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-   -   KEL84 from the recycling centre (http://www.world-designs.co.uk/forum/showthread.php?t=7803)

YummyFuzz 25th September 2019 04:23 PM

KEL84 from the recycling centre
 
2 Attachment(s)
Hello,

I was fortunate enough today to pick up a KEL84 from the rubbish/recycling centre near my work. It is missing the chrome volume knob, and has no tubes, it appears to only have the wiring done for the CD input, of which the black wire is broken, the wiring is messy, and the soldering is blobby in places, but no components seem wobbly, or joints dry. It was sitting in the bottom of a trolley of hi-fi equipment, yet to be put on the shelf and priced for sale. The tube sockets on top and the sheer weight of the thing suggested some value, so I took it up to the counter to inquire on the price of it. "10 dollars" (AUD).

My usual hobby is building, repairing, and modifying guitar pedals of different sorts, with a recent interest to move into working on the several tube guitar amplifiers I have, and though not a guitar amp, this for the price seems like a good opportunity to get started with valves.

I've spent the last several hours reading what information is to be found on the KEL84 to see what I've gotten myself into, and have joined this forum to see if you can help or advise me how to get some build instructions (for purchase?) for the KEL84 as I think I'd like to back out and redo some of this wiring. Next I guess would be getting a matched pair of ECF80, and a quad of EL84 off eBay, though how discerning I need to be on this I don't know, and then run through the test and measure sections of this forum.


I do have a local Hi-Fi amplifier repair person who I'm sure I could pay to source the parts and make the repairs, but where's the fun in that.

I'm new to this forum, or any kind of forum for that matter, so please be kind.

I don't even have a set of speakers (advice for later).

bob orbell 25th September 2019 04:41 PM

Re: KEL84 from the recycling centre
 
10 dollars, you did very well, the forum Administrator Matthew Snell is the person to ask for the instruction papers, go to the very bottom of the page to CONTACT US and give Matthew an email. Valves from ebay will be fine on this amplifier, 4 matched EL84's and 2 ECF80, matched if you wish, but not really necessary. Also lots of help on this forum, and just to add, do not run the amplifier without a speaker load, speakers or a two load resistors. Bob

John Caswell 26th September 2019 08:56 AM

Re: KEL84 from the recycling centre
 
Hi
Nicely done, looking at the PCB it seems to be the Mk1, the only difference being the PCB between 1&2 and also the provision of relay input switching, which you can add if you wish at a later date.
As Bob says matched EL84s are the best way to go and if you can get them E80CF as opposed to ECF 80 as they tend to be closer toleranced than the ECF80, see my comments and FAQ. There is plenty of help here available so if you or your repairer gets stuck just ask.

John

Richard 26th September 2019 09:09 AM

Re: KEL84 from the recycling centre
 
Yes, hello and welcome :) that was a great deal!

I thought I did well getting a pair of Quad II monoblocs from a junk shop window in 1977 for 8 (about 50 today) when they were out of fashion!

If you've not already got them send me a message with your email address and I'll send the info you need :thumbsup:

YummyFuzz 27th September 2019 05:47 AM

Re: KEL84 from the recycling centre
 
Thank you Bob and Matthew. I now have the build instructions, and the valves on order (2 x ECF80 6BL8 EI Yugoslavia - NOS matched pair (Philips construction), and a matched quad of EHX EL84's). I'll also get some cheap 8ohm speakers to test with.

I spent last night removing all the wiring off the 6-way selector switch, volume pot, and the audio connectors at the back which was all single core copper wire that would have been unwieldy to work with. My feeling is that this was an abandoned kit, and may never have worked for the previous owner as some of the audio cables seemed to essentially crimped to breaking in the act of removing the shielding.

I want to get the entire PCB out but is is being held in mainly by the choke and mains transformer wiring.
What is the opinion on having a series of either plug or screw connectors between most (choke and transformers) of the onboard and offboard wiring to allow easy removal of the PCB?

I'll clean the board and reflow the solder as well as suction up some of the excess. I'll be getting some new wire, and a 100ka volume pot as well while waiting for the valves.

A part I will probably be looking for in future is a replacement chrome volume knob, and I notice that chroming the transformer covers is popular, but I was thinking just black paint.

YummyFuzz 27th September 2019 06:43 AM

Re: KEL84 from the recycling centre
 
Those Quad II monoblocs look amazing!

Richard 27th September 2019 03:47 PM

Re: KEL84 from the recycling centre
 
The Quads are lovely and seeing them still brings back nostalgic memories :)

Kel 84 is a really good amp and well worth rebuilding. Look for easy to drive 8 ohm speakers as number one priority, even before sensitivity, as if the load is difficult the response will be lumpy or missing bass etc. It is amazing the difference speaker load can make to some amps.

Older speakers were often of higher and smoother impedance probably due to the designers coming from the valve era and being more conscious of the effects of the amp/speaker interface. There are some great modern speakers too of course.

Good idea about a plug or terminal block for the pcb wiring but not so easy to do as a retro fit in practice. I certainly agree it would be a good idea for any further pcb WD amps to incorporate on the pcb.

On my WAD amps with pcbs I made life a little easier by making a common earth point on the pcb with a tinned copper wire loop to which the earth wires could be more easily soldered to :thumbsup:

YummyFuzz 20th October 2019 01:31 AM

Re: KEL84 from the recycling centre
 
Hello,

In my first test of my refurbished KEL84, the four EL84's lit up, but not the ECF80's. A quick test showed that there was no heater voltage. I've now fixed this (dry joint), but the DC voltage is reading 8.54Vdc, and given that the ECF80 wants 6.3Vdc, I don't want to proceed in case this damages them.

My setup is that I have the PCB out of the enclosure, no valves, with only the heater AC supply connected. I get 6.3Vac between the transformer white wires, and 3.1Vac if I measure between either white wire and ground. I get 8.54Vdc on the BR1 +/- side and at the V1/V2 heater sockets. BR1 tested in isolation seemed fine, and I replaced C22 with a 4700uF 16V instead of the 10V.

Is this normal/acceptable, or do I need to take steps to drop this voltage with some resistors or diodes?

snowman_al 20th October 2019 07:43 AM

Re: KEL84 from the recycling centre
 
The extra voltage on the ECF80s is 'normal' but not necessarily desired.
You must test it with all the valves in though, to get a true reading.
That way the transformer is loaded correctly and you are not measuring just the 'peak' DC voltage. It won't hurt the ECF80s for a few minutes.

Once you know the actual working DC voltage you can work out a resistor to get it between 6 and 6.6 volts. (Assume 2 x ECF80 consume 0.9A) Most people have ended up with a 1 ohm 3 or 5 watt job.
There is a 'Mod' too if you look here: http://www.world-designs.co.uk/forum...ead.php?t=7644

YummyFuzz 24th October 2019 06:13 AM

Re: KEL84 from the recycling centre
 
Thank you snowman_al, and right you were. Once I retested again with everything connected and valves in, the ECF80 heater voltage was exactly 6.3Vdc, and now it all works!

A big thank you all for the support on this forum.

I have it set up in my lounge with a pair of cheap Sony SS-D20's that I got for testing, but I have a pair of 12" 7ohm Phillips AD2500M speakers (1960/70's hand me downs from my father) I'll get serviced as I restore the cabinets. I had a look at the heater BR mod, but I can't hear any hum at all from the amp even when turned right up, so I think I'll leave as is for the time being.

What I will do, which I just noticed when I went to set it up, is that whoever built this, installed all the phono jacks, and speaker terminals in the wrong colour position, probably similar to how I didn't notice while working with the unit upside-down the whole time seeing red on top, and black underneath. Not important, but now I know it's there, it'll keep me awake at night, so out with the soldering iron again this weekend.

I'll post some pictures later, including all the socketing I did on the PCB.


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