6550s on a Twin Reverb PT

[firemedic]

Has anyone done this? Can the PT handle it? I have one that has 400mA 660VAC.
Heater current notwithstanding; I have an auxiliary tranny for that.

[phsyconoodler]

Most Twin Reverb PT's are rated for 500 + Milliamps and a Marshall 100 watt PT is rated at about 420ma.Marshall's have used 6550's but likely not at high current.
  It would depend on how much current you set the 6550's up to draw I would think.

[firemedic]

That's kind of the source of my confusion. The datasheet leads me to believe they would draw too much. I would bias conservatively, it is for a bass amp anyway & I want it clean. Let me check the PT specs again, it's the Weber W022756.

(sound of footsteps going down the hall, then coming back)

640v 400mA. So ballpark 400-420v on the plates under load, if that. Let's say @55mA bias per tube. The datasheet has a pretty big jump in current draw between idle & wide open (double or more), so it's a bit of a gamble I guess. Has anybody tried 6550s on this PT?

BTW I am talking about a quad.

[PRR]

What is the plate load?

> The datasheet has a pretty big jump in current draw between idle & wide open

Idle current is whatever you want it to be.

At FULL ROAR the current IS what the B+ and load DEMAND.

As a useful approximation for not-hot idle in push-pull:

• Divide the nominal OT P-P impedance by 2.
• Divide the B+ by that number.

That's the full-roar demand.

Ampeg VT40 with original OT at 6.6KCT but PT replaced for 400V supply.

6.6K / 2 = 3.3K

400V/3.3K = 121mA full roar. This job was idled at 110mA so there's little rise.

OTOH, a Twin with 2KCT and 400V is 400V/1K = 400mA full roar. Such an amp might be idled at an arbitrary 160mA (40mA per jug) so there's a large rise from idle to loud.

OTOOH, four 6550 at 400V with a hypothetical 20KCT load would suck 40mA (plus whatever idle current). "Hypothetical" because such a rig is nearly as expensive as a 100W amp but only puts out maybe 10 Watts... not a sensible setup.

So what is the proposed plate load?

[firemedic]

1.8kZ/2=900
@420VDC/900.....
The back of the napkin says 466mA wide open. Plus the preamp load which is suddenly a factor.

With 3.8A heater current being carried by a separate heater tranny, the PT will only see...
about 3.8A also. It has a 6A 6.3v capacity. 2.2 unused amps on that winding should help with the total PT load & this may be doable.

[PRR]

> 466mA wide open. Plus the preamp load which is suddenly a factor.

How many preamps you gonna have? 5 bottles of 12AX7 is 10mA-15mA. 3% of the final stage current. Not worth worrying about.

If you build a big-bottle (6F6/6V6) reverb driver that may be another 30mA. Still no big deal next to 466mA.

And as you say, off-loading the heaters lightens the load on the PT.

You are using Twin OT with Twin PT. This will work. Hardly matters what tubes go between. 6L6GC/5881 loaded this way will swing 80%-85% of B+. The bigger 6550 might swing a wee bit more, but not significantly more output or PT heat.

[firemedic]

It'll be a Marshall 100w OT. I'm designing (adapting existing plans for) a bass amp using EL34/6CA7 quad output. I know I will try 6550s or KT88s sooner rather than later so I'm aiming for an output section that will be convertible to 6550s w/ a minimum of circuit mods.
My preamp design currently uses 2-12AX7s, a 5879 and a 12AU7. As always, subject to change.
I'll attach the schematic as currently envisioned: the bias circuit needs a little attention & I'll tighten that up soon.

[tubenit]

Firemedic,

That is a VERY interesting amp! Looks cool.

Can you talk about the 5879 presence control, please?   Have you used it?  What does it do tonewise?

Thanks, Tubenit

[firemedic]

T, I have no idea what this amp will sound like. It's a Weber CS100 almost exactly, with a B25 preamp as 2nd channel and a quad output. I was also curious about the presence control.
I have seen varying reviews about the CS100 but the ones I chose to pay attention to were, someone here mentioned it was T Weber's personal favorite, and one person said it had tons of clean power. Or words to that effect.

At any rate, this one will take a while for various reasons, I'm just trying to get my ducks in a row & anticipate potential issues.

[firemedic]

My mistake, 6S100, not CS100. A little dyslexic moment.