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Author Topic: 200S-a-like amp project  (Read 5387 times)

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arskas

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200S-a-like amp project
« on: November 04, 2010, 11:56:04 am »

I'm building an amp based on the 200S schematic diagram. I'm not attempting to make a perfect clone. I'm just trying to make a good simple amp. For example I'm using Hammond transformers because they are easily available here. 
The cab will fit nicely on top of my Aurinko)))-speaker.
 8-)

loudthud

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Re: 200S-a-like amp project
« Reply #1 on: November 04, 2010, 02:43:35 pm »

I see you've gone the red knob route :)

There is a mistake on the schematic at the volume control. None of the amps I have are wired that way. I posted a corrected 190B schematic in a thread about building a 200S clone started by EdBass. I think the title was "What do you think?". What Hammond transformers did you use?

Edit, Here's the thread: http://sunnforum.ampage.org/sdp/index.php/topic,4961.0.html

Look on page 3.
« Last Edit: March 19, 2013, 10:04:35 am by loudthud »

arskas

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Re: 200S-a-like amp project
« Reply #2 on: November 06, 2010, 02:19:18 am »

Thanks for the tip! The OT is 1650RA and PT is 378X. Choke is 193H.

arskas

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Re: 200S-a-like amp project
« Reply #3 on: November 09, 2010, 08:37:05 am »

there it is now:

arskas

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Re: 200S-a-like amp project
« Reply #4 on: November 10, 2010, 01:23:03 am »

Well, the amp is now ready and I have done some testing with it. It's eating a lot of main fuses when I switch on the standby.

Amp works fine when I do a cold start with stdby on. All voltages look ok. A 3.15Aslow blow holds the stdby-switching but that is about three times too large at 230VAC(!). There is an arch inside the JJ gz34 when I switch on with the 3.15A fuse.

I have read bad things about the jj tube. Could it be that I have a bad tube?
I have slightly modified the power supply filtering. I have two sets of 100uF/350V in series and a cap can 32+32uF/500V. The choke is 5H 65ohm 600V. Can the power supply overload the rectifier?

Soundmasterg

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Re: 200S-a-like amp project
« Reply #5 on: November 14, 2010, 11:24:47 pm »

Take all the tubes out and replace the fuse, then turn the amp on. If you blow a fuse in that situation, then you have something incorrect with your wiring or a bad component. If you don't blow a fuse in that situation, then stick in just the GZ34 and repeat. If you blow a fuse in that sitation then the GZ34 could be bad. If it is ok, then sticking in the power tubes and lowing a fuse could indicate a bad power tube, incorrect bias, etc. Most likely though, you will discover that you have a wiring problem or the rectifier is the problem. I never fire up an amp for the first time with a new build or a repair with the tubes in.....and always do the first power through a current limiter, just in case there are problems, and also to save the cost of blowing a bunch of fuses too....

greg

Soundmasterg

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Re: 200S-a-like amp project
« Reply #6 on: November 14, 2010, 11:26:50 pm »

If you correctly wired the two 100 uf caps in series, then you have a 50uf cap essentially, and the GZA34 should be ok with that, because it's max is 60uf, but you don't know with today's low quality GZ34's.

Greg

arskas

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Re: 200S-a-like amp project
« Reply #7 on: November 15, 2010, 04:03:14 am »

Thanks Greg. The wiring is ok because the amp works when I don't touch the standby-switch. I have been reading this: http://www.freewebs.co.uk/valvewizard/fullwave.html and it looks like I need to add limiting resistors to the rectifier circuit. I'll wait and see how the amp will work then.

I have considered a ss-rectifier but I think I would need to replace filter caps to hold the higher voltage.

Soundmasterg

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Re: 200S-a-like amp project
« Reply #8 on: November 16, 2010, 05:28:23 pm »

If the amp is in standby, then the B+ is not going through the circuit past the standby switch, so your wiring may still be suspect. If the problem occurs as soon as the standby is moved to the operate position, then you have to look at what occurs when that happens. Suddenly a surge of high voltage goes through your circuitry, and because the tubes are in the amp along with the rectifier, then there is a large current draw too that your power supply and caps have to supply to the circuitry. If you have a defective power tube, rectifier tube or incorrect wiring, then you will have a problem with the fuse blowing when the current draw for that part of the circuit is exceeded. Testing without the tubes will verify if you have a wiring problem or not. If it works without tubes, then try the rectifier and see if it works that way. If it doesn't then having a solid state module that you can try will show if your rectifier tube is a problem or not. Regardless, the first time an amp is fired up, it should be on a current limiter and should be run without tubes to verify that there is not a problem. This way you don't blow through a ton of fuses to try to find the problem. Once you've verified that it works without tubes, then you know your circuitry is correct and you can proceed step by step to find the problem as I mentioned. A solid state rectifier would work fine with your amp the way it is now. Adding limiting resistors might help, but it is not your problem. Try the tests.....

Greg

arskas

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Re: 200S-a-like amp project
« Reply #9 on: November 18, 2010, 12:52:27 pm »

I put in a ss-rectifier today and replaced the 15k resistor (in power supply) with a 25k to drop the voltages a little. Amp works nicely now but
it feels a little weak. Somehow I expected a little more punch. A bias check must be done next I think...

loudthud

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Re: 200S-a-like amp project
« Reply #10 on: November 18, 2010, 05:51:23 pm »

Once you get the output stage biased, check the voltage on pin 3 of the 6AN8. It needs to be 80-100V. If it's too far off, drive to one of the output tubes will be weak. A 200S won't have a great deal of overdrive, but it should be loud an clean with the right speakers. The 1650RA is not really the right choice, the original Dynaco transformers had a primary of 4200 IIRC. The 1650RA needs a B+ of around 600V to make 100W so you still should be close to a factory 200S.

arskas

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Re: 200S-a-like amp project
« Reply #11 on: November 22, 2010, 12:22:53 pm »

Well, I put the 15k back to the power supply, increased first cap value to 50uF(100uF+100uF) ...and there was all the power I was looking for  :-D
Look like all the voltages are good now.. Thanks for help.

arskas

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Re: 200S-a-like amp project
« Reply #12 on: December 14, 2010, 04:24:48 pm »

Just thought I'd share this photo here.

I finally got the name plate for this one. ...I've been totally happy with the amp now.

Johnny Guitar

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Re: 200S-a-like amp project
« Reply #13 on: December 14, 2010, 09:36:22 pm »

Cool amp arskas!! 
Greg, what do you use for a "current limiter"?
Johnny G.