New BF Champ-type build Q's FINISHED!!!

[rlh5599]

so I just finished up a BF Champ type build, Hammond 270 bx PT w 275-0-275 hoping to get a reasonable B+ with a Mojo 771 OT w 4 and 8 ohm taps to run through my 8ohm cabinet.  All is well, but the V2 bias is WAY high.
Here is a list of the voltages:
A: 356
           w 1k per schematic
B: 353 
           w 10k per schematic
C: 331

V1: 12ax7
1: 220
2: 1.4
3: 164
6: 219
7: 1.6
8: 1.69

V2:  6V6 GT RCA
3: 349
4: 353
5: 6mv
8: 27  (It was 56 with the 470 ohm, I got it down to 27 with a 1000 ohm)

v3:  (5y3 GE)
2: 358
4: 2.5
6: 2.5
8: 356

I don't understand why the bias is so high, I went through the schem and layout and everything is where it's supposed to be, all new parts doubled checked when installed with VOM.
It works, but it sounds like crap, any thing I can look for??  Just a hobbyist, just need a suggestion where to look...

Thanks!

[sluckey]

Try a different 6V6.

[PRR]

> 56 with the 470 ohm, ... 27 with a 1000 ohm

That does not make sense. Re-check your readings.

[rlh5599]

Thanks guys, got some stuff to do, will check more later.. 
I tried a couple different 6v6's and no appreciable diff.  (another RCA @ 27v, and an old  tungsol at 24.5v)
rechecked the voltages and nothing jumped out.

[mresistor]

can you post a picture of the PS end (6V6 and 5Y3 and circuit board) ?


Probably wouldn't hurt to put a 470 ohm 1 watt resistor on the screen grid to get the screen voltage lower than the plate.

[rlh5599]

So, just so I have my terminology correct, a 470 ohm resistor on pin 4 (grid) of the 6V6, correct??
and here's a pic:

[mresistor]

Yes. Pin 4 is the screen, should be kept lower than the plate. But its not that critical with your voltages as measured., and Champs have been running without them for decades without adverse results. It was just a recommendation for max tube life and better operation.  With that 1K cathode resistor in there you are running 9.5 watts dissipation. Should not sound bad. 

[rlh5599]

Yes. Pin 4 is the screen, should be kept lower than the plate. But its not that critical with your voltages as measured., and Champs have been running without them for decades without adverse results. It was just a recommendation for max tube life and better operation.  With that 1K cathode resistor in there you are running 9.5 watts dissipation. Should not sound bad.

I'll make the change, and recheck all the voltages a little later, got some honey-dos..
I still don't under stand why the V2 cathode bias voltage is so high?

[mresistor]

what is your cathode capacitor ?

[rlh5599]

22uf-50v

[rlh5599]

new voltages with a vintage Tungsol 6V6, and 1k bias resistor.
A: 339
           1k res
B: 336
            10k res
C: 315

5Y3
2: 337
8: 337

6V6GT
3: 333
4: 337
5: 3.6mv
8: 23.4  with 1k resistor

12AX7
1: 213
2: 1.2mv
3: 1.52
6: 212
7: .9mv
8: 1.6

I put a 470 ohm on the screen grid of the 6V6  (pin 4) and it dropped the voltage to 230??  Didn't seem right to me, so I pulled it off.

I checked the wall voltage today, and it was 120, at times it gets to 125, but everybody here in the Mobile Home Park I live in have their AC units on right now.  I'll check again when it cools down later.
It sounds OK through my cannabis rex, not much headroom at all though, breaks up real early.  I also have a tone stack bypass switch, when that gets flipped it'll blow your hair right back.

I still don't understand why the bias voltage is so high...

[mresistor]

I put a 470 ohm on the screen grid of the 6V6  (pin 4) and it dropped the voltage to 230??  Didn't seem right to me, so I pulled it off.

That is strange. Why would a 470 ohm resistor drop 100+ volts on the screen grid?  Are you sure your meter didn't read 330v? Actually one should probably use a 1K to 2K ohm to get the screen voltage lower than the plate voltage in a Champ.


With that Tungsol  and the numbers you posted your now only running 7.8 watts. You could probably use a 750 ohm or 680 ohm cathode resistor.


With that PT your using I wouldn't worry about the wall voltage being higher than 120. Your PS voltages are , to me , actually lower than I would expect in a BF Champ and could be contributing to an early breakup.

[John]

Your first post showed grid voltages on V1 around 1.5. They should be millivolts or zero. Your second voltages showed them more normal. I agree you shouldn't need a 1K cathode resistor, 470 is the default. I would lift the bottom leg of your cathode bypass cap on V2 and check the voltage. If no change, start looking for a leaking coupling cap somewhere (you say it sounds like crap).


But, does your meter have a fresh battery in it? A weak one will drive you nuts. ;)

[HotBluePlates]

... get the screen voltage lower than the plate.

This (plate voltage lower than the screen) is a non-issue.

The screen resistor is to ensure the screen's dissipation rating is not exceeded.

I put a 470 ohm on the screen grid of the 6V6  (pin 4) and it dropped the voltage to 230??  Didn't seem right to me, so I pulled it off.

You hand-numbered your socket's pins. Could you have the numbering wrong? (I can't see where the guide pin is to sort out the correct numbering)

[mresistor]

The screen resistor is to ensure the screen's dissipation rating is not exceeded.

So, HPB,  isn't it important to have a screen resistor in place if you plan to play the champ on Vol 10 (dimed) a large portion of the time ? A screen meltdown wouldn't be a good thing. Regarding the screen voltage , what if your screen voltage is 20v higher than plate?or 30v?  I know his is not, this is hypothetical question. Would it still be a non-issue? Personally, since the bigger fender amps have one, I use them on my Champ clones.


I just checked my Champ clone and it's running 330 volts actual plate voltage (plate v - cathode v) with 28 v on the cathode with a 750 ohm cathode resistor.  I don't consider 28 v high at all.

[shooter]

Would it still be a non-issue

I make sure G2's current can't take the tube to melt-down, I just let the volts go along for the ride.

[rlh5599]

Thanks guys, off to pick up a new battery, then go through one at a time with changes.

I am sure about the socket numbering, I just highlight the raised numbers. 
Kinda silly really, I always count around from number one anyway.

I had forgotten that some of the 'Magic' of the BF Champ was the higher plate voltage,
I will try the 115v tap, easy swap with the IEC conn.

Will report back later!

Thanks Again!

[HotBluePlates]

So, HPB,  isn't it important to have a screen resistor in place if you plan to play the champ on Vol 10 (dimed) a large portion of the time ? A screen meltdown wouldn't be a good thing. ...

A screen resistor is good to have, especially if you think the screen dissipation rating may be exceeded a significant portion of the signal cycle.

Most Fender amps didn't have screen resistors until the 60's. Some (like the Champ & Princeton models) didn't even get them then. There are a lot of Fender amps out there without screen resistors which seem to be getting by. If you're home-brewing, it might be good to spend a dollar or two & use them; just be sure to keep it in perspective.

Screen current should be fairly low until the plate is pulled down to its lowest voltage when the incoming signal peak equals the bias voltage.

... Regarding the screen voltage , what if your screen voltage is 20v higher than plate?or 30v?  ...

Draw a loadline for an output tube; perhaps the 6V6GT. If you follow the typical guidance, one end of your line is at the corner (the "knee") where the 0v gridline departs from running essentially-horizontal and drops steeply towards 0mA @ 0v plate. Look at the upper graph of page 3 of the linked sheet for a visual example.

Where the knee is, plate voltage is ~40vdc for a screen voltage locked at 250vdc. Let's say your plate voltage is 250vdc at idle. Anywhere along the signal swing which does not drive that tube into cutoff, the plate voltage is lower than the screen voltage. Yes, the dashed lines show screen current will rise at the very lowest plate voltages near the knee.

Other than that, what's the harm?

[mresistor]

So, in basic operation the screen being 20-30v higher than the plate isn't really an issue. One would have to drive the tube pretty hard to get plate voltage that low, but I suppose it could go that low briefly in a dimed amplifier with heavy bass notes being played. I see your point HPB and thank you for the information.

[mresistor]

HPB - in the time before your last post here I found this thread and it is really insightful ...


http://el34world.com/Forum/index.php?topic=12281.0

[PRR]

Twice you report major change with a "470" resistor. Check that part. Could it be 4,700? 47K?

[rlh5599]

Twice you report major change with a "470" resistor. Check that part. Could it be 4,700? 47K?

Geezus...  Yess, it's a 470K, meter reads ".465m"  instead of ".465k"...  I ordered it wrong, I measured it wrong, it's what I had in the v2 cathode bias, pulled it out to use on the grid when I put the 1k on the bias....

Back to work....

Thanks PRR.....

[rlh5599]

So, I think I got it figured out.  After I finally realized that I had a 470K ohm resistor mixed up in there, instead of a 470 ohm, things started to fall into place.  I put a 500 ohm res on the bias, (all I had) and retook my voltages, still with the 125 volt tap.
Much better, but too low, so I hooked up the 115v tap and got the 2nd column,

125V Tap                   115v Tap

V1:
1:183v                       203v
2: 1mv                       2.6mv
3: 1.6v                       1.8v
6: 188v                      204v
7: .9mv                      2.0mv
8: 1.6v                       1.8v

V2:
3: 307v                       341v
4: 313v                       347v
5:  4.5mv                      7mv
8: 18.4v                      20.8v

V3:
2: 321v                       351v 
8: 321v                       351v

A: 321v                       352v
B: 319v                       346v
C: 295V                       323v

sounds much better, a LOT of gain, but it can be dialed back a bit at the guitar.  still got some tubes to roll, and I might mess with the tone stack.

I have a bypass switch on the NFB at 2.7k/no nfb, And a tone stack bypass switch that both change the sounds dramatically.

All in all I am happy, and it's an easy one to mess with, now that I have it up and running.

Thanks again to all you folks for all the help.  So easy for ONE part to screw everything up. 
470k.......  lucky I didn't burn the house down....
and Thanks to Doug for the custom turret board!! 
(even though I screwed up on ONE turret.. AND I cut it down. it started out regular width)
Gratuitous pics..

[mresistor]

Glad to see you got her done - and will be making some music.  ;-)

[rlh5599]

Ever have one that had something that just bothered you?  I couldn't stand those big ole' orange drops hanging up there so I got some mojo dijon's.  They fit on the board better, and I can't tell one bit of difference in the tone. 
I think my next one will be a PP with same basic preamp, 2 inputs, maybe add a middle and a presence, (Variable NFB?)  and an MV.  Since I sold my 5E3 I kinda miss having a 6V6 12-15 watt amp.  We'll see. 

Thanks again for all your help.