Champ 5F1 B+ voltages

[Yazoo]

I am trying to understand the Champ 5F1 circuit. I have a printout which lists the B+ voltages, B+1 as 360v, B+2 as 325v and B+3 as 250v. I wanted to confirm that these are close to the measured voltages when the amp is operating with tubes in. I am chasing my tail because the datasheet for the 6V6GT lists the maximum DC plate voltage for a class A amp as 315v. I realise there is a voltage drop across the output transformer but would the voltage still not exceed 315v?

[willienillie]

Fender very regularly exceeded the maximum plate voltage for 6V6s.  By much more in the Deluxe Reverb.  The old US-made tubes could take it, their max ratings were actually fairly conservative.

Look at datasheets for 5881 or 6L6GC, for fixed bias operation they list max grid resistance as 100K.  Fender almost always used 220K.  The tubes held up and the amps sounded great.

I think I mention this every time I see 5F1 come up, but there is a bypass cap missing from the Fender schematic and layout, yet the actual amps had the cap:



It works either way, but sounds much better with the cap.

[amptramp]

If you want an amplifier with plenty of derating for long life, use a storecast or public address amplifier.  I have a Muzak 920-B that gets 15 watts out of push-pull 6L6's.  The output tubes are just loafing at this power.  There is a resistor from the power transformer centre tap to ground that you can bypass to get 20 watts out of it.  Still not much stress.

Something a little closer to the Champ is my Bogen Challenger CHA10 amplifier that gets 10 watts out pf a single-ended 6L6 running 325 volts on the plate and 290 volts on the screen.  A 6L6 barely notices this kind of load.

Guitar amplifiers are designed with little or no (or negative) derating (where derating is defined as the ratio of applied stress to the maximum operating stress the device is under).  It is considered that these amplifiers are not on for very long compared to the storecast amplifiers that are on from 12/6 to 24/7.

Since your Champ is cathode-biased, the maximum grid resistor value to ground is 500K.  The 100K is only for fixed bias.  BTW the plate and screen voltages are not above the rated values on your 6V6 if you count the cathode resistor setting the cathode bias about +10 volts.  The 6V6 can stand 350 volts on the plate and 315 volts on the screen.

[Yazoo]

Thanks to both of you. The voltages are from Rob Robinette's site. I also watched one of Uncle Doug's excellent videos. He gives the B+1 of the Champ as 325v, not the 360v on the Robinette site. This would more or less fit in with the increase of US voltage from 110v to 120v, which would push the measured voltages up. Does that sound right?

[mozz]

Doesn't really matter. It's a Champ. Your output tube will eventually go bad, your output transformer will fry along with your speaker. Besides that, they are great little amps. 360v is perfect. Do not worry about your dissipation, use old tubes for all 3, no chinese junk. 4 ohm speaker to correct transformer.

[willienillie]

Since your Champ is cathode-biased, the maximum grid resistor value to ground is 500K.  The 100K is only for fixed bias.

I guess I should have been more clear.  I was just giving another example of where Fender (and others) exceeded published maximums without problems.  Fixed bias push-pull 6L6 type amps, i.e. Bassman, Super Reverb, Pro, etc.

use old tubes for all 3, no chinese junk

Not sure if you're being sarcastic at all, but I would agree with that part.  Nobody makes a good 5Y3 now.  The JJ brand 6V6s are okay, but I prefer an old RCA, etc.  Not too expensive since you don't need a matched set.

I would consider dissipation, though.  Red plating in Champs is pretty common, but an easy issue to address.  Swapping a resistor is cheaper than buying another tube.

[PRR]

...B+1 as 360v....there is a voltage drop across the output transformer but would the voltage still not exceed 315v?...

There is also a significant drop across the cathode resistor. Say 360V, minus 10V in OT, minus 20V in Rk, makes 330V. Which is 315V for all practical purpose. If you have to hike a mile up a mountain to replace the tube, you quickly learn to run 6V6 at 250V. But a working gigging in-town amp, the 6V6 is quick and easy to replace, and getting a little "more" is worth the occasional expense.

There's also history. 6V6 was intended to steal seats from 6F6. So it cited about the same ratings. May even have been valid, since they were built of similar material. And they did not over-promise, because there was a depression and radio-sets were cut-throat pricing. But even a 1937 6V6 will not go "boom" at 316V. Post-war, 6V6 were used in many things including TV. And improved materials for the massive TV market back-leaked to the radio/g-amp type like 6V6.

315V leaves much slop for careless designers/builders and consumers with extremes. If you actually measure what you make, 350V is specifically allowed.

[Big Monk]

Keep in mind the all important fact here (for the purposes of the visual, picture me shaking my fist fervently): They don't make them like they used to. Mechanically speaking, tubes were made more rugged "back in the day" and because Engineers are naturally a conservative bunch, we covet and value margin as if it were gold.

So you tell the masses certain voltages knowing that pushing things past their limits is inevitably what people will do, and you keep it in your back pocket that they are rugged enough for way beyond what you quote. It's a minor study in human nature combined with the concept of safety margins from engineering design.

[mozz]

The JJ 6v6s is kind of heavy duty and almost a 6l6. The 5y3 they used to sell put out too much voltage.  As to upping the cathode resistor to get dissipation in check, there is a point where your sound suffers , I'd rather drop the B+ s little and leave the resistor stock. I think the silverface or blackface ran even higher voltages than the tweeds. I have a original 59 champ here I can measure if you want real life voltages.

[vigilante397]

I was going to measure my Champ build, but it was built with scraps lying around my garage, and the power transformer is only delivering 220VAC into the rectifier, so my 6V6 is seeing much less than 360V. Sounds great though.

[willienillie]

I'd rather drop the B+ a little

With a zener on the CT?

I think the silverface or blackface ran even higher voltages than the tweeds.

True that.  I've definitely seen more red-plating on BF/SF Champs, but I've also seen way more of those amps in general.  Tweeds are fairly rare around here.  But some 6V6s just bias hotter than average, I'll up the resistor to compensate rather than drill a hole in a (customer's) chassis.

[mozz]

But what are you gonna bias at? 100%? Some recommend 90% for a class A. I've also read they sound like crap until you are at least 14w dissipation. 14w, for a older tube, would be max for plate and screen added together. Sounds like a good starting point.

I have never used the zener trick. I think the more common ones are reverse polarity, so the uncommon ones are what you need, they can be bolted to a chassis for a heat sink. Heard they introduce noise but i have not tried to measure that. You can also use a large dropping resistor and also a bucking transformer but then it is being way overthought. Use a JJ if you are afraid of being way over limits.

[willienillie]

I've used the zener trick several times on my own builds, never had a noise issue.  It's been many years, though.  Better to just get the right power transformer to start with.

For a low-power, cathode-biased Champ, I'm prone to just sticking a 6V6 in there, and if it sounds right and doesn't red-plate, that's enough for me without measuring current.  But if it does red-plate, then I change the resistor.  I'm sure I've gone up to 1K in BF/SF Champs.  I don't recall changing it in a tweed Champ, but like I said I don't get to work on those often.

When I built my own 5F1 clone (sold years ago to a buddy with a studio), I did check against the datasheet.  IIRC 470 ohms was still good though.  That amp is still running like new with the same tubes.

[Yazoo]

Thanks to everybody for your help. If you could measure your voltages, Mozz, that would be very helpful.

I am going to use a 12V6GT rather than a 6V6 because it simplifies the power supply, 12 volt back to back transformers.

I have been trying to work out what load the high voltage sees. The figure I am getting is around 8K, using a figure of 360v with a current consumption of 0.043 amps, the amperage from a circuit analysis of the 5E1 version which is almost identical. Does 8K sound right? I thought I would try using a resistor before connecting the power to the board so I can safely adjust voltages. I am going to use a voltage doubler to get the voltage from my transformer up to around 400v.

As they say in the UK, it's doing my head in.

[mozz]

You won't get 360vdc by doubling 120vac. Maybe a voltage tripler but then you only get 1/3 of the current. I think you can get the proper transformer for what you are going to pay for 2 others. Besides, champ chassis are small and not much room for extras. With all the changes you want to make, it will no longer be a champ. You may be better off buying a radio or record player with the tube lineup you need and just using all the parts.

[Yazoo]

It's one of the few advantages of living in Limey land. We are on 230V, so it should work out OK. I did originally buy 2 12v 10va transformers which were fine for my original build, just the preamp section.

Late last Thursday night I realised they would be no good for a voltage doubler so I have two 12v 50va transformers on the way. I think they should be fine and they only cost £31 pounds. I am sure I'll find a use for the original transformers at some point.

[PRR]

> around 8K, using a figure of 360v with a current consumption of 0.043 amps

Good insight.

You may also note that most single-ended self-biased pentode power amps, the supply V/I is similar to the load V/I (impedance). The Champ OT is normally called 7K. If you deduct all bias and losses, the 6V6 is really closer to 300V plate to cathode, and 300V/0.43A is 7K (for practical purpose).

[Yazoo]

OK I've built it. There were further complications with the transformers. I realised if I used the voltage doubler with a 230v transformer, I would end up with around 650 volts (not taking into account real-world losses). So, to avoid having to wastefully drop big voltages, I bought another transformer with an 18v secondary. That way, with back-to-back 12v and 18v transformers, I would get much closer to the desired HT voltage. I thought I would still need a dropping resistor but in the event, I didn't. I have ended up with around 340 volts which is fine. I got it biased today to 10.9 watts, pretty much bang on the recommended 90% for the 12V6GT.

And...I have already found a use for the smaller transformers, the Rick-Tone Trem-o-Drive. It certainly beats online gambling!

[merlinb]

the datasheet for the 6V6GT lists the maximum DC plate voltage for a class A amp as 315v.

That's a design-centre value, not an absolute maximum. Guitar amps routinely operate outside the recommended 'hi-fi' enviroment.

[PRR]

the datasheet for the 6V6GT lists the maximum DC plate voltage for a class A amp as 315v.

That's a design-centre value, not an absolute maximum. Guitar amps routinely operate outside the recommended 'hi-fi' enviroment.

6V6 specs were not even about "high fidelity". (That would be triodes.) 6V6 was better-grade home radio. Aunt Matilda is not going to change a bad tube herself. Or even move a large console herself. Conservative design makes less customer friction. OTOH a working musician has to move the amp every night, and can learn to change tubes. A loud design is worth a somewhat shorter life.