Tonemaster 214 Clone harsh increasing buzzing sound- no signal

Hi, I have just completed my build of a Magnatone 214 and it was all ready to fire up, I had checked the entire circuit to the best of my abilities, bearing in mind this is my first amp build. Plugged it in and here is a recording of what happened;

https://soundcloud.com/jcandmeirality/tonemaster-clone-trial

The amp turns on, lights to the heater filaments and lamp on, but then again they all run from the PT wires directly so they aren't exactly going through the circuit I have made. Then the first buzz comes on fairly mild and sort of holds, followed by an exponentially increasing harsh buzzing sound until the bypass capacitor linked to the power tubes and powered by the 50v bias tap from the PT blew. The capacitor was rated 25uF/50V according to the schematic, but I thought that it must have blown because it was cutting the voltage rating from the PT a little too fine. So after that blew I have temporarily replaced this with a 20uF/450V, which doesn't seem to have blown... yet hahaha. However the results are exactly the same with the increasing buzz (as recorded) AND no sound from a guitar I have plugged in. Fuse has not blown.
Here is some gut shots of my build if you can spot anything wrong or out of place?

Tonemaster Amp Clone - a set on Flickr

Thanks for your help :)
Sam
 

Tesla88

Member
2011-12-21 8:37 am
Italy
Can you please post a schematic ?Google search lead to many different results...

When powering a amp for the fist time expecially for a newbie , is best to remove tubes and check voltages first! So at least you don't damage tubes and output transformer that are expensive .
Also insert a 100W filament lamp in series with main transformer primary winding to act as a "ballast" limiting the current , in case of short lamp light up , if all is ok lamp stay off or just little on.

In your build you have at least 2 problems , cap explosion and buzz.

An electrolitic Cap explode due to wrong polarity or too much voltage . Is common error in bias circuit to wire the cap reverse due to the fact you used to see the negative terminal connected to ground , in bias circuit the negative terminal goes to the negative rectified voltage (bias) and the positive terminal to GND , so check this first . Also the rectifier could be wired reverse by a mistake , the cathode must go to transformer tap , anode goes to negative terminal of cap.

Then measure the voltage across this cap , it must be lower than cap max voltage.
 
Yes, I have followed the original, with the addition of the three way switch from the second schematic from KO amps.
Hmmmn I will check that now I have a funny feeling that I have wired that bias capacitor "seemingly correctly" with the negative to ground :S not the real right way. I rewired the rectifier and it blew the fuse so I had that right the first time.

Original:

Magnatone 213 Schematic - original | Flickr - Photo Sharing!

Second Schematic:

ammo can 214 | Flickr - Photo Sharing!
 
@kevinkr Sorry if I have posted this in the wrong section, I thought it was :p Let me know how to sort this out.

Cheers for the replies! Here is the thing I used that 50V tap (from the PT) because it looked like a similar centre tap to that shown on the schematic diagram when comparing it to the PT diagram, probably not my smartest idea yet. I also managed to wire in the original bypass cap that blew backwards thinking that negative side of cap went to ground, so I have fixed that with a new one wired correctly (positive to ground). Also have wired in an artificial centre tap from the heaters using 100K resistors to connect 2&8, 7&8 on one power tube. The wire running from pin 8 on both power tubes going to the 250 Ohm 10W resistor has also been connected wired to the 20uF/50v filter cap (that originally blew) following as I understood the connection in the schematic. One of the two 50v bias taps from the PT is connected to the busbar for earth, and the other has been isolated and taped. Does this sound right? The one that I have earthed is the red-yellow stripe which I thought was the centre tap for the 5v rectifier heater wires.

I have taken the power tubes out and check the voltages which are recorded in this photo, compared to the schematic. My voltages seem to be WAY off!:

IMG_2856 | Flickr - Photo Sharing!

Also to clear up some other wiring here is a photo of my layout on the turret board:

IMG_2857 | Flickr - Photo Sharing!

The wiring for my ground is based off another tube amp I had open called a Gunn 50 made in new zealand. The busbar is connected to the chassis at only one point (near the AC power input, via a nut and bolt through the chassis with the busbar soldered to an eyelet lug) I have checked the resistance of all earths back to this point and they range from 0.00- 0.4 Ohm generally. All earths are wired to this single busbar running to a single earth on the chassis. This may still be incorrect let me know your thoughts?

As of now I have not yet replaced the power tubes and tried to see if the amp was working as the voltages were quite high at the points I checked on the Power and preamp tubes.
Thanks for all your help,

Sam