Troublesome silverface Champ

[alange5]

A buddy asked me to take a look at his silverface Champ.  According to him, he was playing it through a mismatched cab and it "fried".  Upon inspection, the fuse was blown, the cathode bypass cap was blown (quite literally... it was in pieces), two 100 ohm artificial center tap resistors fried (previous owner must have rewired the heaters - wiring looks OK), and both voltage dropping resistors on the filter caps looked toasted.  Some power tube arcing left a nice burnt spot on the socket.


  I replaced all the parts per the schematic, and replaced the cathode resistor just in case.  I installed a new socket for the 6V6.  I put in new (old) tubes.  All other parts are stock (except for the new heater wires and "death cap" removed).  Can cap is original.  The amp fires up and plays, albeit with a decent amount of hum at higher volumes.  I measured voltages, and they seem extremely high.  Here's what I've measured:


From wall: 122v


5Y3 (NOS RCA)
Pin 8: 440v


6V6 (NOS Sylvania)
Pin 3: 413v
Pin 4: 422v
Pin 8: 26v


Bias meter hits about 51ma!  What gives?  Is the PT bad?  I tried another 5Y3 and the voltages were only lowered about 10v.

[sluckey]

Those voltages seem fine to me. I would not suspect a bad PT solely based on 'high' voltage readings. Those readings are probably due to your high wall voltage. Look at the voltages on this schematic...

     http://el34world.com/charts/Schematics/files/fender/Fender_champ_cbs.pdf

You may want to use a 5 watter for that cathode resistor. Increasing the value will bring the bias current down. Hum is normal for a champ, especially if it's being played through a good speaker. Replacing the cap can may lower the hum. Connecting the artificial center tap resistors to pin 8 of the 6V6 (rather than ground) may lower hum a bit. Adding a 20µF cap directly to pin 8 of the rectifier and a 100Ω to 250Ω 10W resistor between pin 8 and the cap can will usually improve hum considerably.

[mresistor]

What is the value of the cathode resistor you used? If it is 470 ohm be advised that, like Sluckey says, due to wall voltages being higher these days, the current can be quite high on the 6V6 in a Champ using a stock cathode resistor. I would try a 750 ohm 5 watt cathode resistor as a starter to bring down the current levels to something more reasonable for a 6V6. I try to shoot for 21 milliamps bias current knowing that some of the current is going to the screen. The amp might  actually sound better when the cathode current is more in the ballpark. Most Champ guys I know like to add a screen resistor to lower the screen voltage below the plate voltage and it also adds a measure of safety.

Adding a 20µF cap directly to pin 8 of the rectifier and a 100Ω to 250Ω 10W resistor between pin 8 and the cap can will usually improve hum considerably.

Adding a Pi filter node does a very good job of quieting the Champ. It also lowers the high voltage some. This is a very good thing to do to the amp. You can also use the spare cap in the cap-can to do this, since most cap cans have four 20uf caps inside. It's then super easy to just add the resistor. I have used 250 ohm 7 watt wire-wound to do this.


Since this amp is form the late 60's or 70's era, the filter capacitors are likely to be pretty dry inside the cap-can. Yet some still seem to work OK. It is a good idea to replace these old electrolyitcs and it only enhances the amplifier. As long as the amp isn't a museum piece.   

[mresistor]

It's possible one of the filter caps in the cap-can could be partially shorting.  Here is a pic of one of my Champs. It is what you can do if you don't want to spend money on a cap-can.

[alange5]

interesting... I've been looking at the AA764 schematic to reference voltage.  I didn't realize the CBS/silverface ran them so much higher.

I did use the stock 470 ohm value for the cathode resistor.  I'll raise the value and see what happens.


What's the advantage of using the cap/resistor on the rectifier over using zener diodes on the center tap to tame the voltage?  Or is the idea to simply beef up filtering, with the voltage drop being a byproduct?

[HotBluePlates]

What's the advantage of using the cap/resistor on the rectifier over using zener diodes on the center tap to tame the voltage?  Or is the idea to simply beef up filtering, with the voltage drop being a byproduct?

Yes, the second thing you said, because you also said:

...  The amp fires up and plays, albeit with a decent amount of hum at higher volumes.  ...

To which Sluckey said:

... Hum is normal for a champ, especially if it's being played through a good speaker. ... Adding a 20µF cap directly to pin 8 of the rectifier and a 100Ω to 250Ω 10W resistor between pin 8 and the cap can will usually improve hum considerably.

A lot of folks around here have built Champs as an early DIY project. Most used speakers considerably better than the original 8" speaker, which much more bass response. As a result, most have fun into more hum than they expected because a SE circuit is sensitive to the ripple at the 1st filter cap in a way push-pull amps aren't.

The answer most folks have found is adding an extra stage of filtering (C to ground, R in series) to lower the hum. And if too-high supply voltage is an issue, it provides a handy place to dial that down.

It is interesting to me that the Valve Junior eventually was changed to add such a pre-filter stage. I wonder if they read about it on an amp forum like this...

[PRR]

> two 100 ohm artificial center tap resistors fried

Probably plate-cathode short plus incidental damage. Do NOT use that 6V6 again!

400V is not too absurd. 51mA is IMHO high. 387V*46mA= 18 Watts Pdiss is way past 6V6 original ratings, and right-at what Fender routinely runs modern 6V6 at (with no or minimal warranty!).

I think in 1967 CBS was pushing the little beast for MAX sound under short warranty. As wall-voltages have risen (and new-made tubes may vary a bit more toward the "more" side), these tubes have hard lives, maybe short lives. And the mega-heat inside may cause hard shorts, which burn-up more than just the tube. (A shorted 7027 started a fire in "my" Ampeg VT40's PT, what a bummer.)

*My* suggestion is to add an R-C filter, 470r+47uFd, between the raw B+ and the OT. These SE amps always want a bit more filtering. The 470r will waste-off another 6% of voltage (and loudness) and 11% of heat. (Might even consider 1K dropper.)

However that is a rad mod and maybe a tight fit.

You could plop in a 6L6. 19W is dead-on for original 6L6 and very tame for modern 6L6GC family. The standard Champ used the DeLuxe PT, two 6V6, so it has the heater current.

You could rig a 120V:9V 1A transformer as 122V:114V, the amp thinks it is back in days of 115V wall-power. A 12V step-down is not excessive, unless he also plays in dives with 108V walls (I used to work there). 12V CT with a switch can select 5% and 10% drop, but this implies the user has a clue how to set it. (199VAC LED/LCD voltmeters can be bought cheap.)

[mresistor]

PRR   I think the black/silverface Champ and Vibro Champ uses the same PT as the Princeton. 

[alange5]

Thank you for all the suggestions.


I replaced the can cap with individual filter caps (all 22uf, 450v), and added another 22uf 450v cap from 5Y3 pin 8 to ground.  10w 270 ohm resistor added between 5Y3 pin 8 and first filter cap.  Swapped the cathode resistor for a 560 ohm 5w.  New voltages:


5Y3 pin 8 - 410v


6V6 pin 3 - 385v
       pin 8 - still 26V


Bias meter reads about 45ma.  If my math is correct...17 watts plate dissipation? Isn't that still high?  Does the cathode resistor need to come up a bit?


Sound-wise, it's definitely "tamed down".  It's a bit less noisy, way less aggressive, and there's been a noticeable volume drop all around.  I think it sounds nice, and I'm sure my friend will like it.  I just want to make sure it will hold up over time.


Thank you all again for being so helpful.

[sluckey]

You may want to use a 5 watter for that cathode resistor. Increasing the value will bring the bias current down.

Don't be afraid to take it on up to 750Ω or even 1000Ω if necessary.

[alange5]

Don't be afraid to take it on up to 750Ω or even 1000Ω if necessary.

Will do.  Thanks a bunch!

[alerich]

You may want to use a 5 watter for that cathode resistor. Increasing the value will bring the bias current down.

Don't be afraid to take it on up to 750Ω or even 1000Ω if necessary.

I wanted to be able to swap out 6V6 or 6L6 in my SF Vibro Champ. The PT can easily handle the extra heater current. I ended up using a 1K 5W cathode resistor that works well with both tube types. As I was working I A/B'ed the 1K and a 470R with the 6V6 and really could not hear a difference in tone in that little amp with an 8" speaker. I can't really hear all that much difference between a 6V6 and a 6L6 in that amp, either. Even with a 1K the bias is on the warm side, as I recall. At 470R I think it's mostly wasted in heat, losses in that wimpy little OT or tones that the little speaker just isn't up to reproducing. 

[PRR]

> Does the cathode resistor need to come up a bit?

I say the load impedance and cathode bias resistor, with the same tube, in a reasonably well designed amplifier, should stay in a similar proportion. Roughly Rk ~~ Rl/Mu(g2) times a fudge factor in the 0.6 to 0.7 range.

Upping the cathode resistor starves the tube and hurts power more than just dropping the supply voltage would.

With that proportion reasonable, you drop the supply voltage until the tube isn't too-too hot.

I do suspect that many modern 6V6 are sucking more than classic grandpa-radio 6V6 did, and 750r may be the new 500r in this case.