M49 Microphone from 6S6B-V to AC701k

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distorter

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Hi, I have a FLEA 49 microphone with stock F47 capsule and stock 6S6B-V tube.
I want to change the stock tube to an original AC-701k.
Could anyone share the knowledge of modifications needed to be done on the circuit and PSU?
Thanks in advance🙏🏻
 

mad.ax

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IIRC Flea offered their 49 in two flavors:
-AC701 coupled with an AMI BV11 transformer
-6S6B-V coupled with an AMI BV11R transformer

The BV11R has different ratio max output, and inductance.
 

distorter

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Schematics ?
FLEA is not opening their schematics. And they refuse to give any information on modding my stock FLEA to ac701 tube. Bad customer experience…
But here’s the schematics of original Neumann m49 and 6S6B tube on the internet.406A6BD4-C152-4026-B5A4-1E94F1FCDEC8.jpeg B76CB4D2-AC70-4CB0-BB39-B3443C4A1DA1.jpeg
 

distorter

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IIRC Flea offered their 49 in two flavors:
-AC701 coupled with an AMI BV11 transformer
-6S6B-V coupled with an AMI BV11R transformer

The BV11R has different ratio max output, and inductance.
Thank you so much for the information! So is there any more modifications needed to be done on the circuit and PSU? I think these two tubes takes different voltage?
 

mad.ax

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In order to answer your questions we need to know what are the differences between the original Neumann M49 and Flea's clone. So you're gonna need to trace the schematic, which shouldn't be too difficult since it must be pretty similar.

But first it'd be good to know if the M49B is the closest... Look in the PSU, is the heater regulated? If yes, which regulator?
 

tomas.borgstrom

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Flea have been helpful to me over the years. It's standard for a company to not share documents like schematics of their current products.

It's pretty basic to change the tubes but there are several ways that you can damage the AC701. If it was a tube that wasn't as expensive and rare I would encourage you to do it yourself, but in this case I think it would be a good idea to consult someone with experience in servicing tube microphones with AC701.
 

distorter

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Flea have been helpful to me over the years. It's standard for a company to not share documents like schematics of their current products.

It's pretty basic to change the tubes but there are several ways that you can damage the AC701. If it was a tube that wasn't as expensive and rare I would encourage you to do it yourself, but in this case I think it would be a good idea to consult someone with experience in servicing tube microphones with AC701.
Nice suggestion, thank you. I never asked schematics from Flea, only technical suggestions on how to replacing tube to ac701k, which flea had as an option before. I had very nice experiences with companies like Manley or Chandler Ltd., they won’t refuse to share this kind of simple information.
 

tomas.borgstrom

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If you send me multiple pictures of the internals of the microphone I can describe the circuit to you. If you solder the AC701 correctly in the microphone and lower the filament voltage to 4V the microphone will work fine. It's possible that you have to change some resistors, especially at the anode and the cathode to make it exactly like an M49.

You will damage AC701 permanently if:
- It gets above 4.2V on the filament
- The leads get to hot when you solder (the glass will crack and the tube will get noisy). You have to coil the leads and I also recommend to heat sink it with a clip when you solder.
- If the anode lead tuches any of the other leads when the microphone is on

Always use plastic gloves when you touch the internals of a microphone.
 

distorter

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If you send me multiple pictures of the internals of the microphone I can describe the circuit to you. If you solder the AC701 correctly in the microphone and lower the filament voltage to 4V the microphone will work fine. It's possible that you have to change some resistors, especially at the anode and the cathode to make it exactly like an M49.

You will damage AC701 permanently if:
- It gets above 4.2V on the filament
- The leads get to hot when you solder (the glass will crack and the tube will get noisy). You have to coil the leads and I also recommend to heat sink it with a clip when you solder.
- If the anode lead tuches any of the other leads when the microphone is on

Always use plastic gloves when you touch the internals of a microphone.
Thanks a lot for all this information!
 
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