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PostPosted: Wed Jun 29, 2022 3:53 am 
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I build a 15 watt Champ based on the 5F1 circuit. It's using a 6L6 power tube into a 12" Celestion 75Eighty speaker. I am using a Telecaster with single coils.

At a low talking volume, the amp already has a lot of distortion and the bass is muddy. I need to use an EQ to drop the bass 5db.

I have a silverface Champ and had a clone of a tweet amp. Those were both 5 watts and sounded great. Can someone suggest some possible issues to me or things I should check.


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PostPosted: Wed Jun 29, 2022 10:03 am 
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Are you using a 15w output transformer?

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PostPosted: Wed Jun 29, 2022 12:51 pm 
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byoc wrote:
Are you using a 15w output transformer?


These are my transformers. The output transformer should be good enough to output 15w on a Champ.

Output transformer is Classic Tone 40-18031

The main transformer is a Classic Tone 40-18028

As a test, can I run this setup with a 6V6 power tube?


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PostPosted: Wed Jun 29, 2022 2:11 pm 
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sherwin wrote:
byoc wrote:
Are you using a 15w output transformer?


These are my transformers. The output transformer should be good enough to output 15w on a Champ.

Output transformer is Classic Tone 40-18031

The main transformer is a Classic Tone 40-18028

As a test, can I run this setup with a 6V6 power tube?


That is a 15w OT. Yes, you can use a 6V6 interchangeably.

Is this from a kit, or is this your own layout?

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PostPosted: Wed Jun 29, 2022 2:41 pm 
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byoc wrote:
Is this from a kit, or is this your own layout?

I got this as a kit from BYOC and followed their directions. I've visually rechecked the wiring, but admittedly, it's very messy. I may go back to clean this up.

I'll test a 6V6 and as a data point soon.

Are there points I should check the voltage?


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PostPosted: Wed Jun 29, 2022 3:56 pm 
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sherwin wrote:
Are there points I should check the voltage?

I would start by checking the DC voltage levels at the + side of each of the three large filter caps. Refer to the schematic for the nominal voltages that you should see.

If those look good, I'd check the plate voltages on the output tube (pin 3) and both triodes of the preamp tube (pins 1 and 6).

As always, we caution amp builders to be extremely careful when measuring these potentially lethal DC voltages and keep one hand in a pants pocket.

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PostPosted: Thu Jun 30, 2022 1:12 am 
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I tested with a 6V6 and it behaves much better. I am only able to crank it to 1/2 volume since it's midnight. Input 2 has good tone all around. The bass volume on input 1 is still too high, even with my single coil Tele bridge pickup.

So I see two possibly distinct issues. One, bass volume is too high. Two, why does a 6L6 cause so much distortion? I am guessing the distortion is due to the output transformer not being able to handle the signal from the 6L6.

I'll check the voltages later this week and I'll double check my wiring for the output transformer, esp anything related to 5 watts vs 15 watts.


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PostPosted: Thu Jun 30, 2022 9:59 am 
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sherwin wrote:
I tested with a 6V6 and it behaves much better. I am only able to crank it to 1/2 volume since it's midnight. Input 2 has good tone all around. The bass volume on input 1 is still too high, even with my single coil Tele bridge pickup.

So I see two possibly distinct issues. One, bass volume is too high. Two, why does a 6L6 cause so much distortion? I am guessing the distortion is due to the output transformer not being able to handle the signal from the 6L6.

I'll check the voltages later this week and I'll double check my wiring for the output transformer, esp anything related to 5 watts vs 15 watts.


You've probably got some other problem and the 6L6 only exacerbates it. Please take voltage readings and post pics of your build as well.

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PostPosted: Sun Jul 31, 2022 6:14 pm 
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Time flew by quick. I finally got back to working in this amp.

The readings for the Filter Caps are as follows.

C3 - 355V
C4 - 315V
C5 - 270V


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PostPosted: Sun Jul 31, 2022 8:53 pm 
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I found this circuit diagram with voltages for a 5F1 with 6V6. Just a reminder that I am running a 6L6.

I highlighted the voltages I measured and wrote my amp's voltages with a red color pencil.


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PostPosted: Mon Aug 01, 2022 11:32 am 
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Something I overlooked in your previous posts is that you mentioned there is a difference between inputs 1 and 2. Input 2 has a lower input impedance than input 1. You could try tacking a 68k resistor between the lug where the two 68k resistors meet and any ground point. This would give both inputs the same input impedance.

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PostPosted: Mon Aug 01, 2022 2:14 pm 
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sherwin wrote:
I found this circuit diagram with voltages for a 5F1 with 6V6. Just a reminder that I am running a 6L6.

I highlighted the voltages I measured and wrote my amp's voltages with a red color pencil.

I will defer to the more learned opinion of Keith on those voltages, but they sure look very reasonable to me.

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PostPosted: Mon Aug 01, 2022 6:22 pm 
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It looks like you are using a 470 ohm cathode resistor in the output section. That is typically used for a 6V6. The 6L6 bias is likely pretty far from optimum.

Try measuring the 6L6 bias: https://robrobinette.com/Tube_Bias_Calculator.htm

You'll need to measure DC plate voltage between 6L6 pins 3 and 8 [black lead on 8], voltage pin 3 to ground, and measure the ohms of the actual cathode resistor.

I think that most single ended 6L6 amps want something like a 300-ohm cathode resistor.

Here's another way:

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Last edited by sjaustin on Mon Aug 01, 2022 6:37 pm, edited 1 time in total.
Removed parentheses to get rid of sunglasses emoji caused by 8 + )


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PostPosted: Mon Aug 01, 2022 10:50 pm 
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The bias voltage is currently 25.3V and the diagram shows the expected value is 19V. Changing the cathode resistor to 300 Ohm should reduce the bias voltage down to the expected 19V.

I'll find a ~830 Ohm resistor and solder it parallel to the 430 to make 300. This way, I can easy remove it should I decide to go with a 6V6.

Won't this effectively give me less headroom and distort the signal sooner.


Regarding tying the two input jack together, I think I'll just use input 2 for now. Maybe I will also filter out some of the bass frequencies as suggested.

I am guessing right now that the circuit I followed may need some minor modifications to fully support 15 watts from a 6L6GT. Based on the instructions I followed, the only changes made for 15 watts were some wiring changes to accommodate the higher power transforms. None of the resistor or capacitor vlaues were changed.

I'll try to get a a signal generator and scope on there later this week and see what's going on. But it kinda of feels like I plugged a bass into a Fender champ and cranked the volume to 11. It may just be too much low end signal.

I really appreciate all the help and am glad I ran into a problem with my first build. I've learned a lot more diagnosing this issue than building the amp itself.


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PostPosted: Tue Aug 02, 2022 9:31 am 
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sherwin wrote:
The bias voltage is currently 25.3V and the diagram shows the expected value is 19V. Changing the cathode resistor to 300 Ohm should reduce the bias voltage down to the expected 19V.

I'll find a ~830 Ohm resistor and solder it parallel to the 430 to make 300. This way, I can easy remove it should I decide to go with a 6V6.

Be a little careful with comparing what we refer to as bias (how much current a tube is drawing) against a listed bias voltage. They are not always the same thing. And the voltages listed on that schematic are for a 6V6, not a 6L6. Also, 'they say' that 6L6's typically want to see a higher plate voltage than a 6V6. There could be something to that that is causing the low headroom.

sherwin wrote:
Won't this effectively give me less headroom and distort the signal sooner.

It depends on what your bias is now. If the bias is way out of spec, then than could cause low headroom, and adjusting the bias to the recommended ranges would increase performance.

sherwin wrote:
I really appreciate all the help and am glad I ran into a problem with my first build. I've learned a lot more diagnosing this issue than building the amp itself.

It is a great community! :D

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PostPosted: Tue Aug 02, 2022 3:37 pm 
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One thing just occured to me. I wired the amp speaker output as 8 Ohms, but the speaker cabinets I tested with were both 16 Ohms. I am not sure if that'll make any difference. I'll test with an 8 Ohm cabinet and report back.


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PostPosted: Thu Aug 04, 2022 10:29 pm 
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I got a scope and was able to look at the signal waveform.
Everything looks good after the 1st preamp gain stage. I put the scope on output tab of the Volume pot and measure the open D and open E string signals. They both amplified linearly. However at the output of the 2nd preamp gain stage, the Low E string signal was amplifed twice as much as the E string with the volume at noon.

I tries the amp with a 8 Ohm speaker cabinet and it didn't help.

At this point, I suspect this is a negative feeback issue. I'll diagnose more later.


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PostPosted: Sat Aug 06, 2022 7:00 pm 
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After this exercise, I think everything is working as expected. I did modify the output ohm to match the speaker. I rerouted the wire to be much cleaner, esp the signal path. I trim a lot of wires to shorten them as much as practical. This actually helped a lot.

What I notice at max volume with 6V6, the distortion gets too high and chords sound really aweful to my ears. With the 6L6, it gets to this point so much sooner, that there's no point turning the volume beyond noon. I think I'll stick with 6V6 for this setup.

I'll try to get a recording laster, but this has been very educational.


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