|
Audio Asylum Thread Printer Get a view of an entire thread on one page |
For Sale Ads |
71.50.21.91
In Reply to: RE: Here's a reworked schematic posted by sideliner on February 23, 2017 at 08:29:39
I do NOT think Pat has a center tap ,on his power transformer's Ef voltage, so he showed us a pair of 50 Ohm Rs in series, the joined leads acting as a Center Tap, to feed each 2A3's 1K Rk to ground.ONE BIG QUESTION 330 VDC versus 322 VDC ????
Is there any decoupling of the front end B+, from the Final's B+?? None shows in any of these schematics?? The Finals B+, connected directly to the Driver's B+, will goof-up the stability of the Input stage big-time
If there is NO decoupling, the circuit would need a higher overall B+, to allow an R/C for the front end's B+, hopefully down to one mVAC or so. Then, the Rk of the Finals will need to be increased in value to maintain the same goal of 43 mA and 265 VDC P-K. All doable. Is Pat listening now with no front end decoupling ??
Jeff Medwin
Edits: 02/23/17Follow Ups:
Jeff / Sideliner / 91derlust - I just remeasured the voltages and updated my schematic to reflect the voltages. The B+ going to the driver tube and final tube are the same. I have not heard about decoupling the driver from the finals, so any input would be appreciated.
Pat
PS - 91derlust, I am going to lookup driver decoupling online but just remember that I am admittedly not the quickest learner regarding circuitry. For me, baby steps. :-)
Pat, I am not a fast learner either and I don't have much time to spare these days... and it is getting less.
keep at it.
Cheers,
91.
"Confusion of goals and perfection of means seems to characterise our age." Albert Einstein
Good catch Jeff. I hadn't noticed that. I had turned my attention to the driver stage because the calculated plate voltage / plate current didn't jive with the tube's bias voltage as shown. That's why I reworked the operating point with B+ being kept the same.
You are right, there should be a resistor between the main B+ filter stage and the PSU filter for the driver. If we assume a ~9 mA draw as per the OP's original schem, then this series dropping resistor should have been 880 Ohms for that 8V drop.
The output and driver stages seem to be taken off the same node... but, the voltages out of the PS according the schematic are different. Not sure if Pat has either not updated the voltages or has not drawn in the dropping/decoupling resistances. It appears that the resistances have been intentionally removed at some point therefore my guess is the former (same node).If my guess is correct - and this is where my knowledge starts to reach its limits - for this design, would the signal of the stages be at 180 degrees phase to each other and therefore draw from the same node remain stable(ish)? Also, Pat has created 150uF capacitance node, FWIW.
Note: I'd prefer to have a RC stage to decouple the driver as well, but perhaps it will work as drawn?
91.
"Confusion of goals and perfection of means seems to characterise our age." Albert Einstein
Edits: 02/23/17
Please see my response to Medwin's post. Also, Here is what I found online regarding decoupling resistors/cap:
Isolating one stage from another by adding resistor in series and a large value cap to prevent oscillations and other noises that may feedback thru the power supply connections. It also provide further filtering.
Yes Pat. Now, you need to figure out what sized resistor to use, and how much B+ voltage you need to drop across it, to get ABOUT 1 mVAC at the "C3" spot with the higher driver current you use. "C3" can be a DC LINK WIMA 50 uF, bypassed.
So, your entire supply will be like so: L1/C1/L2/C2/R1/C3. C3 can be shared L-R or split into two R1s and 2 C3s, you likely don't have chassis room to do that splitting.
So, model the supply in PSUD, and change the value or R3 with 50 uF held steady as C3, till you get 1 mVAC at C3. That will be the value, "ball park" needed of R1 ( AND a voltage drop across it ) that you will need.
So Pat, lets say you need to drop 50 to 100 VDC across R1, ( for 1 mVAC at C3) where will you get an extra 50 - 100 VDC , with a fixed power transformer and tube rectifier ???
Easy, Model "645 mHy at 10 Ohms" for L1, ( Hammond 159Y ) and SEE what that does to raise your C2 B+ VDC value. THAT is how you get the extra B+, with a smaller value, lower mHY, lower DCR L1. The VDC at C2 will then dictate the R1 value you need.
Also, Pat, with eg: a 50-100 VDC higher B+ at C2, you will need to use a higher value 2A3 Rk, to maintain 265 VDC at 43 mA. P-K across the JJ 2A3-40.
Go for it. But when you are doing this, remember one thing, one truth. A cap couple is the worst way to couple two stages. A transformer couple is better than a cap couple, easily. And, a direct couple is THE best way to couple two stages. Have fun.
Jeff Medwin
You got it, basically. Read on "Time constant" to get an idea of what values of R and C are ideal for your circuit. And the other point Jeff made is that you may need a higher B+ ahead of the RC decoupling network, so to account for the necessary voltage drop across the new R. That's if you want to keep B+ to the output stage the same as it is now.
Lew,
Thanks for the TC comment. I just read on Time Constant. Here is what I came up with:
Fc = 1 / (2*pi*R*C)
I want to target an Fc of roughly 2-4hz.
My C is already determined by my existing caps which are 50uF. So, if I pick an R value of 1000 ohms, this gives me an Fc = 3.18 hz. This will drop the B+ going to the driver, but I think that will be OK because I will reduce the existing 22K Plate Resistor that drops the Plate Voltage to 133V. I can work on that offline.
let me know if this sounds correct.
Thanks,
Pat
No !! You let us know !! :-)Plug the entire filter into PSUD, and tell us what you need to get 1 mVAC at C3. That would be L1/C1.L2/C2/R1/C3. C3 equals 50-51 uF. Do not worry too much about a low time constant, but concentrate on 1 mVAC of ripple at C3, as your design priority.
Low Mu
Edits: 02/24/17
Jeff - I would like to use PSUD, but I have a MAC computer and PSUD is for PCs only. I got the Time Constant to work with an Fc = 3.1 hz by using a 1k R1. Pat
Don't tell soul I told you this, but it can be three times or more higher, and NOT be a problem. But I want 1 mVAC at C3. Call me some times, and I will ask you for data to input, and I can PSUD supply for you.Is the new Pontiac GTO running well ?? Still liking the purchase??
JM
Edits: 02/24/17
Post a Followup:
FAQ |
Post a Message! |
Forgot Password? |
|
||||||||||||||
|
This post is made possible by the generous support of people like you and our sponsors: