Vibro Champ build advice for a first timer?

[thetragichero]

1. i am unfamiliar how everything mounts in this particular combo, but the chassis has another metal part, no? like a shielding cover? that should be sufficient
2. i am not a scope expert (i plugged mine into a fuzzbox and turned knobs until stuff showed up on the screen. real scientific but for tracing and looking at the signal works well enough for me) but have seen similar things scoping some of my own builds. the peaks get further apart as you turn up the volume which is a great sign. how does it sound?
3. looks fine to me. i have seen some pretty horrifying-looking mods done by fairly well-known companies that sounded killer so... how does it sound? (to me that's what matters at the end of the day... if it sounds good and isn't going to electrocute you or set the house on fire, it is good)

[Johnny_March]

There is no additional piece to cover the chassis. It goes into the cabinet as shown. There is some metal shielding inside the cabinet but that's it

It sounds pretty good as far as I can tell. I guess I was just surprised to see how the amp reacted to a single focused tone from my signal generator as opposed to a guitar signal that's full of harmonic content.

At this point, I think I can call this build successful. I guess the most important thing is that the filter cap section is bolted down securely. One side is bolted to one of the existing circuit board risers. The other end wasn't long enough to get to the next riser, but I got a very stiff piece of cardboard to make a small extension that bolted into the next riser. The eyelet board is firmly in place and shouldn't give any issues.

I used the original spec value of 470 ohms to the 6v6 cathode which gave me a current of about 39mA. The plate voltage was about 358V and I ended up with a plate dissipation of almost 14W which was high. All I had was a 1k on hand to substitute in to bring the dissipation down and it took it to 8.6W. I'll shop around for another resistor that's between 600 and 800 ohms. I'm hoping to get the dissipation to about 12W.

Now that this amp put a little bit of experience under my belt, I think I can conquer the vibro champ build that I was originally going to do with more confidence.

Oh by the way, I got my first taste of high voltage while I was working on this amp. It went through one hand and there was no harm done, but it sure as hell taught me some respect.

[thetragichero]

can you believe that the vibro champ came stock with a 470r 1w carbon comp resistor there?! the one i had (68/69) that resistor was torched and took out the bypass capacitor next to it in the process
how does it sound with 470, 680, 820, 1k there? (i replaced with 1k 5w and it sounded great to me so left it in)

[Johnny_March]

I have to admit that I don't have the best frame of reference to judge how good or bad it sounds. This champion 600 that I got, along with the botched mods, also had a brutally messed up mod to the baffle to make it take an 8" speaker. Whoever put it in didn't even use T-nuts and machine screws to mount the 8" speaker. The baffle had to come out. I have a new one ordered and coming in soon. So far, I've only been able to test the amp visually by probing the speaker jack at my scope and by hooking up the speaker in the open air. A speaker without a cabinet is never going to sound very good IMO. I definitely can't hear the full frequency response, I can really only observe headroom and distortion at this point.

I'll be able to tell much better when I can put the proper baffle and speaker into the cabinet. Just from the open air speaker, I can tell that it sounds good with both a 470 ohm cathode resistor as well as the 1k. I did want to take the 470 out though because the person I got the amp from actually did put a NOS tube in it that I'd like to preserve as much as possible. the 14W of dissipation with the 470 ohm resistor would burn out the tube before its time. In fact, the previous owner/modder had sort of the right idea as far as components. He jumpered in some nice F&T caps into the filtering section which I reused in my build.

[Johnny_March]

I ended up having to do quite a bit of troubleshooting on this amp. It had very low, distorted volume, the kind that you hear when you turn your amp off but keep playing through it. It turned out to be a bad speaker. By the time I replaced it, I had already rebuilt the amp. Here's a list of what was done.

Rebuilt the amp from scratch
Replaced the speaker baffle and installed a new 6" Jensen speaker

This amp was also very susceptible to vibrations, so I had to experiment and add some rubber dampeners in places to get the amp to not sound like it was falling apart when I played through it. The end results were great. No more noise, and the amp sounds great. I ended up with a 680 ohm resistor in the cathode of the 6v6 which gave me a 11.6W plate dissipation. I'll be leaving it there. I'm super happy with my first build.

Onto my second build.... I already cut a proper hole for the power transformer and wired it to the rectifier tube and to the pilot light. I believe the rest should follow pretty swiftly. Just a few hours and I should have a vibro champ.

Thank you for all the support so far. I'll include pics of my first build and what I have going for my second.

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[thetragichero]

I've had similar experiences with a bad speaker (had voice coil rub) and also discovered overtightening speaker mounting screws can cause 'cone cry' (sort of like an out of tune octave up, usually above the 12th fret on the e and b strings). i now do initial testings on a known-good speaker to rule that out

i have also powered up a new build and shouted all sorts of profanity when no sound came out, only to discover the guitar volume was off... stupid human tricks

[Johnny_March]

Hi everyone. I think this may be one of my final postings on this thread. My project is finally complete. Here's the rundown on what I did.

I decided to wire up the filter capacitor board on the same side of the chassis as the rest of the circuit since there was plenty of room given this is an oversized chassis for a vibro champ build.

I followed the Fender Vibro Champ AA764 to the letter and everything came up splendid. It fired up the first time with no issues. The only two modifications I did to this circuit was to use a 40uF capacitor instead of the 20uF for the first filter capacitor and I used a 680 ohm cathode resistor for the 6v6 which gave me a 11.5W plate dissipation. It's biased as hot as is allowable per the 6v6 specs.

Since the output transformer I got had multiple speaker impedance taps, I wired up a switchcraft slide switch and the speaker out can accommodate a 4,8, or 16 ohm speaker out.

It sounds fantastic, although I was surprised at how different it is than my first 5F1 build that I did just before this one. The 5F1 distorts very early even though the plate dissipation is the same as this amp. This vibro champ stays clean until I get to around 7 or 8 and I hit notes pretty hard. It's a great pedal platform as a result. The tremolo sounds awesome too.

One thing I'm not completely satisfied with is the sweep of the intensity on the tremolo. The Fender documentation calls for a 25k Reverse Audio pot, but I wasn't able to find one. I could only get a 25k linear. Does anyone have any suggestions on what pot I can get that will get me a better sweep of intensity? Currently there isn't any substantial effect until the intensity is up to about 5.

Thank you to everyone for your support and guidance. These two builds taught me a lot about electronics and building techniques. I also was able to make lemonade after getting horribly ripped off with both the Fender Champion and this FrankenFender crap build I got in exchange for my good working Ampeg amp.

Let me know what you think of this build!

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[ChopSauce]

Using a 25k log pot for trem intensity and wire it reverse?

Congrats, by the way: it looks (very) good!

[Phil_S]

https://www.tubesandmore.com/products/p ... -angle-25k
You may need to get creative when mounting it, particularly if you already have a hole that is too large.

50K RA pots are available at many places like Mouser, Digikey, etc. You may not like it with a 50K RA, but it might be OK.
Here is R. G.'s article on taper: http://www.geofex.com/article_folders/p ... tscret.htm Nice graph about half way down the page.

Congratulations on a nice build!

[Johnny_March]

I looked at alternatives for the 25K Linear pot I have... I can't get a 25K audio pot for this build. At least not a CTS pot that matches the rest of the pots in my amp. The only 25KA CTS pots available are long shaft for guitars and won't be a good fit. I'm just going to have to stick with the one I have now. The quality of these CTS pots is really good and I think it's worth it to keep a higher quality pot with a less-than-desirable sweep than to switch it out for a lesser quality pot that will have a more favorable sweep. Keeping this pot is definitely not a show-stopper for me.

[thetragichero]

get a larger value pot along with a resistor across the outside lugs (250k or 500k pot with 27k resister oughta do)

https://www.tubesandmore.com/products/p ... a1902b1052

[BartVV]

hi all
first time here on the forum
As we are rebuilding a Fender vibro champ , a problem came up during the testing.
when putting the cathode bias resistor of 560 ohm there was a voltage of about 75v
the cathode - plate voltage was 225 to 275 V depending on the tube we installed (all 6v6)
with the calculation , there is a massive power dissipation of more than 300%
any body any idea to solve?
to get voltage over resistor down and the other voltage consequently up?

[sluckey]

With power turned OFF, measure resistance between V3 pin 5 and ground. Should be 220K. Also measure resistance between V3 pin 8 and chassis. Should be 560Ω (or whatever size cathode resistor you are using.) What are your measurements?

Disconnect the NFB wire at the speaker jack. This wire goes to the 2700Ω resistor on the board. Any better?

[BartVV]

thanks for the advice
v3 pin 5 and ground gives OL
v3 pin 8 and ground gives 560ohm , as installed
disconnected the nfb but didnt test yet as i want to get the correct value first on pin 5

[sluckey]

v3 pin 5 and ground gives OL

This is a problem! Not much to go wrong... A bad/missing wire from pin 5 to the 220K, bad/missing 220K resistor, or a bad/missing ground connection from the top side of the 220K resistor. Should take a couple minutes to spot the problem and another 10 minutes to fix. See attached pic...