Vibro Champ Amp Modification

[Thom]

My mom bought me a Fender guitar and a Vibro Champ Amp when I was 12. I loved both and played often. When I graduated from high school, both went into storage. That was many years ago. I recently dug thru storage boxes and found both. I used a VARIAC to gradually being the amp up to full AC power. Lots of hum (expected) and low audio output. I tested the tubes (originals) in my tube tester. All are in perfect condition. I was positive that the capacitors were shot, and thus ordered a new set. Which leads to a question (Disclaimer....I am NOT trolling for positive comments and not advertising anything): I have the idea to remove the rectifier tube and then install a diode bridge. Larger filter caps and add a 5 or 6 Henry filter choke, all in the effort to “stiffen” the B+ voltage. Plus, I would add a dual input audio matching transformer to the two audio input jacks. Plus, a more substantial speaker. I’m not planning to sell either, and will used this amp for the rest of my life. I did mention these modifications to my son and he nearly had a stroke, pleading with me to not do any of this besides the capacitor replacement. Am I nuts? Should I leave it as original as possible?? Or modify it to suit my desires?? Thanks in advance for your opinion.

[didit]

Hello -

Moderate powered single-ended Champ style amp is an ideal, preferably through an efficient 12" speaker. As you clearly understand from the framing of the questions this will require major modification from a stock Champ. Building anew from scratch in an adequately sized chassis fitted to cabinet that's sized to take 10" or 12" speaker will be similar effort with less compromise. And then selling or keep, and gently restoring, your original seems sensible.

Best .. Ian

[joeboo88]

I would do what your kid says and just do a cap job. After all your mom bought you that and it’s a great keep sake.
Make it workable.. and play with something else to build or mess with.
I just made a vibro champ in a 1940 Philco radio. I shouldn’t have messed with the radio but this one was a basket case, so I feel ok about it plus My mom never gave it to me
Peace

[echuta13]

If you go ahead with solid state recto, make sure you check your voltages, and check your output tube bias as well.

[ChopSauce]

Should I leave it as original as possible?

I think I would do that, so far.

Yet dismantling a working amp may be financially more intersting than building your ideal amp from scratch, I think that because you ask, you're unsure. That's a good reason for thinking twice about that.

If this amp was mine, it probably would end up with smaller filter cap values and maybe a better speaker, at some latter point, though.

[Thom]

Appreciate the comments. For now it will be only to replace old caps, one-for-one. I forgot to mention couple other things. It is a black face. And, for the tubed audio chain, I would leave it alone, other than addition of a balanced input transformer. I’ve always hated to mix audio thru a resistive divider. If I ever do rebuild the power supply chain, yes I would certainly confirm voltages before hooking it all up. Or at least verify what voltages the audio tubes can work with. That is a very good point, tho.

[thetragichero]

your amp is worth about a grand in good working condition so if you're interested in messing with mods to a champ-style circuit you could build one for a couple hundred bucks
i always think of the princess bride "there are a shortage of perfect breasts in this world. it would be a pity to damage yours"

[Mark]

Your son is dead on the money. You would be insane to do the mods you suggest. You will destroy the value of the amp.

I wouldn't replace the rectifier with solid state diodes. You don't want to exceed 360VDC plate voltage. If you want more voltage try a 5AR4. The first thing I'd get rid of on a Champ amp is the output transformer. They are never have enough bass response, again I wouldn't do this to this amp.

I think you ought get a Tweed Princeton chassis and build a Champ/Princeton on that, you can also have a 5E3 type cab made to accomodate the Princetone chassis.

[Stevem]

Since that amp is class A there's no need for SS diodes ( since the amp idles full tilt all the time ), or a running a 5AR4 recto unless the 5Y3 is shot and all you have is the AR4!

Kicking up the power supply voltage will also mean that to keep the output tube idling in class A and also not making the output tube idle at over a good safe 12 watts you would also need to change the outout tubes Cathode resistor from its stock 470 ohm up to a higher resistance and in doing that increase its wattage from 2 watts up to a very safe 5 watts.

If you do anything recto tube wise for just a bit more power supply voltage the use a 5V4 recto as that one also has the amp saving benefits of a slow warm up like the 5AR4 which the stock 5Y3 does not have.

A nice easy mod I have done to all my Champs I have ever owned to lower the high 60 HZ noise floor that they have is to do away with the single heater wire and ground at each tube and run a new twisted pair of wires from the pilot lamp and then to the outout tube and in your case the 2 preamp tubes.

By far the best thing you can do with these amps is to suff in a 10" speaker as there is room to do that, if you do not want to chop up the amp then you can solder on to the terminal board on the speaker a 1/4" switching type jack do you can plug in a bigger speaker and the function of the jack will cut out the amps internal speaker.

This mod does require that you upgrade the amps outout transformer to a mojotone one which has the needed 8 ohm tap, as the stock transformer is only wound for 4 ohms.

Having this 1/4" jack on the speaker and the Mojo output trans will open a big number of options in regards to speakers to try out.

[TUBEDUDE]

I'm of the leave it be and build your dream amp opinion.

[Mark]

Since that amp is class A there's no need for SS diodes ( since the amp idles full tilt all the time ), or a running a 5AR4 recto unless the 5Y3 is shot and all you have is the AR4!

Kicking up the power supply voltage will also mean that to keep the output tube idling in class A and also not making the output tube idle at over a good safe 12 watts you would also need to change the outout tubes Cathode resistor from its stock 470 ohm up to a higher resistance and in doing that increase its wattage from 2 watts up to a very safe 5 watts.

If you do anything recto tube wise for just a bit more power supply voltage the use a 5V4 recto as that one also has the amp saving benefits of a slow warm up like the 5AR4 which the stock 5Y3 does not have.

A nice easy mod I have done to all my Champs I have ever owned to lower the high 60 HZ noise floor that they have is to do away with the single heater wire and ground at each tube and run a new twisted pair of wires from the pilot lamp and then to the outout tube and in your case the 2 preamp tubes.

By far the best thing you can do with these amps is to suff in a 10" speaker as there is room to do that, if you do not want to chop up the amp then you can solder on to the terminal board on the speaker a 1/4" switching type jack do you can plug in a bigger speaker and the function of the jack will cut out the amps internal speaker.

This mod does require that you upgrade the amps outout transformer to a mojotone one which has the needed 8 ohm tap, as the stock transformer is only wound for 4 ohms.

Having this 1/4" jack on the speaker and the Mojo output trans will open a big number of options in regards to speakers to try out.

The valve rectifier will sit at the idle voltage and the valve passes signal the rail goes up, however when the signal clips the rail voltage sags.

I have a 10” speaker in my Champ now and it’s not fat sounding enough I don’t think the cab is big enough for the 10” speaker.

[Stevem]

It does not sag by much now does it?

[Mark]

I know about the sag as I inputted the signal into the amplifier and watched the rail voltage. I noticed that the rail voltage increased while the signal was undistorted, then the rail voltage began to sag again and I noticed that the signal was in fact distorting.

[Thom]

I received/installed replacement caps. I did replace the power supply caps even tho, to my surprise, the old multi-section can measured well within tolerance (cap meter, VOM, ESR). Instead of a one-for-one can, I found a 500V, 4 section, 30uF can, thus increasing the filter action slightly. Oh, I also added a 10 ohm, 10 watt resistor In series before the first filter cap, hoping to shave off some of the instantaneous current spike when power is turned on. I also rewired the tube filament chain, avoiding the previous use of the chassis for one leg. Added a 3-wire power plug, adding ground to the chassis, hence also removing the blue power input cap. I wired my own Manufacture dual input, single output audio matching transformer to the two audio input jacks, and installed it.....thus replacing the existing resistive divider input. I also ordered and installed a new 8 inch-replacement speaker By Eminence. I considered a 10 inch, ultimately not wanting to cut the box. I have kept all removed hardware...and if for some reason someone wished to return it to the way it was....fine with me because I’ll be dead. Whether this amp was worth $10 dollars, or $10,000 dollars is all meaningless to me, because i will never sell it. So, I “cut a deal” with myself to add mods that make me happy, with the idea it can be returned to original if someone ever wished to do so. Many, many thanks for the comments! It did help! i.e., I did not add the SS diodes, realizing I was trying to fix something that wasn’t broken. I guess I love to have SS bridge diode power supplies in all of my Ham Radio tube transmitters....thinking I’d do the same here, finally realizing Ham Radio and belting out my favorite AC/DC riffs is not the same thing!

[didit]

Hello --

Sounds excellent, and pretty certain it actually does.

Note something you likely already realize, which is one can put silicon rectification in via your tube socket with zero modifications to see it's to your taste.

Best .. Ian