DaveP

OP-6 from scratch
« on: October 02, 2019, 06:46:55 PM »
I thought I had retired from making gear for a while, but my wife is a lot better and our singer has broken his ankle!

So I have a few weeks to make something from stuff in my workshop, like this lot:-



A 60 year old box from a vacuum gauge.



A western 862 vu meter

I have replaced the burned out bulbs with two warm white LEDs in series with a 560 res to run on 6V, works a treat.



I have adapted the circuit's gain from 88-90dB to a switched 37-70dB in 3dB steps.



The lower gain does not need a pentode output stage so I subbed a 6J5 triode stage.

I redesigned the ladder pad through trial and error and whatever common sense I might have left.

Should be fun ;)

DaveP

Soundcloud: Delayed Action.


Re: OP-6 from scratch
« Reply #1 on: October 02, 2019, 06:55:45 PM »
Nice one Dave.  Happy to read your wife is a lot better.  Also sorry about the singer's ankle.  But selfishly looking forward to watching this new adventure.

DaveP

Re: OP-6 from scratch
« Reply #2 on: October 06, 2019, 12:51:52 PM »
The original steel chassis was pierced all over so it was easier to make another from ally sheet.



It has an interesting design as the whole thing is secured by four screws on the front panel to the main steel box.



The ground, B+ and 6.3V for the heaters comes in from a separate box via the Bulgin plug and socket.

The supply is DC so no need to twist, hopefully these measures will give better hum performance  than the original.

DaveP

Soundcloud: Delayed Action.

ruffrecords

Re: OP-6 from scratch
« Reply #3 on: October 06, 2019, 02:26:47 PM »
Hi Dave,

Very pleased to hear you wife is better. Health is number one priority at our age!

I think you have successfully cracked the gain control. It is a clever combination of NFB plus output attenuator.

The only thing I do not like about this design is the 47K input resistor. Of course it is essential for the NFB but its noise is directly in series with the input. Hmmm, I wonder if there is away of achieving the same ends without the series 47K???

Cheers

Ian
www.customtubeconsoles.com
https://mark3vtm.blogspot.co.uk/
www.eztubemixer.blogspot.co.uk


'The only people not making mistakes are the people doing nothing'

DaveP

Re: OP-6 from scratch
« Reply #4 on: October 06, 2019, 03:16:33 PM »
I did think about that...........

When I do these projects I am always torn between how much to change and how much to leave!
I thought about changing  it to a 20k, but then all the other feedback resistors would have to be reduced  and that would change the load on the tube.  In the end I thought that people are using the originals with a very old 47k so it may be the lesser of two evils.  At least mine will be a decent metal film.

I decided that it was best to stick to the original as far as possible because in the end it has to sell itself.  Buyers often don't  understand the changes we make for the better, they have so much blind faith in original components.  Carbon comp resistors being a prime example.

DaveP

Soundcloud: Delayed Action.

ruffrecords

Re: OP-6 from scratch
« Reply #5 on: October 06, 2019, 05:24:51 PM »
I hear what you are saying. I will give it some thought anyway.

By the way, is not C13 a little on the low side at 220nF? I reckon it gives a low end -3dB point around 50Hz, or was that intentional?

Cheers

Ian
www.customtubeconsoles.com
https://mark3vtm.blogspot.co.uk/
www.eztubemixer.blogspot.co.uk


'The only people not making mistakes are the people doing nothing'

DaveP

Re: OP-6 from scratch
« Reply #6 on: October 06, 2019, 05:43:09 PM »
Ian,

C13 is the original schematic value at the moment, I plan to optimise that under test.   The triode output gives a lower output Z so it may need a bigger cap.  As I'm also using a different OPT, I will need to test various cap values to avoid resonance peaks in the sub region.

Dave
Soundcloud: Delayed Action.

Re: OP-6 from scratch
« Reply #7 on: October 07, 2019, 08:40:55 AM »
Adding a cathode (or dare I say MOSFET!  ;) )  follower to the first stage, reducing the impedance of the feedback network, and returning it to the bottom of the input transformer would lower the noise.  But then it's not an OP-6.
In terms of altering the design as a whole i say, if it's for you, do what results in the best performance.  If for someone else, whatever makes them happy 👍

Re: OP-6 from scratch
« Reply #8 on: October 07, 2019, 10:39:31 AM »
Better apply the feedback to the cold end of the input transformers secondary.
Andreas

DaveP

Re: OP-6 from scratch
« Reply #9 on: October 09, 2019, 07:03:21 AM »
Thanks for your comments and suggestions, much appreciated.

The pre-amp is nearly finished with most of the components in place.



I found some neat grid caps on ebay for the 1620's,  These tubes are from Heintz & Kaufman who made transmitting tubes in San Francisco, I'm not sure if they are RCA rebranded or made in-house.  This is an original interior (courtesy of Retro Gear Shop) it looks very busy compared to mine with modern components.





Here you can see the Hammond 156C choke and Edcor OPT.  Because the output Z of the 6J5 is much lower than that of a 6J7, the inductance can be lower to allow use of this choke.  The OPT is parallel fed so an Edcor WSM15K:600 does the job.  I have connected it so the phase between input and output remains the same.



The gain switch was quite easy to make using 0.6W mf resistors, I just tied the ladder "rungs" together for the earth connection, this will come from just one screened cable to avoid hum loops

I now have to paint the front panel before assembly.  I don't have time or inclination for an exact copy, which is not possible because I have put the sockets at the back anyway, so it will be an "homage" to the original.

DaveP
Soundcloud: Delayed Action.


rob_gould

Re: OP-6 from scratch
« Reply #10 on: October 10, 2019, 03:52:02 AM »
Looking good Dave!

Nice to see another project, and good news about your wife 👍

alexc

Re: OP-6 from scratch
« Reply #11 on: October 11, 2019, 10:06:41 PM »
Glad to follow this thread  - good news about one's loved ones health is always uplifting; what better way to celebrate than a new DIY Audio build  :)

Intriguing circuit, with regards to the nfb arrangements and so on.  I will enjoy looking and thinking about the OP-6.

I love the metal work, as is often the case with Dave's builds ....   especially so this one, with the nice framework - the bent gold colored pieces.  Damn fine!

Looking forward to seeing the progress  ..   more power to you Dave.
I ping therefore I am

DaveP

Re: OP-6 from scratch
« Reply #12 on: October 12, 2019, 05:38:59 AM »
Thank you Rob and Alex for your kind words.

This is the front plate after 1000, 2000, 5000 & 7000 grit papers on the sander, then a buffing on the wheel.



This is how it will look with the meter and knob in place.



Now I have to apply the lettering and spray on some varnish to seal it.

DaveP
Soundcloud: Delayed Action.

Ricardus

Re: OP-6 from scratch
« Reply #13 on: October 12, 2019, 10:47:07 AM »

Now I have to apply the lettering and spray on some varnish to seal it.

DaveP

You're going with the natural metal color?
Audio mastering for hire..

DaveP

Re: OP-6 from scratch
« Reply #14 on: October 12, 2019, 11:33:05 AM »
Yes, I've done it before here:-


It has a mirror finish after polishing

DaveP
Soundcloud: Delayed Action.

Ricardus

Re: OP-6 from scratch
« Reply #15 on: October 12, 2019, 04:26:48 PM »
How are you doing the lettering?
Audio mastering for hire..

DaveP

Re: OP-6 from scratch
« Reply #16 on: October 12, 2019, 05:58:19 PM »
Letraset or whoever makes decals nowadays.

DaveP
Soundcloud: Delayed Action.

Ricardus

Re: OP-6 from scratch
« Reply #17 on: October 12, 2019, 07:14:08 PM »
Letraset or whoever makes decals nowadays.

DaveP

I was just watching a video from a maker who did some research and didn't think Letraset was being made anymore. Whatever you find please let us know so I can let her know.
Audio mastering for hire..

DaveP

Re: OP-6 from scratch
« Reply #18 on: October 13, 2019, 04:24:30 AM »
Ricardus,

Try Amazon with this link

Chartpak Dry Transfer Letters and Numbers, 12PT Helvetica Font, 1521 per Pack (00100)
by Chartpak

Good luck
DaveP
Soundcloud: Delayed Action.

Re: OP-6 from scratch
« Reply #19 on: October 13, 2019, 01:08:16 PM »
Nice one Dave. 
I'm interested in how you find those 1620 valves stack up.  Glancing at the data they seem pretty much the same as our EF37a valves from the same era.  With a good one, I think the EF37a's are really quite nice, either in pentode or triode mode.


 

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