EL34 power amp project

[Richard]

Good to hear you've sorted the noise Andy. All I could think of was possibly the chassis has a sympathetic resonance at mains frequency and is acting as a sounding board. The chassic top looks well supported; do you have a base plate which could be damped or left off for a trial?

[bikerhifinut]

The red plate effect was a faulty EL34 grrrrrrrr.
I have heard these valves are a bit fragile compared to El84 and Kt88 types and they are getting well battered in this amp.
So its a new set of EL34's............. or and possibly cheaper as I have a stash of them, Kt88/6550/KT90. just need to fiddle the putput stage a wee bit and tweak the feedback. I'll use the Leak TL50 circuit as the input and driver stages are pretty much identical and all it will cost me is a handful of resistors. I have a matched quad of GE genuine USA made 6550 crying out for a good home.

The transformers are still a bit "tizzy buzzy" but I reckon i have got em as good as I can.

Ah well its late and tomorrow is domestic duties and then feet up with a beer....................

Andy

[Phil Y]

Hello Andy,

Glad to hear you have the noise down to a better level. What wax are you proposing to use on the old transformers ?
Just to say, as others have, lovely looking amps.

Phil.

[John Caswell]

Hi all,
A lot of modern txs are just varnish dipped as opposed to vacuum impregnating, and also if the windings are a bit loose, say a bad day for the winder, then rattling/humming will result.
I have had several that I have sent to a transformer manufacturer for vacuum impregnating that have come back as silent as the grave.
You pays your money.......

John

[bikerhifinut]

Originally Posted by John Caswell

Hi all,
A lot of modern txs are just varnish dipped as opposed to vacuum impregnating, and also if the windings are a bit loose, say a bad day for the winder, then rattling/humming will result.
I have had several that I have sent to a transformer manufacturer for vacuum impregnating that have come back as silent as the grave.
You pays your money.......

John

Oh I paid my money allright John.
I got them from Majestic in Poole, a company with, as you probably know, a very good reputation for custom builds and replacements and rewinds for classic amps.
They were proper vacuum impregged and baked, and when I sent them back they did it again for good measure.
They came back with the comment that the noise was "acceptable". There's still a small amount of residual "tizz" that I reckon is coming from the windings. I have to say I am rather OCD about noise both mechanical and electrical, and for me nothing other than silence is an acceptable amount of background noise. In a quiet room playing low level music or even just between passages of music, I find any sort of audible noise or hum carries a long way. I am cursed with decent hearing acuity.
I will probably take them to my local transformer guru in the new year for an opinion and take it from there.
My other main problem is the very high mains voltage here, seldom below 240V and usually between 245 and I measured it at 253 last night. This is giving me headaches with HT and heater voltages. I don't think 7V on an EL34 heater is good for it. And my attempts to control the HT with anode resistors on the GZ34 have been a bit hit and miss as the results dont tally up with my calculations, meaning I have to have a stock of high power resistors and install them on a "suck it and see" basis. Totally unscientific.
I might just throw in the towel and if I can get away with a 3A 5V heater I may rework the PSU and just use a 5U4G in each monobloc. It will mean using a smaller capacitor off the cathode and as it happens I have a couple of polypropylene 4.7uF at 630V in the toybox. or I guess I could buy a couple of 10uF electrolytics. Not sure how that will affect the rest of the circuit but if I parallel the 50 + 50 uF to give 100uF after the choke I should be ok?
The other worry is the lack of slow voltage rise with Directly heated rectifiers which could mean 600V on my 500V rated electrolytics and that means replacing them also. Not a trivial modification then.
The consolation has been that the circuit itself works and sounds really well.
The reason i am unhappy about anode resistors on the GZ34 is that they run bloody hot. I calculated they should not dissipate more than 5W in the worst case scenario with 255V mains and around 460V coming off the nominally 410V secondary. But the 10 Watters i have in at the moment on each anode have got hot enough to burn off the printing on the resistor body. White "coffin" cement cased jobs. And something tells me large amounts of resistance in series with the HT isnt a good thing.
However they are running low on the HT at 390V on the anodes with this value and I figure i should go with the original 270 to 330 ohm I had with these transformers which seems to get me close to the 460V I want off the cathode of the rectifier.

Andy.

[bikerhifinut]

Transformer noise still too high.
It's not just me, the missus is distracted by it.
It's just a matter of what action to take now.
Obviously the first thing is to check them out of the Chassis in case there is a sort of coupling or other effect that can cause the noise.
I somehow think I am going round in decreasing circles.

The projects on hold for other reasons too, Voltages all over the place mainly due to our excessively high mains voltage at night. Varies from a low of 235V to a maximum of 253+V which is within the 10% allowed........ just.
But I have been assured by another that the higher voltage should not cause any issues with the EL34's.

A

[bikerhifinut]

Originally Posted by Phil Y

Hello Andy,

Glad to hear you have the noise down to a better level. What wax are you proposing to use on the old transformers ?
Just to say, as others have, lovely looking amps.

Phil.

I have been advised by someone who rewinds transformers that paraffin wax is the stuff to use.
I am trying to find out if there are different grades/melting points as it's got to be more than just melting down a couple of boxes of "prices candles".

A.

[bob orbell]

Maybe worth trying the transformers bolted down hard with no rubbers or gaskets, failing that, get in contact with the makers and kick ****. Do they still make a lot of noise with no secondary's connected, can't think of any thing ells that may help you, sorry. BOB

[John Caswell]

Hi all,
You will need resistors in the anode feeds of the GZ34 to make up minimum transformer resistance as per Mullard, to avoid damage if you hot switch
I use the aluminium cased types bolted to the chassis in my 50 watters (feeding a par of 6D22s) and yes they get hot but nowhere near max dissipation rating.
Personally not very keen on the white ceramic cased jobs, I have had a lot of trouble with them going H/R in several commercial amplifiers. I prefer the Welwyn/Vishay green vitreous enamelled types, I also don't find the Mills types very good under high dissipation.
As you no doubt know paraffin wax is nothing like candle wax
Just as a thought is the chassis aluminium or steel?

John

[bikerhifinut]

answers as follows:
Bob: I have tried bolting down to the chassis without compliant washers or foam tape, just using the fibre washers to break the magnetic circuit and the result was the worst with the chassis acting as a soundbox. The best result so far has been 4 tap washers of all things, one under each mounting bolt.
They have been back already to the manufacturers once and had an extra impreg and bake, and I have been told the noise is "Acceptable".

John:
Thanks for the heads up on the wax, I didn't know the difference in grades of paraffin wax but had a suspicion and so far internet research hasn't thrown up any edifying results or sources.
The chassis is Mild Steel, rather a heavy duty beast. I was a bit unsure about using steel as I have read during my initial research before starting the build that Steel chassis need a bit extra care in view of magnetic fields and coupling of noise and hum etc etc. Thats why the original transformers were mounted via an insulating adaptor gasket and the output transformers are mounted on grommet spacers to make an air gap off the chassis.
I think modern construction methods have a part in this also. I reckon that it sounds like bobbin noise to me.
Thanks for the info on WW and high power resistors, I aint too fussed on the white cement ones meself, they seem fine as cathode resistors but in the PSU I think something else. I need them wire ended as i have no space to bolt metal cased ones to the schassis.
Anyone recommend a good supplier? I need to get a stash of 2watt metal films too, and some high voltage electrolytics with a decent ripple rating 630V.
Apart from they never stock the right values, Maplins are getting very expensive now. And HiFi collective are ok for some items but I dont want boutique resistors, I'm happy with garden variety metal films.

A.