I'm confused about one thing on my SF Champ build.Hope some can shead some light.

[catnine]

I posted years ago about it. I used Weber transformers he sells for Champs and since I have only 8 ohm speakers I got the Weber OT with a 4 and 8 ohm taps.

I do know how to check the bias . Yet what I'm confused about is the screen voltage. If I check the Plate on the 6v6 to ground and the screen the same way the screen at idle is 9 volts above the plate . The OT has 250 DC ohms across the primary yet the screen has 1K ohms off the 1st 1K dropper . It seems the screen would be lower . If I look at the fender schematics of the Champ and vibro champ and bronco they all use the same PT and OT yet they all differ so much .

In order to read the correct screen compared to plate should I check them both to ground or to cathode or does it matter.

I never saw a Champ SF or Black with any screen resistor yet their schematics show the screen either the same as the plate or 2 volts lower and if you look at the B+ where the plates get the voltage the Bronco drops quite a lot with the same tranny's. So I can't say what the fender schematics relate to .

My main question is does it matter if the screen is 9 volts above the plate? I had real SF champs yet never checked what the screen was at I just checked the bias and kept them under the 14 watt 6v6 so the amps didn't sound so grainy and to keep from killing tubes.

[mozz]

That's fine. The plate is drawing a lot of current at idle and the screen draws a lot less, that's why the screen is reading higher. The 1k is there to protect the screen from melting, a lot of older amps never used them, it's a better safe than sorry thing.

[Justin Thomas]

+\-Re: using the same transformers, the internal Fender part number may have stayed the same, but the specifications for that part number when ordered may have changed between runs. Also, why was the spec'ed wall voltaheon the BF, vs. SF? That might have something to do with it, too...

Justin

Edit: just checked both AA764 & AB764 VibroChamps. AA says 315-0-315 AC; AB says 365-0-365 AC. I bet that explains the B+ variance, combined with "Voltages shown +\-20%"...

[Enzo]

I think his concern was not the voltage per se, but the fact the B+ at the screen was higher than the B+ at the plates.

[Justin Thomas]

No argument there. I was addressing the comment that "they all use the same PT & OT and yet they all differ so much,"cuz that "same" tyranny was indeed changed.

Re:screens higher than plates, I don't worry about that... I'm more worried about what tubes go in my 1980 VC. It ate 2 EHs within a few hours, so I keep UOS in it.

Justin

[catnine]

Yes I am mainly concerned about the screen being higher than the plate. Lets forget about the fender schematics because who knows what is what on the voltages they have .
When my line is 119 and most times it's not steady it can jump from 117.5 to 120 in a second . I caught it at 118.5 to 119 and got B+ of 383 , plate to ground 365 , screen to ground 374 and a 3.25 drop across the 1K . 337 plate pin 3 to cathode pin 8 . 678 measured cathode resistor , 25.5 across the 678 cathode puts me @ around 12.66 watts.

The 12ax7 Pin 1 237, pin 3 1.69 pin 6 236, pin 8 1.82. A bit high yet not bad . It does sound fine and I had a 470 ohm screen across the 6V6 socket yet it did not drop voltage at idle and I never tried to drive the amp to see the current drop. I read they kill the feel on these amps and no champ I ever had used them and I had three all had to sell and no screen issues. On the last one a 73 SF Champ I used a JJ 6V6s and have one in this build. The Weber PT has a 120 and 125 line , I used the 125 because even at 119 line the heaters are @6.3 volt and that was my reason.

I used the 680 ohm 5 watt cathode because the next value is 500 from AES and that might bring me well over 14 watts and I DON'T like ratty breakup. My 73 was @ near 19 watts so I know how that sounds. This built sounds fine . the Jensen C10r is a bit bright so I paralleled a 25 uf cap with the stock 2uf cap on the 2nd gain 12ax7 and this chassis had a slide switch so I used it to go back to stock if needed and it did increase the bottom notable.

All that said my main concern is the screen higher. and no SF Champ I ever had burnt the screen . I don't recall why I put the 470 on the screen in the first place. I removed it.

See I do like SF champs and built this one to replace the three others I had . I never did anything to the first 2 I had until 1994 one was a 78 and the other was an early SF that still had the 2 wire stock cord . I have no idea if anything was ever done on the 78 yet it was pretty new when I got it in 84 . It blew fuses so I asked at a local music shop here and was told I need a new 5Y3 and a fuse and that fixed it. The other one had a cap added to one of the three on the can and it buzzed some. They both had the stock speakers and sounded great to me. It was until 2005 when I got the 73 by that time I knew about bias . It sounded great once I tamed the bias down and put a weber sig 8s alnico in it the stock fender speaker didn't hold up to the bass notes . The others I got for less than $100 and was happy to have an amp again after decades of not. So they sounded great to me and each one is different.

[Enzo]

Don't worry about it. yes, on paper, ideally, we should have lower voltage on the screen. The Champs and other similar circuits have worked JUST FINE this way for what 60 years now? Look at the circuit. The B+ through the OT primary faces the winding resistance at the relatively high current of the class A power tube. The screen has a very low current through a resistor. Do the math, you have current and resistance for both OT and screen resistor. Your 35-40ma across the OT will drop more volts than your 3ma through the screen node resistor.

[Chuck H]

I say DO worry about that screen voltage. Not because it's a big deal operationally, it's not likely anything will blow up or fail, but there is a tonal benefit to getting the screens down IMHE. A lot of great recordings have been made in "olden days" with "Champ" amplifiers because they're small and easily managed at full tilt. They sounded like they sounded and great players can find the musical qualities in just about anything. But from a designer/evaluation perspective the tone of most stock Champs (BF, SF and even tweed era) can be a bit harsh and spikey. Adjusting the circuit so the screen doesn't over dissipate and the plate manages the clipping smooths out the tone a lot. Increasing the HV rail resistor between the plate and screen can decrease the bad qualities and bring the best stuff forward. When clipping, the signal "looks" a lot like DC to the tube. But the "impedance" of the OT presents a much higher resistance than the screen circuit. In this case the easiest path to ground for the current is through the screen circuit. Since Champs are often run heavily clipped (nowadays) it only makes sense to me to redesign the circuit to accommodate that. On my last build I used a 3.3k between the plate and screen and a 1k screen grid "stopper" to good effect. I've read of some guys using as much as 5k between the plate and screen. In a perfect world "sag" shouldn't be an issue because the class A circuit should draw as much or MORE current just sitting there as it does conducting signal.

Your amp will work fine with the stock 1K value. And it will sound just as a stock amp would. But you might at least try a 2.2k, 3.3k and/or even a 5k resistor between the plate and screen and see what you hear. In any case DO use a 1k screen grid "stopper" resistor.

If you DO increase the plate to screen resistor and you want to get picky you can DECREASE the value of the HV rail resistor between the screen and the preamp tube/s to get back to stock voltages in the preamp.

[Enzo]

How's this then:
The higher screen voltage is not a problem requiring a fix. CHuck is recommending a modification to the circuit for tonal reasons. I am not disagreeing with that. My comment only involves whether there is a "problem" or not.

[frus]

http://www.triodeel.com/6v6_p4.gif

On this graph, the screen voltage is fixed at 250V.
You can see that the screen current (the lowest dashed line) is virtually the same until the plate voltage falls under 100 V. Then the screen starts dissipating more than it's rated power (2.2W / 250V = 9mA)
So if it's 150 V lower than the screen voltage, then you should start worrying about it.
Of course the voltages are overall higher in your circuit, so the margin is lower, but screen 10 volts lower than the plate is definitely safe

[Chuck H]

Like Enzo said. There's nothing to "fix". And a lot of stock Champ's are enjoyed by a lot of people. Obviously you picked a model to build, so you must already want "that" tone. I digress.

[catnine]

I guess I'll just leave it as is. I'm not really into experimenting or improving a very old fender design. I don't play the Champ cranked any way and have absolutely no idea what the tone result would be upping the 1K that much . I imagine the 1K screen grid stopper only allows the screen to draw less current yet I've read this for some reason also changes the feel of the amp . Some say it makes it less responsive.

[catnine]

I wanted to thank everyone who offered their help. I wish I had checked the actual voltages at every point in the 73 Champ I had . I was only really focused and concerned about the 6V6 bias . All I have is the B+ and plate and heaters and the diff between the stock resistor and the one to get the bias down and the resulting voltages. I never thought I'd need to sell it. I had it in a tall cab I built just like the music master bass amp only tall enough to allow 2 twelves one above the other and could slide the Champs chassis right in and have a 4 ohm load and really liked it. Then I took a music master bass amp chassis and cab I got that the PT was dead and turned it into a AA964 Princeton minus the trem and used the tone stack from a 6G2 Princeton and since it began as a 5E3 with two inputs and 1 vol , 1 tone V1 was only the preamp so I used a 12 ay7 since an ax7 was to much gain added v2 socket for the 2nd gain and PI and added a tube rect socket and used Allen Amps TP25 PT and his TO25 OT and added the stock but switchable feedback resistance and really like the sound and tone .

Point is as I went on . I put the 73 Champ in the MM pine cab , made a 3/8" thick plywood adapter ring to mount the Weber Sig 8S Alnico and surprise the small 8" opened up just by the larger cab . I did get a weber SIG10S alnico 4 ohm and it really only added a bit of bottom and a slight turn of the bass pot and sounded just like the 8" . Yet if I would have checked every voltage in that 73 I could have come closer to it's tone . This one is missing something , it all seems to point to the speaker. If not then I might try the higher screen and lower preamp HV rail . I do know many still play these SF Champs with the stock 470 ohm cathode resistor so the voltages with more current draw pull down all across the amp yet that heat on the 6V6 is much higher so either your drive the tube to clip sooner and hotter for than harsh OD sound some really dig . I can't say which would affect the screen in a more negative fashion . If I recall since I'm much older now I put the 470 screen resistor on the socket because it seemed like a good idea on my AA964 build . Completely different amps.

I can't say I heard any difference with the 470 in the champ and since the champs 6V6 is at almost full on at idle and had no measurable voltage drop on the screen at idle would that mean with a guitar plugged in the 1K already there as stock will keep the screens current in check to a degree? Isn't that the reason it's there in the first place?