transformer noise

[vitalstates]

Hi Greg

many thanks for the info...been a bit hectic here so I hadnt got around to chasing that up......

Steve.... I thought you were on Holiday otherwise I would have reported back........Got the mullard GZ32s in this afternoon..wow it all came together. Also switched to 6sn7 for the driver, not sure about the sonic change here.

so many changes my heads spinning.

son of rocky has been connected to the stats for the last 3 days. This proves good for observing changes in the configuration ...but the music is pants. Son of rocky is not up to driving stats. All the detail is there and it sure goes loud(not a hint of distortion) but its extremely two dimensional and flat. Having said that the bass is very tuneful and definately equal to the wad6550.

back in the studio connected to the ariels, now this is a different matter. We're rocking now!!!!Its hard to believe that rectifiers make so much difference.... I just love this sound. The speakers have completely disappeared, brilliant stage and its definately got front to back depth.

Now the wierd bit...back on thread...I took the the first ps cap back to 8uf as per James suggestion and the transformer still made the noise. Curses! I then took the amp back to the studio and fired it up......No transformer noise!

The only thing I can think of is that the mains conditioner(3 stage 3kw) in the studio makes a difference to the transformer. Anybody got any comments or ideas about this?


Ed

[Darren D]

Does the mains conditioner change the input voltage by any chance, both of mine do, one higher one lower, as much as 20V

[The Shadow]

Hi Ed

As James says I would definitely go back to using 8uf capacitors.

I stayed with 8uF when I doubled up my GZ37s and the mains TX now gets slightly warm when before it was cool even after a full day's action.

I'm dropping my preamp first cap tomorrow to 8uF too because as you know my HT off the end of the PSU is rather high due to me using a Hammond 372BX meant for a power amp. It is getting too hot for comfort with 33uF.

Nick suggested dropping the first cap about a fortnight ago and I can see what he means now.


Steve.

[James D]

Two possibilities, I guess, one as per Darren's post - the mains voltage is lower in the studio.

"nd is that your mains has a vary poor mains shape with a triangular waveshape that is generating a higher peak field through the transformer than the filtered supply in the studio.

The difference between the Mullards and the others is due to different equivalent resistance of the two types and a slightly higher peak current with the GZ32s.

caio

James

[vitalstates]

Originally Posted by Darren D

Does the mains conditioner change the input voltage by any chance, both of mine do, one higher one lower, as much as 20V

Not sure Darren, I havn't measured it before but I'll get on it.

Hi Steve
I have gone back to the 8uf and I think I'll leave it there now. I will do a better comparison between the 2 drivers now that the amp is back in the rack.

Ed

[steve s]

Hi Ed, glad the gz's work ok, I remember the difference when simon pluged one in his amp, the old stock output el34's will make a difference too.. more 3d ...try them when i get back..

we're going to day...
the lads at home looking after things.. there should be a few good party's whilst we're away... mmmm

I was thinking those kt 44 's/vt75a I showed you.. they have the early kt66 anode....and should sound very good...

a kt 44 is a kt 66 with a 4 volt heater, old type base and top anode connector
the old st shaped kt66 go for a fortune.. i have a pair..would be worh trying, if there a marked difference you could then look at mods for the kt44, as i have loads of them..?

bring the amp round in july and we can have a go at trying what i have..
cheers steve

[The Shadow]

Hi Ed,

I put the preamp back to an 8uF first cap this morning and the preamp mains transformer has quietened down considerably. Heat is no longer an issue. Although the 374BX transformer gets quite warm, it did that even when it was on my Loftin White.

I've also had a look at the specs for the rectifiers (5U4Gs) and carried out Nick's suggestion of placing a series resistance between the rectifier centre tap outlet and the first cap to damp any current spikes.

My data book specifies a minimum series resistance of 70R for this rect.

I have 117R (4x470R 10W wirewounds in parallel) in series with the PSU.

I get a lovely soft start to the pre now, a gentle hum that fades to silence after about 5 seconds.

I'm sure the two 6SN7s will appreciate it.

Steve.

[vitalstates]

Hi Steve

I've been wondering about the rectifier resistors. The specs for all the rectifiers give 1st cap figures related to resistance in place. As I dont have any resistance(other than in-situ transformer resistance) do you think I should be bothered? As you can see I'm like a Jack Russel with this power supply business.

Ed

[The Shadow]

Hi Ed.

James might correct me here but if you look at your circuit diagram, with a full wave rectifier using centre tapped secondary and heater feeds the transformer resistance is across the rectifier rather than in series.

The data specifies a resistance in series with the rectfier output, either from a resistor in each leg of the cathode feed or as in the case of Hammond transformers in line with the centre tapped output from the heater windings.

Phillip at Bluebell specified a 47R wirewound resistor for this purpose on the Loftin White 2A3 amp of his that I built, and advised me to do the same when I built Rocky.

According to Phillip it is supposed to limit the current spike at startup and increase the life of the rectifier.

As I said, my preamp starts up beautifully now, so there can't be any harm in trying it.

Steve.

[NickG]

James might correct me here but if you look at your circuit diagram, with a full wave rectifier using centre tapped secondary and heater feeds the transformer resistance is across the rectifier rather than in series.

I think if you trace the path you will find its in series, remember only one half conducts at once.