I am experimenting, from as design point of view, not sure if I will actually try to build it, with the idea of a Para-UL, Parafeed configuration for a 12L6 power tube with a Bogen OT. I have lots of these inexpensive OTs and was thinking I might be able to boost the performance of the 12L6 family of tubes using this OT.
This is more of a MidFi experiment so don't rain on my parade, the object is to improve the specs if I can. This is in line with typical mass produced radios and portable turntables used in the day. The Bogen T72510 is actually much better than the cheap OTs they used back then.
I'm looking at about 3W 50-20K. THD TBD, running the 12L6 at 188V with a 5k:8 Ohm OT tap. (How realistic that is, is questionable, but that is the specs from Bogen and the 12L6).
T72510:
https://www.bogen.com/products/pdfs/specialelectronicspdfs/WMT1As.pdf
12L6:
https://frank.pocnet.net/sheets/093/2/25L6GT.pdf
Anyways, I appreciate any comments on this design. The values I have are my first shot attempt so I expect they need some tweaking. My theory is not all that solid and I have very little experience with MOSFETs, but for you all I bet this is a fairly simple circuit mod.
This is more of a MidFi experiment so don't rain on my parade, the object is to improve the specs if I can. This is in line with typical mass produced radios and portable turntables used in the day. The Bogen T72510 is actually much better than the cheap OTs they used back then.
I'm looking at about 3W 50-20K. THD TBD, running the 12L6 at 188V with a 5k:8 Ohm OT tap. (How realistic that is, is questionable, but that is the specs from Bogen and the 12L6).
T72510:
https://www.bogen.com/products/pdfs/specialelectronicspdfs/WMT1As.pdf
12L6:
https://frank.pocnet.net/sheets/093/2/25L6GT.pdf
Anyways, I appreciate any comments on this design. The values I have are my first shot attempt so I expect they need some tweaking. My theory is not all that solid and I have very little experience with MOSFETs, but for you all I bet this is a fairly simple circuit mod.
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All I need is a standard coupling cap then? Say 0.22uf or 0.47uf?
The Parafeed examples I was looking at in Tubecad had a 10uf cap on the output, not sure why. In fact one example used a 100uf cap. Another site used a 4.7uf cap. That is why I posted here for some feedback.
The thing I was most unsure about was using MOSFETs in this instead of just standard transistors, but I think they will be better in this application?
The Parafeed examples I was looking at in Tubecad had a 10uf cap on the output, not sure why. In fact one example used a 100uf cap. Another site used a 4.7uf cap. That is why I posted here for some feedback.
The thing I was most unsure about was using MOSFETs in this instead of just standard transistors, but I think they will be better in this application?
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Many brain storms here & started in other threads. But haven't seen much in the way of results yet. When will you be finishing any of these & posting some photos & measurement data for us to see??🙄
That is good to know, I got the idea from the attached, but have not worked out any of the values. Are you saying it is complete horse$^%T and can't work or it's just HS because I have not worked out any of the correct values?
I have yet to try to figure out actual values on the plate, I just want to know the viability circuit of this overall. Would it require cascaded FETs?
2N7000 need replace with a higher voltage/power version, thanks. 2SK3301 was posted before, I'll go with that.
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You'd want about 400V of B+ to end up with 188V on the 12L6 plate. This will put a metric ton of dissipation across the CCS, which is why you typically see a plate choke in a parallel feed amp instead of a CCS. You might be able to use another 12L6 as the CCS.
Damn, ok my B+ is 188V sounds like I might just want to scrap the CCS at the plate and stick with a choke. What is the criteria I need to use to choose the right choke?
I was thinking that using parafeed to the Bogen would be better than the standard configuration, getting the DC off the transformer.
As for the Para_UL portion I was also considering using a sub-mini tube instead, like the 5703W, the Max plate V is 200 . It's a Cathode follower in the circuit but I am uncertain how to determine the best values for R57 and R58, I guess it will have to be experimental.
I was thinking that using parafeed to the Bogen would be better than the standard configuration, getting the DC off the transformer.
As for the Para_UL portion I was also considering using a sub-mini tube instead, like the 5703W, the Max plate V is 200 . It's a Cathode follower in the circuit but I am uncertain how to determine the best values for R57 and R58, I guess it will have to be experimental.
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If you are even considering springing $30+ for a parafeed choke so that you can use a Bogen line matching transformer for an OPT, go here and spend $22 for a real SE OPT that sounds good, and has a frequency response from 30 Hz to 40 KHz when driven by a triode wired 6V6 using zero feedback at 2 watts.
The 12L6 is a 6W6 with a 12 volt heater which has a lower plate resistance than the 6V6, so you could go lower than 5K for a bit more power. If your B+ is limited to 188 volts, go to the 2.5K version for more power.
EDCOR - XSE15-5K
There is also a bigger version for $35, but I have not tried it myself.
The 12L6 is a 6W6 with a 12 volt heater which has a lower plate resistance than the 6V6, so you could go lower than 5K for a bit more power. If your B+ is limited to 188 volts, go to the 2.5K version for more power.
EDCOR - XSE15-5K
There is also a bigger version for $35, but I have not tried it myself.
The tube will take care of itself with good input and output and power around it, just like any other. You won't be boosting the performance of the tube, just not underutilizing it, ....perhaps. I'd use a $15 PT-31 for first try kicks. But remember, crap in = crap out, too. Don't out smart yourself.