OP-6 from scratch

[fazer]

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

I’ll start rounding up some parts.  I have some 0uncer 016 input with 50k output mic transformers and the 6j7’s.  Plenty of old cases so will need a choke and output trans .  Also have some Modutec VUs I don’t have any old ones.  These have to be 50 years old but nothing that goes back to the 30’s.

 

[emrr]

I’ll start rounding up some parts. 

I have original OP-6 inputs in the BM.

I can make the original gain control for anyone wanting that, including modified Daven's that will retrofit a bad original OP-6. 

 

[DaveP]

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.

That would work OK, just need a bit more heater current.

DaveP

 

[mjrippe]

I just posted the schematic of the OP-6's grandpa, the 62-A, in the Tech Docs section if anyone is interested.  I will be scanning the entire manual from 1937 soon.

 

[emrr]

I just posted the schematic of the OP-6's grandpa, the 62-A, in the Tech Docs section if anyone is interested.  I will be scanning the entire manual from 1937 soon.

Grandpa is true, interesting to note the 62-A was the budget alternative to the OP-5, and like many other things the fancier gave way to the more inexpensive as things progressed into the 1940's.  I have an OP-4 and an OP-5, both are intended for battery power only.  Have seen the OP-2 and OP-3, nearly scored an OP-3 once. 

 

[mjrippe]

Complete RCA 62-A manual uploaded here: https://www.dropbox.com/s/hna6milje5hq5xe/62-A%20Manual.pdf?dl=0

 

[emrr]

Complete RCA 62-A manual uploaded here: https://www.dropbox.com/s/hna6milje5hq5xe/62-A%20Manual.pdf?dl=0

Wow, that PSU is same concept as the 96-A.  Very surprised to see such complexity in the lower range remote amp, and no similar supply offered with the OP-4/5. 

 

[DaveP]

mjrippe,

Thank you for that!

Best
DaveP

 

[mjrippe]

Wow, that PSU is same concept as the 96-A.  Very surprised to see such complexity in the lower range remote amp, and no similar supply offered with the OP-4/5.

Agreed, Doug.  I was sharing this manual with a friend who designs audio gear and mics and we were marveling at the PSU.  Four tubes and DC regulation is a HUGE expenditure at the time.

 

[rjb5191]

Greetings,

May I ask what type of cable and  power connection did you use between the power supply and preamp?

best,
Ryland

 

[DaveP]

It was about 5A cable, but the most important thing was the diameter, it had to fit in a Bulgin Plug  313-993
and socket  313-970.

These are Farnell part numbers, I believe this is Newark in the US.

They were wired HT/B+,  6.3Vdc and Earth/ ground.

Best
DaveP

 

[fazer]

Hey Dave I have question on the gain switch.  I’m assuming it’s non-shorting.

 

[DaveP]

I have question on the gain switch.  I’m assuming it’s non-shorting.

I use MBB (Make Before Break) switches for this kind of use to lessen switching noise.

I use BBM types when switching B+ on meters.

Hope that answers your question, happy 4th of July and stay safe!

DaveP

 

[fazer]

Thanks Dave.  We are all safe.  I ordered the shorting type. 

 

[syn]

Very nice work Dave. Is your attenuator a drop in replacement for the original attenuator? If not, what are the differences apart from 21pos of the original vs 12pos that you made? I'm in the process of building OP-6 clone myself and have the iron ( not original but quite close, 10:1 out etc) and the tubes,  so I want to bring the attenuator part as close as I can.

Thank you

 

[C12VR]

Did I read correctly that the meter being in circuit increases EIN by 20db? what is the best possible noise figure ?

 

[DaveP]

Hi C12VR,

I have never heard that before. Many vintage designs have the VU meter in circuit without problems. Maybe some of the early diodes were a bit temperamental though. I have attached an article that explains about the EIN noise.

Best
DaveP

 

[emrr]

Did I read correctly that the meter being in circuit increases EIN by 20db?

Where do you read that?

 

[C12VR]

"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."
PG 3 of this thread
looking back on it it was S/N not EIN, but
EIN was -108db; Is there any way of raising this significantly? Isn't this quite noisy?

 

[Stevie342000]

Hi C12VR,

I have never heard that before. Many vintage designs have the VU meter in circuit without problems. Maybe some of the early diodes were a bit temperamental though. I have attached an article that explains about the EIN noise.

Best
DaveP

I am sure this issue cropped up on the Ampex mailing list.....if my recall is correct it has something to do with the VU meter not being an active circuit and thus in circuit not isolated and the diodes back EMF? If I again recall correctly at the time of the Ampex 440 it was a known issue or some years later it became a known issue.

It has to do with the meter being in the circuit and essentially not isolated...BBC PPM avoids this as it is an active circuit and many think much more accurate than the VU Meter....incidentally there is an VU Meter that uses Neons for each level it was designed by RCA...Do not recall model number.....memory seems to think about 1939.....a very early version of the later LED meter nothing new in this world....or not much ....that wheel keeps turning and gets reinvented.....