Blending 12ax7, ef86 channels into PI

[MattB930]

I was wondering if this is possible or if there is a better way to do it. I was wanting 1 input into both a 12ax7and a ef86 input stage where both input tubes share the input and blend back together into 1 PI input. I've seen, for example, on a dc30 where the two channels have separate inputs into separate inputs of the PI grids but really just want to be able to blend and/or footswitch between the two. I know there are gain differences and other considerations but am wanting to try this somehow.

Are there circuits someone can point me to to study? What are your recommendations?

[mooreamps]

1. I've blended them before.. I don't think they mix too well, but tone is subjective.
2. When I blend a two channel pre-amp, each channel goes into opposite inputs of the P/I.

-g

[MattB930]

thank you sir. Thinking about it last night, I would rather just make them footswitchable rather than blending them. Could I just add a footswitch to the input to switch between input stages or do I need to come up with a more complicated relay switch? I'll wire each channel to each input of the P/I.
Thanks!

[mooreamps]

a more complicated relay switch for a dc30 config ? no.. In fact, if you are not intending to blend the two channels, you can put the relay contacts to feed either the same P/I input, or both. Doesn't matter... Personally, I use the octal Broom and Potter relay for my channel switching, and the footswitch just provides the ground for the relay coil.. I use this part because they are easy to source and built like a tank.. This type of channel switcher will never fail you....You get two pair of contacts with a this type of relay. So, you can feed the signal through one pair of contacts, and then I use the second pair of contacts to drive a front panel lamp showing which preamp channel is on line...

-g

[MattB930]

Is there a schematic or resource online showing how to do this? Where can I find a relay like that one specifically? I like that idea a lot!

thanks.

[pdf64]

Using a relay to switch channels on a low gain amp seems way over the top to me.
Simplicity is generally the best option.
I can't see any benefit from anything more complex than an external A-B selector box in this application.
Leave relay switching till you've got a channel with 3+ gain stages. Pete.

[mooreamps]

Using a relay to switch channels on a low gain amp seems way over the top to me.
Simplicity is generally the best option.
I can't see any benefit from anything more complex than an external A-B selector box in this application.
Leave relay switching till you've got a channel with 3+ gain stages. Pete.

The benefit is the man is seeking to learn how it's done... Good enough reason for me..

-g

[Steve Conner]

Did you mean Potter & Brumfield? (Now owned by Tyco)

Using such a big relay for channel switching is kind of pointless. Not only are the contacts made for power switching, and susceptible to tarnish that will cut low-level signals, but the relay itself is so big that it means lots of stray capacitance to pick up bleed and hum.

Any relay you use for signal switching should have gold contacts, or better still mercury wetted. Commercial kit often uses the ordinary silver contacts, but this is purely a cost-cutting measure, and the relays usually give trouble later.

I prefer the little Hi-G sealed ones, of which I got about a half dozen from some scrapped equipment from a TV studio. I saved them because they looked expensive. They must be 20, maybe 30 years old and are still happily switching channels in one of my amps, having long outlived the TV camera control panel they were in.

On the upside, an octal based relay will fit in a spare tube hole, I guess.

[Tilman]

Hi,

you could shunt switch the channels with FETs. I did this with an 18WTMB to make the two channels switchable. I used J201 N-channel FETs , one for each channel at the volume pots wiper to ground. The negative gate voltage was derived from the HT voltage. See KOC books.
I found this method very elegant and noiseless.

Tilman

[mooreamps]

Hi,

you could shunt switch the channels with FETs. I did this with an 18WTMB to make the two channels switchable. I used J201 N-channel FETs , one for each channel at the volume pots wiper to ground. The negative gate voltage was derived from the HT voltage. See KOC books.
I found this method very elegant and noiseless.

Tilman

yes it is, and yes they are, and I already have most of all of kevin's TUT's.. It's just that when i build my equipment, I wanted to take a more "old school" approach to it... I also don't like FET's... In fact, even to the point where, even if I did do a VVR style plate rail "Power Scale", I would still use a tube instead of a FET....

-g

[greekie]

On the upside, an octal based relay will fit in a spare tube hole, I guess.

Only an upside, until the relay is plugged into the wrong tube hole by the user! ;-)

Jake

[mooreamps]

Only an upside, until the relay is plugged into the wrong tube hole by the user! ;-)

Jake

Will, then you would want to mount the relay inside the chassis, and leave the tubes on the outside.. or an even better way is to have all the tubes on the inside and don't give the user any chance to plug something in wrong....

-g

[Steve Conner]

Using JFETs for switching in tube amps needs a little care, because the voltages they can stand are limited, to maybe 30 or 50V. You need to pick a point in the circuit where that won't be exceeded.

The volume wiper in an 18 watter would be a good place to start, and I'm sure that works OK. I just want to point out that you can't just use them anywhere.

400V MOSFET solid-state relays are available now, I believe you can substitute these for relays and LDRs in the usual channel switching circuits.

[greekie]

Will, then you would want to mount the relay inside the chassis, and leave the tubes on the outside.. or an even better way is to have all the tubes on the inside and don't give the user any chance to plug something in wrong....

-g

I guess your question is a rethorical one where you do not expect the answer to teach you anything. But yes, I would mount the relay inside. The relays I use have gold contacts and are rated for at least 100,000 electrical operations and 100,000,000 mechanical ones. The characteristics of the smaller PCB relays like the ones from Omron match or beat those you advocate. And the user can't ruin his amplifier with them.

I'm not calling anyone stupid, but there's a reason we don't use a jack-socket for the mains AC input.

Jake

[mooreamps]

Using such a big relay for channel switching is kind of pointless. Not only are the contacts made for power switching, and susceptible to tarnish that will cut low-level signals, but the relay itself is so big that it means lots of stray capacitance to pick up bleed and hum.

Well, that's what shielded signal cable wire is for...

On the upside, an octal based relay will fit in a spare tube hole, I guess.

They will fit inside a standard octal socket.. Just one less part number I have to deal with on the BOM..

-g