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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: 15 Jun 2011, 23:01 
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I've just completed a new amp and I thought I'd share the results.

This is a fairly straight forward 6V6 SE UL design with a 6SN7 driver stage (unbypassed cathode). Peak undistorted power is 3.2W RMS into 8Ω. Input sensitivity is 1.56v peak (1.1v rms) for full power. The amp is absolutely dead quiet. Here's how it looks.
Attachment:
Finished Amp.jpg

This amp was built as an experiment. I wanted to see what effect AC heaters had if I made the power supply dead quiet. So I designed a power supply with a ripple that is well below the detection limit of my test equipment. It calls for two large inductors and four filter capacitors. Then the driver stage B+ has an additional stage of RC filtering on it to make sure it's as quiet as possible. The tubes are all heated with a split 6.3vac circuit with the center tap tied to the overall signal ground.

Here are the schematics:
Attachment:
Amp Schematic.png

Attachment:
PS Schematic.png

Since I wanted the amp to be as quiet as possible so that I could see what effects AC heaters really have, I used typical robust build techniques. I separated the PS section from the signal section, I used twisted pairs for heater wiring and major signal runs, and I used a heavy copper signal ground bus tied to the chassis at only one point. Here is a picture of the underside wiring.
Attachment:
Wired Chassis.jpg

As you can see, the underside is fairly tidy but not obsessively so. I think this is a fairly typical neatness level for most DIY amps.

The result of all of this work is an amp that is dead quiet. With the volume control at full and a paused iPod attached to the inputs, I could not hear anything with my ear against each speaker. I am very pleased with the results. The amp circuit itself was designed for a -3dB bandwidth of 18Hz to well over 20 KHz. However using the Edcor GXSE10-8-5 the amp's low end starts to roll off around 35Hz and is about 4.5dB down at 20Hz. But the Edcor transformers sound really good. I have a NOS 6SN7 driver in the amp and brand new JJ 6V6S power tubes. The JJs started off a little bright but after about only two hours they really started to settle down. The amp has a very smooth tone and I expect it to get better with time. Obviously there isn't a lot of power here but the amp will be used in a little 10ft by 14ft reading room. This is plenty of power for this type of area.

And the one thing of which I've convinced myself is that the worry about using DC heaters in unipotential cathode tubes is greatly overstated. My suspicion is that most of the time when builders complain of heater hum in the signal, it is due to either poor lead dressing in the chassis or insufficient B+ supply filtering. If there were any tendency for the ACheaters to influence the signal path, I should have heard it in this amp.


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PostPosted: 16 Jun 2011, 00:23 
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Joined: 28 Dec 2010, 22:07
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Your craftsmanship is impeccable. Did you build everything from scratch or did you have help? This thing seriously looks like something that would sell in a hi-fi store for at least four figures.

This is similar to an amp I built recently. I can post pictures if you like, but I'll not steal your thunder without your approval. The amp I built uses a 12AT7 and Electro-Harmonix 6V6s instead of JJs, and the output transformers are cheap, 70v line output devices. The tubes are biased at 22 mA each so the output power is somewhat lower than what the 6V6 can usually achieve. That, and I put a 22K resistor in series with the screens.

I like the idea of separating the power supply section from the rest of the amplifier with a metal partition. This reminds me of a hundred TV or radio circuits I've pulled apart which possess a portion of the circuit board encased in a metal shield.

Ed

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PostPosted: 16 Jun 2011, 02:32 
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Location: Chilliwack, BC
Another beaut! :D

What is lacewood there? Only comes in 1" up here at my supplier and is something like $40/bf :eek:

Cheers!

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PostPosted: 16 Jun 2011, 08:22 
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Fantastic build, Matt! Looks great and I love the simplicity of the design! :thumbsup:
Suncalc wrote:
My suspicion is that most of the time when builders complain of heater hum in the signal, it is due to either poor lead dressing in the chassis or insufficient B+ supply filtering.

I agree 110% with that statement! The only reason I consider & use DC for filaments is hoping to preserve tube life. Basing the theory that DC heated filaments do not suffer the reversing surge that comes with AC. It is proven that an incandescent light burns longer on DC than AC, and I apply that reasoning to heaters.

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PostPosted: 16 Jun 2011, 09:30 
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Beautiful!

Cheers,
Greg


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PostPosted: 16 Jun 2011, 12:57 
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Location: Lisbon, Portugal
Gorgeous amplifier Matt! :)

Very nice overall job!


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PostPosted: 16 Jun 2011, 16:16 
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Nice ! Out of interest , have you measured or listened to that amp with the 6V6 strapped as triodes ?

BDA


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PostPosted: 16 Jun 2011, 17:58 
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Location: North Carolina,USA
Beautiful and EMP proof!Nice build.That wood really looks good with the black deck.Humless with pretty efficant speakers?Cool, tell us more.What kind of music are listening to and what kind of speakers are you driving with it?

David

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PostPosted: 16 Jun 2011, 20:02 
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BowToEd wrote:
Did you build everything from scratch or did you have help?
No help, just a lot of patience. I build my own wood bases in my shop, do the amp layout with a drawing program on the computer, buy Aluminum 6061-T6 80mil sheet stock, cut and drill all the pieces, spray paint the top and bottom, and then put it all together. For example, here is the metal all cut to size and ready for drilling.
Attachment:
metal.jpg
To be honest, the most tedious part was drilling all the vent holes in the base of the amp. (Please ignore the finger prints, hadn't done the final cleaning yet.)
Attachment:
Vent Holes.jpg

Geek wrote:
What is lacewood there? Only comes in 1" up here at my supplier and is something like $40/bf
The base is built from cut down 1" 2S2 stock from my hardwood supplier. I think I paid about $8 a board foot. If you ever get down to Seattle, you should stock up. (CrossCut Hardwoods)
BDA wrote:
Out of interest , have you measured or listened to that amp with the 6V6 strapped as triodes ?
I thought about doing that (even installing a switch) but it requires a bias point change which would mean switching both the cathode resistors and the screen connection. I may do it in the future but right now I'm just enjoying the amp as built. I have a design for an 807 triode strapped amp, but that's still a couple of projects away.
blackriver wrote:
What kind of music are listening to and what kind of speakers are you driving with it?
I really like the 6V6 family (6V6, 7408, 6AQ5, 6005, etc.) in the UL mode for female vocal jazz (Melody Gardot, Rene Olstead, Erin Boheme, etc.) and that's what I using it for mostly. I'm not driving anything exotic; just a pair of generic bookshelf speakers or a set of BIC DV62si (only 90dB so I don't get much volume out of them, but it's a small room). I switch back and forth trying to decide which I like better.


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PostPosted: 16 Jun 2011, 21:00 
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Suncalc wrote:
The base is built from cut down 1" 2S2 stock from my hardwood supplier. I think I paid about $8 a board foot. If you ever get down to Seattle, you should stock up. (CrossCut Hardwoods)


Thanks! I definately will! :D

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