Vibro Champ Clone wants some breakup

[ampgaragemc]

I finished my first aa764 build and it sounds pretty amazing. I only wish it would break up a little. I installed an NFB switch but it only seems to boost highs and still no breakup. Then I did the mod where you reverse the 10k and the 1k on the power rail. I like the sound of that mod a little better but it's still clean. I've also tried different tubes in v1. It has a WGS 10" Veteran. I copied voltages from the aa764 and ab764 schematics and compared with mine in the attached pdf. I'm hoping someone has some ideas for what I'm missing.

Fender_VibroChamp_layout.pdf

[xtian]

I think the only way to get more gain is to bypass the tone stack.

[maxkracht]

I installed an NFB switch but it only seems to boost highs and still no breakup.

You can bypass your NFB resistor with a cap to let more highs through, cutting highs, but you won't get any more output than no feedback at all. Vibro champ is pretty low gain, so the preamp isn't going to distort much unless you have a really hot pickup or a pedal in front of it. The power tube should distort if you play it loud enough. You could experiment with biasing the preamp a bit hotter, but it's not going to get you that much more gain.

You could also get rid of the Vibro part if you really want a higher gain preamp, but boost pedal is probably the easiest solution. You can also get some weird distortion by turning the vibrato off with a foot pedal and fiddling with the intensity knob which shifts the preamp bias.

[lonote]

Depending how much you are willing to depart from the regulation Champ schematic, you could add a 1MA as a MV in place of the 220K grid leak resistor feeding the power tube. This allows driving the second 12AX7 stage harder, but still be able to turn down your overall volume.

The idea of a MV on a Champ sounds sort of silly but volumes are generally way lower these days.

I built a harp amp for a guy I play with, which was basically a Champ with a different tone stack, & we tried that MV idea, which worked well enough to keep it.

You work how hard you drive the second preamp stage against how hard you drive the power tube, to give it more or less "hair" at an acceptable volume.

You could probably temp it in to see if it gets you where you want to go.

[Stevem]

You only have two gain stages in this amp that can be clipped, ( the ax7 driver and the output tube ) the first gain stage never clips unless you have very high output pickups or an active preamp in your guitar with excess output.



You do not want a master volume as then the great feel , sound and dynamics of clipping the output stage will be lost and that’s not what these single ended class A amps are about.
These amps are also about saturating with signal the small OT to get the frequency roll off that takes place when driven hard to compress the tone.
You loose this with a master volume!

You need to overdrive the first gain stage also with a decent overdrive stomp box ( tube type preferred ) in front of the amp.

If you have another amp with reverb take the output from the send jack and pump it into the Champ and audition that with overdriving the first gain stage.

Just note that the reverb send output is many times trimmed of some low end for use in the reverb circuit .

[ampgaragemc]

Thanks for the info. I admit I couldn't figure out what MV stood for until it was explained. I hear other champs that have easy break up and a lot of times I read where they would like less of that and more headroom. I've got a ton of headroom and wouldn't mind a bit less. Also, I have and use a real tube pedal in front and that's a lot of what I was looking for, but I was hoping to not have to use it. I might test out the tone circuit bypass.

You mention saturating the OT. I'm using the 8k primary tap of the OT now and I'm curious if changing to the 5k tap would saturate more? Although I guess the 6v6 is better off using the 8k.

[Stevem]

What rectifier tube does this have in it a 5AR4 or the correct 5y3?

What are your plate voltages on the first preamp tube
Pin 1 ?

Pin 6 ?

[lonote]

You only have two gain stages in this amp that can be clipped, ( the ax7 driver and the output tube ) the first gain stage never clips unless you have very high output pickups or an active preamp in your guitar with excess output.

You do not want a master volume as then the great feel , sound and dynamics of clipping the output stage will be lost and that’s not what these single ended class A amps are about.
These amps are also about saturating with signal the small OT to get the frequency roll off that takes place when driven hard to compress the tone.

You loose this with a master volume!

While I more or less agree that driving the power section in a SE amp is much of what they are about, I don't find it that black & white, & it really depends on what YOU are going for.

I have heard some nice tones (with a guitar) out of the harp amp I mentioned, with the preamp being more driven than the power tube.


One question, does the amp break-up at all or even close to what you are looking for, when the volume is max-ed out?

[ampgaragemc]

at wide open volume and pushing the strings hard I get to the edge of breakup. But it really sounds great at any volume, and I love it. The tremolo is amazing. The sustain is great. I was just looking for that point where you can dig in without having to be wide open then ease up when you want to clean it up some. And I think the 10" speaker doesn't break up as fast as an 8" would either.

It has a NOS 6087 rectifier
V1-Pin1 205v
V1-Pin6 257v

[lonote]

Another thing you could try is to attenuate the speaker with a simple & inexpensive L-Pad.

I have made a couple of them for friends that are mounted right in the cabinet & they are able to dial back the volume right at the speaker. I have added a 4uF non-polarized cap across the L-pad on a few to help keep things sounding balanced as you roll it down.

Having one mounted in an external box is a handy amp tool to have around. Parts Express has 100W 8 & 4 Ohm units for around $18.

ALL of these methods will alter the sound in one way or another, but most things are a trade-off and you can often find a suitable & usable combination of settings.

L-Pad.JPEG

[Stevem]

Your voltage on pin 6 is very high!

What's the cathode voltage you have on the cathode for that section of V1B?

Go look at a aa764 schematic.

If you have low cathode voltage then that section of the tube is not conducting well.

This means that section of the tube is shot, it also means lower then needed signal voltage that might otherwise be clipping the driver and then that tube in turn clipping the output tube.

What do you have for voltage on the red wire powering the OT?

[ampgaragemc]

I took all voltage measurements again-
B+ - 392v
B+1 - 374v
B+2 - 306v
B+3 - 305v

V1 Pin-1 - 202v
V1 Pin-3 - 1.5v
V1 Pin-6 - 269v
V1 Pin-8 - 2.7v

6v6 Pin-3 - 365v
6v6 Pin-4 - 306v
6v6 Pin-8 - 16v

Makes no sense to me that pin 6 is so high since it's connected at the same supply as pin 1, then both have 100k resistors that measure good.

I've tried 3 different tubes in V1 and they all act about the same, so I think the tube is good.

OT is connected to B+1 so it's 374v.

[Stevem]

I am at a loss right now as to why you have that 269 volts.

Hopefully someone else will participate with a solution.

[martin manning]

Check the value of the cathode resistor on V1 pin 8.

[Craig B]

I like that your design has an extra node of power supply filtering compared to the original fender design. That really reduces hum and reduces the (my opinion) excess B+ in the ab764 version of the Vibrochamp.

If you want power tube distortion you need even lower B+.

To reduce the excess voltage more, replace the 470 ohm/5 w resistor with a larger one. 1k ohm should get you in the ballpark.