DIY pentode preamp?

[Migs 31]

Royer 6AK5 mic pre:

The lower right corner shows what he suggested when I told him I'd be using it without an input trafo, and wanted input level control.

Thanks for sharing this @k brown,
"I told him I'd be using it without an input trafo..."
So he (D. Royer) proposed a 100K pot with a 200Ω resistor on the whiper, then going to grid?!?
If I understand well your drawing, on the bottom right corner?

I have two 6AK5 rolling around here, I'm tempted.

M

 

[rock soderstrom]

"I told him I'd be using it without an input trafo..."
So he (D. Royer) proposed a 100K pot with a 200Ω resistor on the whiper, then going to grid?!?
If I understand well your drawing, on the bottom right corner?

or you can leave out the pot and install a 10/20db pad in front of the transformer. The 43dB a often quite right for the job without pads.

 

[davemoog]

Here's another pentode preamp (highly regarded) from the Ampex 601 portable tape machines.

 

[rock soderstrom]

Here's another pentode preamp (highly regarded) from the Ampex 601 portable tape machines.

Interesting! Is the amp modified for studio use? It has plenty of gain!

 

[davemoog]

Yes it has been modified for studio use...and as well as added a phantom power option for condenser mike. Currently the tape transport doesn't work due to bad belts/etc. There's quite a few articles on the web regarding mods.

 

[Matt Syson]

Hi
The Vortexion uses a grounded cathode and I think as said, a 10 Meg grid resistor which is a 'convenient' way of biassing it.
Vortexion (Clarke and Smith in Wallington) was my first 'proper paid employment' under the watchful eye or Jim Prouton who is probablt not still with us as he was late middle age in 1978. Vortexion were making transistor power amps qnd using FETs in their 'module input stages (a 2 transistor preamp inside qa mic transformer can with the mic transformer attached. Vortexion wound their own transformers at the time where 3 or 4 ladies wound them and they were potted in varnish afterwards. I blagged 12 input transformers which, years later I used on a preamp remodel 'upgrade' for a guy in England since the original transformers designed for 30 Ohm rivvon mics was way too far extreme for almost all modern mics to be useful.
Happy days!

 

[Mickeyvee]

It might be worth taking a closer look to determine if its repairable ,
You wont get any modern equivalent that looks the same ,
Only thing I can think of is maybe some old BBC or maybe Austrailian Broadcasting unit might have something similar ,

Sometimes if a moving coil meter is subject to a drop the pointer gets hung-up and doesnt move or wont zero out , you might need to re-adjust the set screws where the needle pivots , a tiny fraction of a turn anticlockwise and the needle should once again freely return to around the set 0 position , thats fine tuned with the screw adjust slot on the front of the meter .

The other thing that can happen is the glass panel in the front of the meter can get displaced and hamper the movement of the meter ,

My advice is take the meter apart , carefully dust it out with a soft paintbrush , check for bad connections or the mechanical missalignments I described above ,

Hi Tubetech im sorry for the delay i am working my way around this forum i would like to thankyou my tech Andrew Kay managed to fix the meter thanx to you so a big

 

[Disco Volante]

I have a few of those meters lying around... Also own a good selection of 5654/EF95..