• 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.

Valves glow fierce

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
When I turn on my little valve amp, a couple of the valves instantly light up fiercesomly bright, then dim down to normal, and the amp works as it should.

can any one give any advice as to what to do to stop this?

it started when I moved the volume pot.

the amp is (I think) a New Zealand made Fountain brand (from a radio gram maybe, its just a chassis, with a front pannel)

10watt per channel push pull running EL 86's



looks the same circuit as my old Fountain Gemsonic 10-10 (if anyone is familiar to that circuit)



Thanks alot,



Matt
 
I have one of them kicking around somewhere. PP ECL86 with 2x EF86s IIRC.

If its a white flash at the base of the valve, and its a mullard one, then nothing to worry about. Happens with a lot of 9-pin valves.

Main problem with my fountain amp was the anode (big grey metal bit) of the valves started glowing a dull red after about 30 seconds. Sounded pretty good up till then :)

I thought it might be leaky caps so I changed every single cap in the damn thing, and lots of resistors. However it was some kind of oscillation I think.

Its in the pile somewhere...
 
It's got a real higgledy-piggledy lot of valves in it, there are a number of mullards (an ECC83, 2x EF86 and two of the output valves (ECL86))

but the other two ECL86's are TESA and Telefunken (I think, most of the silk screening has come off the tubes. But the types are printed on the chassis of the amp.


the other thing I'd been told, is that its bad to run a tube amp with no load on it? I was just wondering weather it is alright to run it with a 10:1 voltage divider so I can use it as a pre-amp? or if I can just tap straight on the output from the preamp valves without running a load on the output stage?




it use to be a tuner amp, but I removed all the tuner components, and moved the volume control from next to the tuning dial, to the center of the chassis (it was the stereo/mono switch, now hardwired on stereo), and I've just rememberd that I'd also replaced the two rectifier diodes (the old ones burnt out) but I'm sure it worked fine when I'd done that for a while.


CHeers
 
Photo

this is what its like now.


the volume is in the middle.
 

Attachments

  • fountain valve (1).jpg
    fountain valve (1).jpg
    75.1 KB · Views: 511
DON'T run it without a load on the output transformers, you may get a lightshow that destryos tubes and OPTs.

If you want a preamp, buy a 1960s table radio or reel-to-reel recorder, grab the power transformer. Use a metal box from jaycar and build one, using ECC88 or 6SN7. If I wasnt in th UK I could send you some stuff :p

If you want a power amp, rebuild what you have. Those fountains do actually sound quite good.

I seriously doubt you would have damaged the thing by changing the volume control. As posted already, a bright flash at turn on isn't too big a deal...

Buy a new set of ECL86s, change any out of spec components, and use the thing every day.
 
Ahhhh. Love the sound of rusty iron! ;) Just kidding!

Well, the only thing I can think of is that since you've removed much of the original circuitry (more than just the volume control has moved, I see), the 6 volt tap is now providing much more current to cold heaters. The initial resistance, if memory serves, of cold heaters is very low. So maximum current is drawn/supplied. That is why the flash. But again, not a real problem.

As for running the amp without a load, better to just remove the output transformers and tubes. Or really just the output tubes.

Gabe
 
Righto,

At the moment, all I'm really after is a pre-amp, so when I've got some time over the next day or so, take out the output tubes and then connect my power amp to the preamp outputs on the valve sockets.
and see what that does for me.

I'm using an old Sansui AU-101 that the output stage exploded in, as a preamp at the moment, and it definately leaves something to be desired... my power amp is a dirty old Hafler DH-200 that I aquired from some crappy bar up the road from me, looked like it had consumed quite a bit of beer over its life, it was also growing things in the heatsinks...

the Hafler I think was supposeto want a max input voltage of 1.5 volts or something like that? What would I be looking at getting from the fountain preamp circuitry, enough to get it up to a solid volume?

Just the Sansui, when on max volume isn't very loud, I live in an appartment building, with my old Rotel preamp I was getting noise complaints from 9 floors up, no I'm only getting them from 3 :(


Thanks
 
Well that seems to have worked fine, no big bangs or anything,

and its producing quite a nice sound now (sansui goes into the trash)

just one question now, the volume control, should it be log or linear?

just wondering if ones better than the other or if its just a prefference, I've got a horrid log in it, and it doesnt really do all that much till it gets to about 90% then goes nuts....



I think I'll jam this thing into a rack case or something more usable when I've time in the holidays, I'll let you all know what happens with that.

thanks for your help :)
 
Cool!

Volume pots should always be log. Sounds like you may have a linear one in there?!?!?

You should rock into jaycar with a multimeter and get the best balanced dual one you can- ie when one channel measures x, the other measures x +/- 2% or whatever.

If your aren't using the speaker outputs on the fountain, at least put an 8 ohm 5Watt resistor across each one.
 
At power up a valve heater is pretty much a short circuit so takes lots of current.
As it warms up its resistance rises and so it takes less current.
If it worries you then a constant DC current source would fix the problem.

The problem is worst with a regulated DC supply as the regulator tries to source too much current.

AC heaters from a properly sized transformer self limit.
 
Nigel, do you know a ratio, cold vs. warm on filaments off the top of your head? I'm going to be running a 317(Fully heat-sinked) to drive a single 12AU7 on the front-end of my 12B4 SE. At 1.2A capacity of the 317, I was thinking this regulator would be loafing along with a single 12AU7. I have a split-bobbin 28VDC TX available for this one tube.*** But if the cold surge is too high, I will have to re-design.**


_____________________________________________--Rick.........
 
I don't know the ratio off hand.

I just used a regulated supply with my mixer which has a single 12AU7 and it worked ok.

Dropping voltage from 28 to 6.3 is huge drop and will require a big heat sink and maybe a dropper resistor too.

I think you can use the LM317 in constant current mode, that is probably your best bet.
But again you are likely to need a big heat sink when dropping from 28 volts.
 
No, I'll go 12.6 for the 12AU7..........I guess I could monitor current with an Analog readout & watch the swing. Best guess here I suppose....I will look up 317s again to see if there is a surge rating. They do have a short protection but if I'm not off, it was a power/heat protection(seconds) & not an instant protection.




_______________________________________________________Rick.....
 
The tube filaments may all be wired in series and connected to the mains directly. That was done in some older amps. This will cause the driver tubes (the smaller tubes) to light up like lightbulbs until the power tube filaments heat up.

That's obviously not up to current electrical code, but it was common back in the day.

If your plan is to restore the amp, I suggest replacing the hand grenade (supply capacitor without safety release valve). When that blows, it launches shrapnel like a hand grenade. Seriously! If you want to preserve the original look, you may be able to disassemble the cap, keep the aluminum can, and fit a modern cap inside.

~Tom
 
Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.