Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Colas LeGrippa on April 09, 2012, 08:44:56 am
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Hello everybody, salut papa and all ma family.Hope you had a happy easter.
I am working on a MIG 50 amp and I experiment strange things. This amp is fitted with 5881's , on data sheet, 5881 max rating is 350V, there is 535V on the plates, 527V on the screens.
1. too much voltage in there ?
2. for what purpose have they installed diodes between gnd and the plates ?
3. on the back of the amp, a sticker says the amp is biased at 43mV on each cathode ( fixed bias -55V on G1 ). When I install a 1 OHM resistor between gnd and the 2 cathodes, the reading constantly fluctuates between 25 and 150mV, going up to 150, going down to 25 and so on. I would have expected the meter reading to be quite stable at around 86mV ( for both ).
4. is there a milliamp meter for biasing, that has a clamp, like the one we use to check a water heater element, eg , if not, how can an amp be re-biased without un soldering a wire?
5. does it exist any reliable math formula that would calculate that a 5881 with 535V on G2 and P and with -55V on G1 gives exactly a current of 43mA ?
Best regards
Colas
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2. for what purpose have they installed diodes between gnd and the plates ?
for protection purpose
Kagliostro
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Salut Colas ,
1- Yes it is too much , that the spec said , IMO I Will reduce it or put more powerful 6L6= 5881
2- Diode is for protection , normally we put some in series
3- Bias of 43 mv only work with a special tube made hat this time . This 43 mv is not good for any new or other tubes than the original. Use Weber Bias calculator ; http://www.webervst.com/tubes1/calcbias.htm (http://www.webervst.com/tubes1/calcbias.htm)
Bias fluctuated = bad socket contact , bad solder , tubes defect ....
4- I don't think so, no , milliamp is too small for clamp meter . Those I know need some amps to work. A voltmeter each side of an 1 ohms is the best and cheaper way
Bon travail.
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grazie stratele, sei com'un padre per me !
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Nulla di Nicholas, ma non parlo italiano, ma francese.
(Google translation )
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Petit cousin va, mes ancêtres sont normands (et la France me manque terriblement, je ne peux y aller cette année pour poursuivre ma tournée des châteaux et cathédrales).
En deux mots, pourquoi ce pseudo à consonnance italienne ?
Colas
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If those are Sovtek 5881's they can take everything a 6L6GC can and more. I wouldn't worry about the voltages in that amp with those tubes.
Greg
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> 5881 max rating is 350V
That's super-conservative, what Tung-Sol wanted to claim. New-made 5881 are not built with the same materials. AFAICT, with so few tubes made and most tube-uses over-volted, they don't bother keeping 350V plate-stuff around the factory.
> I would have expected the meter reading to be quite stable
Yes. It is motorboating, just not enough to matter YET. Replace the first second and third B+ filter caps.
> does it exist any reliable math formula that would calculate that a 5881 with 535V on G2 and P and with -55V on G1 gives exactly a current of 43mA ?
No. It is the relation of cathode area and activity, grid wire spacing and distance, side-electrode gaps, end-effects, and other dimensions which are NOT tightly controlled in production. You can look-up triode curves and estimate the "typical" V/I, and for high currents most tubes will be within 20% (that's the best a tube-maker can do). But 43mA is very LOW current for this 400+mA tube, so the tolerance is worse.
Adjust G1 for 35mA-50mA in each tube. This isn't precision work. The idle current has no effect on the FULL ROAR power or sound. It has some effect at very soft level, but you don't buy a MIG to play soft.
A clamp meter might be possible but very difficult/expensive. The no-solder tool is a Bias-Rite or the kit that Hoffman sells on the Tube Sockets page.
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the kit that Hoffman sells on the Tube Sockets page.
Been wanting to put a few of these together for a while. Not listed under tube sockets are they out of stock right now??
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I think he changed thing around a little. It's in tools and batteries.
http://www.hoffmanamps.com/MyStore/perlshop.cgi?action=template&thispage=Tools&ORDER_ID=133469817 (http://www.hoffmanamps.com/MyStore/perlshop.cgi?action=template&thispage=Tools&ORDER_ID=133469817)
:icon_biggrin:
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It's all understood, thank you all.