Powering preamp tube heaters

[swipesy]

Hello all. I've been a lurker here for a long time and have gotten lots of great info from all of you, and I appreciate that very much! I've built a couple small preamps which sound great but I 'm wanting a little more flexibility. So, I'm now designing an 8-channel preamp and have pretty much finished the switching system. The channels will be based on the pres I've already built, which are basically Twin and Marshall clones.

I have a question about powering the tube heaters. The pre will be using a multiplexer/latch-based switching circuit and will have some op-amp circuitry as well (mostly for a cab sim and noise gate). Can I power the heaters from the same supply that powers the switching/op-amp circuits, or will they cause noise in the other circuits? I'll already have a 15V rail, and was thinking of using 12V regulators off the 15V for the tube heaters. I don't know if that's a good idea or not though. Can anyone tell me if that's good or bad? I can always build a separate supply for the heaters if I need to, I'd just like to avoid that if I can.

Thanks a bunch!

[jjasilli]

Doug sells 5V relays meant to be run off the 6V heater supply; so it's do-able. With that many channels I'm guessing there will be hi-gain preamps, more susceptible to noise.  If a series supply from the switching circuit makes noise, maybe use a parallel circuit to supply the heaters, with it's own rectification, filtering, and regulation.  That might isolate the heaters from any switching noise.  You could also regulate a 15V series supply down to 6V which might also squelch any switching noise.

[stingray_65]

I think JJ is on to something about the separate paralleled supply.

The current draw of 8 12A_7's is .15A*8 = 1.2A, that's pushing a 7812 kinda hard, but a heat sink will make it ok, If you keep the ambient temp around the heat sink low, I don't think it's possible to chassis mount the heat sink without electrically isolating it.

One plus is it takes 14.8V min in, so a minimal amount of heat is wasted regulating down to 12V.

Because the  heaters are supplied with DC, there should be no hum issues.

I can see no reason it would not work well (assuming your 15V supply can deliver the current)

but again, I think it would be prudent to keep it some what isolated, just because of the current draw.

Ray

[RicharD]

He's talking about DC so there's no reason to go to 6V.  12V is actually better better because there is less current draw.  As long as your bipolar supply has ample ampacity, there's no reason you can't grab the positive rail, regulate it down to 12V, and light your filaments.  It's DC, noise is not gonna be an issue. 

[Willabe]

As long as your bipolar supply has ample ampacity, there's no reason you can't grab the positive rail

              

I call dibs on "Ample Ampacity" for a band name!                   

But I'm still not gonna grab the positive rail!        

[RicharD]

Actually, if you're not exceeding heater to cathode ratings, there's no reason you couldn't light your filaments with a negative DC supply.

[swipesy]

Ample Ampacity ... perfect! Second dibs!

Thanks guys, I really appreciate it. I've been using my two preamps through an A/B switch, and while that has worked ok, I've been wanting something more flexible. Plus, I use different settings on both preamps for different songs. I often use chorus and delay too, so it's quite the tap-dancing at times. I thought about getting a MIDI preamp such as a Rocktron Piranha and matching it to an effects processor like the G-Major, but I just love the tones I get too much, you know? My "Marshall" preamp is pretty much like Doug's hot-rodded rack tube preamp with a few minor changes, and I love it. (Thanks Doug!) I've been thinking for a long time about making an "all-in-one" preamp, and recently decided to just do it. The controller pedal will be a little bigger than what I'm used to, but that's ok as long as I can hit one switch to get the tone I want for the song I'm playing.

For those interested, here's the plan ...

Eight discrete channels, six "Marshall" and two "Twin", with a ninth solid-state channel for acoustic (my guitar has a piezo-loaded bridge). Each non-acoustic channel will have four switches: 1) no effects; 2) with chorus; 3) with delay; and 4) with chorus and delay. Add three more global switches for normal reverb, long reverb, and mute all (for tuning). Yup, that's 36 switches, but it will cover everything I play which is mostly classic rock. I plan on laying out the switches similarly to the Rocktron All Access controller, so it will be slighly larger than that. Not too bad really, when you think about it. The preamp will give me the same sounds I already get, will allow me to hit one switch at a time, and will make me smile all day long!

The front panel will offer the usual channel controls for each channel in vertical fashion, along with a dedicated noise gate control for each high-gain channel, and push-button switches for the same control as the controller pedal offers. There will be a master volume control and a cab sim output based on a Marshall circuit. I need to play around with the cab sim circuit before putting it in the preamp for good.

Main power will be supplied by a 6.7-amp Mean Well LPS-100-15 SMPS. I use a smaller SMPS on my workbench and didn't notice any switching noise at all when building the other two preamps, and that was with a 65KHz switcher. The Mean Well PS has an even higher switching frequency of 134KHz, so switching noise from the SMPS won't be a problem. HT power will be supplied by two Hammond 162F12 (12.6V, 500ma) transformers in back-to-back configuration with a standby switch between the secondaries. Plenty of power for everything. I'll use individual 12V regulators for each tube (they're certainly cheap enough) to keep from having to create a separate heater-only rail. The op-amps will be powered from the main 15V rail and use charge pumps to generate the needed -15V. I use those all the time with no noticeable noise.

(Continued on next post ...)

[swipesy]

(Continued ...)

I decided that I should first work out the channel switching, because if I couldn't get that to work then everything else is moot. After a lot of research I finally came up with what I consider to be a good plan. The first problem to address was coming up with something that wouldn't require a hundred wires (exaggerating, of course) in the connecting cable. Enter the Velleman K8023 two-wire remote control kit. It offers ten independent channels of remote control through two wires. While that's not enough for 36 switches, I came up with a solution for that. Using analog switches and diode arrays in the controller and having an 8-bit multiplexer and latches on the receiving end, only four of the remote channels are needed to select a single preamp channel. Two channels will be used for the chorus and delay, and the remaining four channels for the acoustic-only switch, short reverb, long reverb, and mute all switches. Works out perfectly in theory. The multiplexer prevents two or more channels being active at any one time, and the latches hold the currently selected channel and effect (if any) active. The "acoustic only" switch will shut off all the tube channels, leaving only the acoustic channel active. Solid state relays will be used to open and close/ground each channel on both the input and output. One of the users here posted about using SSRs, which is where I got the idea for that. (For whatever reason, I can't find that post now. If it was you, thanks!) Another concern was what to do about the controller pedal LEDs. If I set those based only on what switch was pressed on the controller, that wouldn't take into account what button might be pressed on the front panel of the preamp. My solution is to use a second Velleman remote kit. The preamp will have a second sending unit to send data based on what is currently active on the preamp and the pedal's receiver will set the LEDs accordingly. After looking at the Velleman kit schematics, I found that I can wire the kits so they use only three wires for both remote systems. One of those wires is power, with no control signal, and adding a fourth wire for ground allows me to remotely power the LED receiver. With only four wires needed, that means I can use standard CAT5 cables for hookup. Cool!

Ambitious? You bet it is. I'm a US Customs Broker by day and a PHP programmer by night, and still have never taken on a project this large (though a few PHP projects have come close). Eight channels, 14 tubes, 36 switches, and I still haven't figured out what to do about a chassis for this beast yet. But, I'll take my time with it, and eventually I'll have what I've been wanting for a long time. I think the effort is worth it, and it will be fun at the same time. Only after the preamp is done will I consider building a stereo tube power amp. Time will tell, as it always does.

Comments are welcome, as are criticisms! I'd be interested in hearing what anyone has to say about it. If you're interested, I've attached the .sch file for the pedal switching curcuit. You might find that interesting ... or not!

Later!

[RicharD]

Crazy cool ambitious!  I need to pull the cut sheets of your switching chips so I'll know what I'm really looking at.  I assume you're gonna design a PCB.  I would definitely bread board this before going all the way.  Please keep us posted on how it progresses.  We like pictures!

Welcome to the forum BTW.  You're doing something quite different and we like that here. I could really use a cheerleader emoticon right about now.

[danger-russ]

Cool idea, that's sort of what I want to end up with once I get competent building tube circuits. Iv been spoiled by multi efx pedals for too long. I wonder if it might be easier to just add a couple micro controllers instead of using the vellman kit. You might be able use i2c over just a instrument cable, many even go wireless. I'm not sure, haven't looked at what microchip has for chips lately but they were coming out with chips with wireless transcievers built in to them. Don't know what the max range would be, but that might work.

[swipesy]

Thanks guys. Yes, I'll definitely be using PCBs for the switching circuits. There isn't any audio on the lines so it's a no-brainer. Some of the audio circuitry will use PCBs as well, but I'll be careful with it.

Actually, I noticed an error is my original switching schematic. The attached one is corrected, and you can see I've started working on the LED control circuits. Breadboarding this would be almost as ambitious as the entire preamp, so I'm not going to do that. I will be breadboarding small sections of it though, just to make sure I'm on the right path.

Pictures? You got it. :) It's still in the design phase, of course, but when I start doing anything that can be photographed I'll create a little pictorial on a web page to go along with it. Glad to!

I did look at the various controller chips, but they all seemed a little beyond me. I have a logical mind, but some of that digital stuff just goes over my head. Analog I can understand though, hence the design I can up with. However, if you do find something that would be easier - and I can understand it - then that would certainly be cool!

I thought about going wireless, but I prefer the solid connectivity of wires for something like this. Using a wireless for guitar is one thing, but this is something else. If a guitar wireless isn't working well because of something in the area, then you can just use a cord. For something like this, you can't. Not easily, anyway. You'd still have to design for the wired option in the event the wireless failed, and a Cat5 cable isn't enough to worry about. Plus, they're relatively cheap, and you can get LONG ones. I've calculated voltage drop over a 100-foot cable and it only reduce power to the pedal by roughly 2.5 volts (if my calculations were right), which means all the chips would still work fine. The LEDs might be a bit dimmer, but functionality wouldn't be affected. I doubt I'll ever use a 100-foot cable though, so it's all good.

I'll keep you all updated as much as I can. Between this and my PHP projects, I've got a lot on my plate.

Later!

[EDIT] Oops! I forgot to attached the updated file. Sorry! [/EDIT]

[danger-russ]

I'm at work at the moment so I can't see you schematic. Here is a link that has the commands for picbasic. http://www.rentron.com/PicBasic2.htm
It's a really simple language, but it gets kind of complex because you'd have to use it to move bits around the different registers on the chip, and all the chips are a little different.
Here is PDF for one of microchips older chips the p16f84   ww1.microchip.com/downloads/en/devicedoc/35007b.pdf . It wouldn't work for what you need, but would give you an idea about how the chips work. I haven't played around with them for years but I picked it up pretty easy and didn't have any programming skills when I started.

[swipesy]

Thanks danger-russ. PICs are actually one of those things I looked at and just didn't want to take on. If I was going to market this thing then I would probably go that route, but this is just for me and the multiplexer/latch scheme is good enough. Besides, I'm familiar with them and, with all due respect, I'd rather stick with what I know, especially on a project this large.

I've just finished the initial schema for the controller pedal. It's attached for anyone's perusal. :)

[swipesy]

Ok guys, attached is the final circuit for the controller pedal. I've made a few changes, but the main one is that I changed my mind about using Cat5 cables for hookup. I just don't think they can handle the power, so I'm going to build a 6-pin cable either using DIN plugs/sockets or even NL8 speakons with 6-conductor 18ga wire. I like the idea of speakons because they lock into place. Regardless, that part of the circuit is now complete and has been thoroughly checked twice.

Next up: The preamp's internal switching circuit. That will be a bit more complex, but still not too bad.

Later!

[RicharD]

18g is good for 10 amps.  Are you using starter solenoids?  Kitty5 will be fine.

[swipesy]

Really? From what I've been reading it wouldn't. According to the AWG table, 18ga wire can handle 2.3amps power transmission, and 24ga (used in kittycables) only 577ma. Am I reading those wrong? The Velleman specs show the remote units require 300ma, then add the power needed for the other chips in the pedal. How much can a kittycable really handle? That is, based on two of the wires for V+ and another two for ground.

[RicharD]

Really.  2.1A

http://wiki.xtronics.com/index.php/Wire-Gauge_Ampacity#AWG

I once upon a time stumbled onto a wall receptacle (yes a 120V outlet) wired in CAT5.  Ironically that isn't what I was called out to fix, but of course I did.  It was still working supplying a day trader's PC at the time.  I was called out to add another outlet.  We joked about using CAT6.

[swipesy]

Ouch! And I thought our house was bad when we bought it! During our initial inspection the light over the sink didn't work and the owners told us the lamp was just burned out and they just didn't feel like removing the lens to replace it. (It is a built-in ceiling light.) No biggie, right? Come to find out later the entire FIXTURE was missing and live wires were just hanging out of the ceiling. Then they put in a 5-globe yard light and mis-wired it. They cut off the plug's ground lug to prevent popping the breaker. Needless to say, I fixed those and other wiring problems shortly after we moved in.

So, 2.1A. Is that for just one wire?  Meaning, I can use just two of the wires for V+ and ground?  That would be beautiful!

[swipesy]

Update:

I had to make some changes to my "verified" pedal schematic. I noticed one major flaw in my design. The outputs from the Velleman receiver are originally designed as open collectors, which I knew weren't going to work with my circuit. I had already decided to omit them and simply use the outputs from the PIC that are normally sent to the darlington array.  What I failed to take into account is that the PIC outputs are at most 5V, and the rest of the circuit runs on 15V. For awhile I couldn't find anything suitable for what I wanted to do, then I came across source driver chips. Putting those right before the latches to get the voltage back up to the rest of the circuit seemed to me to be a good solution. I also decided to add a multiplexer on the reverb switches to prevent accidently selecting both at one time. The last change was to use open collectors to fire up the LEDs instead of driving them directly from the multiplexers. I believe it's now fairly bullet-proof, but I won't know for sure until it's built. I'll be looking it over many times before actually designing a PCB. It looks pretty complicated as it is now, but it really isn't. I should be able to get it all on a smallish PCB, maybe 5x3 inches, something like that. The smallest traces my PCB program creates is 0.006", which I believe are more than wide enough for nearly all but the power rails. Anyhoo, there it is. I almost had the preamp switching circuits done but will have to rework those to include the source drivers, open collectors, etc.

The updated schematic is attached, if anyone is interested.

Later!

[swipesy]

Hi guys,

Well, it's been awhile, and I've been making some progress. First of all, I found someone who is going to program some PICs for my project, so that's going to change everything I have designed so far. That's ok though, as it's going to be MUCH simpler than what I had originally designed and will work a whole lot faster. I will have to wait to redesign the switching cards until the PICs are ready so I know exactly which pins need to go where. I have an idea of that now, but there's no rush on it. Also, I've been looking into switches/LEDs for the pedal and front panel. There are some cool combination switch/LED units out there these days. Kinda pricy though. We'll see. :)

I originally planned to have a 9th SS channel for acoustic that is always on. A custom "pan" pedal would be built to chose between the acoustic and electric channels or mix between the two. However, I'm now considering using one of the tube channels for acoustic use. (Single tube, a la Twin.) That would make the build much easier to design and implement, but would also eliminate the option of mixing acoustic and electric sounds. I can think of at least a few times I would like to have the ability to mix, but it's not a huge deal. Still, since I'm building this from scratch and will be using both PTP and PCBs, I can go either way. Which would you do, and why?

Now I have some interconnect questions, as I've never built anything like this before. This preamp will have 8 (or 9, if using a SS acoustic channel) separate channel cards, all with inputs and outputs. Each tube channel will be completely isolated from each other, with its own tube(s), etc. Switching will take care of which channel is "on". Each "off" channel will have its input tube's grid grounded and its output disconnected by an SSR to reduce noise. How would you suggest wiring up the inputs and outputs? Direct wires from the input jack to each channel? Balanced line driver and individual receivers? FYI, I've used balanced drivers/receivers on various projects before using NE5532 chips over unshielded twisted wire with no noise. What connectors would you use on the PCBs? RCA? F-type? BNC? "Free hanging" wire plugs and headers? There are just so many choices. My thoughts are to use balanced plug/header connections over unshielded twisted wire, as I've used that before and I know it's reliable. Still, I don't know if that would be the best choice for this project. Any opinions?

Thanks everyone!