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Old 12th Jan 2016, 10:34 am   #61
Diabolical Artificer
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Default Re: 807 (maybe) amplifier build.

Thanks David for clarification. I tested and adjusted the CCS see pic left yesterday but was puzzled that when I tested it, as LED 1 went off. I then redrew the schematic see pic right
and concluded that current must be flowing through the top transistors collector because the LED 2 is drawing too much current and starving LED 1 of current turning off the top tranny. Does that make sense?

I'll test with the LTP today and post results.

Andy.

Apologies for the dire schematic. in a rush.
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Old 12th Jan 2016, 11:50 am   #62
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Default Re: 807 (maybe) amplifier build.

Get the constant current source working before putting it into the long tailed pair.

Something's odd. In the left diagram, there should be about 7mA down the bias resistors, zener, LED path, and about 2mA down the two transistors.... well, only 2mA if there is 2mA available from whatever the top collector goes to.

In the right diagram, the top transistor should be on, the bottom one off, no current in the zener, no current in the transistors and LED2 should be in trouble with nothing limiting its current.

A good enouch constant current source would only need one transistor. For s really fab one look for "Ring of two reference" in wireless world.

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Old 12th Jan 2016, 3:30 pm   #63
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Default Re: 807 (maybe) amplifier build.

Thanks David, your right, circuit on right = test LED 2 and ammeter connected, LED 1 goes out, LED 2 won't light as insufficiant current. At least I think that's what is happening. I tried the circuit with a 2k2 R and I got 1.9mA But for some reason it won't work with the LTP as connected- see pic.

I've attached the article I used to work out my component values. With an HT of 300v and bias at 2v one triode of a 6SL7 will be at 1.1mA anode current. Whether it's sinking it or sourcing it in regards to the CCS I don't know. The emitter resistor of the CCS sets the current, so with the zener holding the base of the bottom tranny at 4.7v we need to drop 4v at 2mA across the emitter R to put the base at .6v = 2k or 1.8k for 2.2mA. Both transistors are BC337s

Found the article you mentioned David and am in the process of reading the articles Tricomp attached so I'll hopefully have more of a clue and will try again tomorrow.

Andy.
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Old 12th Jan 2016, 7:51 pm   #64
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Default Re: 807 (maybe) amplifier build.

Quote:
Originally Posted by Dr Wobble View Post
Easily done David, thanks for your explanation. My question concerned using a CCS instead of the tail resistor in an LTP, put simply I wanted to know how to bias the LTP, as the CCS does away with all resistors in an LTP - correct?

Thanks T, I'll read those;as I said, I have a rough idea of how a LTP works, it's the practice at the moment that's tricky.

I knocked up a little board at the weekend with a variable up to 30v PSU for the biasing of the OP stage and a CCS a la Morgan Jones bolted on the side as it were, which to my untutored eye looks like a pair of transistors in cascode, a voltage reference - a red LED and a few resistors to set current. The DIY audio boards and your CCS T on your 2C34 amp is shown supplied by a positive and negative voltage source, EG + 12v and -12v and -78v which puzzles me as both have no 0v reference shown.

Not to worry, I'll suss it out, thanks all for your help, Andy.
Apologies Andy,
My comment was directed at the use of a LM317 type CCS in the power output valves.
Radio wrangler correctly has put me right on the why's and wherefores.
I found a new section in the latest edition of Morgan jones' "Valve amplifiers" 4th ed, (thank you Mrs nut for the Xmas present!) which deals with the use of a 317 type regulator as a CCS and it explains why it's only really suitable in the cathode circuit of an output stage. The advantage as all here will know, is holding each side of a push pull transformer primary at equal currents by forcing the Valves current draw.
I think that's how I understood it anyway.
I think I'll try that trick on my EL34 project amps, nothing to lose and maybe something to learn?
As an aside I noticed at times on my SE EL84 amp that it would go into motorboating on one channel and I am fairly certain it was post CCS mod. I think? that the cause was perhaps the reg dropping out before the EL84 was fully warmed up and conducting fully. Then again it could just have been marginal stability from the regulator. I have a memory of being told that the LM317 regulator could have stability issues that other varieties of adjustable voltage regulator were more immune to.

Well done Andy so far, All I did was tweak about with a known and proven design of amplifier. Nothing compared to your research and experimentation.
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Old 13th Jan 2016, 12:17 am   #65
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Default Re: 807 (maybe) amplifier build.

That circuit may not add up,Andy.

4.7v on the zener, another couple for the LED and the top transistor base is very close to the 6.3v you've labeled on the top collector. Transistor current sources have a minimum voltage below which they saturate and don't work properly. There is no real need for that top transistor or the LED. I'd drop the zener down to 3.3v and pick an emitter resistor for the wanted current at 3.3-0.7=2.6v.

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Old 14th Jan 2016, 12:17 pm   #66
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Default Re: 807 (maybe) amplifier build.

Thanks Andy for your kind words, I'd encourage to have a go yourself, it's good learning experience.

After 4 days of mucking about, I have the LTP running on the CCS. thanks again David for your help, I found a 2v7 zener, took off the top tranny and diode and got it running after lots of loosing my way and mucking about. I was really down yesterday after I finished as I couldn't get it to work.

Even though inexperienced, I'm slow sometimes. After getting the CCS running I spent all yesterday with -2v on g1 with obvious consequences, distorted OP and confusion. After a night reading and thinking about it on my walk this morning I figured it out. With -2v on g1 and the collector of the CCS at about +2v and hence +2v on the cathode, the grid won't know whether it's coming or going. So no grid bias applied and the LTP works ok.

That said, the non inverted signal is a bit out, about a 3v disparity, so not too bad. After a coffee I'm going to measure the anode R's and adjust. I have one other anomaly, well two; the non inverted sine is rounded at the bottom - see pic 1 and 2. It's not because of cut off as this oscillogram is at 10v P-P, the 6SL7 as biased should knock out 170v P-P per anode, so I should have sufficiant for NFB. I need approx 20-30v, so even halved by the Z of the OP stage I shouldn't need a driver stage.

I'm running the heater on DC off a bench supply, so the saw tooth we had seems to have diminished, there is now a sawtooth of 100mV P-P on the LTP anodes. I'm going to de-couple the HT from the LTP as I can loose a bit.

One other anomaly I've noticed is a distorted square wave - see pic 3. Is this down to bad frequency response? The input is 1khz 500mV P-P square.

Andy.
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Old 15th Jan 2016, 4:33 pm   #67
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Default Re: 807 (maybe) amplifier build.

Well after spending all week mucking about with trying to get the LTP working with a CCS
my opinion is, it's not worth it. It might be for someone with more knowledge and experience, but I don't think it's for me.

I have had it going with beautiful balanced signals but at the cost of output and stability. With a R as the tail and a bias resistor I had a lot more output whereas with the CCS at about 40v P-P, the CCS crashed and the two sines went out of whack.

I also have a big problem with biasing and grounding the grids as well as finding the right value for the grid leak R's. That and the CCS circuit is very sensitive, by that I mean balancing the resistor that dictates how much current the zener gets with the resistor that dictates the emitter/CCS current seems nigh on impossible. I've seen schematics where split rails are used, but can't see how this alters the situation.

To sum up I,m going back to using resistors; I'll come back to current sinks in the future.

Andy.
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Old 15th Jan 2016, 7:22 pm   #68
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Default Re: 807 (maybe) amplifier build.

Going back to the LTP 6SL7 circuit in post#63, isn't the fundamental problem that it can't work unless the common cathode is high enough in voltage relative to ground to allow for a signal swing of the same order as the input drive? A resistive tail does this with the valves biassed by an additinal low value bypassed resistor between the cathodes and the tail, grids leaked to the top of the tail.

It doesn't have this-the grids are at 0V ish so at the running current of 2mA the cathode will only be at 2V or so. Nothing in that circuit will work right, there's not enough collector volts for the ccs or anything.

So..... pick a LTP anode current so that the anodes sit at about 3/4 of HT feed volts. Bias the input grid to about 1/4 of HT feed volts using several resistors- say 1meg from HT, decouple bottom end to ground with 100n//330k then take a 470k resistor to each grid from the 1meg/330k junction and decouple the second grid to ground with another 100n.Everything should now sit happily with the cathodes up at a couple of volts above 1/4 HT and the anodes at about 3/4 HT. Ensure that the LTP pass transistor Vce is rated for at least half the HT supply volts.

Hopefully it'll now do something like it's expected to!
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Old 15th Jan 2016, 10:37 pm   #69
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Default Re: 807 (maybe) amplifier build.

That should work, Chris.

THe circuit is actually a constant current SINK, and it needs several volts to work. The biasing of the whole arrangement needs to be planned, then component values calculated.

David
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Old 16th Jan 2016, 10:13 am   #70
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Default Re: 807 (maybe) amplifier build.

Like this Chris? See pic. I knew the bias was wrong but couldn't figure out how to fix it.

I know what you mean David, I've built this off the cuff, but would have benefited from some planning, but I didn't know what or how to plan at the start, now I have a better idea.

Thanks, Andy.
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Old 17th Jan 2016, 2:20 am   #71
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Default Re: 807 (maybe) amplifier build.

Caught up with this.....

See attached for my ideas
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Old 17th Jan 2016, 9:05 am   #72
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Default Re: 807 (maybe) amplifier build.

Chris' bottom circuit is what you need. You need to have the two grids at equal DC bias voltage, and having two 470k resistors from a common bias point makes everything symmetrical and shares the bias current evenly between the two halves of the double triode.

It relies on the two triodes being fairly similar, though obsessive-compulsive matching isn't required.

If you want to take out the DC balance issue and have an AC-coupled long tailed pair, separate the two cathodes, couple them with a non-polarised capacitor and provide them with individual current sinks, each of half the original current. The current sources can share their base bias circuits.

David
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Old 17th Jan 2016, 4:05 pm   #73
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Default Re: 807 (maybe) amplifier build.

Had a SPICE at the CCS I sketched- it looks like about 200k resistor as the long tail. A real resistor in the same role could only be about 15k. Not sure whether it would make much difference in practice.
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Old 18th Jan 2016, 6:49 am   #74
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Default Re: 807 (maybe) amplifier build.

Thanks Chris for all your work, much appreciated. I ordered some MJE340's from CPC to use in a HV CCS, but as usual they're not in stock, even though the site say they are, ce la vie.

Actually, when I had it running on CCS, the DC balance was very good, within a few volts. Just couldn't work out how to bias it properly. One interesting thing happened while I was testing, I made two mistakes, I wired the CCS up wrong and didn't power it up, but the CCS still worked. I'd connected the zener straight to the emitter with the E R to ground.

For now I've used resistors to get it up and running and it all works, sine in, clean sine out only louder, no idea what THD at present, but can't hear anything untoward. However, this design doesn't make good use of the OPT which is capable of up to 120w or the 6 807's.

I was getting about 30w, which with HT at 300v and connected as UL is all I'm going to get. Don't get me wrong 30w is sufficiant, but I could get the same power for less iron and valves. I was under the impression my speakers were inefficiant, so I cranked the volume up on my Yamaha amp and took readings. I knew you don't need mega watts but was still surprised to see 2w was all it needed to generate enough SPL to make my young daughter run away.

So, I either change valves to EL34's and get a bit more out in UL or not bother with UL and increase HT on the anodes which will give a lot more power. One other thing I noted while testing is the amp goes down to 10hz and up to 50khz ( maybe higher, my sig gen only does 50) so I need to roll off the top end, as well as applying some NFB. So far I've tested it with PA speakers, I'll connect to my hifi speakers today and have a good listen and do some more testing.

Andy.
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Old 18th Jan 2016, 3:56 pm   #75
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Default Re: 807 (maybe) amplifier build.

Just finished testing the amplifier on a HP334A distortion analyser and with a 1khz sine input
I got a reading of .18% distortion. This is the first time I've used the HP so there is a distinct probability that I've not tested it and not quoted the above figure correctly. Interestingly I got the same reading using a 10khz sinewave as input.

I also tried it through my usual speakers I listen to music on, the bass was more prominent and sounded rounder than through my tranny amp - 2nd harmonic distortion?

The MJE340's came today so will knock up the CCS that Chris kindly drew for me. I'll try disconnecting the screens and using more HT on the anodes tomorrow but I won't be able to test with a speaker load as I don't have anything here big enough.

Andy.
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Old 18th Jan 2016, 4:22 pm   #76
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Default Re: 807 (maybe) amplifier build.

Well, distortion can improve subjective quality. Generating harmonics of low bass fundamentals makes the result reproducible by a speaker that may not be capable of reproducing the fundamental. And if the harmonics are present, the ear is fooled into believing that it's hearing the fundamental too. This principle has been used for a couple of centuries by pipe organ builders with 'harmonic bass' pedal stops. If they can't fit in a 16-foot pipe (32Hz), then sounding 64Hz and 96Hz simultaneously provides a convincing imitation.

In order to hear the effect of 2nd and 3rd harmonic distortion in an amplifier, then it's generally recognised that you need at least 1% for it to be audible. It's interesting that it's as sine wave distortion begins to be visible on an oscilloscope, it generally starts to be audible as well.

The problem with distortion on real life music is generally not the harmonics generated, but the intermodulation products between different frequencies which can lead to an unpleasant muddy muddle.

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Old 22nd Jan 2016, 6:45 am   #77
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Default Re: 807 (maybe) amplifier build.

After some weeks of digging into photobucketdotcom, ( seems they had some sort of major crash) I managed to find two only pictures of the 807 amp I constructed a while ago.
This is merely push/pull and way less power than DR Wobble's monster.
Its in fact triode mode, but different to the Williamson, as this runs into a load of 3400 ohms. These pics were taken during construction, and the empty 4 holes on the left of the chassis are for the phono (RIAA) preamp.

I have not heard from the owner,so I assume its still working

Joe
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Old 22nd Jan 2016, 7:10 am   #78
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Default Re: 807 (maybe) amplifier build.

Nice work Joe,you can see you've put a lot of thought into layout. If I can build a chassis as nice I'll chuffed. I can see you've got a decent PSU too, BTW what are those two objects bottom right- meters?

Andy.
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Old 22nd Jan 2016, 8:54 am   #79
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Default Re: 807 (maybe) amplifier build.

I think they're for adjusting how spine-tingling the music is going to be once the record starts playing... Portentiometers (Terry Pratchett would have been proud of that one)

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Old 22nd Jan 2016, 6:44 pm   #80
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Default Re: 807 (maybe) amplifier build.

"Out of Cheese error"................................
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