Ceriatone Forum

Hey guys I just built a 2203 jcm and ran into an issue. I turened it on and the indicator light is on.  I went to bias it but i am reading 0v at the end of the 1ohm resistor with respect to ground. The tubes are glowing dimly. Im not really sure how bright their supposed to be with full power on ive never used a tube amp before. I checked my wiring a few times and dont see any issues. I tried hookin up a guitar and playing it at a very low volume but i get no sound, no hum, no static nothing. Where should i check? I have a volt meter. This is my first time building an amp.

Kevin

I would pull the power tubes and make sure you have:
1. Negative DC bias voltage at Pin 5
2. AC Heater voltage at Pin 2 and 7

If either of those are missing you can just work back wards through the circuit until you find where the loss originates.  Let us know what you find.
Good Luck

I read 0 on pins 2 and 7, and -850v on pin 5 all with respect to ground. What should my voltages be. There was some confusion on the layout for me as for the heater let me explain what I did.

I daisy chained all the pin 2's with red and pin 7's with black. Then i tied in the 2 brown cables from the transformer (3.15v each according to layout) which also runs to the indicator.

Kevin

OK -
Just to make sure, check both fuses with you meter to make sure they are good.
Pin 2 and 7 are AC and you read that with one lead on Pin 2 and one lead on Pin 7, not to ground... and you get Zero volts? That means you have no filament supply and the tubes will not function.  Disconnect (unsolder) the heater wires from the first power tube and measure those two wires just like you would a tube, one lead to each wire.  You are checking for 6.3 VAC.  Make sure the wires do not touch.  If there is no voltage there, do the same to the pilot light.  I am thinking you have a short in one of your tubes, and not a bad power transformer.  If you get 6.3 Volts from the transformer then you must have a bad tube that is shorting the heater supply to ground.
I can see Pin 5 having -85 VDC, but not 10 times that much. I think that tap of the PT only supplies 100 volts. You might want to check that again, meter on VDC, black lead at ground, red lead at Pin 5.
Good Luck

ok, i checked pins 2 to 7 incorrectly the first time. With my meter across pins 2 to 7 on AC i measured 11.2 v. As for measuring from pin 5 to ground I have my meter on 200v dc and it says like 872 or something around there. Must have a decimal in there otherwise it wouldnt read that high so 87.2 makes sense. So you think it might be a bad power tube? Is there a way to check them, This is the first time i've ever even seen a tube :P

Kevin

Usually a bad power tube will pull the AC to ground, so the reading would be Zero. Your number of 11.2 is suspiciously high.  The transformer does not normally supply that much voltage. You need to desolder the heater supply leads from the first power tube so you cam measure the supply from the transformer. If that supply is 11 volts, your PT is wound wrong and needs to be replaced.  It is possible, but bad trannies do not happen very often.  So you need to measure the 3.15 tranny wires and see what they give you.  Just make sure they are disconnected from the power tube and the pilot light.  Like I said, if that says 11.2 then your PT is bad.
Regards the DC, yes, I assume there is a decimal you are not seeing. 87.2 VDC sounds reasonable at this point.
Good Luck

Is there a way of testing the tubes? For the PT test I desolder the 2, 3.15v brown wires then touch one lead to each end and measure ac volts correct? Im at school so I cant test it for another 2 hours.

Kevin

I remeasured the voltage from the 2 brown heater wires with a different analog meter I have and its reading around 7.5 volts. I think my digital meter is reading AC incorrectly cause its reading 170v out of the wall outlet, this meter reads correctly. Is that closer to what I should have?

Also that -85v on pin 5 is now -50v with this other meter.

Kevin

That is close, but it is still pretty high. You may want to change the meter battery for a new one and try it again.  It would not hurt to measure your wall voltage again also.  You might have a lot of voltage from the wall, it would be good for you to know, although that can change throughout the day.
You can hook it all back up and see what you get with the tubes all drawing current.

I checked my wall outlet and its exactly 120V

Kevin

That negative dc bias voltage reads -48v with the trim pot all the one way and up to 68v with the trim pot the other way. This doesn't change weather it is on standby or not on standby.

Kevin

I was checking voltages and noticed the voltage selector is wrong. It reads 100v, 120v and 220v, However the layout says 120, 220 and 240 so I flicked it down to 100v which should be 120v. Now my heater voltage is 6.3, my reverse bias voltage is 42.5 volts but my voltage across the 1ohm resistor is still 0.

Kevin

When you say 42.5, you do mean negative don't you? Is your bias cap positive ground?  I think on those JCM's it was a 10uf/150 volt or somewhere in that range?

yeah its negative sorry

Just to double check.... you are reading the negative VDC from Pin 5 to ground, and your 1 ohm resistor is attached from Pin 8 to ground.
Is your meter auto ranging?  If not, you need to select the lowest possible range for VDC.  This will allow you to read milli volts DC across the 1 ohm resistor.

Yeah I did all that. I noticed when I measure from Pin3 to ground I dont get any voltage on the power tubes. Their directly connected to the OT plates, shouldn't i be reading some voltage?

Kevin

Is your HT fuse good?  Otherwise you need to just backtrack from the screens ( Pin 4 ) and their resistors, across the choke, filter caps, rectifier, stand by switch, to the transformer.  It seems like you do not have any B+ supply, it must disappear somewhere.  Do your screens have DC voltage (Pin 4 to ground) ?
Is this EL34 or 6550 power tubes ?
Do you have a light bulb current limiter?

I have -42 volts at pin 4 with respect to ground. I have the EL34 tubes. At point E on the schematic Im reading voltage and it seems I am everywhere across the board. I checked my fuses with an ohm meter their both reading 0 ohms.

I have 0 voltage at K on the schematic and at both plate voltages. It seems my output transformer isnt putting out any power.

Kevin

I am not sure what point K is, I assume it is right after the fuse where the choke and the OT meet.  If you turn the standby switch off (open) what do you read at the two HT wires?  They will be/should be high voltage AC, before it hits the solid state rectifier.
I am always a little confused by Zero ohms. Is that the same as continuity, is that what a new fuse would measure with your meter?  Just wanting to make sure your fuse is OK

well its not really 0 ohms that wouldnt be possible, its a very low resistance meaning its good. If it was broken it would read infinite resistance.

Kevin

whats the HT point?

Kevin

The two wires from the PT to the SB switch. That will tell you if the high voltage AC from the transformer is conducting. If that is OK, you can follow it across the SB switch to the rectifier diodes.  It is possible you have a bad SB switch. I just replaced a bad one that was grounding the B+.... so you never know.
Do you have your diodes oriented the right way?

The diodes came assembled on the board and are facing the right way. It sends 242 volts through each of the transformer wires going through the SB switch and the switch works correctly.

Kevin

I just noticed my 1A fuse is blown, I just replaced it with a new one cause my dad dropped it and it broke. I never though to check a fuse right from the store but maybe thats the problem. I followed to voltage and it stopped there. I doubt thats the problem cause i think it was good at one point. Im gonna stop at the store tomorro and buy a few fuses.

Kevin

I am sure the fuse was good. I am starting to think you have a short somewhere in your B+ wiring.  Maybe in the rectifier diodes or maybe in the filter caps.
A light bulb limiter would help, saves on fuses, and is pretty easy/cheap to build.

what is a light bulb limiter?

Kevin

Basically you put a light bulb in series with the hot lead of a cord that attach's to another outlet. Plug your amp into that outlet so your wall supply is in series with the light bulb. If your amp has a short, the bulb will be bright, if the amp is good the bulb will be dim. I can send you a picture of mine along with a schematic link.

that would be cool

Kevin

Now its playing this loud squeel through the amp when you shut off standby and the volumes are all turned to 0

Kevin

Try switching your OT leads.  I assume yours are Brown and Blue.  Those feed one set of power tubes to the OT.  Just swap them, Blue to where Brown is,  and Brown to where Blue is.

Yeah, thats what Nik told me to do. I swapped the leads warmed it up and biased it. Then plugged a guitar in and it played for about 30seconds and popped the 1A fuse. I touched up the 2 ends i swapped to make sure they were a bit cleaner and tried again and about 6 seconds after i turned it on it popped the 1A fuse again.

Kevin

You must still be pulling to ground somewhere. What were the plate voltage and current when you set the bias?

I didnt measure the plate voltage, Nik told me to shoot for 35mV-40mV for the bias So I set it to 35

Kevin

The HT fuse is usually power tubes, but in a new build it could be a number of things.  I would check all of your wiring against a schematic, not the layout.  Check every connection on the schematic with a bright marker as you verify it.  It is also a good idea to run through the chassis with your meter and check continuity. Make sure nothing is shorted to ground, if your meter talks, all the better.
I would do those two things; check continuity, and verify your connections to the schematic.
If you have or can borrow a set of power tubes that is another quick way to see if your tube(s) are bad.

I dont have access to any other tubes. Does Nik post the schematics anywhere?

Kevin

Nik does not normally supply a schematic. I imagine his reasoning would be that his amps are either a matter of public record, or they are proprietary; and not available at all.  Your amp circuit is pretty much communal at this point.
Good Luck

http://www.ampwares.com/schematics/marshall/jcm800_lead_mstvol_100w_2203.pdf

Whats a good way to go about learning about how a guitar amp actually works? Are there any good sites or books that breaks down why everything is used how it effects the circuit and what not. As for the amp I'm still in the process of checking for shorts, I found someone nearby that said they would give me a hand. Hopefully we can get it working.

Kevin

I got it going now but its all crunchy sounding and the volume is really really low. I brought it to a local amp builder and he said he thinks it might be a bad resistor. He referred me to another guy with proper test equipment. Any of you guys have any ideas?

Kevin

Got it working, turns out it was 2 bad tubes.

Kevin

Congratulations!!!!
You really "hung in there" with this amp.  Tubes are one of the first things to suspect when you have a problem. They can, and do, go bad at any time.  They can be fine at the dealer, and go bad during shipping.  They can be perfect at rehearsal, and go gad at the gig.  They can work fine while you trouble shoot three other problems, and then die themselves just when you think you have it all fixed.  If you have a tube amp, you must have spare tubes. It is that simple.
Good Luck

This guy that told me what the problem is is an amp designer. Hes an awesome guy he showed me how I can change some minor things to get more distortion

Kevin

So all I need now is a set of power tubes. Any recommendations I noticed theres a tone of different types. I think i got the JJ EL34s in it now. I play stuff from Clean chili peppers to metallica, Im looking for mostly that heavy sound. I noticed they have selection of early and late distortion n stuff. Just looking for some advice on tubes I know little.

Kevin

Canada has a very good tube dealer (maybe more than I know)  http://thetubestore.com/

We use mostly SED and TAD.  The fact is you will just have to try a few to see what you hear.  I do not think you need to spend the large amount of money that is NOS tubes, unless your income is not a factor.
For guitar amps there are basically only three tube manufactures:
1.  JJ
2.  New Sensor
3.  Shuguang
Virtually every (modern day) tube you buy will be made by one of these three companies, including the tubes I recommended. Do not pay extra for Gold anything (pins etc.)  do not pay extra for balanced preamp tubes of any kind.  Do not hesitate to pay a few extra dollars to a good dealer for the same tube you might be able to buy from an unproven source.  A good dealer will burn in, and subsequently, cull a large number of the tubes they purchase.  The cull rate might be as high as 20%. It is worth a few extra dollars to know the tubes you buy have gone through at least this cursory 24-48 burn test.  Do not pay more for "quiet tubes".  There is no way to guarantee micro phonics, or rather the lack of it. 
Matched power tubes are not "necessary" but it is a big convenience, and has always been worth the extra money to us to have matched power tubes. That's about it.
Good Luck

Nice, after playing for a bit I noticed just by setting my front pickup volume i can go from a cleaner sound to a nice heavy distortion with a pickup switch.  How could I go from the really clean tone to a heavy distortion during a song with this amp?

Kevin