Hello,
Been a while since I have posted, but recently I was given 6 NOS 7695 Tubes. I have searched and found several remakes of original designs but I would like help finding a different design other than SE. I would like to find something that is working AB1 - push-pull, uses a safer power supply design and good matching outputs (8ohm).
Don't know what it is about the tubes, but I really want to build a nice amp out of them, maybe because they are oddballs or just look cool.
Thanks for the help with my past projects,
Jim
Been a while since I have posted, but recently I was given 6 NOS 7695 Tubes. I have searched and found several remakes of original designs but I would like help finding a different design other than SE. I would like to find something that is working AB1 - push-pull, uses a safer power supply design and good matching outputs (8ohm).
Don't know what it is about the tubes, but I really want to build a nice amp out of them, maybe because they are oddballs or just look cool.
Thanks for the help with my past projects,
Jim
I have two or three of those and looked at them for the $100 guitar amp project in the Instruments and Amps forum. I discovered that they are rather uncommon and not well stocked at the usual tube sellers, so I decided to use a different tube.
The 7695 has a 50 volt 150 mA heater so you can series wire them with any other 150 mA tube including the 12AX7 which is why I looked at them for a guitar amp. I created a rather unique power supply that used an isolation transformer for line isolation and then ran the heaters in series from half wave rectified, unfiltered 120 volts. This has the same RMS energy as raw AC, but allows one end of the heater string to be grounded. The 120 volts AC from the transformer also fed a voltage doubler for 320 volts of B+. Despite a maximum plate voltage rating of 150 volts the tubes seemed happy on 320 volts.....the screens however do not want to be fed more than 140 volts. I was using an Antek toroid for a low cost OPT, and it worked out to be about 3000 ohms. Output power was 15 watts at 2% distortion and over 20 watts of overdriven guitar.
The deal breaker was not the 7695's, it was the 12AX7's. The amp was made for a "contest" that required the total parts cost to be under $100, with all parts available from a major retail source. I have a big box full of 12AX7's but the retail price of new ones is $12 each. I think the 7695's were $5 for a total tube cost of $34. (2 X 12AX7 and 2 X 7695)
I decided to go with a 100 mA series heater string for more audio power for far less money. I used a pair of 26AQ8/UCC85's for $3 each and a pair of 45B5/UL84's for $3 each. Total tube cost $15. (3 X 26AQ8 and 2 X 45B5). The entire series heater string ran directly off of my DC B+ voltage of 160 volts. Three 26AQ8 provided more gain than a Marshall cranked to 11 and a pair of 45B5's make 25 clean watts on 320 volts and well over 30 watts of overdriven guitar. The 45B5 is a 45 volt 6CW5 and sounds a lot like an EL84, so it turned out to be a good combination for guitar.
The Sylvania data sheet gives the operating conditions for class AB1 push pull.
The 7695 has a 50 volt 150 mA heater so you can series wire them with any other 150 mA tube including the 12AX7 which is why I looked at them for a guitar amp. I created a rather unique power supply that used an isolation transformer for line isolation and then ran the heaters in series from half wave rectified, unfiltered 120 volts. This has the same RMS energy as raw AC, but allows one end of the heater string to be grounded. The 120 volts AC from the transformer also fed a voltage doubler for 320 volts of B+. Despite a maximum plate voltage rating of 150 volts the tubes seemed happy on 320 volts.....the screens however do not want to be fed more than 140 volts. I was using an Antek toroid for a low cost OPT, and it worked out to be about 3000 ohms. Output power was 15 watts at 2% distortion and over 20 watts of overdriven guitar.
The deal breaker was not the 7695's, it was the 12AX7's. The amp was made for a "contest" that required the total parts cost to be under $100, with all parts available from a major retail source. I have a big box full of 12AX7's but the retail price of new ones is $12 each. I think the 7695's were $5 for a total tube cost of $34. (2 X 12AX7 and 2 X 7695)
I decided to go with a 100 mA series heater string for more audio power for far less money. I used a pair of 26AQ8/UCC85's for $3 each and a pair of 45B5/UL84's for $3 each. Total tube cost $15. (3 X 26AQ8 and 2 X 45B5). The entire series heater string ran directly off of my DC B+ voltage of 160 volts. Three 26AQ8 provided more gain than a Marshall cranked to 11 and a pair of 45B5's make 25 clean watts on 320 volts and well over 30 watts of overdriven guitar. The 45B5 is a 45 volt 6CW5 and sounds a lot like an EL84, so it turned out to be a good combination for guitar.
The Sylvania data sheet gives the operating conditions for class AB1 push pull.
Thanks for the information, maybe it's because the tubes are oddballs i'm determined to make an amp out of them. I found a link to one called a Matrix Amp on line thats single ended, not sure if I can use it to go push-pull. I like to tinker and build but not too good at designing new circuits. I'm sure I'd have to switch out the transformers in the Matrix Amp with the addition of two more tubes, and if so I'd rather go with better quality ones like edcor. Even the designer states the original would sound better with different one. If someone would like to help with design I'd be more than happy.
Thanks,
Jim
Thanks,
Jim
Tubes 7695 and 50FE5 has similar characteristic, but different bases. You can see small diy-amplifier with 50FE5 tubes at:
http://www.diyaudio.com/forums/tubes-valves/155832-50fe5-push-pull-stereo.html
http://www.diyaudio.com/forums/tubes-valves/155832-50fe5-push-pull-stereo.html
Hello,
For now I think I'll just build the Matrix Amp at:
http://www.jacmusic.com/techcorner/SBENCH-PAGES/analogengineer/7695mat.pdf
I've found the toroidal transformers listed on Digi-Key, but the VA isn't listed in the schematic, so I may just get big ones (3.5 - 5 va) and see how in sounds. After that I may try using same power supply idea and the data sheet drawing for a push-pull.
If someone else were to look at the schematics and figure out how to add a tone control(s) I would add them to the project. My hearing isn't as good as it used to be.
Thanks for the help so far,
Jim
For now I think I'll just build the Matrix Amp at:
http://www.jacmusic.com/techcorner/SBENCH-PAGES/analogengineer/7695mat.pdf
I've found the toroidal transformers listed on Digi-Key, but the VA isn't listed in the schematic, so I may just get big ones (3.5 - 5 va) and see how in sounds. After that I may try using same power supply idea and the data sheet drawing for a push-pull.
If someone else were to look at the schematics and figure out how to add a tone control(s) I would add them to the project. My hearing isn't as good as it used to be.
Thanks for the help so far,
Jim
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.