UK Vintage Radio Repair and Restoration Powered By Google Custom Search Vintage Radio Service Data

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > Other Discussions > Homebrew Equipment

Notices

Homebrew Equipment A place to show, design and discuss the weird and wonderful electronic creations from the hands of individual members.

Reply
 
Thread Tools
Old 4th Jan 2019, 11:14 pm   #1
bikerhifinut
Octode
 
Join Date: Nov 2013
Location: Penrith, Cumbria, UK
Posts: 1,499
Default EL34 power amp reworked as a stereo amplifier

Some will remember the headache I had with a pair of mono power amplifiers I built based on the Leak TL25+ circuit with a couple of tweaks and mods to suit modern sources.
I eventually cut my losses a while back and sold off the mechanically noisy mains transformers as an expensive dead loss.
Enter Joe Bog who kindly donated me one of his own wound Toroid mains transformers. Massively over specified for this job as it will power 8 x EL34 and associated driver/preamp valves.
As the heater secondaries were wound for 9V and I didnt really want the extra parts, complication and heat from a rectifier and regulator to bring em down to 6.3V I managed with advice from Joe and Ed Dinning to rewind one of the secondaries to get me 6.5V AC. Pleanty close enough bearing in mind our mains usually runs at over 250V right on the allowable limit.
Anyway, silicon rectified the 325V secondary gives me 431VDC under load, which after reservoir capacitor aand chokes on each side gets me 410V on the output Transformer CT, again enough for the leak circuit as although the TL25+ ran at 450VHT, the Stereo50 variant ran at 385V on the anodes so I'm nicely in the ball park there, and what's more my 500V capacitpors are running inside their limits.
I settled for 100uF reservoir which is split into two HT rails via a 5H 150mA choke on each side into 50uF each and fed to the output transformer. the subsequent supplies to the phase splitters and preamp valve are dealt with by 22k and 100k resistors and decoupled by 50uF. I guess its a sort of Pseudo "Double mono" design with the split rails.
Anyway the output stage is more 5_20 as I couldnt easily find 440R wirewound resistors and rather than bung a pair of 220R in series I settled for 10W 470r as they would still be inside the 10% tolerance of resistors in the day.
The rest is pretty much by HJL except the input EF86 is configured as a triode and the feedback was tweaked to take that into account, my back of a fag packet calculation/flog took me to the use of a 2.7k resistor with 800pF compensation cap in parallel, thats the loss of gain with triode and also no 16ohm tap so forced to use the 8ohm output. I think i could increase that and reduce the feedback a bit with a 3.3k or even 3.9k but it seems stable but as yet I havent had it on the scope to see if theres any HF issues.
I'm so far very pleased with it, had a couple of teething issues, the usual dry joint stuff and a couple of schoolboy errors like forgetting to wire up the cathodes of an EL34! and connecting one end of the coupling capacitor to the anode of an EF86!
It sounds like a nice hi fi amplifier, which is to say it hasn't got any particular valve "flavour" if such a thing exists.
Theres a couple of issues I'd like to iron out, not serious but there's a low level hum that I can't identify the cause of yet, baffling in as much as with a shorting plug in the input socket both channels are deathly silent, very impressive for a valve amp of this kind of power, but as soon as a source is connected I get a low level hum that I think is 50Hz, This ghappens with a "passive" volume control (50k log pot in a box) also with a good quality commercial solid state preamp (Rega Cursa3 set to unity gain) and with my own homebrew Cathode follower buffer stage with active load which also is quiet as a mouse with my other kit. It's not loud enough to spoil my enjoyment as its inaudible even 5 feet away and I guess I could just ignore it.
I paid attention to the heater wiring, all properly referenced to ground and the small valves are DC heated via an LM338 based regulator.
I am minded to junk the earth lift of 10ohms and take the signal star earth to the mains earth potential as i think its not the issue here and my feeling is if I dont need to lift the signal earth then its better that way. I did pay attention to the earthing runs to the star tag and my feeling is it paid off as the amp is hum free with a shorting plug in it which if there had been a loop would not be the case?
I didnt use screened wires from RCA sockets to the gridstopper of the EF86 as it was only a couple of inches if that. I have subsequently twisted the hot wire with the earth wire to see if that helps any. It doesnt. I may try that tweak as its an easy job to sort.
ok I'll attach some pics of my scruffy handiwork.
Andy.
Attached Thumbnails
Click image for larger version

Name:	20181230_175034.jpg
Views:	142
Size:	61.9 KB
ID:	175883   Click image for larger version

Name:	20181230_175044.jpg
Views:	142
Size:	88.2 KB
ID:	175884   Click image for larger version

Name:	20181230_175102.jpg
Views:	141
Size:	81.2 KB
ID:	175885  
bikerhifinut is offline   Reply With Quote
Old 4th Jan 2019, 11:16 pm   #2
bikerhifinut
Octode
 
Join Date: Nov 2013
Location: Penrith, Cumbria, UK
Posts: 1,499
Default Re: EL34 power amp reworked as a stereo amplifier

some more pics
Attached Thumbnails
Click image for larger version

Name:	20181231_110347.jpg
Views:	121
Size:	69.7 KB
ID:	175886   Click image for larger version

Name:	20181231_110634.jpg
Views:	124
Size:	55.6 KB
ID:	175887   Click image for larger version

Name:	20181231_163546.jpg
Views:	118
Size:	80.4 KB
ID:	175888  
bikerhifinut is offline   Reply With Quote
Old 4th Jan 2019, 11:21 pm   #3
bikerhifinut
Octode
 
Join Date: Nov 2013
Location: Penrith, Cumbria, UK
Posts: 1,499
Default Re: EL34 power amp reworked as a stereo amplifier

Some of the first stages starting from scratch.
Please note I did put grommets in the holes for the mains transformer wiring and also double shrink wrapped the leads to give extra protection.

Andy.
Attached Thumbnails
Click image for larger version

Name:	20181229_125610.jpg
Views:	82
Size:	48.2 KB
ID:	175889   Click image for larger version

Name:	20181215_161317.jpg
Views:	74
Size:	55.0 KB
ID:	175890   Click image for larger version

Name:	20181215_161258.jpg
Views:	74
Size:	55.4 KB
ID:	175891   Click image for larger version

Name:	20181215_160813.jpg
Views:	76
Size:	87.6 KB
ID:	175892   Click image for larger version

Name:	20181210_174354.jpg
Views:	65
Size:	42.3 KB
ID:	175893  

bikerhifinut is offline   Reply With Quote
Old 4th Jan 2019, 11:30 pm   #4
bikerhifinut
Octode
 
Join Date: Nov 2013
Location: Penrith, Cumbria, UK
Posts: 1,499
Default Re: EL34 power amp reworked as a stereo amplifier

A few more.
The woodwork etc isnt up to the standard of Davids but I had to do what I could with what I had.
A.
Attached Thumbnails
Click image for larger version

Name:	20181210_174312.jpg
Views:	72
Size:	40.2 KB
ID:	175898   Click image for larger version

Name:	20181210_174245.jpg
Views:	74
Size:	42.6 KB
ID:	175899   Click image for larger version

Name:	20181115_193600.jpg
Views:	73
Size:	45.6 KB
ID:	175900   Click image for larger version

Name:	20181115_193547.jpg
Views:	75
Size:	48.6 KB
ID:	175901  
bikerhifinut is offline   Reply With Quote
Old 5th Jan 2019, 8:15 am   #5
Diabolical Artificer
Dekatron
 
Diabolical Artificer's Avatar
 
Join Date: Jul 2009
Location: Sleaford, Lincs. UK.
Posts: 4,125
Default Re: EL34 power amp reworked as a stereo amplifier

Nice work old chap, see you got your toroid cover, guess you changed your mind about using a saucepan. good to see it finished mate.

Re the 50hz hum, pull the EF86, is it still there? Are all heater's referenced to ground? did you connect the EF86 screen? That's all i can think of for now apart from if those suggestions don't change anything pull all the valves, power up and see what you hear.

Tother Andy.
__________________
Curiosity hasn't killed this cat...so far.
Diabolical Artificer is offline   Reply With Quote
Old 5th Jan 2019, 9:22 am   #6
Electronpusher0
Pentode
 
Join Date: Jun 2016
Location: Bognor Regis, West Sussex, UK.
Posts: 225
Default Re: EL34 power amp reworked as a stereo amplifier

Very Nice!
re the hum have you tried connecting the heaters across a low value pot say 200 ohm with the wiper to 0v (obviously there should be no other connection from the heater circuit to 0v) Varying the wiper either side of centre position should affect the hum allowing you to minimise it.

Peter
Electronpusher0 is offline   Reply With Quote
Old 6th Jan 2019, 12:29 am   #7
joebog1
Heptode
 
joebog1's Avatar
 
Join Date: Jun 2015
Location: Mareeba, North Queensland, Australia
Posts: 741
Default Re: EL34 power amp reworked as a stereo amplifier

Nice work Andy!!
maybe lift the heater rails ?? to fix the hum?
Maybe use a 12A*7 either parallel, or two low gain stages?
Have a look with the cro and perhaps re route your feedback network away from everything else.

Just a few thoughts.
Joe
joebog1 is offline   Reply With Quote
Old 6th Jan 2019, 7:49 pm   #8
bikerhifinut
Octode
 
Join Date: Nov 2013
Location: Penrith, Cumbria, UK
Posts: 1,499
Default Re: EL34 power amp reworked as a stereo amplifier

I was wondering the very same Joe. I figured to lift the all the heaters by about 40V, the EL34 run at 30V on the cathodes, bang on the circuit spec. I feel that the LM338 regulator on the EF86 and ECC81 which has its negative end at signal earth potential should sort out any heater induced hum. I did get an awful hum on first power up which was quickly diagnosed as a broken wire to the earth refernce on the DC heaters, really hard to see as the wire had parted just at the solder joint on the tagboard. I think lifting the heaters on the power valvres may help also do you think the wire runs to the outside EL34 might be compromising things as they cross at right angles to the feedback components which i mounted on the spare tags on the EL34 cathode resistor/capacitor board. I could easily mount a small tag board nearer each preamp board and run a cntinuous length of screened cable back to there. I also wonder if the EL34 heater passing 1.5Amps and running very close to the output transformer primary taps is the culprit, inducing hum directly into the transformer. This would not be cancelled out by the push pull output pair. Thats an easy fix once i get in the garage and twist some more wire up. its the way the hum cancels out when a shortin plug is inserted in the input sockets that suggest a hum loop on the source but why that should also be on a simple unpowered pot in a box too beats me.
As you say I need to scope it and see what that throws up if anything. It is a fairly low level of hum and this is the perfectionist in me wanting the hum down below the noise of the valves and resistors. It's already reduced sensitivity/gain by triode wiring the EF86, theres no advantage in fiddling about with triodes there. I initially mulled over using a 12B4 there but decided against it in the end. Around half a volt for full output is in the ball park for me.
by the way Joe the slight mechanical noise from the toroid stops as soon as the amp is right way up and the transformer is supported be the top plate and not hanging from the bolt so to speak, I also think the metal cover was coupling noise into the table top like a soundboard.
bikerhifinut is offline   Reply With Quote
Old 7th Jan 2019, 2:12 am   #9
joebog1
Heptode
 
joebog1's Avatar
 
Join Date: Jun 2015
Location: Mareeba, North Queensland, Australia
Posts: 741
Default Re: EL34 power amp reworked as a stereo amplifier

I don't think the EL34 heater wiring passing close to the OPT will cause any hum, at least in my experience I haven't seen that. It could be a possibility though.

I usually run my feedback loops through good quality coax, after measuring the capacity of the length of cable required. I always run the feedback via the SHORTEST path possible and avoid ALL components from the input stage as far as is practical and all other components in general. Which brings me to another point !! 800 pF as correction capacitor on the feedback?? To me that sounds awfully BIG. I use an oscilloscope to choose the capacitor sizing/value while testing the amp with square waves. I choose a value that doesn't produce overshoot spikes, or rounding off, of the square wave.

You will never get this perfect however!! as any output transformer has its limitations. With my own transformers I get in the vicinity of 100 pF as the biggest value with smaller transformers. I start with a value of about 47 pF and select as testing proceeds.

Somewhere I have an excellent article from Radio Television and Hobbies magazine from the late 1950's that explains the technique in terms I can even understand. I will post it when I dig it up.

In the meantime enjoy the music.

Joe
joebog1 is offline   Reply With Quote
Old 7th Jan 2019, 10:53 pm   #10
joebog1
Heptode
 
joebog1's Avatar
 
Join Date: Jun 2015
Location: Mareeba, North Queensland, Australia
Posts: 741
Default Re: EL34 power amp reworked as a stereo amplifier

Did lots of thinking last night and came up with a few more suggestions!
Are both your input RCA jacks connected to chassis? If so fit an insulated variety to one of the channels. It could be the problem.
Main earth for the WHOLE amplifier should be at the input socket.
That means your star earth point as well. The earth point for the input socket "should" be across the grid to earth input resistor, obviously the end connected to ground. Also connected at that point should be the cathode cap ( if your using one) and the cathode resistor of the input bottle ( EF86 in your case)

Another thing to try is to lift the whole heater wiring above ground and fitting the standard sharing resistors ( 50 -220 ohms) and tying the centre tap of these resistors to the output stage cathodes of one channel. That will set the heater line at EL34 bias voltage, about 30 volts or so with your amp. If that helps you can fiddle around with a voltage divider across the HT line and set the "sweet spot" voltage with a pot, and then replacing the pot with a fixed resistor.

Joe
joebog1 is offline   Reply With Quote
Old 8th Jan 2019, 1:00 am   #11
bikerhifinut
Octode
 
Join Date: Nov 2013
Location: Penrith, Cumbria, UK
Posts: 1,499
Default Re: EL34 power amp reworked as a stereo amplifier

Thanks for the input.
A couple of possible design errors of mine have come to light.
One in particular is my idea to reduce the grid leak on the input valve from 1 meg to 100k, My thoughts were as it would reduce noise a bit and all my sources are very low impedance it wouldn't be a bad idea. What I haven't taken into account is that I left the grid leak at 22k. With the input at the junction of the 22k and a 100k to ground I've inadvertently created a substantial potential divider at a point where I don't really need it. Shorting the socket with my shorting plugs has the obvious effect and the hum totally disappears. But with a source and even with a potentiometer on the input it's a different tale. So thats the first point of action in the morning. I'm minded to restore the 1meg grid leak resistor rather than reduce the grid stopper. But I'll have a scribble round the input circuit.
Another thing is with the EF86 valve triode strapped and a 150k anode resistor, theres potentially more gain than needed and a reduction here with a commensurate alteration of the compensation network on the anode might make a big difference too. Thanks to Terry for pointing that one out. And back to the curves.
I'm fairly certain the problem, such as it is, lies in the input stage. The output is totally hum free with no input to it So the heater wiring isnt injecting 50hz where it isnt wanted in that part of the amp. And none of the power valve heater wiring gets anywhere near the signal valves.
All fairly simple fixes I reckon. But one at a time so I can see what if any effect they have. I've already lifted the heater ground references to 40V above 0V which if nothing else gives the cathodes of the phase splitter an easier life. No Hum advantage as they are already warmed with regulated DC.
The amps going under the scope tomorrow if I get time and that hopefully will identify the issue better. I'm having to do some hasty revision on oscilloscopes as my memory aint what it was these days.

A.
bikerhifinut is offline   Reply With Quote
Old 8th Jan 2019, 8:34 am   #12
Diabolical Artificer
Dekatron
 
Diabolical Artificer's Avatar
 
Join Date: Jul 2009
Location: Sleaford, Lincs. UK.
Posts: 4,125
Default Re: EL34 power amp reworked as a stereo amplifier

It is almost certainly the EF86 grid Andy, as you've sussed out. Disconnect the grid resistor and temporaraly ground the grid - what hum do you get? Be careful when mucking about with grid stopper, it will effect bandwidth. If you reduce gain you might not need the stopper at all. The problem is that as you reduce Ra, THD goes up, so it might be provident to reduce HT as well as reducing Ra a tad. Is the EF86 decoupled from HT?

If it were me I'd disconnect the NFB and find out how much open loop gain you need, then work from there. As mentioned earlier do you really need the 86? Would you be better using a triode? It is better to design the IP exactly as it should be rather than trying to make the old IP stage fit.

Just some thought's, keep up the good work, Andy.
__________________
Curiosity hasn't killed this cat...so far.
Diabolical Artificer is offline   Reply With Quote
Old 8th Jan 2019, 2:50 pm   #13
bikerhifinut
Octode
 
Join Date: Nov 2013
Location: Penrith, Cumbria, UK
Posts: 1,499
Default Re: EL34 power amp reworked as a stereo amplifier

quote: Disconnect the grid resistor and temporaraly ground the grid - what hum do you get?
Already done that, it's why I test initially with shorting plugs in the input sockets to eliminate external hum sources. Result.......... silence. So I reckon the amp itself is working as planned.
I increased grid leak back up to 1meg but that made no difference, nor has reducing the grid stopper to 1k.
Reduced open loop gain with a more suitable anode resistor, I should have seen that one coming as The original circuit used different anode resistor values depending on how much gain was needed into the phase splitter section. Leak altered their designs over time and I have a comprehensive history here documenting it all.
The argument about designing with a triode is a bit of a red herring, it's already designed around a triode first stage, albeit an EF86 triode wired but it's a pretty good audio triode at that with a gain somewhere between the ECC81 and ECC82 as far as I can ascertain from the data sheet.
The point is this, it's dead quiet until a source is hooked up. The amps predecessors didnt do this, the issue there was mechanical noise from crap mains transformers.
I suspected a hum loop but from where? my main sources are not earthed to the mains, the Rega Cursa is double insulated as is the Cd player so no mains earth pin on the mains sockets. Right now theres a 10 ohm earth lift backed up with reverse parallel rectifiers and i was tempted to wire the signal earth to chassis/mains earth, if nowt else its an added safety factor.
I did have an issue with valve amps with long interconnects to the power amps, so I reverted to the amp on the rack with long (very long) speaker leads and a 1m lead from pre to power. But this made not a jot of difference.
Its 50Hz mains 'um but where is it getting picked up? The toroid mains transformer on the power amp doesnt radiate like the old one did nor like the EI transformer on my KT88 amp does where it needs a good couple of feet vertical separation from any sensitive sources.
I may just be a bit fussy but it strikes me that a hum loop if that is what it is should be resolvable.
Oh for fully balanced connections!
One thing i havent done is connect the screen pin on pin 7 as according to the Mullard valve data its internally connected to pin 2 also. This has me thinking a bit. Its about all that I have left. I also did remember to earth the centre spigot of the base as per Mullards data.
If it goes boobs up then for sure its a dead easy fudge to rebuild as an all ECC83 amp and it will be a Stereo50/60 clone. It'd be no quieter though.
Anyway I need to do some proper measurements now.


Last edited by bikerhifinut; 8th Jan 2019 at 2:56 pm.
bikerhifinut is offline   Reply With Quote
Old 8th Jan 2019, 5:16 pm   #14
snowman_al
Pentode
 
snowman_al's Avatar
 
Join Date: Dec 2018
Location: Llandeilo, West Wales, UK.
Posts: 240
Default Re: EL34 power amp reworked as a stereo amplifier

Just thinking out loud here....

If you short the inputs individually - silence.
When you connect any input source you get hum.
Is the common factor that the 'sources' have their grounds connected together? So as you have effectively two mono blocks, when you plug the sources in you short the two RCA outer rings, and hence the ground paths of each mono block, together. That might be the loop?

What happens if you add a 10 ohm resistor to the ground path of one of the inputs? (In the green lead, between the RCA ground ring terminal and the common point on the tag board.)

Alan
snowman_al is offline   Reply With Quote
Old 8th Jan 2019, 6:37 pm   #15
bikerhifinut
Octode
 
Join Date: Nov 2013
Location: Penrith, Cumbria, UK
Posts: 1,499
Default Re: EL34 power amp reworked as a stereo amplifier

BINGO!
Didnt try the 10R but with one side shorted the other input goes "Humphrey" (Hum free).
That's intriguing.
I did try tying the phono socket outers together which went counter intuitively as that seems to create a loop between each channel if you understand me. The result was exactly the same.
Do you mean that because I have in a way, two separate mono amplifiers with their own ground returns to the star earth and their own power supplies individually decoupled at each stage that I inadvertently create a hum loop when I plug the preamp in? In which case it would make sense to common the earth returns after each stage and back to the star tag? But then would there be issues with the individually decoupled HT rails and would it be better to common those too?
So do I tie the preamp and phase splitter earths together and take a single wire back to the star? If that is the answer, I assume the point to tie em together will be at the Earthy end of the tail resistor on the phase splitter valves? Not at the bottom of the input Valves as that would upset the convention on grounding, i.e main PSU closest to the chassis earth, followed by output valves, then LTP and finally preamp.
Just trying to understand the process here. I thought I understood grounding but patently I don't!
By the way the hum is very low level and more or less inaudible from the sofa. Maybe I expect too much but I do know from past experience that it is possible to build a hum free valve power amplifier.
I'm sure the problem is a basic error in my earthing. In a previous iteration this circuit with the same output transformers was buillt as 2 monoblocs and i got them hum free. I may also be too fussy.

Thanks for the insight, now i need to understand whats happening and apply the fix.

A bit masochistic but it is actually part of the pleasure to understand whats happening when a plan goes off piste.

A.
bikerhifinut is offline   Reply With Quote
Old 8th Jan 2019, 7:31 pm   #16
snowman_al
Pentode
 
snowman_al's Avatar
 
Join Date: Dec 2018
Location: Llandeilo, West Wales, UK.
Posts: 240
Default Re: EL34 power amp reworked as a stereo amplifier

Phew, well there you are...

I guess you are amplifying the very small difference in potential from each ground line back to their common node?
Try the small value resistor first, that is what Radford and others did so you are in good company there.
In the future you might consider a different arrangement as joebog1 says. Think of the 'noise' each element adds to the ground line, then make sure that does not get mixed with the signal (return) and so amplified.
Anyway, grounding is an art as well as a science and what works for this will be different next time! You will be lucky to get it right first time even.

All valve amplifiers make some very slight noise, but no reason hum should be one.
Alan
snowman_al is offline   Reply With Quote
Old 8th Jan 2019, 7:35 pm   #17
bikerhifinut
Octode
 
Join Date: Nov 2013
Location: Penrith, Cumbria, UK
Posts: 1,499
Default Re: EL34 power amp reworked as a stereo amplifier

I removed the LH channel earth return where it connected to the Tail resistor, then cross coupled it to the corresponding tag on the RH channel. Thus only having one common return from phase splitter and preamp section.

Result!

And a lesson learned. Star earthing isn't always as simple as it sounds. Good bit of practical eddication.

There's a very slight residual hum on the LH but only audible with ear pressed to loudspeaker. I ain't worrying about that.

When I get back in later tonight i will tidy the earthing up properly and take both returns to a common point on one of the PSU tags and thence to the star. It'll look neater and should share the current more evenly.

I think there's mileage in doing the same treatment to the output valve section ie the returns from the cathode resistors and grid leaks. if nothing else it will look a lot neater with one less wire run to the star earth point.

Thanks to everyone for their input.

The amp will now go on the scope again and i can check the open loop gain and work out a sensible feedback ratio.
I have a few tricks up my sleeve still to try as I hope to get the amp down to around a volt sensitivity or up to 1.4V which will be right for my preamps and high output sources. This should if my electronic theory is right reduce unnecessary background noise to a minimum.

Andy (happy Bunny)r
bikerhifinut is offline   Reply With Quote
Old 8th Jan 2019, 10:41 pm   #18
joebog1
Heptode
 
joebog1's Avatar
 
Join Date: Jun 2015
Location: Mareeba, North Queensland, Australia
Posts: 741
Default Re: EL34 power amp reworked as a stereo amplifier

I found one article from Radiotronics. This magazine was published by Radiotronics which is the Australian arm of RCA.
Anyway, it might help in the future Andy

Joe
Attached Files
File Type: pdf 1960_08_AWV_Radiotronics_25_08.pdf (120.1 KB, 21 views)
File Type: pdf 1960_08_AWV_Radiotronics_25_081.pdf (110.9 KB, 17 views)
joebog1 is offline   Reply With Quote
Reply

Thread Tools



All times are GMT +1. The time now is 9:06 pm.


All information and advice on this forum is subject to the WARNING AND DISCLAIMER located at https://www.vintage-radio.net/rules.html.
Failure to heed this warning may result in death or serious injury to yourself and/or others.


Powered by vBulletin®
Copyright ©2000 - 2019, vBulletin Solutions, Inc.
Copyright ©2002 - 2018, Paul Stenning.