blowing fuses ga 5

[john_t]

I am blowing fuses in my schedule 40 ga5. Its fine until I put the tubes in. I just realized i have the capacitors in backwards. WOULD THIS CAUSE THE FUSE TO BLOW. To late to change tonight. Maybe tomorrow.

[JayB]

sure will.

[john_t]

jayb thanks il flip them around tomorrow, I cant believe I did that.

[john_t]

Ok, I flipped the caps around. No longer popping fuses. the amp is plenty loud for 5 watts. However there seems to be significant buzz/ hum at about half volume. All the way up the buzz is way loud. I have a 1960 Gibson ga 5. It has some hum cranked way as well but no where near this build. What should I check.

[LooseChange]

Just maybe you killed the caps.

[john_t]

Loose change thanks. The voltages seem good. Would the voltages be ok if the caps were toast.   Also I have buzz with no signal into the amp. Should I change out all the caps. Wow what a pain.

[john_t]

ok, I replaced the electrolytics. While it seems better, the amp still has a  buzzing problem. Half volume is tolerable. Turned up to 12 not so good. The buzz is present even without a signal into the amp. I plugged in my original ga-5 to compare. I would compare the buzz to my original cranked all the way up with a 1/4 jack plugged in with the other end open (no instrument).

Any advice on this !

[HotBluePlates]

Do you have a center-tap or artificial center-tap for your heaters? If not, it can result in a buzzy-type hum.

[john_t]

HBP, I used the center tap from the transformer. I grounded it to one of the bolts on the tranny. I did no use the two 100 ohm resisters on the heater wires. As for my grounds, I ran ground wire to the pots, jacks, and the ground bus, and ran them over to the tranny as well on one of the other bolts. I used shielded cable from the input jacks to the 12ax7. For what its worth I really like the amp. For 5 watts it is plenty loud. Nearly twice that of my 60 ga5. Very similar characteristic otherwise. I think I am going use it with a 10 speaker though.

[john_t]

My GA-5 is nice and quiet. I am not totally sure why. I pulled the center tap and temp hooked up a pair of 100 ohm resisters. NO SOUND. Put the center tap back in, left in the resisters. Have sound and a no buzz. I was under the impression I needed to cap off the tap if using the artificial tap. Is it ok having both.

[HotBluePlates]

It's overkill to have both, but you won't be harming anything.

You're probably getting no hum now because of reattaching the center-tap. If it works now, it should have worked before, although perhaps good, solid contact wasn't being made.

[john_t]

I think I will leave well enough alone as long as its working. So far I am really happy with this little amp. I appreciate all the help.

[sluckey]

I pulled the center tap and temp hooked up a pair of 100 ohm resisters. NO SOUND.

Sounds like you 'may' have pulled the CT for the HT winding instead of a CT for a filament winding?

[HotBluePlates]

Brilliant again, Steve! That would explain all issues.

[plexi50]

I need some of Steves brain cells.

[john_t]

Could some one explain the difference between the ct and the heater tap. I used the 125p1b. the wiring diagram does indicate ct not ht.

[sluckey]

CT is center tap. There may be a separate CT for the B+ winding and the filament winding on a transformer with multiple secondary windings.

HT is High Tension (or high voltage) winding. HT does not mean heater tap. The HT winding is used to develop the B+ in an amp. The leads are typically red and the CT is typically red with a yellow tracer.

Your 125P1B has a CT for the HT winding only. There is no CT for the 6.3vac heater winding. Look at the pic.

You need to create an artificial CT for your filament winding since it has no real CT. Do this by connecting a 100Ω resistor from each green filament lead to chassis ground. Actually, since this is a single ended amp, you may benefit more by elevating the filaments to a dc voltage rather than referencing to ground (Do one or the other, not both!). You can do this by connecting a 100Ω resistor from each green filament lead to the cathode of the output tube. Probably easy to do right on the output tube socket.

HBP asked "Do you have a center-tap or artificial center-tap for your heaters?" You lifted the CT for the HT winding and this is equivalent to opening a standby switch. Doing so killed the B+, therefore no sound.

[john_t]

The amp is running pretty quiet at this point. I am not opposed to trying something new to see how it sounds. Let me make sure I have this right. I pull the 100 ohm resisters from the heaters to ground. I will still run my heaters to the lamp, from there I will go to the cathodes of the power tube with 100 ohm in place and on to my preamp tube.

[sluckey]

Leave the 100Ω resistors connected to the heater wires but disconnect the other ends from ground. Twist the two flying resistor leads together and connect a wire from the junction of the flying leads to the cathode of the output tube.

If the above looks too messy, just rip the two resistors and wire out. Assuming you have an EL84 output tube, connect a 100Ω between pins 4 and 3. Connect another 100Ω between pins 5 and 3. That's all.

[john_t]

Sluckey, That should be a easy mod. I have the two 100rs on my lamp with the two 100rs tied together at the opposite end, from there they run over to the tranny for a ground point. Let me make sure I have this correct. So I just need to pull that ground off the tranny and place it on pin 7 of my el84.

[sluckey]

So I just need to pull that ground off the tranny and place it on pin 7 of my el84.

No! Connect the resistors to the cathode of the EL84.

[john_t]

I did not explain that very well. It would be the two 100 ohm Rs over to the cathode of the power tube.   I would just be changing them from the ground point of the tranny over to the power tube. Is that correct.

Sorry for so many ?. I just want make sure I have this right before attempting. Thanks for your patience.

[Geezer]

and place it on pin 7 of my el84.

Pin 7 is the plate.....DO NOT connect it there! (as Sluckey said)

Connect the wire from the 2x100ohm resistors to pin 3, which is the cathode.

EL84 pinout>>>

[john_t]

Thanks for reminding me, and I just looked at the tube data sheet for the cathode pin. Il give this a try.

Thanks again

[sluckey]

and I just looked at the tube data sheet for the cathode pin.

Now you're cooking...   

[john_t]

First off, I want to thank everyone for all the help. I did want to post my final outcome. Changing over to the cathode on the power tube was fairly simple. By using a jumper with a alligator clip I was able to compare the dual 100 ohm(heater wire) to ground as per schematic against  using the cathode to lift the ground. While I think the cathode was slightly quieter the difference was not significant. I wound up just putting the ground back in place as the schematic calls for. The amp has some serious grind. Now I just have to decide on a speaker and cab. I think this amp sounds better with a 10" as opposed to a 8.

Thanks again I was about to cry uncle on this.