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10W Push Pull Tube Amp [updated with testing]

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tobias

Mon Jan 20 2014, 01:25AM

Registered Member #1956 Joined: Wed Feb 04 2009, 01:22PM
Location: Jersey City
Posts: 172

Yeap, yet another tube amplifier.
This one is based on the Amplifiers book (York) posted by PM at a different post. (thanks!)

Schematics can be found at page 101. And before that, 80 pages of good theory =)
The idea is to have a setup as flexible as possible to allow for different testing / configurations.

Valves (or tubes) are ECC81 for Phase Splitting and First Stage, and EL84 for the output stage. I am buying the cheapest options online (they go for around $10 each). If it works, I will look for better ones.

After much consideration and some advice I decided to draw a power supply on a PCB using today's components and the rest will be point to point, at least on the first attempt.

PSU design goals:
- Allow use of Choke (Pi) or Resistor voltage regulation
- Include space for up to 1000uF of filtering for the B+ line (~300VDC)
- Allow the use of AC or DC (rectified or filtered) power for the heaters
- Standby relay (to turn power to the output stage after allowing the valves to heat up)
- Include fuses for all Input Power Transformer secondaries
10w Amp Psu2

EDIT: New PSU layout

Still to do / figure out (if you can help please step in!)
- Need a 5H to 10H Choke. Flea markets not being of much help...
- Need to come up with a procedure for testing, since the idea is to be able to find out the best configuration. I have a USB Signal Generator and a Oscilloscope. It has to be enough!
- Find a box

I will update this thread as I progress with the project.

[EDIT 23-mar-1024]

So I spent a month waiting for all components to arrive.
Two weekends working on the box
One weekend mounting the PSU on the box and testing it out.
And this weekend, finally, working! (one channel only)

Almost no Hum (even with messy wiring!). I am impressed how much presence 10W has.

Next weekend will be devoted to iron the second channel, and make it neater. I am happy with the hum level already, so if it gets better will be a plus!

Need to find the amount of feedback I want now (on Phase splitter). Running on naught.

[EDIT 1-apr-2014]

Finished the second channel and had some music playing on it!
After applying feedback from the output to the voltage amplifier the background noise became inaudible.

Real amplifiers glow in the dark =)

Now fixing some old 92dB sensitivity JBL that had the foam edge falling apart to match the new amp =)

[EDIT 18-apr-2014]

So I fixed the JBLs that were given to me. And for the past weeks I spent some great deal of time listening to this amp. Great sound! But I am not aficionado, so I will not characterize it as warm, deep, or whatever. Instead I did a few simple measurements using my scope and function generator.

Starting with sinewave, trying to find the clipping point. At around 9 W:
Newfile6

Then I tried to measure using the FFT feature on my scope. I have no idea how to read it as now. Will start reading about it after this post. I am trying to calculate the THD at two different power levels, 2.5 W and 7 W:

Newfile2

And finally some square wave on to it:

Newfile10

It looks like I have a 'tendency to parasitic oscillation' as per

Maybe sometime I will try more measurements, but I was very happy to see the nice crisp sinusoidal coming out of it! =)

Proud Mary

Mon Jan 20 2014, 04:24PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Hi Tobias,

Re: PSU.

In the Thermionic Age (1930 - 1970) high value electrolytics were not available at the high working voltages needed in valve power supplies, so smoothing circuits used small electrolytics in the range 4 - 16 Î¼F together with an LF choke in a Ï€-filter circuit. Looking at typical consumer applications such as radios or record players of the post-war period , we will often see a line up like this: 4Î¼F-5H-8Î¼F or 8Î¼F-10H-16Î¼F. The first capacitor was called the reservoir capacitor, and the second one was called the smoothing capacitor. Very often, both capacitors were included in a single can, the "dual electrolytic."

Today, high value aluminium electrolytics are available up to 450V, so I would suggest an RC filter, rather than using the two-caps-and-a-choke that was used in olden times.

PS: If you decide you still want to use an LF choke, you must specify the current capacity, as well as the inductance e.g. 10H 100mA.

Dr. H.

Mon Jan 20 2014, 08:55PM

Registered Member #931 Joined: Mon Jul 30 2007, 05:25PM
Location: Bulgaria
Posts: 486

Hi Tobias,

just a quick suggestion - some of the clearances on the board look too small for dealing with 300+V. I suggest you revise the layout on few spots and increase the isolation.

I will be following the project with interest.

Cheers

tobias

Sat Jan 25 2014, 02:22PM

Registered Member #1956 Joined: Wed Feb 04 2009, 01:22PM
Location: Jersey City
Posts: 172

Thank you Dr. H.! I updated the design on the first post =)

During the week I received a bunch of components: capacitors, tubes, sockets, fuse holders, rectifiers...

Doing a final fit test before ordering the boards. I will try a chinese manufacturer that does 10 of these for $20.

tobias

Thu Feb 20 2014, 02:06AM

Registered Member #1956 Joined: Wed Feb 04 2009, 01:22PM
Location: Jersey City
Posts: 172

Finally some progress!
Got the PSU board and components together:

But now I have a question: I have the option of referencing the rectified Heater voltage to any point within B+.

Considering a B+ of 320V, should I reference the heaters to 0V, to 320V, to 160V or not reference it at all?

Multiple sources on the internet advise all different options.

Thanks!

Mattski

Thu Feb 20 2014, 02:50AM

Registered Member #1792 Joined: Fri Oct 31 2008, 08:12PM
Location: University of California
Posts: 527

How's the quality of those PCB's and how long did it take to receive them?

$20 for 10 10x10cm boards seems like a good deal as long as you don't need super tight tolerance.

tobias

Thu Feb 20 2014, 02:53AM

Registered Member #1956 Joined: Wed Feb 04 2009, 01:22PM
Location: Jersey City
Posts: 172

It did not work. Or if it is working it is taking a LONG time. I ordered the one on the pic from OSH Park.
When (if) the chinese ones get here I will compare them =)

tobias

Mon Mar 24 2014, 04:04AM

Registered Member #1956 Joined: Wed Feb 04 2009, 01:22PM
Location: Jersey City
Posts: 172

Updated first post with progress.
Smoke test and first light this weekend! (one channel)
Very impressed so far.

tobias

Wed Apr 02 2014, 02:40AM

Registered Member #1956 Joined: Wed Feb 04 2009, 01:22PM
Location: Jersey City
Posts: 172

Updated first post to show last week's progress.

Will clock a few hours of music until it gets a pre-amp and a FM radio with nixie display =)

Steve Conner

Wed Apr 02 2014, 08:11AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Cool!

Referencing the heaters to a small positive DC voltage can reduce hum. I used +40V in one build, and in another I used the voltage developed across the output stage cathode bias resistor.

160V or 320V would probably be too much, and it's also not advisable to leave the heater circuit floating. So if you don't have a suitable DC voltage, just reference it to 0V.

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