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 NEW  Matt presents bias and operation data for the 6V6 tube in SE operation - 6V6 Single-Ended (SE) Ultra Linear (UL) Bias Optimization.

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PostPosted: 28 Oct 2013, 11:24 
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Joined: 28 Oct 2013, 11:09
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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


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PostPosted: 28 Oct 2013, 13:53 
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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

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PostPosted: 28 Oct 2013, 17:40 
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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

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PostPosted: 13 May 2019, 15:22 
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Joined: 19 Oct 2018, 15:30
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Location: Montréal, Québec
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.


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PostPosted: 28 May 2019, 19:22 
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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


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PostPosted: 06 Jun 2019, 10:45 
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Location: Montréal, Québec
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 !


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PostPosted: 08 Jun 2019, 23:46 
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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.


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PostPosted: 09 Jun 2019, 16:05 
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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


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PostPosted: 10 Jun 2019, 16:36 
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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


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PostPosted: 10 Jun 2019, 17:08 
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Location: US Pacific Northwest
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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