PPP 6CB5A schematic? Look OK?

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Re: PPP 6CB5A schematic? Look OK?

Postby Blair » Sun Jun 16, 2013 7:01 am

Hi,

That's a 6BC5 in your link? The 6CB5 does not have the amplification factor on the data sheets that I can find.

I see the "triode" amplification factor of 3.8, but not pentode mode. Are they different?
Last edited by Blair on Sun Jun 16, 2013 7:04 am, edited 1 time in total.
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Re: PPP 6CB5A schematic? Look OK?

Postby DeathRex » Sun Jun 16, 2013 7:03 am

kheper can you show us how you calculated the gain?
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Re: PPP 6CB5A schematic? Look OK?

Postby kheper » Sun Jun 16, 2013 7:55 am

At the bottom of page 2:

http://tubedata.tigahost.com/tubedata/s ... 6/6BC5.pdf

"Typical Operation and Characteristics:
<snip>
Amplification Factor..................... 40 42"

You can calculate it with mu = gm * Rp. Again, at the bottom of page 2:

"Plate Resistance(Approx)..... 6000
Transconductance..... 6000"

mu = .006 * 6000 = 36 (close to 40)
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Re: PPP 6CB5A schematic? Look OK?

Postby Blair » Sun Jun 16, 2013 8:00 am

Right. That's the 6BC5 though.

Not the 6CB5. The 6CB5 only has the "triode" gain listed as 3.8.
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Re: PPP 6CB5A schematic? Look OK?

Postby kheper » Sun Jun 16, 2013 8:03 am

Blair wrote:Right. That's the 6BC5 though.

Not the 6CB5. The 6CB5 only has the "triode" gain listed as 3.8.


Argh. My error. I'll look at the 6CB5.
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Re: PPP 6CB5A schematic? Look OK?

Postby kheper » Sun Jun 16, 2013 8:13 am

http://www.mif.pg.gda.pl/homepages/fran ... 6/6CB5.pdf

It comes out to 44.

Plate Resistance..... 5000
Transconductance... 8800

mu = .0088 * 5000 = 44

What is mentioned on page 1 is "Mu-Factor, Grid No.2 to Grid No.1". That cannot be the Amplification Factor.
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Re: PPP 6CB5A schematic? Look OK?

Postby DeathRex » Sun Jun 16, 2013 8:29 am

OK, that seems easy enough. But wouldn't the 7591 be 295.8?

Plate resistance = 29000
Transconductance = 10200 umhos

29000 X 0.0102 = 295.8
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Re: PPP 6CB5A schematic? Look OK?

Postby DeathRex » Sun Jun 16, 2013 8:33 am

That can't be right. (???)

I do get 8 for a 6L6 though.

Plate resistance of 1700

Transconductance of 4700umhos

1700 X 0.0047 = 7.99
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Re: PPP 6CB5A schematic? Look OK?

Postby kheper » Sun Jun 16, 2013 8:36 am

The 7591's "Triode Amplification factor" is listed as:

"16.8"

Strapped as triode its Plate Resistance decreases.

http://www.mif.pg.gda.pl/homepages/fran ... 7/7591.pdf
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Re: PPP 6CB5A schematic? Look OK?

Postby DeathRex » Sun Jun 16, 2013 8:41 am

16.8 triode gain. Would the gain be 295.8 in pentode mode?

Or do you use the load resistance of 3000 to come up with 30.6?
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Re: PPP 6CB5A schematic? Look OK?

Postby kheper » Sun Jun 16, 2013 8:52 am

Look at the 6L6's Plate Resistance as a pentode and as a triode.

Page 2:

Pentode = 22500

Triode = 1700

The Amplification factor is 8. It's only mentioned in triode mode. So, the 6CB5a's mu is 3.8, not 44.

The 6CB5a will be harder to drive than the 7591 or the 6L6, not easier as I erroneously wrote before.

http://www.mif.pg.gda.pl/homepages/fran ... 6/6L6G.pdf
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Re: PPP 6CB5A schematic? Look OK?

Postby Blair » Sun Jun 16, 2013 9:14 am

Well,

This was an emotional roller coaster. LOL! Looks like I'm back to a high gain first stage into my 6GF7A stage.

Will the 6CG7 have enough gain into the 6GF7A to give me enough swing from ~275-300V source?

Thanks!

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Re: PPP 6CB5A schematic? Look OK?

Postby kheper » Sun Jun 16, 2013 9:26 am

From the datasheets, Class A1 specs:

7591 Grid1 Voltage = -10V
6L6 Grid1 Voltage = -14V - -18V
6CB5a Grid1 Voltage = -30V

You'll need to swing more with a 6CB5a, than the other two.

Now that I remember, only triodes (and tubes strapped as triodes) have amplification factors. The "Grid No.2 to Grid No.1" phrase in the 6CB5a sheet threw me. I thought that they were measuring something there.
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Re: PPP 6CB5A schematic? Look OK?

Postby Blair » Sun Jun 16, 2013 9:39 am

Something like the KT88 with G1 @ -30V needing 60V g-gV

So, aim for 70-90V swing capability from the front end?
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Re: PPP 6CB5A schematic? Look OK?

Postby kheper » Sun Jun 16, 2013 9:55 am

From the KT88 (and other tubes like the EL34) data I've seen:

G1 Voltage = -15V

http://www.mif.pg.gda.pl/homepages/fran ... k/KT88.pdf

KT88 tubes are not hard to drive. The 6CB5a looks to need twice the swing. Vinyl Savor uses an interstage, and Tubelab uses sand. But, they use them with most of their amps. I think Vinyl Savor made another version with no interstage, but I cannot find the link.
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