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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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 Post subject: OPT for Push-Pull 6V6
PostPosted: 30 Jul 2018, 06:04 
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Joined: 30 Jul 2018, 05:54
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Greetings

I wish to ask concerning the push pull 6V6 design that is shown at this web page (DIY Audio Projects), the one with 6SL7 input/driver stage.

An 8K centre tapped primary is specified for the output transformer. I have good quality 6.6K output transformer (Partridge, and Hashimoto), and I wish to know if a 6.6K primary might be OK for use in this schematic? I have seen 6V6's used with 6.6K a primary, perhaps a different operating point is required for a lower load? ... or perhaps the exact load is not crucial here? ... any advice on this would be most appreciated!

A most elegant schematic, btw!!

Warm Regards
Jonathan


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PostPosted: 30 Jul 2018, 21:39 
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Joined: 04 Jun 2008, 20:59
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Location: Arizona, USA
Hi, Which schematic....it doesn't show in my PC. For class A operation at up to about 250 volts it should be fine. For AB operation you can go a bit higher for the B+. Roughly the difference between 4-5 watts and 10-12. My personal preference is for 8-10K at 225-250 B+ class A. The higher impedance will reduce the output only slightly and will reduce the distortion more.

Good listening
Bruce

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PostPosted: 31 Jul 2018, 04:11 
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Joined: 30 Jul 2018, 05:54
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Hi ... thanks for this, much appreciated!

The circuit I have in mind is this one:
http://diyaudioprojects.com/Schematics/ ... Amplifier/

Is a 6.6K primary OK in this context ... I think from what you said it would be advisable to lower the B+ a bit, to lower current draw through the 6V6's for use with a 6.6K primary ... is this correct?

Thank you!
J


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PostPosted: 31 Jul 2018, 10:21 
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Location: Arizona, USA
Hi, I would use 250 for B+ with the 6.6K transformer. The project author would also be likely to assist you. Choice of output impedance (and subsequent B+) is to a fair extent a designers choice. I would use 8K as well, but the 6.6K ought to be OK. New production 6V6 tubes are generally fine with up to about 350 B+ as long as you keep the dissipation within spec. Older ones are sometimes not good for that level and I run them in the 250 range. FWIW...a 6AQ5 would work fine in this design as they are quite similar in spec to the 6V6. Subbing type 5751 for the 6SL7 is the same. A non-octal amp would be more compact (not as neat IMO) as the octal version and perform the same. It is BTW a class AB design.

Good listening
Bruce

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PostPosted: 31 Jul 2018, 16:49 
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Joined: 19 May 2011, 05:38
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Use them in triode mode ab1 and enjoy some very nice results.


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PostPosted: 31 Jul 2018, 17:22 
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Joined: 06 Oct 2015, 22:05
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gofar99 wrote:
<snip> It is BTW a class AB design. </snip>


Is my understanding correct that if the grid of the output tube is connected to the cathode on the preamp it is AB and if it is connected to the plate on the preamp tube it is class A?

Is it that simple or is there more to it?

Jonathan Noble wrote:
<snip>
The circuit I have in mind is this one:
http://diyaudioprojects.com/Schematics/ ... Amplifier/
</snip>


Please post updates on your build. I've been tempted to try this one for awhile.


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PostPosted: 01 Aug 2018, 04:10 
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Joined: 24 Nov 2010, 14:39
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Location: California
I built an amplifier like this (stereo). I used solid-state rectification, and have about 300 volts B+ to the output tubes. I used Edcor GXPP15-8K output transformers. I made a couple of changes that Bruce recommended. I upsized the coupling capacitors, from .1 uf to .22 uf. I increased the value of the electrolytic bypass capacitors, on the cathodes of the output tubes, from 47 uf to 470uf. There changes gave the amplifier much better bass response.

Since I had 12 volt heater voltage available on the power transformer, I used 12SL7 and 12V6 tubes. I kinda wanted to use NOS tubes (nostalgia), and the 12 volt heater versions are in much less demand that their 6-volt cousins, so they are very inexpensive about $6 each at AES. The amp wont exactly shake the walls, but it does sound really nice.


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