EL34 power amp project

[colin.hepburn]

Hi Andy
For Resistors try the links below

For 1/2/3 watt cheap and fast http://www.bitsbox.co.uk/index.php?m...cPath=83_89_90
Or try cricklewood
http://www.cricklewoodelectronics.co...Resistors.html

[John Caswell]

Andy,
The four main suppliers RS, Farnell, CPC and Rapid all stock various types of high power vitreous enamelled resistors, plus of course the ones Colin suggests.
I think CPC and Rapid do not need you to open an account with them.

John

[Greg.]

It is also now very easy to open an account with Farnell covered by a credit card. Also, no minimum order and still free delivery. A very user friendly efficient service.

[bikerhifinut]

Originally Posted by Greg.

It is also now very easy to open an account with Farnell covered by a credit card. Also, no minimum order and still free delivery. A very user friendly efficient service.

Sounds useful,
Cheers greg.

A.

[Greg.]

No problem, Andy. They are great for all basic components and several less than basic ones too. I have recently used them for caps, resistors and tools at very competitive prices. You should get the same service with CPC as they are a sister company. RS still require a 'trade, account but you can simply give yourself a company name to set this up and they have a minimum order of £20 for free delivery. I have never needed to use any of the other suggested companies as I have found for my use, the ones I listed have been adequate. Obviously I still go to the likes of HFS for boutique components but for basic stuff, at present, Farnell is my favourite.

One point on customer service. A while back I was doing a rebuild for someone. I put in an order to RS, requiring next day delivery to complete the build on time. When the order arrived, four components in the order were missing. Total value was about £2. At 10.00am I phoned them about this dilemma and they motorcycle couriered the missing parts to me from Birmingham to Bristol before 1.00pm. Fantastic service that cost them a lot more than it cost me, but enabled me to meet my deadline.

[bob orbell]

A very similar thing happened to me with a RS order some years ago, fantastic company, had my account about 10 years now with RS, may I also comment about RS and Farnel web sites, the former is very easy to use, even too me, , but the latter is in my opinion, horrid, also I think Greg may be wrong about minimum orders from RS, but I will stand corrected, said the man in orthopaedic shoes. BOB

[bikerhifinut]

Thought I'd kick this one off again.

After the internal pyrotechnics in a GZ34 incident you will know I did some head scratching. It turned out both amps had rectifier anode resistors gone open circuit. I couldn't see any wiring faults so a bit of research and asking silly questions brought me to the conclusion that i was using poor quality wirewounds and also i was running them on the edge of their wattage rating.
I've replaced them with 14watt WW tycos from RS online and so far touch wood no dramas.
Now heres a thing. The transformers still buzz a bit but nothing like they used to, although having half a cycle down when one resistors blown, thus a half wave rect is gonna stress everything innit? That said they are still noisier than I like. I tried lifting the amp by grasping the transformer and just taking the weight.............. the noise dropped by a considerable factor so i am wondering if I have them too tightly fastened down. Theres a definite mechanical coupling through the steel chassis and a resultant amplification of the buzz.
I also now know for sure that Majestic do NOT vacuum impreg their products. I asked them not long ago, and they just varnish dip. Combine that with poor windings not tight enough round the former and it's a hiding to nothing. I have been told that even if I found someone who could vacuum impreg them that it wouldnt work as the varnish wouldnt get past the double thickness of existing varnish around the windings.
I think I have few options. I could rebuild the amps on a pair of nice wooden and ally chassis from Bluebell in the hope that the would would deaden/damp any vibes, but no guarantees.
I could try and find a company that would attempt to encapsulate or impregnate them at what cost?
I can make or get made a cover for the transformers with some sound deadening wadding or felt type substance plus a few ventilation holes.
This last option strikes me as being worth a punt, along with mounting them less tightly to the echo chamber ooops sorry, chassis.
Now I have been looking at some mods to the circuit too.
I'm getting a wee bit of hum, sounds like 50hz, and I think its possibly down to having the input socket at the back, with a foot or so of screened cable running to the first valve. So i'll put the input socket at the front with an inch of wire to V1. Also i have been thinking about the earth lifts, I'm only using 2,2ohm resistors and i wonder if that's insufficient to prevent an earth loop? So i'll get hold of some 10 ohm jobs and see. I think 100 ohms might be a bit unsafe for the purposes of earthing but as the chassis will always be at mains earth potential that may be unfounded? I am happy with my heater wiring and the signal valves run on regulated DC.
Why i hear you ask, am I still persevering with these amps? Simply because every time I put them in the system, it sings like nothing else I have put there. I can't explain it rationally but there's a sort of "mojo" to the sound, it opens right out and has an almost visceral feel to it on good source material.
probably inaccurate but it makes my feet tap!

ok chaps enough.................

A.

[bob orbell]

Hi Andy, could you fit these dampers under your transformers, RS No 360 2867, I have on my ceiling a punker fan that was bloody noisy, so I dismantled and fitted 6 of them between fan and base so the fan hangs on the rubbers, now silent of course, it will look stupid but if it works you could do as you have said and have two covers made, I may be able to help with that. BOB

[Richard]

Hi Andy, can you post the circuit? It sounds like it's being run pretty much on the limit. Have you tried increasing the EL34 cathode Rs to lower the current, perhaps try 560R if they're 470R at the moment? They, or 680R would let you try KT88s in there too.

[colin.hepburn]

Hi Andy/Richard

As Richard has just suggested
Is what I did with my JE labs single ended EL34 Except I added a switchable bias Resistor
I have 470R for the EL34s and 680R on the other side of the switch allowing me to use KT66/6L6/5881s running at about 62mA for the EL34 and around 56mA for the KT66/6L6/5881s
Colin