Fender champ rebuild

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Spencerleehorton

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Hi all,

I have previously built a fender champ but it was a while ago and it's been dismantled and all the parts scattered!!

I've managed to find the PT I used from a broken line 6 spider but the power transformer was fine.

It has the following taps
195v
60v
25v
100v
6.5v
6.5v

I had used this PT but cant remember how I had done it.
Now looking at the champ PT specs it needs 650v, 5v and 6.3v

By the looks of this txf the most i can get out of this txf is 380v, 6.5v and 6.5.

I didnt use the rectifier tube before and used a diode bridge.
So if I'm correct if I put the 195+60+25 that will give me 280v ac, with bridge diode should then give me 280 x 1.2 = 336v dc.
Then I would need to adjust the first two resistors to get 325v and 255v.

Cant for the life of me remember how to calculate but will look through my notes, if any could pitch in please?
 

dmp

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650v is much to high for a fender champ.
If you look at the schematic for a champ it is 325v with a center tap. Yes, that measures 650 across both but it is spec'd as a 325v winding. If you have a 195v with a center tap you could run it in the same kind of circuit and get a B+ of about 195v
Are you sure the 60v and 25v are separate windings and not just different taps off the same winding?
You're not going to be able to use a rectifier that needs a 5v winding with that transformer, but you could switch to a recitifer that runs on a 6.3v winding. Some people have used resistors to drop the 6.3v down to 5v for the rectifier also. Or a diode rectifier.

You calculate the resistors with ohm's law with the current draw of the tube stages. 

 

Spencerleehorton

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Well I'm pretty sure they are all taps off the same winding but I did try out a few and got 250vac with connecting up the 195, 60 and 25, the 100v tap then became 250v?
This doesnt seem quite right so I'm going to try connecting the 25, 60 and 100 and see what the 195 tap becomes!
Hopefully one combination gives me 270+ ac then I can make a diode bridge and that should then give me what I need, then one of the 6.5vac I can use for the heaters
 

CJ

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DC is usually around 1.4 times AC, so 100 AC would be 140 filtered DC.

i would worry about breakdown insulation on those lower voltage windings,  so it might be unsafe to series them.

however, you could take the 100 volt tap and put it thru a voltage doubler to get decent 6v6 plate voltage. and be sure to use NOS Tungsol 6V6, :D
 

Spencerleehorton

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Well as I said earlier this PT was used before to power this circuit and seemed to be fine, but now I've taken it all apart and several years have passed I've forgot how I had it configured.
Need to really find the spec sheet for the line 6 spider amp to be sure on power and how that was wired to be sure.

This was the head

https://uk.line6.com/legacy/spidervalvehd100/
 

peterc

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Spencer

In your first post, you mentioned 650v? I am assuming that was meant to be 350v?

It might be worth looking at Uncle Dougs Champ vids, there is some excellent content in them.

https://www.youtube.com/user/Stratosaurus1/search?query=champ

Terry from D-lab also as come great content

https://www.youtube.com/user/Stratosaurus1/search?query=champ

Hope that helps.
Peter
 

Spencerleehorton

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Yes, I love the uncle Doug vids, subscribed and slowly getting through the relevant vids.
I did mean centering tapped 650v, 2 x 325v.
I am hopefully getting a spec sheet on this line 6 PT which I suspect I can get a good 270vac out of and diode bridge it to give me what I need.
Alternatively I'm open to other PT for this build and maybe use this line 6 PT in another build.
It looks like the line 6 PT has 3 main windings on it, a HV wiring which is CT and two other CT windings, just need to work out what they are and what power they each are.
 

Spencerleehorton

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Might need a little refresher on working out resistor drop and power of resistors as I think I'm a bit off?

Correct me if I'm wrong please.

If I have 300vac at diode bridge that would then give me 360vdc after?
Then to drop 55v at 2mA = 27k ish for the 306v?
Then drop 51v at 2mA = 25k ish for the 255v?

Both need to be minimum of 2W?
 

Lee_M

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There power transformers look fairly decent for smaller amps, and the price is very nice.

160mA @ 270VAC, along with 3A @ 6.3VAC for heaters. Should be enough juice for a Champ.

https://www.ampmaker.com/shop/0-190-275v-20w-power-transformer/
 

Spencerleehorton

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But CJ says to use x1.4 rather than x 1.2 for the diode bridge?

I've always used x 1.2v as its 0.6v per diode and 2 diodes per half of waveform?

If x 1.4v is correct than I should be fine with what I've got as 250vac x 1.4v = 350vdc.

Also could someone confirm my workings, I've worked out the current through the 10k resistor is 3.5mA and the 22k is 3.4mA, is this correct?
 

Spencerleehorton

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Also my power workings out

35v drop on 10k
35 x 0.0035ma = 0.1225, 122.5mA

Double mA for power of resistor = 245mA which would only be 1/4 watt???

I thought it was meant to be 1W and then double to 2W for these first two resistors and the 470R resistor to be 5W?
 

Spencerleehorton

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Right, I've made some headway on this PT, by looking within the windings I can see how they are all paired.
There are 4 secondary windings and I have tested they are all separate.
Winding 1 is 370vac with centre tap
Winding 2 is 60vac
Winding 3 is 20vac with centre tap
Winding 4 is 6.7vac but in 4 leads?
1+3 = 6.71v
1+4 = 6.73v
2+4 = 6.72v
2+3 = 6.71v
When I test from 1 to 2 I get no voltage but it tests with continuity?
 

Spencerleehorton

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Well now I'm quietly confident that I can get 270vac from this PT with 6.3vac for heaters.
185+60+20=265vac

270 x 1.4 = 378vdc

That should give me some power

6.7vac is well within 10%.

What current does the B+ need as I'm pretty sure the 370, 60 and 20v windings are all at least 100ma.
Pretty sure the 6.7v windings are at least 2A.

I'll order a proper txf for this next month
 

Spencerleehorton

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Having a bit of a nightmare with this line 6 PT, when I measure in ac on the MM I can connect up the 185v + 60v + 20v windings and get 265vac.
But when I then connect that to a diode bridge, with it off I get a weird 0.546v on DC then when I turn it on it's so much lower than 265v, more like 240vdc?
Anyone know what's going on?
 

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