OP-6 from scratch

[DaveP]

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.

I think the original OPT is around 50k : 600, which I believe is unobtainable unless you use an IPT reversed.  This is the reason I used a triode for the output, because at around 5mA , the ra is fairly low and with the feedback becomes low enough to drive a 15k:600 OPT successfully.  With a 6J7 pentode the ra is probably around 600k in parallel with the choke impedance, so it is much higher than a 6J5 and needs a high ratio OPT to have a chance.  There are not many chokes available to choose from, but the Hammond 153C at 150H does a very good job with a low Z tube and its affordable!

DaveP

 

[Tubetec]

Happy big 7-Oh Daithi boy 

I have to admit to really liking the big fat caps on the HT too, ok it may well take a few minutes to reach peak volts after power up ,thats not a problem as tubes take anything upto 30 minutes  to reach optimum(to my ears anyway) , theres a reason why your 1950's tube radio still lives and some modern thing that ramps ht instantly through the roof  with no load current drawn  dies young .

You'd hardly  take your brand new V8 upto 155mph on the  Autobahn before the block has  reached  temp would you ? 
My 14 year old ride on mower always gets a little warm up time before load is applied , the oil runs like honey , starts first time everytime ,still on the original spark plug and battery was replaced once .

 

[beatnik]

I think the original OPT is around 50k : 600, which I believe is unobtainable unless you use an IPT reversed.  This is the reason I used a triode for the output, because at around 5mA , the ra is fairly low and with the feedback becomes low enough to drive a 15k:600 OPT successfully.  With a 6J7 pentode the ra is probably around 600k in parallel with the choke impedance, so it is much higher than a 6J5 and needs a high ratio OPT to have a chance.  There are not many chokes available to choose from, but the Hammond 153C at 150H does a very good job with a low Z tube and its affordable!

DaveP

Thanks Dave

If using the higher ratio output transformer, what inductance value would be ideal ? I assume it needs more inductance ?

 

[beatnik]

Sowter has output transformers for the Telefunken V72 / V76 in two versions, the original ratio of 11.2:1 and an alternative 9:1

They also have the Telefunken plate chokes :

- 750H approx at 1KHz 7 ma max DCR  18.4 Kohm
- 250H approx 12 mA max DCR 6.8 K Ohms approx

Do you think any of these could work in the original OP-6 circuit ?

 

[dmp]

Beatnik,
Have you seen the thread in the drawing board on the OP-6?
https://groupdiy.com/index.php?topic=60889

Lots of info there on the circuit and transformers.

OPT is 48k:500, Choke DCR is 4k,  Choke inductance is 400Henries.

If you really want the pentode output of the OP-6 you could try a 10:1 input transformer wired backwards with the choke feed and see how it sounds.  Or I used a Jensen JT-10k61-1M wired 8:1 in my V76 build.
But Dave's solution of a triode wired output is really good. It's similar to the BA-2 output stage (triode wired 1620 into 5:1 OPT).

 

[DaveP]

If using the higher ratio output transformer, what inductance value would be ideal ? I assume it needs more inductance ?

Look at it this way...........a 150H choke has an impedance of almost 1M at 1kHz, this is a massive load resistance, it is even way higher at higher frequencies, so the loadline is almost horizontal, which equals maximum mu and gain.  At bass frequencies the impedance goes down to  47k at 50Hz so a big drop in gain, you could use three Hammond chokes in series to get 450H and it would still be cheaper than a Sowter if you live in the US.  Of course all those pentodes give you plenty of gain to spend on NFB which gives some relief from the loss in bass.

The 47k at 50Hz of the choke is in parallel with the 600K of the rp of V3, so still 43K, but with a triode with rp say 10k in  parallel you get 8k before NFB which means that a 47k load does not look so bad at low frequencies.  This is how you decide on which choke and tube you need, you do the math,  this is why groupdiy is different from other forums

They decided they wanted 88dB gain for the lowest output mics of the time.  Are you going to use mics like that nowadays?  I would guess not and remember what I said about the output level being limited by the VU meter, at 88dB gain the noise might well be unbearable unless you pad the meter and wind up the output level to max.

I hope this is helpful

DaveP

 

[beatnik]

Thanks Dave, that is certainly helpful and good food for thought

 

[beatnik]

Beatnik,
Have you seen the thread in the drawing board on the OP-6?
https://groupdiy.com/index.php?topic=60889

Lots of info there on the circuit and transformers.
If you really want the pentode output of the OP-6 you could try a 10:1 input transformer wired backwards with the choke feed and see how it sounds.  Or I used a Jensen JT-10k61-1M wired 8:1 in my V76 build.
But Dave's solution of a triode wired output is really good. It's similar to the BA-2 output stage (triode wired 1620 into 5:1 OPT).

thank you that thread is good information

I already have a ba-2 but have a few 6j7 laying around and wanted to try a different preamp, the op-6 output stage seems a good candidate

 

[emrr]

Ba-1, BA-11.  Could do either of those with parafeed.  Same ratios. 

 

[alexc]

Nice end result there Dave.  Lovely.

My only 'mod'  would be to make the 'vu meter'  switchable bypass  ...  for the absolute purists  

I also like a 'hi-lo sensitivity' switch on the meter too  for those occasional ocaasions where one does want to bash the levels...

..

Apart from that, I would say - another one

[carpentry metaphor alert!] 

'nailed'  [meaning to nail a nail into wood fully and completely leaving no doubt about it]

 

[IBart]

Hi everyone , i thought i'd feedback about Dave's RCA OP-6.

I’ve recorded through allot of pre’s from API to Neve to Siemens to Telefunken to UA to Electrodyne to Lachapelle and i’d not ever used an RCA OP-6. But I’ve read glowing reviews online like ‘ its a Holy Grail mic preamp ‘.
So when Dave mentioned he was building one using allot of orignal / vintage parts , i was keen to have it here
But i had not real idea what it would sound like at all , or would it suit what i do here.

So when it arrived using my U67 then my C12 , i recorded some A / B’s with acoustic guitar and sang some as well into my new OP-6 . On this A / B it was up against an API 312, a Lachappel pre and Daves very own 241-76 which i use each week esp for vocals. 

And once my ears had cleared after a days recording, i noticed that how thick and chocolatey the OP-6 is.
Its also soo smooth and gives this 3D affect to the audio. A fantastic sound…. kinda put a smile on your face sound.

I can quite see why the RCA OP-6 has gained it reputation, and has raving reviews online !
With more listening and recording more with the OP-6 today, its got a lovely subtle distortion / saturation going on.

It also looks fantastic with its glowing 1939 meter !

I’m not big producer but its my living  (and i;ve had 5 songs on the major radio stations in the last two years) and over the years aside from gear lusting, having the right tools is a must and gets you to your destination quicker .
I've had vintage Siemens sitral desks here, vintage Neve pre’s and all sorts but I continue to be wowed by the sonic personalities of Dave's amazing audio boxs.

Ian

www.ianbarter.com

 

[octavez]

Congrats Dave and thank you for sharing, amazing work!

I was curious if there would be any changes on the power supply to make it work on 110v? I'm from France but live in the US now so 110v is more available than 220v here

 

[DaveP]

I was curious if there would be any changes on the power supply to make it work on 110v? I'm from France but live in the US now so 110v is more available than 220v here

The only changes would be a 110V LED indicator and using the 115V power transformer windings in parallel instead of in series.
Best
DaveP

 

[vinyvamos]

Really great to see your skills put to yet another project DaveP. I have built two OP-6-ish preamps so far in line with the methods at preservationofsound.com, where he wired the first stage as triode, and used an interstage pot volume control instead of the bespoke gain/NFB rotary switch. He also used an Edcor 15K:600 in the output and Hammond choke but he did not change the 1620 pentode output stage. I followed his method, but I think that the output stage needs revisiting due to much too low Z for that pentode.

What I am wanting to bring up as an idea, is that of lifting the centre-tap wires on the secondary of the Edcor, then giving two 150 ohm windings which can be wired in paralllel. This will reflect a primary Z much higher then, which could suit the pentode, and will give a ratio of about 10:1. You would also need to increase choke L but this could be done cheaply with a pair of the Hammonds in series (humbucking too?). I measure Primary L of the Edcor 15K:600 at 85H at 100Hz on my Protek LCR meter, which is getting close to the 120H of an actual OP6 output trafo that I have measured with the same Protek. Another possibly even better candidate could be the VTB2380 from Carnhill/AML UK. This measures a total primary L of 115H and with secondaries parallel it gives a ratio of 10:1.

Am I barking up the wrong tree here? Will the two suggestions above yield poor results?

 

[DaveP]

What I am wanting to bring up as an idea, is that of lifting the centre-tap wires on the secondary of the Edcor, then giving two 150 ohm windings which can be wired in paralllel. This will reflect a primary Z much higher then, which could suit the pentode, and will give a ratio of about 10:1. You would also need to increase choke L but this could be done cheaply with a pair of the Hammonds in series (humbucking too?). I measure Primary L of the Edcor 15K:600 at 85H at 100Hz on my Protek LCR meter, which is getting close to the 120H of an actual OP6 output trafo that I have measured with the same Protek.

I have separated the ct wires  on an Edcor before, it is tricky, but possible with a pointed knife when the solder is melted.  You have to use minimum heat to avoid melting the plastic terminal holder.  You will also need another terminal but there is a space for one.

This is not the complete solution because the preservation circuit does not have the same feedback circuit as an OP-6.  The feedback is important because it reduces the output impedance of the final pentode, this enables it to drive the OPT better.

DaveP

 

[emrr]

What I am wanting to bring up as an idea, is that of lifting the centre-tap wires on the secondary of the Edcor, then giving two 150 ohm windings which can be wired in parallel. You would also need to increase choke L but this could be done cheaply with a pair of the Hammonds in series (humbucking too?).

Yes to the humbucking chokes, I posted that somewhere here.  You could also try a pair of humbucked Edcor outputs primaries series and secondaries parallel.  Dave did that with inputs once.  The Carnhill should be tried. 

 

[fazer]

So glad your wife is better Dave. 

This is a lovely project that always seem to work beautiful with your metal work and you electronics know how.  Have you recorded something with it yet? 

I have some 6J7s and 6SN7s.  It looks like a cool project to work on.  I'm making a record of it .  Very nicely done.

 

[DaveP]

Hi Fazer,
It was bought by Ian Barter's studio and he says it's his go-to pre for vocals, there is something about these old tubes and circuits that just knocks chip based stuff out of the ring.

The wife is almost back to her old self now which makes life better for both of us.  There will be an RCA 96-A from scratch project starting soon to further use some  tubes from the 30's.

Best wishes
DaveP

 

[fazer]

I was thinking about using a 6SN7 as the triode with the second section as a CF to replace the 6J5.  It could be a Parallel connection also.  rp is the same as a 6j5 .  Thoughts.

 

[IBart]

Hi Fazer,
It was bought by Ian Barter's studio and he says it's his go-to pre for vocals, there is something about these old tubes and circuits that just knocks chip based stuff out of the ring.

The wife is almost back to her old self now which makes life better for both of us.  There will be an RCA 96-A from scratch project starting soon to further use some  tubes from the 30's.

Best wishes
DaveP

I’m falling more and more in love with it’s delicious tone