18th Mar 2016, 10:12 pm | #121 |
Moderator
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 22,876
|
Re: Solid state preamp for EL84
You'll form your own set of default settings for your scope and automatically use them except for special circumstances.
Keep the two variable gain knobs for the two input channels in their 'cal' positions and then you know what the vertical scale is in volts per division. Do the same with the variable control on the timebase speed. Sometimes you may want to fit a trace to spread some feature across a chosen number of divisions.... for example there will be dotted horizontal lines on the screen to help you make pulse rise time measurements and to do this you use the varible control to scale the pulse amplitude to fit between one pair of lines while you count the divisions between where it crosses the other pair of dotted lines. It's harder to describe than to do. Not often needed in radio work. Some scopes have warning lights to make it more obvious when you have any of the three variable controls out of their cal position. So, keep them in cal. Leave the input coupling set to DC so the scope shows you whereabouts the voltage of a signal is as well as the AC component. Switch to AC when you need to look at a small signal on a big offset. You'll start using your scope as your bench voltmeter and prodding something that should be a power rail with several volts on it and thinking there is nothing there until you eventually realise you left the input in 'AC' coupling is something we all do. There isn't that much skill in making a pair of speakers, it's a satisfying day's work with the right tackle. There are plans on Wilmslow Audio's website. A tilting circular saw that cuts exact mitres was something I should have got years earlier and saved a lot of careful chiselling of joints. A router makes life easier too, and power screwdrivers aren't essential, but they save a lot of time. Modern adhesives are great too. It's a learnable skill but inclined to suck you into doing progressively larger projects. The speakers got followed by a kitchen all made out of solid hardwood, and now it's stable roofs being renewed. All hobbies are bottomless pits. You're a bit too far away or we could knock up a pair of cabinets in a day and have them working. David
__________________
Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done |
19th Mar 2016, 6:35 am | #122 |
Dekatron
Join Date: Jul 2009
Location: Sleaford, Lincs. UK.
Posts: 7,662
|
Re: Solid state preamp for EL84
Some good tips on driving scopes both. I don't know that much about half the controls on some of my scopes. It can be tricky for those us neophytes who didn't go to college or use a scope in work.
The "bung in a 50hz signal" was on an ancient basic HP scope, David. Thanks for explaining what another function on the scope does. Completely understand the need for space Tim, but here's a couple of things might help. Measure the cathode of the EL84, should be around 2-6v, this will give you it's bias point or operating point. Easier to finnagle a preamp with this info. I'd then connect your sig gen up to input/g1 of the EL84, scope on the speaker out,volume control at 3/4 or so start with a low signal and increase till the sinewave clips. You'll know how much gain is too much then. I'll try and do a short video over the weekend and put it on Utube if that helps. Andy.
__________________
Curiosity hasn't killed this cat...so far. |
19th Mar 2016, 10:32 am | #123 |
Moderator
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 22,876
|
Re: Solid state preamp for EL84
A high performance scope has loads of facilities and controls that are rarely needed, so all you need to do with them is to know what position to park the knobs and buttons in so they don't do anything to spoil more basic measurements.
I bought a fairly good slide-rule when I was a schoolkid and used it through most of a degree course. I still don't know what some of the scales on it are for. Working at HP as a student, I bought an HP45 calculator at employee price and boy did that make life easier on the rest of the course. I now know what the functions on the trusty '45 do, but some have never been used. They're there should I ever need them. David
__________________
Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done |
19th Mar 2016, 1:03 pm | #124 |
Moderator
Join Date: Jun 2003
Location: Oxford, UK
Posts: 27,947
|
Re: Solid state preamp for EL84
Let's stay on topic please. Poor old Tim is still struggling with his preamp and is looking for practical advice.
|
19th Mar 2016, 7:55 pm | #125 |
Heptode
Join Date: Aug 2011
Location: Bristol, UK.
Posts: 538
|
Re: Solid state preamp for EL84
Hello guys,
I couldn't help myself and went into the workshop today. First off I dropped the collector resistor from 150k to 100k and I still had distortion so I plugged the preamp into a 50W valve amp. I didn't have any distortion! So I dropped the collector resistor down to 47k which seemed to fix it. There was a bit of distortion but perfectly acceptable on such a crude player. I did have quite a loss of gain though. I'm going to put 68k in the collector and see if that's a happy medium. I think though that I have proved its the el84 clipping. I will report back on my results on Tuesday. Many thanks, Tim. |
19th Mar 2016, 10:10 pm | #126 |
Dekatron
Join Date: May 2008
Location: Derby, UK.
Posts: 7,735
|
Re: Solid state preamp for EL84
What value capacitor are you using to couple the signal from the collector to the valve grid?
__________________
If I have seen further than others, it is because I was standing on a pile of failed experiments. |
19th Mar 2016, 10:42 pm | #127 |
Heptode
Join Date: Aug 2011
Location: Bristol, UK.
Posts: 538
|
Re: Solid state preamp for EL84
.01 uf
|
19th Mar 2016, 10:53 pm | #128 |
Dekatron
Join Date: May 2008
Location: Derby, UK.
Posts: 7,735
|
Re: Solid state preamp for EL84
That sounds quite reasonable, so probably not to blame. Where is the volume control? Between the cartridge and the preamp, or between the preamp and the valve?
Some may not like the idea, but a modern, hand-held, battery-powered digital storage oscilloscope -- even a crude single-channel one -- is much better than no oscilloscope at all.
__________________
If I have seen further than others, it is because I was standing on a pile of failed experiments. |
19th Mar 2016, 11:08 pm | #129 |
Moderator
Join Date: Jun 2003
Location: Oxford, UK
Posts: 27,947
|
Re: Solid state preamp for EL84
If you read back through the thread you'll find that Tim is using a scope.
|
22nd Mar 2016, 11:52 pm | #130 |
Heptode
Join Date: Aug 2011
Location: Bristol, UK.
Posts: 538
|
Re: Solid state preamp for EL84
Hi guys,
I worked on it all day. To cut a long story short I changed the .01 to .001 and the bass and distortion was a lot better. I eventually settled on 100k for the collector load. Anything above .001 and the excessive bass would come back, even with .0033 I want to put this thing through its paces before declaring victory. I thought it started creating feedback with a high pitch whine just before I left today. I've taken another video. I know the deck is playing a bit sluggish I will sort it tomorrow. https://m.youtube.com/watch?v=oz_9FyLPJEA&feature= I will report back tomorrow. Thanks again, Tim. |
23rd Mar 2016, 12:19 am | #131 |
Octode
Join Date: Nov 2013
Location: Penrith, Cumbria, UK
Posts: 1,993
|
Re: Solid state preamp for EL84
it could be instability rather than feedback. your amps oscillating?
|
23rd Mar 2016, 12:56 am | #132 |
Moderator
Join Date: Jun 2003
Location: Oxford, UK
Posts: 27,947
|
Re: Solid state preamp for EL84
You really do need to establish what the fundamental problem is rather than randomly tinkering about. You have relatively sophisticated test gear so this shouldn't be difficult. Your preamp should be a linear amplifier and shouldn't be introducing distortion at all. The EL84 is after the volume control so won't be being overloaded at normal listening volumes. You need to adopt a methodical approach, as others in this thread have advised you. This 'pin the tail on the donkey' approach won't yield a satisfactory result.
|
23rd Mar 2016, 8:00 am | #133 |
Dekatron
Join Date: Jul 2009
Location: Sleaford, Lincs. UK.
Posts: 7,662
|
Re: Solid state preamp for EL84
Glad your making progress Tim, but could you take a few measurements for us like voltage of AC input, use scope to get this. Then knowing DC conditions is helpful, EG DC at BCE, at EL84 cathode, anode and AC signal from transistor. I'd also like to know how your volume pot is connected. One end to ground, but where is the signal going in, on the wiper or "top" of pot.
Altering values of collector R and coupling cap will effect frequency conditions, as Paul says, a methodical approach is best,, you need to know or have a general idea of why changing a resistor or cap has such and such an effect, and what it does to rest of the other parts of the circuit. Andy.
__________________
Curiosity hasn't killed this cat...so far. |
23rd Mar 2016, 6:08 pm | #134 |
Heptode
Join Date: Aug 2011
Location: Bristol, UK.
Posts: 538
|
Re: Solid state preamp for EL84
Hello guys,
I haven't had a chance to look at this today. I'm going to take measurements asap. I agree a methodical approach is needed. But I was going around in circles not knowing what I was doing. I will get pp data probably next week now. Will keep you all informed. Thanks. |
5th Apr 2016, 10:00 am | #135 |
Retired Dormant Member
Join Date: Jul 2005
Location: Somerset, UK.
Posts: 334
|
Re: Solid state preamp for EL84
Hi Tim
Any news of a breakthrough? Anxious of Somerset. |
6th Apr 2016, 12:33 am | #136 | |
Hexode
Join Date: May 2015
Location: Northampton, Northants, UK.
Posts: 380
|
Re: Solid state preamp for EL84
Quote:
It now suffices as a quick'n'dirty 12-0-12 bench, er, desktop, er, whatever power supply. There must be a lot of these useful tidgy transformers in landfills. |
|
6th Apr 2016, 1:10 am | #137 |
Heptode
Join Date: Aug 2011
Location: Bristol, UK.
Posts: 538
|
Re: Solid state preamp for EL84
Hi guys,
Sorry for the delay. I haven't had a chance to get on with it recently but I will let you all know when I do. It will probably be in a week or two now. Many thanks, Tim. |
16th May 2016, 1:24 pm | #138 |
Heptode
Join Date: Aug 2011
Location: Bristol, UK.
Posts: 538
|
Re: Solid state preamp for EL84
Hello guys,
sorry for not getting back sooner. I've measured peak to peak voltage in the transistor preamp only. I have .4 V with a 10.75khz frequency coming out of my signal generator. I have measured this with nothing connected to the signal generator output. I get with the vol pot 3 quarters of the way up 2.1 V at the collector. The frequency stays the same. Now once I go beyond 3 quarters on the vol pot the top of the waveform starts to clip quite harshly and I measured this in the preamp output so the clipping seems to be happening in the preamp even though the pot is at the output. Anyone have any thoughts on this? Many thanks, Tim. |
16th May 2016, 2:39 pm | #139 |
Octode
Join Date: Aug 2003
Location: Welwyn Garden City, Herts. UK.
Posts: 1,906
|
Re: Solid state preamp for EL84
Surely you should be using 1KHZ.
|
16th May 2016, 3:14 pm | #140 |
Heptode
Join Date: Aug 2011
Location: Bristol, UK.
Posts: 538
|
Re: Solid state preamp for EL84
Well, unless I've got something wrong that's what it's fixed at from my sig gen.
I counted 9.3 divisions at 10 uS per div. 9.3 x 10 = 93 divide 93 by 1 million = .000093 divide by 1 (f=1 over t) = 10752.688 divide by 1000 = 10.75 I should say I haven't calibrated my sig gen but I don't think it matters so long as the signal is in the audio frequency, does it?. Many thanks, Tim. |