Hello...
I built my very first Fender 5F1 champ with help from you guys on this forum. Thank you so much for that! Now I'd like to pursue something of an issue with my build.
First off, I inadvertently bought a Fender Champion 600 reissue that had a lot of issues, including a previously blown OT that the previous owner didn't bother to tell me about. I ended up gutting the amp and building a 5F1 from scratch in the chassis. The build is pretty much to the letter of the schematic except that the 12ax7 stages have the cathode bypass capacitors (20u and 10u, respectively) to give me a little better bass response. The Output Transformer is a Revisit TF22905 output transformer which seems to be the correct replacement transformer for the original reissue OT.
My only issue is that my amp seems to distort very very early on the gain pot. On 3, I am already at a very hot and very loud distortion. From there and onward on the gain, the amp pretty much just goes wild on how much distortion it has. It's not something that is inherently "bad" on this amp, but I would like to get a little more clean headroom or at least have the amp distort say on 4-5 instead on 2-3. I guess I'm just not hearing what I was expecting from a 5F1 circuit.
I am using a 680R 5W resistor at the cathode of the 6v6 to give me about 11.5W of plate dissipation which is right about at the edge of what a 6v6 should dissipate. I understand that having it biased that hot means that I would have the highest amount of headroom possible without burning up the tube.
I'm looking for any advice anyone has to help me tame a little bit of the gain I have on tap without going to a lower gain preamp tube. I'm posting a schematic of my build here. Thank you so much in advance!
Fender 5F1 Champ Distorts too early
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Johnny_March
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Fender 5F1 Champ Distorts too early
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Last edited by Johnny_March on Sun Jun 13, 2021 4:52 pm, edited 1 time in total.
Re: Fender 5F1 Champ Distorts too early
Cathode bypass caps will give you more gain and therefore more distortion. Try removing one or both and see how you like it. Lower value caps will give you less output. Maybe try .47 or .68uf on the second stage or leave it unbypassed. You could also play with the 22k negative feedback resistor to get a bit more clean. Wire up a 20k or 50k pot and see where you like it, then put a fixed resistor in its place, or a couple on a switch.
Re: Fender 5F1 Champ Distorts too early
Did you use an audio taper pot for the volume?
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martin manning
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Re: Fender 5F1 Champ Distorts too early
Is the feedback loop connected properly? Looks to me like you should ground the yellow OT secondary lead if the red primary lead is connected to B+.
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Stevem
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Re: Fender 5F1 Champ Distorts too early
Please double check what you posted for the output tube cathode resistor being 680K, as 680 ohms is more like it.
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Re: Fender 5F1 Champ Distorts too early
Yes, turn it back to stock and start from there.
Shouldn't that be 470?
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Johnny_March
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Re: Fender 5F1 Champ Distorts too early
Sorry about the typo from earlier. The cathode resistor on the 6v6 is in fact a 680R not 680K. The annotated schematic I included on this post does show 680.
As far as the negative feedback, It is a 22k just like the original spec on the 5F1 schematic. One side of the secondary is grounded and the other side to the speaker hot.
Aside from being solid-state rectified instead of tube, and the two cathode bypass caps, the circuit is pretty much bone-stock.
The value of the 6v6 cathode resistor was changed from 470 to 680 because the plate dissipation was around 13.5W with a 470 in place. This would exceed the recommended maximum dissipation for a 6v6. The 680 ohm value gives me about 11.5W which is within spec but still biased pretty hot.
As far as the negative feedback, It is a 22k just like the original spec on the 5F1 schematic. One side of the secondary is grounded and the other side to the speaker hot.
Aside from being solid-state rectified instead of tube, and the two cathode bypass caps, the circuit is pretty much bone-stock.
The value of the 6v6 cathode resistor was changed from 470 to 680 because the plate dissipation was around 13.5W with a 470 in place. This would exceed the recommended maximum dissipation for a 6v6. The 680 ohm value gives me about 11.5W which is within spec but still biased pretty hot.
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Johnny_March
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Johnny_March
- Posts: 46
- Joined: Sun Feb 28, 2021 6:19 pm
Re: Fender 5F1 Champ Distorts too early
I tried removing the 10uF at the second stage of the 12ax7 and the gain seems perfect now!! Thank you for the advice. I left the 20uF in the first stage, but with the second cathode bypass removed, the gain is right where it needs to be. The sweep is so much more gradual.
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Stevem
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Re: Fender 5F1 Champ Distorts too early
If you built it without the tube rectifier then you V+ is some 25 volts too high and that is increasing the gain level a lot also!
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Re: Fender 5F1 Champ Distorts too early
The 10uf you removed wasn't just adding more gain on that triode stage, it was also allowing the negative feedback signal to go straight to ground. Removing it lets the negative feedback work correctly.Johnny_March wrote: ↑Sun Jun 13, 2021 5:51 pm I tried removing the 10uF at the second stage of the 12ax7 and the gain seems perfect now!! Thank you for the advice. I left the 20uF in the first stage, but with the second cathode bypass removed, the gain is right where it needs to be. The sweep is so much more gradual.
To elaborate on that a bit: by using a much smaller cap and a pot instead of the 10uf, you would have a presence control, which works by allowing the high frequency portion of the negative feedback signal to go straight to ground.
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Stevem
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Re: Fender 5F1 Champ Distorts too early
You can have a cap there if you want, just go and look how it’s done on a black or silver face Champ, or the vibrio champ models.
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Re: Fender 5F1 Champ Distorts too early
Sure, you can put caps a lot of places if you want to.
The point remains: the 10uf cap drawn into the schematic in the first post prevents the negative feedback from working.
The point remains: the 10uf cap drawn into the schematic in the first post prevents the negative feedback from working.
Re: Fender 5F1 Champ Distorts too early
Good catch nworbetan, I have made that mistake more then once and scratched my head as to why the neg feedback doesn't work very well. I think Stevem just wanted to point out partial bypass as another option.
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Johnny_March
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Re: Fender 5F1 Champ Distorts too early
I had originally placed that 10uF cap because I thought it would help with bass response. It would have done so if that wasn't where the negative feedback was coming in. Either way, the 20uF in the first stage did more than enough for the bass response. I may even substitute the 25uF in the first stage with the 10uF I took out from the second stage. I think it's almost too bassy as is right now.
Great discussion!
Great discussion!