Custom Vibro Champ Build Complete!

[Potato Cakes]

Hello, Everyone,

I wanted to share my almost completed custom Vibro Champ build. It's a 10W version that has a tone stack defeat control, negative feedback bypass, half power (with negative feedback bypass), effects loop, and 4, 8, and 16 ohm output transformer selection. The reason it is almost complete is that the vibrato section is not functioning, which should be an easy fix.

When I built the board, I had it oriented as though it was going into a standard Fender type pan, but forgot that it was going into a chassis where the components are facing up. Instead of rebuilding it, I just installed it upside down, which made wiring a bit more interesting.

The amp itself works and sounds fantastic. Going into a 4 ohm load (2x 10" speakers) allows for an increase in SPL that makes turning up this amp a lot of fun. Taking out the negative feedback with the volume past 5 creates a very smooth overdrive effect.

I do still get a bit of heater noise as I have with all of my other builds. Granted, the instant you play anything through it you cannot hear any hum and it is not detectable on a recorded track where the guitar is present. But I still try to get my amps as quiet as the audio gear I build which may be wishful thinking when constructing tube guitar amps. I ran my filament power lines up and over all the other wires to make sure the audio path was not affected. I did an elevated ground with the center tap off of the transformer's 6.3V winding but now that I think about it this method may only work well for push pull amps and not single ended ones. I will mess around with that when I open it back up to fix the vibrato.

The effects send is right before the power tube so as to be able to have the vibrato function from the amp to proceed any effect pedals. I do not currently have an attenuator/make up gain circuit in the amp so this will have to be done externally for now. I also need to do some voltage measurements off of the preamp tube versus the input level to the output tube to see how much gain adjustment I will need at the send and return connections.

I had to drill out the pan chassis as it was blank. I don't particularly enjoy doing metal work but this one wasn't too bad. I would much prefer to find somewhere that can make custom pans or at least have the flats drilled to spec and then bend them with a brake.

I am using a Neutrik True1 power connectors with the output option. Some of the old Fender models have the two prong Edison power plug which I see replaced with a three prong receptacle, but those particular ones that are a direct replacement are a bit too expensive for what they are. The True1 connector allows for power to be locked in place and it has a through that can go to my pedal board which also uses the same connector.

This build has a Soursound power transformer and a custom 15W transformer Bryan wound for me for this particular amp. He said he had always thought about making a single ended 10-15W transformer and this build gave him the excuse to do so. As always his work is exceptional and he is a great guy with whom to do business.

There are still a number of tweaks I would like to do but I was just happy that it powered up and passed audio on the first attempt.

Thanks!

Paul

 

[rock soderstrom]

Nice amp, I bet this thing sound killer!

When I built the board, I had it oriented as though it was going into a standard Fender type pan, but forgot that it was going into a chassis where the components are facing up. Instead of rebuilding it, I just installed it upside down, which made wiring a bit more interesting.

Ha ha, I'm trying to imagine the service technician's face in 50 years' time when he sees the upside-down turret board during an upcoming repair the first time...

 

[Potato Cakes]

It does indeed sound killer. I am using 1uF cathode bypass caps for the preamp 12AX7 tube and it might need to go to a 2u2 or around that to get a little more of the bass response back. It is very clear and articulate but my impression after playing through it for an hour is that it could use a little more low end when turning up the bass control. I could be wrong. I'm now in the part of the process where information used to determine changes is nearly 100% subjective.

Ha ha, I'm trying to imagine the service technician's face in 50 years' time when he sees the upside-down turret board during an upcoming repair the first time...

I do hope that technician is someone else!

Thanks!

Paul

 

[Potato Cakes]

Seems that I wired the foot jack backwards. I am using switched 1/4" and not the historic RCA style connectors and I wired the tip to short to the sleeve with no pedal connected but failed to remember that RCA jacks are not switched so with no pedal those connections are not shorted. Now the vibrato works and sounds very nice. I do get a low level thumping when not playing and the intensity is in the middle range of the potentiometer. This noise is again not audible when playing guitar. I recall seeing on one Vibrochamp style schematic that a 22pF cap was connected somewhere to prevent ticking noise, but I cannot seem to find that particular drawing anywhere. This is a very minor thing but while I still have it out of its cabinet I would like to try to fix this.

I did change the heater ground to the dual 100 ohm resistors to the chassis and disconnected the center tap transformer connection and no change in the level of hum. I believe what I am hearing is what is considered "quiet" in tube guitar amps with AC heaters. I'm going to see about getting some really nice boards made for this circuit and try DC heaters on both 12AX7s and see if that changes anything. But this is perfectly fine for playing and recording as is.

The last thing I have observed is when the vibrato pedal connection is shorted (bypass), is that there is a loss of volume with the vibrato controls turned up past 0. I build a standard Vibrochamp last year and I did not notice this happening, but I also did not test for this as I did not know at the time that there was potential for this issue. Again, not really a major issue as this is meant for studio purposes, but if I could get no loss between vibrato in/out that would be ideal.

Thanks!

Paul

 

[Potato Cakes]

I added a 1N4007 across the intensity as I found that this sometimes helps. It changes the characteristic of the vibrato itself which I like but the thumping is still there when not playing and the intensity is in the middle range of the potentiometer. Again, this is only audible when absolutely no guitar signal is present. I'll look around a little bit more to see if I can find what I think I am after but will not spend all day trying to fix this minor flaw. This amp is undoubtably the best sounding amp I have every built and possibly any that I have heard in recent memory.

Thanks!

Paul

 

[Potato Cakes]

I moved the vibrato cathode connection to the cathode of V1A of the preamp 12AX7 so that it proceeds the tone stack instead of the V1B which is section before the output tubes. The effect can now to go to full on/off, which I think is very cool. The value of the intensity control pot does have to be changed to 5k as with the 25kRA pot, the full cut off happens when the dial is at 4. This means I can now have a passive effects loop without the need for an additional gain stage and have the vibrato effect before my pedal board.

The thumping is still there. I have not yet changed the tubes as I don't have another 12AX7 that would work well. At some point I will get some more to do a swap and test. Still not a real issue when playing.

Thanks!

Paul

 

[AnalogPackrat]

Seems that I wired the foot jack backwards. I am using switched 1/4" and not the historic RCA style connectors and I wired the tip to short to the sleeve with no pedal connected but failed to remember that RCA jacks are not switched so with no pedal those connections are not shorted. Now the vibrato works and sounds very nice. I do get a low level thumping when not playing and the intensity is in the middle range of the potentiometer. This noise is again not audible when playing guitar. I recall seeing on one Vibrochamp style schematic that a 22pF cap was connected somewhere to prevent ticking noise, but I cannot seem to find that particular drawing anywhere.

I think that fix is for the amps using the photocell. Lead dress can help sometimes.

As for the heater hum, it may actually be residual AC on your B+. On one of my two 1977 original VCs I added an extra RC filter stage to the B+ which both lowered the voltage slightly (good since 120-125V service was not common when these amps were designed) and reduced 120Hz hum/buzz. Check your B+ for AC.

The last thing I have observed is when the vibrato pedal connection is shorted (bypass), is that there is a loss of volume with the vibrato controls turned up past 0. I build a standard Vibrochamp last year and I did not notice this happening, but I also did not test for this as I did not know at the time that there was potential for this issue. Again, not really a major issue as this is meant for studio purposes, but if I could get no loss between vibrato in/out that would be ideal.

This behavior is normal, unfortunately. Both of mine do it. The intensity pot is still a voltage divider in the circuit when the oscillator stops so it changes the bias on V1B cathode. Check with a volt meter and you'll be surprised. The amp sounds bad with the intensity past halfway and vibrato off.

Nice build. They are great little amps. One of mine is driving a big 2x12 cab (8 ohm load). The small output transformer really limits the low end. With your oversized OT I would put the stock cathode bypass caps in or wire a push-pull fat switch on the first stage to switch a bigger value in. Enjoy.

Thanks!

Paul

 

[Potato Cakes]

I did notice that a different Fender type build that I did I used diodes for rectifier and I think it was a bit quieter. I have seen it reported that using slightly larger filter caps and reduce hum, but the Internet has lied to me before. I'm not worried about the hum. It is impossible to hear when actually playing guitar through the amp.

I did read a thing on Effectrode's website that talked about the thumping issue. Routing of the wires and grounding was mentioned. I haven't dug too deep into it yet. But the other Vibro Champ I built was pretty much to spec and I don't remember hearing any thumping noise. Again, it's not audible when playing guitar.

I did find one last thing that needed to be tweaked when moving the vibrato connection to the V1A cathode. With the intensity turned all the way down, the signal is significantly high passed and overall output is lowered. If I add a 300 ohm resistor from CCW to ground, the full signal response returns and the impedance to ground is low enough that the vibrato effect is not audible. The quiet thumping noise is present but again not noticeable when playing. I suspect that even though the potentiometers I am using are new, the one for the intensity might be bad as the first 10% of rotation produces a scratching sound. If I short the foot switch jack the thumping goes away, so I think as a temporary fix I'll have a shorted 1/4" jack at the ready just in case. When I get around to buying new tubes for testing I'll change out the potentiometer as well.

For now it's buttoned up and ready to use. Always feels good when a pile of parts made into a useable piece of gear and is moved off the bench.

Thanks!

Paul

 

[Potato Cakes]

I started to think about the motor boating sound I was getting and I was lead in the direction of putting an electrolytic across the vibrato voltage return to the V1 cathode and ground. I started with 2u2 and it helped slightly, so then I went to a 22uF and the choppy motor boating was gone as well as the scratchy pot sound through the first part of the turn from 0 to 2. Then I realized that all did was create a parallel cap to the 1uF cathode bypass for V1 which approximately equaled the value used in the original schematic for the 5W Vibro Champ. It now works perfectly and sounds really great. I found myself playing through it at a louder volume than I normally would for a longer period of time than I should have. I think it's a good sign when testing turns into fun without even realizing the one process has morphed into the other.

Thanks!

Paul