5F1 Champ and European wall voltage

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modman
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5F1 Champ and European wall voltage

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Re: 5F1 Champ and European wall voltage

Post by xtian »

I prefer lower voltages in most of my builds. See what you can do to lower the B+ voltage, and if that doesn’t do the trick, raise the value of the cathode resistor.
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Re: 5F1 Champ and European wall voltage

Post by martin manning »

For sure use the 240V primary if you have 238V mains. I'd stay closer to the Fender dissipation numbers, and you will need a bigger Rk. Also, I'd recommend 8k primary impedance for the higher plate and screen voltages. JJ's 5Y3 is pretty close to the right voltage drop.
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Re: 5F1 Champ and European wall voltage

Post by martin manning »

modman wrote: Sun Feb 04, 2018 4:55 pm... but now realized that
higher bias = earlier distortion
lower bias = more clean sounds
right?
I would say the opposite. Since your voltages are high, the operating point will be more centered at higher idle current. See: http://www.valvewizard.co.uk/se.html I would try JJ 6V6S, which is rated at 14W, and run it at 100% using cathode current and Va-Vk.
modman wrote: Sun Feb 04, 2018 4:55 pm Also, what should be wattage rating on the cathode resistor? I see huge 2W resistors there in most Champ builds, while there is only 20V DC...
Do the math... P = V*V/R, so in your example above 22.5^2/470 = 1.08W, so I'd use at least a 2W.
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Re: 5F1 Champ and European wall voltage

Post by tubeswell »

A 5F1 will not malfunction if the wall voltage is 3.4% higher than spec. Tubes are tough. All those gain stages are cathode biased (and therefore the bias is auto-correcting*). Also, 238VAC is only going to make 6.3Vac heater at ~6.5V, which is acceptable. You want 6V heaters to be operating within a range of 6.3VAC +/- 10%.

*however, if the 6V6 bias is a problem, then change the bias resistor, or get the HT voltage down.

But if you want to put the PT at 240, then feel free to do so. This will make the heaters run a little colder and lower the HT voltage slightly, but these will not be a problem either.
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Re: 5F1 Champ and European wall voltage

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Re: 5F1 Champ and European wall voltage

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Re: 5F1 Champ and European wall voltage

Post by martin manning »

Nice demo. I liked the 1k cathode resistor, and the bypassed Rk on the input stage, which is basically giving you a 6dB boost across all frequencies. What value did you have on the 6V6 cathode when you did that? Notice you saw the plate voltage increasing as you lowered the idle current... That's normal, just the power supply showing its lack of regulation.

Were you able to verify the output transformer primary impedance? I'd expect 8k, but I have seen other values used.

If you have settled on a value you like for the 6V6 Rk, what is it, and what are the Plate, screen, and cathode voltages? I'm interested to se where the operating point landed.
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Re: 5F1 Champ and European wall voltage

Post by tubeswell »

For getting guitar tone out of it, I'm all for keeping stock Fender values and lowering the B+ to around 340-350ish. A couple of methods for lowering voltage spring to mind:

1) Zenering the HT winding voltage down with a string of (say 3 or 4 x 12V 5W) reverse-biased axial-mount zeners between the CT and the ground return. (The banded end of the zeners point to the ground return). This will achieve tight voltage regulation.

2) Putting current-limiting resistors in series between each end of the HT winding and each rectifier tube plate pin. You could experiment with a couple of 50R to 100R limiting resistors. Watch the resistors' power rating. This will result in a saggier power supply.
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Re: 5F1 Champ and European wall voltage

Post by martin manning »

tubeswell wrote: Mon Feb 05, 2018 11:41 pm1) Zenering the HT winding voltage down with a string of (say 3 or 4 x 12V 5W) reverse-biased axial-mount zeners between the CT and the ground return. (The banded end of the zeners point to the ground return). This will achieve tight voltage regulation.
A MOSFET would be a better option. Either way the regulation will not be improved, the voltage will just be reduced by a constant value.

Modman, what rectifier tube do you have installed now? A NOS 5Y3 or a JJ 5Y3 will be best for reducing voltage. Don't use a Sovtek.
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Re: 5F1 Champ and European wall voltage

Post by tubeswell »

About the regulation - I was talking about zenering in comparison to series resistance. :roll: keeps the voltage supply tight.
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Re: 5F1 Champ and European wall voltage

Post by martin manning »

Ok, Pete, I see what you mean.

Short of throwing sand at it, and in summary:

- Use a NOS 5Y3 or a JJ 5Y3
- Use a JJ 6V6S and run it at 14W plate dissipation (that would be about 14.7W using cathode current, which includes the screen current).
- Increase the 22k dropping resistor to bring the preamp node voltage down to Champ spec, around 250V.
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Re: 5F1 Champ and European wall voltage

Post by tubeswell »

If you decide to try zenering down the HT winding with a string of axial-mount zeners (of 9-15V each) between the HT winding CT and ground, see the attached schematic. This works perfectly well for amps with cathode biased output stages. Mount each zener with appropriate clearance from each other/hot components. Each zener will see about 50mA (tops) in a 5F1, so for 15V zeners that's 15V x 0.005A = 0.075W. 1W zeners would be fine - 5W ones even better.
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Re: 5F1 Champ and European wall voltage

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Re: 5F1 Champ and European wall voltage

Post by martin manning »

Russian 5Y3's are said to be more like a 5AR4/GZ34, probably adding ~20V or more. I don't know about TAD, but JJ is like a vintage 5Y3.

USA line voltages were typically 110-115VAC in the 1950's and 60's. Now they are more like 120-125VAC. Vintage gear running on modern line voltages will result in higher voltages throughout the amp. See below for Tweed Champ voltages. If you adjust preamp voltage, do it with the 22k and keep the 100k plate loads the same. Note the picture you posted with 400V on the plate is not a Tweed Champ.

7k primary on the OT is close, but in the wrong direction if you are running higher voltage. The drawing does say Blackface champ.
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