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bikerhifinut 18th Oct 2015 11:07 pm

EL34 power amp project
Now that I have got the chassis sorted and painted etc, I am about to start on a build of a pair of monoblocs based on the Leak TL25+ circuit.
I know there are other circuits but I like the TL25+ and these will be my idea of an Homage to Leak.
There will be a couple or so differences, mainly down to the power transformer, which has a somewhat higher HT at 410-0-410 as per the Mullard 5-20. I see no real problem with this as any issues will be more or less sorted out and the output section will resemble the 5-20 in a lot of respects. I will need to tweak the feedback values as well to suit the output transformer which has a single 8 ohm secondary.
I will keep you informed as I go along (slowly) and hopefully there won't be too many cries for assistance.
The Valve line up is EF40, ECC81, 2xEL34 and GZ34 rectifiers, but the mains transformer will allow me to use a 3amp heater valve such as the 5U4G which may be a crude but effective way to lose an extra 30V HT if it's too high.
I chose the EF40 because they are relatively plentiful still and many times cheaper than the original EF86 and I don't need to calculate different circuit values as they are effectively the same valve.
No "boutique" parts will be used, just ordinary 1% metal film modern resistors, observing the areas where the voltage rating is critical. And wirewounds on the high power resistors.
Capacitors are general run of the mill decent quality electrolytics and polypropylene films from my stock of BVWS shop purchases and ones from the toybox.
More to follow and photos too.


Radio Wrangler 18th Oct 2015 11:34 pm

Re: EL34 power amp project
If you want to be close to the Leak, then you'll need to worry about the recipe for the output transformer.

I remember Ed saying he could get close.

You only need an 8 Ohm output nowadays, but which tapping is the Leak's feedback taken from, you might need an extra winding just to keep that the same.


bikerhifinut 19th Oct 2015 12:01 am

Re: EL34 power amp project
As I mentioned, I think the output transformer is nearer to the spec for the 5-20. It's suited to the EL34, but can also be used at up to 50 watts output from a pair of KT88 according to the spec sheet i had from Danburys. So I am sure it'll be fine.
The Leak circuits always took the feedback from the 15ohm tap, so there will be a recalculation of values. Shouldn't be insurmountable.
It's more of an "Homage" to the Leaks, I am perfectly aware that I will have to make adjustments.
The layout is going to be my biggest challenge, and I don't intend to rush it. I have proved to myself that its possible to get a totally hum free power amplifier using AC valve heaters with my "virgin" project of a Single ended EL84 power amp, so this one;s going to be a challenge. I get a bit obsessed with earth routings and heater wiring, but I reckon if I get that part right that's the job mostly cracked.
I confess to a certain amount of "twitchiness" but there's only one way to learn. I have built a KT88 based stereo power amp from a kit so the construction and soldering techniques aren't too daunting. And I'm well conversant with the dangers and pitfalls of playing with High voltages so no need to worry about my safety ha ha.

bikerhifinut 20th Oct 2015 8:56 pm

Re: EL34 power amp project
Ok, Chassis Painted, adaptor plates made for mains Transformer. Hardware fitted to one chassis.
I will take some photos and post tomorrow if anyone is interested, before I do any sort of wiring.
I am trying to work out the best way of wiring the heaters, bearing in mind the basic principles. i.e. Twisted wires, referencing to Earth via the CT and tucking the wiring into the corner edges of the steel chassis. As I understand it from reading various tomes, in particular Morgan Jones' books, it's essential to feed the Power output Valves first and then the phase splitter and finally the input valve so as to reduce the current flowing in the wire and presumably the hum field? All assuming parallel heaters which these are.In order to do this, I think it will be necessary to run a twisted pair along each side of the chassis as the EL34's and effectively have them fed by separate wires from the 6.3V taps. Am I on the right track? It will also reduce the load on the heater wiring I imagine. And will it be better to run separate heater wiring to the signal valves or, as I suspect, make no material difference to run a feed off the EL34 heater pins?
I may seem to be asking schoolboy questions here, But I am keen to make this amplifier work as well as it can within it's constraints.
Thanks in advance gents, all your comments are very much appreciated.


mole42uk 21st Oct 2015 9:46 am

Re: EL34 power amp project
I've tried running the twisted pairs from the transformer to the output, phase splitter and onwards as you say, but then returning the pair from the input to the transformer - 'ring main' style. Don't yet know if it made any difference!

bikerhifinut 21st Oct 2015 9:27 pm

Re: EL34 power amp project
5 Attachment(s)
Ok first batch of photos

bikerhifinut 21st Oct 2015 9:30 pm

Re: EL34 power amp project
5 Attachment(s)
and the next lot. In reverse order ooops. But should give you an idea of progress so far.

joebog1 21st Oct 2015 9:53 pm

Re: EL34 power amp project
Nice paint !!

er hmm with respect sir, wouldnt it be nicer to use countersunk screws in your tag boards ? Keeps components nice and flat :-)

As far as heater wiring goes, I run separate heaters for the EL34's and the input stage.
If you want really low noise, use balanced mic cable for the splitter/preamp valves and earth the screen to the star point. Twisted wire for the EL34,s.

bikerhifinut 21st Oct 2015 9:54 pm

Re: EL34 power amp project
As you can see I had to make some "gaskets" so that the mains transformers would fit the slightly oversize openings. I used FR4 PCB and painted it the same colour as the chassis. Seems to work fine, the weight is still borne by the steel chassis, the gaskets merely close the gap left at the sides.
I also mounted the transformers on plastic grommets, this hopefully will give magnetic isolation from the steel chassis as well as offering a bit of accoustic isolation.
So far I have wired the heaters in for the valves and also the rectifier, nicely tucked into the chassis sides/corners.
The job has ground to a temporary halt as I have decided to source a pair of chokes for the monoblocs, I decided that although the Leak circuit used RC filtering on the PSU, I'd be better using a Choke. Tossing a coin as to whether to use the 200mA x 10H Danbury choke if it will fit ok, or the cheaper (half the price) Hammond 5H x 150mA Choke. The total current draw should be 145mA so I figure either will be fine. My gut feeling is to go with the Danbury choke as it is a match for the mains and output transformers.
The eagle eyed will also note I haven't made the Earth Bond to the chassis yet, I will be making a dedicated point near to the mains socket. I intend to lift signal earth/0V rail with a 10 ohm resistor in parallel with a .047uF capacitor to avoid earth loops with source equipment. This is how my Kit KT88 Amp was grounded.
I have also decided on a "Star" earth for 0V rail by using a loop of thick copper wire between the -ve tags of the PSU capacitors, again a technique stolen from the kit amp I built. I worked out I will need to run 4 separate earth returns to the star from the signal and power valve sections. This is a technique that has worked well for me and although not as artistic as a "bus bar" and discrete component hard wiring, I feel the tag board sub assembly and star earth has advantages, especially if mods/repairs have to be made. I may yet move the input RCA socket to the front of the amp and open out the hole that was intended for the indicator LED. This would put the input more or less directly underneath the input valve and I feel improve matters in respect of sound quality and hum reduction.


bikerhifinut 21st Oct 2015 10:09 pm

Re: EL34 power amp project

Originally Posted by joebog1 (Post 794223)
Nice paint !!

er hmm with respect sir, wouldnt it be nicer to use countersunk screws in your tag boards ? Keeps components nice and flat.

As far as heater wiring goes, I run separate heaters for the EL34's and the input stage.
If you want really low noise, use balanced mic cable for the splitter/preamp valves and earth the screen to the star point. Twisted wire for the EL34,s.

thanks for the compliment, it was standard acidic etch primer onto the plated steel, followed by "Plasti Kote" (Now Valspar as i discovered when buying extra cans) Acrylic "enamel" in Antique Gold. I could have saved a few quid by using Black or another colour but I liked the gold. Then I just sprayed it with a standard Lacquer. And it's the first time I have sprayed paint, you don't want to know about my "practice piece" or the rubbing down of one chassis when I spotted a mistake...........................

I couldn't get hold of any M3 countersunk screws easily so used what I could easily obtain. Not sure what you mean by keeping components nice and flat, the tagboards are mounted on insulated hex pillars and so are held above(below?) chassis and theres plenty of room to get resistors and caps on. I like to "stand off" resistors even if they arent dissipating any power just to give them as much ventilation as possible.
yeah balanced mic cable, I know the stuff and that is EXACTLY how I would wire the input if I used the rear phono socket. Although I figure I wouldnt need the extra wire as the return would go through the star earth in any case. Can somebody tell me the error of my thinking?


joebog1 22nd Oct 2015 3:47 am

Re: EL34 power amp project
I actually meant that you should use the balanced mic cable as heater wiring!!
That way almost ANY layout will be noise free. At some point the heater wiring will have to attach to the valve bases, and it ALWAYS seems to occur near to sensitive areas. The screened balanced cable takes care of that. Any balanced mic cable is twisted pair inside a screen, and then inside a nice "pretty" plastic outer casing. As you only need a few hundred milliamps current, and the run is short, you can use cable that is only 3 or 3.5 mm diameter outside. makes VERY neat and quiet wiring.


bikerhifinut 22nd Oct 2015 4:06 pm

Re: EL34 power amp project
Wish I'd read that before i did the wiring! I could always undo the signal valve heaters. But I am using tightly twisted ( I used a hand drill) ) 0.6 mm solid core into the signal valves and the sockets are close to the chassis edge. The wires come in at right angles to the heaters. i made sure the EF40 socket faced so that the heater pins (1 and 8) are nearest the chassis.
I think I have done as much as I can. But I will consider the mic cable option. I managed to build a stereo Single Ended EL84 amplifier that's totally hum free using AC heaters so I am reasonably confident of my ability in that department.
Famous last words!


bikerhifinut 22nd Oct 2015 4:20 pm

Re: EL34 power amp project
I want to use a choke in the PSU rather than RC smoothing. I would have preferred to use the Danbury 10H 200mA choke, but the chassis isnt deep enough to take the thing, its 60mm deep as measured and the danbury choke is 58mm high.
I found a Hammond one, 5H @150mA DC resistance 105 ohms and it will fit nicely underneath. And its got the advantage of being less than half the price of the Danbury choke.
Can't see what I have to lose really, so I am going to order a pair. That will sort the power supply out.
I havent calculated the voltage coming off the GZ34 yet, and I have the option of using a 5U4G to drop a few volts if necesarry.


bikerhifinut 22nd Oct 2015 4:59 pm

Re: EL34 power amp project
I ordered a pair of Hammond Chokes from Bluebell audio. 5H @150mA. That should sort the power supply out.
Very helpful chap.


joebog1 23rd Oct 2015 8:00 am

Re: EL34 power amp project
Chokes are the way to go. BUT they are "soft" and supply that slightly saggy HT line that gives the valve "warmth".
The hammond chokes are OK, but they are not made like they once were :-(

5 Henries is heaps, especially if you heap the uF's up AFTER the choke!!
The little 6BW6 amp that I recently built uses a voltage doubler, ( 260uF plus 260 uF) into a choke, then into 680 uF :o WHY ?? because I have a heap of them.
The choke is the saviour of the rectifier, be it sand ( solid state) or vacuum! it adds ohms as well as impedance, especially at switch-on.


bikerhifinut 23rd Oct 2015 9:48 pm

Re: EL34 power amp project
Thanks joe.
I will try and make room at the PSU end to add extra capacitance after the choke. I had Audible success doing this with a Leak stereo20 adding an extra 150 uF after the first filter resistor, and it seems to give a little extra weight to the lower frequencies without it sounding "BOOMY". It doesnt always work though. I tried a similar trick with dad's old Rogers Ravensbrook Transistor amp and all it did was make the bass ponderous and slow and gave the amp a propensity for blowing the output protection fuse, probably due to the extra capacitance giving a push to the instantaneous current capability on loud passages. I was young and daft at the time, now i'm old.............. A caveat to those thinking more reservoir capacitance will always be an improvement.
The project is on hold until the chokes arrive and are fitted.

Diabolical Artificer 24th Oct 2015 5:43 am

Re: EL34 power amp project
Nice work Andy, were the chassis's laser cut? They're very neatly cut.The OPT's look substantial too, plenty or iron so should have good bass response.

I suppose you could have gone down the all octal route using a EF37a and 6SN7 but it won't make a difference, and as you say EF40's/ECC81's are cheaper.

Maybe the EL34's would have been better sited further back, with short connection's to the OPT?


bikerhifinut 25th Oct 2015 9:33 pm

Re: EL34 power amp project
I don't know how they were cut, The chassis were spotted at the February audiojumble in Tonbridge by a friend and kindly bought and stored for me.(thanks terry :)) If anybody knows who the guy was I would be grateful. They had all the holes cut that I needed.
The transformers are Danburys, obtained from another contact on an audio forum. Pretty much the set that's still marketed for the 5-20 circuit. The mains transformer has enough reserve to run a 5U4 rectifier, this was confirmed by Danbury themselves when i spoke on the phone. The caveat is to reduce the 6.3 draw by an amp, this isnt a problem as its a 5A s winding and a couple of EL34n ECC81 and an EF86/40 draw a total of 3.5A keeping me under the self imposed 4A limit.
The output trannie according to Danbury's blurb is good up to 50W from a pair of KT88 in UL/tetrode so its running nicely under for 20 or so watts from the EL34. I like the idea of under running wound components.
I originally did consider the all Octal route as I had a set of Chassis cut out for Octal valves. Another member has them now and I look forward to seeing his take on the EL34 power amp. To that end i laid in a stock of 6SJ7 Russian and NOS RCA US made which are pretty much the same as EF37/40/86 as far as I can ascertain. But no top cap and a lot cheaper than EF37's, an advantage in both cases as far as I am concerned. I also got a couple of NOS Sylvania 6SL7 for the LTP on the basis that they were nearer to the ECC81 than a 6SN7 is. I will hang on to them as I have some preamp ideas that can make good use of those octals.
I am fairly agnostic about brands and country of origin of valves, so the makes of those mentioned are for information rather than a recommendation. They do seem better value than the NOS B9A stuff................. so far. I got a batch of RFT ECC81 for the project for a very good price which shows there is still value to be had if you shop around.
I would have put the EL34's further back if the holes weren't already there, but in fairness there isn't a very long run to the output transformer and there's so much room I can have a nice spacious "logical" layout I hope.


joebog1 26th Oct 2015 12:53 am

Re: EL34 power amp project
That will be fine EXCEPT that a 5U4 rectifier is directly heated!!!
That means that all heaters in the amp will be running at full raw HT level.
Unless I have read you wrong, by that I mean you have a single 6.3 volt winding rated at 5 amps.
The 5U4 requires a separate winding completely for its filament. This is usually 5 volts, but a 6.3 volt winding could be used with appropriate dropping resistor inserted into the filament line. In fact to do this its usually more elegant to use two resistors, one in each filament leg.
If you intend to run the heaters for the amp valves AND the 5U4 rectifier from the one 6.3 winding, your amp will fail. In SPECTACULAR fashion !!!!

I hope I havent rained on your parade :-(


bikerhifinut 26th Oct 2015 1:07 am

Re: EL34 power amp project
No You've read it wrong.


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