OP-6 from scratch

[DaveP]

The two parts are now finalised, that's the pre-amp and the remote power supply.

These are the final schematics:-



C13 was chosen to maximise the low end which is now flat down to 15Hz.

C19 rolls off the response over 35kHz as there are a lot resonances around 75kHz otherwise.

C4/R6 make the response flat to 20kHz, after which it declines .



R16 is there to bleed off the voltage after switch-off.

C16 is to remove any HF hash that escapes the smoothing caps.

I'll post some pics later on today along with some more input/output data.

Ian Barter has kindly bought the amp and he will no doubt give his impressions further down the line after I ship it to him, next week.

Best
DaveP

 

[DaveP]

Three pics of the remote power supply.







Here's the finished OP-6 interior,  the choke has a mumetal foil screen held in place with copper foil.
The small 12,000 hour 180uF 420V caps have transformed the power supplies I make since I started 13 years ago.



The audio path is all polypropylene.  Heaters are all DC.



Before I started this project, I decided that I would only do it if I could get a genuine Weston meter and RCA Daven knob, they were on Ebay within a week of each other, those two alone were over $260, so a big investment in pure cosmetics!

DaveP

 

[Fuzz Face]

Wow great build. The power supply looks impeccable, the meter and knob are very cool, and I love the way you did your grounding.

Thanks for sharing all these beautiful pics and schematics.

 

[Winston OBoogie]

Wow great build....
Thanks for sharing all these beautiful pics and schematics.

+10.  Thanks Dave  
I vicariously get to enjoy the building of DIY valve amps thanks to threads like these.  Your "start to finish" threads are top notch.

Do you have any thoughts or opinions on how it performed and sounded in situ yourself? 

 

[DaveP]

Thanks for your support.

I turned 70 in October and I'm obliged to understand I'm in the last chapter, so I guess there is inevitably a legacy element about these projects.

My generation grew up with DIY so that has been theme of my projects, to show what is possible with consistent effort.

I hope that it inspires later generations to continue to work with tubes and components and get their hands dirty!

Best
DaveP

 

[rmburrow]

Three pics of the remote power supply.

Before I started this project, I decided that I would only do it if I could get a genuine Weston meter and RCA Daven knob, they were on Ebay within a week of each other, those two alone were over $260, so a big investment in pure cosmetics!

DaveP

Very nice build.  I am very familiar with the OP6.  (have one, missing p/s!)  Is the 2 amp fuse a little too much, with 230 v primary?  One would think that 1/2 A or 1 A would be sufficient...we don't like seeing power transformer burnouts in the event the diodes short, etc.  I recall the original power supplies for the Western Electric, Raytheon, and similar remote amps had a simple 5Y3 type rectifier for HT and simply ran AC for heater power...

 

[DaveP]

You are probably right, 2A is excessive, but because I now use such big smoothing caps, I was concerned about the initial current inrush.

DaveP

 

[ruffrecords]

You are probably right, 2A is excessive, but because I now use such big smoothing caps, I was concerned about the initial current inrush.

DaveP

Which is why it is always a good idea to specify slo blo fuses with tube gear.

Cheers

Ian

 

[L´Andratté]

I turned 70 in October

My very best wishes to you and yours!

 

[Winston OBoogie]

I turned 70 in October...

Best wishes Dave.

 

[DaveP]

The project is now finished, the meter went in today.

I forgot to put it on the schematic, but it's just fed from the output via a 3.6k resistor like normal.



Also the two series LEDs are fed from the heater supply through a 560 ohm resistor which gives around 10mA and a nice light as you can see:-



Finally the back goes on and its serial number..............Done

.

I will post some results of my level & noise tests in another post.

Best
DaveP

 

[zamproject]

Hey Dave

Very nice as usual !

Bravo 

Cheers
Zam

 

[DaveP]

Before I fitted the VU meter I ran it up to max output, it actually produced a clean 60Vp-p on the 'scope into a 10k load, this is 21.2Vrms or +28.6dB at 1kHz.  However, it cannot reproduce a clean 10kHz at that level.  It's worth noting that 60Vp-p at the secondary is 300Vp-p at the primary which is down to the amazing choke loading.

With the VU meter fitted we are limited to 1 or 2V output unless we want to bend the pointer or burn out the diodes!  At this modest level the full undistorted frequency response  is available.

I will attempt to test the S/N ratio as soon as I have time, I suspect it may well be better at the lowest gain (most NFB).  I don't have the facilities (or ears!) to test the sound quality, but I expect Ian Barter will report on that for me.

Best
DaveP

 

[DaveP]

OK, I tested the OP-6 for noise at three different gain settings, the lowest 37dB, 55dB and the highest 70dB.

What soon became obvious was that the maximum output of the VU meter limited the S:N ratio because it topped out at around 5Vrms.  So as the EIN was around 3uV in all cases, the lowest gain gave the lowest noise at the output as this table shows:-

Gain    Gain factor  Noise    Output noise  Output  S:N Ratio    dB
37dB            70.8          3uV            212uV              5V            23584      87.4

55dB            563            3uV            1.689mV          5V            2960        69.4

70dB          3126            3uV            9.378mV          5V              533          54.5

It would be possible to disconnect the VU meter and run the output right up to 50V and then pad it down again to improve on that 70dB figure which would then add 20dB to the S:N ratio.

But to be honest I doubt it would ever need to be run at much more than 55dB in practice.

Hope you all enjoyed the project and thanks for the support

Best
DaveP

 

[dmp]

Fantastic build!

So the EIN is -108 dB. That's really good! 

Looking at the thermal noise of the 47k resistor, it's about 2.7 microV, so almost all of your noise is coming from that resistor. Could that be right?

RMS Johnson (thermal) noise = sqrt(4*k_b*T*R*dFreq)
Where I used T=300K & dFreq=10kHz

 

[ruffrecords]

Fantastic build!

So the EIN is -108 dB. That's really good! 

Looking at the thermal noise of the 47k resistor, it's about 2.7 microV, so almost all of your noise is coming from that resistor. Could that be right?

RMS Johnson (thermal) noise = sqrt(4*k_b*T*R*dFreq)
Where I used T=300K & dFreq=10kHz

That looks right to me. 20KHz bandwidth noise in 150R is -131dBu, so in 1500R it is -121dBu so in 15K it is -111dBu. 47K is just over 3 times that which is another 5dB giving a final value of -106dBu. 10KHz bandwidth will be 3dB less or or -109dBu.

Cheers

Ian

 

[dmp]

If you ground the grid to V1, would we expect the EIN drop to 0.3 microV (3 - 2.7), -128 dB?
Or are noise contributions not summed? i.e. tube noise would show up once the 47k resistor is grounded out.

 

[beatnik]

awesome build as always Dave

if using the original 6J7 output stage, what sort of parameters would you advise for selecting  the anode choke and output transformer ?  I couldn't  find out the specs of the original RCA parts.

 

[ruffrecords]

If you ground the grid to V1, would we expect the EIN drop to 0.3 microV (3 - 2.7), -128 dB?
Or are noise contributions not summed? i.e. tube noise would show up once the 47k resistor is grounded out.

Noise sources are not correlated so they do not simply add. Their powers add instead so two equal noise sources combine to make one 3dB noisier. You can do it with squares so:

X^2 = 3^2 - 2.7^2 = 1.71 so X = 1,31

Noise will drop 10log(3/1.31) = 3.6dB

Cheers

Ian

 

[dmp]

Noise sources are not correlated so they do not simply add. Their powers add instead so two equal noise sources combine to make one 3dB noisier. You can do it with squares so:

Thank you - great info.