Tbone SCT 800 Mod (Apex460)

[ricardo]

So it work the same with a tube?

Yes.  The OPA circuit is from Microphone Design Handbook - Gayford and can represent a FET, high u triode (ECC83) or pentode (EF86).

It's easy enough to try in any high gain tube circuit.  Only needs a 10p COG/NPO ceramic.  If it doesn't do what you want, it's easy to remove too.  Doesn't need Unobtainium hard carved by Swedish virgins .. ;D

WHAT IT DOES

• applies feedback which
• reduces gain
• reduces distortion
• reduces output resistance of the stage.  Useful for high Ra tubes like ECC83

 

[gct]

OK Will get the shopping list together.

Many Thanks

GCT

 

[gct]

Just checking ceramics with the local supplier and they have 10pF 50V.

I measured 65V in the circuit so will 2 x 20pF 50V in series be OK?

Cheers

GCT

 

[ricardo]

I measured 65V in the circuit so will 2 x 20pF 50V in series be OK?

Yes.

Be very neat and have very short leads.  Best on the valve holder pins directly.

 

[gct]

Just checked the transformer. With a 1KHz sine wave and input voltages between 1.5 and 5.5V I measured an average of 11.6 : 1 output.

Any comments about this transformer?

Cheers

GCT

 

[gct]

Got some of my components soldered on.

The 10pF cap C12 behind the tube gave a 10dB gain reduction over the 1pF. Really pleased with that. So my Philips ECC83 can stay for now.

The RFI caps were a bit of a problem sourcing so I went for smd 4mm and they just sat nicely in place of C9/C10 on the board- I was going to put them on the XLR but it looked fiddly. If I change the transformer I will move them. Didn't hear any difference but didn't hear "Radio One" either, so hope they're doing their job.

Still waiting for more parts.

Cheers

GCT

 

[pasarski]

My experience has been that removing the RFI caps makes the mic sound better, but it could have been just confirmation bias in work.

Nice information about C13. I see how 12AX7 and the feedback loop with one ceramic cap is cheaper option for mass production, but I would just use more suitable tube (12AT7, 12AY7...) for a DIY project. I know it's a kind of "audiophile-/phool" approach maybe.

 

[gct]

I was planning on changing the tube for the 6072 but when I realized I already had an old Philips tube I wanted to try it at less gain. Its tons better  than the Chinese tube so I'll probably stick with it. Also don't forget that the original RFI caps DO affect the sound as they are cutting the high frequencies. 

 

[gct]

Recorded our first project with the TBone and I'm very pleased with the results. The components I finally used were the following:

RFI caps SMD 10nF 50V Vishay Farnell code 1702690 (soldered nicely in place of the originals)
Tube feedback cap 10pF 200V Farnell code 1672257 (soldered directly on the tube socket pins)
Philips ECC83 from an old audio amp.

The input and output caps were kindly provided by Steward Tavener - Xaudia.com - He sent me a selection of caps to "enjoy". Thanks again Stewart! I chose the vintage 1000pF mullard mustard cap for the input (although the polystyrene was also very good) The output cap was a 1uF 230V WIMA film and the bass of the voice was  clearer compared with the one I had purchased from Farnells code 1200792.

I would describe the mic as a little hard still but for the price - apart from the chinese tube - it's great value for money.

Thanks again to all who contributed.

Cheers

GCT

 

[ViciusbeatZ]

Regards! It has a photo of open microphone ? I am a beginner in the world of DIY and I would like to see him finish inside . thanks for everything, and sorry for my weak English : :