ST70 rolloff from 300hz

Okay guys....

I have a pristine ST70 Dynakit ( yes with selenium diode I know
I have to replace do it ,,, do it ) but it starts rolling off
the bass at about 300hz or so.
How do I know... I have a Umik-1 and use REW with a pair of
Spica TC50 and REW says a big NO to anything below approx. 300hz.

With another amp bass shows up fine.

What is the cause of this ?
I would like to step through the solution instead of just
upgrading to newer parts, just because I'm curious.
I have a suspicion it would be some magnetic flux capacitation..
hahahha

Maybe the tubes are old ???
Like I said this thing has not one replacement part since it
was assembled....

What should I do...
New cap can first from tubes4hifi ??

Speak to me...

hahaha thanks for reading

How do you know that you have a rolloff at 300hz ?
Any measurements ? And what is the load and inputs ?

Measurements are from my Umik-1 ( calibrated microphone from MiniDSP )
feeding into REW ( Room EQ Wizard, a sound capture program )
that captures sweeps from my Spica TC50 speakers.
The other amp does not display any such anomaly of frequency.

Thanks for responding.......

Maybe your speakers are ill-suited, i have no knowledge of Spica TC50.
You could try 4-ohm output if speakers are very resonant.

To be more systematical load the st-70 with
resistors and feed it with sweepgenerator. Then you would
also have an opportunity to see power limits.


**EDIT**
What is needed here is a simplification of your object. You have a bass rolloff,
how come ? Well that's why you should simplify in order to measure each
component at a time.
Measuring the st-70 to figure out if it fullfills the performance stated
is one step. If it does not - well time to fix it. But if it
fullfills all it's spec then it's a waste of money and time to start
replacing components.

Maybe your bass-rolloff comes from a interaction of amp and speakers.
Maybe the other amp has a huge bass-increase ( it could have loudness control
active). There is lots of reasons for the perceived rolloff.

Start by ruling out the components that does not causes bass rolloff.

Regards

To accurately determine what the amplifier is really doing, you need to measure the output with an AC voltmeter or on a scope while it is driving a noninductive dummy load matching the output tap to which it is connected. Otherwise your microphone is measuring not only the amplifier but the loudspeaker and the room acoustics.

I guess some would question why I would ask questions
without much previous thought or action about my amp.
Well, firstly I have no experience with really troubleshooting
( as in signal generators, scopes, DMMs )
secondly....

I was asked to work on a Cadillac CTS v6. This car had a
p0305 code ( misfiring on cylinder 5 ). Conventional wisdom
points to inspecting and/or replacing spark plug/ignition coil.
Well one guy out of thousands on a YouTube video just happened
to mention one thing that fixed this car. ( its a big job
to replace spark plugs, the intake top plenum has to come off )
He said that oil pressure regulates the tension on the timing
chain.

Low oil pressure causes the timing chain to jump around to produce
2% - 10% variance and up pops a code.

Soooo, I learned to ask, because instead of taking hours to fix
a problem, there may be just that one expert that has just that
one bit of advice that can make life soooooo much easier.

Thanks

I'd also like to know how different parts affect soundstage, dynamics, freq response etc.. Like a choke? Does replacing that affect the sound? Once all pets are new, is it just a matter of different tube combos for noticeable changes. Thanks for your patience.