6 V 6 Plexi Repair Advice

[purpletele]

Greetings,

I have a 6 v 6 Plexi head that I have had out in rotation as a back up amp.  I recently played it and brought it home for troubleshooting.

I observed two .022 Caps in the Phase Inverter circuit that appear to be blown out.

I went through and tested voltages throughout and there is definitely an issue at V4-1.

I intend on replacing the two caps and then retesting voltages to see if I have other issues.

Any thoughts on what might cause a blowout of the two .022 caps?

Thanks,

Brian V

[DummyLoad]

you are soldering too close to the body of the caps. raise them 3/8"-1/2" above the turret. keep iron time on each lead < 10 seconds.


--pete

[dude]

 Would close proximity of the leads to the soldering iron initially, cause the caps to blow, a while later...?   

[sluckey]

When you replace those yellow caps use Xicon chicklets like Doug sells. They won't melt when you drag a hot soldering iron across them.

[purpletele]

you are soldering too close to the body of the caps. raise them 3/8"-1/2" above the turret. keep iron time on each lead < 10 seconds.


--pete

That's what I thought when I looked at the photo. 

I looked at the original installation and those caps definitely blew out.  I will definitely raise the next set.

Thanks

BV

[purpletele]

When you replace those yellow caps use Xicon chicklets like Doug sells. They won't melt when you drag a hot soldering iron across them.

Thanks Steve,

I checked a before photo and I definitely didn't torch those babies at installation, I would have received a ration of crap from the group at the time for poor workmanship.

I can imagine that I might have weakened the ends of the cap wrapping by the heat.

I'll replace the caps and see what happens.

Now that I am into the amp repair mode it I might as well plan a new amp project!!

Thanks

BV

[sluckey]

 Based on your voltage chart the 6V6s are probably toasted. When you replace those two caps pull the 6V6s and check voltages on all pins. Pin 5 must have a negative voltage present before you plug in the 6V6s. If pin 5 is OK plug in the 6V6s and recheck voltages on all 6V6 pins. Then strum a tune.

[purpletele]

Based on your voltage chart the 6V6s are probably toasted. When you replace those two caps pull the 6V6s and check voltages on all pins. Pin 5 must have a negative voltage present before you plug in the 6V6s. If pin 5 is OK plug in the 6V6s and recheck voltages on all 6V6 pins. Then strum a tune.

Got it, Thank you!

BV

[purpletele]

Based on your voltage chart the 6V6s are probably toasted. When you replace those two caps pull the 6V6s and check voltages on all pins. Pin 5 must have a negative voltage present before you plug in the 6V6s. If pin 5 is OK plug in the 6V6s and recheck voltages on all 6V6 pins. Then strum a tune.

Looks like I have something else going on.

1. I replaced the .022 caps with some Orange Drops that I had on hand.  They are too big but they are in place.

2. I tested pin 5 on each power tube and it was reading -37 mV.

3. I installed the power tubes and proceeded to take readings.  I was getting 3.8 mV on V5-8 and 26 mV on v4-8.

4. I had recorded all of the voltages and I noticed it red plating, so I shut it down.

If the voltage going into V4 and V5 is consistent at 36 V and V4 can be biased at 26 mA, then I'm thinking there might be a problem with the V5 tube socket, primarily pin 8.   

Any input would be very much appreciated.

BV

[sluckey]

2. I tested pin 5 on each power tube and it was reading -37 mV.

Yikes! 37mV is like nothing. Pull the tubes. Look at the bias supply. Don't put the tubes back in until you have approx -35 VOLTS on pin 5 of both sockets.

[purpletele]

2. I tested pin 5 on each power tube and it was reading -37 mV.

Yikes! 37mV is like nothing. Pull the tubes. Look at the bias supply. Don't put the tubes back in until you have approx -35 VOLTS on pin 5 of both sockets.

Correction: I have -37 V at V4 & V5-5.

[sluckey]

Switch the tubes. Do the pin 8 readings follow the tube?

[purpletele]

Switch the tubes. Do the pin 8 readings follow the tube?

New tubes and V5-1 has a reading!  Apparently I fried a new set of 6 V 6's yesterday.

Re-set the bias and it has been stable for 1/2 hour. 

I think you and the crew pulled me to shore again, thank you guys!

BV

[dude]

So, the issue was one or both tubes went and took those caps out, or the caps blew and took the tubes with it?

[sluckey]

Caps shorted first. This put 276 volts on the 6V6 grids which normally operate with -36V. This quickly destroyed the tubes.

I'm still concerned about the voltage readings on V3. They may have corrected when the caps were replaced. Recheck those please.

[dude]

Thank you Sluckey, much appreciated. What could be some of the the reasons those two caps blew?

[shooter]

does it look like the circled cap is smoked?

think it's the PI input coupler?

[purpletele]

Shooter, the cap that you circled doesn't look scorched from this live view, the picture sure indicate something, but it appears normal.

I have had the amp running about 45 minutes and I retested the voltages, there seems to be an issue at V3-6.  The rest of the voltages are consistent with the Hoffman voltages. 

I'll be checking and verifying resistor values. 

New voltage chart attached.

Thanks for the input

BV

[sluckey]

Check R21.

[purpletele]

Check R21.

I pulled apart R21 and R39 and cleaned up the posts.  Both resistors measured within range, which was 99K ohms and 20K ohms respectfully. 

I replaced the resistors and remeasured and the voltage was even higher at 305 V.

R39 is the point where the choke is cut in.

Damaged choke? 

[DummyLoad]

steve, there is an error in the schematic: there are two components with the same ref. des. - R24. there's a 10K and 1M with the same ref. des.. apologies if this has been pointed out already. link below to the schema i'm referring to.

https://el34world.com/Hoffman/files/Hoffman_Plexi_6V6-V2.pdf

LTPI V3 inverting triode is probably not conducting. try a new tube, but make sure that you can read ~1MΩ from pin 7 to pin 8 of V3 before inserting a new tube. move V3 to V1 to test that triode. you should see 1-2V on pin 8 and at pin 6 about 1/2-2/3 of that B+ rail. 



--pete 

[purpletele]

steve, there is an error in the schematic: there are two components with the same ref. des. - R24. there's a 10K and 1M with the same ref. des.. apologies if this has been pointed out already. link below to the schema i'm referring to.

https://el34world.com/Hoffman/files/Hoffman_Plexi_6V6-V2.pdf

LTPI V3 inverting triode is probably not conducting. try a new tube, but make sure that you can read ~1MΩ from pin 7 to pin 8 of V3 before inserting a new tube. move V3 to V1 to test that triode. you should see 1-2V on pin 8 and at pin 6 about 1/2-2/3 of that B+ rail. 

Pete,

I measure 12K Ohms between V3-7 and V3-8.

[DummyLoad]

you should see the resistance of the 1MΩ grid leak (R24) between pins 7 & 8. you have the wrong value resistor installed or something is miswired.

--pete

[sluckey]

Disconnect that .1µF cap (the one circled in the pic above). Now recheck the resistance between pin 7 and pin 8. Do you measure 1MΩ  now? If so, replace the cap.

[purpletele]

does it look like the circled cap is smoked?

think it's the PI input coupler?

I need to post a correction to my original observation, this cap does look smoked.  When I inspected the cap originally I looked at the wrong one.

Sluckey's latest suggestion to check the .1 cap has humbled me again, it looks smoked!!

[sluckey]

Disconnect that .1µF cap (the one circled in the pic above). Now recheck the resistance between pin 7 and pin 8. Do you measure 1MΩ  now? If so, replace the cap.

[purpletele]

Disconnect that .1µF cap (the one circled in the pic above). Now recheck the resistance between pin 7 and pin 8. Do you measure 1MΩ  now? If so, replace the cap.

I disconnected the .1/250v cap and measured between V3-7 & v3-8 and measured 10.5 K Ohms, no change.  I replaced the cap, it did look like it was damaged.

Still getting 308 Volts on pin 6 after I put it back together.

I am focusing on wiring and resistor sizes

[sluckey]

Unsolder that 1M resistor and measure it's resistance. And while the resistor is out measure the resistance between pin 7 and 8.

[purpletele]

Unsolder that 1M resistor and measure it's resistance. And while the resistor is out measure the resistance between pin 7 and 8.

The 1M resistor is measuring 1M when pulled out of circuit.

The resistance at V3-7 and V3-8 is 10.5 K

[DummyLoad]

Unsolder that 1M resistor and measure it's resistance. And while the resistor is out measure the resistance between pin 7 and 8.

The 1M resistor is measuring 1M when pulled out of circuit.

The resistance at V3-7 and V3-8 is 10.5 K

phil, you're measuring from pin to pin right?  ( (+) probe on pin 7 & (-) probe on pin 8 )

--pete

[sluckey]

OK, I see the problem. The white wire that connects to pin 7 is connected to the wrong side of that brown .1µF cap. Move that wire to the junction of that brown .1µF cap and the 1M resistor. Refer to Hoffman's layout.

Has this white wire always been connected like this?

EDIT... Yep. I just looked at your posts from several years ago. It was wired wrong then too!  

[sluckey]

Heads up! This wiring error affects the NFB loop. Don't be surprised if when you make the correction the amp squeals or howls. If so, just swap the OT primary plate leads so the NFB phase will be correct.

[purpletele]

OK, I see the problem. The white wire that connects to pin 7 is connected to the wrong side of that brown .1µF cap. Move that wire to the junction of that brown .1µF cap and the 1M resistor. Refer to Hoffman's layout.

Has this white wire always been connected like this?

EDIT... Yep. I just looked at your posts from several years ago. It was wired wrong then too!  

That is remarkable!  I'll be home in a couple of hours to make the correction!

Thank you!

[sluckey]

Several years ago you had a lot of positive comments about the sound of this amp. I hope you still like it with the correction.    I guess you could put the mis-wire on a switch, label one position Plexi and the other position Purple Haze or some catchy name. Then you'll become rich and famous!   

[purpletele]

Several years ago you had a lot of positive comments about the sound of this amp. I hope you still like it with the correction.    I guess you could put the mis-wire on a switch, label one position Plexi and the other position Purple Haze or some catchy name. Then you'll become rich and famous!   

Ha!  I looked at that section until my eyes were crossed.  Good catch, and thank you.

I'll think about a name for the Mis-wired Plexi Mode Switch.  Maybe I'll name it the Blah Switch

[sluckey]

Come on, you can do better than that! Use a catchy phrase. Remember, people will be coming to you for the secret. You don't want to be known as the god of Blah!   

[DummyLoad]

plexi god mod.


--pete

[purpletele]

Hey I guess wire positions matter!  Thanks Steve.

The amps sounds fantastic and has power to spare.  I am documenting voltages and V3-6 is now under control at 194 V.

Thank you to all that helped solved my issues!!

I wish you all could help with another issue.  Starting Prostate Cancer treatment today, got it early but it is spooky.

Brian V

[DummyLoad]

>> Starting Prostate Cancer treatment today, got it early but it is spooky.

dang! hate reading this news - wishing you a speedy recovery and complete cure.

cancer sucks! kick it's ass!

--pete

[purpletele]

Final voltages for Reference.

The amps sounds great!

[sluckey]

Good deal! Now maybe you can give us one of your great demo videos with this amp?

[purpletele]

Good deal! Now maybe you can give us one of your great demo videos with this amp?

I will!

I owe good demo video on the following:

1. Dual 50'
2. 6 v 6 Plexi
3. AB763 Combo

[Ritchie200]

Come on, you can do better than that! Use a catchy phrase. Remember, people will be coming to you for the secret. You don't want to be known as the god of Blah!   

 

Jim

[Big_Mike]

Good deal! Now maybe you can give us one of your great demo videos with this amp?

I will!

I owe good demo video on the following:

1. Dual 50'
2. 6 v 6 Plexi
3. AB763 Combo

How would you compare the 6v6 Plexi with the dual 50?  I am sure the Dual 50 is louder, is that true?  What about tone-wise?

[purpletele]

Good deal! Now maybe you can give us one of your great demo videos with this amp?

I will!

I owe good demo video on the following:

1. Dual 50'
2. 6 v 6 Plexi
3. AB763 Combo

How would you compare the 6v6 Plexi with the dual 50?  I am sure the Dual 50 is louder, is that true?  What about tone-wise?

Here is a video of the 6 v 6 Plexi in a Mojo Blues Junior Cabinet.

The 6 v 6 Plexi is much more forgiving than the Dual 50 from an attack standpoint, primarily from the rectifier differences.
The 6 v 6 Plexi can clean up really nicely with a single coil and also be chimey with humbuckers when EQ'd correctly

The Dual 50 is a Full on Marshall JCM 800 and JMP.  The JMP is a lower gain deep chewey circuit which is very nice, sounds good with single coils, the JCM 800 side is a full on Marshall circuit, it needs to be cranked up to find the sweet spot.  Not so good with Single coils unless you can harness it correctly.  Sounds great at high volume with a nice humbucker.

Tone wise the 6 v 6 is easier to manipulate the amp to achieve different shades of tone at lower volumes. 

The 6 v 6 is more useful to me because I can get a great blues tone with a strat at a lower volume.  The JMP sounds killer with a strat but it becomes a volume thing in most cases.

BV

[Big_Mike]

I really like my Plexi 6V6.  Just exceptional tone at low volumes, and sounds great with single coils and humbuckers.  A truly great all-around amp.

[66Strat]

Good deal! Now maybe you can give us one of your great demo videos with this amp?

I will!

I owe good demo video on the following:

1. Dual 50'
2. 6 v 6 Plexi
3. AB763 Combo

How would you compare the 6v6 Plexi with the dual 50?  I am sure the Dual 50 is louder, is that true?  What about tone-wise?

Here is a video of the 6 v 6 Plexi in a Mojo Blues Junior Cabinet.

The 6 v 6 Plexi is much more forgiving than the Dual 50 from an attack standpoint, primarily from the rectifier differences.
The 6 v 6 Plexi can clean up really nicely with a single coil and also be chimey with humbuckers when EQ'd correctly

The Dual 50 is a Full on Marshall JCM 800 and JMP.  The JMP is a lower gain deep chewey circuit which is very nice, sounds good with single coils, the JCM 800 side is a full on Marshall circuit, it needs to be cranked up to find the sweet spot.  Not so good with Single coils unless you can harness it correctly.  Sounds great at high volume with a nice humbucker.

Tone wise the 6 v 6 is easier to manipulate the amp to achieve different shades of tone at lower volumes. 

The 6 v 6 is more useful to me because I can get a great blues tone with a strat at a lower volume.  The JMP sounds killer with a strat but it becomes a volume thing in most cases.

BV

Great sounding amp!   Very nice playing!