Newby first build amp 832.

[andrew ivimey]

what's wrong with crocodile clips, just to get things going?

'swatI'veused

[pre65]

Hi-thanks to Matt (Lord) for the resistors (have Paypal'd you) and Andrews advice re the croc clips .

I have slaved away and "finished" the 832 !!!

When i plugged it in i think one of the 832 's is duff as its heaters dont light up (correct voltage on pins) and i got a noise which i think was motorboating.i disconnected the feedback wire and that noise stopped.

So i thought i would see if i could get sound on one channel.No such luck,just extremely loud hum.So really fed up now.

I am going to France for a couple of weeks on Wednesday so it will have to wait untill my return.Perhaps i can thrash it with a branch (ala Basil Fawlty)

Still i have started on the 401 plinth at last and the Fostex cabinets are looking better,so its not all bad.

Philip

[NickG]

You may hate me for saying this but...

One of the reasons that some of us don't build nice tidy amps, its that reality comes in and messes it all up when they don't work. After some messing they do, but by then the nice tidy amp looks a bit distressed.

Sorry. :-(

[Darren D]

Umm, this PX4 amp is my 1st propper tidy build and look at the problems that has caused.

I demo'd my breadboarded PX4 at Pauls and that didn't hum at all.

Darren

[andrew ivimey]

Philip, Basil Fawlty was not all wrong.

Sorry it doesn't work, but this is only 'first attempt'. Assuming that all is as should be in the circuit my first switch on with 832s was a bit disappointing. One valve spectacular but no sound and then with all things glowing, (admittedly, my own version of an SE amp,) not very good sound.

I have stripped it down, checked the stages and dug out a pair of 6S41s instead but last night - Yuk! - and all because I am too proud to use el34s!

Have a break in France.

Don't take an axe to things till (after) you get back and you may have a few more ideas. I am happy to help if I can.

[pre65]

Hi-i think my first step will be to review the earthing system i am using.

i would now like to star earth each baseplate and then connect the 3 stars to the IEC earth.

i must then source some more 832's (none on e-bay when you need them) and check,and re-check all my wiring




Philip

[andrew ivimey]

okay Philip, it does look good so definitely worth carrying on with.

832s will re-appear don't worry about that. I wouldn't be surprised if Langrex etc didn't sell them either.

You really need to check voltages. If you disconnect the HT right back at the mains trafo then switch on and 1. check the heater supplies. 832s do light up quite nicely (after about a minute) so you can't mistake that.

Without heaters you will hear nothing!

2. Check the wiring - there may be more than one mistake.

As you are using solid state rectification for HT, (with working heaters in situation) 3. switch on and measure HT. You should get a quick sweep up to a peak and then the voltge will fall back to about 50volts less than the peak and then remain stable. If it doesn't...something could be roasting! . It should be around 300volts (he guessed wildly, not using your PSU at all and only vaguely remembering the schematic).

If all 1,2,& 3 are okay, then you should get something. I found that it took about a minute to get full volume out of my SE832 but this might not mean anything.

I also got motorboating sort of noises; grid stoppers helped and some amazing v.low frequency oscillation (and a nice 8 or so Hz vibrato that would have helped my fender strat back into the 1950s,) that looked like someone was slugging one of the 832s with a lead filled snow shoe. When that was 'cured' I had lost enormous loudness (one driver stage rather than two) and it all sounded a bit like an old transistor (AM) radio. But in amongst all the conditions I tried, there was the possibility of a very high quality sound. It was there but I couldn't sustain it.

[Primalsea]

Phillip,

That looks really good, if a bit weird 1950's scifi. I was half expecting to see aguy with a bolt through his neck in the photo somewhere. Yep, always a shame when the project you've put so much effort into doesn't go well. I'm sure after a bit of reflection you will sort it.

Another possible thing to look at IMHO is to use a DMM to check for good low impedance connections on the croc clips.

Also if you think the hum is an earthing problem have a check to see if your're getting rogue earth currents from the heater grounds into the audio grounds.

Hope it goes well and have a good holiday

[pre65]

Hi-just having a last minute fiddle.

i have sorted out the output transformer and feedback wires and now when powered up there is no noise.

also the "dud" 832 which doesn't glow is actually getting warm !

the good one which glows is HOT so why is the other one only warm ?

if the gas had escaped would the heater filaments not glow ?

when i have finished my apple crumble and custard(yum-yum) i might try connecting the good channel up again.

Philip

[Primalsea]

Compare the resistance of the heater of both valves, are they the same? If the dud has higher resistance for some reason it won't get as hot as the good one.

Also might be worth checking for shorts between the heater and the other pins.

I'm not familiar with 832 valves but most, if not all valves are called Vacuum tubes because they dont have anything (Vacuum) in them. This is so the electron stream does not collide with any gas particles which may deflect them.