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
Last edited:
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
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
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.