5f1 circuit - testing for a bad tube?

SDM · Post by **SDM** » Tue Apr 25, 2006 4:40 am

Always DC amps doing the transformer shunt (pin 8 to pin 3 here).

SDM · Post by **SDM** » Tue Apr 25, 2006 4:41 am

Wondering if you damaged your meter when you hooked it up to pin 4 of the rectifier and pin 3 of the output tube. Which meter do you have exactly?

rockstah · Post by **rockstah** » Tue Apr 25, 2006 4:43 am

so when i check pin 5 im using dc volts - in this case its set on 200dcv and i get -30.0
when i do shunt i do dc amps (dca) as in milliamps correct? if so i get no reading here.

correct?

beckman hd100

SDM · Post by **SDM** » Tue Apr 25, 2006 4:47 am

rockstah wrote:so when i check pin 5 im using dc volts - in this case its set on 200dcv and i get -30.0
when i do shunt i do dc amps (dca) as in milliamps correct? if os i get no reading here

correct?

Yes that is correct DC volts to measure pin 5. Yes the shunt is measured with DC amps (preferably the smallest range you have in milliamps). A lower range for the DC voltage on pin 5 for better accuracy would be good too, but your meter seems to be having issues. Seems like the lower current /voltage reading circuit might have taken a hit.

rockstah · Post by **rockstah** » Tue Apr 25, 2006 4:57 am

weird - i have another radio shack meter here set to 200m DCA pin8 rect - pin 3 powertube - zero reading..
switch to dcv with same pins 8 i get 1.1 dcv

SDM · Post by **SDM** » Tue Apr 25, 2006 5:16 am

Mark, a zero reading in DC milliamps is okay for starters with the meter shunting on pins 8 & 3, and the negaive voltage on pin 5 still at -38 volts DC. If you can't adjust that shunted current reading up with your bias pot, something else is wrong.

SDM · Post by **SDM** » Tue Apr 25, 2006 5:33 am

You do have the 6V6 in when you do the shunt test right? Just have to check on that one, sorry.

Actually the DC voltage you are reading of 1.1 volts is the voltage drop the resistance of the OT causes as current flows through it. This would usually mean that there is indeed current flowing through. I wonder if the OT you are using has a very very low resistance and/or your meters have too high. That could throw you off, but you should still read some current at least. You can measure the resistance from pin 8 rec to pin 3 with the tubes pulled (this meaures right acrossed the OT's primary). If it's extremely low that could be a problem. You can also measure the current from voltages alone after you have made that resitance measurement. With the resistance of the OT known you just need to know the voltage drop through the OT to calculate the current the tubes are pulling. You said you measured 1.1 volts acrossed the OT, so Ohm's law V=IR, or rearranged I=V/R, can be used. V is 1.1, the voltage drop, R unknown until you measure it (tubes out). So if your OT measures 40 ohms, then I= 1.1/40 =.0275, or 27.5 milliamps the tube is pulling.

rockstah · Post by **rockstah** » Tue Apr 25, 2006 6:44 pm

Steve,
can you tell me what tests here to run on my metro build to verify the meter is working properly?

(i love how this is such a learning process. every step of the way there is something be learned that will later be applied as well!
I love it.)

Mark

SDM · Post by **SDM** » Tue Apr 25, 2006 11:01 pm

Super easy check to see if your meter is working right is measure a cathode resistor on V1. Set meter to DC milliamps. Either resistor is fine. Clip your black lead to ground (chassis), and just briefly hold the red lead to a cathode resistor (or pin 3 or 8 ). This will shunt the cathode to ground so you should read about 1.5 milliamps DC. 1.5 ma DC won't hurt the 12ax7 tube at all, but just do the measurements quick anyways. Amp is fully on here, speaker plugged in, volumes all the way down.

Easy enough to do the transformer shunt on your Marshall too. Marshall OT's are often low enough in their resistance to give you an incorrect reading for accurate biasing (depending on your meter), so it might not be too accurate here. Just keep that in mind. But anyway the meter should still draw most the current, but keep in mind it's also the total plate current for the two tubes on the half of the OT you are shunting. First I'll assume you have your amp up running good, biased correctly by the 1 ohmers there. Turn the amp off completely, connecting the meter to do the shunt with the amp on can easily blow the meter's fuse. Okay with amp off, pick a pin 3 on any output tube, connect your meter lead securely - shorting things out doing this will provide nice fireworks, and possible OT/tube damage. Just be carefull. Okay one lead on any pin 3, now connect the other lead to the center tap of the OT, which is likely on the first turret on the power end of the board (also choke lead on it, and has a wire running to the HT fuse holder). Okay now set your meter to read DC amps, preferably the 200 DC milliamp setting. Turn all volumes all the way down, make sure speaker cab is hooked up. Turn the amp on, let it warm up, take off standby, and watch the meter. You should see the current for the two tubes connected to that half of the OT now, so around 70-80 milliamps likely. If that's what you get, your meter is fine, even a lower reading may be okay, as to the accuracy thing I mentioned above.

rockstah · Post by **rockstah** » Wed Apr 26, 2006 1:14 am

Steve,

thanks for the post. with one lead to pin 3 of output tube, and the other lead to OT tap on board( with choke and fuse hodler wire as well), meter set to 200m DCA... i get zero reading.. tried another cheapy radio shack meter, same thing... zero reading... hmmm

i just noticed this... im such a newbie here, my meter has three different holes.. V-Q , COM , A - i have them plugge dinto red= VQ black = COM..maybe i need to plug the red lead into A for... * clears throat* AMPS?!

...and if so does it matter where the red leads go now as far as pin 3 and OT tap? when i plug into the A FOR AMPS?

Mark

rockstah · Post by **rockstah** » Wed Apr 26, 2006 1:22 am

+ 81.2

rockstah · Post by **rockstah** » Wed Apr 26, 2006 1:27 am

the champ reads + 24.1

glad we figured that out , thanks man!

so now.. what to bias it to and undertsanding that in this case, its a 6v6 tube. i know its somewhere in between 40mA and 60mA

im not asking alot am i?

Mark

SDM · Post by **SDM** » Wed Apr 26, 2006 1:36 am

Doh, usually meters have the low current setting in with the V-Q jack in the meter, The A jack only used for high current measurements. Guess your meter is a bit different, glad you figured it out.

Anyway biasing can be a bit tricky here. You'd think that since it's class A you'd just bias the tube at idle to 100% max diss, and you can if everything in the output circuit is designed right. Won't go into all that, but see here for a good explanation:
http://www.aikenamps.com/ClassA.htm
Scroll down to read the two topics starting with "Does true class A operation require any particular current or bias point?"

That aside try 100% max diss, and see how it goes. So just the usual formula, 12 watt max diss for 6V6, bias current = (12*1)/your plate voltage

rockstah · Post by **rockstah** » Wed Apr 26, 2006 1:47 am

Steve,
u can see the sub assembly here - 1 wire from it joins the lifted 220k grid resistor on board, one wire is ground and one wire is going to pin4 of rectifier tube. the only other thing is i put a wire across the powertube cathode cap and resistor instead of removing them for quick ground as well mod reversal if need be.

thanks for the help in figuring it out!
ill check the link for info - thanks man!

is 12 watt max diss for 6V6?

Mark

SDM · Post by **SDM** » Wed Apr 26, 2006 1:54 am

Nice little Champ Mark, one of the first amps I ever built (like alot of people). I modded the hell out of it, but never even thought about putting fixed bias in it. Interested to see how it sounds compared to the old cathode bias. Good idea about just jumpering the cathode for quick mod reversal.