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Tweed Champ Question..

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6/27/2000 10:59 PM

Tweed Champ Question..

I just replaced the filter caps in my 5F1 champ and was wondering if they are supposed to have 2 25mfd caps across the cathode resistors. In the schems that I have, there is only one. My amp had two, and it looked like it was original. I put in two new ones to replace the old ones. BTW, it needed new filter caps, but should I have left in the original cath res caps? Some people are weird about everything being original, but the filter caps could not be salvaged. BTW, I like the sound of both of those cathode resistor caps being in there.

6/28/2000 12:19 AM

Are you talking about having 2 cathode bypass caps in parrallel across the output tube cathode resistor or one by pass cap on the second gain stage and one on the output tube cathode. The latter is how most, if not all of the 5F1's came from the factory. There is no by pass cap on the first gain stage on these amps. If you put one on and plug in a humbucker equipped guitar, you'll most likely get some pretty ugly blocking distortion on the second gain stage when the amp is fully cranked.

Peter S

6/28/2000 2:45 AM
Rob	One for the output tube, one for the seccond gain stage. I just don't understand why the schem says there is no cap across the second gain stage cath res.

6/28/2000 3:58 AM
Bruce	Because the 25uF cap shunts all the AC negative feedback voltage off to ground, thus rendering the NFB loop nearly useless. Bruce

6/28/2000 4:02 PM

ooops...I was wrong...Bruce is right, there is no bypass cap on the second gain stage for the reason he mentioned....it would defeat the negative feedback loop. 5F1 only had the bypass cap on the output tube cathode. I've built quite a few amps based on the 5F1 circuit, but I've made some pretty significant changes to the circuit design in an effort to get a different response from the amps. Sometimes I forget where they originated from. I have a model called a K-5 that was derived from the 5F1 circuit. Compaired to a 5F1 it has slightly decreased gain on the first gain stage, slightly increased gain on the second gain stage, and no negative feedback loop on the output stage. These amps use no cathode bypass caps at all. K-5's have a detailed sparkling bouncy clean tone with very full mids and bottom and sound incredible for cranked up distorted tone. They use a single 12" speaker and they are extremely loud for a single ended 6v6 amp, easily as loud as my Ampeg jet, which is a 2-6v6 push pull cathode biased amp.

Peter S

6/28/2000 4:36 PM

Bruce /Mission Amps

Right! No feedback with a reduced preamp output ROCKS!
However, on my Soulkicker Jr. amps, (a hot rodded SE tweed Princeton), I also hold the triode section's 1k5 cathode resistor (bypassed with a 2.2uF to 4.7uF cap) up from ground with a 130 ohm resistor and then apply some NFB voltage into the junction of the 1k5 and 130 resistors, very much like a BF Princeton Reverb and it works great.
This can be made switchable for a looser/tight sound.

Bruce

6/29/2000 3:10 AM
Peter S	Bruce, That's a killer idea......I'm sure it sounds fantastic! The K-5 amp I mentioned in the earlier post uses negative feedback on the first gain stage, but the implementation is much different than yours. I really like the results I have gotten from lowering the preamp gain and removing the negative feed back loop from the output stage. As you said.....it rocks! Peter Schmitt Kimerik Amps

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