Tube Amp Archive

240VAC where both lines are hot. I think this is the case in England Nope. UK domestic mains supply is single phase 230V and neutral is tied to earth. The UK (and EU) regulations are that if a device has a fixed mains cable then both live and neutral must be switched, if it has a removable (IEC) ma...

So the resistor is literally from the 16 ohm tap on the switch to ground? how does that work when you're using the 16 ohm output into a speaker? :oops: The resistor is in parallel with the speaker so you will have a total load of 15 ohms, the resistor being a much higher value than the speaker will...

When a device is powered from a wall wart or external low voltage power supply the device itself is not regarded as a class II appliance, it is a low voltage class III appliance that does not require any of the safety features that apply to class I or class II appliances. The power supply itself of ...

sluckey wrote: ↑Mon Jun 15, 2020 8:09 pm Don't measure filament voltage from chassis to socket pins. Maybe a pic...

Ok, see what you were referring to now, it was your reference to "filament wires" that confused me. To me that refers to the tungsten filament inside the valve, not the hook up wiring.

Yes, and it also makes it clear that it is perfectly feasible to design a class II device that could be used between a guitar and and an amplifier that would meet all the requirements whilst still maintaining a direct ground connection between the guitar and amp without needing any decoupling or tra...

A class II device cannot have any exposed conductor, grounded or otherwise. Guitarist are grounded via the strings-bridge-sheilded audio interconnect. Nope. Plenty of class II devices around with metal cases. The IEC definition of class II is here (see note 3 about continuity of protective circuits...

What you said was "You need to measure voltage ***BETWEEN*** filament pins, ie, put one probe on pins 4/5 and the other probe on pin 9. You will measure 6.3v if both filament wires are OK"

You will also measure 6.3v if either or both filaments are not OK, it tells you nothing about the filaments.

That behavior can be caused by a bad socket. But it's more likely that one of the filament wires is broken or not making contact to the tube pin. You will still be able to measure 3.2v on each pin even if one of the filament wires is broken or completely missing. You need to measure voltage ***BETW...

if you run quarter cables to connect your all your effects and amplifier, you cannot have double insulation. By nature, you are electrically coupled via the grounding conductor in your instrument cables. Right? You would have to have some physical decoupling via audio transformer, RF, or ADC to opt...

Unless either of those units has a serious fault (if they have they should blow fuses) and they both have mains safety grounds then there is no potential difference, both chassis will be at 0V,

if you take a look at the heater supply on the schematic there's both a negative and a positive voltage regulator (7812 and 7912) with the 0v point being common. half of the preamp is off the positive side, half of the preamp is off the negative side I think that was fairly obvious from both the OP...

If the amp ever worked before then they can not possibly be wire the way you discribe because then that regulated 12.8 volt D.C. Supply would be partially shorted to ground thru the heaters of the pairs of tubes you say are wired differently! Not sure what you mean. The heaters are a resistive elem...

Have to admit to cheating a bit and using an online bias calculator which I guess was using 5% for screen current. 5mA or so is probably closer to the mark for a SE EL84 circuit like this so yes, probably OK.

Assuming those voltages are with one EL84 then they indicate a plate current of 51mA and power dissipation of 13 watts. A little on the high side but not out of the ordinary, many amps routinely run EL84's above their recommended ratings but if you wanted to bias it a bit cooler using a 150 ohm cath...

You need to know how much current the light uses then use ohms law to calculate the resistance (you should also calculate the power dissipation to ensure you use a suitable wattage resistor)