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use of the 1 ohm 2w resistors - power tube sockets
Posted: Thu Mar 08, 2012 7:02 am
by Xplorer
Hi ! i realised that in my jtm45/100 part list i missed these 1 ohm 2w resistors, accross two pins on each power tube socket. it figures on some layouts available on the forum, and on some modern clones, but i don't see these resistors on the originals, where the two pins are just linked together with a connection, and one side going to the ground.
Anyway, what's the use of these resistors ? could i safely build my clone without these resistors ?
And what about the use of some welwyn 1k 7 w instead of 5w , also on the power tube sickets ? i think i read some saying that it was maybe better to use some 3w here, so in case of electrical chock or something, the tubes wouldn't be damaged, the welwyn would instead.
Thanks !
Re: use of the 1 ohm 2w resistors - power tube sockets
Posted: Thu Mar 08, 2012 7:05 am
by bill bokey
Aren't the 1 ohm resistor added there to help bias the amp ? Instead of using a bias probe ?
Re: use of the 1 ohm 2w resistors - power tube sockets
Posted: Thu Mar 08, 2012 7:18 am
by Xplorer
i really don't know. how would they help to bias the amp ? turning the bias pot a bit more in one way or the other wouldn't get the same effect ?
Re: use of the 1 ohm 2w resistors - power tube sockets
Posted: Thu Mar 08, 2012 7:52 am
by bill bokey
toner wrote:
1) If not using a bias probe, you need to have a 1 ohm resistor on each power tube socket (one end of resistor to ground; the other end tied to pins 1 & 8 ) Use a 1% tolerance resistor.
http://forum.metroamp.com/viewtopic.php?t=12286" onclick="window.open(this.href);return false;
Re: use of the 1 ohm 2w resistors - power tube sockets
Posted: Thu Mar 08, 2012 7:59 am
by Xplorer
thanks Bill, from beaujolais hills ^^ : are you from France ? cause you know ... we drink beaujolais nouveau by here ( not my favourite but .. )
Re: use of the 1 ohm 2w resistors - power tube sockets
Posted: Thu Mar 08, 2012 8:02 am
by bill bokey
Not my favourite either !
100% french
(although in lived in the UK for a long time...)
Re: use of the 1 ohm 2w resistors - power tube sockets
Posted: Thu Mar 08, 2012 11:39 am
by neikeel
The original amps did not have these resistors but if you want your amp to sound its best and your valves last as long as they can whilst sounding good you need to bias them correctly.
If you use a bias probe then you can leave them out, simply tie pins 1 and 8 together with bus wire and then to ground. 50w amps (ie 2 pots vs 4) can be biased using the transformer shunt method, assuming you have a good quality meter (ie mid range or better Fluke). I use this for my vintage amps.
For the DIYer (that includes me too of course
) the bias resistor method is great and done neatly is quite inconspicuous (even better if you use some heat resistant paint to paint the body of the resistor black. 1% or 0.5% metal oxide at 1 or 2w are more than adequate. When you measure the voltage drop across this resistor in mV you ar extrapolating the current (1mV = 1mA) to be the bias current draw at idle - ie no signal. Then calculate what is should be ideally between 60 and 7)% of max dissipation, which for a KT66 is 25watts. 70% x 25 = 18watts. Break the eqaution down and rearrange 18/plate voltage on pin 3 of output valve = bias current in mA.
ie 18/520(typical 45/100 plate voltage) = approx 35mA
This is the maximum bias current. Go higher it may sound ok but you will shorten the life of your valves appreciably and you may toast tem and your amp.
I would use 3 or 5w scren resistors btw. 7 are very big and if you get a valve failure a 3w will go open circuit which is marginally better than a short which could take out your OT (to an extent the bias resistors wil do the same thing so do not go too high a w rating on those).
ps not keen on Beaujolais Nouveau myself, prefer Paulliac. Margaux, St Emillion etc although on a hot day Fleurie and Cote de Beaune can be good. Problem is the Aussies are cheaper and more consistent with their Shiraz and Cab Sauv etc and the Euro makes it more expensive for us too
Re: use of the 1 ohm 2w resistors - power tube sockets
Posted: Thu Mar 08, 2012 12:35 pm
by Xplorer
thanks ! i guess i'll wait for some 3w welwyn, and some 1% metal oxyde 2w resistors.
i'll have to prepare myself for the building part as soon as i have all the few parts left to get. your advices are much appreciated ^^
i'll post something about my build, a bit like vintage charlie did, but with all the steps of the build, and a BOM check before i start, with all the options i'll have.
Yes, my preference goes to a good Lalande de Pomerol ^^ . or a chateauneuf du pape. some old 60' Sauterne...
the pleasure depends of the Castle, of the year ... of the grade ... premier cru, deuxième cru.. grand cru, cru bourgeois etc.
Some aren't classified. it's an extremely wide "science, poetry", i'd say : a bit like a good amp but so much more complex and rich. some think they can import french grappes anywhere in the world, thinking they're gonna make the same wine, but ts ts no ! : D then, peoples on the other side of the planet buy some wine with a frenchy name, a variety of grappe, they drink that shit i'd spit from my mouth right away and think this is wine but sure nooo !! 40 000 % different from that. ha ha
Yes, bottles can be expensive, but you can find some at 7 -10 euros that are decent , quite often. some other bottles can be
30 000 euros loool. i wish i could try one sometime.
Re: use of the 1 ohm 2w resistors - power tube sockets
Posted: Wed Jul 06, 2022 7:17 pm
by uiovbged332