Cathode voltage to power 12V fan

[RockinDoc]

Having some heat problems (especially on half power) with my 36W EF86. Increased half power cathode resistor values a bit (thanks Holmes!), but mostly have been running her on full power.
Hooked up a 12 volt cooling fan off the heaters, rectified, but with a voltage doubler, I'm only getting about 5 DC volts (and about 95mA current).
This PT (from Nik at Ceriatone) has two 3.15 VAC heater leads, center-tapped; I can only assume that I'm getting just about 3VAC off the heaters, with my rectifier/multiplier circuit altering it to 5VDC.

Two questions: I know my EL84's require 6 VAC, and the preamp tubes 12 VAC for their respective heaters. Can someone explain to this novice why I'm just getting (and measuring) roughly 3 volts off these heaters? (relative to chassis ground, that is).

Second: Looking around the amp for more voltage, I got the bright idea that I could run another fan off the cathode line, which runs about 10 to 11 VDC. This way, I'd avoid the need for a rectifier, and I'd have a bit closer to my 12 VDC for the fan. I get the part about lower voltage running the fan slower and quieter, but I run pretty hot, plus with this set-up I won't have an extra AC line running around the chassis.

I have this configuration set up, but have not yet powered it up to try it out; figured I'd check with you all to see if I'm setting myself up for an explosion of sorts, or some other catastrophe. Again, novice status here.

Thanks for any help, I've used the search and I came up dry.

[morcey2]

First, running the fan off the cathode: Just say no. You'll burn up your output tubes and possibly your OT before you pull enough current to turn the fan.

Second, the heater voltage: The preamp tubes can run off either 12V or 6V. In 18-watts, they run of 6V, just like the el84s. When you measure each heater leg to the chassis ground, you'll get about 3V so that's correct. If you measure across the legs, you'll get just over 6V.

If you're going to use a fan, I would suggest just rectifying the 6V and running it off of that.

Matt

[1950]

Dont be worrying about your tube amp getting too hot, I did a test with a blast of cold air directed onto my "bared" tubes, the sound got awfull thin as the tubes cooled, hot and hotter valves get to sounding better and better. Trust me, the tubes need to be hot, I mean, why d'ya think ther is heater elements in them.
Just make sure you take along a fire extinguisher!

[sub]

+ 1 Don't use the cathode voltage.
first reason: what Morcey2 said. (anyway i didn't known that)
secound reason: when you play with the amp the cathode voltage higher, about 16-18V.

[ny_racer_xxx]

What about tapping off the B+ some place with a 12v regulator?

CR

[PeterS]

What about tapping off the B+ some place with a 12v regulator?

Probably not enough current to run the fan.


If you rectify the heater voltage with a diode bridge, and DON'T ground the output of it, (do add a filter cap) you should have about 8 volts, which is enough to run a 12 volt fan.

[RockinDoc]

First, running the fan off the cathode: Just say no. You'll burn up your output tubes and possibly your OT before you pull enough current to turn the fan.

Second, the heater voltage: The preamp tubes can run off either 12V or 6V. In 18-watts, they run of 6V, just like the el84s. When you measure each heater leg to the chassis ground, you'll get about 3V so that's correct. If you measure across the legs, you'll get just over 6V.

If you're going to use a fan, I would suggest just rectifying the 6V and running it off of that.

Matt

Thanks,
That's what I was afraid of, and why I never switched it on (thankfully).
As for the 6V, I'm afraid I may have wired it wrong; I took the wires off the pilot light instead of one of the tube socket lugs. The pilot also runs off the same 6V line, and was a more convenient location for running up out of the chassis.
I took those two lines and wired them into my rigged circuit, which is a two diode, two filter cap rectifier/multiplier, as per several schematics I found. It gives me enough current, about 100 mA, to start up and turn the fan, which I guess is actually in line with the lower voltage than expected that I got, per Ohm's Law.
Actually, i suppose maybe that's enough to blow a little air over the tubes/trannies.

To answer another comment, I built this off one of Ceriatone's kits; first build (obviously). Nik uses the same PT he uses for his Trainwreck Express clone, which runs 2 EL34's at 50 watts, I believe. Should that be enough to run my little fan?

BTW, I had considered the outboard fan, but I wanted to learn something about SS rectification, etc, so I chose this route.

Thanks to all.

[RockinDoc]

Dont be worrying about your tube amp getting too hot, I did a test with a blast of cold air directed onto my "bared" tubes, the sound got awfull thin as the tubes cooled, hot and hotter valves get to sounding better and better. Trust me, the tubes need to be hot, I mean, why d'ya think ther is heater elements in them.
Just make sure you take along a fire extinguisher!

Yeah, I really like the tone I get, but I thought that a little moving air just helped cool the glass of the tube, which wouldn't really affect the tone. The heat radiated off the tube is carried away, by convection (?), and this limits the ability of the hotter glass from radiating heat BACK to the plates and causing redplating etc.
Is that not the case? I kept coming across this fan idea in other places, to allow running the tubes hot without cutting lifespan too much, seeing red plates, etc.

[RockinDoc]

What about tapping off the B+ some place with a 12v regulator?

Probably not enough current to run the fan.


If you rectify the heater voltage with a diode bridge, and DON'T ground the output of it, (do add a filter cap) you should have about 8 volts, which is enough to run a 12 volt fan.

Thanks Peter,

Could you elaborate? The two 3.15V heater lines are grounded at the PT via center tap (is that right?)
So my two wires come off the heater lugs or 6V pilot or whatever, run into my circuit as + and - and then connect to the fan wires. How can I not ground the output?
Dense, I know, but bear with me. If I could get 8V, that would be nice, because the fan runs sweetly off a 9V battery.

[RockinDoc]

You're having so much heat trouble running a quad or duet of EL84's at their normal output that you need a fan? Something sounds truly hinky with that amp. I have an early 70's Traynor YBA-1A Mark II that shipped with a built-in 120VAC fan but that cookie has a pair of EL34's cranking out 90+ watts so it makes sense. An EL84-based 18 or 36 watt amp shouldn't be nearly that hot.

KennyO

I dunnno. It's quad of EL84's. I built according to Nik's specs, with which my full-power plate voltage is about 340V, screen about 325V. Well above tube specs I know. Don't really use the half power as I prefer the full power tone (and half power voltages are even higher).
I asked Nik about this and his voltages are high too, but more like 320-330V plates.
I have read of others running EL's at much higher voltages then me and doing fine; I had a couple of red plate episodes on half power early on, so I'm a little gun-shy about excess heat. I've heard everything from using Zeners to drop my B+, to using 7189's or E84L's, to upping the cathode resistor values a bit (which I did).
I guess if I was running them at 300V it wouldn't be a concern, but it seems this rig is designed for higher than spec on the tubes.
BTW, I've also heard that I might increase the screen resistors to 1K ohms or so, at least for extended life. They are currently 82 ohms by the Ceriatone design.

Thanks to all.

[1950]

Hey Rockindoc. Its really good seing all the electrical current calculations your making dude, but if your amps red plating, it may need some attention but..... If you make some carefully thought out holes in you amps box, like in one sides at the lower part, then a few at the top on the other side, maybe some nice plastic vents carefully fitted in the holes, you will gain a fenonima called convection cooling (I think), costs very little and is totally quiet and uses no currenmt from anywhere inside, also no speaker buzz either.
If your amp has a mesh top and your getting red plating, then check your cathode voltage. Mine did that too, I tried a fan, but it really is irritating when your last note fades and the fans there buzzing away.
Good luck with the idea though.

[PeterS]

Could you elaborate? The two 3.15V heater lines are grounded at the PT via center tap (is that right?)
So my two wires come off the heater lugs or 6V pilot or whatever, run into my circuit as + and - and then connect to the fan wires. How can I not ground the output?

What I mean is don't ground it anywhere else besides where it is already grounded (i.e. at the centertap). In other words the negative connection to the fan is NOT at ground.

If you have redplating the solution is a higher-value cathode resistor.

For tube life you might want to increase the screen resistors. You lose some output power but not enough to worry about.

[ny_racer_xxx]

Here's a stupid idea, why not just find a fan that runs on 120 volt?
I think the moving air, although not needed, defenitly would not hurt and may help the tubes last a little longer...

CR

[1950]

Water cooled??

[PeterS]

Another approach I've used is a 12 VDC fan powered by a 9-volt wall-wart style AC adaptor....

[RockinDoc]

Here's a stupid idea, why not just find a fan that runs on 120 volt?
I think the moving air, although not needed, defenitly would not hurt and may help the tubes last a little longer...

CR

Well, I thought of that, and I have seen where others have hooked in a 240V fan to their mains of 120, in order to have it running slower and quieter. Of course, then it's running all the time you're plugged in (unless you put in a switch, which is more to do). But the same folks say 120 may not be quite enough for a240 fan. Aaarrgghh!
It started as a cool (!) idea, and it seems to have turned in a time-consuming fiasco, but I have learned something.

Thanks everyone for the help. This seems to have been a common idea when I searched the archives over at Ampage, one that was easy and had no real downside. I guess I'll see.

BTW, I DID increase my cathode resistor values, and haven't red-plated since, although I've not been using the half-power mode, which runs hotter.

[RockinDoc]

Another approach I've used is a 12 VDC fan powered by a 9-volt wall-wart style AC adaptor....

Yep, along those lines I had just now mounted a little bracket for a long life rechargeable 9V battery up under the inside of the head cab, it's easy access...time will tell if I get much time out of a battery charge. Runs pretty fast on the battery, must be getting a little more current than my rectified 8VDC off the heaters!

Thanks again

[RockinDoc]

Water cooled??

Like those old-timey machine guns?

Yeah, thought of that too, but seems I read SOMEWHERE that H2O wasn't the best insulator

Think I'll go play the guitar for a change... imagine that!

[Phil_S]

Here's a stupid idea, why not just find a fan that runs on 120 volt?
I think the moving air, although not needed, defenitly would not hurt and may help the tubes last a little longer...
CR

IMO, you are on the right track. Actually, I think the thing to do is find a 240V fan and hook it to 120V supply so it runs at half-speed. This will make it reasonably quiet.