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 Post subject: Re: Champeg guitar amp build
PostPosted: Nov Wed 29, 2017 10:17 pm 
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Location: Vancouver, WA
westcoastjohn wrote:
This amp has a 5V6 on a variac, and maybe a lowered B+ is affecting the power output.

How about a voltmeter on the B+?
Also it will be interesting if you find a different output transformer to compare waveforms.
Congrats on your success so far.


All power output measurements are being done with the Variac turned up all the way which puts the B+ at 336 Volts.

It would be interesting to see how a different output transformer changes things (if at all). That little experiment will likely have to wait until after the holidays. I will be sure to post the results when I do it.


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 Post subject: Re: Champeg guitar amp build
PostPosted: Jan Mon 29, 2018 8:36 pm 
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Still haven't picked up a larger output transformer. I think that may end up being just the ticket to cleaning up the "fartiness" of this amp when driven hard.

My guitar has a switch that puts the pickups out of phase and a separate volume control for the neck pickup. What I've had to do when driving this amp hard is flip the out-of-phase switch then dial up the neck just a bit to pull some bottom end out of the bridge pickup. It's a super cool "Low-Fi" early garage rock sound but that's pretty much all it can do.

Playing around with it a couple nights ago, I tried putting a 0.0033uF cap in series with the 0.02uF cap feeding the output tube. A switch was wired across the new cap so it could be flipped on and off while playing. There was almost no difference. I was going to try a 0.001uF next but decided to go with a 0.0005. That definitely gave a dramatic bass cut when switch on. Unfortunately, it is not a pleasant sounds. It cleans up the "fart" but just sounds weak and thin without any of the jangly character of the out-of-phase pickups.

After that little experiment, I'm even more anxious to try a bigger output transformer. Should help when the guitar volume is dimed but will it be too clean at lower levels?


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 Post subject: Re: Champeg guitar amp build
PostPosted: Feb Tue 06, 2018 9:01 pm 
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A new output transformer is on it's way. In the mean time, I had a somewhat related side project.

I picked up a junior-size Jackson guitar for the kids to play. It has some powerful humbucker pickups that would absolutely blow away this little champ amp. The guitar volume had to be turned almost all the way down and it had a really muddy sound. I tried a treble bypass cap across the volume pot which helped brighten it up but made the volume control more switch-like.

After messing around with several cap values and configurations I finally settled on the scheme below. Putting the tone pot parallel to the volume pot, with the tone cap between the wipers, allows it to work like a treble bypass when the tone pot is turned up, and a more conventional treble shunt when the tone pot is turned down. The treble would roll off really fast when the pot was turned down so the 330K resistor was added to make the pot more linear. Likewise, the 220K resistor was added to the volume pot to eliminate the switch-like volume increase in the lower range.

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This guitar was kind of a one-trick-pony but now is much more versatile and usable.


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 Post subject: Re: Champeg guitar amp build
PostPosted: Feb Wed 07, 2018 7:08 pm 
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I replaced the ClassicTone 40-18110 output transformer with a ClassicTone 40-18115. Both transformers are rated at 5 Watts but the new transformer is physically larger. I took the time to measure the output of each transformer at multiple B+ voltages. It wasn't a lab quality setup so the numbers aren't exact but it still gives a good picture of the general trend.

Image

The new transformer delivers almost 50% more power with no other changes to the amp. The trace on the oscilloscope looks pretty much the same but now it cuts off [just a bit] before it saturates.

All this was done while the kids were sleeping so I haven't had a chance to plug in a guitar and see how it sounds.


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 Post subject: Re: Champeg guitar amp build
PostPosted: Feb Thu 08, 2018 6:43 pm 
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There is definitely a big difference in sound between the two output transformers.

With the smaller transformer, the stock Jackson guitar was useless. After modifying the guitar wiring it was playable when plugged into the #2 (attenuating) input on the amp. Turning it up at all plugged into input #1 would only give massive distortion. And not a cool musical distortion. It sounded like the amp was broken, it was just a drone with no notes getting through.

With the bigger transformer, I can plug the Jackson into input #1, crank the volume to max, and notes will make it out of the speaker. Yes, heavily distorted notes but it still has a musical quality that could be cool in the right context. Back off the volume and the sound is much fuller than it ever was with the smaller transformer.

I'm not sure why ClassicTone even sells the "tiny" output transformer. Maybe I can use it as a reverb driver in a future project. It's certainly not going back into this amp.


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 Post subject: Re: Champeg guitar amp build
PostPosted: Feb Wed 28, 2018 4:36 am 
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Location: Oblong, IL
Curious, how did you measure your impedence on the output transformer? What did you use to drill the holes for your tube sockets?

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 Post subject: Re: Champeg guitar amp build
PostPosted: Feb Wed 28, 2018 6:16 am 
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Madmurdok wrote:
Curious, how did you measure your impedence on the output transformer?


I hooked a function generator to the secondary and set it to 1 kHz sine wave. With an oscilloscope across the secondary I adjusted the amplitude of the function generator to 1 volt peak-to-peak, then moved the oscilloscope to the primary and measured the peak-to-peak voltage. Take that number and multiply it by itself (square it) to find the impedance ratio. Multiply that by the speaker impedance rating to get the primary load impedance.

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What did you use to drill the holes for your tube sockets?


I used a step drill to make the holes for the tube sockets. It leaves a nasty bur on the backside of the hole. Not a problem if the piece is flat, just flip it over and run the step drill through from the other side to debur the hole. In this case I was drilling the side of the box so I had to clean up the bur by hand with a cold chisel and file.

If you can afford it, a set of knockout punches is the way to go.


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 Post subject: Re: Champeg guitar amp build
PostPosted: Mar Fri 23, 2018 7:35 pm 
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Just wanted to give a quick update and say that I'm still loving this little amp. In the evenings, I use it in the living room where the kids are playing. Turn the Variac way down and it's quiet enough to talk over but still makes a sweet overdrive sound for practicing power chords.

Even when the kids are gone, I hardly ever turn the Variac up over half way. That's roughly 0.75 Watt output! It's loud enough that I feel like a rock star but the neighbors aren't calling the cops.

I'm reminded of my first tube amp, a Sano (not Sanyo) accordian amp that I fixed up and modified for guitar. It was a cathode-bias push-pull pair of 7591's, so probably around 25-30 Watts into a single 12" speaker. When Mom would leave the house I would crank it up to try and get that tube distortion that everyone told me was so great. It physically HURT to be in the same room as that amp and it never really broke up. Being a dumb teenager I just assumed that it was too small to get any good "tube" sounds and pined for a 100 Watt 4x12. :roll:

Let this be a lesson to anyone building a guitar amp. LESS IS MORE! Not just less power but less of everything.

This amp is small so it's easy to take out to the living room. It's easy to setup, just plug it into the wall and plug in a guitar. Turn the Variac knob on the amp to set the appropriate volume then start playing. Turn the "volume" knob on the guitar to set the desired distortion. That's all there is too it. No pedals, no fiddling with knobs for hours on end to find the right sound, and nobody yelling at you to turn it down.

The end result is more time practicing. That, after all, is the real key to better sound.


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