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Old 2nd April 2012, 02:16 PM   #11
lordoff is offline lordoff  Bulgaria
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Quote:
Originally Posted by Shoog View Post
...The obvious way still requires cap coupling to the IT - which to my mind is just adding complexity without any technical gain.
Personally I would just stick with the input transformer. If I was going to try anything it would be DC coupled.

Shoog
Yes, with cap and IT may be will work, but this is not so elegant
No sense...
So, if we cannot avoid cap in cascoded diff. stage, stay only with cap.

Bob, sorry for my mini-theme - I don't want mess, just to add more details in big pictures.
Finaly - my judgment and advice:
Go your attention to PP2C, or Kevin Carter designs (more expensive option I think)
What about Output stage: if we use pentode.
I know that with DHT is the best option for driver and output, but for me it is too early
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Old 11th April 2012, 01:54 PM   #12
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Default Made a decision

Thanks for the input!

I decided to build the uniamp. I think I will build the PP-2c down the road sometime. I think the uniamp will be easier for my first build.

I'm going to use James iron throughout. Except the input transformer. I got quotes from Sowter for the 3575 $170 shipped to the U.S. or China where I'm living. The other option is Lundahl LL1690 but these are $140 each plus shipping. I have read the input transformer is very important. Anyone have any opinion on these two?

I've ordered these cases. They are 17" *12" *3". They should match the black James transformers.
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File Type: jpg case2.jpg (44.9 KB, 588 views)
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Last edited by nanchangbob; 11th April 2012 at 02:04 PM.
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Old 11th April 2012, 02:25 PM   #13
chrish is offline chrish  Australia
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Nice chassis. Where did you order them from?
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Old 11th April 2012, 03:19 PM   #14
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Default Taobao

Quote:
Originally Posted by chrish View Post
Nice chassis. Where did you order them from?
Thanks,

It's the Chinese ebay called Taobao. They were about $63
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Old 1st July 2013, 06:00 AM   #15
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We'll I'm finally back to my projects. I had a couple people write and ask me how the amps were doing. I took a contract in Western China and couldn't finish everything. Now it's time to finish.

I found out a few things while doing these builds. I built the first amp and turned it on and everything seemed fine. But the power supply ripple was unacceptable. I hooked up a test speaker and I was getting some motor boating. I pulled the driver first and that was where my problem seem to be. I tried turning the choke and that didn't help.
The James output transformer is positioned horizontally in an encapsulated case. I thought I had it positioned properly but as usually my assumption was wrong. Rotated the output transformer and all is well.

I'm doing a few more test and will check the o scope signal before hooking up my speakers. I'll post some pictures soon. The black aluminum chassis look good. Cutting the holes in the thick aluminum chassis was also a lot of work.
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Old 1st July 2013, 07:32 AM   #16
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Default Just after cutting the holes

This was when I first cut all the tube holes and mounted the transformers. I finished all the wiring on one amp now and it's on my table to be tested.
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Old 21st July 2013, 05:06 AM   #17
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Default Help with this problem

Hi guys, I need a little help. I have been away from home for a year and I've just returned to my project. I thought my problem was fixed but it is not. I've attached the pictures from the scope. When the driver tube is plugged in you can see the AC ridding on the B+. When I pull the driver tube the problem goes away. I thought it was the output transformer.

When I rotated the output transformer it seem to go away. Actually I lifted one of the eyelets for the driver heater. I really don't like heaters on the PCB'S.

I have also rotated the choke and that didn't make any difference. I have not rotated the choke again after I rotated the output transformer.

I strapped a big film cap near the driver B+ and it improves but there is still AC present. Suggestions will be most welcomed. This is my first tube amp build and I'm pretty bummed.

I am thinking this is something to do with my layout or orientation of the transformers?
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File Type: jpg acproblem.jpg (116.3 KB, 275 views)
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Old 23rd July 2013, 08:33 AM   #18
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Wow, 400mS/division... this is very low frequency motorboating!

Change R11 to 4.7k, and R21 to 1k. That should help a lot.

Also, depending on your output tubes, you may very well need to reduce the value of R7 and R18. I found with some new KT88's this needed to go as low as 47k to stabilize the bias, as the tubes pulled a little grid current. This can contribute to the motorboating.

Pete
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Old 23rd July 2013, 08:51 AM   #19
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Thanks Pete,

I will pick up some resistors tomorrow. I've tried a couple different sets of KT88 and I have some EL34 in there now. Not much difference.

I thought about changing R11 and R21 but I thought it was my set-up or layout. I haven't read many issues with people building this amp. Is this have anything to do with the James iron I am using?

I'll make the changes and then post.
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Old 24th July 2013, 08:35 AM   #20
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Replaced R11 and R21 to values you suggested. Motorboating stopped and the amp is stable. I haven't yet tried to decrease the resistance yet to find out where it becomes unstable.

I put a 1k square wave through it and there is ringing. The square wave looks pretty bad.

I believe this is where C14 and C15 come in? What are suggested values to start with on C14 and C15?

I added the schematic and wiring diagrams

UNIAMP2_sch.pdf

uniamp_hookup.GIF

Thanks,
Bob
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Last edited by nanchangbob; 24th July 2013 at 08:43 AM.
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