EL84 current draw help

[lion]

I need to work out the specs for a suitable replacement PT for a vintage echo unit. It's just three 12AX7's and one EL84, but the later is working in a bias oscillator circuit which I'm not sure about.

Can anyone help me calculate the HT current draw of the EL84 in the circuit shown. TIA.

lion

[soundguruman]

I need to work out the specs for a suitable replacement PT for a vintage echo unit. It's just three 12AX7's and one EL84, but the later is working in a bias oscillator circuit which I'm not sure about.

Can anyone help me calculate the HT current draw of the EL84 in the circuit shown. TIA.

lion

You need to give all the details, because there are not enough to calculate any specifications.
You have the voltage, but you need a second fact...
"R" and "X" may be the record head, but what head??? exactly???
Vintage echo unit? What unit??? From What year?

The people here are not members of the psychic friends network.
In Ohm's Law, you need at least two measurements to derive a conclusion from them...

[nickb]

A couple of suggestions:

1) If you know the B+ you can hook the whole thing up using an external power supply o some kind e.g (transformer + variac and set for 240V DC) and measure the current. Also use an external heater supply. You can get the heater current from for data sheet.

2) Just looking at the transformer and comparing in size to data sheet for current production one will get use pretty close to a VA rating.

[Jazz P Bass]

They are hiring, if anyone is interested.
Psychic Jobs ? HiringPsychics.com ? Recruiting the best psychics in the industry

[nickb]

They are hiring, if anyone is interested.
Psychic Jobs ? HiringPsychics.com ? Recruiting the best psychics in the industry

Funny ... I knew that before you said it! Amazing!
Now, is there any out there where the letter "a" means anything to them? Aha! I knew it!

[olddawg]

I would be willing to bet that a PT from an Epiphone Valve Jr would work just fine. There are always some on eBay for $20 because people upgrade to MM transformers for some reason.

[lion]

Thanks for the replies and suggestions.

The unit is a ´63/64 Meazzi Factotum. Circuit similar to the Meazzi PA306 schem attached.

"R" and "X" are the rec and erase heads. Rec head specs are 650 ohms/650 mH - don't know about the erase head.

lion

[lion]

Another schem - probably clearer and easier to read

lion

[lion]

EL84 current draw help - UPDATE

Took the advise of setting up an external supply and measured/calculated the HT current draw.
Turned out at a mere 11 mA.

Found a nice vintage PT specified at 275V/30mA and 6.3V/2A. The HT is a bit on the high side, but it should be possible to drop it I think.
The PT has an unusual 260V prim tap - if I use that with the 220V mains we have here it's down at 85% = 234V x 1.414 = 330V. The highest B+ tap in the unit is 280V. To drop the rest I'll use a zener and/or adjust the dropping resistor(s). I'm no expert on this but I think it would work - or am I missing something?

lion

[pdf64]

'The PT has an unusual 260V prim tap - if I use that with the 220V mains we have here it's down at 85%'
Heaters? Wouldn't want them to drop much below 6V.
Pete

[lion]

Heaters? Wouldn't want them to drop much below 6V.
Pete

That was my belief too - but just recently stumbled accross this on the Valve Wizard site: The Valve Wizard

"Voltage considerations: The heater voltage specified in the data sheet is the optimum value (usually specified as +/- 10%). Running them at higher voltages will considerably reduce valve lifespan and should be avoided. Running them at lower voltages, within reason, will increase their lifespan and reduce hum but also reduce their emission, although the grid curves and general performance remain much the same;- only the saturation current is reduced. Running heaters under-voltage is therefore perfectly acceptable, and provided the voltage isn't too low there will usually be no noticeable difference in sound. Normal heaters rated at 6.3V can be run quite happily between 5V and 6.9V, maybe even lower, but not higher. The exception to this rule is rectifier valves, which should not be run below 10% of their rated voltage since they usually operate very close to saturation."

"....increase their lifespan and reduce hum but also reduce their emission, although the grid curves and general performance remain much the same;- only the saturation current is reduced...."
What exactly does that mean???

lion

[eschertron]

That was my belief too - but just recently stumbled accross this on the Valve Wizard site: The Valve Wizard

"Voltage considerations: The heater voltage specified in the data sheet is the optimum value (usually specified as +/- 10%). Running them at higher voltages will considerably reduce valve lifespan and should be avoided. Running them at lower voltages, within reason, will increase their lifespan and reduce hum but also reduce their emission, although the grid curves and general performance remain much the same;- only the saturation current is reduced. Running heaters under-voltage is therefore perfectly acceptable, and provided the voltage isn't too low there will usually be no noticeable difference in sound. Normal heaters rated at 6.3V can be run quite happily between 5V and 6.9V, maybe even lower, but not higher. The exception to this rule is rectifier valves, which should not be run below 10% of their rated voltage since they usually operate very close to saturation."

"....increase their lifespan and reduce hum but also reduce their emission, although the grid curves and general performance remain much the same;- only the saturation current is reduced...."
What exactly does that mean???

lion

I was looking at that myself, just the other day while trying to determine just how Hammond rates the windings on their transformers. From the above, it seems that Merlin is asserting that heater voltage is OK +10% / -20%, which allows me to sag the heater voltage quite a bit.

As to the saturation current, Merlin states in his preamp book that what guitarists refer to as 'tube saturation' is not even close to the physical phenomenon of the tube in saturation. I believe he's referring to the absolute maximum limit of electrons that can travel from the cathode based on the temperature. Something, that given the limits to heater voltage that he's prescribed, shouldn't bother the amp designer.

[lion]

..As to the saturation current, Merlin states in his preamp book that what guitarists refer to as 'tube saturation' is not even close to the physical phenomenon of the tube in saturation....

Exactly waht I was wondering about - thanks

The idea of reducing hum induced by the heater AC by lovering the voltage appeals to me, as the vintage echo I'm working on is very crammed and following the standard rules for heater wiring is near dead impossible. I'm gonna give it a try.

lion

[soundguruman]

Exactly waht I was wondering about - thanks

The idea of reducing hum induced by the heater AC by lovering the voltage appeals to me, as the vintage echo I'm working on is very crammed and following the standard rules for heater wiring is near dead impossible. I'm gonna give it a try.

lion

Reducing the heater voltage shortens the life of the tube quite a bit. That is, anything more than 5% below normal, usually.
AC hum comes from the AC heaters in the preamp, and DC heaters are a better approach for the first two tubes of the preamp...
Follow what the hi fi guys did. Look at McIntosh tube preamp schematics for a DC heater supply circuit.

Reducing the current of an EL34 is probably best done by converting it to Triode operation.
This will knock a 50 watt Marshall down to about 17 watts. Or a 100 watt Hiwatt down to 36 watts...

But it's still plenty loud for almost anything, except stadiums. And it sounds the same or better.

You are just disconnecting the screen grid resistors from the usual power supply point,
and re-connecting them (each half) to the corresponding half of the output transformer.
This lowers the HT current draw, reduces the volume to "usable." The bias remains essentially unchanged, and the preamp voltages remain about the same.
However, you can now crank the amp up without blasting too loud, and get a nicer sound, from output tube overdrive.

As I have said before, output tube overdrive is the real thing.
Preamp distortion and effects pedals are just an imitation of the real sound.
So, running in triode makes this easier, without the profound hearing loss.

I am a technician, and a very experienced electric guitar player. Based on those tools, I offer you these tips above...which I have used in my own shop.
What did customers think? They loved it.
Less noise and heat, more crunch.

[g1]

The Valve Wizard:
"Running them at lower voltages, within reason, will increase their lifespan and reduce hum... Running heaters under-voltage is therefore perfectly acceptable, and provided the voltage isn't too low there will usually be no noticeable difference in sound. Normal heaters rated at 6.3V can be run quite happily between 5V and 6.9V, maybe even lower, but not higher. The exception to this rule is rectifier valves, which should not be run below 10% of their rated voltage since they usually operate very close to saturation."

Reducing the heater voltage shortens the life of the tube quite a bit. That is, anything more than 5% below normal, usually.

Hmmm, now I'm confused, who should I believe? .