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Is there a secret to using the 7A4?

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Posted on January 6, 2017 at 20:34:25
DAK
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I hooked up according to the base diagram. it is taking the place of a section of 6sn7. B+ is 300. Plate resistor 80k , Cathode resistor is 360 ohms. Filament voltage is 6.4vac. I am using it as a driver tube.
I put signal on G1 and get nothing. I just did the same circuit with a 6j5 with no problems

 

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You do know it's a different pinout?, posted on January 7, 2017 at 04:04:05
andy evans
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Different pinout, but otherwise should work the same.

 

RE: what about the central locator pin?, posted on January 7, 2017 at 08:17:29
DAK
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Different than 6j5 and 6sn7 yes i know that. What about the weird central alignment pin. Is there supposed to be something hooked up to that?

 

RE: what about the central locator pin?, posted on January 7, 2017 at 11:06:28
maxhifi
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Ground it.

 

RE: Is there a secret to using the 7A4?, posted on January 7, 2017 at 11:20:06
Tre'
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With an 80k ohm plate resistor and only 300 volts supply you can't run much more than 2ma. or there will be no voltage left at the plate.

Try a 2.7k ohm cathode resistor or keep your 360 ohm cathode resistor and replace the 80k plate resistor with a 22k.

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

 

RE:Actually, i had a circuit problem. But, your suggestion is good, posted on January 7, 2017 at 14:39:33
DAK
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Thank you for the help. I ran across a wiring mistake. Since, i am using fixed bias, which i hardly do i wired the grid to ground resistor to ground forgetting that the fixed bias circuit establishes the ground. But i will try your suggestion as the 7a4 plate voltage is too low. Btw, is it better to have bigger plate resistor (80k) or smaller cathode resistor of 360R?

 

Ok, thank you , nt., posted on January 7, 2017 at 14:41:41
DAK
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.

 

If i use a Blue LED ?, posted on January 7, 2017 at 15:05:15
DAK
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Or any other color for that matter. What should the plate voltage be? I was originally aiming for around 250vdc but i got nowhere near that. Like you said, the Av for the 7a4 was low only 60vdc. It was working though and sounded ok.

 

RE:Actually, i had a circuit problem. But, your suggestion is good, posted on January 7, 2017 at 18:52:17
Tre'
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If your circuit uses fixed bias why do you have a cathode resistor.

With the grid resistor feed by a negative bias supply the cathode should just be grounded.



BTW I read your other post and if you are going to use a LED in the cathode for cathode biasing you still need a proper amount of plate current and a proper amount of plate voltage.

You want to operate the tube somewhere that the tube is linear.

A 6sn7 with only 60 volts on the plate and only 2ma. (or so) is not a linear place.

___

I like 12-14ma 200 volts on the plate but that operating point is out of the question it you use a plate resistor.

I use a CCS plate load.

It makes everything easier and makes for a very linear stage with a lot of plate voltage swing (if you need a lot of swing, like in a driver stage).


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

 

I'm open to suggestions and a CCS would be something i'd like to try, posted on January 7, 2017 at 19:47:44
DAK
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for the driver tube. I know i can get a B+ of 300v for the driver stage.
As it stands now, i have 2 LEDs in series, which puts the 7a4 cathode at 3V. This also raised the Plate voltage to 115vdc.
I got some SS devices although i am not sure if they are rated for hi voltage. The ones i have are LR8N3G, TIP50, LM337, LM317HT. Can i make a CCS for the 7A4 plate with any of those. regards, Dak

 

RE: I'm open to suggestions and a CCS would be something i'd like to try, posted on January 8, 2017 at 08:08:07
Tre'
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Posts: 11851
Location: So. Cal.
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Something is not adding up here.

If you look at the plate curves for a 7a4 at the spot where 115 volts plate and -3 volts bias intersect. That shows about 4.5ma. plate current.

4.5ma of current across 80k ohms will drop 360 volts.

To end up with 115 volts at the plate you would have to start with a 475 volt supply feeding the top of the plate resistor.

As for CCS......DN2540 are cheap. $1.42 each at Mouser.

Check the link below.

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

 

RE:I changed the plate resistor between posts, posted on January 8, 2017 at 08:38:55
DAK
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The plate resistor is now 50K. I am thinking of adding another LED. That should bring the voltage up to a good point. Thank you for the link i will definitely check that out. cheers, Dak

 

RE:I changed the plate resistor between posts, posted on January 8, 2017 at 17:33:29
Tre'
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"The plate resistor is now 50K"

Ok, now things make more sense.

"I am thinking of adding another LED"

That will drive the current even lower.

The plate voltage will be higher, like you said, but I don't think the tube will be very linear at such a low current.

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

 

RE:I changed the plate resistor between posts, posted on January 8, 2017 at 19:57:30
DAK
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Hi Tre, with 3 LEDs the voltages are now; Av 168. Kv 5.57vdc. and the plate resistor is actually 56k. The B+ to the plate resistor is 360 under the conditions set by the 3 Leds.

 

RE:I changed the plate resistor between posts, posted on January 9, 2017 at 09:41:04
Tre'
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Posts: 11851
Location: So. Cal.
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Ok, those numbers all line up like they should.

With a 56k ohm plate load the load line will be pretty horizontal but with only 3.4ma of plate current that tube will not be as linear as it would be if there was more current.

More current with a 56k ohm plate resistor would take a lot of B+ (supply) voltage.

A CCS will give you your cake (a horizontal load line) and you can eat it to (get about 10 or more ma. of current going).

Then that tube would be very linear (low distortion).

Just something to think about.

Tre'

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

 

Do you know someone who makes one that i can buy?, posted on January 9, 2017 at 09:56:01
DAK
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In this situation, with my new amp, i would like to keep the variables as few as possible, so i would rather get one that has seen a lot of people using them and so would be vetted and less worrisome? In other words if i build one i still would not be sure if i had an poor interaction between the CCS and my amp. So, i would rather buy one with a good "track history" as it were.

 

RE: Do you know someone who makes one that i can buy?, posted on January 9, 2017 at 14:54:26
Tre'
Industry Professional

Posts: 11851
Location: So. Cal.
Joined: February 9, 2002
K&K Audio (here on the AA) sells kits.

Look for "Two Terminal Constant Current Source" on the page linked below.

You want the Cascode ones (2 MosFets per CCS).

I just make my own using a pair of the dn2540 MosFets.

All it takes is 2 of the MosFets, 2 gate stop resistors and a current set resistor. That's it!

Tre'


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

 

RE: Do you know someone who makes one that i can buy?, posted on January 9, 2017 at 15:27:19
DAK
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Right, Kevin sells them. i will check that out. But later, do you mind if i contact you about building my own? thanx for the link. regards

 

RE: Do you know someone who makes one that i can buy?, posted on January 9, 2017 at 16:20:12
Tre'
Industry Professional

Posts: 11851
Location: So. Cal.
Joined: February 9, 2002
Just send email through the AA.

I'll help.


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

 

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