12b4 bass rolloff problem

[surfstu]

Thanks Eli and everyone else who contributed

I will measure the output caps, I have found the bigger brothers to my caps on ebay...

4.7uF 250V PIO capacitors HI-END K75-10. Lot of 4 NEW | eBay

I will also change R4 to 1.5M

Can I just add a power resistor to my anode choke to see if I get better bass response? Not sure about the CCS on the anode yet, I'm not confident enough to build one!

cheers Stuart

[surfstu]

Forgot to mention that I am considering making the first watt F5 amp, this is why I'm concerned about impedance but the F5 is forgiving at 100k I think...

[pieter t]

Originally Posted by Eli Duttman

X_C for 80 H. @ 20 Hz. is 10053 Ω. That can easily be too small working into the comparatively low SS power amp I/P impedance. Replace the choke with a good CCS. Then, even the nasty IHF 10 KOhm "standard" I/P impedance will not be a problem.

I don't get that.
The 80 H choke is a 10k AC load at 20 Hz; that is plenty enough for a 12B4 with its 1k5 or so Rp.
Replacing the choke by a CCS will not lower the output impedance, so there will not be better driving capacity doing that.
Reading the posts I think the choke is the suspect part; its inductance should be checked before IMO unnecessary changes to the circuit are being made.

[euro21]

Originally Posted by surfstu

Not sure about the CCS on the anode yet, I'm not confident enough to build one!

Why not buy it (Cascode current source kit) from K&K audio?
K & K Audio - Lundahl Transformers, audio DIY kits and more

First watt F5 amp....maybe average sensitivity speakers...perhaps silver interconnects...typical bass rolloff system. :-(

[Eli Duttman]

The 80 H choke is a 10k AC load at 20 Hz; that is plenty enough for a 12B4 with its 1k5 or so Rp.

You're forgetting that the choke's reactance is in parallel with the I/P impedance of the downstream device. The NET load will always be less than that of any entity that contributes to the set of parallel paths. Yes, even at 1.5 MOhms, R4 makes a negative contribution.

Another issue with choke loading is the HF "peaking" that occurs. "Peaking" can be very useful, but not here. The OP's complaint about poor bass behavior dovetails all to well with gain rising as frequency rises.

[pieter t]

Originally Posted by Eli Duttman

You're forgetting that the choke's reactance is in parallel with the I/P impedance of the downstream device. The NET load will always be less than that of any entity that contributes to the set of parallel paths. Yes, even at 1.5 MOhms, R4 makes a negative contribution.

I am not forgetting that .
Even with the 33k input impedance of the Rotel amp the net load load will be some 7k7 at 20Hz (rapidly rising to some 0.5 Meg at 1 kHz), which is a very decent load for a 12B4.
There is still no explanation for the bass roll off, presuming the choke is really 80H.

[pieter t]

Originally Posted by Eli Duttman

Another issue with choke loading is the HF "peaking" that occurs. "Peaking" can be very useful, but not here. The OP's complaint about poor bass behavior dovetails all to well with gain rising as frequency rises.

HF peaking does not occur with all chokes; quality chokes will show a ruler flat frequency bandwidth up to hundreds of kHz (depending on source impedance).
I question the quality of the choke; the 2k DC resistance already indicates this, and is a bad match with a low Rp tube to begin with (almost 50V DC drop across a choke is not what you want; I agree that the 50V could better be used for a CCS instead of a bad quality choke).
Based on a quality choke (with some 5 V of voltage drop at 20 mA of plate current) and a 1k cathode resistor, there is nothing wrong with the circuit.

[Eli Duttman]

"Peaking" is a natural consequence of inductive loading. X_C = (2)(Π)(F)(L) Inductive reactance varies directly with frequency. Increase the load and gain goes up.

[DF96]

Once the load significantly exceeds the anode impedance further increases do not increase gain. As the external applied load is much higher than the anode impedance, the only cause of a genuine change in bass with change in load is the output coupling cap being too small.

[pieter t]

Originally Posted by Eli Duttman

"Peaking" is a natural consequence of inductive loading. X_C = (2)(Π)(F)(L) Inductive reactance varies directly with frequency. Increase the load and gain goes up.

That is too simple.
The math is right, but practice is a bit different.
For example: inductance is not a frequency linear parameter; pick a socalled 80H choke and measure the inductance with a good quality meter at 100 Hz, 1 kHz and 10 kHz. With standard grain oriented silicon steel you will see the inductance drop at the higher frequencies (there you go with the math...).
Adequate inductive or CCS loading gives a gain of the triode of about mu, there is no significant difference, and up to very high frequencies (very much over 20 kHz). For triodes: check load lines and see when they become flat.
When speaking of the gain going up by increasing the load, that is generally right for pentodes.