5E3 no cathode voltage mystery - PLEASE HELP!

DJH83004

I think you are pretty much on the limit with the 291BEX, especially considering the the HT supply is actually rated at 138mA. Stick with 6V6s, which is after all what it was designed for   

ecc83

DJH83004 said:

I think you are pretty much on the limit with the 291BEX, especially considering the the HT supply is actually rated at 138mA. Stick with 6V6s, which is after all what it was designed for   

+1 and don't forget, that poor traff has also got deliver another 5.6W of heater power for L6s.

Dave.

Modulus_Amps

It is the heater current that is the main issue really not the HT current, you can always re-bias if needed to control the heater HT current.
 if you leave the amp stock and plug in 6L6's it will likely be fine, maybe run a bit hotter, the 291 series has better lamination's than the 290 so should last the pace, if it does run really hot you can always get a small 6v transformer to run the heaters off or install a silent computer fan to help keep it cool.

There are lots of deluxe's with smaller PT's running 6L6's, some very famous brands too.

I prefer the deluxe with 6L6's, just has more vibe

knuckleberryfinn

The transformer's rated for 3 amps 6.3VAC heater current, so surely 6L6s are fine?  0.9 + 0.9 + 0.3 + 0.3 = 2.4A.  Am I missing something here?

I just threw a load of computer fans away - maybe I'll fish one out of the bin.  What's the best way to power it?  Rectify DC off the 50VAC bias supply (which I'm not using) and drop to 12VDC with a voltage divider?

ecc83

knuckleberryfinn said:

The transformer's rated for 3 amps 6.3VAC heater current, so surely 6L6s are fine?  0.9 + 0.9 + 0.3 + 0.3 = 2.4A.  Am I missing something here?

I just threw a load of computer fans away - maybe I'll fish one out of the bin.  What's the best way to power it?  Rectify DC off the 50VAC bias supply (which I'm not using) and drop to 12VDC with a voltage divider?

What you are missing is if you are exceeding the TOTAL power rating of the transformer. If you are pulling more HT current then you really don't want to impose an extra load on the heater winding.  I don't know the ratings of that traff (can't be arsed to find out!) but I have a personal horror of running things AT their limit, leave alone above it but it does see a 'feature' of some guitar amplifier designs!  Note, not only will an overloaded transformer get bloody hot it might also be noisy and chuck out a nasty hum field.

Like the separate filament traff idea.. a lot!  Nice lil' 20 VA toroid should not be much dosh.

IF the traff will stay in its rating but you just want to cool it a bit I would run the fan off the heater winding.  The extra power will be insignificant to the 3A winding but might be too much for the bias winding. Try the fan with a 1A bridge rectifier and 1000mfd cap, should give about 6-7V. If fan is very slow try a '2 diode, 2 capacitor' voltage doubler which should sort it easily.

NB. The fan supply should 'float' since I am guessing the winding will be ctr tapped to earth?

Dave.

knuckleberryfinn

Hmmm, maybe I could do the filament transformer idea then.  Would this one be okay? 

https://airlinktransformers.com/product/chassis-mounting-toroidal-transformer-standard-range-cm0030206

Rated at 230V input for 6+6 output, so with 240+ volts mains, that would bring it up to around 6.3 wouldn't it?  I would then use the CT and one end of the secondary winding for the filament supply, and artificial centre tap with 2 x 120ohm resistors, yes?  Or does only using one half of the winding affect the amount of current the tranny can supply?  

DJH83004

Don't forget to take into account the 5v rectifier heater, I think the 5U4GB you were thinking of using is around 3amps, the 5Y3 around 2amps. Hammond spec the 291CEX for the Vibrolux Reverb (2x6L6GCs @35watts), total wattage 250mA, HT 207mA   

knuckleberryfinn

Sod it, I'll just make a plug-in SS rectifier for use with the 6L6s and not bother with the valve.  I had a 5U4GB hanging around, but if it's going to cause trouble I'll leave it in the cupboard.

RiftAmps

knuckleberryfinn said:

Sod it, I'll just make a plug-in SS rectifier for use with the 6L6s and not bother with the valve.  I had a 5U4GB hanging around, but if it's going to cause trouble I'll leave it in the cupboard.

Just buy a 5V4...

knuckleberryfinn

@RiftAmps I am too poor 

ecc83

knuckleberryfinn said:

Hmmm, maybe I could do the filament transformer idea then.  Would this one be okay? 

https://airlinktransformers.com/product/chassis-mounting-toroidal-transformer-standard-range-cm0030206

Rated at 230V input for 6+6 output, so with 240+ volts mains, that would bring it up to around 6.3 wouldn't it?  I would then use the CT and one end of the secondary winding for the filament supply, and artificial centre tap with 2 x 120ohm resistors, yes?  Or does only using one half of the winding affect the amount of current the tranny can supply?  

That transformer can supply 6 volts at a TOTAL of 2.5A so ideally you would just run the OP valve heaters from it (1.8A) and keep the CTapped original traff for the pre amp valves.  In fact 'I' would wire it as 12V, 6V to chassis and run the OP valve heaters in series. Blackstar have done this for all their amps except the Artisans, never a problem and it keeps I sqrd R losses down.

Dave.