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Old 21st Sep 2018, 5:00 pm   #1
Diabolical Artificer
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Default 120W PP EL34 amplifier re-born.

Having finish the big amp, that started as an 807 amp, but morphed into a EL34 amp, I wasn't happy. The amp weighed over 60kg! Also the case just wasn't right, so I started pretty much from scratch, this time opting to make two monoblocks.

I kept the amplifier chassis's but redid the PSU's housing them in wooden boxes. One is pretty near finished, see pics, the other is about half way through. I've yet to apply a finish on the wood, which is sycamore and at the moment the PSU is wired up temporarally.

The fan is there to cool the cathode follower resistors and once I've drilled some holes around the EL34 OP valves, jets of cool air will cool them too.

It's meant a lot more work, but I'm a lot happier with the end result and the monoblocks don't need a crane to lift them.

TFL, Andy.
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Old 21st Sep 2018, 5:02 pm   #2
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Default Re: 120W PP EL34 amplifier re-born.

More pics. I've yet to fit the panel meters (VU meter) on the new amps, they'll be the same as on the MK1.

A.
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Old 22nd Sep 2018, 1:11 am   #3
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Default Re: 120W PP EL34 amplifier re-born.

Nice work - looks like you are enjoying doing it.
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Old 22nd Sep 2018, 8:12 am   #4
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Default Re: 120W PP EL34 amplifier re-born.

Thanks. " looks like you are enjoying doing it." Sometimes, I've made a few mistakes entailing undoing hours of work, but once you stick music in one end and it comes out the other it does make you smile.

Andy.
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Old 22nd Sep 2018, 5:32 pm   #5
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Default Re: 120W PP EL34 amplifier re-born.

Looking very nice Andy. Well done. I'd like to pop across and hear this when your a bit further. If you wind it up enough I might hear it from here!
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Old 23rd Sep 2018, 1:24 pm   #6
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Default Re: 120W PP EL34 amplifier re-born.

Phenomenal build and re-build, Andy!

Really shows a great deal of resilience and determination. It must sound quite ‘out there’.

Nice one !
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Old 25th Sep 2018, 1:41 pm   #7
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Default Re: 120W PP EL34 amplifier re-born.

Thanks both, at the moment it sounds dire, it's giving me gyp...again. I have a really odd fault. I have a basic common cathode triode gain stage AC coupled into a LTP. Signal in without power, sinewave at the grid of the triode gain stage is perfect. Power on and the sinewave distorts badly, OP is terrible and limited to 1v P-P. Remove the valve in the LTP socket, all is well.

Tried different sig gen's, different valves, same problem. Scoped everywhere, wondered if there was some spurious oscillation, no. Checked DC conditions, fine. Thing is, it was working fine yesterday. Blocking distortion? Doesn't make sense.

Just when I think I'm getting somewhere, wham! Another problem. So resilience, determination, sheer stubborness or just plain stupidity is needed in bucket loads.

It's a nice day, off for a walk and a think.

Thanks for your kind words and interest, Andy.
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Old 25th Sep 2018, 6:59 pm   #8
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Default Re: 120W PP EL34 amplifier re-born.

Hi Andy,

That sounds like an irksome situation! I hope you had a nice walk and some relaxing time away from it for a bit.

I’m thinking, if your LTP stage was all sweet-as ,yesterday, and it’s not today, and your’re suddenly getting just 1V PP there, how about catastrophic failure of a passive component , specifically a coupling capacitor?

Perhaps you’ve transiently exceeded a voltage somewhere and now you have a big old leaky thing in hiding.

Alternatively, wildly thinking outside the normal range of options .... is this beast in a building outside? With drastically lower temps over the last few days, what if you’ve got some condensation in there?

These ideas don’t need circuit-specific knowledge , I know , but just putting them out there !
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Old 26th Sep 2018, 6:38 am   #9
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Default Re: 120W PP EL34 amplifier re-born.

Thanks Al, some good idea's there. It is a puzzler, it's almost as if something is being transmitted back to the ECC83 triode gain stage or the LTP IP has gone very low Z, which I discounted as everything works from the LTP on.

It's a pain this, it should be coming together not falling apart, that said I'll find the fault and it's honing my fault finding skills. I start by wildly unplugging things, then actually start to reason. Ho hum.

Andy.
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Old 26th Sep 2018, 11:54 am   #10
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Default Re: 120W PP EL34 amplifier re-born.

Think I've found the problem - a crashing 6.3v PSU that powers the heaters of the gain stage and LTP, it's still an odd fault though.

Test sequence =
1) ECC83 in socket, power up, take DC readings, they were ok. Next 1khz sine on the input = bigger sine out, as per load line calcs. All as should be.

2) Hot swop ECC82 into LTP base, IE with HT etc applied, whilst monitoring 6.3v rail = 6.3v crashes to 1.2v, LM317 heatsink getting V hot, but not too hot to touch. Result both IP and OP of ECC83 attenuated. Is the ECC82/LTP acting like a diode?

3) Repeated # 2 a few times, it did it again, but other times not - odd. Is LM317 oscillating?

4) Powered up both valves in place a few times, all well, odd. Can't replicate fault of yesterday.

Tested all passives, all seem ok. Will test PSU further.

Andy.
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Old 26th Sep 2018, 11:59 am   #11
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Default Re: 120W PP EL34 amplifier re-born.

Quote:
Originally Posted by Diabolical Artificer View Post
2) Hot swop ECC82 into LTP base, IE with HT etc applied, whilst monitoring 6.3v rail = 6.3v crashes to 1.2v, LM317 heatsink getting V hot, but not too hot to touch. Result both IP and OP of ECC83 attenuated. Is the ECC82/LTP acting like a diode?
Are the ECC83 and ECC82 heaters both powered from the LM317? If so, the extra current drawn by the ECC82's heater when it's cold may well be sending the LM317 into foldback current limiting, hence the output crashing to 1.2V and staying there.

Chris
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Old 26th Sep 2018, 12:16 pm   #12
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Default Re: 120W PP EL34 amplifier re-born.

Heater/cathode short?
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Old 26th Sep 2018, 3:10 pm   #13
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Default Re: 120W PP EL34 amplifier re-born.

Ehm, that's an idea Chris, but the LM317AHV datasheet says it should be able to handle 2.2A. I've also had the problem with the heaters warmed and running for a while. the fault occurred yesterday when the IP signal was increased, in amplitude that is.

That's what I thought PJL, I tried different valves , both different ECC83 and 2's, no difference

Attached are some oscillogram's of said fault.

Andy.
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Old 26th Sep 2018, 4:11 pm   #14
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Default Re: 120W PP EL34 amplifier re-born.

Quote:
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Ehm, that's an idea Chris, but the LM317AHV datasheet says it should be able to handle 2.2A. .

Hey Andy...

that sure looks wrong, don't it!

have you decoupled the input of the regulator with an 0.1uf capacitor and the output with a 1uf capacitor, as close as possible to the pins?
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Old 26th Sep 2018, 10:51 pm   #15
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Default Re: 120W PP EL34 amplifier re-born.

Andy,
daft idea maybe but have you tried feeding the preamp and LTP valves with 6.3VAC? i.e. use a separate filament transformer as I guess you are using a 9V or similar AC supply into the rectifier and regulator for that supply? Or even feed a 6.3V DC supply to the heaters from a bench PSU. That would eliminate the regulator/heater PSU if the fault still manifests.
Then I'd be thinking maybe the 317 could be faulty/damaged, these things can happen and they are pennies to substitute, or maybe a faulty transformer winding? again these days I prefer something like a LD1086 or similar as I feel they are a bit superior to the 317 but that may just be my bias.
Personally my feelings these days are DC heater supplies are overkill on circuits running at line level signals which on a power amp in my opinion sould be of the 1V mark or even a bit higher assuming an input from preamp/source of around 2V.
At those sort of sensitivities good wiring practice should eliminate heater induced hum.
Just my take on things, nowt personal!
Andy.
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Old 27th Sep 2018, 9:42 am   #16
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Default Re: 120W PP EL34 amplifier re-born.

"have you decoupled the input of the regulator with an 0.1uf capacitor and the output with a 1uf capacitor, as close as possible to the pins?" Yes Al.

You make some good point's there Andy, will have a look at the LD1086, it's easy to get stuck in your ways I guess and use LM317's as default.

Managed to fault the circuit repeatedly, by jumping a bigger load onto the PSU, it is probably as Chris says, the reg is going into foldback current limiting mode. I scoped the OP and got this odd "signal", hash whatever you call it.

So any sudden current demand is what is causing the "fault". Odd thing is, the tfmr winding is beefy enough and the 317 should cope with what's asked of it, so either the 317 is faulty or the circuit is wrong.

The trouble I have with the LM317 and other 3 pin regs I've used is figuring out what IP voltage to use. Too much IP voltage the reg is working too hard and can fail, too little and OP voltage drops. It's a fine balance. I'll admit to struggling to deciphering some parameter's on the datasheet.

Back to bench...Andy.
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Old 27th Sep 2018, 4:18 pm   #17
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Default Re: 120W PP EL34 amplifier re-born.

I reckon Chris has it, foldback limiting. The 'LM will do everything it can to mantain it's ref pin at the ref voltage, short of self distruction ! There are current and thermal limits operating so it can look quite complex. But, if it drops out its just not man enough for the load!

dc

PS. that scope trace will look better in ALT mode
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Old 27th Sep 2018, 4:46 pm   #18
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Default Re: 120W PP EL34 amplifier re-born.

You'll need to design the regulator to deliver 6.3V into the 'cold' resistance plus a margin. You won't need to spec the heatsink for that much current though, as before its gets super hot the heater will be hot and the load current will drop. On a shorted heater the LM317 will protect itself (but they are quite cheap anyway).

Current limit is 1.5A Min / 2.2 Typ @ < 15V In to out
This implies cold R is less than 4R2 ?

Does that fit with reality ?

dc
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Old 28th Sep 2018, 7:02 am   #19
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Default Re: 120W PP EL34 amplifier re-born.

"I reckon Chris has it, foldback limiting." It has to be it DC, however the I max of a LM317AHV is 2.2A, see attached "Does that fit with reality" With a 5ohm load, it goes into current limiting mode.

My winding is beefy enough 16AWG = good for 3.7A less a bit @ - 10v (13.4v), I'm using a 2200u reservoir cap.

In the end I used a TIP3055 as series pass Q, no more issue's.

Andy.
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Old 28th Sep 2018, 5:35 pm   #20
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Default Re: 120W PP EL34 amplifier re-born.

After more testing and mucking about I've hopefully sorted things out, no more oscillation or crashing heater supply. There's still some tweaking to do, but hopefully I can crack on and finish the other monoblock.

It'll be nice to do some more woodwork, give the old brain a rest for a spell.

Andy.
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