300b O/P transformers

[pre65]

Hi All

Today i have been "playing" with the 300b amp that i have been using to drive the 845 project.

I have clipleaded in the Ask Jan output transformers that were originally fitted,but this time set to 8 ohm for the speakers.

They can be wired up for 4K or 5.2K on the primary but they sounded the same to me,should that be so ?

As a further experiment i have connected the 6.8K output transformers i am using for 845 and they sound good as well.

It has been said before that the output transformer resistance is not of paramount importance which this "test" confirms.

Now the sound seems less prominent in the mid range than i remember but it now has DC heating for the 300b,is that likely to be the cause ?
I am tempted to try the DIY hi-fi supply heater module for 300b and will be interested to see how Steve gets on with his PX25 amp when his ones arrive.

[Nick]

Using higher reflected loads will tend to decrease distortion, but reduce volume, as long as replacing the extra volume doesn't increase distorion beyond the starting point there should be a improvement. But in my experence its not something that you can hear as its mainly the level of 2nd harmonic which is benign in sound.

It might be the heating, I find that AC heating can tend to muddle up the midrange, but thats just me YMMV.

[Andrew]

It might be the heating, I find that AC heating can tend to muddle up the midrange, but thats just me YMMV.

That's an interesting observation.

I have always wondered if AC heating was truly blameless beyond the usual heater hum we all know and love.

Does alternating heater current cause some sort of odd (diode?) effect that generates harmonics at 200hz, 400Hz and again 800Hz and even 1.6khz?

I would expect the fundamental to be at 100hz, would that be right?

-- Andrew

[Mike H]

It might be the heating, I find that AC heating can tend to muddle up the midrange, but thats just me YMMV.

Is that a fact? *rushes off to investigate DC heater supply*

How about noise on the mains? Might be amazed what's on there if you put a 'scope on it. One time when I did it was like a very bad square wave

[Nick]

Yep, my mains is very poor, varies in level a lot, and has harmonics all the way up to 10k or so, and they vary during the day. Think thats why I spend so much time messing with power supplies.

[richardcooper2k]

that would fit with using small value chokes on heater supplies. they don't get rid of all of the 50hz wave but they reduce it to a fairly inocuous nice smooth sine ?

[Mike H]

Yes that's a good idea, maybe like those 1mH high current ferrite bobbin types. Have one on each side

Yep, my mains is very poor, varies in level a lot, and has harmonics all the way up to 10k or so, and they vary during the day. Think thats why I spend so much time messing with power supplies.

Same here! You just reminded me one time I turned it on and there was this background buzz, like it had a bad earth or something. N-no...

Only need some guy up the road using a variable speed power tool and we get the triac switching pulses *huh?*