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Old 30th Dec 2015, 6:45 pm   #41
tri-comp
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Default Re: 807 (maybe) amplifier build.

Replacing the 20K pots with 100K ditto will not cure the problem.
You need more negative Bias voltage.
I suggest to feed the Bias-circuit around -25 ~ -30VDC.
Then your 20K pots will fit fine taking the amplifier to class AB1 which I find more sensible.
About -20V at Grid1 should set the cathode resistor drop around 800mV, each, depending on Va & Vg2.
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Old 3rd Jan 2016, 8:28 am   #42
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Default Re: 807 (maybe) amplifier build.

I made the bias more negative on the last test, but the 20k pots still didn't give me adequate adjustment. For instance on one valve a pot reduced cathode voltage, whilst on another I couldn't adjust downwards as the pot was on it's end stop. Not too worried about this at present as I'm going to use auto bias until I get the front end sorted.

Re the front end, I have problems here. I've taken out the second stage as it complicated things and wasn't really doing anything except volume control. That and the PS has stopped working. I also had a problem with attenuation of the signal from the PS by the OP stage. The OP stage has a g1- g1 low impedance, 100k ish when measured.

So I tried building a cathode follower first using a 6V6 triode strapped. I encountered problems here trying to bias the grid. I used a resistor above the cathode load resistor with the bias tapped off at the node where they join.No matter what value R I used, I couldn't get the grid to bias properly. I tried changing the value of the grid leak R also but feeding the bias from a pot across HT and 0v got the best results. However, I ended up with distortion and a square wave riding on top of the signal. The other problem I had was that I really had to reduce my input signal to get a clean output. I tried a triode as well, with similar results. In the end I thought I may as well use bigger grid leak/stopper R's on the OP stage to get bigger Z for the PS to see.

So after two days at the bench, I'm no further forward, with as I said the LTP phase splitter not inverting. A bit frustrating, but hey ho that's how it is sometimes. I have a few ideas to try today, if I still hit a brick wall I will put the amp aside for a few days, I havn't a deadline to meet fortunately. I'ts good to step back as you can come back refreshed.

Andy.
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Old 6th Jan 2016, 4:14 pm   #43
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Default Re: 807 (maybe) amplifier build.

I've rebuilt the whole front end using 1/2 of a 6SN7 as the input stage and another as a LTP phase splitter. I tried direct coupling but it was tricky to keep it stable. I connected it all up this afternoon and got 45v P-P before onset of clipping into an 8 ohm dummy load using a 1khz sine. So that's 32w "RMS" output power. Also used a speaker in series with 1/2 the dummy load, there was hum but's that's expected as my "layout" is untidy.

I'm quite chuffed, this is my first homebrew amp that I've designed and built myself, with your help here and on VRAT. There's lot's to do to make it into a finished amplifier like applying NFB etc. I'd advise anyone wishing to really learn about valves and amplifiers to have a go and build one.

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Old 6th Jan 2016, 11:20 pm   #44
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Default Re: 807 (maybe) amplifier build.

Andy,

So far, so good.
It's very rewarding listening to music streaming from an amp you created yourself
I take it you're only going with one pair of 807's at the moment ?
32 Watts isn't terribly much for a pair so at least you may expect the valves to live long.
Is there a chance to have a glimpse at the schematics ?
It would also be interesting to know what the 807 is now dropping across it's 10R cathode resistor i.e. are you still in Class A or more reasonable into Class AB1 ?
For AB2 you'll of course still need the cathode-follower drive and it isn't all that difficult getting it up and running.
You'll notice the difference in bass-output quality if you do. VERY firm !
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Old 7th Jan 2016, 7:14 am   #45
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Default Re: 807 (maybe) amplifier build.

That's three pairs of 807's with .6v across the sense R so 60mA quiesent. I'm running them with a common to all 50ohm R ( with the 10r sense R's on each cathode ), - 20v bias at the moment to get it going, so it will kick out a bit more. I'll use fixed bias in the finished amp.

The power output is 200w if calculated from it's P-P output - 40v which brings us back to the old chestnut, " What is the power rating of my amplifier?" It's loud on my WS test speaker (100w 4 ohm car speaker in series with 4 ohm dummy).

Thanks T for your interest and comments,I'll post a schematic later, Andy.
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Old 7th Jan 2016, 5:29 pm   #46
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Default Re: 807 (maybe) amplifier build.

Here's the schematic of progress so far. I need to reduce the 62.5k R on the PS. Yesterday the two signals were spot on. Today I've tidied up the layout a bit as I have quite bad 100hz hum/ripple as a result the in phase OP is too big.


TFL,A
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Old 7th Jan 2016, 11:31 pm   #47
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Default Re: 807 (maybe) amplifier build.

Some serious overkill for driving a car speaker

I like your schematic showing the LTP with a CCS in the tail.
You need to get as high an impedance as possible towards the cathodes and using just 1 transistor isn't optimal.
The amplification factor of a single transistor is too low.
The CCS should eliminate any need to balance the P/I output with different anode loads.
Do you know how to calculate/dimension the CCS circuit ?
If not I'll be happy to post a page on the subject from a Morgan Jones publication.
Very easy to follow, theory as well as practical dimensioning.
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Old 8th Jan 2016, 2:01 pm   #48
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Default Re: 807 (maybe) amplifier build.

I'm going to try the tranny CCS when I've sorted other issues. Thanks for the offer T, but found MJ's article online, that and I have a few books about CCS's and current mirrors. A lot of "designs" use LED's or diodes as the reference but I think a zener may be better. From what I've read a low voltage CCS is better as were not dumping heat as would be the case using HT. I'm going to use fixed bias so I'll knock up a little PSU for that and tap off a bit for a CCS and any other LT applications I might use.

Talking of fixed bias, what can I do to protect the OP stage in the invent of the bias supply failing?

I've eliminated the hum completely, now it's quiet as a the proverbial mouse. I upped the capacitance and tied all grounds to one point, still hum, so I gave it a poke with a plastic pen, the hum stopped after a poke on anode 2 of the PS, looks like a dodgy pin on the base.

The power OP is feeble for what it should do, so I need to alter the input stage and maybe the PS for more gain. I may need to add a driver stage too.

I have a problem though that I could do with a bit of help on. There is a 4v P-P saw tooth on the OP of the phase splitter - see pic. So the sine or music is riding this, not good. It looks like ripple, IE ramping up/down as the caps are charging and discharging. Could it be not enough capacitance on the PSU? (I have two 220u's in parallel as reservoir caps to drive the OP stage with a 47u for the input and a 47u for the PS) Therefore the HT is sagging? I'm sure the transformer and variac I'm using to power it all is big enough.

Andy.
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Old 8th Jan 2016, 8:53 pm   #49
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Default Re: 807 (maybe) amplifier build.

What is the frequency of the sawtooth and is it related to mains frequency?
Also is the nfb connected yet?
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Old 8th Jan 2016, 9:14 pm   #50
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Default Re: 807 (maybe) amplifier build.

To protect the OP in case of bias failure simply drive a relay from the bias supply. If the bias fails the relay will relax and cut HT to the output stage.
An even better method would be to disconnect the cathodes using the relay when bias fails.

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Old 8th Jan 2016, 9:36 pm   #51
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Default Re: 807 (maybe) amplifier build.

An idea from a relative novice re CCS.
I had a lot of success with a single ended EL84 amp which used a LM317 type regulator as a Constant current sink under the cathode, bypassed by the usual 47uF. It used a 27 ohm resistor to set 46mA draw on the EL84.
I reckon as long as you dont have more than 30V on the cathode it'll work with an appropriate setting resistor between the output and ground pin. At currents like 50mA or so you don't need a heat sink either.
No doubt more experienced and knowledgeable members can make a suitable comment on the relative merits of this approach.
here's a link to the article on Alex Kitic's Blog.

http://rh-amps.blogspot.co.uk/2013/0...sion-2_26.html

regards

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Old 9th Jan 2016, 8:04 am   #52
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Default Re: 807 (maybe) amplifier build.

Ta for the link Andy, I was aware that you can use the 317, I guess you could use a TL783
for higher voltages.

Thanks Jo, sound idea and I have plenty of relays.

Definately mains related Rob, the same waveform is on the mains at about 200mV so the ripple could be somehow getting into the valves and is being amplified. If that's the case, I can't understand how it's getting in? I wonder if it's being picked up on the tail of the LTP because of it's its high ish potential? Or it could be the HT sagging, which I don't think is the case as I'm using a 1kVA isolation tfmr and 500VA variac.

I suppose if I make a CCS for the PS then it could solve the problem. I'll have a probe about today and get back to you.

Thanks again for all your input, Andy.
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Old 9th Jan 2016, 8:22 am   #53
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Default Re: 807 (maybe) amplifier build.

Three terminal regulators are feedback-controlled machines, and with feedback comes the issue of stability. In planning their compensation timeconstants to get a stable condition of loop gain and phase, they make some assumptions about the capacitors which will be connected to ground from both their inputs and outputs. Not only the capacitance of the capacitor gets used, but also its effective series resistance. These assumptions are made for the intended operation as a power supply regulator.

In a constant current sink application, such as in series with a cathode, the cathode bypass capacitor is effectively the input decoupler seen from the point of view of the regulator. So in choosing this capacitor, take a look at the full datasheet for the regulator and see what the manufacturer recommends. Sometimes they don't talk about this explicitly, so you have to go by what there is in their applications circuits usually at the end of the data sheet. Usually there is a minimum amount of C needed at the input pin and a maximum at the output pin. As the constant current circuit just has a resistor on the output, the output is fine.

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Old 9th Jan 2016, 8:44 am   #54
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Default Re: 807 (maybe) amplifier build.

Andy, I think h-k leakage in the phase splitter may be causing the problem. Try running the heater from a separate DC supply.
I doubt it's HT ripple, your scope is your friend so use it and trust it.
Rob.
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Old 9th Jan 2016, 3:29 pm   #55
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Default Re: 807 (maybe) amplifier build.

I'm having a bit of trouble getting my head round using a CCS instead of a tail R in an LTP.
An LTP has a R to set the bias as in a normal common cathode stage which raises the cathode at X times the control grid which is at 0v, and an R "tail" as a CC sink. If I replace the tail with a LT CCS do I need to apply fixed bias to g1 or can I just replace the tail and leave the bias resistor in situ? No that doesn't make sense as one end need to be a 0v, so I presume I need to feed the CCS with a negative voltage with one end to ground. So therefore, I would reckon I can supply the anode at full HT.

Rob, I'll give that a go and let you know.

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Old 9th Jan 2016, 3:46 pm   #56
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Default Re: 807 (maybe) amplifier build.

A constant current source in the cathode of a valve will let the cathode potential float to whatever voltage gives the chosen current. If g1 is at ground potential, then the cathode will rise to the same voltage it would sit at if you'd used a cathode resistor of the right value to get the quiescent current bang on.

Now, if you change the valve for another with a bit different characteristic, the cathode resistor circuit will settle at a bit different quiescent current. the constant current source circuit will change the cathode voltage and get the quiescent current bang on again.

So CCS biased stages don't need bias adjustment and accommodate changes in valves and ageing all up to a point, of course.

The fun bit is what would happen without the cathode decoupling capacitor. A CCS would allow the cathode voltage to vary as much as necessary to kep the valve current constant. So the cathode voltage will rise and fall following the grid voltage, and the current will stay constant. So that means the anode current is constant, and there is no signal output so the gain is zero, not a sausage. This will be true over the bandwidth the constant current source can work over... Ie most of the audio band.

The cathode decoupling capacitor's job is to hold the cathode voltage fixed over the audio band, but not at DC. The fixed cathode voltage for AC restores the stage's gain, and relegates the constant current source to only control the DC current component.

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Old 10th Jan 2016, 12:20 pm   #57
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Default Re: 807 (maybe) amplifier build.

I was possibly lost somewhere on the way in this discussion.
Wasn't the CCS suppose to go in the LTP tail and NOT in the output stage ?
Putting it in the cathodes of the 807's will set the amp back in class-A which may be fine but very far from the original idea of a cathode-follower drive ending up in class-AB2.
About dimensioning the CCS have a look at the attached articles.
A very straight-forward, non scientific explanation on how it may be done.
Remember, the articles are very old and today, in order to raise the AC impedance and the lower the capacitance at the output (towards LTP cathodes), you would use high-gain silicon and happily 2 in a cascade as Morgan Jones explains.
You'll end up with an extremely high impedance, low capacitive load and hence a very fine frequency responce.
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Old 10th Jan 2016, 12:31 pm   #58
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Default Re: 807 (maybe) amplifier build.

Sorry, I got thrown.

A CCS in a long tailed pair does good things, but it shouldn't have a decoupler across it. It's job is to provide bias current while letting the pair of cathodes move in voltage according to the input signals. The cathode of the other valve looks to each valve as trying to drive a cathode follower backwards - IE a low impedance.

Where we run into trouble is that 3-terminal regulators make good constant current sources at DC, but most lack the bandwidth to handle the full range of signals that can be found in an amplifier. Also they need an input decoupling capacitor for their own feedback system, and this not a good thing for a long-tailed pair.

For a long-tailed pair, use a discrete transistor current source. You not only need enough bandwidth to handle the adio range, you need more to make sure it's not disturbing the gain/phase characteristic of the main amplifier feedback loop.

Mention of cathode in singular and decoupling capacitors and remembering that this thread was working around bias schemes for the 807s diverted me.

David
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Old 11th Jan 2016, 7:45 am   #59
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Default Re: 807 (maybe) amplifier build.

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.
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Old 11th Jan 2016, 7:58 am   #60
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Default Re: 807 (maybe) amplifier build.

It can do away with the tail resistors, but you still need base/grid/gate bias and collector/anode/drain loads for the pair itself.

Sometimes you'll see a long-tailed pair with small value resistors to each cathode/emitter/source from the constant current source in the tail. What these do is to provide a little degeneration and trade some gain for linearity.

The constant current source in the tail does several good things for a long-tailed pair. It not only makes biasing a bit better defined, it also provides a significant reduction in common-mode gain, so that the pair is sensitive to only the difference of the two base/grid/gate voltages, and any movement of the two voltages together is ignored.

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