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Old 26th March 2004, 03:01 PM   #1
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Default 6B4G Amplifier Questions

Am currently planning to build a PP 6B4G amp similar to Ralph Power's unit. I would like to drive the finals with an ECC99 and have read through the threads. I have two questions;

1. I have a pair of Magnequest Trans from an ST-70 clone. They have 4.3 K primary impedance. Would that match up well with the 6B4G?

2. What would be your suggested configuration for the drivers looking into a Lundhal LL1630 interstage Trans?

Sorry for the basic questions but my tube theory is just good enough to prevent me from killing myself or burning down the house!


Would appreciate your input.

Regards;
Pete
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Old 26th March 2004, 03:55 PM   #2
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Thumbs up 4k3 opt

Hello Peter Menting

4.3k is an excellent choice! You should get around 15W. PP class AB1 fixed bias, 38mA plate current and 12.3W plate dissipation per tube at 325V plate voltage. Vg1 about -68V.

Sorry don't have an suggestion for #2.

Wayne
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Old 26th March 2004, 04:13 PM   #3
SY is offline SY  United States
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I'd be more inclined to use a driver stage that can swing 68V peak (not a terribly large amount) and avoid the interstage transformer entirely. Joe Curcio's cascode diff amp from his ST-70 mod published in Glass Audio would work well- the sample I tested had no problem swinging over 100V.
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Old 26th March 2004, 05:00 PM   #4
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Default Re: 6B4G Amplifier Questions

Quote:
Originally posted by Peter Menting

2. What would be your suggested configuration for the drivers looking into a Lundhal LL1630 interstage Trans?
That depends a lot on what you're feeding it with. Does your linstage have balanced outs?

Peace
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Old 26th March 2004, 06:47 PM   #5
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Thanks for your responses; I believe that I would only require 6-8 Watts since I am using speakers whose sensitivity is 94db and only above 125hz. I had thought to keep the finals in class 'A' if possible thereby only asking for perhaps a -45/+45 swing from the ECC99's.

I am currently using the output of a CD player to drive the system and had not planned on building a linestage with a balanced output.

Pete
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Old 26th March 2004, 08:26 PM   #6
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Quote:
I had thought to keep the finals in class 'A' if possible thereby only asking for perhaps a -45/+45 swing from the ECC99's.
Here you go:

Vb = 250V
Vg1 = - 46V
Vg1 sig = 45 Vac pk
Ra-a = 4.3k
Ip no sig = 107.6mA (both tubes)
W out class AB1 = 7.75W
W out class A = 6.23W
Pd = 13.46W

Wayne
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Old 26th March 2004, 08:44 PM   #7
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Default LTCP?

Hi,

Quote:
Joe Curcio's cascode diff amp from his ST-70 mod published in Glass Audio would work well- the sample I tested had no problem swinging over 100V.
Hmmm....Vaguely remember that one.
Was this a cascode of 6922s as LTP with a CCS or a negative Vref in the tail?

I think this would have tremendous gain (1000+) and very low noise as well using the 6922.
Alternatives could be the 6SN7 ( 420) or a 6SL7( 5000).

From this a ECC99 could be driven which in turn would rattle the grids of the 6B4Gs.

Cheers,
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Old 26th March 2004, 10:14 PM   #8
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Yes, that's it. 50K plate load, 400V B+, 10ma CCS in the cathode circuit. Gain is a bit lower than your guess, about 300-400, IIRC. I don't know if you'd even need the extra ECC99 driver tube- the 6922/6DJ8 family can pump pretty well into triode grids.

You'd implement it with CCa, if I know you.
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Old 26th March 2004, 10:42 PM   #9
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Hi,

Quote:
I don't know if you'd even need the extra ECC99 driver tube- the 6922/6DJ8 family can pump pretty well into triode grids.
Nah...You probably won't even need the extra stage.

Your gain "guesstimate" seems rather low considering the humble SN7 can make it to 420 (checked the math on that one).

This is a formidable circuit idea that's both simple and powerful, I'm starting to like it already.

Quote:
You'd implement it with CCa, if I know you.
The few I've left are sitting in my headamp....
With 24 VDC on their roof and 6V on the heaters they're going to last forever....
Not a bad thing either, considering the asking price.

BTW, the LTPC was first implemented_that I'm aware of_ and publicised by L.B. Hedge in a 1956 Wireless World issue page 283. "Cascode A.F. Amplifier".

No need to tell J.C.
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Old 26th March 2004, 10:59 PM   #10
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Agreed, mu is very high for a cascode, but it suffers in the same way as a pentode from high output resistance, and that drops its gain. I recently screwed a gain of 200 out of a 6SN7/FET differential pair cascode. (Predicted gain = 235)

Edit: Should have said that the gain of 200 was to one anode.
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