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My parafeed 300B prototype and questions

66.138.43.98
Posted on May 17, 2004 at 21:05:30
Tanker
Audiophile

Posts: 368
Joined: April 17, 2001

Hi, I just recently breadboard the amp below and just wonder what would you consider as an acceptable minimum ripple at the B+ or the plate of 300B? The voltage values shown on 6SN7-300B are the actual value taken from the amp after a few adjustment with the ripple shown. I did not install the C2. Here are my questions

1) what's recommended C1, C2 and C3 in this case? How do you calculate C2 and C3?
2) I am concern about the ripple as shown in the schematic. Should I increase the capacitance of the power supply caps to reduce the ripple? or there's a better way to do it?
3) What else would you recommend?

Thanks in advance

Here is the link to the picture of my 300B prototype

 

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Re: Like so Mike? Parafeed compact 2A3, posted on May 18, 2004 at 17:27:31
direct
Audiophile

Posts: 10
Location: nj
Joined: May 16, 2004
 one thinking:
If you do fixed bias, you can achieve cancelation by tweaking bypass caps.

 

Where are all expert on Parafeed? Comment and opinion please, posted on May 19, 2004 at 18:11:14
Tanker
Audiophile

Posts: 368
Joined: April 17, 2001
 .

 

Re: My parafeed 300B prototype and questions, posted on May 19, 2004 at 18:55:50
keto
Audiophile

Posts: 1006
Joined: September 29, 2001
 Its my understanding that parallel caps provide the sum of the two capacitances and half the voltage rating. If so, those 4 20uF caps will be equivalent to a single 80uF/165VAC cap. Maybe stack them series-parallel, to yield 40uF/660VAC, or put a dropping resistor between each. Looks like an intesting circuit. How does it sound and how do the two drivers compare? Is the 4uF coupling cap before the TFA2004jr rated for such high voltage? --keto

 

Re: My parafeed 300B prototype and questions, posted on May 19, 2004 at 23:01:36
Paul Joppa
Industry Professional

Posts: 7295
Location: Seattle, WA
Joined: April 23, 2001
 Huh - I though I had posted, but it's not there now. Maybe I punched the wrong button.

Your hum is not from the power supply, which if built as shown should have about 1mV ripple. The 300B cathode hum vltage is 0.1 times the plate voltage, but mu is less than 4, so there must be a lot of hum at the grid also. Perhaps your plate chokes are too close to the power transformer? Hard to say; hum-busting is a pretty long and frustrating experience unless you get lucky. I wish you good luck!

C3 of 4uF works well with TFA-2004. Can't say about the others, maybe someone who has worked with that topology can comment?

 

Keto, parallel caps will add the capacitance, but voltage will, posted on May 20, 2004 at 05:07:06
Tanker
Audiophile

Posts: 368
Joined: April 17, 2001
 remain the same. Series caps will halve the capacitance, but double the voltage (assume same rating). The total capacitance in power supply is 120uF with 660VAC rated oil caps.

Yes, the coupling is also an oil caps rated at 660VAC. I have not plug into the system yet. I am still concern about too much ripple in the circuit.

 

Paul! is the 1mV ripple mentioned regarding to at the 300B plate or, posted on May 20, 2004 at 05:12:14
Tanker
Audiophile

Posts: 368
Joined: April 17, 2001
 at the cathode end? Also if you look at the picture, the plate choke is really far away from the power tranny. The power tranny is a military potted one.

Using Duncan model, the ripple should be much much less than 2.5V at the plate, but not really sure why I get that much.

 

What Type of Cap for Parafeed?, posted on May 23, 2004 at 06:25:40
MikeT
Audiophile

Posts: 610
Joined: July 25, 2001
 Oiler? Solen? Other?

 

Re: What Type of Cap for Parafeed?, posted on May 23, 2004 at 11:51:24
Paul Joppa
Industry Professional

Posts: 7295
Location: Seattle, WA
Joined: April 23, 2001
 From another post on another subject:

"I have no idea how different caps will sound (to you, in your system...) and to be honest, I wouldn't admit it if I did. When it comes to caps, you pays your money and takes your choice. I avoid the subject of cables too. :^)"

People's reactions to caps vary a lot, and so far it appears that taste is the predominant criterion. There is virtually no consensus on what is best, and very little on sonic descriptions. To complicate things, PIO caps are said by some to sound less soft in parafeed applications than in interstage coupling applications.

I have enough Scotch ancestry to use the cheaper alternatives when there is no clear preference, so I use Solens. But then, I also have not pursued studies to compare caps - I'd rather change the circuit to get an effect. I don't question the value of parts selection, it just interests me less.

FWIW (not much!) here's my summary of what I hear others saying:

Mylar sounds muddy
Polypropylene sounds plasticky
Teflon sounds buttery but brittle and edgy
Oil sounds smooth and euphonic

Construction details are said to have a big effect, confusing the issue. Steel leads are bad, foil instead of metalization is good, winding tension counts, etc etc.

 

Re: My parafeed 300B prototype and questions, posted on May 24, 2004 at 00:13:18
dead_SHORT


 
Hi Tanker.
I use almost exactly the same circuit with the major difference being using a 6C45Pi rather then a 6SN7. My resistor values, choke loads and currents are different as a result. I bypassed the cathode-anode resistor (R2 in your schematic) of the driver tube with a 47uF film cap rather then the cathode-ground resistor (R3) as you did.
C2 is a 47uF as well and I got this value basically as I had this size of motor-run oiler on hand.
My ripple at the speaker terminals is 3.5 mV of mostly 60 Hz. The 120 Hz ripple is largely canceled by a Steve Bench type nulling circuit. This I can live with on my 94 dB Tannoys. Without the nulling circuit, I had ~8 mV hum which is perceptable to me.
For C3 I use a 4 uF 1000V oiler.
Regards, David

 

Thanks For The Info (nt), posted on May 24, 2004 at 07:55:48
MikeT
Audiophile

Posts: 610
Joined: July 25, 2001

 

Hi David, are there any a reason behind using, posted on May 25, 2004 at 20:37:52
Tanker
Audiophile

Posts: 368
Joined: April 17, 2001
 cap bypass through R2 instead of R3? Also what did you use in Steve Bench nulling circuits? I assume you use AC for the 300B heater.

By the way what the total capacitance you use in your power supply?

I did manage to cut ripple in half after posting and now using 12uF for C2. One thing I do not understand is that without circuit connected, my power supply only registered 200mV ripple max with purely dummy resistor load but increase quite a bit when connected to the circuit.

 

Re: Hi David, are there any a reason behind using, posted on May 26, 2004 at 20:46:58
dead_short


 
Hello Tanker.
I biult the amps almost five years ago now and frankly don't remember exactly how I got to the layout I have. 8^)
Blowing the dust off of my memory...
This design when I biult it was much discussed on the Joenet and was then referred to as the monkey amp or full monkey amp(don't ask me why). The design originated with Thorsten Loesch I believe.
I think the voltage across R2 is basically the PS for the driver tube and one wants a low impedance AC path around this resistor as in any PS. As R3 is not in this 'loop', I didn't see any reason to bypass it. The 'ground' of the driver tube is the bottom of R2, not the actual ground ground. If I'm wrong in this assumption, someone please explain.
I remember trying the design with and without this cap and liked it better with it. Personally I like resistors best when they are eliminated with some iron or if that is not possible, bypassing them. Do a search on the web for monkey or full monkey and you will find a few variations. Some don't use bypasses in the resistor 'tree' at all. Some bypass R1 & R2 like I did. Check out http://www.diyparadise.com/monkeydarling.html for starters.
As noted on this site, the lower the Rp of the driver the better. That is why I used the 6C45Pi and I run them low and hot to get the Rp down.
Steve Bench's hum bucking circuit cut the really bothersome (to me) 120 Hz hum without having to resort to current regulated DC heating which would have been my next option. It is such a cheap option to try and is easy to retrofit.
My PS is choke input 10H/47uF/10H/47uF from a full wave 400-0-400V winding. I never felt the need to use more capacitance as I can live with 3.5 mV of ripple at the speaker terminals. You seem to be using lots of filtration. I don't think that the PS is the source of your problems. Try bypassing R2.
Regards, David
PS: I also added UTC A20 input transformers (600 ohm:600 ohm) which cured some nasty ground loop problems I had.

 

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