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Can I use a resistor instead of the choke for coupling to the 813?

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Posted on October 29, 2014 at 12:53:54
vinnie2
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I don't currently have a choke I could use and am wondering if I could use a resistor instead for now? If so, how would I figure out what resistor to use?

 

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RE: Can I use a resistor instead of the choke for coupling to the 813?, posted on October 29, 2014 at 13:11:30
dave slagle
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 What Choke?

I see an OT and a CT filament winding but no choke.

dave

 

RE: Can I use a resistor instead of the choke for coupling to the 813?, posted on October 29, 2014 at 13:25:08
vinnie2
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 The choke between pin 2 of the 6em7 and opt. At least that is what I assumed it was since it says 10h. I guess couple was probably the wrong word to use; is it more part of the plate load?

 

RE: Can I use a resistor instead of the choke for coupling to the 813?, posted on October 29, 2014 at 13:51:13
Tre'
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 That 10Hy choke is between the B+ supply and the driver tube, not between the opt and the driver tube even though that's how it looks. The B+ supply is feeding both.

You could use a resistor in it's place but you would have more PS ripple on the plate of the driver cathode follower stage and the plate of the voltage amplifier stage.

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

RE: Can I use a resistor instead of the choke for coupling to the 813?, posted on October 29, 2014 at 13:54:09
dave slagle
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 That is a PS ripple reduction choke to decouple the output from the driver. Its goal is to provide a high AC impedance to ripple and low resistance to DC.

dave

 

RE: Can I use a resistor instead of the choke for coupling to the 813?, posted on October 29, 2014 at 13:56:22
vinnie2
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 Ok, I see. Thanks. My B+ is pretty clean, so I would like to give it a try until I can get my hands on the choke. How would I calculate the ohms/watts for the resistor?

 

RE: Can I use a resistor instead of the choke for coupling to the 813?, posted on October 29, 2014 at 14:05:49
Tre'
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 I would use a 100 ohms resistor.

Calculate the total current draw though the resistor.

The voltage at pin 6 divided by 12k ohms would be the current draw of the input stage and the voltage at pin 3 divided by 3.2k ohms would be the current draw of the cathode follower stage.

If you add them together, that will be the total current through the 100 ohms resistor. You can figure out how much voltage drop there will be across the 100 ohms resistor. Current times resistance equals voltage.

Voltage times current is the power in watts (heat) that the 100 ohms resistor will have to dissipate.

BTW With a resistor instead of a choke I would use a larger value filter cap. Instead of 10uf I would go 100uf to try to keep the ripple down.

Tre'


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"Still Working the Problem"

 

RE: Can I use a resistor instead of the choke for coupling to the 813?, posted on October 29, 2014 at 14:47:50
mwiebe
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Contributor
  Since: January 5, 2006
 Vinnie,
You have to know the operating points you want to run the power half of the 6EM7 in order to figure out what you need as a decoupler. For example lets say you want to run 150Vs from plate to cathode, which is straight off the RCA data sheet. Grid bias is -20V below cathode, plate current (also straight off the data sheet) is 50mA.

50mA running thought that big 8K2 cathode puts to top of the cathode resistor at 410Vs, which is kind of a non-starter. That resistor is dissipating 20Ws so your 10W is not a good idea either. So basically you need to trim down that 6EM7 cathode resistor to where the voltage on top of the cathode resistor, plus the 150 plate to cathode voltage is below B+. Then use ohms law to figure out what size of decoupling resistor you need to get from B+to the plate voltage you need.
Matt

 

RE: Can I use a resistor instead of the choke for coupling to the 813?, posted on October 29, 2014 at 14:59:25
vinnie2
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 Thanks Tre'

 

RE: Can I use a resistor instead of the choke for coupling to the 813?, posted on October 29, 2014 at 14:59:47
vinnie2
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 Thanks Matt

 

RE: Can I use a resistor instead of the choke for coupling to the 813?, posted on October 29, 2014 at 18:08:18
Chip647
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 Dave, Great to have you back.

 

RE: Can I use a resistor instead of the choke for coupling to the 813?, posted on October 29, 2014 at 20:30:21
Caucasian Blackplate
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 Let's just assume that the 10H choke is ~200 Ohms. If you use a 200 Ohm resistor, you can jack up the 10uF cap to a 100uF cap and probably be just fine.

 

RE: Can I use a resistor instead of the choke for coupling to the 813?, posted on October 30, 2014 at 03:59:41
vinnie2
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 Thanks. I will probably try between 100-200 ohms for starters with a higher value cap.

 

RE: Can I use a resistor instead of the choke for coupling to the 813?, posted on October 30, 2014 at 04:00:57
vinnie2
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 And your suggestion for a resistor to replace it temporarily?

 

RE: Can I use a resistor instead of the choke for coupling to the 813?, posted on October 30, 2014 at 07:01:08
dave slagle
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 Does anyone want to discuss the merits/goal/purpose of the cathode follower cap coupled to the 813??

dave

 

RE: Can I use a resistor instead of the choke for coupling to the 813?, posted on October 30, 2014 at 08:22:07
Tre'
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 To better drive the Miller of the 813 but not enable A2 operation?

Not my argument, just a thought as to what might be an argument.

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

RE: Ok, what am I doing wrong, posted on October 30, 2014 at 15:12:58
vinnie2
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Location: North Carolina
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 Well I have bread boarded it just like the schem in my original post above, but I am not getting any sound out of the other end. I have double checked my wiring hook ups and i think they are correct. I have a guess that there may be some problem with the way I wired the 813 for triode mode. The two methods I tried were:

1. connected g3 & g2 to each other and then to the plate via a 200 ohm resitor.

2. connected g2 to the plate via the 200 ohm resistor and connected g3 to ground.

Neither worked. Are both of those wrong? I have an o-scope so I can try to trace the signal through the circuit if need be, but was hoping something in the schem or my post might jump out at some one as wrong.
Thanks

 

Re: Found it...., posted on October 30, 2014 at 15:26:31
vinnie2
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Location: North Carolina
Joined: September 28, 2013
 Decided to go back through the wiring one more time, and dang if there wasn't a wire hooked up wrong. Sound is coming out the other end now, and tomorrow I can hook up the other channel and start tweaking.
Would still appreciate it if some one sees anything wrong with the triode connection if you let me know.

 

RE: Can I use a resistor instead of the choke for coupling to the 813?, posted on October 31, 2014 at 15:27:55
Ejam
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 Vinnie2

You might want to adjust the cathode resistor on the first section of the 6EM7 from 12k to 1k2. The 6EM7 configuration looks remarkably like Eric Barbour's front end to his Class A2 SV811-10 amp featured in Glass Audio back in the 90s.

 

RE: Can I use a resistor instead of the choke for coupling to the 813?, posted on October 31, 2014 at 18:04:24
vinnie2
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Posts: 4481
Location: North Carolina
Joined: September 28, 2013
 I appreciate the tip, but i already have taken the bread board apart for now. It sounded pretty good, but I think it might have real potential with higher b+ and plate current. You can get into a very linear area on the plate curves that way and still not be pushing the 813 too hard at 500 vdc b+ and 100 mA current. I want to do some more work on that at a later date, and I will try the 1k cathode resistor then if I do.
Thanks

 

RE: Can I use a resistor instead of the choke for coupling to the 813?, posted on November 1, 2014 at 10:32:13
dave slagle
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Location: NYC
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 I guess that is plausible but the 800 ohm Rp of the EM7 would take care of the miller just as easily and give you some gain to boot.

dave

 

RE: Can I use a resistor instead of the choke for coupling to the 813?, posted on November 2, 2014 at 07:04:17
Tre'
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Location: So. Cal.
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 Agreed, so what would be the point?

If one is going to use a CF in this case wouldn't it be better to have a negative supply at the bottom of the CF and direct couple to the output?

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

Miller effect and tubes with plate caps, posted on November 3, 2014 at 05:22:51
Jim Dowdy
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 One of the benefits of plate caps is that it offers optimal distance between the grid and the plate. This makes for extremely low input capacitance, which tends to make Miller effect a non-issue.

I am in agreement that the cap in the cathode-follower makes no sense here.



 

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