Gain of output valves

[Mark]

Unfortunately, despite my Google searches I’m unable to find info on the gain of output valves. It’s my understanding that output valves apart from EL-84’s don’t have much in the way of voltage gain.

The reason for my thread is I have a SF Twin Reverb putting out 66 watts RMS. The voltage output of the P.I. stage is greater that the voltage on the secondary of the output transformer.
I’m expecting at least the same voltage on the secondary.

Thanks for your assistance.

[R.G.]

It's a little tangled. The valves/tubes themselves have a decent amount of voltage gain and power gain. The plates swing a couple of hundred to a few hundred volts from the plate supply voltage down to 80-90V from ground. The output transformer itself converts this down to a lower voltage/higher current for the speaker outputs. The effect is to match the output impedance of the output tube plates to the load impedance of the speakers. This has very different results from voltage gain only.

The output transformer is designed to take the full swing of something less than 2X the HT voltage (in a plate-to-plate output stage) and convert that voltage to speaker output voltage. Generally OTs have voltage ratios of between 15:1 to 25:1 step down - and current step up.

[pdf64]

...
The reason for my thread is I have a SF Twin Reverb putting out 66 watts RMS. The voltage output of the P.I. stage is greater that the voltage on the secondary of the output transformer.
...

That may indicate the load impedance is rather low.
Edit - oops, that's nonsense, I misread the quote

[Ten Over]

The reason for my thread is I have a SF Twin Reverb putting out 66 watts RMS. The voltage output of the P.I. stage is greater that the voltage on the secondary of the output transformer.
I’m expecting at least the same voltage on the secondary.

66W is kind of interesting because I get 64W with an eight Ohm load in my silver face Twin Reverb. What is the voltage output of the phase inverter and the voltage output of the OT secondary?

[Stevem]

The simple answer is a speaker due to its small impedance load is a low voltage high current device.

The voice coil of a speaker rated for 100 watts could easily be blown open by pumping a 45 volt audio sign wave into it.

[Ten Over]

This shows the gain of the things you seem to be interested in. This is open loop gain, so the gain of the LTPI would be without the NFB connected.

Screen Shot 2024-06-23 at 11.15.50 AM.png

[Mark]

The reason for my thread is I have a SF Twin Reverb putting out 66 watts RMS. The voltage output of the P.I. stage is greater that the voltage on the secondary of the output transformer.
I’m expecting at least the same voltage on the secondary.

66W is kind of interesting because I get 64W with an eight Ohm load in my silver face Twin Reverb. What is the voltage output of the phase inverter and the voltage output of the OT secondary?

The SF Twin is putting out 55vpp on both sides of the P.I. stage. I will check what is on the plates of the output valves.
There is 46vpp on the secondary of the output transformer. I’m running the amp into a 4 ohm load.

[dragonbat13]

This shows the gain of the things you seem to be interested in. This is open loop gain, so the gain of the LTPI would be without the NFB connected.
Screen Shot 2024-06-23 at 11.15.50 AM.png

This may be a really dumb few questions but I like that diagram.

What exactly is "A" referring to? Anode Voltage? Amperage?

What does "Z" refer to? I assume that's primary impedance, correct?

[Ten Over]

This may be a really dumb few questions but I like that diagram.

What exactly is "A" referring to? Anode Voltage? Amperage?

What does "Z" refer to? I assume that's primary impedance, correct?

"A" is gain. "A" sub "o" is open loop gain, i.e. without NFB.

"Z" is impedance.

[dragonbat13]

Thank you very much.

[Mark]

I had a look at the rail voltage with respect to the output of the amp. I have a mains voltage of 235Vac which is a bit low. The rail voltage without a signal is 432VDC, when I have full undistorted output the rail voltage has dropped to 401VDC.
I would imagine that there would be a drop in rail voltage, but would a drop of 30VDC be considered normal for a S.F. Twin Reverb?

[Stevem]

At full clipped output,yes.

[Mark]

At full clipped output,yes.

Steve, thanks for taking the time to answer my question. The rail voltage is 401VDC at clipping, so this normal, I didn’t check the voltage beyond clipping as it wasn’t relevant.

[Ten Over]

The SF Twin is putting out 55vpp on both sides of the P.I. stage. I will check what is on the plates of the output valves.
There is 46vpp on the secondary of the output transformer. I’m running the amp into a 4 ohm load.

The AC voltage on the output valve plates would be good to know.
What kind of 4 ohm load?

[Helmholtz]

The SF Twin is putting out 55vpp on both sides of the P.I. stage. I will check what is on the plates of the output valves.
There is 46vpp on the secondary of the output transformer. I’m running the amp into a 4 ohm load.

Power tube grid signal Vpp needs to be at least 2 x grid bias for full power, so roughly clean 100Vpp minimum.
It is normal that output voltage is less than grid signal.
Power tube voltage gain should be around 6.5.
Don't forget that the OT steps down voltage by roughly 12.
When scoping plates, make sure to use a100x probe with a voltage limit >500V peak.