Having a nightmare with simple txf wiring!!

[Spencerleehorton]

How about this?

 

[Whoops]

That 270 ohm will smoke very fast  ;D
More like 2x100 kohms tied into the centre point of the caps

Ian Suggested a 270K resistor and not 270R

 

[Spencerleehorton]

But 2 x 100k would be ok right? as I have these on hand

 

[ruffrecords]

Those values should be fine for testing. They are not critical. 270K was chosen because it gives about a 1mA or so load - enough to discharge the cap safely and quickly once power is removed but not so much as to be a heavy load. 100uF was pretty arbitrary but a smaller value than I use on most HT supplies.

Cheers

Ian

 

[Spencerleehorton]

Pretty sure I have a box of some big caps 400v or more somewhere in the shed!!!
Will have a look in a bit.
Have got some 100k 2w and plenty of 1n4007 or some Russian military diodes.

 

[Spencerleehorton]

I found a 470uf 450v cap with 4 terminals, perfect for mounting the diodes and resistor on.
I mistakenly put a 470R 2w resistor on it thinking it was 470k!! It was fine on the 2 x 18v toroidal which measured 52vdc which came out spot on as the ac measured 37vac.
On the 270vac txf the 470R whent out in a flash so I quickly flipped it off.
Looked at the colours and tested another one, using a different MM and nothing reading it correctly!! Shouldn't really do these things when I'm working night shifts!!
Anyways put in a 100k 2w as that's the highest I have spare and flicked it carefully on, expecting the worst but it was fine, sitting a solid 360vdc.
Which is perfect if I use it on this champ.
Only thing is I've ordered a champ txf and it's probably being delivered this week!!!
Still I've got the testing done on a practical level now, just need to learn circuit total current and then I can learn again how to get correct load resistor.
Loads of unhelpful stuff out there which doesnt make sense to me, so I'm all ears if anyone can explain simply these two equations please.

Below is a photo before the 470R died!!

 

[Whoops]

Loads of unhelpful stuff out there which doesnt make sense to me, so I'm all ears if anyone can explain simply these two equations please.

You should register and post on the Hoffman EL34 forum, it's a great forum dedicated just to tube amps, a lot of helpful and knowledgeable people there also, PRR is over there:

https://el34world.com/Forum/index.php

 

[Spencerleehorton]

Ok great stuff I have registered and await approval.

 

[RuudNL]

For smoothing capacitors I usually use 1000 mfd/Amp.
In general this works fine for me.
But things can be different if you want an extremely low ripple.

 

[Spencerleehorton]

Adam,
Thanks for your wise words, dont worry I am being very careful even though yes I'm in totally out of my depth.
I am looking at my safety setup and I'm going to build the lightbulb tester for current and move forward very slowly and carefully with these projects.
I'm not going to be connecting up anymore power until the chassis are made and will go through it all in stages.
I did have a MM connected the whole time so could see the voltage draining and didnt go anywhere near it.
I drained it down to 5v using the diode setting when I blew the 470R as I've had this happen a few times and use this.

Thanks everyone for all your help so far, I'm trying to read up and write down as much as I can so not to make anymore mistakes.

 

[Spencerleehorton]

My next question would be, could I use a flood light for the bulb current limiter?

Pretty sure it's a 400watt bulb.

 

[Spencerleehorton]

I'm 240v here in uk so would 400 watt be ok?

 

[Spencerleehorton]

So two setups should be good, for low powered amps 5 - 18 watt = 100 watt bulb, for higher power 50 watt and 100 watt use the 400 watt bulb?

 

[nedonnelly]

I'm assuming you have already seen this Spencer, but just in case you haven't here is Uncle Doug explaining his light bulb limiter setup.

https://www.youtube.com/watch?v=wRFRwOnLsZI

 

[Spencerleehorton]

Yes that's best video which explains it.

 

[abbey road d enfer]

Loads of unhelpful stuff out there which doesnt make sense to me, so I'm all ears if anyone can explain simply these two equations please.

Please don't put the blame on those who tried to help you and took the opportunity to try to inculcate some theory.

 

[Whoops]

I built my current limiter 12 years ago, I use a 100Watts 230V incandescent bulb.

100Watts worked pretty well every time to test amplifiers being it 15 wats amp or 150watts amp, I've only used this current Limiter and it works great. I think you are overthinking this in wanting to make different Bulb Current limiters, most people only have 1

 

[Spencerleehorton]

im certainly not having a pop about anyone on here or the valuble information they have shared, its the internet im meaning, and everyones google search is different.

Even though im shakey (to put it mildly) on the theory of it all, i do have a fairly good grasp of the basics and have had many sucessful builds of valve stuff (fender champ, LA-2A's, many other tube microphones), its been a while since i have done any true valve stuff and i have been preoccupied with various house stuff, so have forgotton a fair bit.

I just need to brush up on it all again, when you get a bit older things tend to push out of your brain when you put new stuff in!!

 

[Spencerleehorton]

ok whoops, 100watt it is then, just needed someone to be specific, which you did, thanks bud.

 

[abbey road d enfer]

I think you are overthinking this in wanting to make different Bulb Current limiters, most people only have 1

It depends very much on context.
If you have to work on a tube amp that has sat for years unused, you want to start slow, just to make sure there is no hard short, like a shorted tube or capacitor, then you can increase the bulb power to make sure the idle voltages/currents are correct and stable. Only after that you can use a bulb that's dimensioned to the (almost) full operation.
It used to be easier when there was a larger choice of incandescent bulbs.
Actually, light bulbs are far from being adequate, because of their own high intrinsic non-linearity; during the turn-on period, they don't provide much protection. A cold bulb measures  less than 1/10th of its nominal resistance, which means it can let pass about 10 times its nominal power during the first 10-20ms, which is enough to make a lot of damage.
Fortunately, most of this surge is absorbed by the xfmr core for its initial magnetization, but still, until the filament is hot enough, there is not so much protection.
A much better protection would be achieved by using power resistors, which value does not change so much, as long as the nominal dissipation is not exceeded. The only problem is the cost and the bulkiness.
The big advantages of light bulbs are:
Cost and availabilty.
Ease of implementation, with all the hardware commonly available.
Signalling effect.
When incandescent bulbs will be effectively banned, we'll need to find a clever solution.

PS: sorry for TMI, but I never want to loose an opportunity to perorate (and hopefully enlighten).