Problems with Matamp GT120 clone build

[dwrockdoctor]

Hi all,

Long time lurker, first time poster.

I built a clone of a matamp GT120 a little while back and have always struggled with issues since completing the build. The main problem is lack of gain. The first few minutes of playing it roars at full power but quickly gets stripped of a lot of its gain. If I then stop playing for a bit it'll return somewhat before fading back out once I start playing again. Output volume is still pretty healthy when this happens.

I have pulled out the PCB and reflowed solder over all the points, I also did this on the preamp sockets and drive switch as well as changed out the screen and grid stopper resistors on all the power tube sockets as well in case one was failing which didn't fix the problem. Triple checked all components with DMM and they all measure up and the right values are where they are meant to be. Changed out different pre and power tubes in case it was that, still no go. All DC voltage readings come up fine at idle at all pre and power tube socket points

I tinkered more today and discovered with the NFB loop disconnected the amp becomes a lot more stable as far as gain content. As soon as I reconnect the NFB loop the problem returns including on and off red plating on one particular power tube socket if the NFB is connected to the 16 ohm tab on the selector.

Any idea what this might be pointing too as far as a problem is concerned?

Any help would be hugely appreciated!

Edit: I just realised like a total noob that I put this in the wrong section of the forum instead of in the trouble shooting section. Mods feel free to move it to the appropriate section, I couldn't figure out how to delete the thread to repost in the right section

[DrGonz78]

Can you please post the exact schematic of your build?

[DrGonz78]

It is the same amp as in this thread... Which was posted by NateH who built this Hilbish design.
http://music-electronics-forum.com/t38447/

Okay here is the schematic below>>> OP messaged me to let me know this is the correct one for his amp.

Edit: Oh yeah... I forgot to ask which tube red plates when neg. feedback is connected to 16 ohm? Also, is that red plating happening when you apply signal or just at idle?

[dwrockdoctor]

It was one of the inner power tubes and it was doing it at idle pretty much directly after turning the amp on. The red plating was kind of random though, it did it a few times in a row, but after I switched between trying the amp with NFB disconnected and then on the 8 and 4 ohm taps also, when I attached back to 16 ohm it didn't do it again. The phasing in and out of gain was still very much apparent though.

[DrGonz78]

Perhaps rule out bad tube socket by wiggling the output tubes while in idle, wear an oven mit. Just got to know whether that causes anything abnormal to happen. Unfortunately/Fortunately, the red plating is intermittent and makes it hard to use that symptom as a constant. The idea of moving the red plating tube to another socket to see if it follows or is the socket might not be possible in this case. What is your bias voltage reading? Check that reading at turn on and even after running for 10 minutes. Also, in the same manner, what measurements do you have at the grids?

You said you changed out the preamp tubes and does this include the PI? Do you have the ability to use a scope or signal tracer? Also, please post your general voltages for all to see.

Edit: One more question... With the amp on for at least 10 minutes and volume turned up a bit, does the OT seem a bit hot?

[dwrockdoctor]

Thanks DrGonz.

I will measure bias and grid voltages as described when I get a chance. If not today, definitely tomorrow.

The red plating tube doesn't follow the tube when I move it to another socket. Though at one instance in another socket I got gain jumping in and out when I wiggled it. I then retensioned the sockets and that problem went away when I wiggled the power tubes, but the coming and going of gain still was happening on its own with NFB connected.

Could I have inaudible parasitic oscillation happening and thats why I get the problems as soon as NFB is connected?

Oh, and unfortunately I don't have a scope.

Also: I'm not sure if it is of note but a week or two ago I hooked my DMM to the plates of the power tubes while I played and I was getting significant voltage sag in the area of 60-80 VDC when playing a hard strummed open E chord. Is that normal behaviour?

Seems like a large sag to me.

[DrGonz78]

Also: I'm not sure if it is of note but a week or two ago I hooked my DMM to the plates of the power tubes while I played and I was getting significant voltage sag in the area of 60-80 VDC when playing a hard strummed open E chord. Is that normal behaviour? Seems like a large sag to me.

To be honest I am not very well versed to answer such a question. However, I have read somewhere that it might be 10-20% sag as a general rule of thumb, but this is a hard one for me to explain with hard data. Let's say it is 10-20% and your plate voltage is 500v, well then that (60-80vDC) is 12-16% sag. So if 10-20% is the general rule (of course this might be wrong) well then it is in tolerance. We also would want to know what your exact plate voltage reads. How did you bias this amp and what percent of idle dissipation are you measuring?

[DrGonz78]

Check out this thread...
http://music-electronics-forum.com/t29066/

The whole 10-20% thing I mentioned is probably something I read that someone made up as I have yet to find where I read it. Just ignore all that and set the bias for a reasonable idle current for your tubes.

[dwrockdoctor]

Interesting. I have a hunch it might be related to an inaudible parasitic oscillation eating up some of the output power that is exasperated when the NFB loop is connected. But would that explain the coming and going of gain when the circuit is hit with a hard signal?

Hopefully a poke around with my DMM tomorrow can shed some light on the situation.

I have been setting bias to around 70% dissipation or a bit under at idle.

[dwrockdoctor]

After everything I've tried I got out the old trusty chop sticks and found some problems around the gain pot with gain coming and going as I chopsticked the leads coming from that pot, if I hit a sweet spot, there is the gain!

Hopefully with a nice new pot and leads I will be back to having stable gain structure again. Thanks for all the help!

P.S. The red plating tube was down to a dodgy solder on the grid stopper on that socket.

[DrGonz78]

So yes two different issues completely!! Glad to hear you got it all sorted out too. Perhaps it's time to build a simple circuit tracer to add to your list of tools, but nothing beats a pair of chopsticks. Bon Appetit

P.S. The red plating tube was down to a dodgy solder on the grid stopper on that socket.

I had a feeling that there was something wrong in the socket connections since it didn't follow the tube.