5f2a Power supply filtering question

[morton]

I'm preparing to build a stock 5F2a and came across this thread about adding addition power supply filtering c-r stages and increasing the values in existing stages to quiet down a 5f1. Just wondering if anyone had made any changes to the 5f2a PS filtering?

http://el34world.com/Forum/index.php?topic=14464.0

Thanks,
Gary

[Willabe]

You would do the same thing by adding an extra filter cap stage before the filter cap that feeds the OT/power tube plate with a power resistor or a choke between the new 1st filter cap and the original filter cap in the Fender schematic.

If you go to the very bottom of any page in this forum you will see some links, click on Tube Amp Schematics Library and print out the 2 schematics, 5F1 and 5F2a to compare them. The power supplies are very close to being the same.     

 

[HotBluePlates]

I disagree with Marsh a little. Regardless, look at the 5F2-A schematic; add a choke or resistor between the existing parallel 16uF filter caps (where the rectifier connects, and also the output transformer).

This is essentially same-as what the 5E2 Princeton and 5D2 Princeton did. One used a resistor, while the later one used a choke. These were between 2 filter caps, and the output transformer connected to the 2nd filter cap.

The net result isa reduction in ripple in the B+ supply where the OT takes power, which reduces the chance of hum in the speaker.

[Willabe]

I missed the Marsh link.

If you scroll down to the 2nd reply in the link you posted you will see PRR's reply, that's what I was suggesting more or less. Which is also what HBP just posted too. 

[Willabe]

I have a 5F1 set up on my bread board right now and it's very quite.

> 22uF > Doug's choke for a Fender 2x6L6 amp > 22uF feeding the OT. Then a 470 ohm B+ dropping R > 22uF feeding the power tube screen.

[morton]

Thanks. I have one of Doug's small Fender chokses, 50ma 4h, would that work? Any other changes that would need to be made? If I went the resistor route what value would I need?



Thanks,
Gary

[Willabe]

I have one of Doug's small Fender chokses, 50ma 4h, would that work?

See reply #7;

Maybe, but it's cutting it real close to burning up or not. I wouldn't do it.

Any other changes that would need to be made? If I went the resistor route what value would I need?

If you scroll down to the 2nd reply in the link you posted you will see PRR's reply, that's what I was suggesting more or less. Which is also what HBP just posted too. 

Edit; I changed this post as to HBP's in reply #7, I trust his math. 

[HotBluePlates]

Thanks. I have one of Doug's small Fender chokses, 50ma 4h, would that work?

I think it could, unless your B+ was very low.

12w/300v = 40mA idle
12w/400v = 30mA idle

240v is about the lower limit (idle at 50mA for 100% dissipation), and you're likely to be above that.

[morton]

Thanks, I'll get the parts on order from Doug, maybe get a larger choke just in case. Would the choke drop the b+ to the point that I would need to change the value of either the dropping resistors?

Willabe-think I was posting that last one at the same time as you.

[HotBluePlates]

Would the choke drop the b+ to the point that I would need to change the value of either the dropping resistors?

No. The point of the choke is to get the filtering effect of a large resistor, but without the same amount of voltage drop. DCR of the choke is unlikely to be even as high as 200Ω (probably well below), and if you had a 200Ω resistor it wouldn't drop enough voltage to be worth jiggering the other dropping resistor values.

[morton]

Great thanks all for the help. I'll get going on the build and report back. I'll be drilling out a stock chassis, so I wanted to make sure I had one with enough real estate to add the choke.

Thanks again.

[Willabe]

Doug's Fender chokes are fully shielded so you can place them tighter to other iron on a chassis. 

[Fresh_Start]

FWIW my first build was a 5F2-A.  If I remember correctly, the power supply was stock at first.  I added a choke like HBP suggests in Reply #2 above and it greatly reduced noise without affecting tone or touch sensitivity.  The basis was the 5E2 power supply (note that there isn't a separate power rail node for the screen grid in that circuit).  I think that the choke is rated for more than 50ma but could be wrong about that.

I also think that I experimented with bigger filter caps and did not like the impact on touch sensitivity and bass response depending on where the bigger cap was.  Honestly, I've tweaked this amp so much and re-built it twice to the point where I can't remember the details.  But IIRC a bigger cap (22uf) before the choke in the "reservoir cap" location helped noise without impacting touch response.  I think that the next node (plate supply) also is 22uf now but wouldn't swear to it.  The screen grid and preamp caps are still/again 8uf.

"Grounding" the heater virtual center tap on the 6V6 cathode should help with noise also, especially in a single-ended amp.  Blatant appeal to "expert":
http://www.valvewizard.co.uk/heater.html

In addition, putting some aluminum flashing inside the back panel to act as shielding seemed to help also.

A 10" speaker is a really good idea IMHO.  Mojo used to make a 5F2-A cab with a 10" baffle.  Might have called it a Harvard.

Last but not least, a bigger output transformer is not necessarily better.  My amp currently has a Allen TO8C in it but the original Mojo OT will go back in the next time I open it up.  This amp isn't supposed to have a tight bottom end, at least not for me.

Cheers,
Chip

[HotBluePlates]

Here's another example of C-R before the power supply node which powers the OT and output tube plate: Epiphone Valve Jr Rev. 3 (see below).

C6 and R10 were not in the original circuit (there was a "C6" but not a "C9" and only 3 filter caps total). Early modders were adding a filter cap in parallel with what is now C9 due to high hum levels. Eventually, Epi got around to adding C6 and R10 to create an extra stage of filtering before the OT, which is something we advocated around here for a long time for Champ/tweed Princeton type amps.

[morton]

Thanks for the info. On the 5f2a do you think the choke added will be efficient in reducing the hum level, or should I look at adding another c-r stage? Also, in looking at the Hoffman 5f2a layout, the first 2 16uf caps are combined into a 33uf. How would you modify that layout for added filtering-off board c-r filtering stage?

Thanks,
Gary

[sluckey]

I would install one more turret in the lower right corner of the board. Mount the resistor between that turret and the turret for the positive end of the 33µF cap. Now move the wire from the rectifier that currently connects to the 33µF to the new turret. (Alternately, if the recto tube socket is close enough, you could mount the resistor directly between the recto socket pin 8 and the 33µF on the board). Move the OT wire that's currently connected to the recto socket to the 33µF turret. Finally, connect a new 33µF cap directly to the recto socket pin 8 and a PT bolt ground lug.

[morton]

Sluckey, on the new resistor to mount between the existing 33uf+ turret and the new turret what value should I use?

Thanks,
Gary

[Willabe]

Did you ever look at the drawing that PRR posted in his reply #1 in the link you posted in your original post of this thread?

 

[morton]

Yes, I just didn't know if the value would change as the 5f2a circuit existing c-r's were different.

[Willabe]

A larger filter cap will take a little more current to keep it charged up. But The difference between keeping a 16uF/20uF or 40uF is not much and PRR had the larger 40uF after the 5w dropping R in his drawing. 

[morton]

Understood. I'll get a quick layout tonight to send for review. Thanks for the help.

[morton]

Apologies in advance for butchering the Hoffman 5f2a layout.  Resistor value is a 5w 250ohm.  Let me know if this would work.

Thanks,
Gary

[Willabe]

Yes but with 1 major mistake. (Edit; And 1 small mistake, see Sluckey's next post.)

The positive end of the new 33uF cap goes to pin 8 NOT to ground. You hook up that cap like that it will kill it. 

[morton]

Thanks man, that was why I put the stripe and the + on it! thanks again for the help.

[sluckey]

In addition to what Willabe said... Move the OT wire to the 33µF positive turret.

[Willabe]

Missed that, thanks Sluckey. 

[morton]

Thanks, meant to slide that one over to the other side of the 5w. Thanks for the clarification guys.

[PRR]

> didn't know if the value would change as the 5f2a circuit existing c-r's were different.

It's still a one-6V6 amplifier. General power demand and cleanliness issues will not change enuff to matter.

An upper "limit" on the added R will be roughly the cathode-resistor value. That will cause just enough max-loudness drop to notice. In bedroom or recording studio, that's very acceptable. If you need to maintain "full" output but lessen the buzz-curse, half the Rk value would be a good compromise. (About what I show.) Going much lower won't give the added cap enough leverage to work against, buzz won't go down much.

If you are brave, put ten 100r resistors in series. Use jumper-clips (BE CAREFUL!) to short-out to get 100, 200.. 900, 1000 Ohms. Play and listen. Small resistors don't cut buzz or power much. Big resistors cut buzz more than power, and 1K would make a Champ a champ (less annoying to parents).

[Fresh_Start]

A choke would have better filtering and less reduction of power output (provided resistance spec of choke is good). But it takes more space and a lot more $$$.

Chip

[morton]

Finished this AM, got rid of a good 90% of the hum.  Used a tag strip, board was already mounted.  Thanks again everybody for the help.

[Willabe]

Hey now, that looks real nice! Great job!     

[sluckey]

I like it. What size is that chassis?

[morton]

Thanks. 9x6.5 from Watts. Kind of a tight fit but it's nice and compact. Put a MV and a NFB switch on it too. First 5f2a, nice sounding amp, addition c-r stage seemed to tighten up the bottom end, but maybe it's just in my head. Getting ready to start a stock Fender layout, curious how they will sound compared to each other.

[Ed_Chambley]

Look at some of the tweed fenders and look at how Leo matched chokes to PT values. 


Which amp or amps are you referring to?  5F4?  Any others?  I have not noticed others, but would like to see which ones have a choke filtering the power tube plates.

[VMS]

I found these:

5E5-A, 5E2, 5E6, 5E6-A, 5E7, 5E8, 5E4-A.....

[HotBluePlates]

I found these:

5E5-A, 5E2, 5E6, 5E6-A, 5E7, 5E8, 5E4-A.....

Yes, but no data on any of those chokes on the schematic/layout.

IMO, The 50ma, choke is too small unless you filtering only the preamp.

Why?

[Fresh_Start]

Max plate current for 6V6 at 315 volts = 35 ma
Max screen grid current = 6ma
Max plate current for 12AX7 1.2ma * 2 = 2.4ma

Total 43.4ma

Would a choke rated for 55 or 60ma be a bit better?  Theoretically, but not as readily available. I've been running a 50ma 4 Henry choke in this circuit for years. My B+ voltage is lower but the power tube is not biased hot.


Drgonzonm:  if the rectifier tube feeds a C-L-C filter, nothing is different for the rectifier.  Depending on choke specs, it can do a better filtering job with less voltage drop.  A choke input where there is no capacitor on the upstream end of the choke is a whole different story and rarely done in modern devices.


Tube data from Tung Sol sheets here: http://www.mif.pg.gda.pl/homepages/frank/vs.html


Chip

[Willabe]

Drgonzonm:  if the rectifier tube feeds a C-L-C filter, nothing is different for the rectifier.  Depending on choke specs, it can do a better filtering job with less voltage drop.  A choke input where there is no capacitor on the upstream end of the choke is a whole different story and rarely done in modern devices.

Yep, yep, yep!

[Fresh_Start]

FWIW ClassicTone includes the 5E1 and 5F2 in their list of amps using a 50ma 4 Henry choke, so Hoffman's equivalent should be OK.

Chip

[morton]

What about the original 5f2 schematic?

Thanks,
Gary

[HotBluePlates]

I found these:

5E5-A, 5E2, 5E6, 5E6-A, 5E7, 5E8, 5E4-A.....

I forgot that the 5f2a, is a three tube SE amp.  Regardless IMO a 50 ma choke won't handle load, it is marginally too small.   

Why? 5F2 Princeton has B+ at 320vdc. Why won't the choke handle a 6V6 at 320vdc?

No Data:
Use the transformer part numbers as a starting point.  To find data on the chokes look at the specs published by the original suppliers or replacement suppliers.  (I know some of the early schematics on fenders don't even have that limited information).

If we had the amp, we wouldn't need to guess (we could just measure the original part. Where is there choke data/numbers on the schematics of the amps mentioned?

IMO, The 50ma, choke is too small unless you filtering only the preamp.

Why:
I am glad you asked the question, (See my note about it being an SE amp, Later Princetons are PP)  because, a 50ma choke is too large to handle the preamp.   There just is not enough current draw to get the choke to work. 

Too small unless preamp only, now too big for just preamp. Again, why? Contradicting statements, and no one is talking about preamp only, but whole power supply.

  The choke is only large enough to handle the plate current on the 6v6, Resistance losses through the O/T and the step down resistors plus the loading on the tubes will cause the load to exceed the rating on the choke.  I believe you risk "smoke the choke".

Why? A lot of statements but no numbers...

The 5F2 Princeton layout shows voltages, including those across cathode resistors. 19v across 470Ω for the 6V6 indicates 19v/470Ω = ~40mA for plate and screen current in the 6V6. Both preamp tube sections show 1.5v across 1.5kΩ cathode resistors, for 2mA additional total for the preamp. So we have 42mA through a 50mA choke.

Anticipating next thought: yes, those numbers are idle d.c., but this is a Class A amp. Output tube will swing roughly as far negative as it does positive with signal, so average current stays the same (mild rise in average current due to distortion). The whole output stage still stays safely within the 50mA chokes's capabilities.

[Fresh_Start]

My choke don't smoke... 

[Willabe]

Too small unless preamp only, now too big for just preamp. Again, why? Contradicting statements, and no one is talking about preamp only, but whole power supply.

Why? A lot of statements but no numbers...

Why do you keep posting a bunch of nonsensical gibberish that makes no sense? 

Why not take up fishing or something?

[Fresh_Start]

I am not a proponent of using anything larger than a 8 muf, cap as the first cap in the pi filter application.  Norman Crowhurst and a few others recommend a low value cap in first leg of the pi filter. 

Its good to remember chokes were used because high value caps just were not available and iron was relatively cheap.  In our time, iron is expensive and large value caps are readily available. 

I respectfully suggest that you experiment with Duncan's power supply modeling software. Choke vs. resistor. 8uf vs 16uf vs. 22uf.
http://www.duncanamps.com/psud2/

Leo Fender didn't drop the choke in the 5F2-A and go up to 16uf caps from the 8uf caps on either end of the choke in th 5F2 because the change provided better filtering. It cost less and was "good enough" for that amp.  Without the inherent hum rejection of a push-pull power amp, doing a little extra to lower the noise floor is worth the effort IMHO.


I'm completely mystified by the suggested 8uf limit on the reservoir cap value. If you have unequal caps on either end of a pi filter, it is more effective to have the higher cap on one end (assuming Duncan's PSU is accurate).  I just can't remember right now which end was more important though. Also, while the 5Y3 rectifier tube has a theoretical limit of 20uf for the first "reservoir" cap, Leo ignored that and used 32uf in this iconic circuit (two 16uf caps in parallel).


Chip

[PRR]

> the 5Y3 rectifier tube has a theoretical limit of 20uf

Where do you find that?

RCA 1951 gives 10uFd "typical". http://www.mif.pg.gda.pl/homepages/frank/sheets/049/5/5Y3G.pdf

GE 1955 gives 20uFd "typical". http://www.mif.pg.gda.pl/homepages/frank/sheets/093/5/5Y3GT.pdf

Neither has any upper limit on cap size.

GE 1955 Chart III gives a minimum resistance which appears to apply for any cap size. At 325VAC per plate you want >36 Ohms series resistance. Typically your PT resistance will be most all of this.

[Fresh_Start]

EDIT:  found one spec sheet with a max capacitor value - Brimar
http://www.mif.pg.gda.pl/homepages/frank/sheets/155/5/5Y3GT.pdf


32uf - maybe Leo knew what he was doing with the 5F2-A after all!   


Also figured out where I got the idea of maximum cap values for rectifier tubes. Merlin's book and site plus data sheets for other rectifier tubes like the GZ34.

> the 5Y3 rectifier tube has a theoretical limit of 20uf

Where do you find that?

RCA 1951 gives 10uFd "typical". http://www.mif.pg.gda.pl/homepages/frank/sheets/049/5/5Y3G.pdf

GE 1955 gives 20uFd "typical". http://www.mif.pg.gda.pl/homepages/frank/sheets/093/5/5Y3GT.pdf

Neither has any upper limit on cap size.

GE 1955 Chart III gives a minimum resistance which appears to apply for any cap size. At 325VAC per plate you want >36 Ohms series resistance. Typically your PT resistance will be most all of this.

You are correct. GE and Tung Sol data show "typical operating" conditions including 20 uf input capacitor. I copied someone else's incorrect reading of the tube data from a discussion long ago. Still wrong as to value but not maximum value.

Chip

[HotBluePlates]

EDIT:  found one spec sheet with a max capacitor value - Brimar
http://www.mif.pg.gda.pl/homepages/frank/sheets/155/5/5Y3GT.pdf


32uf - maybe Leo knew what he was doing with the 5F2-A after all!   


Also figured out where I got the idea of maximum cap values for rectifier tubes. Merlin's book and site plus data sheets for other rectifier tubes like the GZ34.

View those things as "prudent guidelines". The spec which is a hard limit is the maximum peak plate current. This value is way above the rated maximum d.c. (steady) current. What gives?

You already know the first filter cap charges to some voltage, and only dips below A.C. * 1.414 because there is load current pulled from the cap by the amp. The cap only gets recharged through the rectifier when the rectified pulsating d.c. exceeds the cap's voltage at that instant. So recharge happens in very brief, high-current pulses less then a half-cycle long, and of a current magnitude much greater than the load current. These charging pulses is where the maximum peak plate current rating enters the picture.

Some things make those charging pulses tend to get bigger: higher load current means more charge has to be replaced during that brief recharge window, and larger filter caps mean more charge capacity trying to be filled during each recharge window.

But power supply resistance slows (limits/lowers) the charging current. This is usually PT winding resistance, but actual resistors could be installed between the PT winding and rectifier plate. These could enable you to use bigger filter caps, yet still limit charging pulses from exceeding the maximum peak plate current rating.

So theoretically, you could have a 1 Farad filter cap hanging off the 5Y3 cathode; you'd simply need enough series resistance to keep the charging pulses from getting too big. This is where the Duncan Power Supply program is handy: it does the multiple math steps needed to figure this stuff, and compares peak current to the spec for the tube.