Fender champ rebuild

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Spencerleehorton

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Thought I'd record my voltages on this build and look at what's right and what's wrong.

B+1 = 370v
B+2 = 272v
B+3 = 237.4v

6V6 cathode = 15.2v
V1B
12AX7 Pin 1 plate = 156.6v
12AX7 cathode pin 8 = 1.161v to 1.212v (10k + 50k pot for adjustable NFB)
V1A
Plate = 153.5v
Cathode = 1.210v

Will check with what I'm meant to be getting!!
 

Spencerleehorton

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Well judging by what I can find only B+1 is correct!!
B+2 needs to be 325v
B+3 needs to be 275v

All plates and cathode are a little low
Plates on 12AX7 should be 168v
Cathode should be 1.4v
Plate on 6V6 should be 360v and Cathode should be 19v.

And I seem to have a hum with volume turned down?
I changed 22nf but no change, sounds like a GND problem to me?
Have moved output txf around and no change?

Any ideas anyone please?
 

Tubetec

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If you look at the diagram below what you see represented by the dotted line is what the rectifier output would look like without a smoothing capacitor , when you add in the cap all those dips in the waveform are prevented as the capacitor holds the charge . In the case of the unsmoothed DC your multimeter does some kind of averaging therefore you wont get a good idea of what a proper HT supply will look like until the cap is there .
Again refering to the smoothed DC voltage , if you draw 0 current from the rectifier the small fluctuations in the output voltage will dissappear , you'll get a higher DC output voltage which wont represent what you'll see under real world conditions in an amp . Sorry if my explanations were a bit vague , hopefully this is more understandable.

I'll have a better look at your schematic later and we'll see if we can figure out whats causing the large drop at B+2
Upload not possible just now so heres a link to the image ,

https://chselectricity.weebly.com/uploads/1/3/6/9/13696721/5261081_orig.jpg
 

Spencerleehorton

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Ok bud, would appreciate the help on this one as seem to be learning a fair bit which should definitely help on future builds.
With the current limiter in at full volume there does seem to be sag when I hit a big chord and the 60 watt bulb lights up more!!
Without the current limiter the sound sounds much better.
Also using a treble booster really gets some nice but weird distortion!

I did just notice my filter caps are a fair bit higher than what they should be!!
Got a 47uf then 22uf then another 22uf!!
Could this be my voltage problem.?

But this doesnt account for the hum?? Or does it?
 

Tubetec

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A larger cap after the rectifier would tend towards a slightly higher HT and less hum . Single ended amps like this dont benefit from the cancelation of HT hum and noise in the transformer that push pull amps do . PSRR  or power supply rejection ratio might be 20 db or more worse with single ended .

Fender model 5E1 uses a choke after the first filter , looses it an insignifigant enough 15 volts on the ht but will make it a fair bit quieter if done right  .Thats one possible option, Another is adding maybe 5W 150-220 ohm resistor and an extra smoothing cap before B+1 point , you'll loose a small amount of Ht voltage some of which you can recoup by making the first filter cap larger , maybe a 220 uf or more but be sure the voltage rating is respected with a margin to spare  . You'll take back hum a noticable bit  from what it is now but you will still hear it clearly with your ear from the speaker at minmum volume on the amp . You can add further RC smoothing elements to lower noise more and take a hit on your HT voltage . Ive found for  studio usage where your miking up a speaker or DI' ing a small amp like this your less concerned about maximum output wattage or volume , the trade off between power/noise  in that situation is a no brainer.

I'm looking at the DC circuit voltages you listed and to be honest all is fairly good , there an allowable margin on everything . The 6V6 screen grid does seem to be pulling a touch more current than expected , you could swap it out with another valve just to double check.


Single ended amps do distort differently to your usual push pull tube amps , clipping is asymetrical.
But if the thing truely breaks loose when overdriven and starts ringing it could be on the verge of osscilation . Its often down to layout , a stray capacitance ( or proximity of wiring) between two different paths in the circuit causes feedback of an unwanted kind often accompanied by buzzes/honks/hums even in the absence of signal .

Be especially carefull of the path your output transformer secondary /feedback resistor/ grid and associated wiring takes back to earlier stages of the amp .A matter of millimeters in proximity to the wrong wire can have a huge effect on how the amp handles under overload especially

When in doubt refer back to the chopsticking  methodology by PRR ,
 

Spencerleehorton

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Well I though I had the 12ax7 with a couple of wrong connections but it is correct.
I've taken the 68k off the turret board and put onto the input jack and run a short screened cable to pin 2 of 12ax7, but no change to hum.
I was using a 2.2meg pot with a 1meg resistor across it to make it 1meg, changed this to 1meg pot and no change to hum?
When volume is down the hum is at its lowest, volume up full hum gets louder and NFB pot does seem to adjust the hum and volume as well.
Just cant figure out where the hum is coming from?
B+1 is only 312v btw not 370v, so it could be a suspect 6V6 I suppose.
Worth looking at voltages without 6V6 in?
Or perhaps take out the 12ax7 and see if hum changes or goes?
 

Spencerleehorton

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Thought I'd change the 12ax7 for a millard one I had in my stash, when I turned back on I could hear this fizzing and the HV was all over the show and wouldn't drain  off how it normally does, then it reduced then it went back higher after I had turned it off for about a minute, I quickly grabbed my big resistor and attached to chassis drain drained the filter caps, the filter caps then went into minus figures?

Any ideas on this one?
 

tony hunt

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Spencerleehorton said:
Thought I'd change the 12ax7 for a millard one I had in my stash, when I turned back on I could hear this fizzing and the HV was all over the show and wouldn't drain  off how it normally does, then it reduced then it went back higher after I had turned it off for about a minute, I quickly grabbed my big resistor and attached to chassis drain drained the filter caps, the filter caps then went into minus figures?

Any ideas on this one?
Over the last days I have tried to follow your thread in order to make a contribution. I cannot really keep up.

1. Is this "millard" valve from the stash actually known to work as it should?
2. Hum: I cannot see any kind of reference for your filament wiring in the gutshot photo. If you missed that, or cannot follow what I mean, please resaerch here https://www.tropicalfishvintage.com/blog/2019/7/4/on-vacuum-tube-heaters , or at any good source.
3. Do you mean Hum at around 50/60 Hz or a buzz at 100/120 Hz?
4. The wiring suggest to me that you may have a slight high frequency oscillation dragging you voltages down. Watch your lead dress, even in a Champ circuit. There is good advice in the replies so far and in the link above.

Got to walk the dog now, cheers, t.h.
 

Spencerleehorton

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Well I've drawn out the circuit several times and have spotted a few mistakes and also a few things that I'm unsure of.
First of all the main problem I can see is the audio gnd isnt separated  from the power gnd.
I need to remove the link from the 22uf at B+3 to the negative of the 22uf at B+2.
B+3 22uf negative then becomes audio gnd, where I need to attach both cathode and screens on screened leads.
Also I need to use screened leads for the NFB pot and attach them to audio gnd.
Also I need to rearrange the caps and resistors to the 12ax7 so they are all as short as they can be.

I take it when I have done this I need to make a single connection from B+1 negative 47uf to star gnd, where I have only the IEC gnd connected.

Both Jack's I'm using are isolated from chassis, putting the input gnd to screen of lead and putting to audio gnd is no problem.
But the speaker jack, should I use isolated jack or normal jack which its gnd will connect to chassis?

The other issue is the 6.3v heater centre tap need to go to power gnd or direct to star gnd?

Also my 275ac high voltage from power transformer doesnt have a CT and goes to diode bridge, do I connect negative to power gnd or to star gnd?

Sorry so many questions just want to get all this correct.
 

Tubetec

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Heres a link to a very old article by PJ Baxandall about avoiding hum in high gain amps .  page 57 Feb 1947 edition of Wireless World (starts page 55 of the PDF).
It should answer a lot of the questions your asking , certain advice like top cap grid connection wont apply in your case , but how the grounding is done in the floated input socket still holds good .

https://worldradiohistory.com/UK/Wireless-World/40s/Wireless-World-1947-02.pdf

Thats an article I knew of years back that had eluded me  , Id highly recommend it to anyone who has an interest in quietening tube gear .



 

Spencerleehorton

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Thanks for that, I will peruse tonight.

Today I had a little bit of time for the rewire of the circuit.
Swapped a few positions of components so I could shorten the leads to the 12ax7, make sure I had the audio gnd and the power gnd config, as just had everything all connected to power gnd before!!
Screened volume pot leads and NFB leads and connected to audio gnd.
Once I did all the corrections I nervously turned on expecting a big bang, I was greeted with silence, no hum, so I plugged in a guitar and all sounded good.
Only one problem seem to be at full volume there is some sort of oscillation?
Will check all voltages tomorrow.
Any ideas on what the oscillation could be?
 

Spencerleehorton

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This build seems to be going from bad to worse!!
I've now only getting 0.8v on both cathodes of 12ax7 and no output?
Is the no output due to the cathode being so low?
I will try another 12ax7 but I'm a bit scared the output txf is not working now?

Soldering iron tip has packed up so need to wait until delivery of new tip to continue any work!!
 

Tubetec

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Ive had reversed primary on a few guitar amps , generally the moment you throw the standby switch the oscillations go to full volume almost instantly . If its only happenening  or becoming audible when the amp is cranked its a different issue .

https://worldradiohistory.com/UK/Wireless-World/40s/Wireless-World-1949-05.pdf

page 189 (page 37 of the pdf)of this edition of WW contains an article 'When negative feedback isnt negative' by Cathode Ray (M.G.Scroggie) . There is a bit of mathmatics in it but even if you skip through the minutia and just get a flavour of what he's saying it would be very worth while .
 

Spencerleehorton

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Well I'm not getting any output now, so need to check out all B+ voltages and see if the output txf is still good?
Very weird what's going on.
Such a simple build which is giving me the run around!!
 

Spencerleehorton

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Back up and running now and have hum free output.
cathode on 12ax7 are only 1.2v and on 6v6 it's at 14v.
No oscillation either.
Dont know whether I need to up 470R to get to 1k ish?

It's looking like I'm within about 10% of the voltages,  hope that's ok?

Will change back to the other 6V6 and measure again  as the brimar tube is too big to fit into the case!!
 

Spencerleehorton

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one slight note to mention, my voltages were down due to still being plugged into the current limiter!!
plugged into the wall, i get almost perfect voltages everywhere.

case closed.
 
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