Troubleshooting GAIN problems for preamps?

When troubleshooting GAIN , what is the best way to know the input voltage VS the output voltages?

For preamp amplifers for a 6 stage or 8 stage preamp sections, how do i know the GAIN is correct?

PROBLEM example#1

The preamp has 8 stages, its suppost to have 6 divisions on the oscilloscope set at 20volts per division, The problem is that its only outputting 4 divisions, what should i check for and how do i know ?

I checked all the Bypass capacitors values because bypass capacitors for each 8 stages of the preamp at what set the gain

I checked the voltage dividers for each input going to each preamp tube

But My main check is how do u troubleshoot GAIN problems?? what are some tips and rule of thumbs when troubleshooting gain problems??

Gain of different stages could be different by design. To see, which stage has unregular low gain, you have to know the nominal gain, either from circuit analysis or from comparative measurements with a known good amplifier.

B.T.W.: With a tube amplifier, I would doubt tubes first. After having sequentially exchanged each tube with a known good one for trial, I would start searching other causes. Each preamps operation points could be checked e. g. with a multimeter.

Hi,
You can trace through the gain of each stage, by applying a known input voltage, you can measure the outputs of each stage and see which stage has the unexpected low gain. While making measurements use a VTVM with high input impedance or use oscilloscope with an attenuated probe to ensure that you do not load the high impedance outputs of the tubes.
As FVM has mentioned, one of the major reasons for a tube amplifier to have reduced gain is due to low cathode emission over a period. So, the first thing you can do is to replace each tube one at a time with a known to be good tube and see if the problem gets corrected.
Changes in resistance values and failure of electrolytic capacitors are other common possibilities. Normally, the circuits provide the DC biase voltages at various points on the circuit, you can also check these with out applying a signal and catch the first faulty stage in a chain of amps.

Regards,
Laktronics

what about voltage divider formulas for the input resistors and output resistors X's multiple by the input voltage and output voltage to see if the gains are correct?

Hi,
In order to calculate the expected output voltage, you must know the gm of the tube which is specified as the ratio of the change in plate current to a change in grid voltage. If you know gm, you can multiply it with the AC load at the plate circuit(RL parallel , parallel combination of biasing resistors of the next stage) and calculate expected output for a given input.
If you have standard working amplifier, you can as well compare the stage gains and see which stage is not working ok.
Regards,
Laktronics