Marshall Lighted Rocker Switch hookup-Bias problem solved!

[JustMike]

 I'm building a JCM-800 from a Ceriatone layout but I'm using the newer rocker switches. The power switch (scavenged from a Marshall 3210 Mosfet JCM-800) has a LED (I think) and I need to know what voltage it is and what pins it's wired to. Tried searching but came up empty.
Thanks.

[SILVERGUN]

Here's a diagram from the Hoffman parts page for this lighted marshall rocker switch.
The bulb in that switch runs off of the 120VAC primary input supply.

Good luck with your build JustMike, and don't stop asking questions. 

[JustMike]

OK, thanks. So since you asked, here's another question. The diagram shows what I think is a bridge rectifier in the power supply. I have what I know is a bridge rectifier, it's got 4 leads, A/C in and +/- out. The diagram isn't clear to me and I see nowhere to hook up the - out of my bridge rectifier. Is it just as simple as tying the anode side to ground?

Resized to 1024 pixels wide using the width tag
http://el34world.com/Forum/index.php?topic=464.0

[sluckey]

The diagram shows what I think is a bridge rectifier in the power supply. I have what I know is a bridge rectifier, it's got 4 leads, A/C in and +/- out. The diagram isn't clear to me and I see nowhere to hook up the - out of my bridge rectifier. Is it just as simple as tying the anode side to ground?

That's not a bridge rectifier on the Ceriatone layout. It's four individual diodes. You cannot use your bridge in that layout. You must connect the four diodes exactly as shown on the layout. No part of any diode will connect to ground.

[JustMike]

Thanks Sluckey. How CAN I use a bridge rectifier in this amp? I'm pretty sure I've seen them in Metropoulous builds.

[sluckey]

Thanks Sluckey. How CAN I use a bridge rectifier in this amp? I'm pretty sure I've seen them in Metropoulous builds.

You would need a different PT. Get one that puts out 325VAC rather than one that puts out 650VAC.

I'm building a JCM-800 from a Ceriatone layout

That's a good plan. Stick to it. The original JCM-800 2204 did not use a bridge. I'm pretty sure Ceriatone follower the original schematic. They definitely did in the power supply area.

[SILVERGUN]

What PT do you have now?

[JustMike]

I'm using the Classic Tone 40-18023. 690V CT on the output. Sounds like I need to get some diodes.
Thanks again.

[EL34]

Diodes page
http://hoffmanamps.com/MyStore/catalog/parts20.htm

[SILVERGUN]

Some guys get hung up on thinking they need every part exactly as listed on a schematic or layout...
In this case the Ceiriatone layout lists UF4007 diodes for those rectifier diodes.

You do not have to use those specific diodes, as standard IN4007 will be sufficient. (Like the ones on the Hoffman part page)

The UF designation means "Ultra Fast", but they are not necessary to build a guitar amp.
I usually just order the IN4007s in quantity of 10, because they are cheap enough and used everywhere....if you think you'll do more than one amp build, it's one of those parts that makes sense to stock.

[JustMike]

Thanks Silver,
 I have a local supplier that had the UF's which I know are Ultra Fast and not necessary for an amp build. I try to order everything from Doug, but for $1.25 in parts, it's just not worth the shipping cost. This isn't my first build and it won't be my last. Thank You Doug Hoffman!

[EL34]

I'm building a JCM-800 from a Ceriatone layout but I'm using the newer rocker switches. The power switch (scavenged from a Marshall 3210 Mosfet JCM-800) has a LED (I think) and I need to know what voltage it is and what pins it's wired to. Tried searching but came up empty.
Thanks.

Look here in the helpful info items section
http://el34world.com/schematics.htm

[JustMike]

 So I fired it up and NO SMOKE. But my voltages aren't correct on the 12AX7's. I'm using a Metroamp build guide as a reference and it has a voltage chart. I noticed that the 10K 2W that's across the 50+50 cap is getting very hot when I take the amp out of standby. I checked the wiring and the only thing on the other side of the resistor is the 2nd side of the cap, the choke and pins 6 of the two el34's. It appears to be wired as the layout shows.
 I have 375v across the 10k resistor and only 13VDC on pin 1 of v1.

[Willabe]

I noticed that the 10K 2W that's across the 50+50 cap is getting very hot when I take the amp out of standby. I checked the wiring and the only thing on the other side of the resistor is the 2nd side of the cap, the choke and pins 6 of the two el34's.

I have 375v across the 10k resistor and only 13VDC on pin 1 of v1.

The '2nd' side is actually the 1st side, it's up stream from the rest of the B+dcv power supply (PSU).

Sounds like you have something wired wrong that's causing a large current draw down stream. That's why the dcv is so low and why the B+ PSU dropping 10K R is getting so hot.

Do you have a light bulb limiter yet? (You should always use 1 when you 1st power up a new build or when your blowing fuses or have changed/rebuilt something in the PSU.)

I would use the limiter, turn on the amp and see if gets bright and stays bright. Then I'd pull the 1st preamp tube and see if the bulb goes dim. If it does it's something with that tube wiring. If not I'd pull the next tube (leave the 1st tube out) and so on up to the phase inverter (PI) until I could get the bulb to go dim.

(Or it could be the power tube screens are wired wrong?)

Does your PSU go,  > 1st filter cap/OT CT > choke > 2nd filter cap/screens > 10K R?


               Brad        

[Willabe]

Here's a link to Sluckey's web site with the light bulb limiter and LOTS of other great info!

http://home.comcast.net/~seluckey/amps/misc/Amp_Scrapbook.pdf



               Brad     

[JustMike]

I do have a light bulb limiter in line and it's acting normally (bright at turn on for a second and then dims). I've snooped around a bit and found something odd. At the junction point where the 10k 2w is tied to the choke, cap and ht fuse, when I turn power on, it appears as a short to ground. when I turn the power switch off, it "drains" to an open to ground...?? Is this normal?

[Willabe]

found something odd. At the junction point where the 10k 2w is tied to the choke, cap and ht fuse, when I turn power on, it appears as a short to ground. when I turn the power switch off, it "drains" to an open to ground...?? Is this normal?

NO!

 I think it's miss wired!

Please answer about the B+dcv PSU chain as I posted. Where is this 10K 2w R in the chain?


                  Brad     

[SILVERGUN]

I have 375v across the 10k resistor and only 13VDC on pin 1 of v1.

That's a BIG problem.....that tells you that resistor is dropping 375v
Ohm's law tells you that resistor is trying to dissipate 14 WATTS!  ....that's why it's HOT

found something odd. At the junction point where the 10k 2w is tied to the choke, cap and ht fuse, when I turn power on, it appears as a short to ground. when I turn the power switch off, it "drains" to an open to ground...??

IT IS MIS-WIRED!

NO MORE POWER UPs UNTIL YOU FIND THE WIRING MISTAKE

Are you using the half power switch?

These 2 comments don't coincide with one another-
- At the junction point where the 10k 2w is tied to the choke, cap and ht fuse

-the only thing on the other side of the resistor is the 2nd side of the cap, the choke and pins 6 of the two el34's

Here's a "cleared up" snip of that layout....
I'm still looking at it too, but this is a very easy area to make a mistake because there is so much going on.

[SILVERGUN]

JustMike,,,I was thinkin' 

If you substituted the toggle switch in the Ceriatone layout, with the lighted power switch,,,AND used the little wiring notes from the Hoffman library...
YOU would have to compensate for the difference in wiring...it has a different location and hookup for the standby switch.

The example in the Hoffman notes is just one example of how that switch is used, but it DOES NOT match the Ceratone layout, so if you just "subbed it in", that could create a wiring problem...

[JustMike]

So I backed out of that particular design and used this one just to eliminate things like the half power switch, etc. I built this one before so I'm kind of familiar with it. In the process I found my mistake. And I'm not too proud to admit here that I had the polarity of the board mounted 50+50 backwards. In my own defense, I don't have a board mounted 50+50 so I used 2 47u caps that are off the board...anyway I'm still using the rocker switches but at this point they're just wired as switches and there's no light yet.
So now, all appears to be working properly and I've checked voltages with just the 3 12AX7's installed as per a Metroamp 50w Plexi building guide. One odd thing; on pins 4&5 I'm reading 3.15VAC but on pin 9 I see 6.3VAC. The wiring is as shown in the diagram.

[JustMike]

OK, I'm going to ignore the voltages because the amp works! And it sounds like a JCM-800. The only issue I have is  the bias voltages are different by about 8mv between the 2 output tubes. I'll try swapping the tubes around, but is there anything else I should be looking for? And it's kind of odd that I remember having this exact problem on my last build. I could never find the culprit and ended up living with it. But...

[JustMike]

Now I've got a problem with the bias voltage. Originally I had the 50k ceramic bias trim pot and the 56k resistor. I couldn't seem to get the voltage over 30mv. So on a whim I changed the resistor to the first schematic's 47k resistor and I changed the pot to a regular 50k pot that I mounted on the back of the chassis. Now when I adjust it I get close to 40mv and it just takes off to 45+mv and I hear a hum in the speaker. My GF was out in the shop just now while I was doing this and she heard a loud high pitched sound. (I'm half deaf from too many years in front of too many Marshalls). Could it be the potentiometer I'm using? Is there anything special about the 50k trim pots?

[sluckey]

Now when I adjust it I get close to 40mv and it just takes off to 45+mv and I hear a hum in the speaker. My GF was out in the shop just now while I was doing this and she heard a loud high pitched sound.

You may have a NFB phase reversal issue. You can temporarily disconnect the NFB wire going to the board to see if bias current behaves and the squeal goes away. If so, the fix is to reverse the OT primary plate leads going to pins 3 of the output tubes.

[JustMike]

 I disconnected the NFB and checked bias voltage. It's still slowly drifting up and there is about a 10mv difference between the tubes. I believe these are 2 different issues. The difference between them may be the tubes. But the drifting bias voltage is odd, eh? I'm not getting any redplating and the other odd thing is when I reversed the OT leads I didn't hear the loud squealing I heard on my first build. I played thru the amp like this for 10 minutes or so and no redplating or squealing.

[SILVERGUN]

I disconnected the NFB and checked bias voltage. It's still slowly drifting up and there is about a 10mv difference between the tubes. I believe these are 2 different issues. The difference between them may be the tubes. But the drifting bias voltage is odd, eh? I'm not getting any redplating and the other odd thing is when I reversed the OT leads I didn't hear the loud squealing I heard on my first build. I played thru the amp like this for 10 minutes or so and no redplating or squealing.

Let's clean up some terminology so sluckey doesn't have to....
-The bias voltage is the negative voltage that is applied to the grid of the power tube on pin 5 (give us - VDC)

-The bias current readings that you are taking are a result of having a 1ohm resistor between the cathode and ground....by taking a reading in millivolts (mV), we are then using Ohm's law to convert that to (milliamps)mA which will be equivalent to the mV reading because of the fact that we are using a 1 ohm resistor....so you have a 1/1 conversion.

So when you say that the bias voltage is drifting, we assume that you mean the negative voltage reading on pin 5 is changing.

But, what you are reporting is that the bias current is drifting,,,,which may not be a result of the actual bias voltage...
If the negative bias voltage that is on pin 5 stays constant but the plate voltage changes, that will also effect your bias current measurement.

So take a look at that and tell us:
- Does the bias voltage change as you're seeing the bias current drift?
,,,,or does the plate voltage change as you are seeing the drift?

 

[JustMike]

Thanks SILVERGUN.
My bias VOLTAGE is -35.8 rock solid on both pin 5's
Pin3 voltage is 477
Pin4 voltage is 473
Pin6 voltage is 475


Bias current variation between the tubes is about 10mA. And it appears to creep up at about .1ma per second or so...??
Gotta go gig with a real Marshall. I'll check back tomorrow.

[JustMike]

 OK, I'm getting frustrated. I swapped the output tubes and the bias "reading" follows the tube. So I changed the tube set and the reading is a little different as I'd expect, but the ratio between the tubes is still there.
 When I power downn the amp there is a small pop. I don't tink this is related, but thought I'd throw it out there.

What else in the circuit could be responsible for this? All of the parts are new except the 12ax7 sockets.

[HotBluePlates]

I swapped the output tubes and the bias "reading" follows the tube. So I changed the tube set and the reading is a little different as I'd expect, but the ratio between the tubes is still there.

This sounds normal. All it means is the 2 tubes don't draw exactly the same current. FWIW, few "matched tubes" are ever 100% matched; it's very normal to have several-mA's of difference between the idle current draw.


The drifting current is also normal. Early in their life, all tubes have characteristics which drift somewhat, especially slowly drifting idle current. In critical circuits (Read: "where any change makes things go 'Boom' or kills people"), there may be circuits which lock down the tubes and prevent any drifting. Almost no audio amps fall in this category, except possibly 300-400w amps.


Had the tubes been burned-in for a couple-hundred hours, the drift would have largely settled down. That first period in a tube's life is when characteristics change the most. But that's time and $$$ to the manufacturer and distributor. And no one's gonna die if your tube current drifts a bit. If you're concerned about it, button it up and re-check bias in a month or so after it's been played a while. As long as you haven't had redplating, there is no significant issue with the amp's bias.


It's worth pointing out that voltage regulator circuits using tubes had problems with drifting tube characteristics, and some ingenuity was spent in figuring out how to lock them down (after all, the point was to have an unvarying output voltage). However, anyone used to modern solid-state regulated power supplies almost has to laugh at how tube regulated power supplies are less precise and repeatable. It's really just the nature of the tube...

[JustMike]

Thanks HBP. That's exactly what I need to hear. Being a noob at this, I just don't know what "Acceptable" is.

[JustMike]

 New problem found.
 I've been playing the amp and it sounds wonderful. But I just noticed that when I switch to standby, the audio fades to nothing as it should but I randomly discovered that when I touch the tip of the guitar cord that is plugged into the amp, I still hear a ground buzz...and it's level is controlled by the volume pots.
 When I hold the sleeve of the cable, there is no ground buzz (as I would expect).
 This tells me that the amp isn't fully going into standby(?). It's wired exactly as shown on the 2nd diagram I posted. Could it be the 100k 3w resistor across the standby switch? I don't even know what that does anyway.

[sluckey]

Could it be the 100k 3w resistor across the standby switch?

yes

[JustMike]

OK, then this behavior is normal? I think this resistor has to do with slowly draining the filter caps when the amp is put in standby, no? So maybe if I wait, the caps will fully drain and there will be no audio passed?

[sluckey]

OK, then this behavior is normal? I think this resistor has to do with slowly draining the filter caps when the amp is put in standby, no? So maybe if I wait, the caps will fully drain and there will be no audio passed?

Just the opposite. It allows the caps to slowly CHARGE while in STBY mode. Leave it sitting in STBY for 10 minutes and you might even be able to get one loud power cord out of it while on STBY.

I suppose Ceriatone had a reason, but I'd remove it if it were in my amp. As much as I like to tinker I'd probably forget and get bit while on STBY. I hate those kinda surprises!

[JustMike]

Thanks Sluckey. So I searched the web for a standby switch resistor and came across this from Valve Wizard;

Worst place for a standby switch:The worst place for a standby switch is immediately in series with a smoothing choke: When the switch is thrown one end of the choke is left completely unconnected to anything. Not only is this likely to lead to pop sounds, but the choke will develop a massive fly-back voltage which can cause serious arcing in the choke and switch, and may even break through the choke insulation. This could be allayed by adding a snubbing diode in parallel with the choke.

and here's a link for reference;
http://www.valvewizard.co.uk/standby.html

This appears to be what we have here in this Ceriatone layout. Could this be the why for the resistor?

[sluckey]

This is the applicable statement from the wizard...

"However, if we are willing to forgo the complete muting facility we can instead redesign the standby switch for a 'soft start' to truly help extend component life (especially the rectifier, whether valve or solid state). If we arrange the switch to allow the anode current to increase very gradually at the same time as the heaters warm up, then both inrush surge and cathode poisoning can be completely avoided. This can be done simply by placing a resistor in parallel with the switch. Obviously this works best if the switch is placed before the reservoir capacitor."

And specifically for your findings...

"if we are willing to forgo the complete muting facility"

[JustMike]

Right. So this is exactly what this is.  And if I understand correctly, it's purpose is to save the life span of the rectifier diodes and filter caps by lessening the inrush current when the switch is engaged.

[JustMike]

OK, back to amp building. I'm having a problem with the preamp of this thing. I know the PI & PA work because when I use the fx loop return with an external preamp, it works. But when I plug into the front inputs I get very, very low output. Sounds like the gain is there...So I checked voltages. My heater voltages are correct. B+ is479v. At the junction of the 2 100k plate bypass resistors for V1 I have 275v. At the V3 plate resistor junction I have 394v.

V1
pin 1 260v
pin 3 1.5v
pin 6 271v
pin 8 .1v


V2
pin 1 306v
pin 6 333v
pin 7 301v

V3
pin 1 321v
pin 2 16v
pin 3 23v
pin 6 323v
pin 7 0v
pin 8 23v
pin 8 15v

 Any ideas?

[sluckey]

V1 pin 8 should be about 1.5v. Since you only have .1v, that means the tube is barely conducting. This is a likely cause for very low sound.

What are the voltages for V2 pins 3 and 8?

V3 pin 7 voltage should not be zero. It should be about the same as v3 pin 2 which you say is 16v.

You have V3 pin 8 listed twice. Since pin 8 should be connected to pin 3 I'll assume pin 8 is really 23v?

[JustMike]

V2 pin 3 0v
V2 pin 8 15v

V3 pin 2 17v
V3 pin 7 17v
V3 pin 8 23v

[sluckey]

V2 pin 3 0v
V2 pin 8 15v

Both of these are wrong. Pin 3 should be about 1v. Pin 8 should be a couple volts greater than pin 7.

Look for wiring errors or wrong value resistors.

[JustMike]

As you can see, it's backwards. My intention was to put this in a combo (upside down). but it wasn't until after I modded the chassis that I realised that Marshall just relabels the front panel. Anyway, it's done and I can't change it back.
 Like I said, it works as it should from the PI out. I've been through this thing a half dozen times and I can't find anything miswired. As stated above, it works, but very low volume. I appear to be getting preamp gain-it sounds like a Marshall, just when it's cranked it's a little louder than bedroom volume. Does anybody see any mis-wiring?
THANKS.

[sluckey]

Did you resolve the voltage issues?

[Willabe]

Is the only thing holding the 2 high voltage filter caps in place on the far left the silicon? 

If so and if that lets loss big trouble.


                  Brad   

[JustMike]

Did you resolve the voltage issues?

No, still the same. That's why I posted the pic. Maybe another set of eyes can spot something I've been staring at.
 And about the silicone and caps, when I originally planned this out, it was to be in a combo amp and would sit as you see it. But if I do end up putting this in a head shell, I'll tie wrap them in place. There are 2 holes in the chassis under the goop.

[sluckey]

Hi rez pics are needed. We need to be able to read resistor values. There's probably just a simple error somewhere but your pic is not detailed enough. And your 'flipped' board makes it very difficult to compare with the layout. That must have been a challenge to build.

[Willabe]

And about the silicone and caps, when I originally planned this out, it was to be in a combo amp and would sit as you see it. But if I do end up putting this in a head shell, I'll tie wrap them in place. There are 2 holes in the chassis under the goop.

They still need to be fastened/tied down.


                      Brad   

[JustMike]

Here's a link to my Photobucket album with more shots;http://s34.photobucket.com/user/emenab/library/JCM

[JustMike]

Here's what I have;
B+ 477
v3 pin1 290v
v3 pin2 14v
v3 pin6 427v! Could this be a bad v3 tube?
v3 pin7 22v
v3 pins 3&8 26v


v2 pin1 364v
v2 pin6 429v
v2 pin7 364v
v2 pin8 40v


v1 pin1 386v
v1 pin3 2.1v
v1 pin6 316v

[John]

I'm assuming V3 is your PI? If so, I'd check the value of the plate resistor going to pin 6.


If V2 is a 12ax7, something is horribly wrong with those voltages.      Forgive my lack of knowledge, but I don't have the schematics in my head the way a lot of these guys do, and I'm guessing that V2 is NOT a 12ax7.


Just read the schematic. Never mind. 

[JustMike]

Plate resistor reads 99k. And v1, v2 and v3 are 12ax7's...been swapped a few times.