Help please to join 2 pre-amp channels into one

[hamfist]

First post here. What a great forum you guys have ! I've a regular over at several other forums but have only really been getting into amp tweakage in the last year, so am starting to need to be involved with slightly more technical fora (to help me out when I get stuck. LOL !).

Anyway, I've just picked up an old 100W Selmer PA, with about 9 12AX7's and 4x EL34's (plenty of flexibility to tweak !!).

My plan is to have two distinct channels, each with their own separate pre-amps .....and then to join the channels straight before the PI.

I've no idea about channel switching, but was hoping I might be able to do it something like the 1st gain stages of the treble and bass channels on a plexi are arranged. ie. they simply just are hard-wired together at the mixer stage, and share the rest of the following signal pathway.

I was intending to select my channel by having a separate input on the amp for each channel, and run these down to an A/B box on the floor, which my guitar plugs into. The A/B box would then govern which channel I sent my guitar signal to.
I've certainly played plexi's and an AC30 like this before, andit worked fine.

Is this a reasonable plan ? And what is involved in appropriately joining the two channels, at the point that they are hard-wired together ? Is it simply a case of soldering the two different pre-amp pathways together at a certain point, and all the subsequent signal pathway would be shared ?

[Chuck H]

You said:

My plan is to have two distinct channels, each with their own separate pre-amps .....and then to join the channels straight before the PI.

And

I've no idea about channel switching, but was hoping I might be able to do it something like the 1st gain stages of the treble and bass channels on a plexi are arranged. ie. they simply just are hard-wired together at the mixer stage, and share the rest of the following signal pathway.

Those are two different things. But if what you mean is that you hope to just join the two signals before the PI as a simple circuit like this, then that should work fine. Notice that most amps that do this (no matter where it is in the circuit) use mixing resistors. These are in place to reduce interaction between the two signal paths. You can count on some signal loss due to these resistors. If you use a split load (concertina) type PI circuit that signal loss could reduce gain by more than you like. Split load PI's usually have a dedicated triode with very little resistance or loading to feed the PI input. If you plan to use a long tailed PI I think it will work fine.

If you can spare a triode another way to go is the virtual earth mixer. It's called that because the input impedance is so low. The mixing resistors can be much smaller and there will be NO interaction between the channels. Also, if you wanted to use a third channel, add an effects loop or a reverb you will still get no interaction between them. It's a very good circuit for joining signals together. You will also get a low impedance signal to drive the PI with, which to my ears seems to add crunchiness to the tone. If your interested I will explain the circuit.

I was intending to select my channel by having a separate input on the amp for each channel, and run these down to an A/B box on the floor, which my guitar plugs into. The A/B box would then govern which channel I sent my guitar signal to.
I've certainly played plexi's and an AC30 like this before, andit worked fine.

I've done this too. In the real world it works fine. But if you want to idealize and reduce background noise (especially for your clean tones) you need to use relays. Amps set for high gain hiss. If one of your channels will be set for high gain that hiss will be heard as background noise even when your not using that channel. With relays you can ground both the input AND the output of the high gain channel when it's not in use which keeps the noise down. It's nit picky, but true.

Is this a reasonable plan ? And what is involved in appropriately joining the two channels, at the point that they are hard-wired together ? Is it simply a case of soldering the two different pre-amp pathways together at a certain point, and all the subsequent signal pathway would be shared ?

Yes. As long as you use mixing resistors and account for the gain loss. But not directly feeding a split load PI. Or use a virtual earth or some other mixing/joining stage behind the following circuitry or even a split load PI.

Chuck

[hamfist]

Excellent information Chuck. Just the sort of advice I needed.

And you have brought up a point that I had completely ignored ....... the fact of high gain hiss ! One of my channels will be a 4 or 5 gain stage high gainer, so it looks like I will need to do some sort of relay circuit to ground out the input and output of each channel when not in use.

OK, basically I have no idea how to do this, or what type of relay to use. I'll do some searching, but it would be very helpful if you could possibly guide me to some circuits or give some guidance as to what sort of relay to use.

I really appreciate your help !

[tubeswell]

For relays you need a lowish power DC rail (- you can get different voltage relays to suit), so tap an AC line off your HT and slap a 1000V diode in series and then a Pii filter and a voltage divider (to pull it down to whatever - 12V, 36V, etc voltage you have to match the relay(s) you have got). Now hook the relay(s) up to that with your footswitch activating the DC supply to the relay. If you have a DPDT relay - nice and simple, you can use one of the poles for grounding the grid in front of the stage you want to cut out of the circuit, and the other pole for grounding the short circuit path from the coupling cap of your first stage to the grid of whichever stage you choose as your 'common' stage. Look at the PV '2 channel' amps e.g. Classic 30/50, DB

Or you can use opto isolators in a similar fashion (see Soldano SLO100).

[hamfist]

For relays you need a lowish power DC rail (- you can get different voltage relays to suit), so tap an AC line off your HT and slap a 1000V diode in series and then a Pii filter and a voltage divider (to pull it down to whatever - 12V, 36V, etc voltage you have to match the relay(s) you have got). Now hook the relay(s) up to that with your footswitch activating the DC supply to the relay. If you have a DPDT relay - nice and simple, you can use one of the poles for grounding the grid in front of the stage you want to cut out of the circuit, and the other pole for grounding the short circuit path from the coupling cap of your first stage to the grid of whichever stage you choose as your 'common' stage. Look at the PV '2 channel' amps e.g. Classic 30/50, DB

Or you can use opto isolators in a similar fashion (see Soldano SLO100).

Thanks Tubeswell. Is there any way you can simplify and spell it out rather more easily for a relative beginner.
You state I need a low voltage DC supply, and then suggest I tap off the amp's AC supply before the PT ? Why not just tap from a higher voltage DC line and reduce the voltage from that ?

Also, what is a Pii filter ?

Is a voltage divider simply a resistor that reduces the voltage ?

I think if I can get my head around the power supply for the relay, I might be able to get it all to work.

[HTH]

Thanks Tubeswell. Is there any way you can simplify and spell it out rather more easily for a relative beginner.
You state I need a low voltage DC supply, and then suggest I tap off the amp's AC supply before the PT ? Why not just tap from a higher voltage DC line and reduce the voltage from that ?

Also, what is a Pii filter ?

Is a voltage divider simply a resistor that reduces the voltage ?

I think if I can get my head around the power supply for the relay, I might be able to get it all to work.

Pi filter is typically used right at the source point of a rectified DC voltage to smooth the ripple, thereby reducing hum. For instance, right after the rectifier diodes on an amp's B+ line, you'll want a Pi filter which is just a cap to ground, then a resistor (or choke) then another cap to ground. You'll find this setup in an old Marshall plexi (checkout a schematic). wiki description... http://en.wikipedia.org/wiki/Pi_filter

A voltage divider is two resistors in series with the 'output' being taken from the junction of the two resistors. one of the free ends is connected to your voltage source, the other to ground - this pads down the voltage. see wiki for more info... http://en.wikipedia.org/wiki/Voltage_divider

I think the voltage would be more stable being sourced from AC (from the PT's secondary HT winding, not the AC mains) and rectified rather than tapping off an existing DC supply for the preamp. Might be total shite what I've said there too as I've never played around with relays yet.

[tubeswell]

Hi hamfist

The HT (short for 'High Tension" which is techno-slang for 'high-voltage') is the secondary AC winding that you rectifiy for your B+. You tap the power supply for the relay circuit from that. Everything else was explained by HTH

[hamfist]

Thanks guys, I think we're getting there.

So, if I'm right, you're suggesting I tap the AC supply from the secondary winding of the PT, but BEFORE the rectifier, or directly after the recifier ?

As for a Pi filter, please could you check this out, but from the link you gave HTH - http://en.wikipedia.org/wiki/Pi_filter - C1 and C2 are obviously caps, L would be a resistor in my case, not a choke, and RL would be my relay(s) ?

Once I've got my rectified power supply then, I can use a voltage divider (seems reasonably simple) to lower my voltage to that required by the relays.

I'm sure I can play around and get the voltage divder sorted, but does anyone have any idea what sort of values for the caps and resistor in the Pi filter I might need ? (or be able to guide me to somewhere that might give me an idea).

This is a tricky subject to research. I've done a load of searching on the matter, but come up with little so far of help, except for what you guys have told me.

[HTH]

Thanks guys, I think we're getting there.

So, if I'm right, you're suggesting I tap the AC supply from the secondary winding of the PT, but BEFORE the rectifier, or directly after the recifier ?

As for a Pi filter, please could you check this out, but from the link you gave HTH - http://en.wikipedia.org/wiki/Pi_filter - C1 and C2 are obviously caps, L would be a resistor in my case, not a choke, and RL would be my relay(s) ?

Once I've got my rectified power supply then, I can use a voltage divider (seems reasonably simple) to lower my voltage to that required by the relays.

I'm sure I can play around and get the voltage divder sorted, but does anyone have any idea what sort of values for the caps and resistor in the Pi filter I might need ? (or be able to guide me to somewhere that might give me an idea).

This is a tricky subject to research. I've done a load of searching on the matter, but come up with little so far of help, except for what you guys have told me.

you're tapping off the HT winding, so that's before the rectfier diodes. the HT winding is still AC - the diodes rectify the voltage to DC.

For the Pi filter, start off with 50uF caps on either side and a 1k resistor.

RL is just the load, in your case the relay. however, you need to pad the voltage down with a voltage divider first as your relay is likely to be 12vdc. try 100k and 3k3 assuming a rectified voltage around 350v, try a voltage divider calculator... http://www.raltron.com/cust/tools/voltage_divider.asp

[hamfist]

OK, now we're really getting somewhere !!

you're tapping off the HT winding, so that's before the rectfier diodes. the HT winding is still AC - the diodes rectify the voltage to DC.

gotcha !

For the Pi filter, start off with 50uF caps on either side and a 1k resistor.

Would a couple of 33uF caps also be a reasonable place to start, as I have a couple of those sitting in my drawer already ?
What effect would using 33uF have instead of 50 uF ?

RL is just the load, in your case the relay. however, you need to pad the voltage down with a voltage divider first as your relay is likely to be 12vdc. try 100k and 3k3 assuming a rectified voltage around 350v, try a voltage divider calculator... http://www.raltron.com/cust/tools/voltage_divider.asp

Excellent link !


Thanks so much HTH. I think this could just about be do-able for me.

[Steve Conner]

I love those 70s Selmers. They're good amps but cheap and ugly enough that you can mod hell out of them without feeling guilty. I made this from a Treble'N'Bass 50 SV:

http://music-electronics-forum.com/s...ead.php?t=5008

I added an extra preamp tube to one channel to make them clean and dirty, and originally I switched between them with the A/B box just like you said. (If you look at the schematics of the T'n'b 50, plexi, or any similar amp, you'll see ways of joining two channels together. For example, the T'n'B uses two 220k resistors as the mixing resistors mentioned by Chuck H.)

When I transplanted it into the 5E3 cabinet, I implemented proper channel switching with relays, which I powered off the heater winding with a rectifier circuit. I wouldn't suggest running a relay off B+, as it wastes huge amounts of power and generates a lot of heat in the dropper resistor.

Oh, and I also used Chuck's "virtual earth mixer" (though I always think of it as an "anode follower") as the reverb return mixer.

steve

[hamfist]

When I transplanted it into the 5E3 cabinet, I implemented proper channel switching with relays, which I powered off the heater winding with a rectifier circuit.

Cool Steve. Can you be a bit more specific about how you implemented your power supply to the relay(s).
The heater circuit is AC , right ? So I'd need to use a Pi filter to tap off a rectified DC circuit from that. If the voltage was then high enough, I could then use a relay appropriate to that voltage. More details, please, please, please !!

My amp had 11 pre-amp tubes running off the heater windings, of which I am now only going to use 7, so I'm sure the PT would cope easily with tapping off some power from that circuit.

[hamfist]

For anyone following the thread, there is a schematic of a circuit used to rectify a heater filament supply, to supply a 12V relay at http://music-electronics-forum.com/s...ghlight=center

[hamfist]

Other excellent info here - http://music-electronics-forum.com/s...powering+relay

[fdesalvo]

Ham,

I'm glad I found your post, as we are up to the same thing! I'm probably where you are technically, but I have a few basic builds under my belt- mostly lightly modified tweed clones. I'm going to build the power section and clean channel of an ab763 Twin and hopefully mate it with a hot-rodded jcm type lead circuit.

In all honesty, it would be easier to just buy a Mesa MKIV or Stiletto and be done with it, but never being the sort to leave well enough alone, I have to build it. The information in this thread has definitely changed my mind about how I'm going to go about designing this puppy- looks like relays are a must. Anyway, I'll be following your progress, brothah!