DIY pentode preamp?

[ChrioN]

I've been looking for pentode preamps but without success.
Would be fun to try one out, but they seem pretty rare.
Anyone?

 

[Rob Flinn]

Vortexion made a 3 & a 4 channel mic mixer using EF86 pentodes. This was used by the legendary Joe Meek, back in the day.

 

[zebra50]

Hi Rob,
Have you seen the schematic for the vortexion around?
I thought it was out there but it's not showing up either here or on google.

ChrioN - Pentode experiments have also been in my mind recently. If you hit the meta-meta and look at the preamp threads, you'll find some ideas. The red preamp looks interesting.

 

[Rob Flinn]

I haven't seen the diagram "out there" but I do happen to own one, which, happens to be sitting next to me as I type. It's not overly complicated, &
I drew the diagram for it a while back but can't remember where I put it.

 

[mcs]

Here's an unusual one (unusual tubes at least). I'm not sure if it has been discussed here before...

http://dogstar.dantimax.dk/tubestuf/graphics/shroom.gif

http://dogstar.dantimax.dk/tubestuf/shroom.htm

The "Not for public viewing" is there because it was meant to be a commercial product. The circuit has nothing to do with me - I just host Fred's website.

Best regards,

Mikkel C. Simonsen

 

[Bryson]

There's the old RCA ones.

http://www.waltzingbear.com/Schematics/RCA/BA-1A.htm

http://www.waltzingbear.com/Schematics/RCA/OP-6.htm

http://www.waltzingbear.com/Schematics/RCA/BA-23A.JPG

 

[zebra50]

[quote author="Rob Flinn"]I I do happen to own one, which, happens to be sitting next to me as I type.[/quote]

Lucky you! The last one I saw went for silly money! They do look very cool.

[quote author="Rob Flinn"]I drew the diagram for it a while back but can't remember where I put it.[/quote]

Ha! That happens to me a lot. Would love to see it if it reappears.

Cheers!
Stewart

 

[hodad]

[quote author="mcs"]Here's an unusual one (unusual tubes at least). I'm not sure if it has been discussed here before...

http://dogstar.dantimax.dk/tubestuf/graphics/shroom.gif

http://dogstar.dantimax.dk/tubestuf/shroom.htm

[/quote]

I found no sign of it here, so it must have been discussed at the old place. If memory serves(and it usually doesn't) there was some concern about its transformerlessness, with prr(possibly?) defending that design decision. I think a couple of folks expressed interest in the design, but I don't know if anyone ever did anything with it.

 

[Mickeyvee]

Hi there guys im new to this forum just wondering my ol valve tech is looking for schematics so he can restore my ol vortexion 3/ppm 3 channel mixer preamp with one large meter can anybody guide me cheerz

 

[Tubetec]

Ive had quite a few of their poweramps over the years , these typically included the EF86 pre-amp circuit , but no metering ,

Its a simple circuit , EF86 pentode connection , either 220 or 270 kohms anode load , with feedback taken from the output and applied to the lower end of the input transformer primary , I think its a 10Meg grid resistor and regular cathode bias R bypassed by C .

Downside of these pre-amps is there usually 30/50 ohm at the input , so not a great match for modern dynamic mics .

The PJ Baxandall preamp contains a circuit very similar to what is used in the Vortexion pre , I'll see if I can find it .

 

[Mickeyvee]

thanx heaps tubetec this is the model i have notice the meter on the right im trying to get a replacement meter that one is not working cheerz Mike

 

[Tubetec]

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 ,

 

[k brown]

Many a moon ago, I e-mailed Dave Royer about the suitability of using 6AK5 in microphones, and to my surprise he responded that not only is it one of his favorite tubes, but sent me a schematic for a simple two-stage mic pre he made with them; it even has the option to be battery powered!

It's a single 43 dB gain stage into a cathode follower; he used them transformerless-out, and built them into ammo boxes.

The battery option is a 12v lead acid alarm battery for the series/parallel heaters, and the B+ is the 12v batt in series with nine 9v batteries, which powers two channels!

I didn't keep the e-mail with his hand-drawn schem, but copied it into one of my notebooks (hopefully accurately!).

As soon as I get a chance, I'll take a shot of that page as post it.

 

[k brown]

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.

Pardon the way the batteries are indicated; I long ago inadvertently got into the habit of drawing them bass-ackwards.

 

[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

 

[Mickeyvee]

Thank you very much tubetech
I have forarded your helpful info to Andrew my tech i will let you onow the outcome this is a fantastic forum Mike

 

[rock soderstrom]

Royer 6AK5 mic pre:

Many thanks for the schematic!
David Royer and I share a preference for the 5654/E95F/6AK5w family of tubes. Really great tube, still good and cheap to get, consistent high quality and small form factor.

I have built similar circuits with them, but also connected as triodes, ala Gates SA70. Also interesting would be a two stage mic preamp in a CF with a pot in between (or a 5654 in a SRPP) for more gain.

 

[ruffrecords]

Interesting power supply. Still trying to get my head around how it works.

Cheers

Ian

 

[rock soderstrom]

Interesting power supply. Still trying to get my head around how it works.

I also had my difficulties with it at first, it is a voltage drippler for B+ and unregulated DC voltage for the heaters. Mr. Royer uses a similar one for his Royer mods, see here.

 

[ruffrecords]

I also had my difficulties with it at first, it is a voltage drippler for B+ and unregulated DC voltage for the heaters. Mr. Royer uses a similar one for his Royer mods, see here.

I guessed as much but what puzzled me was that neither side of the transformer is connected to ground.

Edit: OK just got it - the 'common' terminal is in the middle. Makes sense now.

Cheers

Ian