300b lovers

Try removing the 56K load resistor entirely and give it a listen. You might like it better. Remember, with conventional signal sources, there is almost no ultrasonic content, so the ringing in the transformer is never stimulated. And getting rid the load resistor has no effect on circuit stability or DC stability, since the grid sees the DCR of the transformer, which is a few K at most. All the grid resistor does is make square waves look pretty. It has no other function.

Now, in a cap-coupled circuit, it is absolutely necessary, since grid current, as small as it is, has to go somewhere. With transformers, it just goes through the secondary, and the current is so minuscule there is no effect on the core.

Try removing the 56K load resistor entirely and give it a listen. You might like it better. Remember, with conventional signal sources, there is almost no ultrasonic content, so the ringing in the transformer is never stimulated. And getting rid the load resistor has no effect on circuit stability or DC stability, since the grid sees the DCR of the transformer, which is a few K at most. All the grid resistor does is make square waves look pretty. It has no other function.

Its been my experience that allowing an audio coupling transformer to ring will result in brightness, since any signal presented to the transformer can cause ringing (this is easy to demonstrate- try it!). I'd not be surprised if some people mess with the damping to compensate for a weakness in the circuit elsewhere; IME/IMO you're far better off finding those weaknesses, sorting them out and making sure your transformers perform as good as they are able!

The overshoot, as these things go, isn't bad. You might be able to zero in on it a little bit more. The thing is, the more energy the overshoot has, the brighter/livelier the presentation so if you're going for a warmer sound this is something to avoid (you can see here how easily distortion can influence the tonality of the circuit). I would not reduce the loading resistance to the point it rounds the leading edge. A slight bit of overshoot is OK when trying to hit that critical damping value.

In a zero feedback circuit you have no correction to deal with this sort of thing, so you have to sort out details like this and get them right. The reward is greater detail since distortion and detail really don't go hand in hand. You can see by doing this sort of measurement how different power tubes and different speaker loads affect how well the output transformer can perform- and why people might have contradictory observations about how the same OPT and power tube sounds, because the way the transformer behaves changes depending on the speaker load.

In my case, I compared the sound of my amplifier with different IT resistor load. 120K (Kiwame 2W) vs 56K (Alan Bradley 2W).
The sound with 120K resistor was too bright and rough, with 56K tonal balance was more natural and sound was more refined.
But with RC coupling the sound was even less bright vs IT with 56K resistor. I think the resistors material AB vs Kiwame can make a difference too.
I am afraid if I completely remove the load resistor the sound will be even brighter. So I want to try 39K AB 2W resistors (that I have in my stock) to make the sound a little bit warmer.

Alex, remember, you cannot harm your amplifier if you completely remove the grid-load resistor. With a transformer, runaway from DC instability is impossible. There is always a DC path through the secondary ... as long as the secondary is intact.

If you have an oscillation lurking in there, at some high frequency like 5 to 20 MHz, that's a different story, and unrelated to the value of the grid resistor. 5 to 20 MHz oscillations, even at a very low level, will absolutely make the sound bright and unpleasant. If they are -40 dB down, you will never see them on a scope ... that's no more than a trace width. You need an RF spectrum analyzer to sniff out the little monsters. They look like little spikes rising out of the noise floor (which should be very smooth).

If you suspect this, you need a grid-stopper in series with the grid pin, like 100 to 500 ohms of carbon-comp resistor soldered no further than 1/2" from the grid pin. (NEVER use a wirewound for a grid-stopper.) That will kill self-oscillation.

I would try a grid-stopper first before futzing around any more. The only way you can solidly rule out self-oscillation is use an RF spectrum analyzer that's good to at least 20 MHz, preferably 100 MHz. These things aren't cheap, and only have one use, chasing out RF nasties. Low-level RF oscillations are surprisingly prevalent in high-end audio equipment, with poorly designed regulators as the usual culprit.

Try the grid-stopper first before anything else. After that, play around with various value of grid resistor, including nothing at all. It should not be sounding bright, unless something is wrong.

There are now over the past few months arrived Three 300B Power Amp's within my Local HiFi Group.

Two are from the Designs done by TDP and Andy Groves for World Audio Designs, One of these Two has an upgrade which I believe has Interstage Transformers.

The other is a design from a 'Heart' of Dutch Origin.

I am now becoming re-familiarised with the 300B sonic.

With the recent experiences and the knowing there are certain designers early in their career knowledge been applied to the Amp's, it really does make the idea of listening to the Amp's being discussed in this thread, a experience to be had. 

Who knows a Pair may one day be on the UK to be demonstrated.