pentode vs UL

[ernie_jr]

I am planning for my next build at 2 x kt88 bass amp. I want to be able to switch between pentode and UL mode. On Sunn's UL amp, the OT is taken of the second node after the choke.  Pentode amps take the plate off the 1st node and the screens off the 2nd node.  When switching should I use a 3pdt switch to change the plates and the screen taps, or can I take everything from the 1st node (or would taking only from the 2nd be better) Hope this make sense.
Thanks,
Ernie

[clyde]

I've done this with a dpdt switch, 2nd node to one side, UL taps on the other, common terminals to the screens, with screen resistors always in circuit, or it could be wired so that the resistors are in circuit only in pentode mode.  I can't comment on the Sunn arrangement as my OPT feeds from 1st node, as is usual. 

[HotBluePlates]

I would figure out what you want to be your power supply/OT arrangement, and leave it at that. Whether that's the Sunn way or the typical way is up to you.

It probably won't matter as far as UL goes, because there are plenty of UL amps that use the typical arrangement.

[PRR]

> On Sunn's UL amp, the OT is taken of the second node after the choke.

Is this the one with very small 1st cap for such a large amp?

That's an odd supply. Copy it EXACTLY or don't do it that way.

The voltages are very high. The current is awful high too. With a good cap-input supply, you need a very costly cap and you need very beefy rectifiers (tube or silicon). What Sunn did is a "bad" cap-input. The first cap is very small (cheap) and has HUGE ripple but not so much peak current. There is very little smoothing, it is "crap DC". Then a choke and a second cap gets clean DC.

They must have got a good deal on that choke, it has to be substantial.

With today's cap and choke prices, steal the plan from Dynaco or Fender big UL amp. A big first cap (or two series) and modern Silicon can be clean enough for plates and screens. For the rest of the amp, which does not draw large or varying current, R-C filter is fine.

[ernie_jr]

Thanks PRR,
My plan is to use a 3pdt switch to switch between standard pentode and Sunn/Dynaco UL.  PT is 400 volts, solid state rectifier, choke is Sunn/Dynaco (1.5H 200ma) and first filter cap about 100mFd and second 50mFd. This set up on paper seems to work. Any advice for be most helpful.
Thanks,
Ernie