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Direct-Coupled Audio Amplifier with Cathode Follower
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Author:  Mikeh3 [ 28 Oct 2013, 11:24 ]
Post subject:  Direct-Coupled Audio Amplifier with Cathode Follower

Has anyone built this Direct-Coupled Audio Amplifier with Cathode Follower amp?
If so, where can I get the output transformer?
Were you happy with the performance?

This is a very interesting amplifier.

Thanks,
Mike

Author:  Gio [ 28 Oct 2013, 13:53 ]
Post subject:  Re: Direct-Coupled Audio Amplifier with Cathode Follower

Hi Mike.

A neat looking design indeed. I have seen only a couple built - and just one channel at a time as it is hard to find a suitable output transformer . As noted, the choice of output transformer will be very important in this circuit. The article says 6k with a DC resistance of 250 ohms on the secondary. Look for a transformer with a primary of 5 to 6k, but you have to stay pretty close to a DCR of 250 ohms on the secondary.

Cheers

Author:  gofar99 [ 28 Oct 2013, 17:40 ]
Post subject:  Re: Direct-Coupled Audio Amplifier with Cathode Follower

Hi I believe I would try this one from Edcor.

http://www.edcorusa.com/p/545/gxse5-8-5_5k

If the DCR is not high enough a fixed resistor could be put in series with the primary. It would waste a little output but not really make that much difference. You could if desired use a fairly large value capacitor across the resistor and that should restore nearly all the output.

Good listening
Bruce

Author:  Gab [ 13 May 2019, 15:22 ]
Post subject:  Re: Direct-Coupled Audio Amplifier with Cathode Follower

Sorry to dig this out of the grave, but I have the same question : does anybody have built this amplifier ? I have a pile of 6SJ7, a pair of 5k SE OPT, and pretty much everything else that’s needed. I’m curious about the direct coupled topology.

Author:  ChrisABC [ 28 May 2019, 19:22 ]
Post subject:  Re: Direct-Coupled Audio Amplifier with Cathode Follower

check here:

https://www.avforums.co.za/index.php?topic=44459.0

https://www.antiqueradios.com/forums/vi ... 2&t=329087

does your OT measure approx. 250 ohms?

This one has 230 ohms res.

https://www.tubesandmore.com/products/t ... ngle-ended

Author:  Gab [ 06 Jun 2019, 10:45 ]
Post subject:  Re: Direct-Coupled Audio Amplifier with Cathode Follower

I already have a pair of these transformers. Do you think it is close enough for using it as is ? I dont know how much close to 250ohm it needs to be. Thanks for the links !

Author:  ChrisABC [ 08 Jun 2019, 23:46 ]
Post subject:  Re: Direct-Coupled Audio Amplifier with Cathode Follower

I would measure the resistance of the OT's, they may be closer to the 250 ohm specified. I would try it even if they only measured 230 ohms (because 230 / 250 is .92 or 92 % of the value specified.
You could place a resistor in series with the OT to come to the value of 250 ohms , maybe a power resistor.

Author:  ILoveHiFi [ 09 Jun 2019, 16:05 ]
Post subject:  Re: Direct-Coupled Audio Amplifier with Cathode Follower

If they measure less resistance thats normal, becasue with audio signals its ac. Then inductance of the transformer will increase the resistance as the frequency rises

Author:  Gab [ 10 Jun 2019, 16:36 ]
Post subject:  Re: Direct-Coupled Audio Amplifier with Cathode Follower

As you can see in the schematic description, both impedance and resistance are specified. I assume that since the transformer replaces the cathode resistor, it must have a precise resistance (dc).

Would the impedance of the output transformer go up if a resistor was added in series with it ? Since mine are 5k, they would get closer to the specified 6k.

Image

Author:  Suncalc [ 10 Jun 2019, 17:08 ]
Post subject:  Re: Direct-Coupled Audio Amplifier with Cathode Follower

Gab wrote:
As you can see in the schematic description, both impedance and resistance are specified. I assume that since the transformer replaces the cathode resistor, it must have a precise resistance (dc).
Not quite.

The 6V6 cathode load is the series combination of R3 (1000Ω), R5 (355Ω), and the transformer primary dc resistance (250Ω) for a total of ≈1.6kΩ. This (along with the B+ voltage) sets the 6V6 quiescent plate voltage but NOT the grid bias. The grid bias is set by the resistive voltage divider formed by R3/R5/R6/R7. The 49.5mA cathode current drops 12.4 volts in the transformer primary which adds to the voltage set by the R6/R7 divider to set the cathode at 85v against the grid at 73v. Total grid bias is therefore -12.4v. (i.e. essentially -12.5v, the most common 6V6 grid bias.) Also note that the plate voltage works out to be 355-85=250v, the most common plate voltage for the 6V6 tube.

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