Type 47 Tube Ultra Linear Input SE Power Amplifier
This is actually a follow up on this post: Adjustable Ultra Linear Line Stage. Here we see a slightly different variation of the first post, namely the cathode follower is now moved to the top of the small signal pentode and arriving at some kind of mu-follower circuit.
This CF will act as constant current load to the pentode and will allow us to achieve high gain. On Spice simulation, with 6K7 as shown, the open loop gain for the input stage is about 75x (15Vpp swing with 200mVpp input). This gain level is perhaps excessive so how do we tame it? As shown, the input stage is on pentode-mode as the screen grid is AC-grounded by C2 via the lower 6N8S cathode follower. We can reduce the gain by changing R16 to a 100k trimpot and connect C2 to the trimpot wiper. Adjusting the wiper, we can vary the UL feedback from 0% (full pentode mode, achieved when the wiper is at the screen grid side) to 50% feedback (achieved when the wiper is grounded). Should we worry about running DC current through a trimpot? In this case, we are safe as there is no DC current passing through the wiper contact, which is the main cause for flaky trimpot. Here is a sample of running the input as 25% UL feedback.
How do we go about running triode mode or say 75% UL feedback? Simple, instead of R15, we put the trimpot in place of R16 and the C2 end that goes to ground is moved to the upper part of the trimpot. Here is 75% UL feedback.
This CF will act as constant current load to the pentode and will allow us to achieve high gain. On Spice simulation, with 6K7 as shown, the open loop gain for the input stage is about 75x (15Vpp swing with 200mVpp input). This gain level is perhaps excessive so how do we tame it? As shown, the input stage is on pentode-mode as the screen grid is AC-grounded by C2 via the lower 6N8S cathode follower. We can reduce the gain by changing R16 to a 100k trimpot and connect C2 to the trimpot wiper. Adjusting the wiper, we can vary the UL feedback from 0% (full pentode mode, achieved when the wiper is at the screen grid side) to 50% feedback (achieved when the wiper is grounded). Should we worry about running DC current through a trimpot? In this case, we are safe as there is no DC current passing through the wiper contact, which is the main cause for flaky trimpot. Here is a sample of running the input as 25% UL feedback.
How do we go about running triode mode or say 75% UL feedback? Simple, instead of R15, we put the trimpot in place of R16 and the C2 end that goes to ground is moved to the upper part of the trimpot. Here is 75% UL feedback.
Obviously 100% screen feedback or triode mode is achieved when R16 is fully bypassed by C2 where all of the output signal is fed back to the screen grid.
On this particular Type 47 Power Amp, i choose to keep the pentode mode and use it to my advantage to achieve high open loop gain. Mainly because the input pentode i'm using is not that linear to use in audio (6K7 is remote cutoff) so to lower the distortion, i introduce a negative feedback from the 47 plate back to 6K7's cathode (i dare not include the OPT on the feedback loop with OL gain that high). The higher the OL gain, the more feedback i can introduce and the less distortion i get. If this amp gets build, i will be using 39/44 tube as the input which is also remote cutoff. Why bother with remote cutoff in the first place? It's simply what i have in my stash
I am sure you can use other pentodes will better linearity and end up with even lower THD.
There is a hidden advantage from the particular tubes i am using (a pair of 6SN7/6N8S, 6K7 and Type 47 each for stereo amp) that is simpler heater power supply. A pair of 6SN7 and 6K7 heaters in parallel needs 6.3Volts and 1.8Amps while a pair of Type 47 heater in series 5V and 1.75Amps. If we put those heaters in chain, they're close enough to use 12VDC 1.8A power supply (6.3V + 5V = 11.3V, don't worry about 0.7V difference, heater wiring resistance flowing 1.8A will drop some voltage as well). Of course this means we still need some fixed bias on the 47 grid and since one 47 is on top of the other, the fixed bias voltage will differ by 2.5V (the heater voltage for one type 47 tube). There is one hidden risk though: since all the heaters are in series and they're on ground potential, the upper 6N8S cathode follower will have its Vhk limit breached. Well.. it's up to you if you feel the risk is worth the simpler heater power supply. To me it is..
Using fixed bias on the Type 47 can be scary and cause runaways and self-destruct. 100k grid leak resistor will help reduce this risk but at the expense of loading the input stage. This is where the cathode follower on top of the input pentode comes in handy. It provides low output impedance to drive the output stage easily.
The type 47 amp shown above will output 2W at 0.81% THD with around 950mVpeak input.
On this particular Type 47 Power Amp, i choose to keep the pentode mode and use it to my advantage to achieve high open loop gain. Mainly because the input pentode i'm using is not that linear to use in audio (6K7 is remote cutoff) so to lower the distortion, i introduce a negative feedback from the 47 plate back to 6K7's cathode (i dare not include the OPT on the feedback loop with OL gain that high). The higher the OL gain, the more feedback i can introduce and the less distortion i get. If this amp gets build, i will be using 39/44 tube as the input which is also remote cutoff. Why bother with remote cutoff in the first place? It's simply what i have in my stash
I am sure you can use other pentodes will better linearity and end up with even lower THD.There is a hidden advantage from the particular tubes i am using (a pair of 6SN7/6N8S, 6K7 and Type 47 each for stereo amp) that is simpler heater power supply. A pair of 6SN7 and 6K7 heaters in parallel needs 6.3Volts and 1.8Amps while a pair of Type 47 heater in series 5V and 1.75Amps. If we put those heaters in chain, they're close enough to use 12VDC 1.8A power supply (6.3V + 5V = 11.3V, don't worry about 0.7V difference, heater wiring resistance flowing 1.8A will drop some voltage as well). Of course this means we still need some fixed bias on the 47 grid and since one 47 is on top of the other, the fixed bias voltage will differ by 2.5V (the heater voltage for one type 47 tube). There is one hidden risk though: since all the heaters are in series and they're on ground potential, the upper 6N8S cathode follower will have its Vhk limit breached. Well.. it's up to you if you feel the risk is worth the simpler heater power supply. To me it is..
Using fixed bias on the Type 47 can be scary and cause runaways and self-destruct. 100k grid leak resistor will help reduce this risk but at the expense of loading the input stage. This is where the cathode follower on top of the input pentode comes in handy. It provides low output impedance to drive the output stage easily.
The type 47 amp shown above will output 2W at 0.81% THD with around 950mVpeak input.
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