5F1 arc in tube 6V6

[FREYES_7]

Hello all!

I built a 5F1 from scratch, as there are no kit available in my country,

I ran some guitar tests

1.- sounds fine at 9 and full volume, but awful at 12 and 3.

2.- had a second try and it did not make a sound, the 6V6 tube was arcing.  Disconnected and connected amp and it went back to 1, somehow...

Any insight will be appreciated!

[shooter]

https://el34world.com/charts/Schematics/files/Fender/Fender_champ_5f1_schem.pdf


what's the DC voltage across the cathode R? 
Is it 470 ohm like the schematic?
what is the DC voltage on the "right side" plate and "left side" screen-grid of the 10K dropping R??

[tubeswell]

And can you upload some hi-res gut shots so we can inspect your wiring etc,

[FREYES_7]

Hello guys!

Not sure if I covered this request! "what is the DC voltage on the "right side" plate and "left side" screen-grid of the 10K dropping R??" please correct me if I indicated wrong voltage points,

6v6 cathode voltage = 23V
6v6 Cathode resistor= 478 ohm

B+1= 428V
B+2= 269v
B+3= 239V

full view https://imgur.com/a/wNM0sd4
preamp input https://imgur.com/a/C3gj4H0
12ax7 plate caps https://imgur.com/a/9mdYftR
6V6 cathode cap and resistor https://imgur.com/a/8CapIDM
DC filter caps https://imgur.com/a/4GZQSdP
5Y3 rectifier connections https://imgur.com/a/ZS3Gwni
6V6 tube connections https://imgur.com/a/co29iyo
12AX7 tube connections https://imgur.com/a/63u3gS0

I followed the stewmac instructions, and in them I did not find were to connect the black cable (the other end of the 4ohm output coil) of the OT, so I soldered it to the OT output jack's ground, as shown in last image,

Played through the amp at low levels and worked fine, past 12 it still fuzzes awful when trying chords

[shooter]

23vdc /478 ohm = 48mA


428vdc - 23vdc = 405vdc


405vdc * .048A = 19.44WATTS


you're biased too HOT, hence the arc'E spark'E


make the cathode R something around 700 ohms give our take


fuzzes could be speaker, bias, too much drive signal to the 6V6

[FREYES_7]

Thanks!!
I also noticed I used the 8ohm output instead of the 4ohm output,

I think I will try fixing that first

I hope I didn't ruin the speaker 😅

[Willabe]

Thanks!!
I also noticed I used the 8ohm output instead of the 4ohm output,

I think I will try fixing that first

I hope I didn't ruin the speaker

NO!

Fix the bias 1st! Your killing that tube. It might be toast already.

You can run a slight mis-match with speakers, 4 ohm OT tap into a 8 ohm speaker is fine. Or an 8 ohm OT tap with a 4 ohm speaker, it's fine. Just wont get the max output, slightly less.

[FREYES_7]

Thanks!!
I also noticed I used the 8ohm output instead of the 4ohm output,

I think I will try fixing that first

I hope I didn't ruin the speaker

NO!

Fix the bias 1st! Your killing that tube. It might be toast already.

You can run a slight mis-match with speakers, 4 ohm OT tap into a 8 ohm speaker is fine. Or an 8 ohm OT tap with a 4 ohm speaker, it's fine. Just wont get the max output, slightly less.

I see, I will get the right resistors then, thanks!!

[shooter]

I will get the right resistor

if you're buying, pick up something close to 550 ohms, anything in the 600 ohm range, then something 700-750 range in the 5WATT range
even biased HOT the R is "feeling" about 1.2W so double for safe is 2.5W


There is no "right" in SE except to keep the plate around or Below Max dissipation.  It's usually a trial n error method rather than a mathematical "model"

[FREYES_7]

I see, tomorrow I'll check in local stores and buy lots of 5W resitors just in case, if not I will have to order abroad,

What does SE mean?

[tubeswell]

What does SE mean?

Single Ended.

[FREYES_7]

Hello!
I changed the 478ohm @1W to 680ohm @5W but the issue persists

Will try changing the OT's output from 8ohm to 4ohm, same as speaker during the weekend and check again

[shooter]

what are the new DCV readings??

[FREYES_7]

Hello!

I made the modifications and the amp is behaving in the same way   

Maybe I should try another 6v6 tube? Did not change the tube that arced

New voltages are:
Volume pot at minimum volume, no guitar connected

B+1= 416V
B+2= 379.6V
B+3= 328.1V

12ax7
Plate 1= 208
Plate 2= 210
Cathode= 1.64V

6v6
Plate= 395V
Cathode= 27.17V

[shooter]

Maybe I should try another 6v6 tube? Did not change the tube that arced

that's a good idea
also use the math I did with your new VDC readings and see where you set on tube dissipation, BEFORE you put in a new tube, then re-check after the new tube is installed, BEFORE you apply a signal.

[FREYES_7]

Thanks for the wise advice!

I checked the TAD 6V6GT-STR data sheet and states 60mA as maximum dc current in cathode

Calculating 27.17/680 =39.95mA

(416-27.17)x39.95mA yields to 15.54W

Will double check connections, then try new tube,

If that doesn't work will replace the resistance till I have the 12W indicated in the shared doc

[FREYES_7]

Tried with new tube, and the same issue happened, will change speaker input and cables (I'm using car speaker cables 20awg) and prepare another board, I think I ran put of options 

[tubeswell]

Hmmm For a 6V6

2 and 7 are the heater pins
3 is the plate
4 is the screen
5 is the signal grid
8 is the cathode

Edit - oopsies. - Looks like the cathode resistor is going to pin #1 (and you need it to go to Pin 8 for biasing)

[tubeswell]

The previous arcing may've caused some charring between some of the 6V6 socket pins - inspect this carefully

[FREYES_7]

Thanks for the advice!

I'm not giving up, will check the 6v6 socket and add a grid stopper during the week

The tube hasn't arced in the recent tests, but he speaker still gets spluttery after 12

[SEL49]

Temporarily disconnect that green wire from the 22K resistor (located on the board). Any joy?

[FREYES_7]

Hello! I tried that already and no joy found, issue worsened:(

[66Strat]

 I suggest that you add resistance in the power supply prior to the rectifier. One option could be to add 500 ohm 3 watt resistors for both the A and B resistors. Option 2 could be to add 1,000 ohm 5 watt resistors to either the A or B resistors.

Modified to add text: The result of the added resistance would be to drop the B+ voltage by about 50 volts.

[FREYES_7]

Hello!

I have two 820ohm @5W

Two 470ohm and one 680 ohm at same power

Would the latter 2x470 in the B and 680 in the A do the trick?

[66Strat]

Hello!

I have two 820ohm @5W

Two 470ohm and one 680 ohm at same power

Would the latter 2x470 in the B and 680 in the A do the trick?

It should get you in the ball park. Another option could be to wire the two 470 ohm resistors in series in the A position or a 470 ohm resistor in series with a 680 ohm resistor in the A position. Whatever combination works
to get in in the 350 volt range.

[FREYES_7]

Hello!
Added the 470+680 5W resistors in the A position and voltages improved a lot!

Unfortunately the spluttery sound stayed, I think I'll change the RCA output for a ts output

New voltages are:

B+1= 329.5V
B+2= 300V
B+2= 257.8V
12ax7
Plate1= 163.1V
Plate2= 167.2V
Cathode= 1.34V
6v6
Plate= 315.1V
Screen= 300V
Cathode= 21.1V

[66Strat]

The voltage drop between the plate and screen connections of the 6V6 looks low. Are you sure that there is a 10K resistor there. The voltage drop looks more like that of a 1K resistor (could be a bad solder joint).  Double check your solder joints, maybe reflow the joints with fresh solder. Clean/retension tube sockets. If this doesn't fix the problem and you want a new V6, get a JJ. They are not made in a communist country that invaded a european country.

[AlNewman]

Looks like you're biased safe for testing.
Does it still sputter with the volume turned down?  How about when you pull the pre amp tube?

[FREYES_7]

My 6v6s are TAD I think they are made in Germany hehe and got a spare, I can get jjs tho, they are sold in my country

I will check the 10k, I have 9.1k and 3.somethingk @2W for replacement, maybe those would make a better job (?)

At 9 the amp sounds good, even after the arc episode,

At 12 and onwards it sounds spluttery

There are no signs of burnt socket or any charring

I'll change that resistor and change the RCA output and if it doesn't work I think I'll just change the whole board and check again.  This was my first try so it is awful, now I can be tidier :D

[shooter]

At 9 the amp sounds good, even after the arc episode,

Once you Overdrive a Class A amp, things can and do make ugly sounds, that's were a scope comes in handy.


Have you tried other speakers?

[FREYES_7]

I'm using an eight15 from celestion,

Have another spare, I originally planned to build 2 5f1s in one go, it turned out it wasn't that easy hehe

The speakers are new, but I don't lose anything trying

[shooter]

 the 2nd one will go much easier now 

[66Strat]

My 6v6s are TAD I think they are made in Germany hehe and got a spare, I can get jjs tho, they are sold in my country

I will check the 10k, I have 9.1k and 3.somethingk @2W for replacement, maybe those would make a better job (?)

At 9 the amp sounds good, even after the arc episode,

At 12 and onwards it sounds spluttery

There are no signs of burnt socket or any charring

I'll change that resistor and change the RCA output and if it doesn't work I think I'll just change the whole board and check again.  This was my first try so it is awful, now I can be tidier :D

Don't be in a rush to replace the resistor. Measure the resistor to ascertain its resistance. Then, using ohm's law, you can determine the screen current. There is nothing wrong with having a 1K resistor in this position. The voltages that you have are good. What I was questioning  was the voltage drop. The voltage drop is less than I would expect for a 10K resistor, which could indicate a bad solder joint or a bad tube. Use chop sticks to poke around and identify bad connections. (Tuck your left hand in your back pocket.)

TAD does not make any tubes. TAD rebrands tubes that are tested to meet their specifications. There are only three tube manufacurers in the world, the old Reflektor plant in Russia, Shuguang in China, and JJ in Slovakia.

[66Strat]

FWIW, I believe the TAD 6V6GT-CZ is made by JJ.

[FREYES_7]

Hello!

Today I changed the small 10k@2W for a normal 9.1K@2W, added a grid stopper 1.5k and a Screen stopper 470 Ohm, now I can reach up to 10 o clock with good tone haha, but the issue remains, I corrected the OT output to match the speaker (4Ohm)



b+1= 335.4V

b+2= 306.6V

b+3= 265.8V



6v6

plate= 319.6V

screen= 307.1V

cathode= 21.7V



12ax7

plate1= 168.5V

plate2= 171.8V

cathode= 1.4V

[AlNewman]

16V seems like a lot between B+ and the plates across the transformer. What is your resistance across the transformer winding?

[tubeswell]

My 6v6s are TAD I think they are made in Germany hehe and got a spare, I can get jjs tho, they are sold in my country

I just had an amp on the bench with TAD-branded 6V6s and a problem with loud intermittent hum. I noticed the heater in one of the tubes glowing more brightly exactly when the humming occurred. There was nothing else wrong with the amp apart from that. Looked a lot like poor H-k insulation to me.  I put a new set of JJs in and tested for a couple of hours and problem appears to be solved. Methinks the rebranded TADs, for all their fancy stickers, are probably not worth the hype.

[FREYES_7]

The resistance across de winding is 470+680 @5W so 1050 ohm

I tidied the cables, the ot output was too close to the preamp tube, making that spluttery sound,

Now it works!!!! Thanks to everyone that helped and provided precious feedback!! I have enough materials for a seconf 5F1, hopefully more tidy this time