they usually make a different sound, but I have had them do all kind of weird things.
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Yeah all squeals have stopped for now I think I will replace the ecc83 and 12ax7 at some point later. Thanks for your help. :l2: