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 Post subject: 5F1 Build Advice
PostPosted: Mon Jan 07, 2019 8:31 am 
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Recently my nephew and I began building a multi modifications 5F1 amplifier. Some of the mods include a switchable bypass cap on V1. As of now the values we’ve chosen are .68uf, 10uf & the stock 25uf cap. We also changed the 100K plate resistors to 220K and the .022uf coupling caps were lowered to .01uf. There are 2 additional switches. One to disconnect the negative feedback and another to switch in a conjunctive filter across the O.T.’s primary. For the filter values we chose a 10KΩ 5W resistor and a series .047uf cap and placed them across the 8K primary of the O.T.

After initial testing we both decided to return the plate resistors and coupling caps to their stock values. The switched bypass cap mod did all the tone shaping we wanted so lowering the values of the coupling caps didn’t seem necessary. We both liked the additional dirt from the 220K plate resistors but we both agreed that having a little more clean tone available would be the best way to go.

A note on the build: Once we built the amp we did some math and found that the 6V6 plate dissipation was close to 18W. According to my info the JJ 6V6S we’re using is spec’d at 14W. To lower the dissipation we raised the 470R cathode resistor to 606Ω using a 1KΩ 5W wire-wound pot.

Now for my advice question. Vince wanted to know if we could use a different power tube. Since we only built this to be a 5W amp I thought that maybe we could replace the 6V6 we’re using with an EL84? I figure it’s do-able but I have limited knowledge of tube circuits and have no idea how I would go about making the necessary changes. I’d appreciate any input you guys have to offer.

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 Post subject: Re: 5F1 Build Advice
PostPosted: Mon Jan 07, 2019 9:04 am 
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Location: The Last Place You Look
https://www.yellowjacketstc.com

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 Post subject: Re: 5F1 Build Advice
PostPosted: Mon Jan 07, 2019 9:56 am 
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No go. Won’t fit under the hood of the head cabinet.

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 Post subject: Re: 5F1 Build Advice
PostPosted: Mon Jan 07, 2019 10:14 am 
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Stephen wrote:
No go. Won’t fit under the hood of the head cabinet.


Pull the chassis out of the head cabinet.

Give it a test run with the yellow jacket.

If you like it configured that way, do the math and install a nine pin socket in the chassis for the EL 84.

You're going to find that the necessary voltages are a little off but should be in the ballpark.

The yellow jacket should not be viewed as a permanent solution in any case.

I always use them to try things out and then do a permanent change over if I like the sound.

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 Post subject: Re: 5F1 Build Advice
PostPosted: Mon Jan 07, 2019 10:52 am 
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Scooter, would 6K6's be an option for this amp? I'd think that the plate voltage would have to be lowered some and the cathode resistor changed, since this type is only rated for 8.5W of plate dissipation.

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 Post subject: Re: 5F1 Build Advice
PostPosted: Thu Apr 11, 2019 10:22 am 
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Here are some pix of the modified Champ my nephew Vince and I built a while back. It turned out really well and sounds HUGE thru an Eminence GA-SC64 open back 1x12 cab. The switches in back are from left to right…

Bypass Cap selector: Allows a choice between 0.68uf, 10uf and 25uf. I plan to replace the 10uf with a lower value. Probably somewhere between 1uf and 4.7uf. Too little difference between 10uf and 25uf for taste.

Feedback ON/OFF: Allows you to disconnect the negative FB loop. Considered adding a pot but decided against it.

Filter ON/OFF: Allows the addition of a conjunctive filter across the O.T. primary. We’ve been tweaking component values for this for a while now. We finally settled on a 5.6K/5W resistor in series with a .015uf Orange Drop.

OHMS: Selects between a 4Ω or 8Ω load. Placed the switch inside the cage for protection.

We recently added a 1uf cap across the 2nd cathode bypass resistor. Going to have to look into maybe placing it on a switch too.
Attachment:
IMG_0903.jpg
IMG_0903.jpg [ 141.28 KiB | Viewed 125 times ]

Attachment:
IMG_0908.jpg
IMG_0908.jpg [ 204.68 KiB | Viewed 125 times ]

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 Post subject: Re: 5F1 Build Advice
PostPosted: Thu Apr 11, 2019 1:18 pm 
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Stephen wrote:
Here are some pix of the modified Champ my nephew Vince and I built a while back. It turned out really well and sounds HUGE thru an Eminence GA-SC64 open back 1x12 cab. The switches in back are from left to right…

Looks great! Very professional job.

Stephen wrote:
Bypass Cap selector: Allows a choice between 0.68uf, 10uf and 25uf. I plan to replace the 10uf with a lower value. Probably somewhere between 1uf and 4.7uf. Too little difference between 10uf and 25uf for taste.

You may find Ampbook's online cathode bypass capacitor calculator useful for fine tuning the frequency where the gain rolloff occurs for various cap values: https://www.ampbooks.com/mobile/amplifi ... capacitor/

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 Post subject: Re: 5F1 Build Advice
PostPosted: Thu Apr 11, 2019 4:10 pm 
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Thanks for the calculator link. We’ve decided the next time we pop the hood we’ll replace the 10uf bypass cap with a 2.2uf instead.

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 Post subject: Re: 5F1 Build Advice
PostPosted: Thu Apr 11, 2019 7:34 pm 
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Thumbs up on the build, looks great! Don't forget to post a gut shot when you go in to swap that bypass cap (you know the rules).

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 Post subject: Re: 5F1 Build Advice
PostPosted: Fri Apr 12, 2019 9:42 am 
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As requested…
Attachment:
ChampGuts.jpg
ChampGuts.jpg [ 275.05 KiB | Viewed 90 times ]

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 Post subject: Re: 5F1 Build Advice
PostPosted: Fri Apr 12, 2019 9:54 am 
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Neat as a pin! :mrgreen:

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 Post subject: Re: 5F1 Build Advice
PostPosted: Fri Apr 12, 2019 10:59 am 
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Looks great, Stephen! Some nice inspiration as I look forward to my first amp build.

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 Post subject: Re: 5F1 Build Advice
PostPosted: Fri Apr 12, 2019 1:10 pm 
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You haven’t built an amp yet? Good lord man! What are you waiting for? That very first time you throw the switch and release those hundreds of volts and nothing goes up in smoke is very exhilarating. Firing up your very first pedal build doesn’t even come close. It’s satisfying… but the rush just isn’t there.

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 Post subject: Re: 5F1 Build Advice
PostPosted: Fri Apr 12, 2019 1:44 pm 
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Stephen wrote:
That very first time you throw the switch and release those hundreds of volts and nothing goes up in smoke is very exhilarating.

And if the smoke does come out, that's exciting, too, but in a very different way.... :wink:

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 Post subject: Re: 5F1 Build Advice
PostPosted: Sat Apr 13, 2019 4:28 am 
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Yikes! I can do without that kind of excitement in my life.

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 Post subject: Re: 5F1 Build Advice
PostPosted: Sat Apr 13, 2019 8:00 am 
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Yeah, don't jinx me, Bob! :mrgreen:

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