#1
Hey Guys,

I'm currently in the process of building the SEL amp from AX84. Being a homebrew project, I'd like to be able to play around with different output tubes. The problem is that not all tubes have the same output impedance. This will cause the output transformer secondary wiring to change. I've attached the schematic and hook up data sheet for my Output Transformer (Hammond 125GSE) for reference. I'd like to label my outputs properly so i know what output is what impedance when trying different tubes. I've tried google, but all the information is either ancient or not there. From the schematic for the SEL on AX84.com, i can gather that a KT88/6550 output impedance is approximately 2.5k ohms (although it doesn't specifically say it). I was wondering if anyone had a definitive chart of the most common octal power tubes and their output impedance that i can use as a reference.
Attachments:
OT.JPG
#2
The output impedance is the load in parallel with the tube's impedance. However, if you are interested in designing an output stage based off of a tube's dynamic characteristics, then the Valve Wizard's site is a good place to check out.
The suggestions for which tubes and the number are just that suggestions, you're limited to the devices at hand. For instance, an EL34 tube will draw such a current and have such a limited dynamic grid swing available, the KT88 will draw a different current and will have different curves for the same grid voltage swing. If you would like me to clarify that some I'd be happy to.
#3
I think I get what you are saying. The output impedance of the tube is dependent on the circuitry of the output section. Which is why on some of those data sheets, i saw different impedance values for different applications (push pull, fixed bias, common cathode, etc.).
I already have the schematic and layout (attached). I guess what I'm asking is, if wire this for kt88/6550 use, will the 8ohm output still be 8ohms if i throw in a 6v6 or a 6l6?
Attachments:
OUTPUT.JPG
Last edited by macgregger at Sep 29, 2011,
#4
The reflected impedance on the tube plates varies with the transformer. For instance, if you hook up a 4 ohm load to a 6600:8 transformer, it will appear as a 3300:4, and the tube will be loaded based on the 3300 primary. Transformers have impedance ratios. The given values on the datasheet for P-P and SE are recommendations, not necessarily specific values that must be used.

The output will still be for 8 ohms, however, the impedance on the tube plates will force different currents based upon the bias and the tubes used.

More simply, look at a resistor loaded triode. Say 22kohms plate resistor, put a 12AX7 in, swap it for a 12AU7, the currents will be different as well as the bias points and available grid swings. The output stage is more complicated, but follows this rule, the load will be the same, but how the tubes react will be different. And that is what needs to be taken into account.
#5
i'm assuming you are running cathode biased on the power tubes?

if you use like a 1.8-2.2k primary impedance on the transformer and pick a cathode bias resistor that is in the middle average for all the tubes you want to use it should be ok.

look at the egnater tweaker for example.

however you really won't be able to use a 6v6 since the b+ will be way to high if it is set up to run kt88s.
Prs se Holcomb is the answer
#6
Forget about the output impedance of the tube. You should concern yourself with understanding load lines on a chart of characteristic curves. Keep it simple. It's the primary impedance of the tranny that matters.

So take the diagram of the tranny that you posted. What it is telling you is the varying primary impedances you will have when connecting the secondary to the listed load impedances. For example, if the white lead is connected to an 8 ohm load, then the tranny's primary impedance will be 2.5k ohms. Doesn't matter what tube is in the circuit. White lead on an 8 ohm load will yield a 2.5k primary Z. Period.

It is up to you to analyze load lines for your chosen plate and screen grid voltages on the tube you choose. From there you will determine what primary impedance you require as well as what lead should be connected to what speaker load in order to achieve that (on top of the tube's bias resistor). This will be entirely dependent on the variables of your unique circuit.

Hope that makes sense.
#7
Quote by CECamps
Forget about the output impedance of the tube. You should concern yourself with understanding load lines on a chart of characteristic curves. Keep it simple. It's the primary impedance of the tranny that matters.

So take the diagram of the tranny that you posted. What it is telling you is the varying primary impedances you will have when connecting the secondary to the listed load impedances. For example, if the white lead is connected to an 8 ohm load, then the tranny's primary impedance will be 2.5k ohms. Doesn't matter what tube is in the circuit. White lead on an 8 ohm load will yield a 2.5k primary Z. Period.

It is up to you to analyze load lines for your chosen plate and screen grid voltages on the tube you choose. From there you will determine what primary impedance you require as well as what lead should be connected to what speaker load in order to achieve that (on top of the tube's bias resistor). This will be entirely dependent on the variables of your unique circuit.

Hope that makes sense.



sooo what's your saying is shunt regulated ccs cathode biasing?
Prs se Holcomb is the answer
#8
I think I'm starting to get things now. I was looking at it the wrong way. I'd have to look at the tube characteristics and operating power to get a proper output impedance.

from the valve wizard site:
Because the grid curves of a pentode allow the signal voltage to swing almost to 0V on the anode, and because we would like to run the stage approximately centre biased, there is a rule of thumb we can use to find a suitable transformer impedance:
Z = Va^2 / Pa
Where:
Va = Anode voltage.
Pa = Maximum anode dissipation.

So in my case, the anode voltage is 346V and the kt88 max power is 42W. that gives a Z of approx. 2.8k ohms. where the 2.5k ohm primary is the closest match.

Now correct me if I'm wrong, but if i were to use an EL34, would the math look something like this: anode voltage is still (approximately) 346 volts, max power is 25W. Z = 4.7k ohms. therefor i'd want to use the yellow wire tap on the secondary of my OT for an 8ohm cab.

I'm just wondering if I can do this without having to change the screen and grid resistors on pins 4 and 5.

To answer AcousticMirror, i will be modifying the fixed cathode bias to a variable cathode bias. Attached is the schematic.
Attachments:
BIAS.JPG
Q: What's worse than lobsters on your piano?
A: Crabs on your organ.
#9
A critical factor that you are leaving out here is screen grid voltage. The screen grid voltage will determine the characteristic control grid curves from which you will determine, ultimately, what the best primary Z is.

I highly recommend diving into the analysis and subsequent decision making based on graphs of characteristic curves. The method you describe above is a "loose" ball park estimate type of approach. You seem like you are genuinely interested in getting into the technical details so I think you'd really enjoy going further.
#10
Quote by CECamps
You seem like you are genuinely interested in getting into the technical details so I think you'd really enjoy going further.


Well i have a diploma in electronics technology. I'm a bit of a tech goon. The whole field interests me. It's just I'm fairly new to the tube theory part of it all.

Can you suggest a good place to find proper graphs and curves? I'd like to be able to find the information from one place rather than scabbing bits of info from here and there.
Q: What's worse than lobsters on your piano?
A: Crabs on your organ.