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Old 13th March 2011, 10:13 PM   #1
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Default Fender champ circuit low output.

Hello all this is my first post on here, so a little background. I am no stranger to tube/valve circuits, my main interest is in restoring valve radios TV's and Hi-Fi. But I thaught I would have a bash at building a small single ended Guitar amp mostly from scrap parts.
Looking round on the net I found the schematic for a fender champ and thaught i'd have a bash as it looked a nice simple design for a first attempt, the only changes i have made to the original design is the replacement of the original rectifier with an EZ81 as the transformer I am using has no 5v tap and the fact i am useing a 6BW6 as the output valve instead of a 6V6.
the amp works fantastically with a lovely clear tone which distorts nicely when driven hard. however there is a problem and that is i only seem to be getting about 500mw output out of it, I thought this circuit was good for for about 3 to 5W. anyone any suggestions and comments, all voltages measured concur with the circuit I'm using, and the valves I'm using are all NOS brimmers and test 100% on my valve tester.

Jay
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Old 13th March 2011, 11:57 PM   #2
M Gregg is offline M Gregg  United Kingdom
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Just a thought,

If you have a meter on the HT "B+" Rail and hit a chord do you get sag, i.e. does the B+ drop to a lower voltage which would point to either: not enough current to drive demand or the voltage drop across the rectifier is to high. If B+ is OK just try some diodes in place of the rectifier to see if the output is increased. B+ will increase no volt drop across rectifier! So cap voltages need to have head room!

Are you sure the output is low or is the speaker to hard to drive?

A few things to try!

Regards
M. Gregg
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Last edited by M Gregg; 14th March 2011 at 12:08 AM.
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Old 14th March 2011, 12:24 AM   #3
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Thanks for the reply M Gregg.
No drop in the HT (B+) voltage when a cord is struck. the mains TX and output TX are salvaged from and old radiogram that had a fair old valve line up and should be able to deliver plenty of HT and more than enough LT.
I will try you suggestion of subbing the valve rectifier with a couple of silicone diodes the HT smothers should be fine as they are rated at 450V, HT on the first cap is currently 330V.

the speaker I am intending to use and what most of my testing has been done with is a 10" bass speaker rated at 40w Max, I have tried a 10" driver rated at about 5W but the output was the same level just not as nice sounding as the bass driver.

Jay
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Old 23rd March 2011, 10:11 PM   #4
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Sorry for the late reply only got chance to have a play tonight.
I've subbed the EZ81 with two 1n4007 diodes this has given a rise in HT from 330V up to about 360V and a rise in volume but not by a lot, it's still running at well under 1 Watt output.

Jay
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Old 23rd March 2011, 10:50 PM   #5
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I know that schematics of the champ and its variations are all over the place, but could you post what you built? How about the part number for the output transformer. I agree, it seems like you should be getting more.
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Old 24th March 2011, 12:19 AM   #6
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The circuit I am using is marked 5F2 It's actually a Fender Princeton which as far as i can tell is basicaly the same circuit as the champ, found it here 200 OK
my mains and output TX where salvaged from a scrap radiogram chassis the output TX was matched to an EL84 so 5000 ohm primary which should be fine for the 6v6 or indeed the 6BW6 I am using.

Jay
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Old 27th March 2011, 08:58 PM   #7
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Hi chaps
Sorted it, it was shorted turns in the output transformer, reason I didn't suspect this at first was I had run the chassis up before stripping the radiogram and it was working well. all it has done since then is sit on a shelf in my store room, these things from my experience happen from time to time, like the time i was restoring a leak stereo 20 I had the mains TX rewound only to find one of the two output TX's which had tested fine beforehand failed within a couple of weeks.

Jay
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