Studio Projects TB-1

[EEMO1]

So, this is my first real attempt to contribute something back to the group.

i've got some nice mics whipped together, so i figured i might try to mod my studio projects tb1 and share it with everyone. i don't use it that much anymore, so what the hell...



the mic apparently has a 6072 tube, 797 capsule. sounds pretty good, but the highs are sometimes a bit too apparent. i figured i'll change the tube and a couple of components and go from there.

here's some pics.

to me, the construction doesn't seem that bad. the psu outputs 200v/2.5mA and 6v/100mA, didn't measure it yet though.

 

[ChrioN]

Studio Projects makes pretty descent mics. I got their VTB-1 preamp, but the sad thing is, the preamps onboard my sound card (emu 1820m) is in a totaly different league.
The VTB-1 sounds like crap with horrible distortion at higher levels.
I wounder how much voltage the tube gets. The wallwart is 12vac.

 

[phatmateo]

did you ever get around to modding the tb-1?  Im debating whether to sell mine or not

 

[rodabod]

I bet that probably already sounds fairly decent. There are a few things you may want to try which you might get tips from, from the Nady/Apex threads (eg. check heater voltage, check general circuit configuration, etc.).

If that's a 12AX7 then I'd maybe try the usual sub of a 12AT7, 6072 or if you can find a low-noise one, an ECC82. If it's still bright, try loading the plate with a small cap to roll of the HF a little.

Roddy

 

[phatmateo]

k, thanks for the info Roddy.  Much appreciated

 

[Emperor-TK]

the preamps onboard my sound card (emu 1820m) is in a totaly different league.

Those are Ted Fletcher's mic amps.  They hang with the best of my pre's.

 

[rodabod]

Just realised it was a 6072 from stock. If it's a single plate-out circuit then you could load the plate with something like a 500pF-1500pF film cap to ground. Inject tone at the valve grid and sweep from 10K to 20KHz. You might want to be about a few dB down by 10-12KHz. Use your ears.

 

[phatmateo]

I managed to get a schematic for the TB-1
[img=http://img24.imageshack.us/img24/6758/tb1l.jpg]


Let me know your thoughts.  I'm not experienced with modding microphones, but any ideas are appreciated

 

[rodabod]

http://img24.imageshack.us/img24/6758/tb1l.jpg

That's essentially 1/2 of a C24 circuit, a bit like what some people mod the Apex and Nady mics too. Looks good. Some people might argue that the bias is a little cold, but that's open for argument.

 

[Rossi]

Looks like very nice quality for a low cost mic.

I'm not sure I like the 22n cap across the output. Although it may tame excessive treble, it likely introduces distortion in the upper range because the tube has to drive a pretty low impedance at high frequencies. Take it out and see if you like it better that way; there'll be even more treble but you may like it better anyway because it is cleaner treble. If you don't like it that way, it's no big deal to put the cap back in.

I'd probably replace C2 and C4 by a single polyester cap (at least 250 V) somewhere between 330n and 1u. Perhaps even a little smaller than that.

Those 797 capsules are pretty bright. I have a B1 that has probably the same single diaphragm capsule. You could try a Dale M7, if you'd like a more vintage sound.

 

[phatmateo]

Thanks for the replies gentleman.  I agree that the high end is a little harsh to my ears so maybe I'll try removing that 22nf cap across the output.  I'll also see if any more of Dale's M7's are available since I would want a more vintage sound out of this.   
As for C2 and C4, what is the reason that these are in parallel?  Shouldn't it just be one cap as suggested, or is there a specific reason they did this?

 

[rodabod]

Thanks for the replies gentleman.  I agree that the high end is a little harsh to my ears so maybe I'll try removing that 22nf cap across the output.

 

Removing that cap will make the response more harsh. It's there to reduce treble. I've used a circuit which is similar with the same capsule and added some filtering and it worked very well.

As for C2 and C4, what is the reason that these are in parallel?  Shouldn't it just be one cap as suggested, or is there a specific reason they did this?

The theory is that the smaller film cap bypasses the larger electrolytic for higher frequencies. Some say it's better, some say it's worse. You can see it in a lot of Jakob's designs. You could probably replace this with a single film cap and see if it sounds much better.

If the mic is currently harsh though, I'd consider adjusting the low-pass filter first untill it sounds in the right range before aking smaller adjustments.

 

[melville]

Has anybody had any success with mods on the TB1 at all? Any particular valve choices that worked well?

cheers
Dominic

 

[abbey road d enfer]

Don't forget the transformer's inductance! With the 22n cap, it certainly introduces a resonance in the upper spectrum. This may be the origin of the excess of treble. This resonance is not tamed by the high impedance of the plate (ca. 40k). You may find that loading the output with a 1k-2k resistor may tame the resonance. In fact, the best solution would be a Zobel circuit across the xfmr's primary, but it needs to be tuned with a generator and 'scope.

 

[Gus]

To add a little to abbey road d enfer post.

I would load the circuit with the preamp you are going to use with it and sweep the circuit looking at the output with a scope (or load the circuit with a fixed resistor and look at the output with a scope) to find out WHY the .022uf cap was installed in the first place.

That plastic backside backplate capsule design sounds strange to me compared to the dual brass versions used in cardiod.


At a stompbox forum they have term BUM(blind urge to mod)

 

[melville]

Ok, well I've had this apart now. And it seems a little different to the one above in the picture and the above schematic. The tube is dead and needs replacing, that went some weeks ago. Now the odd thing is, I see no .022 cap on the output anywhere and the capsule in mine looks different?? For instance, you can see the dots on both sides of the capsule in the pix above, but on mine you can't see through the metal disc on the side with the red wire, so possibly a different capsule. I just tried to take a picture of it and the flash just reflects off it and causes a big white circle in the pic. Other than that it seems the same. All the other caps are the same values as the schematic.

Now I do know mine is from the first run of these made, there were only three or so in the UK when I got it directly from the distributor years ago. I have never found it to be too bright, more easy to overdrive and make dirty sounding across the freq range. If anything it seems like the circuit has a tendency to produce a very hot signal easily. I'll check the psu voltages tomorrow, it could well be running on something other than the quoted +200V and +6V. Hmmm we will see.

Thanks for the pointers for what to look for. I'm going to try and suss out the differences between mine and the one above. Since I know the .022 cap on the output is not mounted on the pcb, it does cross my mind that this was added later, possibly at the same time as the capsule pictured above was introduced. Ok enough guesswork, I'll let you know what I find out re voltages and circuit differences.

Thanks
Dominic

 

[gemini86]

At a stompbox forum they have term BUM(blind urge to mod)

So there IS a name for my disease...

 

[melville]

Right, had the psu open today and tested the voltages. 200V output is giving a steady 197, not bad, and the 6V is fluctuating wildly from 2 to 16V! So I presume thats what killed the valve. There is also no cap on the output of the mic for sure.

So fix the psu replace the valve and then see where I am.

A question for those of you with more experience of valve circuits than me, what side effects could a heater supply that fluctuates this much produce in the output? Could this be the source of the hot output and dirty sound I've been experiencing?

Any thoughts welcomed.

Dominic

 

[Gus]

Did you load the Fil supply when you measured it?  You need to load a 3 term regulators to a few ma to take a measurement.  Unloaded they can give non correct voltages.

How is the tubes filament wired?  What pins are used 4,5,9?  

If they use 1/2 of the fil for the triode used,  6VDC/150ma= 40ohms as a test load,  I^2R .15Ax.15A x 40ohms = .9watts OR ExI=P, 6VDC x .15A = .9watts

 

[rodabod]

A question for those of you with more experience of valve circuits than me, what side effects could a heater supply that fluctuates this much produce in the output?

Reduced and increased emmission until the valve gets killed.

Check the regulator, and if there are any pre-regulator dropping resistors acting up.