Groove Tubes "The Brick" Mic Preamp/DI Box

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David Root
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Groove Tubes "The Brick" Mic Preamp/DI Box

Post by David Root »

I have one of these on my bench now. The mic pre is not working. The DI function works fine. I've checked the mic pre input transformer and I think it's OK. I get DCR of 90R Pri & 236R Sec.

The input transformer feeds a triode-strapped 6205 sub mini pentode that sits in cascode mode on top of an FET. It's an unusual circuit, the plate of the 6205 is grounded and the output is from the grid G2. The nearest e-cap to the 6205 reads 12uF OK.

I am suspecting the FET, maybe the 6205 itself and maybe its cathode resistor.

Anyone have any experience troubleshooting one of these?

The schematic I have is indistinct and very hard to read.
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Leo_Gnardo
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Re: Groove Tubes "The Brick" Mic Preamp/DI Box

Post by Leo_Gnardo »

Got high voltage? Just had one of these in. The tiny hi voltage wire coming out of the transformer body came unstuck from the pin it was supposed to be soldered to - that pin soldered to the PC board. Not too much unlike a pickup failure. I was able to get the insulation off @ 1/8" of the skinny wire & get it soldered back on to the pin - then it worked just fine.
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David Root
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Re: Groove Tubes "The Brick" Mic Preamp/DI Box

Post by David Root »

Thanx Leo. I don't think so, this is an intermittent problem. Could be dry joint(s). Owner has had two previous dry joints fixed. I didn't see any obvious bad soldering on the underside of the PCB.

The 6205 heater lights up, so I'm guessing the tube is probably OK. Owner took it back to check out again, I'll see what happens.

I forgot to mention that the original mic transformer has been replaced with one out of an old Yamaha ???? mic preamp.
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Leo_Gnardo
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Re: Groove Tubes "The Brick" Mic Preamp/DI Box

Post by Leo_Gnardo »

The one I fixed, had come in earlier with "into mittens." At owner's insistence I swapped in a new tube, also touched up PC board connections. 2-3 years later, it came back & I found the problem I described above. And with the transformer changed in the one you're looking at, well who knows...

The multipin cable to his mic was going bad at both ends too - I recommended just buy a new one from Gotham. The grounding yoke at the mic end was loosening, causing clicks in the audio.
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David Root
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Re: Groove Tubes "The Brick" Mic Preamp/DI Box

Post by David Root »

Thanx again Leo, I'll keep that in mind. I have a feeling I might see it again soon.
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