DIY Audio Projects Forum

6V6 Marblewood Amp
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Author:  Suncalc [ 04 Mar 2018, 16:56 ]
Post subject:  6V6 Marblewood Amp

I have finally completed my new amp. This one bears the moniker "The 6V6 Marblewood".
6V6 Marblewood.jpg
And here is how she looks from the back.
6V6 Marblewood Back.jpg

This is another 6V6 UL power stage amp operated at "optimum" bias. Unlike some earlier amps, this one is aimed specifically at the first time DIYer. It uses a 4S Universal preamp topology as a driver, it only has a single choke in the power supply design, and it has sufficient margin to run not only 6V6s in the power stage, but 6F6s, 6W6s and 6L6s as well. The design also dispenses with a separate rectifier winding and uses a 9-pin 6CA4/EZ81 rectifier. All this was to add simplicity to the design.

Here are the schematics. First the amp itself (one channel shown):

And the power supply:

I've spent a couple of hours throwing different music at it and it behaves beautifully. Clear, responsive, and fast with an excellent wide soundstage and silent as the grave with no input. The frequency bandpass is good with the upper end rolloff only down 0.2dB at 20kHz and the low end gently rolloff by the transformer, down about 2.5dB at 40Hz. The 4-prong power supply topology used produces excellent channel separation with no discernible crosstalk between channels. Distortion is exactly where one would expect with this topology. Here are the numbers:
Distortion Summary.png

The amp sounds wonderful and, in my humble opinion, looks good as well. I am glad to finally have this one complete. It's been an epic 10 month journey!

So what do people think? As always, questions and comments are more than welcome.

Author:  Geek [ 04 Mar 2018, 17:09 ]
Post subject:  Re: 6V6 Marblewood Amp

She's lovely! :thumbsup:

Author:  Suncalc [ 04 Mar 2018, 23:19 ]
Post subject:  Re: 6V6 Marblewood Amp

Just a little more information here. Here is the pass band data for the 6V6 Marblewood amp; all taken at 1W nominal output.
Marblewood Amp Bandpass Characteristics.png

And here is a shot of the insides all wired up. Well, except for the B+ dropping resistor. Not too crowded and easy in which to work.
You'll note that for the indicator light, instead of using my typical dimmer, I chose to use a fixed 25Ω, 3W resistor. This is a simpler and less expensive option than the 50Ω, 5W rheostat.

One more quick note: This amplifier uses a 4S universal circuit as a driver. This means that you can freely swap 12AU7, 12AV7, 12AY7, 12AT7, 12AX7, & 12BH7 driver tubes. But it also has enough margin in the power supply to use the following power tubes: 6V6, 7408, 6L6, 5881, 6F6, & 6W6. I have tried all these power tubes and they all work acceptably well in the amp. So all combined this is a total of 36 different driver/power tube combinations. Add to this all the different brands and sub-types and it means that there are hundreds, or maybe even thousands, of different combinations to try out. Each one with a subtly different sound.

This little amp is a tube roller's dream!

Author:  gofar99 [ 06 Mar 2018, 16:26 ]
Post subject:  Re: 6V6 Marblewood Amp

Hi, I love the wood.

Good listening

Author:  Suncalc [ 08 Mar 2018, 16:21 ]
Post subject:  Re: 6V6 Marblewood Amp

A complete writeup of the 6V6 Marblewood amplifier project is now on the CascadeTubes web page here:

As always, questions and comments are more than welcome.

Author:  Gio [ 11 Mar 2018, 22:03 ]
Post subject:  Re: 6V6 Marblewood Amp

Very lovely build. Congrats Matt, it turned out great.

I really like all the tube possibilities. Generally, how does the power and distortion output change with 6L6/5881 tubes.

Author:  Suncalc [ 12 Mar 2018, 19:37 ]
Post subject:  Re: 6V6 Marblewood Amp

Gio wrote:
Generally, how does the power and distortion output change with 6L6/5881 tubes.
Verses the 6V6, the power goes up and the distortion is very similar at1W. The 6L6 distortion being slightly lower. Here is a distortion summary table:
Distortion Summary.png
I don't trust the 6F6 low power distortion number, I think I type a wrong number for the 3rd harmonic. I'll have to recheck it.

The peak clean output power for the 6L6 is 3.15W as opposed to 2.54W for the 6V6; the difference being just slightly less than 1dB.

Author:  mwhouston [ 19 Mar 2018, 13:28 ]
Post subject:  Re: 6V6 Marblewood Amp

Mate, what a great build. Never seen the wood before and looks a real talking point.

These days I'm hooked on my 6L6 SE UL builds like this. I've found the 6L6 fast sounding and dare I say a little SS sounding especially with the 12AT7 up front. In the first of my builds (your design of course) I split the HT to the driver stage to get better channel separation. In my 6V6 build I split the HT after the first cap. So two chokes and two double Pi filtered etc. A little more work and cost but I believe I hear real benefits.

My last four builds have all been SE ULs. I think you get more grunt and better more solid mids with this topology.

Author:  gofar99 [ 19 Mar 2018, 21:15 ]
Post subject:  Re: 6V6 Marblewood Amp

Hi, I did something similar a while back with 6L6s. It worked fine, but IMO not as well as the P-P EL84 U/L amps. So I converted it to a 4P1L SE/UL See the schematic. Nice sound a bit more complicated and limited to about 2 watts. Better sounding than the SE 6V6 / 6L6 ones but still not my cup of tea.

Still love that wood....

Good listening
4P1L SEP UL Amplifier ver 3 Feb 13 2018.jpg

Author:  harrydutt [ 31 Mar 2018, 11:40 ]
Post subject:  Re: 6V6 Marblewood Amp....Question about transformers

Hi Matt,

I already have a pair of Edcor GXSE15-5K opt's - , are these okay for this amp ?
They're the same except rated for 15 watts.

Would this Edcor power transformer be suitable - (120V, 60Hz. line to 550V (275-0-275) at 250mA center tapped and 6.3V at 7A.) ?
It has higher current ratings and is only $10 more.


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