6g15 build help

[deadlands]

Hello everyone,

This is my first post here and my first tube circuit build. I have some experience with building pedals and working on my amps/guitars. I want to learn more about tube amps and decided a simple build will help get my feet wet. Plus I really want a 6g15.

I got the Triode Electronics 6g15 ver. 2 kit. A Mojotone chassis and a locally built cab from Beaver Bottoms.

Here's the problems so far.

Signal passes through but there is no reverb. I can tap the tank and hear the springs crashing, but there is no reverb on the guitar sound. I checked the tank I'm using (USA Accutronics 9AB3C1B), I get 1.2 ohms on the input and 200 ohm on the output. I assume that means it's good. I do have a new MOD 4AB3C1C arriving tomorrow.

I checked all wiring, values and cap placement for errors, did not find anything (however, my tired eyes could've missed something). I then checked DC resistance on the resistors and found that the 2.2M resistor that connects to pin 7 of the 12AX7 measures at .5k. Is that weird? or is there a reason it would read that way?

The most alarming thing I found that is that the voltage at pin 6 of the 12AT7 is reading at +220v. The schematic and fender layout say it should have +120v. I checked the 100k resistor and it's okay (I measured before and after the resistor). From what I read that is why I'm not getting the +2v out of pin 8 of that same tube. I get about -.002v on that pin.

Any help on how to proceed will be greatly appreciated. I've tried getting in touch with Triode for tech support but my calls and email have gone unanswered so far.

I'm including some pictures, not sure if they will help. And yes, my wiring is messy. I'm going to rework all the off-board wiring so it's neater and more out of the way of the heater wires.

[Willabe]

First off, Welcome!

Second, looks real nice and for a first build it looks great, IMO.    

The most alarming thing I found that is that the voltage at pin 6 of the 12AT7 is reading at +220v. The schematic and fender layout say it should have +120v. I checked the 100k resistor and it's okay (I measured before and after the resistor). From what I read that is why I'm not getting the +2v out of pin 8 of that same tube. I get about -.002v on that pin.

Try measuring for dcv at the other end of the 100K plate R that's connected to 12AT7 pin 6, measure where it makes a Y with the 10K R.

What do you get?

If you get the same dcv on both sides of the 100K plate R then the 12AT7 is not drawing any current. Which would also be why you don't have any dcv on the 12AT7's cathode, pin 8.

Check that R's value should be 1K5 (1500) if right reflow solder at the tubes pin 8 and that R's ground connection.

Signal passes through but there is no reverb. I can tap the tank and hear the springs crashing, but there is no reverb on the guitar sound.

What I said above would be why you get dry signal through to the input of the amp your pluged into but no verb. Dry signal has it's own path to the output jack and doesn't go through the 12AT7 at all.

It also sounds like the reverb recovery stage is working.



                Brad        

[deadlands]

First off, Welcome!

Second, looks real nice and for a first build it looks great, IMO.    

Thank you! I took my time, but those long off-board wires are starting to bug me when looking at my photos. I'll probably start shorting those tonight.

Try measuring for dcv at the other end of the 100K plate R that's connected to 12AT7 pin 6, measure where it makes a Y with the 10K R.

What do you get?

If you get the same dcv on both sides of the 100K plate R then the 12AT7 is not drawing any current. Which would also be why you don't have any dcv on the 12AT7's cathode, pin 8.

Check that R's value should be 1K5 (1500) if right reflow solder at the tubes pin 8 and that R's ground connection.

At the terminal where the 10k and 100k meet for the B+ I get +240v. Right after the 100k and pin 6 of the 12AT7 I get +220v. The 12AT7 I have is a brand new JJ and I also have a few spares that I know are good that I tried. Same results, swapping tubes did not make a difference. I also get the same result with no tube installed.

What I said above would be why you get dry signal through to the input of the amp your pluged into but no verb. Dry signal has it's own path to the output jack and doesn't go through the 12AT7 at all.

It also sounds like the reverb recovery stage is working.

Which gives me hope that it's some silly mistake I made.




                Brad        
[/quote]

[Willabe]

At the terminal where the 10k and 100k meet for the B+ I get +240v. Right after the 100k and pin 6 of the 12AT7 I get +220v.

That's where your problem is. That 12AT7 is only dropping 20dcv across it's 100K plate R.

Make sure the K (pin 8) has continuity to ground with your meter.


             Brad     

[deadlands]

At the terminal where the 10k and 100k meet for the B+ I get +240v. Right after the 100k and pin 6 of the 12AT7 I get +220v.

That's where your problem is. That 12AT7 is only dropping 20dcv across it's 100K plate R.

Make sure the K (pin 8) has continuity to ground with your meter.


             Brad     

That's good information. Let's say I find that pin 8 does not have continuity. Is the connection to ground through the .002 cap and 220k?

[Willabe]

Let's say I find that pin 8 does not have continuity. Is the connection to ground through the .002 cap and 220k?

No.

That's the coupling cap to the next stage, 6K6 and it's grid return R to ground.

Look at the layout schematic. (don't guess) It shows pin 8, K (cathode = K) to a wire to a 1500 ohm (1K5) R and then to ground.

Here's the link to our host Doug's schemo library for your build.

http://www.el34world.com/charts/Schematics/files/fender/REVERB_6G15.pdf


                  Brad    

[deadlands]

Let's say I find that pin 8 does not have continuity. Is the connection to ground through the .002 cap and 220k?

No.

Look at the layout schematic. (don't guess) It shows pin 8 to a wire to a 1500 ohm (1K5) R and then to ground.

Here's the link to our host Doug's schemo library for your build.

http://www.el34world.com/charts/Schematics/files/fender/REVERB_6G15.pdf

Gotcha. I see exactly what to check on the layout. I'm at work and don't have the unit in front of me, but as soon as I get home I'll start measuring.

Thanks for your help so far, this kind of information is great and helps me learn.

[Willabe]

You'll find it.

Your gonna love that verb unit. I built 1 last year and they sound great, IMO.


                  Brad     

[Willabe]

I then checked DC resistance on the resistors and found that the 2.2M resistor that connects to pin 7 of the 12AX7 measures at .5k. Is that weird? or is there a reason it would read that way?

This I don't know why.

One of the other guys will know.


               Brad    

[deadlands]

I rewired the off-board connections and reflowed all the solder joints. I also double-checked the layout against my build. Everything seems up and up.

Still same issue. This time when I touch the probe to pin 6 I get some tank splash and I notice more hum from the PT. That side of the tube is not working, no glow and same voltages. I made sure the path from pin 8 to ground was solid, I even rewired from the 1k5 to ground to be sure. Voltage at pin 8 is zero. I tried several different tubes as well and verified that there is continuity between the pins and the where the tube pins insert into the socket.

Here's some better photos:

[sluckey]

I made sure the path from pin 8 to ground was solid, I even rewired from the 1k5 to ground to be sure. Voltage at pin 8 is zero.

Measure resistance from pin 8 to chassis. Meter lead directly on pin 8, the other meter lead directly on chassis. Reading should be 1k5. What does your meter say?

[Willabe]

Still same issue.  That side of the tube is not working, no glow and same voltages.

Voltage at pin 8 is zero. I tried several different tubes as well and verified that there is continuity between the pins and the where the tube pins insert into the socket.

You have to have a dc voltage at pin 8, about ~2dcv as per schemo.

When a tube is working it pulls current up from the ground through the cathode to plate. That causes a dcv to be developed on the top of the KR and a dcv drop across the plate R. You don't have either happening. You have to find whats causing this.

Take a copy of the layout schemo and go through each and every connection 1 at a time and verify that it's correct. As you run them down 1 at a time and OK them mark them off with a marker on the layout copy and move on to the next connection untill you find it.

You found that there was the wrong dcv on the plate and no dcv on the K, your going to find what's causing it.    

Keep us updated as you go for help if needed.


             Brad    

[deadlands]

Got it working!

I realized I didn't pay much attention to the heater wiring so I closely examined them and noticed that the jumper from pin 4 to 5 didn't look to be connected. I resoldered that and the tube fired right up.

Thanks for all the help! I knew it to be something completely silly since there was nothing obviously wrong.

The unit sounded fabulous. I couldn't play for long since the fiance needed to get to sleep but I'll get to do some playing tomorrow night after I get the new tank and get it all together in the cabinet.

It did seem to have some noise, so I need to see about how to eliminate that.

[Willabe]

Got it working!

Alright then! Well done.     

1'st build?      Bad solder joint, join the club. Get's the best of them.     

It did seem to have some noise, so I need to see about how to eliminate that.

Next on the plate.


             Brad     

[TIMBO]

Well done, Try to find some good pics of similar builds and this might help with your wireing. Better wireing (sheilded where neccessary )will help with hums and buzzes. 

[deadlands]

Alright then! Well done.     

1'st build?      Bad solder joint, join the club. Get's the best of them.     

Yup, that's a lesson to never assume a connection is good. I think I was putting the probes on the wire and not the pins when I was checking the heaters. 

[Willabe]

I think I was putting the probes on the wire and not the pins when I was checking the heaters. 

Yeah could be, I missed that.

I should have asked you to look and see if the heaters where lit up.


           Brad       

[deadlands]

Well here it is in all it's glorious wet verb. It really sounds fantastic and after getting it in the cab, it's pretty quiet. I won't get to use it at gig volume until this weekend but man, I'm loving this.

[tubeswell]

Nice looking job.

[TIMBO]

Thats GREAT. No problems with the verb tank mounted on its side??? Just a small thing to help with some of your noise/hum/buzz is to maybe to fix a metal divider between the PT and the filter caps although it is along way away from the sensitive end of the amp so it may not do anything. NICE   

[deadlands]

Thats GREAT. No problems with the verb tank mounted on its side??? Just a small thing to help with some of your noise/hum/buzz is to maybe to fix a metal divider between the PT and the filter caps although it is along way away from the sensitive end of the amp so it may not do anything. NICE   

I got the 4AB3C1C tank which is designed to be mounted that way. That's the way the original Fender's were mounted. The only difference between my build and a vintage Fender is the placement of the filter caps. I didn't want to deal with a doghouse and trying to fit the board in with the provided stand-offs.

[Willabe]

I got the 4AB3C1C tank which is designed to be mounted that way. That's the way the original Fender's were mounted.

If you look at the info from the accutronics site they show that orientation is the best way to mount the tank. But you have to have the right tank for different orientation in mounting.


          Brad     

[Willabe]

The only difference between my build and a vintage Fender is the placement of the filter caps.

No it's not. I just noticed you mounted the PT inside the chassis.

Why?

That might be part of the noise your getting. It's supposed to be on the outside of the chassis for shielding for the circuit. Also for free air to cool any heat from the PT during usage. And any heat from the PT the way you have it inside the chassis will heat the components inside the chassis.

             Brad     

[deadlands]

No it's not. I just noticed you mounted the PT inside the chassis.

Why?

That might be part of the noise your getting. It's supposed to be on the outside of the chassis for shielding for the circuit. Also for free air to cool any heat from the PT during usage. And any heat from the PT the way you have it inside the chassis will heat the components inside the chassis.

             Brad     

Actually, the originals have the PT mounted inside the chassis. The chassis shields the reverb tank input transducer from the PT.

Here's the inside of a vintage unit

[Willabe]

Actually, the originals have the PT mounted inside the chassis. The chassis shields the reverb tank input transducer from the PT.

Look at that, hmm...      

They must have felt that shielding the tanks transducer (output) was more important then the reason I sited.

Thanks for posting that.

BTW, where did you get the springs you mounted the tank to the cab with?


               Brad                          

[deadlands]

Look at that, hmm...      

They must have felt that shielding the tanks transducer (output) was more important then the reason I sited.

Thanks for posting that.

BTW, where did you get the springs you mounted the tank to the cab with?


               Brad                          

I don't think heat is much of an issue in these units. There are still many old units running like champs out there. I played mine last night for about 2 hours and it didn't get that hot. The 6K6 was definitely toasty but the chassis was not.

I got the mounting springs from darrenriley.com. It's the only place I've found them.

http://www.darrenriley.com/FenderAmpParts.htm

Scroll down and there is the spring extensions and the spring anchors. They are original Fender parts. He also has the lock bar which was on back-order, but I should be getting that in a few weeks.

[Willabe]

I got the mounting springs from darrenriley.com. It's the only place I've found them.

He also has the lock bar which was on back-order, but I should be getting that in a few weeks.

Thanks!


                Brad     

[Willabe]

Scroll down and there is the spring extensions and the spring anchors.

Scroll down, boy you wheren't kidding!


                    Brad