BF Fender Vibrochamp questions

[Justin Thomas]

I've kept a 750-820R 10W resistor in my Champ for 10 years. First, the B+ is ~430, and I couldn't keep any 6V6 within safe limits with anything lower. My last build, I have a shared 750, so each has the equivalent 1.5k. No artifacts from crossover. I'duse at least a 5W there, and keep the cap as far away as possible.
PS: what'sthe old resistor actually measure at?

Justin

[Chuck H]

I'm tellin' ya..! Increase screen resistance THROUGHOUT! 2.2k for the rail dropper between the plates and screens and a 1k screen grid resistor. Adding more resistance to the screen circuit will cut power a smidge, but it helps the tube to operate within somewhat normal parameters under overdrive conditions. If you don't like th voltage drop on the preamp you can decrease the value of the HV rail resistor between the screens and the preamp tube.

The 8k OT and the low resistance screen grid circuit sort of force the screen and plate to share duty when clipping. The watts need to find a way out via the path of least resistance. With the HV rail at 1k and NO screen grid resistor even the tubes internal resistance isn't enough when the OT is at 8k (and higher at the extreme HF). Even with the 2.2k rail resistor and a 1k screen grid stopper I saw HF spiking on the scope. I added a straight 1000pf/2kV cap across the transformer leads. Problem solved.

[pdf64]

...My last build, I have a shared 750, so each has the equivalent 1.5k. No artifacts from crossover...

Note that the artifacts of crossover distortion in over-biased cathode biased amps are most obvious at high signal levels; rather than at low signal levels, which we are perhaps more used to with fixed bias amps.
I find that it gives a thin, reedy quality to the overdriven tone (compared to bias conditions with less squish).

To get cathode biased amps to a reasonable idle plate dissipation, I prefer to add resistance into the HT side of things, rather than the cathode, eg rather than increase a Champ cathode resistor from 470 to 1k, leave it at 470 and add 470 ohms between the rectifier and reservoir cap.

[Justin Thomas]

Well, maybe there is crossover.. I don't have a scope, and I like the sound of it, so... I might should try that!

Justin

[nosaj]

Got some sound out of the amp now, good!



To be expected. Champ turns out only a couple of watts, and clips asymmetrically. The basis of many a hit song's guitar sound in spite of what it may sound like all by itself.



Ye olde Fender standards why not. 1K5 grid limiter and 470R to the screen grid. As on the larger amps you could use pin 6 as an attach point for your screen grid resistor, pin 1 same for the grid limit resistor. Likely you won't hear much of a difference, just add to output tube's stability & lifetime.

Ok I looked at the Fender super amp
and i see 1.5k between pins 5 and 6
470 grid limiter resistor goes to Pin 1 and where?
I've replaced the Cathode resistor 470 r 10 watt and 22uf/50v cap and the 1.5k between pins 5 and 6.

Thanks,
nosaj

[nosaj]

To install a screen grid resistor From what I see online is on a 6v6 on pin 4 I'd disconnect the B+ and put a 470r inline?

Is that correct?

Thanks,
nosaj

[g1]

Have a look at the Deluxe AB763 layout.
Grid stopper between pins 1&5, screen resistor between pins 4&6

http://www.thevintagesound.com/ffg/s...763_layout.gif

[nosaj]

Ok here's my delima Vibrochamp 6v6 pin6 is not connected to anything nor is there a connection in the tube so anything connected to pin6 is free floating?
I think i may have it now. Connect as show on the layout and move the wire on pin5 to pin1 and the wire on pin4 move to pin6.
Thanks,
Nosaj

[The Dude]

Yes. Those are unused pins. You are basically using the pins for terminal strips.

[nosaj]

Yes. Those are unused pins. You are basically using the pins for terminal strips.

Good I'm glad that's cleared up had some mental spaghetti to work through.

Thanks so much.
Btw have you ever met Jeff Bridges?

nosaj

[nosaj]

Cathode current in excess of 65ma would do it. 3 hours of thrash playing? If not, maybe an ultrasonic oscillation? Screen and grid stoppers should help that. Otherwise, use a 5W cathode resistor. If the resistor toasted, then the cap would fail immediately on overvoltage.

edit: What's your B+ voltage?

305 VDC

nosaj

[The Dude]

Nope. Never met him. But I'm told he hates White Russians and everywhere he goes, people buy him one. So, if I ever do meet him,......I won't be buying him a White Russian.

[Chuck H]

There is another problem to be solved for here. I've never seen a VC with plate voltage below 400!

[nosaj]

There is another problem to be solved for here. I've never seen a VC with plate voltage below 400!

The problem may well be me. What pin on the 6v6 should I check?

Thanks,
nosaj

[The Dude]

Agreed. I would pull the 6V6 and re-measure B+. That will tell us if we're starting off low or if the output circuit is drawing excessive current.