• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

So i was going into Tosche Station to pick up some power converters . . .

ericj

Member
2008-12-08 10:24 pm
Has anyone here actually built and used a tube amp with a switching high voltage power supply? Either an AC to DC SMPS or a DC-DC boost converter.

It's an interesting idea and i'm sure it can work well, I'm just curious to hear about actual experiences.

I picked up one of those "capacitor charger" high voltage dc-dc converters with the idea of experimenting with a 12v powered approximation of an old Fender guitar amp.

A couple 6v6 and three 12ax7 should come in somewhere around 30w of b+ in actual use, right?

This is the one with a small heatsink and one fet, like here:

DC DC Boost Converter 8 32V to 45 390V High Voltage ZVS Capacitor Charging Board | eBay

Some vendors say this is a 70w supply, others say 40w and 70w "peak".

I also noticed this one:

DC12V 24 to DC 100 250V 70W High Voltage Converter Boost Step Up Power Supply | eBay

What's curious is some vendors refer to this one as a 20w supply, others as 40w, and others as 70w. This one is listed with a 100-250v range, others are rated up to 450v. Since it's in a channel extrusion it's probably slightly safer to use. But the variance in claimed power rating is interesting.

I have heard that many switching power supplies don't really "like" a large capacitive load on their output. I don't fully understand what is meant by that, though.
 
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Whoa!
Switching power supplies (or any power supplies) have a common mode voltage isolation spec. (input to output or to case) I doubt if ANY are rated high enough for 18 supplies in series. Not to mention 18X noise sources. 400V or 500V isolation is fairly typical, but do check the unit's specs.

Switching supplies for audio also should preferably use the quiet switchers too. Linear Tech makes some control chips that provide controlled turn-on and turn-off ramps for the switches. A little less efficient, but much quieter electrically. That also applies to radiated EMI leaking out all over the neighborhood too. Maybe the VHF noise doesn't show up in the audio channel, but it will still show up in cell phones, TV, radio ....
Commercial units typically have a metal case around the switcher with feedthrough caps or line filters etc.
 
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B+ from a switcher

I have bought a couple of these.
DC DC Boost Converter 8 32V to 45 390V High Voltage ZVS Capacitor Charging Board | eBay

And yes, loaded with a 350uF cap id does charge it without complains, i have
used it with DC from 12V to 30V.
I have also tried to feed it from a small 220 to 12VDC switcher, but that
does not work well, the switcher overloads and stops, and restarts and so on.
I have not (yet) examined the quality of B+, but i will.

In addition to the two mentioned there is also a 200 - 450V model :
2X DC12V 24 to 200 450V 70W High Voltage Converter Boost Power Supply with Shell | eBay

The really nice withh these is that small amps may be driven with a simple
wall-wart or laptop PSU, nullifying the need for power tranny and all
components that comes with it. It might also be a low-hum powersupply
for lowlevel amps .
 

ericj

Member
2008-12-08 10:24 pm
Yeah, stacking 48v converters seems sketchy to me.

I was wondering if i could power a small amp with an external 4 or 5 amp 12v switcher. May need to be bigger than that, or maybe i can just put a delayed start on the hv supply

.:Sent by pneumatic tubes
 
Yeah, stacking 48v converters seems sketchy to me.

I was wondering if i could power a small amp with an external 4 or 5 amp 12v switcher. May need to be bigger than that, or maybe i can just put a delayed start on the hv supply

.:Sent by pneumatic tubes
Yes it should ve very doable, and no delay is needed.
What could be needed , and actually good, is to reduce the filement CURRENT, as they might overload the mains-switcher initially. The B+ switcher
will limit it's current, so that is no problem here.
As to filament :
I would use a 7805 with proper heatsink , bootstrapped to leave 6.3V to reduce
the mains-switcher to filament voltage. As the 7805 will current limit it will allow the mains-switcher to reach working voltage.
If the mains-switcher is close to capacity, using a switching step-down converter could be a solution, it works sort like a transformer, converting 12V 0.5A to 6V 1A and has very low loss, heatsink might not be needed.
 

ericj

Member
2008-12-08 10:24 pm
Yes it should ve very doable, and no delay is needed.
What could be needed , and actually good, is to reduce the filement CURRENT, as they might overload the mains-switcher initially. The B+ switcher
will limit it's current, so that is no problem here.
As to filament :
I would use a 7805 with proper heatsink , bootstrapped to leave 6.3V to reduce
the mains-switcher to filament voltage. As the 7805 will current limit it will allow the mains-switcher to reach working voltage.
If the mains-switcher is close to capacity, using a switching step-down converter could be a solution, it works sort like a transformer, converting 12V 0.5A to 6V 1A and has very low loss, heatsink might not be needed.

Staged powerup has worked well for large disk arrays for decades, and monostable delayed start-or-stop relays are easily breadboarded with 555 timers or purchased out of china as modules for less than $2 shipped.

A bunch of cold filaments present a similar challenge to a switcher as a bunch of cold DC motors.

Delaying the HV lets the cathodes get hot before the plates get voltage as well.

But you're right that i might not need it.

The current theoretical complement of what I'm thinking about is 2x 12ab5 at 200mA each, 2x 6n2p in series at 340mA, and one 12ax7 (or 7025, or 7058) at 150mA, = a hair under 900mA at 12v. Probably with initial startup a lot harder than that.

Max push-pull plate current of a pair of 12ab5 is given in the datasheet as 79mA, with idle at 70mA (iirc). I guesstimate the 12ax plate currents combined as "not a lot". I've seen people who should know (like PRR) state that the 6n2p, aside from the heater configuration, is as similar to a 12ax7 as any two 12ax7 from different factories are to each other.

I don't plan huge rail capacitance. 1st cap will be 33uf or smaller. The output of the switcher i have here has 10uf, which may be plenty for the top b+ voltage.

Tubes selected by virtue of already being in the parts bin. Most of them new, even.

To my knowledge, most of those heaters don't have a controlled start characteristic.

I'm not sure what the startup surge from these off-the-shelf high voltage switchers is like. The wee cheap one with the small heatsink has an automotive 10A fuse in it.

I have a fair number of 12v-14v bricks in the 3 to 5 amp range.

I'll generate a negative bias voltage with a small diy charge pump.

Yeah, I'll take my build thread over to instruments if/when i actually start, but i figured this discussion would find more interested parties here.
 
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I have been tinkering with a battery powered vacuum tube guitar amp. The exact size and power output is yet to be determined, since I have not yet found the limits of the available cheap power sources.

My last experiment consisted of a "200 watt 220 volt AC outlet Power Inverter" from Amazon for $17.29. Unfortunately I may have purchased the only one in existence, but there are other similar units. I opened up the unit and connected a pair of wires across the only 400 volt electrolytic in the unit.

Amazon.com : SallyBest® 200W Automatic Thermal Shutdown 220V AC Outlets Power Inverter Car DC 12V to AC 220V / USB 5V Inverter Laptop Charger Notebook Adapter with 1 AC Outlet + 1 USB Port : Car Electronics

I then connected the AC inverter to this 12 volt 6 amp "LCD AC Adapter" which I got from Amazon for $6.86. It's now $7.71.

http://www.amazon.com/gp/product/B003TUMDWG?psc=1&redirect=true&ref_=oh_aui_detailpage_o01_s01

I powered everything up and measured 13 volts across the AC adapter output, and 290 volts DC across the inverter wires with no load.

With a 10K load the output was 270 volts.
With a 5K load the output was 265 volts.
With a 3.3K load the output was 260 volts.
With a 2.5K load the output was 256 volts. (26 watts)

No start up issues were seen so I added a 13GB5 tube heater (600mA) across the 12 volt supply. Again no start up issues were seen....so I added another 13GB5. Again no start up issues were seen. I tried adding a 47uF 450 v cap across the load, and the power brick would not start.

At this point it looks like this power solution would have no problems powering a 10 watt guitar amp made with a pair of 6AQ5's and a pair of 12AX7's. Or a similar load without further work and I may just build one.

I believe there are two ways to solve the start up issue.

Add a CL-140 inrush current limiter between the 250 volt supply and the 47uF cap with the load across the cap. This will delay the hit on the 12 volt brick until the tube's are already nearly hot. This is the proper solution regardless of the next option......

Brute force......

I got one of these. but I have not tried it yet:

http://www.amazon.com/gp/product/B00D7CWSCG?psc=1&redirect=true&ref_=oh_aui_detailpage_o04_s00

I also have some other 12 to 14 SMPS's to try. I will need about 14 volts to charge the batteries when I get that far, and the little 12 volt brick is the limiting factor. It is probably over stressed at it's rated 72 watts. I have seen the same brick with a different name rated at 60 watts.

I also got some of these to play with. They put out a raw square wave AC which will give about the same DC voltage when rectified. You must use UF4007 diodes. Ordinary 1N4007's will fry rather quickly. They use the SG3525 chip (DIP in a socket), so a feedback loop can be made to regulate the output. I plan to stack two of them to make a supply in the 400 volt range and a lower voltage screen supply to wake up a pair of those 13GB5's, I have seen them crank out over 100 watts....gonna need a big battery!

http://www.amazon.com/gp/product/B00D7CWSCG?psc=1&redirect=true&ref_=oh_aui_detailpage_o04_s00
 

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ericj

Member
2008-12-08 10:24 pm
I like to live the slow and dangerous life so i order stuff out of china and then watch my beard grow in the mirror waiting for it to arrive.

Might be tricky to get 220v inverters out of Amazon but not so difficult to get them via AliExpress. If you can wait. You should include a message to the seller when buying, letting them know that you are aware that you are asking them to send a 220v inverter to a 117v region.

New 1pcs 12V DC to AC 220V Car Auto Power Inverter Converter Adapter Adaptor 200W USB hot selling-in Car Charger from Automobiles & Motorcycles on Aliexpress.com | Alibaba Group

I've heard it alleged that most inverter controllers are nothing more than a huge 555 timer charge pump. May be possible to modify these for higher voltages.

If you used 3 or 4 cell lipo packs of the high discharge variety it might not have to be so huge, but the expense goes way up compared to something like a Diehard Silver AGM. I was looking at them for a completely unrelated project and was grateful when i did the math that i could get away with much cheaper and more energy dense 18650 cells.
 
Might be tricky to get 220v inverters out of Amazon but not so difficult to get them via AliExpress

That is the exact inverter that I used in my experiments. I took it out of the silver case and added some heat sinks to the switch fets since the case was the heat sink. It wasn't even bothered by the loads that I threw at it, and it will deliver over 300 volts on 14 volts input. I was originally intending to steal its transformer and toss the rest, which might still be the case if I blow it up.

I've heard it alleged that most inverter controllers are nothing more than a huge 555 timer charge pump.

That's not the case here, and I have never seen an inverter bigger than about 10 watts that is made that way. The green board on the left is what's inside the Chinese 200 watt inverter. There are two identical SMD chips, Fairchild KA7500 SMPS controller. These are similar to the SG3525 chip. One of these drives the pair of fets above the transformer that I put heat sinks on. It operates as a push pull step up converter. The transformer is the typical center tapped primary (like a P-P tube amp) and the secondary is a full wave bridge with 4 diodes and a 6.8 uF 400 volt cap. There appears to be some feedback to one of the KA7500 chips.

The 300 or so volts DC are fed to the 4 fet H-bridge seen along the bottom of the board. These have no heat sinks and serve to chop the DC into a square wave with dead time that "approximates" a sine wave. I assume the second KA7500 controls them through some level shifting, but I haven't investigated that circuitry.

The other TO220 device in the upper right is a 7805 for the USB charging port.

I have cracked open a few inverters and this is the way they all work.

If you used 3 or 4 cell lipo packs of the high discharge variety

I have experimented with the LIPO batteries used in the model helicopter industry, and aside from the fire risk, they would be ideal. I'm not so sure I want to use them in a wooden guitar amp that is kept inside the basement of a wood frame house just yet unless I can find a foolproof controller. I have several battery powered projects in the fire, including building my own laptop. The last three commercial units I have purchased from major manufacturers have been disappointments. Weight is a big factor there, so LIPO is the first choice.

A guitar amp is already going to be somewhat heavy due to the speaker, and some of the other stuff mine will have, so a little extra weight isn't a big deal. It may use a LiFePO4 battery. I already have a pair of 12 volt "12 AH" batteries that I got on Ebay. My testing shows more like 8 AH.

cheaper and more energy dense 18650 cells.

That is another option. Most 18650 cells are Lithium Ion which are relatively safe unless overcharged, or completely discharged.
 

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ericj

Member
2008-12-08 10:24 pm
Ahh, enlightening.

I read on another forum where someone tried an 18650-based pack in his quadcopter drone in place of the factory pack and it wouldn't even take off.

My Panasonic NCR18650B cells have a discharge rating of 0.5C, which in this case comes out to about 1650mA if the specs are correct. Peanuts compared to what you find in RC helecopters and the like.

It turns out that our asian buddies also sell ready made boards to protect and balance charge series 18650 cells, so i am building a battery pack to provide 5 and 9.mumble volts for a Koss esp900 electrostatic headphone kit and a very small very good DAC, and an old DAP with optical output. Purely because i can. The Koss kits ship with a 9vdc unregulated wall wart and a box that holds 6 C cells. I found that the e/90 amp wasn't happy with the 8.mumble volts of a 2-cell lipo, and 3-cell would probably be too much, so i am using an adjustable buck/boost board and will adjust it to wherever in the 9-11 volt range i figure it sounds best.
 
I built an amp using a 24v to 220v SMPS power inverter here: http://www.diyaudio.com/forums/tubes-valves/213947-just-what-world-needs-another-el84-amp.html

I used one with a 24v input because the DC barrel jacks are only rated for 5A. I took a SWAG at where I thought the high voltage DC was on the circuit board before it was chopped up and fed out, and I guessed right. I ended up with a fairly clean 260v.

My next guitar amp will probably feature one of those $15 ebay DC-DC converters just for fun.