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 1st May 2017, 12:01 am   #181
joebog1
Hexode
 
joebog1's Avatar
 
Join Date: Jun 2015
Location: Mareeba, North Queensland, Australia
Posts: 496
Default Re: 807 (maybe) amplifier build. Now EL34

For output valve protection I still use the 10 ohm sense resistor in the cathode, but only use 1 watt carbon resistors. I cover them with good quality clear heatshrink to prevent the burnt carbon going all over the place when they do burn, plus you can "check" the colour of them in a quick check of overloads.

Under normal operation the resistors are within " limits", but when an overload happens the resistor cooks and goes open circuit disconnecting the valve itself. Its cheap, works extremely well and the smoke is a dead set giveaway that something has failed.

Joe
joebog1 is offline   Reply With Quote
Old 1st May 2017, 7:05 am   #182
tony brady
Pentode
 
Join Date: Mar 2014
Location: Peacehaven, East Sussex, UK.
Posts: 113
Default Re: 807 (maybe) amplifier build. Now EL34

One thing to remember Andy if you haven't already done so is to put high value, high voltage resistors across the caps that are in series to ensure approx equal voltage across each one. Otherwise one may end up with more than its rated voltage across it.
tony brady is offline   Reply With Quote
Old 2nd May 2017, 8:50 am   #183
Diabolical Artificer
Nonode
 
Diabolical Artificer's Avatar
 
Join Date: Jul 2009
Location: Sleaford, Lincs. UK.
Posts: 2,836
Default Re: 807 (maybe) amplifier build. Now EL34

Thanks, useful suggestions there fellas.

Andy.
__________________
Curiosity hasn't killed this cat...so far.
Diabolical Artificer is offline   Reply With Quote
Old 11th May 2017, 4:14 am   #184
Diabolical Artificer
Nonode
 
Diabolical Artificer's Avatar
 
Join Date: Jul 2009
Location: Sleaford, Lincs. UK.
Posts: 2,836
Default Re: 807 (maybe) amplifier build. Now EL34

Making progress on the PSU. It will still need tweaking but hopefully you'll get the idea. I'm wiring it in temporarily at present to check everything works.

A.
Attached Thumbnails
Click image for larger version

Name:	IMG_20170510_102422.jpg
Views:	72
Size:	87.8 KB
ID:	142412   Click image for larger version

Name:	IMG_20170510_102454.jpg
Views:	62
Size:	78.7 KB
ID:	142413  
__________________
Curiosity hasn't killed this cat...so far.
Diabolical Artificer is offline   Reply With Quote
Old 2nd Jun 2017, 1:22 pm   #185
Diabolical Artificer
Nonode
 
Diabolical Artificer's Avatar
 
Join Date: Jul 2009
Location: Sleaford, Lincs. UK.
Posts: 2,836
Default Re: 807 (maybe) amplifier build. Now EL34

I've nearly finished fettling the chassis or rack unit to accept the two amp "modules" and have temporarily wired in the PSU. I managed to fit it all in though this has taken a lot of measuring, jiggling, trial and error. I've had to make several compromises because I didn't really think everything through. To be fair to myself though unless I'd actually built this beforehand, I couldn't have thought about everything before hand. Still, if I'd made the amp module 1/2" smaller on the length and width, life would have been easier.

Starting with the PSU, the mains enters the rack unit through an IEC kettle doodah, goes into a big 32mm/1 1/4" NOS fuse holder and is switched both L and N with a NTE thermistor on the big toroid tfmr primary. I tried fitting caps across the SW to surpress arc, but this means the PSU isn't properly off. The thermistor should extend SW life somewhat anyway.

Without the thermistor even with a soft start circuit, there is a hell of an inrush current. At first I used 100 ohm resistor's on each capacitor "bank" , but this didn't reduce the current surge or delay the rise of HT voltage. I'd obviously miscalculated. I found some big vitreous enamel resistors in the shed, that came of the Tek 454A scope that donated the ceramic wafers I used on the amp. So I used two these, a 700 ish ohm on the "bottom" 225v HT PSU, and a 1k5 on the top. this delays full HT by around 8 seconds, these R's are then shorted by a relay, two contacts for the top HT, one for the bottom. See schematic.

The relay delay circuit is powered by a little tfmr, GR etc, bottom left, pic one. You can just see it under the cap board. the delay circuit is on the PCB on the left. A TIP122 Darlington switches on the relay. It's overkill for this, but works well. I tried using a 2SD880 at one point, but it oscillated.

Next, middle on the PCB is the bias/CCS PSU. I had to change values of the first RC filter resistor. There were two 1k5 100u RC filters, but the first dropped the OP voltage too much. I'd also put a 80v zener on the OP but had to take it off. There is a bias adjust pot on the amp, this dropped too much voltage. I experimented with the "fools bias resistor" ( a R to stop bias from being set to 0v) to stop too much voltage being dropped. 1M and a 50k pot gives -16v to -40v and doesn't drop too much voltage.

Last, on the right of the PCB is the 12.6v PSU circuit that powers the htrs of the triode gain stage and PS. I screwed up here and wound the tfmr secondary that powers it with too high a voltage, EG 34v when rectified and smoothed. this means the LM317 got very hot indeed. So I had to retro fit a dropping resistor of around 20 ohm's (two big cement R's on right) to give 19v IP to the LM317. Not happy with this bodge but it would mean re-winding the big toroid . No way jose! : )

I keep writting PCB, but it was made using a dremmel and fine diamond tip to cut the traces. As you see it and the relay is on a hinged panel to make access easier. This hinged panel took about a week to get right!

The fuseboard needs re-doing. I used veroboard but am not happy with the result. It has gaffa tape on the chassis to stop any shorts at present which on the finished amp will need something better. Also there are no covers on the HT fuseholders - an oversight because as the bridge rectifiers have caps across each diode to suppress switching noise, HT is present on the fuse holder.

Lastly for now, as you'll see in the last pic is a panel meter. I managed to get two the same off ebay for 20. They are the same NOS military 50uA but have different scale plates. I'm thinking of using these to make PPM/VU meters. I saw there are cheap Chinese VU meters that come with a driver board, but thought I could do better.

That's about it for now, TFL, Andy.
Attached Thumbnails
Click image for larger version

Name:	IMG_20170602_124758.jpg
Views:	69
Size:	92.1 KB
ID:	143778   Click image for larger version

Name:	IMG_20170602_124807.jpg
Views:	63
Size:	80.8 KB
ID:	143779   Click image for larger version

Name:	IMG_20170602_124812.jpg
Views:	67
Size:	97.7 KB
ID:	143780   Click image for larger version

Name:	IMG_20170602_125135.jpg
Views:	61
Size:	84.5 KB
ID:	143781   Click image for larger version

Name:	IMG_20170602_125405.jpg
Views:	60
Size:	80.7 KB
ID:	143782  

__________________
Curiosity hasn't killed this cat...so far.
Diabolical Artificer is offline   Reply With Quote
Old 4th Jun 2017, 6:23 am   #186
Diabolical Artificer
Nonode
 
Diabolical Artificer's Avatar
 
Join Date: Jul 2009
Location: Sleaford, Lincs. UK.
Posts: 2,836
Default Re: 807 (maybe) amplifier build. Now EL34

I tested everything yesterday and all worked well for a while. Previous to that I'd noticed the cathode follower resistors made up of matched 10k and 2k2 5w wire wound, were getting very hot, I could smell scorching. I measured the current and found they were running at 20mA, which is a bit too much. I'd also noticed that the cathode followers wern't biased quite right. They should have been at -10v but were at -3v.

I replaced the 2k2 R's with 5k6 which reduced dissipation to 15mA, the resistors still get warm, but not overly so. After some thought I realised I could set the bias with the adjustable CCS of the phase splitter. This meant moving the bias point of the ECC82 triodes a bit, but found with the ECC82 PS set to -6.6v ( they were running at -11v, center biased, but not running in the linear region), the cathode followers were now set to -10v.

So as mentioned at the start all was going swimmingly, I was testing and setting the bias of the OP stage. This is set by a dual ganged linear 10k pot which balances the two sides of the OP stage. I noticed that it wasn't working as well as it did on the "prototype", and after a while prior to switching off I unplugged the lead from the sig gen. A loud screech issued from somewhere, I hit the off Sw.

I noticed I had nasty oscillation on the OP and after some investigation I found the screech was coming from the OPT. I tested the OPT for shorts and whilst doing so I found the tag of the pin on the valve base that the screen tap of the OPT connects to, was broken. This accounts for the difficulty of balancing the OP stage. These are decent NOS Cinch octal bases so not rubbish, but I did think when i got them that the tags looked a bit thin, and I had to get off some scale off each tag with a file prior to soldering. This must have put undo pressure on the tag. I started to unsolder every tag, then realised I had a spare base, so nicked a pin/tag off it to replace the broken one.

The only problem with doing a good job of soldering leads to valve base tags, IE making a good mechanical joint by wrapping a few turns and cinching with pliers, is that it's a ****** to unsolder without wrecking the hookup wire. Anyhoo, did that, screech still there, OPT not shorted. So I took out the input, PS and cathode follower, screech gone.

So something is broken/amiss with one of these three stages. Today if i get time I'll see if I can get to the bottom of the problem. At least the OPT seems undamaged, thank the gods. It seems odd that disconnecting the sig gen should cause such a catostrophic failure. I've unplugged/turned off the sig gen gen loads of times before, without issue. However I was using a different sig gen yesterday.

Two steps forward, one back. this is a bit like playing snakes and ladders, A.
__________________
Curiosity hasn't killed this cat...so far.
Diabolical Artificer is offline   Reply With Quote
Old 4th Jun 2017, 10:23 am   #187
frankmcvey
Hexode
 
Join Date: Jan 2015
Location: Cottesmore, East Midlands, UK.
Posts: 438
Default Re: 807 (maybe) amplifier build. Now EL34

Aaargh!

Quote:
The only problem with doing a good job of soldering leads to valve base tags, IE making a good mechanical joint by wrapping a few turns and cinching with pliers, is that it's a ****** to unsolder
Andy, me ole mate, you've just succinctly described why you don't wrap a few turns and cinch up with pliers! You're building a piece of electronic equipment, not mooring the Lusitania

There are two good reasons not to wrap your wire end "a few turns" around a solder tag*:

1. It ain't necessary for making a good mechanical and electrical joint**

2. You might be the poor prat who has to take it back off again at some time in the future!

All you need to do is to form the joint shown plan view in the attachment:

Strip off 1/2" of insulation

Form the joint shown with fine needle nose pliers and your thumbnail (if you're using multi-strand hookup wire rather than single core, tin it first to make a solid single conductor before forming the joint). The "neck" of the joint from the end of the insulation to the first bend should be less than 1/16". Two quick movements - takes longer to describe than it does to do it.

Snip off the free end of the joint to size. It wants to be wide enough to accommodate the solder tag, but not so wide that it overhangs the joint significantly.

Tin the solder tag (never mind if it's pre-tinned, do it again!) and your joint (if you haven't done so already). Slide the joint over the tag. You can nip it up gently in the direction of the arrows if needed to hold it in place while you do the joint.

Dab the joint with flux. (Oh yes, you do!***) Load a clean iron with just enough solder to do the joint. Touch the iron briefly to the job. The solder should wick into the joint quickly and smoothly. Remove the iron quickly before the insulation starts to discolour and char.

When the job is done, there shouldn't be a blob of solder over the joint. You should be able to see the outline of both the tag and the hookup wire clearly, with just a tiny fillet of solder between the two.

This is the method we were taught in my apprenticeship in the '60s and was the approved method for aircraft electronic equipment. The NASA standard today isn't significantly different.

Cheers,

Frank

* Yes, I know a lot of manufacturers did it back in the day, but this was for commercial production reasons. One guy on the line could make a ferret's nest of components by wrapping them in as you describe, then the next guy would flash round the job with a soldering iron. Didn't matter - some other poor sap would have to deal with it later if there was a problem. I expect that this is why most of us at some stage have worked on an old radio or somesuch and found an unsoldered joint, that had obviously never been soldered at the factory.

** "Good mechanical" in this instance means just strong enough for the task - the joint shouldn't be under any mechanical strain. If it is, then you have a design problem with your wiring loom, your strain relief, your component positioning, or all three!

*** Yes, solders like Ersin have flux incorporated, but I find that you need to keep your iron in place for slightly longer for the flux to activate and get a good flow, whereas with clean components, a dedicated flux and a hot iron (with the right-sized tip!), the flow happens almost instantaneously without risking burning your insulation.
Attached Thumbnails
Click image for larger version

Name:	Solder tag joint.jpg
Views:	46
Size:	12.6 KB
ID:	143881  

Last edited by frankmcvey; 4th Jun 2017 at 10:53 am.
frankmcvey is offline   Reply With Quote
Old 4th Jun 2017, 1:12 pm   #188
Radio Wrangler
Dekatron
 
Radio Wrangler's Avatar
 
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 8,932
Default Re: 807 (maybe) amplifier build. Now EL34

I just disagree with Frank on one point. I was taught not to carry solder to the joint on the iron, but to feed it onto the joint. Have enough solder on the iron to act as a heat conductor from the iron to the joint. Use it to heat the joint to soldering temperature and then feed enough solder directly onto the hot joint. You should see any cored flux go active, and you should see the solder flow and wet with low viscosity and active surface tension. This too was on an aerospace training course (RR/BAC Filton in the early 70s)

David
__________________
Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done
Radio Wrangler is offline   Reply With Quote
Old 4th Jun 2017, 1:17 pm   #189
Diabolical Artificer
Nonode
 
Diabolical Artificer's Avatar
 
Join Date: Jul 2009
Location: Sleaford, Lincs. UK.
Posts: 2,836
Default Re: 807 (maybe) amplifier build. Now EL34

Thanks for those pointers Frank, my method does seem a bit over the top when you think about it.

I've had part success in finding the fault. I started to take voltage readings of everything and whilst doing so I leaned my arm on the chassis, fault stopped. Aha! So it looks like I have an intermitant joint or connection somewhere. My intuition says it's a valve base or similar.

I'll take a video of the scope trace tomorrow, it's a weird one, one I've not come across before.

Andy.
__________________
Curiosity hasn't killed this cat...so far.
Diabolical Artificer is offline   Reply With Quote
Old 4th Jun 2017, 1:36 pm   #190
astral highway
Octode
 
astral highway's Avatar
 
Join Date: Nov 2006
Location: London, UK.
Posts: 1,943
Default Re: 807 (maybe) amplifier build. Now EL34

Quote:
Originally Posted by frankmcvey View Post
Dab the joint with flux. (Oh yes, you do!***) ...

This is the method we were taught in my apprenticeship in the '60s and was the approved method for aircraft electronic equipment. The NASA standard today isn't significantly different.
Hi Frank,

Interesting you mention the importance of a separate flux. I'd come to this conclusion on my own when soldering larger gauge wires >2mm, but fascinating to see it was taught as good practice on a routine basis. (Although I can't see the point with tiny SOIC IC leads, for example, these days.)

I'd also got in the habit of cleaning the entire soldering iron tip in flux by dipping, after every four or five joints, in plumber's flux, on heavier work.

What (more appropriate) flux would you recommend for the method you describe?
__________________
Al
astral highway is online now   Reply With Quote
Old 4th Jun 2017, 6:16 pm   #191
astral highway
Octode
 
astral highway's Avatar
 
Join Date: Nov 2006
Location: London, UK.
Posts: 1,943
Default Re: 807 (maybe) amplifier build. Now EL34

Quote:
Originally Posted by Diabolical Artificer View Post
I unplugged the lead from the sig gen. A loud screech issued from somewhere, I hit the off Sw.

...It seems odd that disconnecting the sig gen should cause such a catostrophic failure. I've unplugged/turned off the sig gen gen loads of times before, without issue. However I was using a different sig gen yesterday.
Interesting... how did you connect the signal from the SG to your amplifier?
Were the grounds of your SG and amplifier also intentionally connected? Just trying to establish what might have happened as you imply a strong cause-and-effect relationship between unplugging the SG and the fault arising.
__________________
Al
astral highway is online now   Reply With Quote
Old 4th Jun 2017, 8:39 pm   #192
frankmcvey
Hexode
 
Join Date: Jan 2015
Location: Cottesmore, East Midlands, UK.
Posts: 438
Default Re: 807 (maybe) amplifier build. Now EL34

Hi, Al,

I'm currently using TermoPasty Topnik TK83, which is classed as a medium-active rosin flux, a bit more aggressive than pure rosin/alcohol. In liquid form, it comes in a wee bottle with a handy brush in the lid. You'll find it on Ebay.

It says that it's no-clean, but I like to clean up anyway. The very best cleaner that I've ever come across was carbon tetrachloride (CTC), but they decided it was pretty nasty stuff carcinogen-wise, so they issued trichloroethane; then they found that was pretty nasty as well. I use IPA these days (the solvent rather than the beer!) but it takes a bit longer, particularly on old, hardened rosin.

And David, you have me questioning my memory as well; thinking back 50-some years, it may well be that we did just use multicore solder in training and that the flux thing is something I've picked up since. Works well for me, though!

Cheers,

Frank
frankmcvey is offline   Reply With Quote
Old 4th Jun 2017, 9:55 pm   #193
Radio Wrangler
Dekatron
 
Radio Wrangler's Avatar
 
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 8,932
Default Re: 807 (maybe) amplifier build. Now EL34

The simple medium activity flux in multicore solder is pretty good unless you're tackling something awkward like nickel plate or tarnished metal. But injecting the solder onto a heated joint works well. If you try to transfer the necessary solder via the iron, the flux will have gone by the time the iron gets there.

I do most of my soldering under a microscope these days (well, components have got smaller, that's my excuse) and my favourite scope is a Russian stereo one originally developed for metallurgy. You can see the grain in a dodgy joint and it doesn't half cause you to up your standards!

David
__________________
Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done
Radio Wrangler is offline   Reply With Quote
Old 5th Jun 2017, 3:13 am   #194
joebog1
Hexode
 
joebog1's Avatar
 
Join Date: Jun 2015
Location: Mareeba, North Queensland, Australia
Posts: 496
Default Re: 807 (maybe) amplifier build. Now EL34

As far as the oscillation goes in the amp, it sounds like you don't have the input grid tied down and the amp is using the output impedance of the oscillator as a "grid leak". It will probably raise a discussion BUT:

I start at the input stage and plug valves in and check all is well and operation of that particular stage is "as expected". Sometimes it's your layout that is flawed and not the circuit at all. I did notice that your "box/cabinet" is jam packed and that's NOT good in a high gain amplifier. Ideally, when one looks inside an amplifier, you should almost "see" the circuit in the layout, especially if the design uses "English" style circuit drawings.

A circuit diagram has a very logical and progressive design. It shows the input stage on the very left, the phase splitter (or extra voltage amplifiers) in the approximate middle and the output stage (and sometimes the power supply) and output transformer on the right. I always try and set up the chassis in just the same fashion: input on the left and output on the right of my bench. After making umpteen dozen amps for Hi-Fi I try and follow this convention. On a circuit diagram the components are placed vertically or horizontally, there are not any "diagonal bits". Very rarely do circuits cross over each other, by which I mean a plate resistor is far away from the input grid, in fact the grid is on the "bottom" of the valve with any grid leak going directly vertically down to earth, and any series resistor or capacitor from the previous stage is usually drawn horizontally from the left and the plate resistor going vertically up to the supply rail, and any coupling caps will be drawn horizontally to the right. So if you half close your eyes, its an almost straight line for your signal from left to right. JUST as we read a circuit diagram.

I hope Im not preaching to a master at all, but rather you check your layout for logical progression. Sometimes an amp of mine will not work correctly, I leave it on a shelf for a week and don't think about it at all. I then put the amp back on the bench and lo and behold I have made a whopper somewhere, thinking to save a couple of inches of wire.
I then use an extra six inches and the wire that was causing the feedback coupling disappears.

Keep up the good work
best regards
Joe
joebog1 is offline   Reply With Quote
Old 6th Jun 2017, 3:30 am   #195
joebog1
Hexode
 
joebog1's Avatar
 
Join Date: Jun 2015
Location: Mareeba, North Queensland, Australia
Posts: 496
Default Re: 807 (maybe) amplifier build. Now EL34

After re reading the topic I did some thinking about your concerns for saving EL34/6CA7 output valves should a fault occur. I still use 10 ohm sense resistors on the cathodes, but a very elegant solution to excessive dissipation, is to take the suppressor grid to a supply that is negative.
You already have a negative supply to set bias levels so it is available. After rectification and some filtereing you have a negative supply that you connect to the bias pots (s) to set idle current ( perhaps through a resistor to make setting easier by limiting the bias range) Pin 1 or the suppressor grid of all six output valves may be safely connected to the filtered supply BEFORE it goes to the bias setting pot. ( say minus 60 or 70 volts)
I have ever been amazed that the suppressor grid has been made available for use, yet almost nobody has taken advantage of this feature. The maximum plate/cathode dissipation can then never exceed the maximum ratings of the valve.

Joe
joebog1 is offline   Reply With Quote
Old 6th Jun 2017, 4:37 am   #196
Diabolical Artificer
Nonode
 
Diabolical Artificer's Avatar
 
Join Date: Jul 2009
Location: Sleaford, Lincs. UK.
Posts: 2,836
Default Re: 807 (maybe) amplifier build. Now EL34

Input has a 1M on the grid to ground, have tested input, PS and driver stage, all ok, but swapped valves out. After further investigation am sure the OP stage is to blame.

You make good points Joe, my amp follows your convention, IE input left, OP right. I put a lot of thought into layout of the amp, but the amp is temporary lashed up to the PSU with long wires, not good.

Yesterday I tried making a video to demonstrate the fault, however my editing software OPENSHOT is a pile of s*** , making video editing a nightmare. That aside whilst filming the video I was poking it with a stick and playing and thought to give the CCS a tweak. Adjusting the CCS output altered the "oscillation" frequency, then it disappeared. Fault gone. No rejoicing though as now two EL34's are redplating, no input, input grid grounded. This is with a dummy load and speaker load. There is an oscillation on the redplating valves, but before I jump to conclusions I need to do more testing, but there's instability somewhere.

I have a few thought's and improvement's in mind, (ferrite beads on g2 and anodes is obvious) that I'll run past you all later. First I need to do a revised schematic and take some pics.

Havn't forgot your post Al, also Joe's comment is pertinent. The sig gen OP has four OP 4mm jacks. Two red ones are the balanced signal, in between these are two black jacks marked ground, however I was using the other black one, which is shown on the panel as being the middle of the two balanced signals. Think the middle of a potential divider, OP sigs on the ends. Using the ground jack seems a better choice in retrospect.

Whatever the cause, the amp was fine, now it isn't, something broke or changed.

Thanks for your input, pics and more info later, Andy.
__________________
Curiosity hasn't killed this cat...so far.
Diabolical Artificer is offline   Reply With Quote
Old 7th Jun 2017, 11:09 am   #197
astral highway
Octode
 
astral highway's Avatar
 
Join Date: Nov 2006
Location: London, UK.
Posts: 1,943
Default Re: 807 (maybe) amplifier build. Now EL34

Woah, your anodes are glowing red, with no drive?
__________________
Al
astral highway is online now   Reply With Quote
Old 7th Jun 2017, 2:34 pm   #198
Diabolical Artificer
Nonode
 
Diabolical Artificer's Avatar
 
Join Date: Jul 2009
Location: Sleaford, Lincs. UK.
Posts: 2,836
Default Re: 807 (maybe) amplifier build. Now EL34

No, only two valves. I found two "sense" resistors had gone high, from 10 ohm to 400k. This was putting the cathodes at 23v. What I can't get my head round is that 23/400000 = 575 uA cathode current. This obviously isn't the case or they wouldn't red plate. As the OP stage is running on fixed bias, this complicates things. I didn't have chance to take readings to suss out things further as I switched off sharpish.

Earlier in the thread I mentioned ways of protection should any OP valve draw too much current. Putting small wattage sense R's on the OP valve cathodes was muted as a possible protection "device". I've read the same suggestion over on DIY audio. However not all R's go short under high current conditions, as proven by these R's going high. Whether they went high as a result of over heating or are just faulty am not sure. When looking for replacements I found another "new" one, that had also gone high, around the same value, IE 400k.

Anyhoo, replaced said resistors, all well again - ish. However, I still got the "squeak" fault today. It sounds like the squeak you get with dry ice. Adjusting the CCS cures this. It's almost like the pot and some capacitive component, maybe the 0.1u cap across the zener, is acting like a tank circuit. I can only think that the CCS isn't stable, which puts the PS into oscillation, which in turn sends the OP stage into oscillation. The fault usually occurs at switch on, adjusting the CCS current kills the fault.

I've put a 1u and 0.1u cap after the 1235u cap "bank" to shunt HF to ground. Will also put some ferrite beads on the CCS transistor legs, OP anodes, g2's.

When stable and working, it works grand, with 120w OP. Measured ripple voltage is 200mV P-P measured at OP stage anodes.

A.
__________________
Curiosity hasn't killed this cat...so far.

Last edited by Diabolical Artificer; 7th Jun 2017 at 2:39 pm.
Diabolical Artificer is offline   Reply With Quote
Old 9th Jun 2017, 4:27 am   #199
Diabolical Artificer
Nonode
 
Diabolical Artificer's Avatar
 
Join Date: Jul 2009
Location: Sleaford, Lincs. UK.
Posts: 2,836
Default Re: 807 (maybe) amplifier build. Now EL34

The fun continues ... yesterday after putting a tiny ferrite bead on the collector of the CCS transistor I turned it on, checked cathode I, all well apart from two valves have a little high quiescant current. No squeak.

Signal in, all ok until about 30v P-P on the OP, then a bit of thickening of the bottom of the sine was apparent followed by all hell breaking loose with the waveform almost unrecognisable.

Power on, no signal input, scope probe on anode of one side, volume slowly turned up, at about the same place, IE volume knob turned by 90 deg, where we had the problem above, a 100khz 20v P-P almost perfect sine appeared, what the? Got another scope probe out to see where it was coming from or starting. Whilst probing around I noticed that when I touched a part of the circuit, it dissappeared or was attenuated. Interesting. Also noticed that touching the chassis attenuated it somewhat. Haha! Grounded chassis with a jump lead, 100khz sine vanished. Job done (maybe).

So your all probably shouting "what you didn't ground the chassis you pudding ! " Yep, it got forgotten what with doing a hundred and one other things. The big box is grounded/earthed, just not the wee chassis. So, thinking about this the chassis is like one big plate of a cap and will form a capacitor with wiring, components, Aunt Murial's hat and of coarse the builder of the amp. Which in turn, turns our amplifier into a transmitter.

These problem's I keep getting are a bit of a drag. Solve one, get a new one. However I enjoyed solving that last one. It was a chance to use acrued knowledge and put it into practice. I'm learning and if I say so myself getting better at solving problems, though I keep making daft mistakes. It's good craic, though hopefully soon the amp will work for more than five minutes without developing a fault.

Onwards and upwards, Andy.
__________________
Curiosity hasn't killed this cat...so far.
Diabolical Artificer is offline   Reply With Quote
Old 9th Jun 2017, 6:05 am   #200
Radio Wrangler
Dekatron
 
Radio Wrangler's Avatar
 
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 8,932
Default Re: 807 (maybe) amplifier build. Now EL34

Work on the assumption that most devices want to oscillate at any frequency at which they have even a glimmer of gain.

Work on the assumption that semiconductors keep being improved, their gain goes up, as does their frequency capability, so if you design anything intended to be manufactured over a decent lifetime, or repaired over ditto, you need to build in margin for friskier parts.

In a universe far, far away, someone created a theory that you need to return all ground connections to a single point or else you will suffer hum and noise pick-up due to ground loops.

In another universe far, far away in a different direction, someone created a theory that for stability at radio frequencies you need short immediate connections to a groundplane, or else you will suffer horrible oscillations and like Laurel and Hardy's chest of drawers, every time you close one, another pops open. Unused or supposedly unrelated parts and connections can get in on the act.

Here in our universe, both these theories are true. They look and are irreconcilable. So you have to compromise. The star ground threatens you with background hum and fridge clicks if you compromise on it. The groundplane threatens you with large oscillation, meltdown and drama if you compromise. You do have to compromise, but the threat levels say that the compromise is located nearer to the plane than to the star.

On top of this, you need to ask yourself what you've done to each individual device to stop parasitic oscillation.

For every feedback loop, you need to plan its frequency and phase response to engineer some stability margin.

Look upon these things as good housekeeping, good practice, and basic hygiene. Do them automatically every time. Do them thoroughly and don't wait for something to go wrong before you add them.

If you haven't designed in safety margins, then although your proto may work, you can run into trouble if it is ever manufactured or if anyone else builds one to your design, or if you have to change any parts.

This takes the self-discipline to put in parts which don't really seem to be needed. This is also an area where learning some maths pays off.

David
__________________
Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done
Radio Wrangler is offline   Reply With Quote
Reply

Thread Tools



All times are GMT. The time now is 1:56 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 - 2017, vBulletin Solutions, Inc.
Copyright ©2002 - 2017, Paul Stenning.