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Old 8th April 2013, 08:20 AM   #11
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Are you guys talking about the 5F1 Champ? If so, then why do you need such large output transformer (15W)? 2W might be a bit small, but 5W would do job for a single 6V6, no? Anyway, since you are not sure how the NFB will effect the tone, you can add a "presence" control in parallel with the cathode resistor, that way you can make the feedback adjustable. IMO, that will have a greater effect than changing the cathode bypass capacitors and/or the cathode resistors. Of course, as mentioned earlier, the speaker will have the greatest effect of all.

Jaz
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Old 8th April 2013, 06:49 PM   #12
shanx is offline shanx  Canada
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Here is a mod that has been done on the Princeton model, gives some insight on the NFB. Follow this link for mods
GAA's Tweed Princeton Clone
In following what I suggested before, for the variable pot that is added on the NFB, maybe try a cap wired across the wiper and left side of the pot. As wiper is turned more clockwise with increasing resistance to open up the gain loop, but high frequency is bypassing that added resistance and should roll off the highs through NFB. Could do with simply switched in resistance/capacitance once test is done.
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Old 9th April 2013, 09:44 PM   #13
Nicho is offline Nicho  United Kingdom
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I really have very little idea what I am talking about. I am hoping to make a champish creation at some point as my first amp build. With that in mind, what about making the 220k resistor before the power amp into a pot and putting a cap to ground on the wiper to bleed of the desired amount of treble?
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Old 9th April 2013, 09:56 PM   #14
Nicho is offline Nicho  United Kingdom
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Or I suppose that should be part of the 220k resistor so the previous tube is not outputting to a high frequency open circuit to ground
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Old 10th April 2013, 08:06 AM   #15
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Hi everebody, thanks for your input.
@jazbo8 - I got the big OT because it was the smallest one I could find with a 16 ohm tap.
Part of the reason i thought of using a tap switch for nf is the relatively small footprint and repeatable settings. Also, I've heard a champ with switching for less nf, and that wasn't for me. What I'm uncertain of is wheter or not more nf will be better or if the stock value is perfect.
Speaker will be a g12m as and I expect it will take this amp closer to a marshally sound?
The changes Iíll make with the parts that Iíve got now are: Tap switch for nf loop, 1.8k chathode resistor with 1uF (first stage); 0.01uF coupling, 1k resistor and a 330pF (I think) plate bypass (second stage), 0.022 coupling.
How does this sound to you guys?
What Iím really anxious about is the plate bypass. Iím not quite up to the task of calculating the outcome. Will this roll off the excessive highend (and play nice with the circuit in general)?
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Old 11th April 2013, 08:08 AM   #16
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Mine still doesn't have tone controls yet, but I find a master volume more useful. For tone controls, I want a variable treble-cut very late in the circuit, after any distortion from the output power tube, so I'll either put some simple network at the power transformer primary or work up something with an L-pad like a crossover on the speaker side of the output transformer. Usually you put such bandwidth-limiting before the transformer, so the transformer can enjoy the decreased power and bandwidth, but my output transformer is kind of oversized anyway. Could also have 2 tone controls in the feedback loop, if I make it like GAA's Tweed Princeton Clone that Shanx linked to (thanks dude).

Last edited by cyclecamper; 11th April 2013 at 08:10 AM.
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Old 11th April 2013, 01:57 PM   #17
shanx is offline shanx  Canada
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Happy to offer some suggestions, cyclecamper. Good to look at alternatives and maybe come up with some interesting ideas.
For the tap switched NFB, I am a bit leery of opening up the NFB even for a moment as it switches from one to the other. Although this may not be a problem if you do it while in standby.
This is a lower gain amp so may not be an issue, but if this were a high power AB amp I would suggest just variable NFB or switching in/out a paralleled resistor to keep the loop stable.
The rest of your mod values look very reasonable.
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Old 13th April 2013, 07:41 AM   #18
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Thanks, Iīll see if i canīt start assembling sometime during next week. I was planning on putting the tap switch underneath the chassis and switching when the amp is off. I'll try the different nf taps and probably end up liking one more an leaving it there most of the time. After a while (if it stays in on the same tap all the time) I might consider removing it and putting something like a presence control (or a variable nf pot if I feel two of the tap settings are worth keeping) on there instead.
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