Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Leevi on January 18, 2021, 07:08:44 am

Title: Fixed bias 5D3
Post by: Leevi on January 18, 2021, 07:08:44 am

Has anyone built 5D3 with fixed bias?
The PI inverter must probably be changes?



https://el34world.com/charts/Schematics/files/Fender/Fender_deluxe_5d3.pdf


/Leevi

Title: Re: Fixed bias 5D3
Post by: 66Strat on January 18, 2021, 10:53:33 am

The 5D3 PI won't work with fixed bias. The PI from the Ampeg SB12 will work with fixed bias. The SB12 PI is a self balancing paraphrase that is comparable to the 5D3, except that the SB12 has a negative feedback loop. The feedback loop could be eliminated to get even closer to the 5D3 performance characteristics.

https://el34world.com/charts/Schematics/files/Ampeg/Ampeg_sb12_schematic_6l6_ver.pdf (https://el34world.com/charts/Schematics/files/Ampeg/Ampeg_sb12_schematic_6l6_ver.pdf)

Title: Re: Fixed bias 5D3
Post by: 2deaf on January 18, 2021, 12:32:43 pm

Quote from: Leevi on January 18, 2021, 07:08:44 am

Has anyone built 5D3 with fixed bias?
The PI inverter must probably be changes?

The PI itself doesn't need to change.  There would just need to be another set of blocking capacitors before the 6V6 grid leaks.

Title: Re: Fixed bias 5D3
Post by: Leevi on January 18, 2021, 01:14:28 pm

Quote

The PI itself doesn't need to change.  There would just need to be another set of blocking capacitors before the 6V6 grid leaks.

Would the NFB work with the current PI?


/Leevi

Title: Re: Fixed bias 5D3
Post by: 2deaf on January 18, 2021, 01:37:51 pm

Quote from: Leevi on January 18, 2021, 01:14:28 pm

Would the NFB work with the current PI?

NFB could be added by elevating the 1500/25uF cathode of the inverting stage and running a resistor from the speaker jack to the elevating resistor.

Title: Re: Fixed bias 5D3
Post by: Leevi on January 20, 2021, 12:58:58 am

Quote

The PI itself doesn't need to change.  There would just need to be another set of blocking capacitors before the 6V6 grid leaks.


NFB could be added by elevating the 1500/25uF cathode of the inverting stage and running a resistor from the speaker jack to the elevating resistor.

Now the first tests have been done and both changes work excellent.
I used 100n capacitors to separate the inverter part and 820R for NFB. I also replaced the original 50n caps with 100n.
Thank you for the tips.


/Leevi