Tube rewiring (pentode to triode, etc) questions

[dissonantdissident]

Hey Folks,

So, I've been reading a lot about some modders using the EF86, in their DIY tube mic mods. If seems that a couple of pins need to be connected, to each other, but I'm having a hard time finding clear instructions, as to which pins should be connected, do I disconnect any wires(from the tube's socket), or really much of any help. I could just not be looking hard enough, but I'd appreciate any tips I could get.

Also, I've heard that some modders rewire the standard triode valves to run as "parallel triodes." I'm guessing that this is something that you could rewire, at the socket. I think that's less common, but I figured I'd throw that trivia out there, as well.

As always, any help/tips is certainly appreciated.

cheers!

[dissonantdissident]

So, it seems that the only pin that I see going to something that doesn't seem to be in the triode tube schematic(if I'm reading this right), is that pin three seems to be going to something identified as K, which doesn't appear in the triode's schematic. Don't know if that's part of the equation, or not.

[hogfish]

http://www.valvewizard.co.uk/pentode.html Read this. It is pretty informative and useful. K means cathode, which is the low voltage input for the EF86.

Be careful...........

[Matt C.]

Using a pentode configured as a triode usually means connecting the screen grid (g2) directly to the plate, and the suppressor grid (g3) directly to the cathode. I have no idea if it'll be suitable in whatever circuit you're working with but that's how it's generally done.

+1 be careful

[dissonantdissident]

So, If I'm reading this correctly, I would be connecting pin 1 to pin 6 and pin 8 to pin 4, to make the conversion, in the case of an EF86, which seems to be the most popular tube to use, in these Alctron Chinese condenser mods. Can that be done by simply jumpering, between the solder points, coming off of the socket, or would I need to connect the grid pins to their mates, but then clip the wires coming off of the grid pins socket slots? I'm sorry, if this is a dumb question, but I've never messed with any tube rewiring.

I'm not really sure what else would need to be altered, to make it compatible to the rest of the nady 1050 circuit, but more and more folks seem to be giving using them, so I just figured I'd start with figuring out this pentode to triode conversion question. Thanks again, and I really appreciate any insight into using these tubes in DIY mics. I like the 12AY7 tube, in this circuit, but variety is the spice of life, right?

[themagicmanmdt]

if the tube is wired to a PCB, and the PCB is wired as a pentode, you'll have to cut traces and run little jumper wires to have it run correctly.

the main difference you'll do is cut the trace for g2 from the board. make sure it doesn't have any remaining connection. jump g2 to A.

g3 is already either hooked to k or ground. either is fine.

[dissonantdissident]

So, if the rest of the circuit is wired for the pentode circuit(and since this particular mic has a tube socket that is connected by wires, instead of PCB mounted), it seems that all that would be needed to be done is clipping off the wire going to g2, at the board, and use that wire to jumper over to A. This would insure that the g2 pin never touched the pcb, which would save the cutting traces step. That is, if I'm understanding the last reply.

If the rest of the circuit is wired for a triode, I would need to jumper both sets of grid pins, right, or is that pin gonna be connecting to ground or k, regardless, making the second jumper a moot point?

[floid]

I like the 12AY7 tube, in this circuit, but variety is the spice of life, right?

wait, what tube was in the socket yr rewiring? 12ay7 is a dual triode, with a different pinout than the pentode ef86. for starters, pin nine is for heater on the first, input on the second.

[dissonantdissident]

I haven't done anything yet, since I'm still trying to "pre-game" this mod, via here and my head. When folks on the TOMB tell me to be careful, I try and take the advice . The mic I'm looking to try this in is the Nady 1050, which is one of the alctron tube mics, with the 9 patterns, switchable on the power supply. The Apex 460, Nady 1150, and numerous other re-brands. They ship with an 12AX7, but most of the mods recommend using a hi quality 12ATk7 or AY7, in the completed circuit.k In all of these mics, the tube is socketed, and there are wires going from the socket to the pcb, from the solder point coming out of each pin's socket.

I've noticed a number of the modders/upgraded importers(Stellar's CM-6 is one I've seen, in particular) are now using EF86 or EF806 tubes, in some of their offerings. All of them say that they rewire the pentode, as a triode, and I haven't noticed any pcb pictures looking to be too heavily modded. FAR had a post on their blog about using an EF86, instead of a 12AY7.

Since he does his blog posts for the general committee, and doesn't specifically go into his nuts and bolts, for either his U67 or U47 mods(in which he offers the EF86) I think posting the link would be "fair use":

http://www.foxaudioresearch.ca/EF86%20preamp.htm

In this aritcle, and judging from the not-especially-frankensteined pcb's of mics I've seen that use this tube, it seems that this tube can be substituted into the circuit, with wiring changes, at the tube.

I could totally be wrong, and this might just be over my pay grade, but this seems to be a popular trend, and I just wanted to see if I could give this new tube a try. Even FAR's blog states that he doesn't find a big advantage, but hey, I've got all this solder just sitting there...

[floid]

if you haven't already, look up the pinouts for both an ef86 and members of the 12a*7 family. compare these to diagrams of the respective tubes and to the socket you plan to work on, to give you an idea of what's what. They are both nine pin tubes, but NOT pinout compatible.

I would be connecting pin 1 to pin 6 and pin 8 to pin 4, to make the conversion, in the case of an EF86

1 to 6 looks right. but pin 4 on both of these tubes connects to the heater - is it being used as the cold/ground side? Is this common with the ground in the rest of the circuit?
Another consideration: 12a*7 type tubes can be run at heater voltages of either 12 or 6.3 volts, depending on how the three heater pins (4,5,9) are wired. The ef86 is, I believe, strictly 6.3V, which is supplied at pins 4 and 5. If the 12a*7 circuit uses a 12v supply (applied to 4 and 5, no connection at 9), it's doubtful an ef86 will perform very well/last very long unless this voltage is halved. If 4 and 5 are jumpered together and pin 9 is utilized, you're dealing with the correct heater voltage. BUT, as I mentioned previously, pin 9 on the Ef86 is the (input) screen grid (corresponds to both/either pins 2 and 7 on a 12a*y). You will have to separate pins 4 and 5, move the wire from pin 9 to one of them (other retains original connection), then move the wire from either 2 or 7 to pin 9 ....
I'm making this convoluted, maybe you get the idea though. Seriously, the datasheets with pinouts and diagrams will make it all quite a bit clearer if you spend some time with them, figure out what you have, what you want, what you have to do to make it happen, all that.

to configure the 12a*7 as a dual triode as you mentioned, you can tie pin 1 to 6 (plates), 2 to 7 (screens), and 3 to 8 (cathodes). not sure if you need to make any changes to the circuit to do this?

[dissonantdissident]

So, that pretty much makes sense, to me. I'm actually going to get another one of these mics, to be my "tube rewiring guinea pig" mic. I like the two I've already modded, and don't want to risk that money/work I've got into those. When I get the new one, I'm going to try both of these rewiring experiments, and report back with what I learn.

I really appreciate all of the advice!