Magnatone Twilighter 2020--MOD BUILD-Only Tremolo

[sluckey]

Show us a picture of your rectifier socket.

[sluckey]

I checked voltages at the rectifier socket, and I’m getting voltage at all pins except for the yellow wire at pin 8. The other yellow wire at pin 2 is showing 5 VAC, and each of the red wires show 350 VAC. So no power is getting to the OT node on the cap can. Is it a bad PT winding?

You cannot measure the 5VAC at pin 2 or 8 if you have one meter probe on chassis. The only way to accurately read the 5VAC is to put one probe on pin 2 and the other probe on pin 8.

[wsscott]

Here’s a photo of the rectifier socket.

[sluckey]

Your wiring is fine. Your method of checking the 5VAC is suspect.

[wsscott]

Ok, so that I’ll have something to dream about tonight, since I was told to check and write down the voltages before I install the tubes and let the group know the readings, shouldn’t I be seeing voltages other than just at the rectifier socket?  If so, what would you expect me to see? Thanks.

[sluckey]

I've asked several times about the bias voltage on pin 5 of the output tubes. I ain't answering any more questions until you answer my question.

[wsscott]

That’s fair. I’m burned out for tonight, but will get that posted in the morning. Thanks.

[wsscott]

Ok, so pins 2 & 8 read 5.41 VAC

When the bias pot is adjust to the maximum negative VDC:
V-4 reads -0.901VDC—90mv
V-5 reads -0.902VDC

[sluckey]

When the bias pot is adjust to the maximum negative VDC:

So what is that voltage?

[wsscott]

- 90.1 mv

[wsscott]

No, that should be -901 mv! That doesn’t make sense. I’ll check the auto-scale on my meter.

[sluckey]

Where is your meter connected when you measure that 901mV?

[wsscott]

The red probe is on Pin 5 where the 1.5K resistor attaches, and the black is to the chassis.

I checked it with another meter and read the same thing.

Hopefully you are going to tell me that I've got the black probe incorrectly connected to chassis.

[sluckey]

Nope. I'm gonna tell you that's a problem and you will have to troubleshoot to find what's wrong. Plug the amp straight into the wall and recheck pin 5 voltage. If it's not something like -30v to -50v, then the troubleshooting starts. Check for -30v to -50v (or more) at the junction of the diode and the first bias cap. If no good, set your meter to read AC volts and check for approx 50VAC on the other side of the diode. If not good, check for approx 50VAC at the junction of the 470Ω resistor and the wire from the PT. Report findings. Somewhere along this troubleshooting path you should have found the problem. Could be wiring error or bad/incorrect component.

A hi-rez pic showing the bias pot end of the board would be a big help right now.

[wsscott]

Here’s the photo.

[wsscott]

Ok, here are the results.

1.  With amp plugged directly into wall receptacle, and black probe on chassis, and red probe on Pin 5 the reading is -0.936 VDC.

Black probe connected to chassis and Standby ON, ie. not in Play position, for the following measurements:

2.  At the junction of the diode and the first bias cap (470 ohm 3 W) it reads +22.83 VDC.

3.  It reads +0.001 VAC on the other side of diode.

4.  At the junction of the 470 ohm R, and the Blue 50V PT wire, it reads 0.198 VAC.

[wsscott]

Sorry, #4 above does read 54.7 VAC not 0.198.

[sluckey]

Replace that 470K 3W resistor with a 470Ω 3W resistor!

[shooter]

or just paint the 2 yellow stipe brown n call it good 

[sluckey]

Replace that 470K 3W resistor with a 470Ω 3W resistor!

While you're at it, do the same for the two 470K 3W resistors on the tube sockets.

[wsscott]

Wow-I must be going blind.  But I did have 470K 3W resistors for some reason.  Why I don't know because there are none in this layout.  I must have ordered them wrong in the first place.  So all I have on hand is 470 ohm 2W resistors.  Should I use those or wait for a new order arrives?  Thanks as always.

[sluckey]

So all I have on hand is 470 ohm 2W resistors.

Those will be fine in all three places.

[shooter]

Wow-I must be going blind.

fwiw;
I always ohm every resistor before I install it. 
The CCP is real bad at brown, red, and orange

[wsscott]

I think things are going right.

Pin 5 is showing-59.3/-59.2 VDC max. Neg bias.

51.0 VAC at the diode/first bias cap.

-61.2 VDC or 0.00 VAC on other side of diode at the 47uf cap. IS THIS CORRECT?

52.6 VAC at junction of 470 ohm resistor and blue PT wire.

Issues or good to go with next phase of startup?

[sluckey]

    Much, much better. But before we go on, let's fine tune the negative voltage on pin 5. Still no tubes plugged in. We really want the bias pot to be able to swing from approx. -25v to approx -45V for 6V6 tubes. So, we need to experiment with that 470Ω resistor on the board. That resistor is commonly referred to as the bias range resistor and right now it's too little so pull it out and set aside. Now temporarily connect a 1K resistor in that spot. Gator clip leads make this easy. Check the bias voltage on pin 5 and adjust the bias pot from one end to the other and note the max and min negative voltage values. If the range is now close to -25 to -45, you're done. Permanently install the resistor and set the bias for max negative voltage on pin 5.

If you still don't have a proper range, grab another resistor and repeat. A larger resistor will reduce the voltage and a smaller resistor will increase the voltage. You can use 1/2W or 1W for testing but once you have nailed down the value you need, get a 2W or 3W for the permanent resistor.

Your 6V6s will probably want about -35 volts for proper operation. I chose the -25 to -45 range so the pot will sit at about halfway for -35v. This way you can adjust hotter or cooler as you wish. When you get this nailed down report the resistor value and the range of voltage on pin 5. Leave the bias pot set for max negative.

Don't proceed any farther until this step passes QC. I'll be close to the forum for the rest of the evening.

[wsscott]

Thanks. I’ll work on this in the morning. Thanks again.

[wsscott]

Sluckey I’ll check the range again tomorrow before I do anything, but I think the low reading was about -25 VDC.

[sluckey]

Sluckey I’ll check the range again tomorrow before I do anything, but I think the low reading was about -25 VDC.

The max negative voltage is applied to a voltage divider consisting of a 10K pot and a 10K resistor. This means the minimum voltage will be half the maximum voltage. IE, if the max is -60v, the min will be -30v. But if we can drop the max down to -48, the min will be 24, and this is a much better range for 6V6s. The bias voltage for 6V6s will likely be about-35v when tubes are biased to about 70%. This puts the bias pot at about the middle of rotation, giving you plenty of adjustment to bias hotter or cooler if you like.

[wsscott]

What Luck! I have a 1.5K @ 2W that brought it to -25.98 to -48.6 VDC! So I’ve installed that one.

[sluckey]

What Luck! I have a 1.5K @ 2W that brought it to -25.98 to -48.6 VDC! So I’ve installed that one.

Set the bias pot to -36V at pins 5 of both 6V6s.

Next step... With amp plugged straight into the wall and amp in ***STANDBY*** mode, plug in all tubes, turn the amp on, and insure you can see ***ALL*** filaments glowing. 12AX7s can be hard to see so dim the room lights if necessary. Now measure the filament voltage at V1 with meter connected ***ACROSS*** pins 4,5 and 9. Should be 6.3VAC.

Next step... Unplug the amp from the wall and plug it into the light bulb limiter. Connect a proper speaker. Turn the amp on and put the standby switch in ***OPERATE*** mode. If no bright light and no smoke, turn the amp off, unplug from light bulb limiter, and plug directly into the wall. Turn the amp on. If no loud squeal, smoke, or red plates, measure the dc voltage at all four B+ nodes. Play a little guitar. Report the voltage readings and any unusual sounds, and tell us how the guitar sounds.

[wsscott]

How can I "Set the bias pot to -36V at pins 5 of both 6V6s"--the word "both" confuses me since I only have 1 pot.  I can read the bias voltage on each of them, but I can't separately adjust them--can I?

[sluckey]

You're that guy that all instructors just love.   

OK, let's make this easy. Set the bias pot to -36V at pin 5 of any 6V6. You get to chose which one. Now check the voltage at pin 5 of the other 6V6. I'm betting it will be -36v too.

[Willabe]

How can I "Set the bias pot to -36V at pins 5 of both 6V6s"--the word "both" confuses me since I only have 1 pot.  I can read the bias voltage on each of them, but I can't separately adjust them--can I?

No you can't adjust the -bias separately, but, when you adjust the -bias pot, it will adjust both power tubes -bias at pin 5 at the same time.

Because both tubes pin 5 are fed from the same source.

They will be the same -dcv or very close. 

[wsscott]

Thanks. Yes I know! I guess that’s the retired lawyer still hanging around in me. Every word has a meaning and sometimes it’s important.

[sluckey]

I should have guessed. You guys speak and write in such a cryptic manner that the only people that understand is another lawyer. I'm gonna see if jjasilli will rewrite my responses so you can understand them.   

[wsscott]

I see the filaments glowing on the 3 12ax7’s and the GZ34 but NOT on either of the 6V6’s.  I’m reading 6.43 VAC on the 12ax7 filaments.

[wsscott]

I know why. I didn’t wire pins 2 & 7 on them to the 6.3 V filament wire. You don’t need to say anything.

[sluckey]

I see the filaments glowing on the 3 12ax7’s and the GZ34 but NOT on either of the 6V6’s.  I’m reading 6.43 VAC on the 12ax7 filaments.

Too late. jjasilla, you want to take this one?

[wsscott]

Ok, everything lit up, no smoking. Then I went your next step and I got the high pitch squeal. I assume from what you said earlier in this process that I need to reverse the OT Secondary wires at the Internal Speaker Jack. Correct?

Can you achieve the same temporary fix by reversing the wires on the speaker?

[wsscott]

And I did get a low warm glow on the light bulb limiter.

[wsscott]

Sorry, I meant switch the OT leads at Pin 3 on each of the power tubes.

[Willabe]

Sorry, I meant switch the OT leads at Pin 3 on each of the power tubes.

Yes, you can switch either the OT primary leads (pin 5) or the power tube control grid (grid 1) (pin 3) leads.

Edit; Pin 3 = power tube plate, pin 5 = grid 1. If you put the grid stopper across pins 1 and 5 then swap the wires at pins 1. 

[Willabe]

If you want to test it 1st, just disconnect the -feed back wire. If that stops the squeal, then swap leads at the power tube.

[sluckey]

Sorry, I meant switch the OT leads at Pin 3 on each of the power tubes.

That's correct. Looking at your pic I'd say you're gonna have to stretch that brown OT wire a bit.  Another lesson to learn... Don't cut the OT primary plate leads until you are sure the NFB phase is correct.

There is another easy way to accomplish the same thing and it will be very easy to do on your amp. Locate the two green wires that go from the board to pin 1 of each 6V6. Swap those two green wires on the board. Easy, peasy.

[shooter]

I'm gonna see if jjasilli will rewrite my responses so you can understand them.

.........as wsscot is learn'n to translate the Tech language to Lawyer speak 

[wsscott]

I don’t know which language is worse to learn. I guess once you learn it though it’s no big deal. If I can learn these 2 languages, then maybe there’s still hope for me learning Italian!

[sluckey]

Any joy with the amp?

[Willabe]

He must be jammin. 

[shooter]

more like 

[wsscott]

Sorry guys, but we had some thunderstorms and lost power for awhile, so will be getting to it this morning. Will keep you posted. Fingers crossed.🤞