Silverface champ biasing hot

[texwest]

I know this has probably been gone over and over, but want advice on this particular amp.

74 silverface champ

plate 400v
screen 409
cathode 25v

bias ~48ma depending on tube

new 500r 5 wt on cathode  with 25uf 50v cap

slightly redplating every tube I put in there.  NOS GE and Sylvania

recommendations??

[Latole]

400 v X 48 ma = 19.2 watts !!!

Champs use 6V6, a 12 watts tube, yes tube will redplate

Wich Silverface Champ ? Link to schematic please

That one ?  aa764
http://ampwares.com/schematics/champ_aa764.pdf

Recommendations ?  Put 600 ohms or more
If you are sure your tubes are not short

[sluckey]

According to the numbers you provided the tube is dissipating 18.75 watts. Too much for a 6V6. I suggest a larger cathode resistor, maybe even 750Ω.

[pdf64]

What are the actual measured wall and heater Vac? eg 120, 6.3, 5.

[mresistor]

I have a '68 that was running similarly and I used a 1K 2W cathode resistor to bring the P Diss down some. 

[texwest]

What are the actual measured wall and heater Vac? eg 120, 6.3, 5.

Funny, right before I saw your post I checked the heater voltage.  Right now   -   heaters 6.69v Wall voltage 122.6v plate at 398v

And the wall voltage is usually running around 123-124 in this town

I don't think I will be able to ask the guy to run his amp on a variac!  So want to get it running well on wall voltage.

[pdf64]

It seems that the PT was designed for 115V primary. At that level, the plate voltage would drop to to about 375V.
I don’t suggest the use of a variac to achieve that, rather a built in bucking transformer.

[HotBluePlates]

...   heaters 6.69v Wall voltage 122.6v plate at 398v

And the wall voltage is usually running around 123-124 in this town

I don't think I will be able to ask the guy to run his amp on a variac!  ...

It seems that the PT was designed for 115V primary. ...

People like to complain the Princeton Reverb runs on an "undersized Champ power transformer."  Instead, I think the Champ & Vibro Champ are run on an under-loaded Princeton/Princeton Reverb power transformer.

My 1965 Vibro Champ doesn't have 6.3v on the heater wiring until the wall voltage is 106.8vac.  At that wall voltage I had 323-0-323v at the PT high voltage winding, 366v at the 6V6 plate, 371v at the screen, and 17.95v across a 465Ω cathode resistor.  That's still 13.4w dissipation, but okay I suppose if we take the rating to be 14w rather than 12w.

However, the 5v winding was 4.8v for the above condition.  Overall, the 6.3v and high voltage windings were likely high for this transformer because there weren't 2x 6V6 and 4x 12A_7 tubes pulling current to load it down.

Total dissipation was 14.5w if the wall voltage was adjusted up to 110.3 to get 5vac on the rectifier's winding.

[Latole]

To lower B+ I use zener diodes stack with success in few vintages amps. Cheap to buy and easy to built.

https://robrobinette.com/Generic_Tube_Amp_Mods.htm#B+1_Voltage

[texwest]

I think this is going down a path I wasn't expecting!! 

I was hoping for a recommendation for a good value for a cathode resistor.

What about a value for a screen resistor considering the 9 volt overvoltage compared to the plate?

[shooter]

start at 1k 3w and move down as needed


fwiw;
easy math
Vk/Rk = I * (plate vdc-cathode vdc)  = close enough o dial in

[texwest]

To lower B+ I use zener diodes stack with success in few vintages amps. Cheap to buy and easy to built.

https://robrobinette.com/Generic_Tube_Amp_Mods.htm#B+1_Voltage

This sounds like an interesting option.   What value diode should I use to lower the voltage of this champ?  By the rob robinette site he says each diode will lower B+ 8 volts.  So five of them will lower it down to around 360 on the plate?

[texwest]

AES  has a 33 volt 5 wt zener diode.  About how much voltage drop would I get from one of those?

[sluckey]

I think this is going down a path I wasn't expecting!! 

I was hoping for a recommendation for a good value for a cathode resistor.

What about a value for a screen resistor considering the 9 volt overvoltage compared to the plate?

You've had suggestions for cathode resistors. Have you tried any of those suggestions?

[PRR]

> suggestions for cathode resistors. Have you tried ....

Agree. I wonder if that is "best", but it sure is simple and easy. Solder!!

> screen resistor considering the 9 volt overvoltage compared to the plate?

There is NO magic ratio of plate to screen in a power pentode. It does no real harm for G2 to be 9V even 50V higher than plate, as long as tube current is appropriate. And Vg2 is a very poor lever on tube current. In this case you'd have to trim it down awkwardly low, so Vg2 is saggy or PS design is more complex (costly).

If it was designed for 350V, and is running 400V, then cutting the current (cathode resistor) puts it even further from the intended ratio of voltage to current (load impedance).

Also SE amps are prone to buzz. Kill two porcupines with one big rock. Move the OT/Plate feed to the 2nd filter node. Three things happen: plate is now closer to 350V, buzz is reduced (2-stage filtering), and this resistor starts to smoke. So get a bigger more-Watts resistor.

[texwest]

Thanks for all the replies!!

I'm going to have to order some resistor values for the cathode.  Don't have them in my stock right now.  I'm really thinking that just a little change in the resistor would keep it from redplating and would still sound hot.

Interesting idea about moving the OT over to the second node.

You know I just looked up my old sheets of layouts and schematics and found one for a Bronco and low and behold the voltages on that layout are almost exactly what mine are.  It shows 420 at the first node and 410 on the second.  260 on the preamp tube plates. 

[Latole]

I think this is going down a path I wasn't expecting!! 

I was hoping for a recommendation for a good value for a cathode resistor.

 

The only answer is try different resistor values. Stat with 1.5 K
I use 5 watts ciment resistors, available every where and cheap
 

[texwest]

Thanks for the replies.  After reading PRR's post yes I realize the plate voltage will go up if I limit current.  Still very interesting that Fenders layout of the bronco has these exact voltages and still has the 470r cathode resistor.

I'm wondering about the xener diodes. Sounds promising.  I saw that one at Antique Electronics that is 33 volt 5 wt.  Would even just one of those on the centertap bring the B+ down some?  How much voltage drop would I see?

[Latole]

Here what I do on Deluxe Amp. Voltage was 438 Vdc

Sorry I dont remember how much is the voltage drop. Not 30 volts IMO, 15 volts ?
It is three 1N5347, 10 volts zeners.

Both terminals use PT screws. I can return to original amp easily.

[sluckey]

That zener operates at 33V. That's 33v peak when used in the center tap. The actual B+ drop will be a bit less. Just have to try it and see.

Larger cathode resistor is my choice. I would just buy a couple 250Ω/5W (80 cents in Doug's store). Put one in series with your 500Ω/5W. That should make you happy. If not, put both 250Ω/5W in series with your 500Ω/5W

[texwest]

I ended up putting a 500r 5 wt between the rectifier and first filter cap.  I'm down to 355v plate with 42 ma current.  I really like the result.  it's a softer feel but I like it.

[Latole]

 I'm down to 355v plate with 42 ma current.   

More sag with this resistor.

It is for 2 tubes not only one.
About 50 % power dissipation, it is good