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Old 13th Apr 2017, 2:55 pm   #161
Diabolical Artificer
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Default Re: 807 (maybe) amplifier build.

A bit more done but a few mistakes made. The amp chassis is 7 1/2" by 12 1/2" and sits between two end pieces like the cross bar on a H. I mounted this 2" above the amp case floor, but this meant that the OPT hit the amp case top cross bar, so therefore wouldn't be able to slide in. So had to lower it.

Lowering it to 1 and 13/16" maent that the CCS heatsink was too "high" ( see pic 1) and would hit the bottom of the case. So off with the old HS but I didn't have one suitable so I had to cut one off an old car stereo HS that I've had in the junk box for years but was too weird to be used. This meant bodging the CCS board a bit, but not too badly.

I've also hit a potential snag with the chassis power connector (see pic 2) which came off the old Tek scope. It's a McMurdo red series RSD16, which ITW still make. I wasn't going to butcher the Tek plugin so had to buy the male connector. These ain't cheap at 21.83 each, but I found some in Oz at 4.50 each so got 6 which makes the postage cost ok.

However reading the datasheet - see attached - it says contacts = 5A "All contacts (simultaneously)" which may mean each pin/contact is rated at 312mA each. This means that I'll need to double up the pins for the HT and ground and use eight (or would it?) for one 18.9v 1.5A heater supply. Each pin is 19 thou thick by 1/8" wide. The McMurdo octal socket pins are 15 thou thick by 3/16" roughly and I presume they were designed with the idea that valves like the 5U4G which has a htr current of 3A, could be used. looking at similar/suitable type connector's, the Amphenol blue ribbon connector which is similar has a pin rating of 5A. Looking at the Tek plug ins I have two that use 15 small valves like the 12AT7/6AU6. At a rough estimate that would be 15 x 300mA = 4.5A. As far as I can see Tek used only two pins for the htr wiring loom, so maybe I have it wrong and am talking bo*****s. Feedback most welcome.

The amp chassis bolts will be counter sunk and the front plate will have a thin plate on top, as per Tek scope and plug in.

That aside here's some more pics....

Andy.
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Old 13th Apr 2017, 2:58 pm   #162
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Default Re: 807 (maybe) amplifier build.

Attachment and one more pic.
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Old 13th Apr 2017, 3:49 pm   #163
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Default Re: 807 (maybe) amplifier build.

I could interpret that data sheet as meaning you can pass 5A through each contact, on all contacts simultaneously.

Having seen those connectors on Tekky plug-ins, I could easily believe they will carry 5A on each contact. They are not going to be rated at 300mA/pin. And it makes little sense for the current per pin to go down as the number of pins increases.
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Old 13th Apr 2017, 5:39 pm   #164
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Default Re: 807 (maybe) amplifier build.

Odd looking 807s

You're using DC heaters, aren't you, hence the heater buses rather than twisted wiring.

David
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Old 13th Apr 2017, 7:43 pm   #165
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Default Re: 807 (maybe) amplifier build.

Quote:
Originally Posted by TonyDuell View Post
I could interpret that data sheet as meaning you can pass 5A through each contact, on all contacts simultaneously.

Having seen those connectors on Tekky plug-ins, I could easily believe they will carry 5A on each contact. They are not going to be rated at 300mA/pin. And it makes little sense for the current per pin to go down as the number of pins increases.
That would be my take on things, too. I suspect that the "5A on all contacts simultaneously" is as much to do with overall permissible dissipation in a confined volume as much as anything. I've seen other multipole connectors where they might be rated at, say, 5A, Y poles, but if more than a certain percentage in a multipole assembly were carrying less than a given current, then the maximum current for the remainder might be relaxed to, say, 7A or somesuch. The manufacturer comes up with a connector to suit as many applications as possible but some users might only use a couple of poles for power supply, the remaining ones for signal. Others might use every pole to carry appreciable current. What's unfortunate here is that the maximum individual pin current isn't quoted, it's a rather sparse sheet from that point of view.
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Old 14th Apr 2017, 6:43 am   #166
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Default Re: 807 (maybe) amplifier build.

That makes more sense Tony. Looking at my wire gauge load carrying capacity chart,which uses the 700 circular millimeters per amp rule for power transmiss gives 23 AWG gauge at 22 thou can carry 4.7A. This is for a wire in free air. The load carrying capacity goes down if the same gauge wire is wound on a tfmr. Therefore the spec on the datasheet is probably more concerned with heat generated so that the insulation of the plug doesn't break down; so the plug will do the job. As turretslug says, the datasheet doesn't help us much, but as long as i keep within reasonable limits she'll be right.

As you see in the title "yer onner" it says 807 (maybe) : ) I dropped the idea of using 807's David a few pages back because I didn't want to exceed the 807's g2 300v max spec and would have had to connect the 807's in pentode config rather than UL.

Am using DC heater's on the front end and PS but not the EL34's. I'm just connecting them in series with two separate 18.9v 1.5A htr windings. It makes things neater and reduces the wire gauge thickness of the windings.

The PSU to the McMurdo connector goes something like this, provisionally : -
Pins -
1) HT 450v DC @ 700mA
2) Signal in +
3) Htr 1 12.6v DC @ 300mA for front end and phase splitter
4) Htr 2 6.3v AC referenced to 100v for cathode follower
5) Htr 3 18.9v AC @ 1.5A for 3 x EL34
6) Htr 4 18.9v AC @ 1.5A for 3 x EL34
7) Neg Bias - 100v DC
8) OPT sec 4 ohm tap
9) Ground
10) Signal in - Htr 1 18.9v etc
11) Htr 1 12.6v DC etc
12) Htr 2 6.3v etc
13) Htr 3 18.9v etc
14) Htr 4 18.9v etc
15) OPT sec 8 ohm tap
16) OPT sec 16 ohm tap.

Andy.
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Old 22nd Apr 2017, 10:24 am   #167
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Default Re: 807 (maybe) amplifier build. Now EL34

Just when you thought you had it sussed.....

The amp is mostly together so I thought I'd power it up. I've started with the input and phase splitter. I've changed the input stage slightly. Am still using an ECC83 but instead of HT @ 250v and anode R of 82k I now have it at HT400v, anode R is 220k, but cathode R is still 1k2 with a 100u cap across. this bit works well.

However the bldy phase splitter which was working fine in the "prototype" is playing silly bggers. the CCS and circuit are here -

http://www.vintage-radio.net/forum/s...=134126&page=2 post 27.

The CCS supplies around 10mA to the tied cathodes which should sit at + 10v, it was at - 3v.

So did the usual, and triple checked everything and scoped the cathodes for HF oscillation. All connected as it should be, no oscillation. I noticed that increasing the value of the emitter R raises the cathode voltage. The emitter R, R5 on WD's schematic consists of a 330r fixed R and a 300r preset. I swapped this for a 1k pot, with it set to full resistance the cathodes went up to +2.8v.

But this means that cathode current is at about 0.0014mA. Looking at the load line for a 12AU7/ECC82
with a HT of 430v (350 + -80v) and anode R's of 36k the bias should be at - 15v. I can't figure this out, especially as the anodes are at around 74v. An anode voltage of 74v indicates a grid bias of over 0v according to the loadline.

Puzzled.

Andy.
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Old 22nd Apr 2017, 11:47 am   #168
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Default Re: 807 (maybe) amplifier build. Now EL34

I think the reason for lower G2 voltage rating IS pentode/tetrode mode where the anode voltage goes below G2 volts and current switches across (hence the kink)

With ultra linear or triode strapping you may well get away with it.

David
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Old 22nd Apr 2017, 1:05 pm   #169
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Default Re: 807 (maybe) amplifier build. Now EL34

You've got me confused there David, an ECC82 is a triode and has no screen grid. It was the tied cathodes of an ECC82 LTP phase splitter fed by a CCS that was sitting at the wrong bias voltage.

I found the cause of the problem, but still have no idea why the "fault" occurs. I've attached the schematic so we know what's what. Sorry, forgot to label the other valves, but the ECC82 should be obvious, the cathode follower are 12BH7's.

So why with no 12BH7 inserted in the circuit does the ECC82 PS cathodes stay at -3v, but with the 12BH7 in does the cathode tail voltage rise to +10v?. Could it be that because the cathode followers are direct coupled that their grids are pulling more current from the PS anodes, therefore raising PS anode current and thus cathode current?

Andy.
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Old 22nd Apr 2017, 3:34 pm   #170
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Default Re: 807 (maybe) amplifier build. Now EL34

Sorry for the confusion, I was commenting on the 807's low screen voltage rating.

David
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Old 23rd Apr 2017, 4:24 am   #171
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Default Re: 807 (maybe) amplifier build. Now EL34

Thinking about this again, if you look at the schematic, with no 12BH7 present but the wiring and component's there, there is a path to ground via the 1k R, through diode, though the 12k R for DC of the ECC82 PS anodes.

With the 12BH7 in the socket warmed up and drawing current, the diodes turn off cutting the path to ground.

Panic over.

Andy.
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Old 24th Apr 2017, 9:39 am   #172
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Default Re: 807 (maybe) amplifier build. Now EL34

Quote:
Originally Posted by Radio Wrangler View Post
I think the reason for lower G2 voltage rating IS pentode/tetrode mode where the anode voltage goes below G2 volts and current switches across (hence the kink)

With ultra linear or triode strapping you may well get away with it.
There are a couple of glass-foot audio output tetrodes/pentodes that were also listed for video amplifier (resistive load) use- the maximum screen voltage was listed as somewhat lower for the former purpose than the latter, supporting the idea that it's a dynamic behaviour thing rather than a straight DC voltage limitation.

Also, there were quite a few amps that gave EL84s something of a thrashing as regards anode and screen voltage and dissipation, if they did anything sudden and disastrous to audio and mains ironware as opposed to just getting clapped out, it would be common knowledge.
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Old 25th Apr 2017, 5:45 am   #173
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Default Re: 807 (maybe) amplifier build. Now EL34

It's curious that a lot of beam tetrodes or pentodes max G2 is given as 300v like the EL84 807 and 13E1 and some like the 6DQ6B, QQV02-6 are less in the region of 250v. but a lot of these valves designed for use as AF amplifiers, so their data sheets don't give AF amplification info.

Whilst designing this amp I've had some people tell me it's ok to exceed g2 max V as long as the power limit isn't exceeded. One person said this is rubbish as this POV is given by folk who've never designed an amplifier. The general consensus is NEVER to exceed the maximum ratings given on a data sheet.
But then you look at the 807 datasheet which gives total max plate dissapation as 25w, but further on gives examples of PP outputs of 80w!

Valves are pretty hardy beasts, so I guess the best way to find out if a valve can function at Xvolts g2 or whatever is to stick it in a circuit and test it.

I ran the 807's with 400v on the screen grids but not for any length of time. As they were configured in UL on 43% taps I calculated the max power wasn't being exceeded. Still, not being rich and having a box full of 807's to test to destruction, I erred on the side of caution and used EL34's.

Andy.
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Old 25th Apr 2017, 8:55 am   #174
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Default Re: 807 (maybe) amplifier build. Now EL34

Quote:
Originally Posted by Dr Wobble View Post
One person said this is rubbish as this POV is given by folk who've never designed an amplifier. The general consensus is NEVER to exceed the maximum ratings given on a data sheet.
Some folk get really rather didactic about what they "read on a data sheet"- sometimes this suggests that they don't really have a full grasp of a particular situation or have adequate flexibility of mind. Or maybe they just like giving instructions to people. A classic example is audiophile forum discussions on PSU use where someone will cut in with a slightly robotic "Maximum reservoir capacitance for XYZ valve is 12uF" regarding a successful commercial design that uses several times this value- not taking into account transformer secondary voltage, winding resistance, reflected impedance, steady-state current, whether using a cheap commercial electrolytic or high-quality, low ESR block.... The published curves need interpretation, understanding and application, not the quoting from a worst-case header.

All the same, it is worth playing safe and keeping to ratings- I used to do occasional overflow work for a valve specialist and got a bit weary of amps using Chinese valves (however flowery the salesmanship) that were being run at 650V+ when they really ought to have been restricted to the 500V of the original topology. All in the interest of skirting round the elephant in the room that xx solid-state watts are cheaper to achieve than xx thermionic watts and straining every sinew to try and even up the gap.

Keep up the good work, Andy, it's been an absorbing exercise here and I admire your persistence,

Colin

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Old 25th Apr 2017, 11:48 am   #175
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Default Re: 807 (maybe) amplifier build. Now EL34

My datasheet interpretation skills are lacking as is my understanding in depth of most circuits, so i'm slightly in the dark a lot of the time. Still, i pick up a bit more every day thanks to your help.

One aspect that I'm slightly hazy on is on the subject of heater supplies. In one of my threads someone, can't remember who said "Nothing inside a valve should be floating"

I have four separate htr supplies; two for three EL34's, one for the ECC83 gain stage and ECC82 PS and one for the cathode followers. So with this advice in mind I've paid attention to referencing each supply.

The cathode followers as they're direct coupled need a 100v reference from a resistive divider, no worries there.

For the gain stage and PS I'm going to use 12.6v DC, so as I see it it could be referenced two ways. By a center tap or humdinger at the tfmr end or strap the DC rectified/regulated supplies negative to ground. The latter seems best and as a bonus free's up a pin on the power socket.

I've not seen power valve htr's referenced to ground, but is it advisable to do so?

Thanks Colin for your kind words, much appreciated as is any thoughts or advice from you all.

Andy.
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Old 25th Apr 2017, 9:07 pm   #176
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Default Re: 807 (maybe) amplifier build. Now EL34

I've never seen output valve heaters in any serious amps left floating andy. If the heater winding isnt centre tapped just use a couple of 47ohm resistors to make a CT reference to earth. I wouldnt mess about with humbuckers when using DC heaters, why? But a neat trick when using something of the ilk of an ECC82 with 12.6V heaters is to take your reference point from pin9 which is the heater CT, I've done it and it works. I am assuming here that the DC supply is floating and the negative side isnt tied down to earth?
finally, was there any particular reason for using separate heater supplies for the cathode followers and the input /phase splitters? Again I have no qualms about using an elevated supply on the valves at lower cathode potentials, my KT88 amp runs elevated heaters for the pahse splitter and first stage which is an ECC83 with a cathode voltage of 1.5V above ground. Never had a problem.

Hope thats of help and still most impressed with your work.

Andy

Last edited by bikerhifinut; 25th Apr 2017 at 9:11 pm. Reason: clarification.
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Old 26th Apr 2017, 4:41 am   #177
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Default Re: 807 (maybe) amplifier build. Now EL34

Thanks mate, that helps. Going back through a load of amplifier schematics I could only find one amplifier without a reference htr, the Heathkit Williamson, so I wonder where I got that idea from? I've a head full of monkeys juggling eels sometimes.

So if I've read you right and looking back at a load of schematics I can stick the reference in anywhere within reason. The pin 9 trick is a nifty idea, though I can't use it here as the heaters are in series, so no pin 9.

I screwed up when wiring the heaters putting the two valves in series. That's odd I thought, can't see no glowing htr's. Cause you can't numpty, it would take 25.2v to power them. Connected in series parallel all is well. Anyhoo, I'll tie the 12.6v DC supply to ground, job done.

You raise some good points there Andy, I've made a few daft decisions based on false precept's, making things overly complex.

Thanks for your help and kind words, Andy.
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Old 26th Apr 2017, 10:02 pm   #178
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Default Re: 807 (maybe) amplifier build. Now EL34

Within reason, yes. I'd still put the potential divider for the reference at The PSU end of life. It should just make it easier to trace if any fault finding needs done. Also in the interests of avoiding hum loops and other associated issues I'd think you are better running the Earth reference back to the PSU end of things (Or star earth).
Not sure what you mean by series parallel here Andy, I take it you mean you have connected pins 4 and 5 together and are applying the heater volts accordingly which with a 12.6V supply would entail wiring the heater string in series? If that is what you've done then it would work fine but all the heaters will have to have the same current draw otherwise the voltages will get a liitle out of kilter (ohms law etc). I cant see why you cant wire each ECC** valve as 12.6V heaters and then just use a parallel heater string.
No doubt i have got hold of the wrong end of the handle here so sorry if I have.
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Old 30th Apr 2017, 11:25 am   #179
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Default Re: 807 (maybe) amplifier build. Now EL34

Still fettling this amp, am now working on the chassis, PSU and protection circuits.

I'm going to use a NOS ceramic dropper R of 67 ohms on the HT to soft start the amp. This will be shunted by a relay after about 8 seconds. It means I can use a smaller fuse on primary too. Smaller fuse, quicker to blow. All mains tfmr secondaries will have fuses as well as a fuse on the HT after the reservoir caps.

I want to knock up protection in case an OP valve starts conducting too much too. Havn't worked out how to do that yet. But as each cathode has a 10r "sense" resistor, maybe I can do somethiing with the voltage across these but to turn on an SCR or something to trip a relay that will cut power to the mains tfmr primary.

For no load on the OPT secs I was going to use a 1k R across the speaker out terminals as well as diodes on each half of the primary of the OPT.

Am sure there was something else, but if anyone has any suggestions....

I've painted myself into a corner regarding the PSU. I've not left much room to fit a 625VA toroid, two bridge rectifiers, 6 big caps, fuse board, two big relays, protection etc PCB etc.

Regarding the fuses I've had an interesting morning trying to find a lot of capacitance in as small a size as possible, at high voltage for as little as poss 's. I did have 8 220u 385v and two 150u caps in series parallel, but miscalculated capacitance. I've calculated I'll need around 1000u with HT around 500v at times that means a 1000u 550v cap. Not cheap, your talking around 50 each!

The cheapest way I found was to use four 820u 315v and two 470u 315v caps. so thats two 820u and a 470u in parallel across one 230v secondary, with the same again. I'm pretty sure this comes to 1235u at 630v. Series parallel circuits always fox me a little.

I'll have some pics soon.

Andy.
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Old 30th Apr 2017, 7:11 pm   #180
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Default Re: 807 (maybe) amplifier build. Now EL34

Hi Andy, for protection go the silicon route and use an LM311 or similar comparator trip across the 10R resistors. You can then set a trip level with a pot and even add a little time delay if you wish.
311 will drive a small latching relay for the trip.

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