What's Next?

Wntrmute2

Not So Mediocre Member
Well the Gadget rev-3 is breaking in nicely! But it is only mid-January. I need some suggestions as to another build. I could use another phono-stage for the upstairs rig, or..... My Radiotron 2A3/300Bs are crying out for some mods to increase the HF response but I do not want to lose any gain as with the WBPS, the max volume is a little low despite using a 20:1 SUT. Weird I know. Suggestions welcome.PXL_20210112_204628323 (1).jpgPXL_20210113_165217232.jpg
 

Wntrmute2

Not So Mediocre Member
Be happy to help! I'm thinking maybe I'd build another Gadget preamp but make it switchable between 150V and 210V B+. You could then adopt the Cary permanently. I'd like to build something where I could get good balance control as well as remote control without the expense and hassle of the attenuators I sourced for Gadget MK-1. I've got most of the parts except a few sockets and the afore mentioned controls. I just got some Russian 3.3uf caps that are 2 inches in diameter and 5 inches long! I could throw them as hand grenades!
 

Wntrmute2

Not So Mediocre Member
Mark at Blueglow also had some mods that WE used to boost HF response that might not reduce my gain as much as Paul's ideas. Need to do more research!
 
Thorsten Loesch also posted a circuit for doing that for a 310A pentode driver... should work with the 6C6. He also covered similar in his treatise on building a direct-coupled pentode amp back in the day. I can dig them out if you can't find them online. I may try it someday, after my next build perhaps, though I rarely feel the rolled-off HF is an issue with my amp: I really, really dislike accentuated HF or HF that is disconnected from the fabric of the music (i.e. rings along or rides over or is not proportionate, if that makes sense).
 
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Agrre with you on the HF.
Rolled off will not fatigue you and with a good solid presentation won't bother me if it's just the top airy information missing.
Out of context or overly hot will fatigue me very quickly.
 

Wntrmute2

Not So Mediocre Member
Agrre with you on the HF.
Rolled off will not fatigue you and with a good solid presentation won't bother me if it's just the top airy information missing.
Out of context or overly hot will fatigue me very quickly.
I do have HF attenuators on my crossovers so I could dial that out if I ended up overshooting. I agree generally with what you said though.
 

Wntrmute2

Not So Mediocre Member
Thorsten Loesch also posted a circuit for doing that for a 310A pentode driver... should work with the 6C6. He also covered similar in his treatise on building a direct-coupled pentode amp back in the day. I can dig them out if you can't find them online. I may try it someday, after my next build perhaps, though I rarely feel the rolled-off HF is an issue with my amp: I really, really dislike accentuated HF or HF that is disconnected from the fabric of the music (i.e. rings along or rides over or is not proportionate, if that makes sense).
Do you think you could narrow it down a bit? There is a LOT of stuff that comes up searching his name!
 
Do you think you could narrow it down a bit? There is a LOT of stuff that comes up searching his name!

The main thread is here: Oh no - YAWE91B300BA (Yet Another WE 91B 300B Amplifier) - Thorsten - Tube DIY Asylum. The images are no longer available, though I have the important ones in my saved doc.

The relevant info is contained in the last few paras:

A last trick is implemented in a pair of 1nF capacitors between the 300B Anode and 310A screengrid. This is actually positive feedback at high frequencies. This helps to overcome the HF rolloff inherent to the classic 91 Circuit, where usually 20KHz is around 3db down. In my Amp's I measure -3db somewhere at around 75KHz with this positive feedback in place, without -3db is around 20KHz as expected.​
As build the amplifier now measures 20Hz - 20KHz with better than +0db/-1db, I cant measure the -3db point at low frequencies, my generator only goes to 15Hz. Again, not particularly remarkable, unless we consider the performance of the traditional 91 Circuit with the LF and HF positive feedback deleted (more like -3db @ 20Hz & 20KHz).​

Here is the circuit Thorsten is discussing - you can see the pair of 1nF capacitors in series between the 300B anode and 310A screen at the top of the circuit.

Thorsten_91_amp_circuit_highlights.GIF

Maybe Paul or someone else with a better understanding of this stuff than me can comment.

Thorsten also included in his design/discussion other stuff WE did to flatten the FR - messy and probably needed for modern builds using modern capacitors of more typical sizing.
 
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Wntrmute2

Not So Mediocre Member
Really cool. I wonder if I just graft that circuit you highlighted into my Radiotrons if I'd reap the HF benefit?
Thanks.

A lot of the notation as to values and components if confusing for a novice like me if I wanted to build the whole amp.
 
Really cool. I wonder if I just graft that circuit you highlighted into my Radiotrons if I'd reap the HF benefit?
Thanks.

A lot of the notation as to values and components if confusing for a novice like me if I wanted to build the whole amp.
From what I could gather, it is the connection of the 300B anode to the 6C6 screen via two 1nF capacitors that provides the positive feedback and improves the FR. Maybe Paul could confirm.

There is a lot of "stuff" in that amplifier design - much of it not needed or even desirable in a modern build, where we can use correctly sized capacitors etc.
 
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