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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: EL84 SE UL power amp
PostPosted: 07 Nov 2015, 18:06 
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This started as a Decware inspired build but having Edcor OPTs (under the bonnet) with the UL tap and tubes which are pentodes I thought why not make the amp SE UL. I could put a switch in to switch between the two but I have built three SE UL amps in the past and loved the sound.

The Sophia 274B mesh plate may not remain. A smaller 5U4 tubes may replace it. I have some nice looking Golden Dragons plus others. Though this amp is assembled it is not wired. The EL84s are Japanese made AWVs and I have an AWA matched pair also plus a few others. The 6N1P is just a generic Russian.

The base is raw at this point and will get a final sand and beeswax treatment. My base building and speaker box building mate Don made the base and ABS top as usual. More to follow.

Can some one tell me if in UL mode I need any more than 100ohm resistor between the UL tap and grid two. So that would be Rg2 = 100ohms. I'm thinking 1W Carbon film here.
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 Post subject: Re: EL84 SE UL power amp
PostPosted: 08 Nov 2015, 03:03 
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100 ohms is usually fine. I use 470R simply because I have this "5 pound bag" a-la Ebay, LOL! :D

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 Post subject: Re: EL84 SE UL power amp
PostPosted: 08 Nov 2015, 05:16 
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Geek wrote:
100 ohms is usually fine. I use 470R simply because I have this "5 pound bag" a-la Ebay, LOL! :D

Thanks.

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 Post subject: Re: EL84 SE UL power amp
PostPosted: 08 Nov 2015, 09:27 
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mwhouston wrote:
Can some one tell me if in UL mode I need any more than 100ohm resistor between the UL tap and grid two. So that would be Rg2 = 100ohms.
Leave it out. The EL84/6BQ5 does not have a screen transconductance which would promote oscillation in conjunction with the output transformer primary winding. All that a resistor does here is force you closer to pentode mode and hurt the 3rd order harmonic response. IMHO.

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 Post subject: Re: EL84 SE UL power amp
PostPosted: 08 Nov 2015, 16:57 
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Suncalc wrote:
mwhouston wrote:
Can some one tell me if in UL mode I need any more than 100ohm resistor between the UL tap and grid two. So that would be Rg2 = 100ohms.
Leave it out. The EL84/6BQ5 does not have a screen transconductance which would promote oscillation in conjunction with the output transformer primary winding. All that a resistor does here is force you closer to pentode mode and hurt the 3rd order harmonic response. IMHO.


Thanks will do. Hope to have this finished before end of week.

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 Post subject: Re: EL84 SE UL power amp
PostPosted: 13 Nov 2015, 21:07 
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Attachment:
PB133387 (800x571).jpg
This is a Decware inspired EL84 power amp with attenuation. My twist was to make a it a EL84 SE UL (not triode). Also I upgraded the PS a little. One PS tweak was to use a Russian PIO 4uf cap as the first cap in the PS. Also the larger electros are all snubbed.

The inter-stage caps are Audio Note Tin foil, mylar and oil. The amp I called "Tamarisk" - just like how it sounds. The finished amp looks good and not too heavy. Sound is a big step up from my already great sounding 6V6 - Ozzie tube SE UL amp. On live classical every part of the music has a layer. There every instrument is heard in its own layer and decay, reveb etc also have there own layers. If I had have paid $5K for this amp it sounds like what I would expect for that money.
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 Post subject: Re: EL84 SE UL power amp
PostPosted: 14 Nov 2015, 01:40 
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Suncalc wrote:
Leave it out. The EL84/6BQ5 does not have a screen transconductance which would promote oscillation in conjunction with the output transformer primary winding.


Oh? I learned something new. So used to working with EL34 and ilk that make really interesting g2 RF oscillators :blush:

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 Post subject: Re: EL84 SE UL power amp
PostPosted: 14 Nov 2015, 02:28 
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AS usual I rubber mounted the Hammond power tranny, added its own RFI\EMI power filterand ceramic tube sockets. There is a mix of carbon film and metal film resistors. I also added a 10H 150mA choke to the PS. I stuck close to the Decware schematic and suggestion of parts. It's not untill you try and build a Decawre amp like this that you realise some very clever little tweaks which they have done to a simple tube amp. Find a schematic of thier EL84 Triode and look at the PS feed going into the 6N1P. They use of polies to form part of the filtering and audio return for the driver section. I didn't have 3.3uf 400V polies so I used 2.2uf 450V electros but snubbed them with 0.1uf Solens.

The use of a single Rk to be shared by the power tubes is a little odd. But I wired it the way they suggested. Not sure what it is meant to do.

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 Post subject: Re: EL84 SE UL power amp
PostPosted: 14 Nov 2015, 09:12 
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mwhouston wrote:
The use of a single Rk to be shared by the power tubes is a little odd. But I wired it the way they suggested. Not sure what it is meant to do.
The requirement for single or dual cathode bias resistors can be confusing. Most power triodes and high transconductance tetrodes require separate cathode bias resistors in push-pull to help ensure the tubes go cleanly into cutoff and hence minimize crossover distortion. Most beam power pentodes are so well behaved in their transitions that a single bias resistor can be used for both tubes and crossover distortion is still acceptable.

Going to UL mode I'm not really sure how they will behave. I would look at the crossover distortion with the single Rk and if it looks ok then you're set. If it looks excessive, run the separate Rks so that the you get a small auto bias current at crossover that cleans up the transition. We can't count on the inductance of the transformer to do all our work for us. :D

WAIT A MINUTE - THIS IS SINGLE ENDED :eek:

A bypassed single Rk for both channels? The AC is bypassed but the bias voltage is forced to identical values. If the tubes aren't very well matched this is going to lead to channel imbalance. One nice thing about SE amps is no matched tubes. This is just silly, IMHO. I know that Steve Deckert has a reputation, but this is just designing finicky for finicky's sake.

It will be interesting to see how this turns out.

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Last edited by Suncalc on 14 Nov 2015, 09:20, edited 1 time in total.

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 Post subject: Re: EL84 SE UL power amp
PostPosted: 14 Nov 2015, 09:18 
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Suncalc wrote:
mwhouston wrote:
The use of a single Rk to be shared by the power tubes is a little odd. But I wired it the way they suggested. Not sure what it is meant to do.
The requirement for single or dual cathode bias resistors can be confusing. Most power triodes and high transconductance tetrodes require separate cathode bias resistors in push-pull to help ensure the tubes go cleanly into cutoff and hence minimize crossover distortion. Most beam power pentodes are so well behaved in their transitions that a single bias resistor can be used for both tubes and crossover distortion is still acceptable.

Going to UL mode I'm not really sure how they will behave. I would look at the crossover distortion with the single Rk and if it looks ok then you're set. If it looks excessive, run the separate Rks so that the you get a small auto bias current at crossover that cleans up the transition. We can't count on the inductance of the transformer to do all our work for us. :D


I'm just a little confused this is a SE UL build. Where is the Xover distortion?

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