EmRR

Gates Radio 7X tube preamp
« on: February 14, 2011, 03:39:29 PM »
Another little known late 1940's Gates preamp.  Some mod notes on there too; one winding of the input trans secondary was not used, I changed the 6J7 plate resistor, and re-tapped the output connection.   I've since discovered that particular ADC does not like to work in SE connection, but is meant for PP connection.  Gates did a lot of things like that due to supply issues, I suspect.  This amount of gain means you need a really well shielded input transformer, and this ADC would not meet modern needs for hum rejection when this power transformer is used on the same chassis.   Before anyone suggests DC filaments, you can ground the 6J7 grid and it's very quiet.  Very cool sounding unit; another I no longer own, but did for awhile.


Best,

Doug Williams
Electromagnetic Radiation Recorders

"I think this can be better. Some kind of control that's intuitive, not complicated like a single knob" - Crusty

"Back when everything sounde


DaveP

Re: Gates Radio 7X tube preamp
« Reply #1 on: February 14, 2011, 07:51:25 PM »
Another nice no feedback design, thanks Doug.
best
DaveP
Soundcloud: Delayed Action.

Re: Gates Radio 7X tube preamp
« Reply #2 on: February 14, 2011, 11:04:57 PM »
Nice one - thanks for sharing.   Chassis a U style?

Re: Gates Radio 7X tube preamp
« Reply #3 on: February 15, 2011, 03:09:00 AM »
Good stuff, thanks for sharing!
I already bought a bunch of 6J7 and 6J5 tubes, $1 each!
Was looking for some oldschool preamp schemos - you have them all!
 :D

EmRR

Re: Gates Radio 7X tube preamp
« Reply #4 on: February 15, 2011, 04:49:46 AM »
Nice one - thanks for sharing.   Chassis a U style?

Yes
Best,

Doug Williams
Electromagnetic Radiation Recorders

"I think this can be better. Some kind of control that's intuitive, not complicated like a single knob" - Crusty

"Back when everything sounde

Nele

Re: Gates Radio 7X tube preamp
« Reply #5 on: February 15, 2011, 08:03:52 AM »
Thanks!
Any tips on which transformers currently in production would be suitable? 10k:600 edcor as output should be ok..?


Kingston

Re: Gates Radio 7X tube preamp
« Reply #6 on: February 15, 2011, 08:13:07 AM »
Thanks!
Any tips on which transformers currently in production would be suitable? 10k:600 edcor as output should be ok..?

No, it needs to be gapped. Notice the transformer is connected between B+ and 6SN7 plate. For that edcor you need an additional plate resistor, lets say 10k and a 2-10uF coupling cap.

I would love to know which transformers that are still manufactured can handle "single ended" output tasks like this.

By the way is this supposed to be a stereo amp, or is it really wasting a half of that 6SN7?

Nele

Re: Gates Radio 7X tube preamp
« Reply #7 on: February 15, 2011, 08:24:42 AM »
No, it needs to be gapped. Notice the transformer is connected between B+ and 6SN7 plate.
Doh! Indeed.

Eskiclown

Re: Gates Radio 7X tube preamp
« Reply #8 on: February 15, 2011, 08:38:47 AM »
By the way is this supposed to be a stereo amp, or is it really wasting a half of that 6SN7?
Both triodes of the 6SN7 are connected on the output (notice the pin numbers on Doug's schematic).

Thanks for this schematic, Doug !
« Last Edit: February 15, 2011, 08:40:28 AM by Eskiclown »

Kingston

Re: Gates Radio 7X tube preamp
« Reply #9 on: February 15, 2011, 08:45:09 AM »
Both triodes of the 6SN7 are connected on the output (notice the pin numbers on Doug's schematic).

Doh! I don't know why but I just skipped over them as "mA".


gary o

Re: Gates Radio 7X tube preamp
« Reply #10 on: February 15, 2011, 08:54:24 AM »
http://www.sowter.co.uk/specs/9040.htm

any good for op ....lots of iron....but lots of money

volker

Re: Gates Radio 7X tube preamp
« Reply #11 on: February 15, 2011, 09:08:00 AM »
Thanks!
Any tips on which transformers currently in production would be suitable? 10k:600 edcor as output should be ok..?

No, it needs to be gapped. Notice the transformer is connected between B+ and 6SN7 plate. For that edcor you need an additional plate resistor, lets say 10k and a 2-10uF coupling cap.

I would love to know which transformers that are still manufactured can handle "single ended" output tasks like this.

By the way is this supposed to be a stereo amp, or is it really wasting a half of that 6SN7?


The Carnhill VTB2290 might up to the task, but unfortunately there is no maximum current specified in the datasheet. Lundahl/Sowter also have several gapped output transformers.

EmRR

Re: Gates Radio 7X tube preamp
« Reply #12 on: February 15, 2011, 09:35:49 AM »
Output should be rated for 20-25 mA DC. 
Best,

Doug Williams
Electromagnetic Radiation Recorders

"I think this can be better. Some kind of control that's intuitive, not complicated like a single knob" - Crusty

"Back when everything sounde

Re: Gates Radio 7X tube preamp
« Reply #13 on: February 15, 2011, 10:34:32 AM »
Consider also that you likely don't need the current drive of // 6SN7 to feed your DAW . . .

Single SN7 would cover 2 channels if you think that way.  Now you have half the current demand in output section.  Higher source Z too.  40H pri on the Sowter is about as high as you'll find.  No concern though if you go parafeed.

Lundahl makes SE high current OT similar to the Sowters.  Don't recall the model.  $120-130 isn't too bad considering the special purpose.  People are paying sometimes twice that for certain 'plain 'ol'  15K:600 OTs- when Edcor makes an equivalent contender for $13.

EmRR

Re: Gates Radio 7X tube preamp
« Reply #14 on: February 15, 2011, 10:55:47 AM »
If you went with a single section of 6SN7 (change to 6C5 or 6J5), you'd want to raise the output primary to something like 15-20K.   I'd bet you would simply be trading off which difficult-to-source transformer quality you were chasing. 

Cinemag CM-9589 looks good, though they don't go into details about the limits on the butt stack options. 

The James Audio JS-3004N looks perfect.    I think they are very expensive.

You could use Magnaquest if you went parallel feed and used their plate choke.  Though, if you go parallel feed, you could buy the spendy plate choke and then use an Edcor 10K:600 output. 



Best,

Doug Williams
Electromagnetic Radiation Recorders

"I think this can be better. Some kind of control that's intuitive, not complicated like a single knob" - Crusty

"Back when everything sounde

Re: Gates Radio 7X tube preamp
« Reply #15 on: February 15, 2011, 11:11:35 AM »
I forgot about the James.  Yes - exorbitantly $$ (think maybe 250-275$).   Think their speaker amp only SE OTs are about half.


The Sowter 9040 looks like it would give 15K plate load but the low corner looks like it would move up to 30+Hz with single tube plate resistance.

Thinking in the opposite direction - triode 6V6 w/lower supply voltage, pad the output.  Worth the possibility of getting more 2nd harmonic?     

EmRR

Re: Gates Radio 7X tube preamp
« Reply #16 on: February 15, 2011, 11:39:13 AM »
There are RCA line amps with lower current 6V6 driving a 15K winding, no need to pad. 

It is an American design, ya know it's gonna sound F'ed with a foreign transformer.   ;)

The Sowter 9040 looks bad with quoted "20Hz 3 dB point possible with a 5K source".  The Sowter exhibits the modern case where FB is somewhat expected, to lower drive Z.  The original iron would eat that alive.   I guess you might start to get somewhere driving the Sowter with a 45 or 2A3.  WE 417A might work.   Now we're redesigning..... 
Best,

Doug Williams
Electromagnetic Radiation Recorders

"I think this can be better. Some kind of control that's intuitive, not complicated like a single knob" - Crusty

"Back when everything sounde

Kingston

Re: Gates Radio 7X tube preamp
« Reply #17 on: February 15, 2011, 11:58:31 AM »
Cinemag CM-9589 looks good, though they don't go into details about the limits on the butt stack options.

What, really? This has been my goto transformer for several 4:1 outputs but I didn't realise they have a version that can take DC! This is reasonably affordable as well.

Oh and thanks for all the hints for the transformers.
« Last Edit: February 15, 2011, 12:06:32 PM by Kingston »

EmRR

Re: Gates Radio 7X tube preamp
« Reply #18 on: February 15, 2011, 12:28:55 PM »
Let us know what pricing is for both versions of Cinemag, if you get it.   The time I tried to get quotes, they wouldn't even talk numbers!
Best,

Doug Williams
Electromagnetic Radiation Recorders

"I think this can be better. Some kind of control that's intuitive, not complicated like a single knob" - Crusty

"Back when everything sounde

Kingston

Re: Gates Radio 7X tube preamp
« Reply #19 on: February 15, 2011, 01:31:13 PM »
They are really slow with email, but they are very forthcoming with additional information if you just ask. Try calling.

(I'm currently not in the market for a transformer shipment from the US)


 

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