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Old 24th May 2013, 09:49 PM   #11
Matt BH is offline Matt BH  United Kingdom
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Hey Stajo, not sure from the photo but your grounding scheme seems a bit random ie you did your grounding the way you built it? I would recommend that you sort it before any other modifications. Looks like you have room to go to a star ground or a tree of stars. I'm sure there's a lengthy thread on here about grounding.

Cheers Matt.
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Old 24th May 2013, 10:11 PM   #12
DF96 is offline DF96  England
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Another thing to try is reducing the coupling caps. The aim is to get their LF pole well away from the OPT LF pole, so this could mean moving it up or down.
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Old 25th May 2013, 07:35 AM   #13
stajo is offline stajo  Sweden
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Friends! The boating seems to go away with decreased GNF/increased resistor (2k4 instead of 1k atm). Im gonna let it play a while and see if its stable.

I think the sound firmed up a little to, not as wet and fluffy as before.

Any suggestions on how to get to optimum values? Just try until I'm satisfied? Scope and find an optimum squarewave with random decoupling caps?
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Old 25th May 2013, 09:33 AM   #14
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Hi Stajo,

Changing the 1k resistor in the feedback to a 2.4k also moves the 12AX7 operating point - that circuit uses the feedback resistor to set the grid bias. Might have fixed the motorboating, but could cause other problems.

You may know that already, but I was wondering if you had checked the 12AX7 bias voltages after your change to make sure your operating range is still sufficient.

Also, you might try going back to the 1k feedback resistor (which seems to be the "normal" feedback resistor value for that circuit) and increasing the 100uF cathode bypass cap on the 12AX7.

Increase it to 200uF or 300uF. The cathodes should be at AC ground, but at 1Hz the 100uF cathode bypass impedance is about 1600ohms which may be too high.

Sam

EDIT - Well, ONE of the cathodes should be at AC ground. The other is 100 ohms resistive above AC ground. Minor point, just being clear.

Anyway, I think this was a commercial design so cost was a consideration so they didn't go overboard with parts values. Since this is a DIY project you, of course, have the option to increase the design margins a bit.

Good luck!

EDIT 2 - I saw that Artosalo recommended increasing the cathode bypass on the output tubes. Just to be clear, I'm recommending increasing the cathode bypass on the input 12AX7 tube - I think that may be where your problem is.

I think I would also separate the cathode bias on the output tubes - running them in common is another commercial practice to save money which we can spend like Midas on these DIY projects - but that is another story and " if it ain't broke don't fix it " is the best approach here
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Last edited by rfengineer2013; 25th May 2013 at 09:51 AM.
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Old 25th May 2013, 09:40 AM   #15
M Gregg is offline M Gregg  United Kingdom
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Try,

Putting the feedback on the 8 ohm tap to see what happens...leave everything else standard just to try it..

Change grounding to Star grounding..does it do it with no connection to other inputs and the input turned to different levels..you also seem to have output and input very close.... Keep output gnd as close to earth as possible no variation in Gnd resistance. Variation in Gnd resistance creates a voltage drop and possible voltages in the ground system.


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Old 25th May 2013, 09:51 AM   #16
stajo is offline stajo  Sweden
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When you say it I have the feedback on the 8 ohm tap. I dont use an original A-410 as it was designed for by Mr Hafler, but a some salvaged Z 565s.

Its been playing for a while now, and its stable. Firm and controlled in sound. Maybe a little to much controlled to my taste.

Im gonna turn it upside down later, and I find your suggestions to try to go back as close to the 1 k as it allowes. Maybe increase cathode bypass in the splitters and find me a good cap value with squarewaves over the feedback resistor (right now I have none).

Is that correctly percieved from my side?

/Staffan

Last edited by stajo; 25th May 2013 at 09:54 AM.
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Old 25th May 2013, 10:00 AM   #17
M Gregg is offline M Gregg  United Kingdom
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You need to find out if its a stability issue or a grounding issue.

Might sound strange..ie is it wiring layout or is it stability created by resistance in the Gnd.

Output Gnds should be as close to Gnd/earth as possible.

The feedback resistor as stated sets the bias so it should be as close as possible..(to value)

The feedback is already lower if you are on the 8 ohm tap..

I assume the feedback is in the correct phase<<just asking

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M. Gregg
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Old 25th May 2013, 10:04 AM   #18
stajo is offline stajo  Sweden
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Another thing that I find a little odd is that the completely oversized powertranny gets almost to hot to touch after some hour playing. It might be that it has some minor shorts in windings? This one should easily drive a 6L6 PPP or similar and I dont think it should be this hot on this little circuit.

About grounding it might not look star:ish but I have actually worked that a couple of time and I dont have any 50 or 100 Hz tendensies.

Excuse the dust, its been collecting a while.
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Old 25th May 2013, 10:04 AM   #19
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If you are already taking the feedback off the 8-ohm tap, the feedback has been reduced by 6dB from the original design. Increasing the feedback resistor to 2.4k reduces it another 6-ish dB so you're down 12dB from the original.

Seems that you would actually need to decrease the feedback resistor to get back to the original design level.

The compensating cap will almost certainly be different (from the original) due to the different output transformer, but that is a fine-tune you can do with a square-wave test.

This doesn't sound like a grounding issue to me, especially if you are not hearing other noise in your system.
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Old 25th May 2013, 11:25 AM   #20
M Gregg is offline M Gregg  United Kingdom
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Looking at the power Tx,

ER....Its not a good idea to have single cables through holes on AC supplies..

The two AC cables for each supply should come through the same hole..

I guess (two grey) two green (two black)<<each supply? (you can just cut a slot across each pair of holes to overcome this)

Whats the chassis made of?

I also don't see any isolation for the transformer mounting pins...I guess you have a shorted turn on the Iron??? (Not the windings the mountings)
Easy to check there should be no connection between each of the four mounting screws and Tx iron..with a meter set to ohms.
ie no connection across the laminations to the screws so you dont get a loop via the screws and the laminations. (shorted turn)

Does the incomming mains have a grommet (through the chassis)?
Keep Feedback and output wiring away from inputs..check phasing of feedback..

Regards
M. Gregg
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