Heater Transformer Upgrade

[soulminer]

I am tempted to remove the heater wiring for the 300Bs in my 300B PSEs from the original mains transformer supplied windings to some seperate 230v/5v 20VA EI transformers from RS. I have read that doing this can not only give the original mains transformer an easier time (2.4 amps less it has to pump out), but can also reduce hum induced in the heater windings from the HT windings on transformers without winding shielding.

My monoblocs are deathly silent on switch on with the hum buckers tweaked, but get slightly louder and louder as they heat up. I think this hum is on the heaters of the 300Bs as it is there with all other valves removed or the inputs shorted out with valves in. It seems less noticeable with a pair of TJ meshplates fitted than with a pair of EH GGs fitted. Not sure why.

I was going to fit the RS transformers inside the chassis at the back, and use the two 5v AC outputs to heat each DHT. I was hoping that not using the other two 5v windings would keep the original WAD mains transformers slightly cooler and maybe even reduce mechanical noise/hum slightly too.

I love the sound of the amps and the EH GG 300Bs (and their low cost), but am getting a little tired of this low-level drone when they have been on for an hour or more.

Would this little modification cure this do you think, or would I be wasting my time?

Also, the RS transformers are 230v primary -> 2 x 5v secondaries @ 20VA giving an output current of 2 amps. Is this per winding or for both windings in parallel (ie each winding can supply only 1 amp)?

Obviously the 300Bs need 1.2 amps each. Are these RS transformers a little weedy or will they do the job? I have seen a 30VA transformer on eBay that supplies 3 x 5v at 2 amps each, so I am guessing that the RS one will supply 2A each too with each 5v/2A winding being 10VA. Am I correct?

The RS stock number is 805-029

Cheers,

[Ianm2]

well I haven't looked at the rs one, but I know rs stuff is very good, so even if the ebay one is rated higher, you can't rely on that

its a very good idea for a dedicated heater tx, that's a lot of current.

don't forget tube rectifiers draw about 3a heater, too.

but will it cure it? no idea, I suspect not.

as to the 20va, I suspect that's the rating for the entire tx.

if you can post the ebay no. I will hazard a guess if its any good....

had a look at the rs, no idea, I suspect its going to buzz like mad and get hot, get the ebay one, can't be much worse, RS one reminds me badly of one I bought and it did that

nuvotem are good, they stock them...cancel that, those ones are the only ones with 5votls

http://rswww.com/cgi-bin/bv/rswww/se...cape&Nr=avl:uk

[NickG]

I take it you have tried re-tweeking the hum buckers after it warms up?

[soulminer]

Originally Posted by NickG

I take it you have tried re-tweeking the hum buckers after it warms up?

Hi Nick.

Yes. The hum buckers if moved either way give a different type of hum. It's almost as though the hum on the speakers is a different frequency, but I don't have a scope to measure it with. Either way, the hum buckers, once set are in the optimum position regardless of heat or length of time the amps are on. I think the hum I can hear is from the heater windings. I am loathed to go to DC for the 300Bs as they were not designed for use with DC and I have heard that it robs the music of it's life as well as shortening the life expectancy of the valve itself. I would like to keep the heating AC, but remove that low level hum.

Ian, the transformer I saw on ebay was a 110V primary, so not really of any use to me, and it was rather pricy too as I would need two of them. The number is 320104979958.

Cheers,

[Ianm2]

hi lee, give it a try if you want, its not too pricy.

I think it will run hot and buzz tho'.

see if farnell have anything good, too.

try ebay for some oversize old potted ones when they were made properly, but they are hard to come by

[NickG]

You might be better getting a couple of hamond fill TX's

[soulminer]

Originally Posted by soulminer

I think this hum is on the heaters of the 300Bs as it is there with all other valves removed or the inputs shorted out with valves in.

OK. Not sure if I have made a cock up here, but just decided to recheck actual hum levels of the amps using a Fluke DVM. With all valves fitted and warmed up after being on all day, it is reading 0.010V (10mV) AC on the right amp and 0.007V (7mV) on the left amp. A bit high me thinks.

Now - removing the 6AU6 and ECC82s, both amps come down to 0.001V (1mV) and they are completely silent. Just how they should be. I think this may indicate that the 300B heater windings are actually OK - maybe? However, replacing JUST the ECC82, the hum levels increase back up to 10mV on the right and 7mV on the left. There is no difference with the 6AU6 in or out.

Does this mean that I need DC heaters on the ECC82? Does it mean that I am picking up hum on some of the associated ECC82 wiring or is this all just a red herring? The ECC82 is still AC heated directly from the transformer, and its B+ (along with the 6AU6's heater supply) is via the original, crappy rectifier packs that are supplied. Could these rectifiers be the problem rather than the 300B heaters? Surely it would hum all the time if it were a wiring or wire routing problem? Could it be the cheapo rectifiers getting hotter and hotter and getting noisier and noisier?

Sorry if this seems a bit silly after the original title of the thread, but I just realised that I don't think I recall testing with just the ECC82 fitted in the past, and probably tested with just the 6AU6 removed. Doh !!!!