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Old 12th Feb 2016, 10:27 am   #101
Hartley118
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Default Re: 807 (maybe) amplifier build.

Quote:
Originally Posted by Dr Wobble View Post
Hav'nt experimented at all Martin, I did briefly consider putting a 8 ohm load on the 4 or 16 ohm secs, but Rob (AKA Mr Anode) told me I'd loose low freq's. I don't think that's what your talking about though. By using a 10 ohm dummy say on the 8 ohm secs is what I think
it is you have in mind. How does this help in a practical amplifier? Can you elucidate please?

Having thought about it, the pri Z has to be low for multiple/parallel OP valves. It's just that 807's are ill suited as UL valves.

The PSU is running off a 1KVA iso and variac, so enough ooomph. The pre stages were running on low HT yesterday. The problem is every time I change the set up I have to recalculate and change resistors etc. I don't always get it right first time. So the input to the OP stage was low. The LTP/cathode follower combo I have at present works really well, I can turn the volume control up to nearly max before the OP clips, therefore have plenty of drive potential.

Andy.
Just that you don't seem to be getting the power that 6x807s can yield. You've spent an admirable amount of time studying 807 characteristic curves, bias, load lines etc and it might well be worth experimenting with the actual anode load that you're presenting to the output valves ( = 'speaker' load x ratio squared). I've suggested experimenting at mid-frequencies so that results aren't trammelled up by the transformer primary winding impedance which appears in parallel.

By the way, I wouldn't worry about pushing the 807 screen grids higher than their rated voltage - they're tough valves. I ran a pair for years in a 30W amp in ultra linear with well over 400V on the screens.

Martin
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Old 12th Feb 2016, 2:14 pm   #102
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Default Re: 807 (maybe) amplifier build.

Just finished experimenting on the amp, and did as you suggested Martin - 1khz input, 8ohm pot in series with the 8 ohm dummy load - turned the pot up, up went the V RMS, but we arnt getting more power as the load has increased. Before with an 8 ohm load I was getting 20v, so 20 X 20 div by 8 = 50. With 16 ohm load I got 25v RMS, so 25 X 25 div by 16 = 39, so power gone down. Or am I missing something here? One of my whackier ideas was to put a resistor/choke on either end of the OPT primary.

Re the screens of the 807 I've read, as you both say that the 807 can handle more than 300v, it's only a 6L6 in disguise, yes? Which brings me to a puzzling reading I'm getting off one 807. The STC datasheet says PP 807's 360v HT, 270v G2, no sig anode 88mA, max sig 138mA, no sig G2 5 mA, max 16mA. So to drop 90v I'm using a 18k R and a 2k R stopper to limit screen I. 2k is wrong, I used max anode I in my calculation. Anyway, that aside, on one 807 I'm getting 360v at the screen. So I test ed the resistors, replaced them and the 807 with the same result. Odd.

Another oddity (maybe) is with -22v fixed bias I'm getting 60mA approx quiescent current, probably down to low OPT primary. One other thing of note with no signal the other screens are sitting at 300v ish, but at max signal they fall to around 263v. I know there's supposed to be some interplay between anode and screen, but is this normal?

A few questions. Joe, you say to use a grid "stopper" of a few ohms, the data sheet says " a series grid R may also be employed, but should be chosen with respect to frequency response.... should not exceed 100k for Class A and not used for AB2" Will try lowering it as you say. BTW, when calculating the value of a dropping resistor to limit screen voltage in an OP stage, do you calculate for min or max conditions?

Apart from upping the screen voltage I'm not sure what to do at present re the OP stage power. Pretty sure my LTP/driver is ok. Will experiment more and report back.

Thanks for your suggestions/input, Andy.
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Old 12th Feb 2016, 4:09 pm   #103
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Default Re: 807 (maybe) amplifier build.

I suggest that you first sort out the odd valve that doesn't seem to be drawing any screen current. Does the fault move with the valve? If not, it may be a bad valveholder contact - you seem to have checked everything else.

Just in passing, I think you're being conservative on screen voltage, I ran a pair of 807s for some years in UL in a 30W amp with 450V or so on the screens and never had a failure despite anodes glowing a (very) dull red. The 807 is a robust animal!

Your load impedance experiment is interesting - it's pointing to the optimum load being lower than 8 ohms. In other words, need to draw more current from the valves. From your previous power measurement, it looks the amp can push 20V comfortably into an 8 ohm load. Now try dropping the load resistance, perhaps by connecting the pot in parallel with the 8 ohm load. If your pot has good power handling, at least for long enough to make a voltage measurement, you could use that alone to give a 0-8 ohm load. Otherwise, I'm sure you're resourceful enough to fix something up to, say, vary the load between 2 ohms and 8 ohms. The issue is: how much peak current can you draw from the 807 anodes before you start to run them into saturation below that 'knee' in the graph of anode characteristics.

Martin
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Old 13th Feb 2016, 12:14 am   #104
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Default Re: 807 (maybe) amplifier build.

For control grid stoppers, I dont even bother "calculating" a value. I start with say 4k7 and "suck it and see". If I still have oscillation I go for 10k, but for myself thats the upper limit.
For screen grid resistors I use 47 ohm carbon 1/2 watters, they act as a fuse if something really goes haywire.

I used to look after a 5000 watt AWA AM radio transmitter that used 807's as pre modulators. Even with bright lighting it was easy to see dull red plates on those valves.
I worked for that radio station for 5 years and never replaced them. They are a very good bottle.
Without discounting all the work you have done so far, I would like to make a suggestion.
Get rid of current sources!!
For a start use -30 volts bias!
Ground each cathode through a one ohm resistor so we can measure the current from each valve.
Use a simple phase splitter! I single triode with the top trio driven from the splitter plate, Drive the bottom trio with the cathode of the splitter.
Use a simple voltage amplifier in front of that.
Test it!!!
That setup should give something in the region of 100 - 120 watts RMS.

Now that we have the boot fitting the foot, we can polish the toe caps.

In other words, Im suggesting the KISS principle.
Please dont shoot me When things go wrong, I go back to basics.

Joe
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Old 13th Feb 2016, 7:32 am   #105
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Default Re: 807 (maybe) amplifier build.

Was feeling a bit disconsolate yesterday so I upped the HT to 380v, G2 droppers still in place and the OP went up a bit. I then dropped the bias, big change, the Ia went up as did OP AC to 25v, so 78w.

One side effect of this was ringing on the sine in and out. Will take pics later. Today, I'll change a few resistors, find a big trmr and up the HT. As you say the 807 is tough,; I've had one glowing bright red when I left a meter between K and ground.

Fixed the dodgy valve, it was a loose connection of a 10r R to ground, 40v on the K, yep tough valves.

Joe, good advice there, use a cathodyne PS and amp, KISS. I would take it if it weren't for the fact that my pre set up is working, is capable of driving into AB2, I'm fed up of changing components/setup, have lots on round to it pile, et al. Will see what happens today, may have to re think whole amp.

Andy.
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Old 13th Feb 2016, 8:05 am   #106
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Default Re: 807 (maybe) amplifier build.

As you wish Andy

BUT remember!! AB2 sounds like a railway station PA :-(
UNLESS you are using the 120 watts of course!!

SIX 807,s is a bigger audio amp than I have EVER worked on!!

Joe
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Old 13th Feb 2016, 1:06 pm   #107
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Default Re: 807 (maybe) amplifier build.

A little unfair I think to both AB2 amplifiers and to rail station PA systems

If you're heading for AB2 operation, that implies grid current in the 807s, which is tough for the phase splitter. A cathodyne may be OK from the cathode end, because that's a low impedance cathode follower drive, but the anode end output impedance is much higher (roughly equal to the anode load) and isn't good for supplying grid current..

This thread is now extensive enough to deserve its own Google! . In its absence, I've lost touch with your drive circuit arrangement for those 6x807s. I think you're using cathode followers, which will be fine so long as you have enough quiescent current in them to source the 807 grid current, which, with 3 valves per side, could be well over 10mA, depending on how hard you want to drive them into saturation.

You may wish to stick with AB1 operation, in which case you still need to maximise the output current that you're able to draw from those 3x807s on each side. This suggests trying a lower output load impedance than you're using at present. After all, the only reason for paralleling up those 3x807s on each side is to drive more AC current than you can draw from just a single push-pull pair, so you need to make the most of it with the right load impedance.

Best of luck,
Martin
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Old 14th Feb 2016, 12:44 pm   #108
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Default Re: 807 (maybe) amplifier build.

Andy, glad to see that your project is progressing well. Two lots of three 807's in parallel. That's something ! Beats just the three I have in parallel in my Coastal Radio Nimbus marine AM Tx.
Coincidently, my wife has just used a spare 807 as a project for a competition at our local camera club. Rather amusing. I hope that none of yours get too stressed.

Regards, David
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Old 15th Feb 2016, 7:20 am   #109
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Default Re: 807 (maybe) amplifier build.

I can hear you tutting and shaking your head from here Joe : ) Tricomp loves AB2 and has good things to say of it's bass response. I'm not intending to drive it in AB2, I was trying to say that the driver stage isn't the problem re max OP. A few days ago I measured everything and the OP's of the cathode followers were at 55v P-P with the OP stage fixed bias at 22v. The sine isn't clipping as it was before I fitted the followers, so therefore their OP Z must be low and the LTP/follower stage must be capable of driving an undistorted signal at AB2 conditions.

Have done a bit more work on the amp, so far the best approach to obtain Pmax seems to be actual experiment backed up by a bit of calculation. I've taken this approach because I've read 1000's of pagers over the last month or so concerning valve amplification, filled with hair raising equations and formulae, complex circuits and tables of data, but I find sticking a signal in one end and seeing what comes out on the scope more informative.

One bit of data which backs up Martin's theory that the Ra-a is out of whack with Z pri OPT is the sum for finding Ra-a, which took some finding in all of the literature I have. It is Eo x 4/Imax . Eo is output voltage swing. There's a way to find Eo which involves the mutual charateristics graph, but as a lot of these are really dire either because the image is bad or mainly because the anode voltage axis is divided by four, this makes accurate reading of value's problematic. In the end I measured it, = 75v AC. Which gives a Ra-a 2142k which is way off the 6k6 indicated by the data sheet. This value may not be accurate as I'm taking the value of I max off the data sheet.

75v is low from what I've read and intuited, this is because the load line for my OP stage is very steep. So far one thing has improved Pmax and that is lowering the bias to get more Imax thus driving the amp more into class A.This is counter intuitive, as Class A usually results in less power out. This resulted in distortion though.To get more OP out of this amp I've redone the input/LTP/K follower to get more P-P voltage swing and a higher current and will up the fixed bias to around 40v. If you look at the 807 mutual charateristics graph, as G2 voltage increases, the G1 lines spread out and go up, which should be the case with the OP stage at 400v UL, making the 0v line on the G2 = 300v sit higher at = 10v. It's hard to see from the graphs I have, as, as I said above, the image is degraded and I cant see what is the lowest value of G1 is. I've made my fixed bias supply capable of 50v which should give me the ability to drive the 807's into cut off/class B, not that I want to. This is just for experimentation. All this should give a higher voltage output swing and increase Ra-a. Whether this helps or not, we'll see. I'll also try lowering the load as suggested by Martin.

Hope that all makes sense, thanks for your input and continuing interest, I really appreciate it. It's good to have someone to bounce ideas off.

I hope my 6 807's don't dissolve like your picture David (beats the usual cute kittens/amusing dog/misty landscape at dawn) there is some heat coming off them, especially in Class A.

More later, Andy.
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Old 15th Feb 2016, 8:43 am   #110
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Default Re: 807 (maybe) amplifier build.

If your graphs/data sheets are a bit wonky or blurry/hard to read.
for all educational purposes you can also look at
CV428, 5B/254M
KT66
6L6
1625
They are all "similar" in operation for the maths to work.
There are probably others, but I just rattled off what came to memory quickly.
I am going to build a Golden ears laboratory amplifier which uses KT66 in UL parallel
and fetches about 55 watts into the "wrong" load for optimum, and it runs an unloaded HT of 420 volts. It uses fixed bias, and cathode follower drivers.
There are a whole range of Golden ears amplifiers, but the one you could look at is the
"Laboratory Golden Ear" August/September 1956 Radio-Electronics.
If you read ALL the golden ear amplifier history you will have lots of "general information" on Joseph Marshall's engineering style.

It may be data that you dont have, or maybe havent read.
Im still of the idea that 6 off 807,s in ultra linear "SHOULD" fetch over 100 watts with ease!!Even if the "ideal" load is not quite perfect, which, when you connect a speaker is exactly what will happen. Speaker impedance rises with increasing frequency!!

Best wishes
Joe
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Old 15th Feb 2016, 10:28 am   #111
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Default Re: 807 (maybe) amplifier build.

Looking at the RCA datasheet anode curves, an 807 can do a peak of 800v and a peak of 450mA, the latter at +30v on the grid, which is going to be difficult to drive for a linear amplifier.

Let's take just the voltage range, design an output load for 50W from a push-pull pair, see what the currents are like, and then halve the load for a paralelled push pull pair.

So we have to fit the voltage swing between 800v at the top and 50 to 100v at the bottom, depending on the current we allow. Mains voltage fluctuates, so we need some margin at the top. If I pick 400v HT and say we let the bottle go down to 100v on current maximum condition, by symmetry, the anode voltage will swing 300v below HT and 300v above HT so peak anode is 700v which has margin for HT to wander.

So while one valve of a p-p pair is up around 700v its opposing number is at 100v, so there is 600v across the whole of the primary. Swing the other way, the voltages swap over, so there is now -600v across the same primary connections.

So we have 2*600 = 1200v peak-to -peak

or

600/root(2) = 424.25v RMS

RMS volts are useful. set a resistive load and we get power, or pick a power and we get load. Say you want 100W from a quad of valves, that's 50W from each pair

so power = volts-squared/R where we mean RMS volts

50 watts = 424.25*424.25/R

By the magic of algebra

R = 424.25*424.25/50 = 3600 Ohms Anode to anode load for one p-p pair.

In this resistance, for 50W the RMS current will be = Root (50/3600) = 117.85mA RMS

so the peak load current will be 117.8*root(2) = 166.666mA

So for class A, one valve will have to pull 166.66 mA more than quiescent while its partner pulls 166.666 mA less than quiescent. So quiescent needs to be more than 166.666 mA
We don't want to go too high, let's pick 175mA and live a bit close to the edge (well, you could be playing prog rock on it )

So the valves swing between 175+166.66 = 341mA and 8.333mA

With 175mA each and 400v HT, quiescent anode dissipation is going to be 70 watts each. So the anode rating of 25W (30W ICAS) scuppers class A

In class B we let the anode of one valve cut off. In pure class B it would be 50% of the time and each valve would run from cutoff up to 166.66mA and power is dissipated only during programme. For any clas between pure A and pure B we allow more quiescent to move the crossover outwards and away from the low signal case. 25W dissipation quiescent at 400v would be 62.5mA so we would need to pick a value less than this. How about 32mA for a value plucked out of the air. So peak current is 166.666 +32 = 198.666 mA, call it 200.

From the 807 anode curve 200mA and 100v on the anode is at +5v on the grid. 32mA at 400v is about -30v on the grid. I'd have to work out the max sig anode dissipation but it's not too far out for ICAS.

Notice that the one thing we haven't fiddled with is the anode load of 3600 Ohms across the full primary. THat's set by the wanted power and the HT voltage.

Double up the 807s and the quiescent PER VALVE still applies, the power doubles to 100W and the anode load impedance halves to 1800 Ohms across the full primary.

Fag packet calculations for a reality check

David
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Old 15th Feb 2016, 11:35 am   #112
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Default Re: 807 (maybe) amplifier build.

Masterly piece of fag packet work David- that's my kind of electronics!

So from Andy's triple parallel pairs of 807s, 150W should be possible with a load of 1200 ohms across the full primary. Hence the planned experiment to lower the output load could pay off. That's as long as the driver stage can drive G1 of the 807s to +5V: at that peak voltage, they'll be drawing around 5mA each from the driver stage, i.e. 15mA for the three. Needs a relatively beefy driver.

Martin
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Old 15th Feb 2016, 1:59 pm   #113
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Default Re: 807 (maybe) amplifier build.

I see someone's offering four 813s right now

David
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Old 15th Feb 2016, 4:04 pm   #114
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Default Re: 807 (maybe) amplifier build.

Thanks for the heads up Joe, will look them up and thanks for the analysis David, havnt had time to read it properly yet, but will do later when it calms down here- school hols.

With the latest drivers etc I got 91w with 1v P-P 1khz input, HT at 380v ish, using 3 6SN7's. I upped the current to 20mA on the followers and configured the LTP for more voltage swing.

I tried an 8 ohm pot in parallel with the 8 ohm dummy, at four ohms load OP power was cut by roughly a third, adjusting the pot made little difference. I also took out a pair of valves and tried various settings, no improvement. I've also experimented with the bias. I I'm sure I can squeeze a bit more out; I'll look at David's figures. A bit more power would be nice, for NFB.

Andy.
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Old 15th Feb 2016, 4:07 pm   #115
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Default Re: 807 (maybe) amplifier build.

I started those sums too early in the morning and there is an oversight. There is an inconsistancy between the current calculations for class A and class B.

Yep, in class B the one valve which is on has to drive the full load current via only half the turns of the primary so the peak anode current is twice the peak load current of 166.66mA.

So the on valve anode peaks at 333.33mA and 100v. So the grid voltage is somewhat higher, and so is the grid current. The anode dissipation is higher, but it does get a nice rest for half a cycle.

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Old 16th Feb 2016, 7:26 am   #116
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Default Re: 807 (maybe) amplifier build.

Right I've read your figures David and understand them. In a nutshell we need more oomph in the drivers. I thought about using a triode strapped 6V6 as the follower, but I had trouble getting it to work, so I'll try a 6N7. The peak current I'm getting is nowhere near 333.33mA. One question,you mention +5v on the grid,from what I've gleaned so far, I thought that we drive the grid only up to 0v, why +5v and not +10 etc? At full whack my driver OP is at 60v P-P, with a bias of around -25v. The missing 10v I'd put done to loses.

BFN, Andy.
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Old 20th Feb 2017, 11:32 am   #117
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Default Re: 807 (maybe) amplifier build.

I'm finally getting somewhere with this amplifier. After the last post above I put it away and got it back out about a month ago. First I connected a driver transformer to a 3w single ended amp I made back end of last year and that drove the OP stage no problem over 120w so I knew the OP stage with 6 807's could do 120w. However the OP was distorted but it proved the point.

Next I tried just about everything in the book to get over 100 watts out of this setup including a cascode LTP, using two EL84 triode strapped in a LTP and two 6SN7's cathodes and anodes connected with separate anode resistor's.

I was about to give up again when I tried using a ECC83 as the common cathode gain stage front end, an ECC82 as the LTP PS with a CCS on the tail and a 12BH7A as a cathode follower on each signal. The last really makes a difference and will drive 6 807's easily. I got 136w out it yesterday briefly, it still wasn't clipping.

Today I tried a quick frequency response test and it's flat from 100hz to well over 30khz. So in all in all I'm well chuffed. However there is still a lot to do. I'm running the 807's at 400v the OPT configured as UL. The 807 g2 max is 300v, so really I ought to get some 6L6GC's or use EL34's. First thing though is to tweak the front end, LTP etc as they have far too much gain. The OP is with about 200mv on the input, so as most signal sources are line voltage nowadays it doesn't need as much gain as this.

Second I need to sort the PSU out at the moment I'm using an old Tek scope tfmr with two secondaries of 180v and 135v I think. These are connected in series with one big 800v 10A bridge rectifiers and 3 220u caps in parallel on each sec winding. The primary is fused and the secondary has two big thermistors to stop current surge prior to the caps as well as a 500mA fuse on both negative and positve HT.

I'll need to wind a tfmr capable of supplying two monoblocks. I'll use a choke filter as the current setup isn't ideal. I need to build some protection into the circuit, maybe to cut mains to the primary using a SCR and relay if cathode current on the OP stage is exceeded.

After building a chassis for both PSU and one monoblock to start with I'm going to have to apply some NFB and probably deal with any instability that occurs. So lot's do but at least I've overcome the difficult part; it's been a hard slog, this is only the second valve amp I've built and there's a hell of a lot head scratching, research and pile's of sheets of paper covered in loadlines and calculations, most of which have been wrong.

Here's a schematic which has a few bit's missing I see, but you should get the gist. Any feedback welcome. Andy.

PS sorry image is not orientated properly.
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Old 20th Feb 2017, 4:34 pm   #118
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Default Re: 807 (maybe) amplifier build.

12BH7 is a good choice for the driver - it's got more anode-dissipation than the ECC82.

In the past I've used a 6N7 as Class-A push-pull driver for a pair of triode-connected zero-bias Class-B 807s, using transformer coupling - if you're running zero-bias on the output pair you need to be able to drive them into significant grid-current, meaning a low-impedance driver circuit is necessary to avoid waveform-distortion when grid-current starts to flow.

Two triode-connected 807s in zero-bias Class B will give 120 Watts. See here: http://n4trb.com/AmateurRadio/RCA_Ha...amtips0702.pdf though they use a pair of 2A3 triodes as driver... I guess in 1947 they were available at not-so-stupid prices!
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Old 20th Feb 2017, 4:41 pm   #119
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Default Re: 807 (maybe) amplifier build.

Great article. I like the way it says "only" 750 volts on the anodes!

73's

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Old 26th Feb 2017, 8:27 am   #120
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Default Re: 807 (maybe) amplifier build.

I doubt Class B as in your link would make good hifi. There's going to be considrable Xover distortion and I'd need a different OPT, but thanks anyway.

After the high of getting this working I hit a low the next day on start up. There was a weird buzzing every time I turned the volume up. After investigation I found the 1N10007 diodes I'd put between the cathode follower grids and cathodes to keep them at around a volt at power on, were switching/oscillating.

Took them off but then found one half of the NOS 12BH7 I'd got from the states had gone low emission. Replaced it, luckily I'd bought two but found now the sine on the cathodes of the cathode followers was distorted. At first I thought it was 2nd harmonics; it had like a small bite out of the negative going half of the sinewave. After investigation and experiment I found the 150k and 470n coupling cap was wrong. I had the 150k grid reference R's returning to earth. These work better returned to negative bias. Also the bias arrangement I had was wrong. After changing to the 150k for a 47k and the 470n for 220n the amp is working better.

I'm now doubting that I even had the amp working as per post #117, but I know I did. How can an amplifier work one day and not the next? Ce la vie.

After making the changes to the negative fixed bias and reference R and coupling cap as above I got about 60w, but although the OP of the cathode followers is clean, the OP now has distortion on the falling edge of the positive half of the sine. A bit like how the sinewave gets pinched in prior to Xover distortion, but its only on one side. Having done more research and experimentation, I suspected the power supply and especially the CCS might not be optimum. The layout wasn't brilliant either, being birds nest style.

I spent a day putting the components on a tag strip, only to find the paxolin or whatever had gone conductive. After some thought on this I came up with the idea of using fiberglass single sided copper clad board, with turret tags. So you end up with a combined PCB and tag board. I cut the traces with a dremmel making sure there are wide gaps between traces.

Morgan Jones mentions a CCS can introduce noise and hum,see attached PDF I'm getting about 2v P-P of square wave like noise on the cathode of the PS. He suggests using a current regulator diode, but these arn't cheap at around 10 - 15 quid each. I'm using this circuit, see CCS 1 PDF, but will use a zener in place of LED 2 to get better Z.

I've also had to use a separate HTR transformer referenced to 200v as the cathodes of the cathode follower are @ 150v, a bit near the 200v HTR K max. The HTR's of the pre/PS/CF's were all powered from a bench supply. Is there a way to make a 6.3v DC HTR PSU without a separate tfmr?

So getting there, but slowly. It's one step forward two back.

A.
Attached Files
File Type: pdf CCS.pdf (34.0 KB, 86 views)
File Type: pdf CCS 1.pdf (86.9 KB, 120 views)
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