Turning 2x6V6 PP Fixed Bias into SE EL34 or 6L6GC Fixed bias, can it be done?

[RobBozic]

Hi guys,

I have a home brew amp I built with 2 x 12AX7's into 2 X JJ6V6 Fixed bias running about 430V on the plates, GZ34 recto, with a 40W 4K OT. The PT is a Heyboer unit typically used for Deluxe Reverbs, tweed deluxes & Mojotone assures me it can handle 6L6 PP cathode bias.

Can I turn this amp into a Single Ended fixed bias amp running a bigger power tube such as a 6L6GC, EL34 or even 6550?

Thanks
Rob

[tubeswell]

If the manufacturer says the PT can handle 6L6s, then it can handle 6L6s. As for a SE KT88, or EL34. quite possibly yes. For SE OT, you ideally want gapped iron (and a proper SE Pr:sec winding)

[FYL]

Can I turn this amp into a Single Ended fixed bias amp running a bigger power tube such as a 6L6GC, EL34 or even 6550?

The OT is designed for PP balanced operation, with no DC thru it - B+ is applied at the center tap and goes in opposite phase to the plates, thus the DC-generated field is (+1) (-1) = nearly 0 at the core. SE is unbalanced and sees the whole current going thru a single winding, leading to very quick core saturation in in standard OT. You need a larger air-gapped OT.

[RobBozic]

Thanks for your response.

Rob

[HotBluePlates]

It can be done if you insert a volume control leading to 1 side of the push-pull output only, and leave 2 output tubes in the sockets. The volume control (much like a master volume) reduces the signal going into one side of the output, ultimately to zero. The remaining driven tube is essentially operating single-ended.

The idea is that the non-driven tube still passes idle current, which balances the idle d.c. current of the driven tube, and removes the need for an air-gapped OT. Output power is necessarily low, as the OT is not an ideal load in this case for maximum power. But the thought is that maximum power isn't the goal in this situation.

London Power calls this a "Body" control, and sells the mod kit on their website. You need a volume control (say 1M) and an added coupling cap going into the volume control which is at least 10 times the stock coupling cap. The stock coupling cap can go from the wiper of the volume control to the "non-driven" grid.

[RobBozic]

HPP,

I've attached a schematic showing two versions, would you mind please checking it for me.

Thanks
Rob

[HotBluePlates]

Start with schematic B.

EDIT: Marked up your schematic to show the change.

This change makes both side receive the bias connection. That bias voltage has to be blocked from the Body control by the 0.47uF cap, or the control will sound scratchy. It also has to connect to the 1.5k resistor, or that tube will receive no bias voltage.

It really is as easy as 1 pot and 1 cap to incorporate this mod.

Oh... you probably also want a 1M audio pot to have an even sweep of volume.

[RobBozic]

This forum is definately the best.

Thanks very much HPP.

Rob

[RobBozic]

I forgot I have another question.

Could you incorporate a fixed bias VVR into this amp after this mod?

I don't see why you couldn't but I thought I'd ask to be on the safe side.

Thanks
Rob

[FYL]

Could you incorporate a fixed bias VVR into this amp after this mod?

No problemo.

[phsyconoodler]

I have a question for you: What are you looking for in terms of tone here?
  If you do all these mods and then use a VVR,what do you hope to accomplish?I would rather switch to cathode bias,a 5Y3 rectifier tube,add a three way low power switch and then VVR.
  All you get going single ended would be a less girthy tone and lower output power.Easily accomplished with cathode bias,low power switch and VVR.
  I have built lots of single ended amps and I always come back to push-pull for more 'body' to the tone.I get all the squishy sounds from the low power switch and cathode biasing.You could do both fixed and cathode biasing easily too.
  Not to rain on your parade,it's just that I wanted to share my experiences with single ended vs push-pull.

[RobBozic]

I'm trying to lower the power of this amp for home use. I have two other louder amps. With this amp I've tried cathode bias with a 5Y3 & a 5R4GYA & I didn't like neither one. I guess I don't like cathode bias amps with tube rectifiers because they become too spongy for me. Cathode bias with GZ34 & the voltage is too high for cathode bias with a 8K load (that's another topic).

I've ordered a VVR3 for fixed bias from Dana Hall which I'll implement into this amp. I tried the Single Ended mod last night & it does work however I didn't realise how loud the amp would still be.

Therefore, I'm going to ditch the single ended mod above & just go with the fixed bias VVR3 with an inefficient speaker like the new Heritage Greenback rated @ 96db.

But I'm glad I tried the single ended mod, it's just still too loud for me and my neighbours (I got a visit from the cops last week).

Thanks guys
Rob

[phsyconoodler]

ah the neighbor syndrome.
   You are going to love VVR as long as you just regulate the power amp and PI.It will rock your world.
You can play at such low volumes and not lose any highs and it still has tons of tone.You just don't get that pant-flapping experience.

[RobBozic]

Thanks,

So VVR for the Power & PI only, thanks buddy. Do you recomend doing it that way for non-master volume amps?

For NMV amps I'm looking at it this way, if you implement it only on the power & PI won't you lose the preamp / power amp balance of the non-master volume amp?

It makes sense for pre-PI MV's where you can adjust the balance between preamp / poweramp distortion.

Thanks
Rob