EL84 and 6L6 in same amp - how to get 2 different B+?

[Hardtailed]

I've been thinking about this for a while, and the concept is starting to take its final shape.

The amp will have a typical 5F6A/Plexi preamp with a 2X6L6 power amp.

However, I want to also have a SE 1XEL84 power amp with cathode bias!
I want to have the 6L6 on a switch so I can turn them off and use it in 5W mode at home. However, the EL84 output will also be tapped off and reinjected before the 6L6's PI (paralleled to the regular preamp signal, with a mix control).
Also the EL84 will have a switchable internal dummy load, so I can use just the 6L6 power amp if I have a single cabinet (yet still be able to inject the EL84's crunch into it), or use 2 cabs if I so desire.

Anyway, it's all taking form, but one problem I have is I can't run the EL84 on the same B+ line as the 6L6. I want high voltage for the preamp and 6L6, like around 450V, but that's gotta drop to like 300V for the EL84.

Any simple way of doing that, short of having a 2nd PT?

Also, if I run the preamp and 6L6 on the same power rail, will switching the 6L6 off send the B+ voltage through the roof because of the load removed? I know Mesa amps simply lift the ground from the unused power tubes and it doesn't seem to cause them any problems.

[bluefoxicy]

Put a RESISTOR after the lead off the rail to the 6L6, and then bring the B+ off that.

Look at B+[123]

http://www.sewatt.com/files/sewatt/StockVJ_1-3.pdf

[Steve Conner]

Sure, you can use a resistor, although it'll need to be a big power resistor burning about 5-10 watts.

If you use a bridge rectifier along with a center tapped transformer, or a voltage doubler, you can tap off an auxiliary supply at half the voltage of the main one. Fender's 300PS and 400PS used the doubler to get 700V for their plates and 350V for their screens and preamp.

The only problem is that you might struggle to find a transformer with all the right windings. If you found one with the right secondary voltage, Murphy's law says it probably wouldn't have a heater winding or something, and you'd end up using another transformer again.

Someone else once suggested using an ordinary full-wave center tap rectifier, but feeding two sets of diodes, one of which goes to a capacitor input filter and the other to a choke input filter. The choke input filter will give a second voltage about 0.61 times the main voltage. But again you need an extra piece of iron, the choke.

I like your idea, I'm currently working on something similar, with a SE EL84 driving a transistor output stage that can be bypassed. I didn't even try finding a suitable PT for it all, I just used two separate ones.

[Hardtailed]

I was just thinking... the Yellowjackets are said to be good up to 520V.

The cathode biasing resistor is the only thing used to drop the voltage and apparently it's good enough.

So maybe that would be sufficient since the EL84 will be cathode biased...

[pvsage]

You could just add another rail to the power supply. One more metal oxide resistor and another high-voltage electrolytic capacitor, and you're in business.

[pvsage]

I was just thinking... the Yellowjackets are said to be good up to 520V.

The cathode biasing resistor is the only thing used to drop the voltage and apparently it's good enough.

So maybe that would be sufficient since the EL84 will be cathode biased...

BUT the EL84 plates aren't happy with voltages over 310VDC; you need to consider your voltages at both the plate and the cathode.

The cathode biasing resistor doesn't drop the voltage - it raises the cathode above ground. It also adjusts the output tube's power dissipation.

[Bruce / Mission Amps]

Choke input filter

BUT the EL84 plates aren't happy with voltages over 310VDC; you need to consider your voltages at both the plate and the cathode.

The cathode biasing resistor doesn't drop the voltage - it raises the cathode above ground. It also adjusts the output tube's power dissipation.

Well, I have built and worked on plenty of EL84 amps with B+ voltages in the 360vdc to 380vdc range so 310vdc is not a rule of thumb.
Also, the cathode resistor's voltage drop does subtract from the plate's B+ ...since the plate to cathode voltage is all the tube "sees".
The bad news is that not very many cathode biased EL84 amps have more then 10-15vdc on the cathodes.
Using power resistors to drop B+ is a waste of PT energy and will just burn up potential current in heating a resistor.
My idea of using a choke input filter to build up a parallel, lower voltage B+ rail for the EL84 portion will still work but the builder will need at least a 120ma to 150ma choke... more expense.

[hasserl]

BUT the EL84 plates aren't happy with voltages over 310VDC; you need to consider your voltages at both the plate and the cathode.

The cathode biasing resistor doesn't drop the voltage - it raises the cathode above ground. It also adjusts the output tube's power dissipation.

I've run many EL84's at right around 400 volts at the plate in fixed bias. They work fine like that, sound great and provide good service life.

[DirtyGeorge]

Bruce, why would the current requirement be so high? I would expect the average current for an el84 SE @ 300v to be 40 or 50 mA.

[R.G.]

I think the bottom line is - use a separate PT for the EL84. Yes, a choke input filter does the job, but the size of the choke is comparable to the PT you'd need.

As for switching off the 6L6s, why don't you just switch ground the input to the PI? Leaving them there idling uses the same amount of power as you'd need the other way, using a resistor to eat the same current. Yes, turning off the 6L6s will increase the B+ some. I don't know about "through the roof". It depends on your PT, rectifiers, etc, but is probably less than 75V.

[pvsage]

Well, I have built and worked on plenty of EL84 amps with B+ voltages in the 360vdc to 380vdc range so 310vdc is not a rule of thumb.
Also, the cathode resistor's voltage drop does subtract from the plate's B+ ...since the plate to cathode voltage is all the tube "sees".
The bad news is that not very many cathode biased EL84 amps have more then 10-15vdc on the cathodes.
Using power resistors to drop B+ is a waste of PT energy and will just burn up potential current in heating a resistor.
My idea of using a choke input filter to build up a parallel, lower voltage B+ rail for the EL84 portion will still work but the builder will need at least a 120ma to 150ma choke... more expense.

Thank you Bruce. Apparently some misinformation or a poorly-interpreted data sheet has become Gospel in terms of modding a certain mass-produced SE amp - everyone's been saying "first get B+ down to 310-320V".

For what it's worth, many EL84 data sheets list 300V plate-to-cathode as a "limiting value", but I'll take an amp builder's real-world experience over a data sheet any day of the week.

[Wakculloch]

Why not drop the HT to 350V and run everything off that voltage.
You should still be able to get 40W out of a pair of 6L6's, and happily run a SE EL84 and all preamp requirements.

[Bruce / Mission Amps]

Bruce, why would the current requirement be so high? I would expect the average current for an el84 SE @ 300v to be 40 or 50 mA.

You're right a 90ma-100ma choke running a single EL84 and two or three 9 pin preamp tubes would probably be good enough.