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Old 9th June 2012, 05:56 PM   #1231
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Thank you - I am not sure that it makes me more brave :-)

I have just recently discovered CCS, and I am not at home here. But it is something that I have to learn about.

I have the books by Allan Wright and Morgan Jones, so I guess I should just read those chapters again.

The lowest tap on my transformer is a 100v, but of course I could add a small transformer as you suggest.

Thank you for trying to help me out.


Best regards
Arthur.
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Old 9th June 2012, 07:33 PM   #1232
rongon is offline rongon  United States
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Quote:
Originally Posted by ArthurDK View Post
The lowest tap on my transformer is a 100v, but of course I could add a small transformer as you suggest.
Hey, where there's a will there's a way.

A 100V tap could be used with a voltage doubler to get about -250V for the 12AX7 cathode supply. That should work well enough, and would not be difficult. No need to buy another transformer, and the only silicone you'd need would be two diodes.

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Old 9th June 2012, 07:44 PM   #1233
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There is a will, and 2 diodes are probably ok :-) And there are also 120v taps.

Thanks for helping me out. Sometimes obvious things are so far away.

Have just looked at Allen Wrights schematics. The power amps are balanced, but in a foreseeable (spelling doesn´t look right) future I would like to go balanced, because some of my friends have demonstrated how much speaker cables influence the sound, and you can rid of them with balanced mono blocks.

I didnt believe it for one second that cables has so much influence, but they do.


Best regards
Arthur.
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Old 14th June 2012, 09:14 PM   #1234
CSlee is offline CSlee  United Kingdom
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Default Bad transformer?

Hello,

I’ve recently finished constructing a Baby Huey style amp for a gift. Essentially the same but RC biased output stage. The amp sounds good, not as good as the original version but good enough for a present!

However, there was quite a large amount of hum with a shorted input, and one side had more hum than the other. So a little testing and one channel was down at about -65dB, and the other -55dB.

My grounding scheme is a bar of tinned copper wire across the amp and grounded at the input sockets. A little playing around with ground wires, and splitting the ground bar into two channels improved the SNR to where I expected - about -75dB. But only on one channel, the other being at about -55dB.

With a sine wave input, the noisy channel was about 1dB higher than the clean channel. So I went through to check everything was the same between channels.

Swapped valves, checked all voltages, checked each input ccs - all OK.

Now I think there may be some interference from the volume pot (the build is in an old radio and isnt ideal) so I grounded the grid of the noisy channel which dropped the noise down to -65dB - better but not good enough.

The last part I haven’t swapped/checked is the output transformer (Hammond 1608) as it will be quite tricky. Do you think this may be an issue?

Attached is a live trace of the noisy channel.

The peaks cycle at 50Hz, which suggests heater hum, but I have tried a few approaches with heater wiring and makes no difference. Is there any tell tale interference with this picture?

I will keep trying different ground approaches for the bad channel tomorrow, but I exhausted all my ideas today. Will need to keep at it!

Cheers

Charlie
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Old 14th June 2012, 10:28 PM   #1235
BudP is offline BudP  United States
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If you haven't yet segregated power ground from signal ground until you reach IEC and chassis ground you might want to try that. You can also try lifting the OPT off of the chassis, just to make sure you aren't getting chassis transferred circulating currents into the OPT.

From the looks of the wave form your problem may well be airborne EMF, certainly the trace position of the peak is showing that. This will entail revolving core center leg direction to an out of phase position from the emitter, but do try the other things first.

Bud
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Old 15th June 2012, 05:57 AM   #1236
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One could suspect that the input socket on the faulty channel is not isolated from ground.

Maybe sounds stupid, but check again that you made the ground connections exactly the same way on both channels :-)

There is one more thing you could try, for general noise - isolate and elevate the heater ground connection.

I have drawn it, but wonder why it has to be so difficult to attach pictures ??

But here goes: Isolate the ground connection from the heater trafo, so that it is floating. Balance it with 2 100 ohm 2w resistors, and connect this point between a 220K,2w and a 22K, 2w, parallelled by a 1 uF cap, between HT and ground.

Best regards
Arthur.
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Old 15th June 2012, 06:21 AM   #1237
CSlee is offline CSlee  United Kingdom
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Thanks for the tips, I will certainly try them out.

The fault can't be input socket related as it shows with a grounded input grid. I do have segregated signal and chassis ground, but it may well still be a grounding issue.

My worry is that because of the different signal size at the output, it may be something else?

Note: The very sharp peaks in the picture are my soldering iron. But the main shape was there in the other channel until grounding was adjusted, so I am hoping it is that.

Heaters have an artificial centre tap with 2x 150R to ground, so I may play around with that too - thanks for the tip.

Charlie
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Old 15th June 2012, 07:43 AM   #1238
CSlee is offline CSlee  United Kingdom
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OK, just got into work and played around with the heaters. I moved the centre tap to another point and it improved the SNR quite a lot. So I will play around with that again when I get the chance.

The noisy channel is actually lower in output than the clean, not the other way round as I said before. About 0.6dB. So need to figure out what is going on there too.
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Last edited by CSlee; 15th June 2012 at 07:47 AM.
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Old 23rd June 2012, 11:22 PM   #1239
6L6 is offline 6L6  United States
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Hello all!

I have been researching this amp with interest, and am going to build one, provided I can use up some parts that I have on hand...

A few questions -

Feedback or no feedback? What's the general consensus?

What to do about the Blackgate capacitors? As they are practically unobtanium anymore, is there a good alternative?

Power rating of the unmarked resistors in the circuit? 1w? 2w?

PSU - The impetus to build this is to use up a tranformer and choke that have been sitting around unloved for far too long. A Hammond 272JX and 159T (600v C.T. and 2.5H)

I only have Macs, so I can't run PSUDII, and am slightly in the dark about how this will play out...

Click the image to open in full size.

I have a bunch of volts to throw away, I would like to do it with the combo of input cap and the choke instead of some big resistors. (Or I could use a 5R4, but I would rather use the 5U4) Yes, I also wish the choke were bigger in value, but this is the only one I have with the proper current rating.

CCS power will be from it's own 6.3v 1.2A transformer I have.

The output transformers are the only thing I need to buy, I'm thinking of these - http://www.edcorusa.com/p/424/cxpp25-6-7_6k (The single secondary has it's advantages in coupling) or the standard type - http://www.edcorusa.com/p/429/cxpp25-ms-8k Any experience with these or other suggestions?

Thanks in advance for your answers and comments!

-Jim
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Last edited by 6L6; 23rd June 2012 at 11:44 PM.
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Old 23rd June 2012, 11:58 PM   #1240
AJT is offline AJT  Philippines
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you can also add resistances from power traffo secondary to plates of the 5U4...
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