Any reason not to use bias tap to elevate heaters?

[PositiveNegativeMan]

So I've put together a little amp with a 2 EL84 power section, and now I wan't to make another.

The first one was cathode biased EL84s, which was easy but I'm going to have a crack at using fixed bias on the next one. Mainly because the PT has bias tags on it and it's a shame not to use them.
The PT has a bias supply, which will have 70 available volts (if I use a bridge to rectify the supply).
I want to elevate the heaters because of issues with cathode follower and heater - cathode voltage. When I did this before I used a voltage divider off the main HT. I connected that to two resistors on my heater supply (no centre tap on the transformer).
What I'm wondering is... is there any reason not to use the bias supply for this? I'm aiming to have a bias on the EL84's of something like -10v. So for example if I set the bias at -10v I've got +60v which at the moment is unused. Is there any reason not to use that to elevate the heaters?

[tubeswell]

What I'm wondering is... is there any reason not to use the bias supply for this? I'm aiming to have a bias on the EL84's of something like -10v. So for example if I set the bias at -10v I've got +60v which at the moment is unused. Is there any reason not to use that to elevate the heaters?

I recall that there was an amp discussed here a while ago that had heater 'elevation' from a negative DC voltage and it apparently worked (despite some lively debate that it oughn't ta - you have to watch that you don't exceed the tubes' heater-to-cathode voltages, and heater elevation is usually thought as having positive voltage).

But why not? If it has worked on some other amp? I would think it would be better to run a separate supply resistor and voltage divider, that has a sufficient sized coupling cap and tap into that.

But what is wrong with taking if from the HT? Why can't you do that?

[Joey Voltage]

So I've put together a little amp with a 2 EL84 power section, and now I wan't to make another.

The first one was cathode biased EL84s, which was easy but I'm going to have a crack at using fixed bias on the next one. Mainly because the PT has bias tags on it and it's a shame not to use them.
The PT has a bias supply, which will have 70 available volts (if I use a bridge to rectify the supply).
I want to elevate the heaters because of issues with cathode follower and heater - cathode voltage. When I did this before I used a voltage divider off the main HT. I connected that to two resistors on my heater supply (no centre tap on the transformer).
What I'm wondering is... is there any reason not to use the bias supply for this? I'm aiming to have a bias on the EL84's of something like -10v. So for example if I set the bias at -10v I've got +60v which at the moment is unused. Is there any reason not to use that to elevate the heaters?

you can use it, just filter it well. Elevation works *better* the cleaner the reference voltage is.

[Tom Phillips]

Hi ±,

Just my 2 ¢ here,

IMHO I would keep the bias supply dedicated to the bias circuit.

… When I did this before I used a voltage divider off the main HT. …

That seems like a tried & true method and it won’t really cost you any more than trying to use the bias supply.

…for example if I set the bias at -10v I've got +60v which at the moment is unused. Is there any reason not to use that to elevate the heaters?

I must admit that I don’t understand that statement. I don’t see how you plan create a -10V and a +60 volt supply from one 70V (after bridge rectification) secondary.

Regards,
Tom

[Diablo]

I've used the preamp filter cap positive terminal to source the voltage and then to a voltage divider to elevate the heaters. It was handy, and I figured it already had a little better filtering to start.

[Joey Voltage]

Hi ±,

Just my 2 ¢ here,

IMHO I would keep the bias supply dedicated to the bias circuit.


That seems like a tried & true method and it won’t really cost you any more than trying to use the bias supply.


I must admit that I don’t understand that statement. I don’t see how you plan create a -10V and a +60 volt supply from one 70V (after bridge rectification) secondary.

Regards,
Tom

He's not using the bias supply tap for anything, his build is cathode biased. he wants to rectify, and use the currently unused buas tap for the + DC reference voltage, which is fine, so long as it is well filtered.

[Tom Phillips]

He's not using the bias supply tap for anything, his build is cathode biased. he wants to rectify, and use the currently unused buas tap for the + DC reference voltage, which is fine, so long as it is well filtered.

It doesn't seem so.
His first post said "...The first one was cathode biased EL84s, which was easy but I'm going to have a crack at using fixed bias on the next one...."
Tom

[Joey Voltage]

It doesn't seem so.
His first post said "...The first one was cathode biased EL84s, which was easy but I'm going to have a crack at using fixed bias on the next one...."
Tom

alright I re-read it, and I agree it is a little cryptic. Now if he is just tapping off of the B+ line to generate the -supply, and has the bias tap free, than he can use it, but yes as mentioned It is easier to just to tap off of the B+, at a place where the ripple is already pretty smoothed out. less components, easier to set up. now if he is talking about placing an additional load on the already used bias tap, than yeah, I'm with you guys, there is really nothing to be gained by doing it this way.

[PositiveNegativeMan]

I was thinking of something like this.

Sorry I've not put in exact values, but I'm sure you get the idea.

Neither the Bias nor "reference" for elevating the heaters actually need to draw much in the way of current, and this does seem a simple way of doing it...

Anway, as I said, I have used a divider off the HT, I guess I just wanted to try something a bit different to keep it from being "samey".

[Joey Voltage]

to be honest you can just as easily reference the heaters to the - voltage without all the extra jipity-joop, and it should be fine

[ThermionicScott]

to be honest you can just as easily reference the heaters to the - voltage without all the extra jipity-joop, and it should be fine

Not a good idea when a cathode follower is involved, IMO.

[Joey Voltage]

Not a good idea when a cathode follower is involved, IMO.

Oh Damnit!, I missed that part....... at least somebody is on the ball.

[Tom Phillips]

That circuit you showed will not produce +60V and -10V. The resistors are just separate loads on each side referenced to ground. Not voltage dividers.
I can see what you're thinking but the circuit doesn't work like that.

[Joey Voltage]

I honestly just don't see how it would be any easier implement.