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Re: Very soft bass - scratching my head

Posted by J.Rankin (A) on March 1, 2007 at 08:54:52

In Reply to: Very soft bass - scratching my head posted by single_ended on February 28, 2007 at 12:42:32:

First off, congratulations for putting together a very high quality SET system. Definitely not for the faint hearted.
To help you with the bass aspects, we need to work backwards from the power tube.
The Ck value is far too small at 47uF for quality bass.
You do not mention any 20-30 ohm resistors placed between the filament and the Rk resistor thus I must assume that you have connected Rk directly to one pin.
This could be better arranged using a pair of 20 ohm resistors to ensure that the filament is correctly ac balanced with regard to the audio signal path. Once these are fitted then a pair of N type 330uF caps can be placed between the tube pins and ground so that proper ac balance is achieved. There is also another reason as follows.

The IST output should be connected between the filament and the grid and to NOT use the cathode resistor Rk.
This arrangement will give you maximum gain and the absolute maximum signal clarity. Local feedback is removed thus obtaining the benefits of fixed bias but without the hassle of another power supply.
To allow the grid leak to function, a coupling cap has to be fitted between the top of the IST secondary and the grid.
The grid leak is placed between the grid and the grounded end of Rk.
A grid stopper is not required here.
The IST secondary must not be loaded nor frequency corrected despite much advice to do so. Doing so ruins the sound.
If you are against using a coupling cap here then you will have to go to fixed bias.
A 0.22uF VCAP will give you everything that can be desired. You can also use the lower voltage BG caps but beware of the voltage swing needed for full power not forgetting dynamic signal overloading and associated clipping.

The OPT that you have selected will give you good extension of the highs and good *air* but not necessarily at the expense of bass output and low end punch. Very often the extended and very clean highs will fool listeners into thinking that the bass is missing. My experience of SET is that clean and extended highs are relatively easy to obtain; deep and extended bass is very much harder to achieve and comes down to the driver stage having enough current and system overhead to handle the large voltage swings. If there is not enough overhead eg minimum of 6dB, the dynamic range will be compromised and the sound stage collapses on high impact bass notes.
When you get it right, the amplifier will not sound like a tube amp at all. Once that is correct then the amp can
be voiced to suit ones own musical tastes.

Now to move on to the driver stage.
You will find that the Rp of the tube is too high to drive an IST indirectly since as I understand it, you are using an ac coupled input to the IST itself. A genuine IST 5k:5k is designed for dc consequently has an airgap. It needs between 15 and 30 mA to function correctly therefore will not be able to respond properly to ac only high value low bass signals.
There is also the issue of unwanted crossover distortion as the core flux changes direction - it is not meant to.
The good news is that a high Rp triode can drive one successfully since they l function very well with a typical pentode such as the 6V6. As an example, a 6SN7 can drive one, Rp of 7K6.
To recapituate, a plate Rp of 5K can successfully drive a 5k:5k IST when connected correctly.

Other points raised

Coupling caps - using large value BG high end types is unusual but can be made to work well. The exception is where there is NO dc polarisation as in a location such as an IST to a grid. They will fail given enough operating time.
These caps are not chemical, they are electrostatic as a consequence, they do not use metal for the windings therefore the inherent ESL is very low. The ESR is also very low which is why they work out to 100kHz.
I have used the 50uf/50 volt NP type for CD player output and they are clean and fast but only if polarised by at least 10 per cent of their rated voltage. I have a sub woofer that used ordinary electrolytics between the opamps
and all of the caps failed due to no dc voltage. The designer wrongly assumed that the ac ripple current would support
the low frequency signals. Worthy of note, the non-polarised types also failed in this application. I talked with the manufacturer and they said yes we know.

May I suggest that whilst the overall circuit choices are excellent, it is unlikely that you will be able to use the amp over the full 8 octaves therefore some rework may be necessary.
I would propose changing the driver tube to a 2A3 and arranged to drive the IST with 30mA. An IST is normally operated with the quoted mA rather than at say 50 per cent of its rated current.
A 2A3 working into an IST and driving any of the usual power tubes will give you the delicacy that the remainder of the selected tube set is capable of.
An alternative is to retain the driver tube since high Rp power tubes have an inherent sonic delicacy that is hard to give up and change the IST to one having a primary impedance of at least 10k.
I hope that this helps you.
Regards JohnR

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