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RCA OP-6 powerhouse...
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bren-hur
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#1
10th May 2010
Old 10th May 2010
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RCA OP-6 powerhouse...

For the RCA heads out there... here are some droolworthy pics of the OP-6 I just finished restoring/refurbishing...

It sounds as good as it looks... see posts 21, 34, 41 and 56 for sound clips.

RCA OP-6 powerhouse...-handsome-devil.jpg

RCA OP-6 powerhouse...-sexy-fur.jpg

RCA OP-6 powerhouse...-vintage-stack.jpg

RCA OP-6 powerhouse...-my-sick-rca-knob-i-built.jpg

RCA OP-6 powerhouse...-cap-stack.jpg

RCA OP-6 powerhouse...-final-overview.jpg

RCA OP-6 powerhouse...-tubes-trannys.jpg

RCA OP-6 powerhouse...-output-attenuator.jpg
#2
10th May 2010
Old 10th May 2010
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NICE! thumbsupthumbsup

Had an OP7 that I had hoped to restore at some point, but - after 7 yrs and it still sitting there, I had to let it go...

I kept my RCA BN5A tho!

Great job on the restoration!
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#3
11th May 2010
Old 11th May 2010
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She is a beaut mate!

Quote:
Originally Posted by bren-hur View Post
For the RCA heads out there... here are some droolworthy pics of the OP-6 I just finished restoring/refurbishing...

It sounds as good as it looks...

Attachment 172084

Attachment 172085

Attachment 172086

Attachment 172087

Attachment 172088

Attachment 172089

Attachment 172090

Attachment 172091
Looks Stunning, Hows it sounding, Grail Like?. Steven Sank does some amazing mods on these, what are those coupling caps they look a bit like the original Vitamin Q/zenith paper in oils, Even if they are actually audio note, I would swap them out for multicaps, one of the many mods Steven does gives a heck of a lot more capacitance (bypassed by poly caps) to the HT, somehow without damaging the glass rectifier on switch on:

Top Secret Weapon! This is all I can show you of a Stenen Sank ones guts or I will have to Kill you:

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11th May 2010
Old 11th May 2010
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Ok, this could be the start of the ultimate OP-6 thread thumbsup, so I'll bite. Here's mine (fixed by S Sank). Had problems with the attenuator until just recently (had to replace a few of those old resistors), then the 6X5 tube died. Just got a new tube in (from GS member Bowie) so haven't been able to audition much, other than noticing that it is extremely low noise.
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RCA OP-6 powerhouse...-op6f.jpg   RCA OP-6 powerhouse...-op6a-1.jpg  
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11th May 2010
Old 11th May 2010
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Yes Mate!

Quote:
Originally Posted by unit7 View Post
Ok, this could be the start of the ultimate OP-6 thread thumbsup, so I'll bite. Here's mine (fixed by S Sank). Had problems with the attenuator until just recently (had to replace a few of those old resistors), then the 6X5 tube died. Just got a new tube in (from GS member Bowie) so haven't been able to audition much, other than noticing that it is extremely low noise.
Yes mate I love these more than anything else. I had a go at making a upside down version of stevens power supply mods using poly motor start caps to try to reduce noise even further. Was just as good but not really any better.

Mains transformer is out while sowter's rewind it for proper British 240v electricity, not that half arsed american shit. Should have seen him just drop the core out of the UTC bitumin filled pot with heat from a blow lamp and just install the new tranformer and even put the original terminal board back on while you wait, Sowters are very old school and very
Shockingly good signal to noise any way, Even better than my old ms 10. Was one of this pair of rouge resistors the one in series with the daven pot secondrey output, to the negative feedback circuit by any chance?.
Thats four Holy Grails we got then so far.
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11th May 2010
Old 11th May 2010
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Quote:
Originally Posted by Bassmec View Post
Was one of this pair of rouge resistors the one in series with the daven pot secondrey output, to the negative feedback circuit by any chance?
I think so. Had a tech here in Stockholm measuring mine after I found out it distorted on low gain settings. He found that a few of those old (carbon I believe he said) resistors was falling apart, causing the distortion.
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11th May 2010
Old 11th May 2010
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Quote:
Originally Posted by Bassmec View Post
Looks Stunning, Hows it sounding, Grail Like?. Steven Sank does some amazing mods on these
I have so much love for this thing...It sounds AMAZING! ...warm, sultry, thick... all the good words!!

My friends got a BK11 and we did some acoustic guitar tests, Jensen twin servo vs. OP-6... seriously, there is no comparison. My previously loved Jensen sounded kind of plastic and fake compared to the OP-6... the OP-6 is insanely natural... it's like hearing you guitar as it is, but through a wicked hot valve... it puts your sound in a sphere of awesome. Still... I'm gonna be sending mine to Sank eventually, I just wouldn't feel 100% till he gave it the thumbs up! my 44-A is with him right now... biting my nails in anticipation on that one, I can already feel how it's gonna sound...

As far as the internals on my OP-6, heres the rundown...
- Jensen copper foil paper in oil coupling caps. (It was going to be mundorf silver/oil but I couldn't find the right values)
- Obbligato gold caps everywhere else.
- Sprague atom filter caps, stock values and modded to look the part.
- 1 watt carbon comps everywhere.
- Ninja mod input xlr (I spent nearly a whole day getting a female XLR jack inside the original housing...)
- Four step output attenuator (A driven OP-6 is INSANE!)
- Re-purposed the mic selector switch to switch the VU between standard RMS and simulated peak, to kind of match the meters on my computer. It's also handy when driving the OP6 hard, it stops the meter from getting too hammered...
- Ninja power plug mod... i found a perfect square fitting plug!!
- Cleaned up the power supply wiring

This took me a month... and a looonnggg time just staring at the internals and schematic!

I'll try and get some sound clips together... I have some, but my playing sucks. To much time tinkering, not enough time playing...
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11th May 2010
Old 11th May 2010
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Yup!

Quote:
Originally Posted by unit7 View Post
I think so. Had a tech here in Stockholm measuring mine after I found out it distorted on low gain settings. He found that a few of those old (carbon I believe he said) resistors was falling apart, causing the distortion.
Most of the old carbon resistors are fine and don't drift very much its only the ones that are loaded higher and get hotter and thats one of them.
#9
12th May 2010
Old 12th May 2010
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Good!

Quote:
Originally Posted by bren-hur View Post
I have so much love for this thing...It sounds AMAZING! ...warm, sultry, thick... all the good words!!

My friends got a BK11 and we did some acoustic guitar tests, Jensen twin servo vs. OP-6... seriously, there is no comparison. My previously loved Jensen sounded kind of plastic and fake compared to the OP-6... the OP-6 is insanely natural... it's like hearing you guitar as it is, but through a wicked hot valve... it puts your sound in a sphere of awesome. Still... I'm gonna be sending mine to Sank eventually, I just wouldn't feel 100% till he gave it the thumbs up! my 44-A is with him right now... biting my nails in anticipation on that one, I can already feel how it's gonna sound...

As far as the internals on my OP-6, heres the rundown...
- Jensen copper foil paper in oil coupling caps. (It was going to be mundorf silver/oil but I couldn't find the right values)
- Obbligato gold caps everywhere else.
- Sprague atom filter caps, stock values and modded to look the part.
- 1 watt carbon comps everywhere.
- Ninja mod input xlr (I spent nearly a whole day getting a female XLR jack inside the original housing...)
- Four step output attenuator (A driven OP-6 is INSANE!)
- Re-purposed the mic selector switch to switch the VU between standard RMS and simulated peak, to kind of match the meters on my computer. It's also handy when driving the OP6 hard, it stops the meter from getting too hammered...
- Ninja power plug mod... i found a perfect square fitting plug!!
- Cleaned up the power supply wiring

This took me a month... and a looonnggg time just staring at the internals and schematic!

I'll try and get some sound clips together... I have some, but my playing sucks. To much time tinkering, not enough time playing...
The paper in oil caps are nice and vintagy and are original to the circuit
but the multicaps favored by Sank and interestingly modern Manley stuff
are a huge improvement, mine had zenith PIO's very similar to Vitamin Q's
I remember the amp was ever so slightly grained and dull but with the multicaps it went sparkly clean at low gain and at higher gains it just sings along accompanying the vocalist with angelic self invented rich harmonics, However at much higher gains than that (14 at 250 ohms in, with an old U87) with Danny Filth screaming into it, distorting and roaring like mad thing, this my friend is the true voice of Beelzebub reincarnate!.
Guitar with my old custom built sank pre cloud design ribbon on the old Altec 417 speaker driven by my old Matamp on a clean sound, power soaked with a motherload. It sounds like you must have an awesome compressor in circuit when there just isn't one!? must be the output transformer saturation
doing this.
There is nothing I can think of to replace this tool, maybe a very early Langvin pre would get quite close, but no cigar!.
This is IT!.
Bump there must be more out there chaps lets see em!
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12th May 2010
Old 12th May 2010
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Bump

More RCA Porn.
Blackbird audio rentals time machine with mods worthy of HG Wells himself:

How fast do you have to be traveling before you engage the flux gate convertor I wonder.
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12th May 2010
Old 12th May 2010
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So awesome. Mine's in Sank's shop right now.
#12
13th May 2010
Old 13th May 2010
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Want!


Gregory Scott - ubk
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13th May 2010
Old 13th May 2010
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Quote:
Originally Posted by Bassmec View Post
The paper in oil caps are nice and vintagy and are original to the circuit
but the multicaps favored by Sank and interestingly modern Manley stuff
are a huge improvement
Cap selection took me fckn ages... The multicaps were actually on my shortlist, along with Mundorf silver/oils... I went with the Angela branded Jensen copper foil PIO, coz dang... copper just felt right. Nice and musky. I'll definatly be ordering a set of Multicaps too after what you said... give them a bit of a whirl in there!

Also, when you say 250 ohm in, is that an input pad? That was the next thing I was going to add to my OP6. I want to be able to crank it with a tube condensor... which isn't really possible without an input pad... unless you mic from very very very far away...
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13th May 2010
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Quote:
Originally Posted by u b k View Post
Want!


Gregory Scott - ubk
+1
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13th May 2010
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Ahh!

Quote:
Originally Posted by bren-hur View Post
Cap selection took me fckn ages... The multicaps were actually on my shortlist, along with Mundorf silver/oils... I went with the Angela branded Jensen copper foil PIO, coz dang... copper just felt right. Nice and musky. I'll definatly be ordering a set of Multicaps too after what you said... give them a bit of a whirl in there!

Also, when you say 250 ohm in, is that an input pad? That was the next thing I was going to add to my OP6. I want to be able to crank it with a tube condensor... which isn't really possible without an input pad... unless you mic from very very very far away...
The original input transformer has windings for 50 ohms and 250 ohms I have mine hooked up Sank style so that mic switch position 1 is 50 and 2 is 250.
I notice from what I see in your pictures you appear to have a round section transformer like a UTC ouncer rather than the normal square input transformer, what impedances does it offer mate?.
Yes mate 14 on 250 ohms with a condenser mic is gain wise a bit like a marshall JCM 800 on the high gain socket cranked with a les paul.
Filth yeah! but for death metal vocals, what glorious filth.
Its clean at about four or five with a powerful singer though, Wimpy girly
singers you can set at about 9 on the gain control.
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17th May 2010
Old 17th May 2010
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Quote:
Originally Posted by Bassmec View Post
I notice from what I see in your pictures you appear to have a round section transformer like a UTC ouncer rather than the normal square input transformer, what impedances does it offer mate?.
It's got the original input tranny in there i believe, the number stamped on top matches up with the schem, maybe they switched to round ones at some point? The inputs are 250 and 30. I only have the 250 hooked up right now... what do you find the difference between the two are?

I'm putting a -20db pad in mine, just trying to figure out the right frickin values... I HATE impedance riddles. was probly gonna do a U pad with 680-150-680 values, put it on a dpdt switch. That would match the mics impedance... but I don't know how it will affect the input of the OP6? I suck at math.
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28th May 2010
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Seriously guys - when you are ready to sell pm me.
I have a pair of RCA BK-5's that would love to be paired with an OP-6.

PM me or else !!!!!
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28th May 2010
Old 28th May 2010
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Just got mine back from Sank today. Plugged it in for a second but there's another session going on so i couldn't run it through the paces. Early next week I'll blow out a full report.
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#19
29th May 2010
Old 29th May 2010
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44-A is here!!

You are going to have some fun... these things are beasts. Megapowerbeasts. The more i record, the more I love it...

Got my RCA PB-90 ribbon back the other day... fully Sanktified. He did some really really nice work... converted it to a 44-A... It looks magnificent, and sounds like an animal...

sound clips coming very soon...
RCA OP-6 powerhouse...-p1010128.jpg
RCA OP-6 powerhouse...-p1010127.jpg
RCA OP-6 powerhouse...-p1010089.jpg
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29th May 2010
Old 29th May 2010
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Quote:
Originally Posted by Bassmec View Post
I notice from what I see in your pictures you appear to have a round section transformer like a UTC ouncer rather than the normal square input transformer
The round transformer is the original. I have seen a lot of these units, and never seen a square RCA input transformer of that size, or in that era.

What identification exists on your square can input transformer?

I have seen two examples of alternate RCA transformers that were in square cans rather than round. Both examples were on Hollywood RCA Photophone equipment with alternate model #s. The alternate transformers had the same part #s as the standard round cans, but were made by different suppliers.
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#21
12th June 2010
Old 12th June 2010
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*DISCLAIMER* Contains bad singing and sloppy guitar.

SO! finally got round to finishing up my OP6 test:

OP6_44A_TEST_DirtyRain.mp3

Acoustic guitar and vox recorded with the OP6 and RCA 44 shown in this thread, everything else added by laddiemusic... final mix run through a Fatbustard...

It blows my mind how 1930's tech can sound so damn good...

I'm in the midst of more tweaks: 20db pad on input, unbalanced out (to feed a guitar amp) and direct in... so more tests coming...

Heres my first test of the DI:

OP6 DI - Guitar at 3 - OP6 at 10.wav
#22
12th June 2010
Old 12th June 2010
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Sexy on fur.... That is funny.
#23
12th June 2010
Old 12th June 2010
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Quote:
Originally Posted by bren-hur View Post
SO! finally got round to finishing up my OP6 test:

Attachment 177882

Acoustic guitar and vox recorded with the OP6 and RCA 44 shown in this thread, everything else added by laddiemusic... final mix run through a Fatbustard...

It blows my mind how 1930's tech can sound so damn good...

I'm in the midst of more tweaks: 20db pad on input, unbalanced out (to feed a guitar amp) and direct in... so more tests coming...

Heres my first test of the DI:

Attachment 177883
That is a f***in WICKED vocal sound! Bastard swine, now there is another piece of quarrelsome gear I lust after.....
#24
12th June 2010
Old 12th June 2010
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Aagh!

Quote:
Originally Posted by anguswoodhead View Post
Seriously guys - when you are ready to sell pm me.
I have a pair of RCA BK-5's that would love to be paired with an OP-6.

PM me or else !!!!!
You will have to pry mine out of my cold dead hands mate.
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12th June 2010
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Nice sounding vocal (and guitar) as well as the production is good. thumbsup

Sorta regret I never purchased a certain OP-6 when I had the chance, as that thing sounds lovely.

Luckily, I still have my other RCA pre to keep me snug and warm...
#26
12th June 2010
Old 12th June 2010
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Umm!

Quote:
Originally Posted by emrr View Post
The round transformer is the original. I have seen a lot of these units, and never seen a square RCA input transformer of that size, or in that era.

What identification exists on your square can input transformer?

I have seen two examples of alternate RCA transformers that were in square cans rather than round. Both examples were on Hollywood RCA Photophone equipment with alternate model #s. The alternate transformers had the same part #s as the standard round cans, but were made by different suppliers.
The pictures I posted of both my own OP-6 and Steven Sanks OP-6 both show a square radius'd transformer by UTC.
I am fairly sure its original mine dates from 1938 then somehow came into the possession of the RAF during the war, I bought it from a government sale along with a 1350 watt Bell and Howell 240/110 volt transformer with a big western meter in it. I paid £86.00 for the two, did I do OK?.
#27
12th June 2010
Old 12th June 2010
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Salutations...

Gotta LOVE those 1620 tubes!!! I use them in mic's... you've achieved an AMAZING vocal sound, KUDOS!!! Quiet, silky, dynamic....beautiful.
#28
12th June 2010
Old 12th June 2010
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Quote:
Originally Posted by Bassmec View Post
The pictures I posted of both my own OP-6 and Steven Sanks OP-6 both show a square radius'd transformer by UTC.
I am fairly sure its original mine dates from 1938 then somehow came into the possession of the RAF during the war, I bought it from a government sale.
Where do you get 1938? In 1938 they were selling the OP-5 and the 62-A remote amplifiers. I've owned and seen a lot of RCA products, and your square transformer would be a first, if original.

Two problems to me with your assertion:

1) The earliest reference I can find to the OP-6 is the 1941 RCA catalog. All the catalog pictures clearly show the round can input transformer, which is about the size of a UTC A series (if it were round), has two protruding bolts for mounting, a phenolic terminal board with solder posts, and a 900xxx (or 900xxx-501) series number stamped on the top.

2) You do not see UTC identification on RCA amps until 1950 and later. Everything before that time is only marked with RCA part #'s, and the construction does not resemble UTC manufacture. If it's a UTC, it's non-original before 1950.

It is possible if they were RAF property that they specified a different input, or modified them. I have seen the rare odd shaped factory replacement transformer too. I once had a BA-11A input transformer that was made by ADC, and was square rather than round. Never have I seen one on an actual RCA amp. It was the same RCA part #, so I have assumed it was a later replacement stock item.
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#29
13th June 2010
Old 13th June 2010
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WAR

Quote:
Originally Posted by Bassmec View Post
...somehow came into the possession of the RAF during the war,
A WW2 veteran OP-6... very wicked.
#30
15th June 2010
Old 15th June 2010
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