question about tube amp prototyping

[pompeiisneaks]

If I were playing with a circuit, and want to set it up and look at it on the scope, but don't have an output transformer, could I hook up a resistor about the impedance that I'm expecting for the tube output?  I.e. if the output tubes are supposed to push 9k could I put a 9k resistor there between the two in P/P?  I'm assuming you still need the B+ coming into them at the same time, so I"m not sure if I can still jsut connect the b+ via a Y to either and then connect the two output tubes directly with the resistor?  Or is this bad for the tubes and I need the OT?  I don't need to put sound through it yet, as i just want to see if the setup I have seems to be working without too much distortion or oscillation etc, so I don't even want output yet. 

~Phil

[HotBluePlates]

If I were playing with a circuit, and want to set it up and look at it on the scope, but don't have an output transformer, could I hook up a resistor about the impedance that I'm expecting for the tube output? ...

No, because the resistance will drop the output tube plate voltage so much as to give meaningless results.

Say you have a Champ, 350v B+ and want an 8kΩ load. You might want to idle that tube to 100% plate dissipation: 12w/350v = ~34mA. But you have a series 8kΩ plate load which is trying to drop 34mA * 8kΩ = 272v. You're only left with 78v on your plate, minus whatever bias voltage is across the cathode resistor.

The value of a transformer (or choke) is that the direct current voltage drop is very small while the alternating current impedance can be very large (or perhaps frequency-dependent).

You might be able to imagine ways to circumvent this situation (doubling the intended supply voltage), but this path seems prone to mistakes resulting in big damage for results which may not correlate to the circuit's performance with the correct part installed.

[pompeiisneaks]

ahh, damn, guess I'll have to get one then :) Thanks, that makes sense.  I do have an OT in house for an AC30 I'm going to build soon, but its for a 4k primary as I recall and the circuit I'm looking at would be about 8 or 9k (two 6mb8 in push pull pentode side).  I'd have 8k ish output so if I wanted an 8ohm on the other side it doubles so the 4k OT primary would be best used on the 16 ohm tap for 8 ohm output, if I'm doing the math in my head right?  or is it just a 4k primary to 4 ohm secondary is going to be doubled to 8k and 8ohm due to the rough impedance of those tubes?  So I'd use the 4 ohm tap?  (Now I'm making my head hurt lol).

~Phil

[HotBluePlates]

...  I do have an OT in house for an AC30 I'm going to build soon, but its for a 4k primary as I recall and the circuit I'm looking at would be about 8 or 9k (two 6mb8 in push pull pentode side).  ...

The mistake is thinking the output tubes or the transformer have a characteristic impedance of their own (in a way each does, but that's not relevant here). What matters is the transformer's winding (and resulting impedance) ratio, and the load which is attached.

You on-hand OT is for 4kΩ:SecΩ, and I gather there might be 16Ω/8Ω/4Ω secondary taps?

Let's assume you have an 8Ω tap for this example:
OT is 4kΩ:8Ω. You want 8kΩ reflected to the primary. Connect a 16Ω load to the 8Ω tap.

4kΩ:8Ω = 500:1 (impedance ratio) = 8kΩ:16Ω.

The OT has a winding & impedance ratio between the primary and various secondary taps. The 4kΩ (marked) primary and 8Ω (marked) secondary are really just 2 coils with a 500:1 impedance ratio, so any other load impedance you hook to that "8Ω" secondary is levered-up 500 times to reflect some other primary impedance.

[pompeiisneaks]

Yes there are all three as I recall, so that means that if I have a 500:1 then for an 8ohm speaker, I need to use the 4 ohm tap:

4k:4ohm = 8k 8 ohm ?

~Phil
 

[HotBluePlates]

Yes.

4kΩ:4Ω = 1000:1 impedance ratio = 8kΩ:8Ω (meaning 8Ω on the "4Ω tap")

Instead of 4kΩ:16Ω/8Ω/4Ω, you could look at the secondary taps as "250:1" (16Ω tap), "500:1" (8Ω tap) and "1000:1" (4Ω tap). You hook up whatever load impedance you want to one of those impedance transformation ratios to reflect your desired primary impedance.

[kagliostro]

Recently I've read this simple definition, to me very useful as aid to remember

the impedance ratio is the turns ratio squared

the reflected impedence is the impedence ratio multiplied by the speaker load in ohms

http://www.radioremembered.org/outimp.htm

Franco

[pompeiisneaks]

Excellent, thanks guys.  I'll use that!


~Phil