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"Lacewood" Amp V2.0
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Author:  lowhifi [ 09 Jun 2018, 08:13 ]
Post subject:  Re: "Lacewood" Amp V2.0

On an other forum someone said the current is too close to the maximum. I can use the 100 mA version for the high frequency driver in my active set-up but it will be limited to this application.

Also i found very nice military brown base RCA 6V6 but on my calibrated Jackson 648 they read about 30%.
Anyway they sound great on my Lacewood, is there any problem using them.

Regards.

Author:  Suncalc [ 09 Jun 2018, 10:19 ]
Post subject:  Re: "Lacewood" Amp V2.0

lowhifi wrote:
On an other forum someone said the current is too close to the maximum.
Hooey!

One of the primary considerations for max primary current is core saturation. This is controlled via core cross section, and gapping. The "C" of the transformer core controls the propagation of the magnetic field and the gap dimensions control the overall reluctance of the magnetic circuit. It is this circuit that connects the primary and secondary windings. The core and gap are sized to support a certain level of DC induced flux and present a reasonably small magnetic hysteresis such that the coupling distortion is very low. So when a designer sizes a core for an 'X'mA magnetizing current, he has taken into account both magnetization flux and energy transfer.

The Lundahl LL1663/50mA is sized to cleanly handle up to 50mA DC current AND the signal fluctuations without saturating. (LL1663 data sheet attached) The design point of the lacewood is 43mA total DC magnetization current. This is only 86% of the maximum design specification which likely already has a little design margin thrown in for manufacturing yield purposes.
Attachment:
1663.pdf

Go ahead and use the LL1663/50mA. The amp will have fuller low end performance and will have significantly better transient response. You do not need to worry about core saturation. Trust the designers at Lundahl. They know what they're doing.

Author:  lowhifi [ 13 Jun 2018, 14:31 ]
Post subject:  Re: "Lacewood" Amp V2.0

mike567 wrote:
Boost transformer. 12.6 volt secondary wired in series with the primary of your power transformer will put you in the ballpark.
Use higher voltage till you get what you need.


I have an insolation transformer who had many taps on the secondary, maybe an other solution.

https://www.hammfg.com/electronics/tran ... s/line/169

Author:  john55 [ 26 Jun 2018, 01:56 ]
Post subject:  Re: "Lacewood" Amp V2.0

About six months ago I finished building Suncalcs' Lacewood V2 and have probably had 450 – 500 hours of enjoyment from it so far. Yesterday morning the fuse popped and all went quiet.
This morning I opened up the amp and checked for any visual faults, eg. scorched components, loose wire flapping about, etc. Everything looks fine. I put in another fuse and fired up the amp with my finger on the switch at the mains outlet. After 3 or 4 seconds the GZ34S rectifier put on a truly impressive lightning display. Needless to say, I flipped the switch.
I will check the Power Transformer voltages before I plug in a replacement, and I'll give the amp another good visual checkover before switching on, but my feeling is that I've just been unlucky with the GZ34S.
If anyone has any other ideas on what has happened, or thinks my reasoning faulty, please post your thoughts. They really will be appreciated as I'm still very much at the learning stage.

Author:  Suncalc [ 26 Jun 2018, 19:23 ]
Post subject:  Re: "Lacewood" Amp V2.0

john55 wrote:
I will check the Power Transformer voltages before I plug in a replacement, and I'll give the amp another good visual checkover before switching on, but my feeling is that I've just been unlucky with the GZ34S.
Maybe.

I would definitely check the transformer before you go any further. With the rectifier out you can safely check the high voltage windings and the 5V winding. You might want to remove the rest of the tubes and check the 6.3v winding as well.

It is rather uncommon for a failed rectifier to simply blow the primary fuse as you describe. It can happen but what you describe sounds like a transformer short. Before you power up anything, check the resistances on the transformer windings. The HV winding should be relatively balanced between sides (within about 10% to 15%) and in the range of 50Ω to maybe 200Ω. The 6.3 and 5v windings should show small (<1Ω) but measurable resistance. Resistances between windings and between windings and the transformer case should be quite high (>1 to 3 MΩ). Make sure this is the situation before applying power to the transformer again.

And most important... proceed with caution.

Author:  john55 [ 28 Jun 2018, 12:50 ]
Post subject:  Re: "Lacewood" Amp V2.0

I've checked the resistances on the transformer windings. The HV winding is about 100 ohms.
As for the 6.3v and 5v windings, my meter is wildly inaccurate down at 1-2 ohms. I got readings anything from a short to 4 ohms on these two windings. I do have a new, unused Hammond 370FX and I get exactly the same readings eg. Short to 4 ohms. I don't think it's the meter, I think it's the leads. I also checked for shorts between windings and between windings and case.... all looks good.
Powered up the amp with 1 amp fuse for 5 minutes without valves, and all AC voltages are good.
That's enough for today as I still have to earn a living.
I have a 5U4GB spare, so I'll plug in tomorrow and see what happens.
Any final advice?

Author:  Suncalc [ 28 Jun 2018, 18:06 ]
Post subject:  Re: "Lacewood" Amp V2.0

john55 wrote:
Any final advice?
Nope. I think you've got it pretty well covered.

Let us know how it goes.

Author:  john55 [ 29 Jun 2018, 03:11 ]
Post subject:  Re: "Lacewood" Amp V2.0

I've just noticed that I've wired the power transformer primary for 230v. white (0V), then black (120V) joined to brown (0V) and then blue/yellow (110V). Is this okay, or should it be balanced, eg. wired for either 220V or 240V?

Author:  john55 [ 29 Jun 2018, 12:14 ]
Post subject:  Re: "Lacewood" Amp V2.0

Interesting. Hammond give connection details for the primaries showing 100V, 110V, 120V, 200V, 220V and 240V only.
The company that sold me the transformer has come back to me saying that as the primaries are STACKED, 230V (110V on one winding and 120V on the other) is fine. I take it that stacked means one winding on top of the other as opposed to two windings side by side. Could anyone elaborate please?

Author:  Jim_O [ 29 Jun 2018, 17:24 ]
Post subject:  Re: "Lacewood" Amp V2.0

I'd agree with the company that sold you the transformer. The way you have it wired is fine. Connecting the black and brown wires together, putting the two primary windings in series, creating one 240-volt primary winding with taps to "fine-tune it to your local mains voltage. Symmetry isn't needed.

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