Hell-marriage amp (5842-> IT -> 2A3)

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Mike H
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#151

Post by Mike H »

thomas wrote:Various schematics I've seen suggest 220k or so would be a usual value here if an interstage tranny wasn't there...
Oh right, well usually not a lot. The IT winding is like a couple of hundred Ohms DCR anyway. The grid could be grounded if you like, shouldn't make any difference to the o/p valve. The maximum value should be as per manufacturer's guide. (datasheet)


Dave the bass wrote:
Mike H wrote: I presume the valve using the IT is OK with a 12k load?
Yup, it's a chunky little beast the 6C45p, our AndrewL has a spud amp using one. He drove the massiff Sachiko (sp?) speakers with it once at an Eggy-bash.
OK then! :D

Here's a 120Khz sinewave 'getting tidied up'... click pic for mini movie...
Whoo. 120kHz or 120 Hz? What are you doing that makes it go clean or lean to the right?

I'd imagine it's not conventional to have a 12K grid leak pulling 3mA through it to ground on the -45 grid bias feed
AH!!! It's all becoming clear ~ so that created a small DC shift, that explains all :D
 
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Dave the bass
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#152

Post by Dave the bass »

Mike H wrote: Oh right, well usually not a lot. The IT winding is like a couple of hundred Ohms DCR anyway.
175R in the case of the Hammond IT I'm using.
Mike H wrote: Whoo. 120kHz or 120 Hz? What are you doing that makes it go clean or lean to the right?
120KHz. The sine wave becomes distorted up in that region just before it starts falling away response wise, I noticed when added that 12K across the ITx Secondary it pulls the sine back into order.

What i didn't realise at the time I recorded that vid.... was that the biasing of the amp had radically changed. Instead of -45 on the grid I'd pulled it down to approx -26V or so causing the poor little 2A3 to conduct harder than normal (even desired). Maybe it was that large increase in anode current that caused the sinewave to appear to rebuild itself. Certainly not good for the 2A3 I'd imagine. I corrected the bias back to -45V.
Mike H wrote:AH!!! It's all becoming clear ~ so that created a small DC shift, that explains all :D
It does!? Erk :-)

What I do know is that I now have a very small ~3mA current flowing through the IT secondary and the also the 6C45p now has a 12K load reflected back through the IT whereas before it was dependant purely on the inductive reactance of the choke ( I think I've got that right).

Sperrymentalism in Da House :-)

DTB
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Mike H
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#153

Post by Mike H »

Dave the bass wrote:120KHz. The sine wave becomes distorted up in that region just before it starts falling away response wise, I noticed when added that 12K across the ITx Secondary it pulls the sine back into order.
Oo-er! I only queried it 'cause it looks like what you get if it's running out of Henries at LF
 
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#154

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Attention has turned back to this project as little 6EW7 sounds so good I'm loathe to turn it off! Also JTS gets the 'ump if we don't have any music on when she's 'broidering :-) the detour with loading the sec of the IT with a low value resistor has been dumped, in retrospect all I was doing was causing the 2A3 to conduct harder as I was pulling down the grid bias supply with the resistor and causing the 2A3 to conduct harder. Live n learn. So the amp is back to normal.

I'm getting too used to having an amp that doesn't hum at all, 6EW7 on the other thread has none, I like that, a lot.

So I'm hum-chasing on this amp 6C45->IT->2A3.

The 2A3's are AC heated natch but are hum-dingered so I can null pretty well. The 6C45's are also AC heated (for now, haven't tried DC heating yet, might.) but having read the Merlin Blencowe PSU design book he mentions that some hum and rectifier action can be coupled to all other windings if the same TX supplies other supplies. He shows an oscillogram of switching spikes coulpling hash into the grid of a valve.

Here's my 6.3V AC heater supply to the 6C45's...
Image

See the little spikes on both edges of the sinewave? If I power up the amp from cold whilst observing the 6.3V sine on the scope I can see those spikes appear as the GZ34 starts to conduct and supply HT, so those spikes are deffo rectifier induced IMO. It was mentioned in passing a while back in this thread IIRC.

The 6.3V sinewave is a poor shape I know I don't know why this is, I live in the middle of a large town on mahoosive estate with lord knows what being pumped into the mains supply, but the rectifier spikes, there must be summat I can do about them? I've got another stand alone 5V heater supply TX I could try, i also have a pair of Hammond 2.5V heater TX's.

Is it worth trying 3 separate heater supply's ( 1 for each 2A3 and 1 for the GZ34) or is there a way of snubbing the spikes and hope this reduces the slight background hum I'm getting, or am I being a straw-clutcher having tasted regulated smooth DC for 6EW7 amp??

DTB
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#155

Post by Mike H »

That makes sense, as GZ34's cathode is connected to one end of its heater. Separate TX for the GZ34 might sort it.
 
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#156

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Looking where the kinks are, makes me think its the point where the rising voltage from the rectifier reaches the point the reservoir charged down to, and so the rectifier starts conducting. You might want to try a 10nf + voltage rating cap across the two transformer secondaries. If you cant get the voltage rating between the to secondary taps, try between the secondary and the center tap. It may be the sudden inrush of current causing the transformer secondary to ring with the shock.

If you still have that Sound Practice DVD, have a look for the Camile article about removing switching spikes in their 211 amps power supply.
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#157

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Mike H wrote: Separate TX for the GZ34 might sort it.
Clip-leaded in another TX, nope didn't.

However a separate 6.3V supply for the 6C45's doesn't have the spikes (no s%$t Sherlock! :lol: ) so that proved the point that spikes are from some form of rectifier switching IMO.

Sinewave was still and awful shape though, what does other peeps mains sinewaves look like? Does anyone else have un-clipped waveforms?
Nick wrote:Looking where the kinks are, makes me think its the point where the rising voltage from the rectifier reaches the point the reservoir charged down to, and so the rectifier starts conducting. You might want to try a 10nf + voltage rating cap across the two transformer secondaries. If you cant get the voltage rating between the to secondary taps, try between the secondary and the center tap. It may be the sudden inrush of current causing the transformer secondary to ring with the shock.

If you still have that Sound Practice DVD, have a look for the Camile article about removing switching spikes in their 211 amps power supply.
I'm sure I bought one of those 'lucky bags' of ceramic caps that Mapper's do but I'm blowed if I can find where I've put it. meh!.

I've raked through my cap boxes and manage to experiment with 400V rated 220nF from each sec tap to CT and I actually made the spike ring more! I also uncovered a 1nF 1KV ceramic that I tried across the (600V) secondary, nope, no joy, not enough capacitance I'm guessing. I'll pick up a lucky bag tomorrow and have another snubber session next weekend.

DVD, blimey, I'd forgotten about that, yes I'll have a look for it.

Is this next thought process logical though -> I'm wondering, as the grid bias cct only uses 1/2 wave rectification if thats not helping things either and the single 1N4007 is injecting switching spikes into the TX and coupling onto all the heater windings. But would that clip both sides of the waveform even though it's only half wave rectification?
:idea: External grid bias supply provided by a handy bench PSU.

That'd prove it, i think.

Onwards.

DTB
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#158

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Dave the bass wrote:
However a separate 6.3V supply for the 6C45's doesn't have the spikes (no s%$t Sherlock! :lol: ) so that proved the point that spikes are from some form of rectifier switching IMO.


DTB
A while ago Nick and I spent ages discussing and thinking about where hum could be coming from in a small preamp that used AC heating on some D3A. The B+ was clean as a whistle so it wasn't that. Nick tried DC heating on the D3A that cured the problem so we had the culprit but the amp sounded crap so... AC was the way forward. But how does AC heating on a indirectly heated valve translate, in a circuit with very little gain, to really bad hum?

It turns out, the HT and LT were on the same transformer and switching noise from the rectifiers, cap input etc, was getting coupled back into the second winding on the TX, this winding was used to heat the valves. Nick slapped on a separate cheap 6v toroid and the noise went.

The D3A gave no hum with pure sine waves or on DC heating but on a wave that looked like yours above it sound terrible.

One trick I find useful is to work out the umms freq, usually its 100Hz but if it isn't then that can be a clue too.

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#159

Post by Dave the bass »

Ta our-MrL.

Aye, it's a funny one this one. I noticed the spikes a while back on the 2A3 2.5V heater supply but didn't go too deep into it at the time. Then having bought the Merlin Blencowe PSU book and looked at the pretty pictures it dawned on me that 'aha! I think that's what's happening in Hell-Marriage'.

Scoping around today and seeing the same switching spikes on the 6.3V windings too confirmed it really.

I've got the hum waaaaay down now by fitting null pots to balance both the 2A3 heaters and another to null the 6.3V AC supply for the 6C45's.

Its coming to the next Owsters for critical appraisal.

DTB
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#160

Post by Mike H »

Dave the bass wrote:Sinewave was still and awful shape though, what does other peeps mains sinewaves look like? Does anyone else have un-clipped waveforms?
Worse than yours! I think it's pretty much the norm these days I'm afraid
 
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#161

Post by Mike H »

How about small value chokes in series with the rectifier anodes?
 
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#162

Post by Dave the bass »

Mike H wrote:How about small value chokes in series with the rectifier anodes?
Teeny tiny inductance? To block the spike but pass the low frequency AC? Is that what you had in mind Mike?

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#163

Post by Mike H »

Something of that sort. No idea what'll happen though
 
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#164

Post by Dave the bass »

Remember this project from yonks ago? Its still on-going, it lives in another format (again). This time after advice/general pointers and encouragement from Nick and Thomas at the last Owston get-together I've now grafted a 2 stage DC coupled 5687 stage up front on this 1:1 IT -> auto bias 2A3 SE amp.

OP point on the 1st stage is 144V @ 9mA (1.3W Anode dissipation) and 2nd stage is 187V @ 11mA (2W Anode dissipation) so unlike the previous incarnation which was a single stage 6C45pi with IR diode bias and thrashed hard this latest installation is being treated much more kindly I believe.

After a covert top secret need to know basis only meeting of the Kent Massiff yesterday (in a pub natch) I wanted our-Thomas to just look over what I was planning on doing as I've got a bit rusty doing DIY HiFi of late cos of playing funny bikes for the last 18 months but the re-cap on elevated heater supplies was much needed, ta MrT!

The heaters are elevated as the 2nd stage cathode sits at 162V +ve well past the 90V potential on the data sheet. It's funny how you forget this stuff if you don't do this so often innit. Anyway I've raised the heaters to about 80V which is midway-ish up the 2nd cathodes potential.

Only done one channel so far but it works, whahey!

Oblig piccie...
Image
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#165

Post by PeteC »

great to see this one resurrected Dave....long winter nights seem very conducive to amp building.

I am thinking of re-doing the (Shishido) Loftin White as my winter project.
Will be 6N2P-EV into 6C4C with current reg heaters and James TXs. That breadboard was probably the amp I have liked the most for Rock and it needs to get into a box I think. Not the most refined but had the most in-yer-face impact.
Good to see some amp building going on..
..hows it sounding ?

Rgds
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