OP-6 from scratch

[DaveP]

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

I did this test some time back, but it's worth another look:-

These were only single tube tests but the most important thing to notice is the ratio between the plate current and the g2 current.  This is because it directly affects the level of partition noise.

The 1620/6J7 tube's ratio was pretty good for such an early date and the later tubes gradually increased the g2 wire spacing  to reach a maximum with the EF86.  I think the EF37A has a very high internal impedance rp so it draws the least current of all.
I think it's telling that RCA and others, carried on using the 6J7/1620 long after the 6SJ7 was introduced.

On page 510 of RDH4 there is a comparison of intermodulation distortion between a 6J7 and 6SJ7, guess which is best!

DaveP

 

[Winston OBoogie]

Bloody hell Dave, that's great 
I don't think I need to check RDH to guess that the earlier 6J7 wins the IMD battle
Yes I've read that 1620's continued to be used in more critical applications such as test equipment long after top grid cap/double-ended valves went out of fashion. 

I would be interested in a test between 1620 and EF37a in regard to noise, microphonics, thd etc.  in triode mode.  The 37's I played with were a decent alternative in certain applications to the now ridiculously priced grey glass NOS 6SN7.  At lower currents,  I tried G2 as the anode and it was about as quiet as needed for most anything.

Have you looked at the Brimar 6BR7 and  6BS7?  I haven't myself but would imagine them similar to an EF86.

Thanks for the data, very cool of you

 

[Tubetec]

Hi Dave,
Is it your first go at choke load/parallel fed transformer ?

Very glad to hear of the improvement on the home front .

 

[DaveP]

Is it your first go at choke load/parallel fed transformer ?

No I have made a couple of V241-76's for Ian Barter
https://groupdiy.com/index.php?topic=67386.0
And also my own mic-pre that used an EL84 triode output stage, that was not a groupdiy project

DaveP

 

[IBart]

I just started following this post.

And yes can vouch that the 241-76 mic pre that Dave kindly made me is the classiest sounding pre i have ever owned. Its my goto for my C12 and other mic's.

I've had here vintage Neve's & API's and modern ones , also have here right now, Neve, two API's, an Electrodyne, and two Lachapelle... all good for different occasions and i have no idea at all what a choke load / parallel transformer is , but this 241-76 has a special sound ( in my opinion ).

 

[alexc]

With regard to the circuit, it's interesting to note some similarities between the OP-6 and the rs-61  ? Yes, no ? I have to lookup the relevent years ...

In the OP-6, I like the use of the same type of pentode for the first two positions  [of the 3-tube lineup].

Similarly for the rs-61,  except one further  - it also uses the same type at the 'final' tube position. I can see it, I really can!

Both of course have the nice output pentode configured for    'triode mode' and 'choke loading ' with 'cap-coupled' output transformer.

Myself, I prefer Dave's  OP-6 approach with it's different tube  type in the 'final' role   

I once made two channels [of rca tube limiters], more or less identical, but one side with 'metal can pentodes' and the other with 'glass pentodes'    ...  just to check the differences.  I marvelled at how similar they measured once properly dialled in.

...

Just looking at the OP-6 nfb arrangements now - fascinating!

 

[DaveP]

Alex,

I changed the final tube to a triode for this project, but the original has all three tubes the same.

Yes, the RS61 has many similarities.

Thanks for that

DaveP

 

[Winston OBoogie]

...it's interesting to note some similarities between the OP-6 and the rs-61  ? Yes, no ? I have to lookup the relevent years ...

Yes for sure.  From what I've been able to find out, the RS.61 seems to have arrived out of, and after, the formation of REDD in early 1955.  Particular units still in use @ Abbey Road Studios in the 1960's have been identified carrying 560** serial numbers, although there are a few recordings from around October 1955 that I believe are the first to use the amplifier.

 

[alexc]

Thanks for those bits of  extra info re the  rs-61 and the op-6  ... 

Just checking the op-6 lineup and it does indeed have three 1620 in the original lineup.

Here's a pic of the original OP-6  front view, for reference.

 

[emrr]

OP-6 is 1938 or 1939.

 

[alexc]

and  for further ref, the original schem 

and a link to Winston's rs-61 schem thread  for related  interest ..

https://groupdiy.com/index.php?topic=73301.0

......

SO the RCA OP-6 is circa 1938-39  ..  that is so early!  And the somewhat related EMI  at 1955, much much later.

Fascinating!

......

Dave's build threads  never fail to get me jump started at my own bench     

I'm working on a recording preamp similar to the  RCA OP-7  which is  the 'portable  four-chan tube mixer'  presumably of similar family  to this  RCA OP-6 'portable preamp'

.....  for my 'pedal steel'    plus fx-pedals  setup

Kind of reminding me of the 'motown quad DI box'  of legend ...  4 awesome tube DIs with mixdown and feeds for recorder and [tube amps powered]  monitor speakers [for the performance room, I think].

 

[DaveP]

I cleaned up and lightly sanded the box and gave it a new coat of grey Hammerite paint.



The part I hate about these projects is the lettering, it's so fiddly!

However, I have finished it now and the gain switch is labelled in 3dB intervals from 37 to 70dB
I used the pointer knob to line up the intervals.



I have just given it a coat of spray on varnish, which looks fairly opaque at the moment, but will hopefully dry clear.

This will protect the lettering and the shine on the front panel.

DaveP

 

[Ricardus]

Lookin good.

 

[Winston OBoogie]

That looks great Dave, you've got skills mate

 

[ruffrecords]

That looks great Dave, you've got skills mate

He certainly has. Dave sent one of his mic pres to his customer via me so I could check its performance. Dave's build quality is exceptional (and the performance was pretty damn good too!).

Cheers

Ian

 

[DaveP]

That is down to my Grandad.  First Left.



I come from a generation and working class background where "Children were seen but not heard" and were told, "Speak  when you're spoken to".  So although my grandparents lived with us for 17 years (they were bombed out in the war).  I had almost no conversations with  them!   I think many of my generation can identify with that.

So when he did speak to me, I listened.  He said, " If something is worth doing, then it's worth doing well".  That stuck with me and became a way of life, much to the annoyance of those who wanted me to cut corners and just bodge things up.  I wonder what else I may have learned if they had talked like grandparents do now!

DaveP

 

[Winston OBoogie]

I had almost no conversations with  them!   I think many of my generation can identify with that.

I certainly identify with it.  One of my grandfathers went through and, by sheer luck, survived every major battle in the first world war.  Although he passed away before I would have been old enough to have held any type of serious conversation with him, I don't think he spoke more than a couple of times with any of the family about his experiences from then. 
Anyway, that's a great picture you posted, your Grandad would be proud that his advice has stuck with you.
Cheers

 

[IBart]

Great picture !

 

[emrr]

indeed, great pic. 

 

[IBart]

Similarly to you Dave......
My dad said to me, what his father he told me had said to him , which is '' work hard and play hard ''.... and its still my moto to this day