Champ 420V B+; Misload Transformer?

[jeff]

I got a transformer for a Champ 330-0-330 which gives me a B+ of about 420V after NOS 5Y3(Sovtek 5Y3's yeild about 460V so watch out for Sovteks I don't think they're true 5Y3s)

420V seems way too high for a 6V6 rated at 315V but they've been doing it for years so...maybe I shouldn't worry about it....O.K.

The question I have is the output transformer is a 7K:4R. From everything I think I've learned this seems wrong to me. A 7K transformer seems right for 290V at 12W but for 400V(-20V on cathode) would you want a load closer to 13.33K?

Would it be a better choice to use a 8R speaker instead of a 4R? The reflected impedance of 7k:4 = 14K:8
Or is having the wrong load on the tube somehow keeping it in check from blowing up because the voltages are way above the maximum rating?

 To go another route if I wanted to bring the B+ down to a reasonable 320V I need to drop 100V. The amp pulls about 46mA so I'm thinking about using a 2K5 resistor to drop the 100V. Would a 10 watt 2K5 resistor be OK?

[plexi50]

The 6V6 can withstand 420 volts but is not my choice for a sweet sounding amp. And i wouldnt expect the tube to last a long time either. I have had this same problem in the past and the only real cure is to get the right transformer for the job. 340-350 VDC on the plate of the 6V6 is what will produce a nice sweet tone and what these amps were known for. A larger B+ will just make the amp nasty and harder to find that sweet spot.  Believe me i know what your trying to come up with but even trying to get that transformer down 70 VDC to get your 350B+ is not gona happen. But just for having a working 420 V B+ that is no problem.

The 7K OT is fine. Thats what they came with. Even if you were to get the B+ down to where you want to be the PT would be cooking itself with the a resistor big enough to drop it down that far. The 5Y3 as well would not tolerate that for long. Too much stress on the PT and 5Y3.

A single ended EL34 Champ is probably your best alternative for this PT. There was a thread on here not long ago about the SE EL34. I bet it sounds great!

[eleventeen]

I agree that 420 v on the plate of a 6V6 is not a happy condition.

You say the whole amp pulls 46 ma. Looking at a V-Champ AA764, I figure the preamp 12AX7 uses 1 ma + 1 ma, per half. That's based on 1.5 volts sitting top of a 1500 ohm resistor. I'd figure the trem tube uses 2.5 mils for the half with the cathode = pin 8. The other half, I can't tell from the dwg if that cathode sits at 1.6 volts or 16 volts. I've looked at several V-C schematics. Regardless, the 6V6 is thus pulling about 40 mils of that 46 mils, and isn't that a tad high?

If 40 mils, then your 2.5K 10 W resistor should work just fine. I calc 4 watts. So I agree with your math. Only thing left is to solder it in and take cover! 

[PRR]

Put modern (not vintage) 6V6 in there. They may work.

If they die a lot, put in 6L6GC, it will be fine.

If you go EL34, watch your Pin 1 connection.

[bigdaddy]

I would add a 10 watt resistor but not 2.5K. Maybe a 470 ohm at the most but go with a JJ 6V6. They sound great and work fine at those voltages.

I tried a lot of different 6V6 tubes and voltages and the NOS types work and sound best no higher than 350V. Once you go higher all the best of what a 6V6 does is lost, of course this is my ear and opinion. By using the JJ you get the 6V6 sound but those higher voltages do not affect it. The JJs seem to say I will produce that tone no matter what... .

You'll probably need to use around a 600-700 ohm bias resistor to get in the 12-14 watt region which the JJ works fine in. Just to try it I ran a JJ at 18 watts and it worked fine but It sounded best at about 12-13 watts, NOS lower.

[tubenit]

transformer for a Champ 330-0-330 which gives me a B+ of about 420V after NOS 5Y3

I would anticipate a 330-0-330 with a vintage 5Y3 would give about 365v?   (1.1 x VAC)

I would anticipate a GZ34 would give the 420v?  (1.3 x VAC)

Have you actually measured the AC going into the rectifier tube?  Or are you going by what the trannie is supposed to be?

I can NOT see how you're getting 420v if the PT is truly 330-0-330 using a 5Y3?   A low battery in a voltmeter can give false high readings at time. Not saying that is the problem.

I have run JJ 6V6's at 476v for well over a yr with no issues.

With respect, Tubenit

[stingray_65]

transformer for a Champ 330-0-330 which gives me a B+ of about 420V after NOS 5Y3

I would anticipate a 330-0-330 with a vintage 5Y3 would give about 365v?   (1.1 x VAC)

I would anticipate a GZ34 would give the 420v?  (1.3 x VAC)

Have you actually measured the AC going into the rectifier tube?  Or are you going by what the trannie is supposed to be?

I can NOT see how you're getting 420v if the PT is truly 330-0-330 using a 5Y3?   A low battery in a voltmeter can give false high readings at time. Not saying that is the problem.

I have run JJ 6V6's at 476v for well over a yr with no issues.

With respect, Tubenit

I was wondering the same thing last night and ran a few scenarios on the Duncan PSU designer. and came up with the same figure you're getting Tubenit.

The only way I got the OP's readings were if I didn't have a load.

So I have to ask a silly question just so we're all on the same page:

Is this amp running?
Are there tubes in the Amp and you are measuring it under load?
By under load I mean tubes plugged in, speaker plugged in controls set to mid and no signal in.

Ray

[jojokeo]

How about a few zeners on the pt's ct to lower some v's? And/or a combo of these and the dropping resistor to find happy spot?

[bigdaddy]

I just sold a 606 champ PT and it was very high when it was in the amp, I was really surprised. Leo did make a mistake using the Princeton PT and should have stayed with a dedicated champ PT. But Leo was about the bottom line(money) as all businessmen have to be. The numbers only matter if indeed the PT is 330-0-330 which it was in 1965 at 110V.

420V......possible with a champ even under load. I have found that every 5Y3 I tried had very different voltage drops. Even NOS ones can be +/- 20V depending on the B+.

As for OTs, with higher voltages I used Allen OTs at 8K pri and they sounded the best. I call his TO11C the Goldilocks of Champ/SE OTs.

[Tiny_Daddy]

One other thing: I am thinking you are using cathode bias so the plate to cathode voltage will be less than B+. Also there is some drop in the output transformer. So measure actual plate voltage and subtract the  cathode voltage. The result is the "actual" plate voltage that the tube sees.

[jeff]

Thanks for all the input you guys are good. It's late so I'll get back to you on the other stuff.

transformer for a Champ 330-0-330 which gives me a B+ of about 420V after NOS 5Y3

I would anticipate a 330-0-330 with a vintage 5Y3 would give about 365v?   (1.1 x VAC)

I would anticipate a GZ34 would give the 420v?  (1.3 x VAC)

I can NOT see how you're getting 420v if the PT is truly 330-0-330 using a 5Y3?   A low battery in a voltmeter can give false high readings at time. Not saying that is the problem.

This is not necessarily true in all cases. The amount of drop for a tube rectifier is dependant on the current draw. The efficency of a 5Y3 is not always 1.1 X VAC it is greater at lower currents. That's why tube rectifiers sag when cranked.

I like the idea of using a bigger tube. I have a 5881 Not sure if I can use the same OT. The output transformer I got was rated at the Weber site as "6 to 15 watts" does anyone know what this means?

 Also the PT in question was changed to have a 125V input and 300-0-300 secondary taps in addition to the 330-0-330, but they used the same part # so I don't know what the mA rating is on mine(the old one). I remember it was sold for a Champ OR for a Princeton. I think I remember it was 60mA but I'm not sure. Just so I can get a general idea what's the draw on a Princeton?

[bigdaddy]

The weber should be fine with the 5881. Just change the taps, if it's 4/8 that would be 2/4 ohms. If it doesn't have a tap then just run it the way it is until you get a new OT IMO.

I had a lot of champ builds I ended going with the 6L6GC because of the high voltages. It sounded better with that tube then pushing a 6V6 JJ. I started using lower voltage PTs for 6V6 tubes and made some great sounding amps. The 6V6 really likes 350V and below, that's a 6V6 comfort zone I guess. 6L6GCs like the JJ worked the best in SE operation compared to others, Sovtek also handled it well but did not have as good a tone. They liked around 400V or more no problem. Some other 6L6GCs had issues until I changed the screen and dropping resistors to higher values.

I would go with a 6L6GC JJ and buy an Allen TO11C. You'll be very happy with the amp then, I had some hellacious sounding champs like that especially when I removed the NFB.

[jojokeo]

I like the idea of using a bigger tube. I have a 5881 Not sure if I can use the same OT. The output transformer I got was rated at the Weber site as "6 to 15 watts" does anyone know what this means?

5881 @ 350 plateV, 250v screen specs = total current Max signal (which rarely occurs) is 73.5mA, load resistance 4k2, output is 11.3 watts

Can your PT & OT provide this, yes. Can you do it, yes. "does anyone know what this means?"  that's funny.

[jeff]

Sorry I wasn't trying to be funny.
Maybe I'm missing something.
What I meant is, is this a 15 watt ot. It seems wierd to say 6 to 15 you know? Why not 0 to 15 or 2 to 15 or 9 to 15???.
Is it 6 sometimes and 15 others?
Or a mixed bag, some are 6 some are 15 who knows what you get.

[jeff]

The weber should be fine with the 5881. Just change the taps, if it's 4/8 that would be 2/4 ohms. If it doesn't have a tap then just run it the way it is until you get a new OT IMO.

I don't think I have to change the taps.
This is what I'm thinking, Please tell me if I'm figuring something wrong. I was going to run the 5881 with the 420V B+, so ~400V plate to cathode at 57.5mA(23W). This is the part I'm not sure of but I thought to pick an OT for a SE amp the formula was:
V/A or
V/(max watt dissapation/V) or
V²/MWD
so 400X400/23=6956 or ~7K which is prefect, that's what I've got.

If this formula is wrong how do you figure the Ot's impedance?
Does that sound right for a 5881 400V plate to cathode 400V screen at 57.5mA?

Here's what's gonna throw a wrench in the gears. If I use the bigger tube because I have the higher voltage I'll need to draw more current which will cause my rectifier to sag more which will lower the voltage which will mismatch my OT which will .....arrrrrrrg!

[jojokeo]

Who knows what/why manufacturers write certain things but I interpret the "6-15 watts" being that it simply works and is "suggested" for this range. It implies that if you were to run something at less than 6 watts, you should buy a smaller and less expensive OT rated for "upto 6 watts". No biggie.
The "funny part" is that as you'll find, there's many very intelligent people on this site. I thought it humorous while taking things in good spirits & didn't mean anything by it.

I don't think that your formula holds true for the OT's impedance, it doesn't match the spec sheet's suggested ratings. Since it stays in the 4k range at various voltages, 4K in triode mode, and only goes to 5k for push-pull use (class A), I would stay w/ 4k. This is not exact nor does it have to be for an OT's load impedance. It comes down in the end, what sounds best to you anyway. The impedance is always changing in use the same way a speaker's does. You ballpark it and let 'er rip.

[jeff]

No offence taken I just thought I was being unclear. I just wrote it a diffrent way to try to show you where I was comming from. Sorry. Typing can sometimes come off colder than speaking and I DO appreciate all the help. No offence

        Thanks
            Jeff

I don't think that your formula holds true for the OT's impedance, it doesn't match the spec sheet's suggested ratings. Since it stays in the 4k range at various voltages, 4K in triode mode, and only goes to 5k for push-pull use (class A), I would stay w/ 4k. This is not exact nor does it have to be for an OT's load impedance. It comes down in the end, what sounds best to you anyway. The impedance is always changing in use the same way a speaker's does. You ballpark it and let 'er rip.

O.K. Thanks That must have been something I picked up along the way and got wrong in my head.

[jeff]

Cool page It seems to calculate class A at 90% which I understand is right for push-pull but aren't single ended biased at 100%?

The way I got that number was I think that a 5881 max diss watt is 23 and I thought SE amps should be run at 100% max diss watt. So 23/400=0.0575  

I have two questions:
Is the 5881 max rating 23?
and
Should SE amps be run at 100%?

[jeff]

I worked the numbers in the calculator. According to it a 5881 tube is 26W. According to one chart you directed me to its 23W and the other it's 19W?????

[jojokeo]

I worked the numbers in the calculator. According to it a 5881 tube is 26W. According to one chart you directed me to its 23W and the other it's 19W?????

A 5881 is a "ruggedized 6L6 or 6L6G" and it's max diss. wattage is appx 20% higher as a result.
19*1.2=22.8
5881 = 23w according to TungSol's data sheet.

[bigdaddy]

Here's the issue about tube data sheets.....it works for the tube the data is for not only including nomenclature but make. An old 5881 or 6L6GC is different from a new one. The JJ is built like a tank most other ones are not. Some of those Sovteks aren't even what they are labeled. So you can't really rely on those data sheets unless you're using NOS tubes.

It's just a basic idea of what the tube should do. I have had issues with some 6L6GCs, some you can run 90% and some not. The JJ is the closest tube to a real 6L6GC in what you can do with them. But running at 90% is like 90% of what? 25 watts, 23 watts, 30 watts.....I have read data sheets for 6L6GCs that say all three. Better be safe then sorry and eat a tube. I just biased it to about 21 watts and the amp roared. It's not even in SE operation from what PRR said, it moves into AB1 as it goes into higher overdriven states or something like that. Turn the amp up and all that data goes out the window.

That's the issue with finding out the correct primary impedance, its a guessing game too. It's best to error in the conservative rather then to push things unless you want to experiment and don't mind burning things out. 6L6 family of modern tubes and including a KT-66, EL-34 and so on 3K-5K primary in a Champ type amp and 5K-10K in the 6V6/EL84 type tubes. KT-88/6550 down to 2K. That would be my rule of thumb since it's impossible to really know what is correct. I also go by the sound of the amp too but the bottom line is not blowing anything up or burning anything out, so I try to stay conservative.