Tbone SCT 800 Mod (Apex460)

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gct

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Dec 11, 2011
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Hi

Having  checked the Apex 460 circuit against the Tbone there seem to be a few differences.

1, There is a 1pF ceramic soldered between pin 1 and 2 of the tube.
2. There is no R8. R8 is the 270K on an Apex between the  2nd cathode input and ground. This has been replaced (I think) by C11 - 1000uF electrolytic.

Could anyone please explain how these changes affect the circuit.

Many thanks

GCT
 

pasarski

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Interesting, didn't notice these circuit changes in a SCT 800 I've been in.

After a small research, the cap between plate and grid could be a RF bypass cap (???), might affect the sound or not. I would try to remove it (also C9 and C10, my personal experience is that removing these is a improvement soundwise). R8 and C11 goes over my head.
 

gct

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Thanks
I was checking everything to be sure TBone was exactly an Apex 460 circuit. The ceramic is soldered to the tube socket so you don't see it on the component side. I also removed the C9 and C10 from the output. 

Do you know how I can measure the turns on the transformer?

Cheers

GCT
 

pasarski

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Came to think that in a place of R8 the cap should be rated at least 200V so it would be huge, you really have a huge electrolytic in your mic?

Sorry don't know how to measure transformers. It's probably something like 4:1.
 

gct

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Hi
Quite different from what I expected in the end. Looked like an Apex circuit but it's wired quite differently. The 1000uF 10V didn't replace R8 but was for the heater circuit. I noticed R3 and R4 were also reversed as marked on pcb.
GCT
 

pasarski

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Wow, this is really some news! They have changed the design from the CCDA to the more conventional plate follower, which is kind of a good thing. Just makes me wonder if the transformer is really up to spec for the 12AX7. Should have 12:1 ratio. More likely the circuit is designed for 12AT7 but they just use 12AX7 because they're available for cheaper. You should try a NOS 12AT7 in it. Too bad I can't help with measuring the trannie's ratio, it would be nice to know.

But this is great news anyway, makes the modding much easier (IMO).  I would just change the tube, and depending of the quality of the parts used maybe the trafo, capsule, C4 and C8. Then get rid of C9, C10 and C13 (where is C12 BTW?). And maybe also try what removing C7 and replacing C6 for a very high quality (maybe lower value) electrolytic does.
 

gct

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Hi
Thanks.
Whats specs do you have for a good C4 and C8. (On the FAR mini-mod they suggest Polystyrene input and polypropylene out - spent all day looking in Farnells but wouldn't know the right one it I saw it -did see 18€ surcharge for  Vishay and Wima caps) I wasn't sure how much I could drop the voltage spec. Seen 250V used in mods. The voltage at R2/Pin1 is 133V

Sorry missed C12 and went straight to C13 - old age.

Any help with cap choices would be very much appreciated.

Cheers

GCT
 

pasarski

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I suggest to search the forums and try different types. Maybe even the stock ones are good.

400V ones just must have been cheap.
 

micaddict

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Yes, I had heard about the change to plate follower.

Another difference with the Apex 460 of course is that the T.bone STC800 is cardioid only.
So, fellow members  :), for multipattern, what in your opinions would be the most sensible thing to do on the PSU side?
1. Somehow mod the standard PSU and upgrade while you're at it.
2. Get an OEM Apex 460-style PSU and upgrade that.
3. Roll your own from scratch.
4. Other................


 

gct

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Thanks  - all good stuff.

Later I will probably need the PSU upgrade too, when I change the capsule. Do I just need a Lorin Multi switch and a few resistors?

Cheers

GCT
 

ricardo

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pasarski said:
Then get rid of C9, C10 and C13
Don't remove C9 & 10.  They are for RFI.  Removing them will change the sound but not for the better.  C13 makes the circuit a "charge amp" like the U47 etc.  It applies feedback around the tube, stabilizes gain & reduces distortion & output resistance.  It should be a NPO/CGO ceramic of the correct voltage.

[edit]
Du.uuh!  Neumann didn't start using "charge amps" until the U87.  C13 is the equivalent of C3=10p in the U87.  Mea Maxima Culpa.  See Scott Wurcer's articles in Linear Audio for details.
[/edit]
 

gct

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Hi
Surprised that removing the RFI caps will affect the sound adversely. General opinion is that they should be removed. (See FAR Mini-mod) but will check it out by ear/measurement. I read they should be no larger that 0.001uF if kept.

Thanks for the info on the ceramic.

As I have a ECC83 what's the best way to reduce gain so I don't overdrive the tube and circuit gaining headroom?

Cheers,

GCT
 

ricardo

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gct said:
Surprised that removing the RFI caps will affect the sound adversely. General opinion is that they should be removed. (See FAR Mini-mod) but will check it out by ear/measurement. I read they should be no larger that 0.001uF if kept.
As an ex mike designer for Calrec (the Mk4 Soundfield was my baby) I note with interest that Dip Ing Jo Wuttke uses 10-22n like I do though his circuit is a lot different from mine (transformer).

I think I rather trust his ears (and mine) than lesser Golden Pinnae.  There's a lot of liquid BS about caps, supersonic response etc.  eg the trend towards replacing the coupling caps in Schoeps circuits with Golden Pinnae films.

You can be certain that if Schoeps use something, it won't be cos its cheap.

Scott Wurcer, designer of some of the quietest OPAs in the known universe, will show measurements on some Golden Pinnae favourites in the next issue of Linear Audio as part of his series on LN mike electronics.

As I have a ECC83 what's the best way to reduce gain so I don't overdrive the tube and circuit gaining headroom?
C13 is probably the best method with that style of circuit.  Make it bigger for less gain & more headroom.  But keep it CGO/NPO ceramic.
 

gct

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Any particular type for the RFI caps?

Thanks for the cool C13 info. That's COG/NPO 200V OK?

GCT
 

ricardo

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gct said:
Any particular type for the RFI caps?
Ceramics.  Unfortunately, CGO/NPOs of that value are Unobtainium so get the best you can find.  They must be soldered directly to the XLR.

Thanks for the cool C13 info. That's COG/NPO 200V OK?
Yes.
 

gct

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Sorry maybe I have misunderstood because  10-22nF COG/NPO are sold in Farnells: That's COG unless CGO is another thing?
 

ricardo

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gct said:
Sorry maybe I have misunderstood because  10-22nF COG/NPO are sold in Farnells: That's COG unless CGO is another thing?
YEES!  Go for it!
 

tchgtr

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For what it's worth, there is a cap in the MXL 9000 that sits in the same position as C13 in the schematic above.
It's 9pf in the 9000,  and acts as a LPF, with larger values affecting the mic as ricardo describes above, i.e., reducing out output, as well as reducing the HF.
It's a nice way to lessen the HF bump found in a lot of these capsules.
 

ricardo

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tchgtr said:
For what it's worth, there is a cap in the MXL 9000 that sits in the same position as C13 in the schematic above.
It's 9pf in the 9000,  and acts as a LPF, with larger values affecting the mic as ricardo describes above, i.e., reducing out output, as well as reducing the HF.
It's a nice way to lessen the HF bump found in a lot of these capsules.
It isn't a LPF.  It sets the gain as in the U87 I've corrected my post which said U47
 

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