G9-BBC variant - pix

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zebra50

Well-known member
Joined
Jun 4, 2004
Messages
2,943
Location
York, UK
Lots of G9 related stuff recently, so I thought I'd join in.

Having lost my first G9 to the studio forever, I put together second G9 for home use (if I'm lucky), based on leftovers from other projects, and bargain bin bits.

I got two lundahl 1530 xfos from canford audio (1:7) at discount price for the inputs, and used a pair of BBC LL228s for the output (a fiver each!). These are 2.8:1 ratio but work fine - a little less output level than 2:1, obviously. I changed the 10K resistor across the secondary to 5K so that the output stage is still seeing the same load.

Some other changes - I was short on rotary switches and had no relays around so I skipped the whole phantom power and line level selection, and also omitted the phase switch. I'm usually using ribbons or tube mics, so I can live without phantom. HPF is just on-off; I picked a cap value somewhere between the two that Jakob uses - 15nF, I think.

Had some trouble with earth hum on this unit at first. Carful rotation of the two toroidal mains transformers was necessary. I got a great improvement by connecting the -ve side of the output transformers directly to ground using thick guage wire - that fixed it.

Some pictures showing the simpified front - haven't cleaned or labelled it yet so you can still see my marking out for drilling. BIG chicken head knobs always sound better....


G9b001.jpg


Inside, you can see the transformers next to the PCB (one of Swedish Chef's - very good quality :wink: ), and the thick ground bus. It's not quite complete yet. Still need to nail down a couple of components.

G9b002.jpg


Cheesy picture of the tubes lighting up..... :green:

G9btubes.jpg


:thumb: z50
 
That just looks great!!! :razz:

Where did you get your tube sockets? I am looking for sockets in the UK that will fit swedish chef's boards. My sockets have flat pins that doesn't fit.

Thanks

Robert
 
I'm just curious, where is a good point to place LEd's for that purpose? I mean where on the track can you pick up the voltage, and would it change the sound in any way?
 
:green: :green: :green:


(Seriously, in case anyone thinks we're not fooling around, it's just a tripod and a long exposure time - 25 seconds) on the camera, shot at night with the lights off).

I got the tube sockets from farnell. You have to push hard but they go in just fine - I don't think I had to widen the holes but if you do then you can just twizzle a jewellers screwdriver round to widen the hole a little.

The on/off LED on the fron of the unit is hanging off the regulated 12V heater supply, with I think a 10K resistor in series to drop the voltage/limit the current. It won't affect the sound at all!

:thumb:
 
[quote author="Swedish Chef"]Subaru!!!!!!!!!!!!!!!!!!!!!!!!!!! :green: :thumb:[/quote]

possibly more 'morris minor' :cool:
 
ooohh!!!! very nice
One for my future (as I was one of the other people to buy those Canford cheapies...)
No subaru or Morris Minor - definately Inspector Morse Deep Red Jaguar territory

I wish I could ask decent questions - maybe in a few years until then I will just look and smile
 
Just had a bit of fun checking out the HPF on this with a frequency spectrum analyser. I wanted to use a toggle switch rather than a rotary switch for the HPF so I picked a 15nF cap - midway between the two in the original design (10 and 22 nF). To see what this did to my signal I used the pink noise source from the spectrum analyser. Here's a picture of what I got:

BBCG9hpf.jpg


You can just about read the scale on the left and bottom.

The signal is pretty much flat down to 160 Hz and then tails off when the HPF is in play. Overall, the frequency response of this pramp looks pretty good - some slight peaks and dips, but seems to be about ± 1 db from 30Hz up to 20KHz, and the noise source showed a bit of 'flicker' anyway without the G9 in line- these are 'snapshots'!

I'll do the same on my Lundahl-G9 and see how it compares, if I can ever get it back from the studio.

Not sure how scientific my method is, so let me know -I'm sure you will.

z50
 
Hi Fly,

thanks for that. Can you explain what the top chart means? I can see those spikes at 1k, 2k , 3k 4k etc, but I don't know what I'm supposed to be seeing.

One think I did notice with my test is that the frequency response did not seem to change with gain position, which is reassuring.

Stewart
 
Hi,
The analyser is called 'Rocktorch'. I had never heard of them before, but I got this very cheap (along with an MBHO measurement mic) from a university music technology department who were having a clear out. It has an XLR for a calibrated mic on the front, and dynamic mic and line in inputs, A-weighting and sensitivity switching on the front, as well as attack and release settings. If it looks like yours, then i do have documentation I can share.
Stewart

(edited for dodgy spelling!)
 
[quote author="Mark Burnley"]
Someone else who uses those ugly-but-cool BeeBeeCee trannies!
[/quote]

Hi Mark,

I finally found a use for them - I'm pretty impressed, especially at the price I paid. Might see if I can grab some more before they get scarce or fashionable. To my ears (which should not be trusted) they are equal to the Lundahls, but I do still need to do some serious A:B testing.

I guess the real test will be how much it gets used. My 'Lundahl' G9, which I built last Christmas, gets used a lot, so it'll be really interesting to see how I feel about them both in another 6 or 12 months.

Stewart
 
I just bought another one today - why not at £3.50 each they are a rainy day item...
 
Think I grabbed 10 before Rafa and Stewart :cool:

Canford Audio is a nice company! Will buy all Sifam I need in the future from them much better price that I can get in Sweden! They even created a credit account for me. :grin:
 

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