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
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
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
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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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
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
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
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
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.
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
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
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
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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