Need help solving problem on new 2A3 build

[jmillerdoc]

Hello,
I just finished wiring up my most recent build, the Hagtech Clarion 2A3: http://www.hagtech.com/pdf/clarionarticle.pdf


There is something wrong though and I am unsure where to start.

It by nature is a fairy low sensitivity amplifier requiring 4vrms to drive to clipping at a stated 3 watts. I did however bypass the driver cathode and estimate it is now somewhere around 3vrms. The problem is it plays very quiet, like the power is being held back somehow. The reason I say it this way is b/c the strangest thing happens when I power it down with the signal still running to it....the volume increases to what I would think is a normal lever for just a few seconds then fades away. If you power it back up while the volume is coming up it immediately dampens the volume back down to almost a whisper. I have powered it on and off several times and as best I can tell it sounds truly great as soon as you power it down for that 2-3 seconds the volume rises. Anybody have any ideas what could be going on here? Did I wire a cap up backwards or something (don't think I did)?

I have never heard of something like this but I'll bet there is somebody on this post who has. Any help greatly appreciated.

Sincerely,
Jeff Miller
Lawton, OK, USA

[astouffer]

Do both channels behave the same way? If you have a DMM take readings at the grids and plates of each tube. That should be an easy way to identify the problem.

[Boris_The_Blade]

Check all your power supply voltages? That's probably the best place to start.

[azazello]

Try disconnect C2 - 10 uF from PS

[Arnulf]

What is the combined DC resistance at output tube's cathode ?

[jmillerdoc]

Originally Posted by Arnulf

What is the combined DC resistance at output tube's cathode ?

It is supposed to be 680R plus whatever the #47 bulb puts into the path.

It does behave this way on both channels.

I have not yet had the time to put a DDM on it with the tubes in it. I did do some voltage checks without the tubes but they were way off b/c there was no current running through the system. I did a sim on PSUD2 without current running through the system to see if the voltages would behave the same way and they did. The B+ rises to 410v without the tubes, exactly what PSUD2 showed with a 1mA load on the system. So I am pretty sure they are correct but, again, I haven't had a chance to put the voltmeter on it yet with the tubes in and warmed up. I am thinking one of the bias points is off and when the power falls the bias point falls, or something like that.

Another idea is the bulbtype on the cathode of the 2A3 might be the wrong one, I am not home yet but that might explain it, if the bulbis wrong than the cathode resistance would be off and the 2A3 not biased at the proper point, when the power falls off the bulb filament cools a little causing the resistance to drop and the bias point to come in line to where it should. I could just bypass the bulb altogehter and see if that works. Besides that, what else could cause the output to rise as the unit is powered down? I have never seen anything like this before. I am confident that it will sound great once I get it corrected as it sounds really good for that brief period of time when it is powering down.
Jeff

[jmillerdoc]

Originally Posted by azazello

Try disconnect C2 - 10 uF from PS

Why would disconnecting C2 help anything, just curious as to what exactly this "parafeed" cap does anyway to help the amp?
Jeff

[Ty_Bower]

Originally Posted by jmillerdoc

It is supposed to be 680R plus whatever the #47 bulb puts into the path.

Schematic says #49 lamp, not #47.

I was just in Lawton, OK a few weeks ago near the Goodyear plant. Wish I knew you were around, I would have begged to come see your amps. The woodwork on this one is absolutely gorgeous. Hope you figure out the difficultly. I'm guessing a voltage check will tell all, and now I'm double suspicious of the lamp in the tail of the finals. Good luck.

[jmillerdoc]

Ty,
I actually mis-stated what I intended to say. Either way, I could have the wrong value in and need to recheck them. I sent you a PM with my phone number, please call me next time you are in town you are more than welcome to drop in.

I stepped out on a limb on the wood work on this one and it turned out really nice. It took a little bit of time to cut down the Tiger Maple and laminate it all together but it really wasn't hard to do. I find the best results are fairly simple in design but use very high quality materials. If I had done it in Oak it wouldn't be much to mention.

I am in the process of finishing parts acquisition on the JEL 300B dx and will be building a set of monoblocks the same way I did this one since it turned out so well. Spanish Modena is very hard to find and costs about $20 a bdft but, if you find it, make the investment and get some, it is easy to work with and really looks fantastic. I have probably about $50 in Modena in this amp with all the cut offs required to get what I wanted out of the wood. The Tiger maple is also about $20 a bdft and hard to find, but not as hard as the Modena. The trick to getting the Tiger Maple to look so 3D is to do a sanding with 220, 330, then a final with 400 grit. Before the polyurethane goes on I apply several coats of Danish oil. This MUST dry for 4-5 days before the poly goes on or the poly will never dry right. That is it, easy. If you just try to put poly or lacquer on the maple it will not turn out right, the Danish oil is a must.

Jeff

[azazello]

Originally Posted by jmillerdoc

Why would disconnecting C2 help anything, just curious as to what exactly this "parafeed" cap does anyway to help the amp?
Jeff

I case If C2 is not good more some +U, how U will be on cathode, and how will be between
Anode and cathode and what will be work-point of tube. You can measure.