Champ 600 with MM upgrade kit

[Wes]

I was given one of these little guys and it had the typical dead power tube circuitry due to the fantastic
China tube that it came with. Anyway, I fixed it up and it's been laying around for a couple years. I decided
to throw in that Mercury Magnetics upgrade kit and it's up, running, and sounds pretty good with it.

My issue is that I'm kinda freaked out by the operating conditions of the power tube. I didn't modify anything
when I repaired the amp to stock so I didn't check anything per bias, but the conditions with the MM upgrade
have me worried.

I've got 419V on the plate and I'm drawing 42mA and I figure that is 17.6W worth of draw. Of course a 6V6
is spec'd at 14W so....

And looking at the datasheet (grid is at about -32.5):

JJ-6V6-S.pdf

It appears that this is clearly out of line.

What are your thoughts on this?

I see many, many power amps with their tubes running hotter than their specified limits but is this ok? What is the limit?

Thank you

[Tom Phillips]

Most Chaps run the power tube very hot and hard especially with today's higher power line voltages. I personally do not like to leave them that way. If I were in your position the first things I would do are:
1) Determine if the plate voltage is lower with the stock PT. If so then I'd take out the MM and stick with the stock Fender PT.
2) If your amp is using the stock 470 Ohm cathode resistor then I'd increase the value to reduce the bias current.

[Leo_Gnardo]

The JJ 6V6 is a bit tougher than others. Some take the plate dissipation to be more like 18 or 19 watts in this tube. If you're worried, can always up the size of the cathode resistor to 560 or 620 ohms. Self biased tubes often settle right at their plate power limit, as long as other operating conditions aren't out of line.

Considering the high voltage, can't blame a China-made tube especially.

Recently one of my customers showed up with his Vibro Champ. I'd sorted it out for him a couple years ago, and it had a JJ 6V6. He dimed the amp, and was running a fuzz box or overdrive into it besides. Then the fuse popped, with the smell of smoke in the air. When I examined it, there was a carbon track from arcing between pins 2 and 3, both on the socket and tube base. He was beating the hell out of the amp, what could you expect? Lucky all he needed was a fresh output tube and fuse, and I ground away the burnt part of the socket with a Dremel. No further problems, but he's not going to try anything like that again. I also added a set of flyback-quench rectifiers from plate to ground, 3 x 1N4007 as recommended in The Trainwreck Pages. Cheap insurance. Just makes it a little less likely to arc in case of amp abuse in the future by him or anyone he might lend or sell it to.

Lots of folks use a 6L6 instead of 6V6. Never found it to be a problem. Delivers the same power so don't expect a big boost. You could expect a 6L6 to last a lot longer as it's not being run so hard compared to its limits.

[Wes]

Thanks guys.

I'll pop the original PT in and see what I get. As far as the cathode resistor- I have a 750 in there now! Maybe go to 1k. Intense....
The idle wattage with the stock 470 was 24W.

Leo- good call on the flyback diodes. I've come across several amps that have had arcing occur. They usually have really worn out
power tubes that won't conduct that I think are the main reason for the arcing. I may start installing the diodes as well. Some people
just don't like to change tubes!

[Wes]

Yeah, the Mercury PT HT secondaries are putting out 385.9VAC before rectification. The original Fender PT HT secondaries
are putting out 299VAC before rectification. Quiet the difference. I'm not sure the filter caps can take this beating. Upon
fire up, before the tubes start conducting, the HT swell is over 500VDC. I have heard several pops on startup that I think
are coming from the filters.

I'm going to contact Mercury to make sure these voltages are right....

[Tom Phillips]

Yeah, the Mercury PT HT secondaries are putting out 385.9VAC before rectification...

It will be interesting to hear what they say about that when you contact them. Please let us know. Glad you zoned in on the reason for your excessive B+ voltage.

Setting up the Champ single ended power amp properly requires both the correct operating voltage (within a reasonable range) and cathode resistor. When the plate voltage gets as high as yours was with the MM PT you can't really set a proper operating point by increasing the cathode resistor.

[PeanutNore]

Since you are already running an upgraded PT, I would just switch to a 6L6GC and a cathode resistor in the 300 ohms range, and swap in a speaker with half the impedance. Now that'll be one hell of a Champ.

[trem]

It will be interesting to hear what they say about that when you contact them. Please let us know.......

I would also be curious to know what they say.......

[Tom Phillips]

Since you are already running an upgraded PT, I would just switch to a 6L6GC and a cathode resistor in the 300 ohms range, and swap in a speaker with half the impedance. Now that'll be one hell of a Champ.

What you suggest would get you an amp that functions but there would be multiple deviations from the original. Some things to consider are:
-I'm not sure we can even call the MM PT "Upgraded." So far the MM PT that Wes has is inferior for the intended task.
-The 6L6 configuration would respond less like a classic Champ because you'd loose the drive sensitivity of the 6V6. It depends on what you want to end up with.
-A speaker of half the impedance would be a 2 Ohm unit so choices would be way too limited.

[Wes]

Hey guys-

I got a response back from MM. I was informed that 380VAC is correct for the HT secondaries with a mains voltage of 120VAC.
I think the extra 6 volts I measured is negligible and I feel that 380VAC is still bad for business considering these things:

-The DC swell at the startup of the amp is over 500V. Those puny little caps are rated for 450V and I don't trust that they are
the best built anyway.

-With the stock 470 cathode resistor and this B+.... that 6V6 is pushing 24W.

-Even with a 750 cathode resistor the power is too high. I think it was 17W or a bit higher.

-The stock secondaries are 299VAC with the same mains that the MM produces 386VAC from.


I put the original transformer in and dropped a 680 cathode resistor in. The B+ doesn't surge more than 420VDC or so and
the 6V6 is running at about 85%.


Am I looking at this all right? It looks to me as if this MM PT is a bad idea/design. It seems they would realize these issues?

[Tom Phillips]

...I got a response back from MM. I was informed that 380VAC is correct for the HT secondaries with a mains voltage of 120VAC....Am I looking at this all right?...

I think you have the facts correct. I have had similar issues with MM's designs. I'm thinking that the response you got came from a marketing person rather than an engineer. Did you send them the data comparing the stock PT with their PT?

[Wes]

Tom, I did send him the numbers.

This is my actual message:

Hey there-

I bought one of the upgrade kits for this amp and had a question about the
voltages I should be seeing from the power transformer.

What should the AC voltage be coming from the power transformer HT secondaries?

I'm measuring 385.9VAC with the Axiom C600-P before rectification while the stock PT
has 299VAC on the HT secondaries.

After rectification I've got 420ish VDC on the plate of the power tube, but before that
tube conducts, I've got a swell voltage of over 500VDC. I'm concerned for the filter caps
whose rating is 450V. I think I have heard them popping!

Is 385.9VAC correct for the HT output of this transformer? I may have to switch back
to the stock PT in an effort to not destroy the filter caps.

To which the response was:

At 120V in our transformer is 380VAC out and 6.91 to the filament center tapped open circuit.

If your wall voltage is higher, which we suspect yours to be 125 in based on your measurements your B+ will be higher.

Thanks.

I'm just going to save this MM-PT for another project and keep the stock PT in.

[Jazz P Bass]

Wes, there is another thread going on where the OP cannot get the amp to 'breakup' at high volume settings.

Does your amp 'breakup'?

[Leo_Gnardo]

Tom, I did send him the numbers.

This is my actual message:

<message skipped for brevity>

Another fine non-answer from Mercury. He should have added "Thanks for your money! Don't forget to send more."

It's Mercury's practice in recent times (last 15 years or so) to huckster-sell their iron. "Your amp will sound bigger, better, brighter, whatever. Pay a 50 to 400% premium for our iron and you too can wear The Emperor's New Clothes." An unplanned unwanted increase of B+ of 30+ % WILL result in more power all right, but a more stressed tone and will definitely need a serious change in bias to stay out of meltdown. Lower B+, a more relaxed tone, less power but are we aiming for a power figure, or good tone? For those seeking more power, no problem if that's your choice. Make the appropriate adjustments and enjoy the incremental boost in power.

A 4% increase in line voltage would result in 311 VAC on a transformer that was properly wound to match yours. That would be nothing to worry about.

I'm sure Mercury will custom wind you one for mo mo mo money. Why bother when you have a perfectly good Fender PT.

Live and learn. If, as in Fender transformers, it's the same PT for Champ and Princeton, your MM PT may be more applicable to a Princeton type build. More tubes will load it down a bit more; both filament and B+ will be reduced a bit. I often find original Fender Princetons with B+ in the 400-440V range. And put in 500V rated filter caps for the safety margin if nothing else.

"I can hear my filter caps popping." Not a good sign. Put head-shaking grim looking smiley thing here.

[Enzo]

Caps popping? They don;t usually un-pop. Many amps make popping sounds warming up, just as your auto in the driveway will make popping sounds as it cools off after a drive. More likely a tube than a cap. And heat related rather than voltage related.

Caps may have a 500v rating, but that is called a "working voltage" rating. In fact older caps used to be marked "500WV" and that implied - and was often also printed on it - that there was a "surge voltage" rating. the purpose of that was to cover things like turn-on voltage rise.