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blowin' fuses (Gibson GA-8T cont.)

7/2/2000 8:02 PM
Sean Kilback
blowin' fuses (Gibson GA-8T cont.)
Still working on my '64 Gibson GA-8T...  
It blows fuses when turned on if all tubes are in the amp. When ALL tubes are removed (including rectifier) the the fuse is fine, and the voltages look ok, even on the heater winding. Of course, with the rectifier removed the rest of the circuit is not seeing any voltage...  
Using RG Keen's tube amp debugging page (big thanks RG!), the PT looks ok. Next, I checked the OT and the secondary windings are shorted to the chassis. The primary windings look ok (162R and 141R which is within the 1K tolerance RG suggests).  
My question is this: will shorted OT secondary windings cause the fuse to blow? Since the tranny is supposed to block DC I can't see why this would happen. Perhaps I have to look elsewhere (as well as replace the OT).  
Again, thanks for any help you can think ok!  
And now, a word from our sponsors:

7/2/2000 8:03 PM
Sean Kilback

Of course, that should have have read:  
"Again, thanks for any help you can think OF!"  
7/3/2000 7:26 PM

if the secondary of the OPT is measuring no resistance to chassis, it might be intentional since one of the speaker leads (usually the -) is commonly connected to earth. This connection can be made anywhere from the speaker jack to a very shortly trimmed transformer lead connected to the chassis or an equivalent ground point.  
If your OPT turns out to be OK,  
If your first filter cap can handle the B+ produced by your rectifier alone (without the additional current being drawn by the other tubes which can help to drop B+ to within the cap ratings on some amps), it can be inserted by itself and further fuse behavior observed. If OK then take voltage readings, if OK, turn off & add the power tubes. With a preamp or hot pedal (even a signal generator) and a little shielded temporary wiring, you might be able to drive each power tube directly through a 100K resistor to its grid to verify correct operation at this point.  
Keep restarting with an additional tube, moving in the direction of the preamp tubes one at a time until you have either verified operation or more closely pinpoint what tube or other component finally makes the fuse blow.  
Assuming it is a new correct fuse of amperage originally intended for your amp.  
If it seems like there is a cause-and-effect relationship when inserting a certain tube the fuse blows, it might be true. Make sure the tube is a good one or re-try a different tube. There is some risk, since occasionally the amp could be defective enough to damage a new tube, but with good fortune in the design, the fuse will protect it from an extended overload that could cause permanent damage. This is where a variac can be helpful to reduce voltages overall, lengthening the short time you might have to take readings before the fuse goes again.  
Another service aid for this is to use an in-line AC ammeter. I've got one in a BUD box with a 3-prong line cord to the wall socket, the *wall* socket on the box is metered by a zero to 10Amp current meter. The line cord from the guitar amp plugs into it and it shows the current drawn by the entire circuit. On a typical small circuit using a 1Amp slo-blo fuse, you might see a turn-on surge of a few amps, but it immediately declines way below 1Amp unless something is wrong.  
You are using slow-blow fuses aren't you? Most amps are expected to use them, if you have mistakenly replaced with rapid-acting fuses there *might* not really be anything wrong . . . with the amp ;)  
Hope this helps,  
7/4/2000 3:25 AM
Sean Kilback

Thanks again Mike!  
Regarding the OT secondary shorted to chassis...right! I should have figured that one out!  
I neglected to mention before that there were two small diodes taking the place of the missing 6CA4...with these in place and all tubes removed it was still blowing the fuse. I've since removed the diodes and the fuse does not blow.  
The previous owner knew nothing about the history of the amp except that it has never worked, so for all I know the diodes might have been a failed attempt to replace the rectifier tube.  
FYI, I've replaced the filter caps with 22uF/450VDC NTE's (the stock caps were 20uF/325VDC). I'm going to order some new 6EU7s and a 6CA4/EZ81 tomorrow so I'll be able to resume my testing in a week or two. I already have some Sovtek EL84s kicking around so I'm set for output tubes.  
This particular amp calls for a 1A fuse, and I'm using a 1A slo-blo. Thanks for the reminder just the same!  
I'm still excited about this amp. I know I'll get it working with some effort!  
7/4/2000 4:02 AM

First, make yourself a light bulb limiter (see the tube and SS amps page at GEO) and quit buying fuses by the gross.  
OK. All tubes out, no blow. This means everything up to the rectifier is OK, at least for this particular problem. Then you put in two diodes, and something pops. Can't be a tube, can it? If I remember the debugging page right, next I'd suspect the first filter cap, choke, and second filter cap, in that order. Open the choke (or corresponding resistor if it doesn't have a choke) and see if the current's still too much. If not, and the B+ is good on the first filter cap, then check the choke for shorts to ground, per the debug page. If the choke is not shorted, check the second filter cap.  
After the second filter cap, each RC section usually involves a biggish resistor that usually won't blow a fuse, preferring to smoke and die itself. So if you clear the first filter cap, choke, and second filter cap, you may have a wiring short on either the B+ wiring, or on the heater wiring.  
If that doesn't hit it, come back and we'll see if there's something here that the debugging page might miss.
7/4/2000 5:57 AM
Sean Kilback

I checked the diodes and one is ok, the other is bad (it drops 0.133V both ways). This would effectively short the secondary of the PT and poof the fuse wouldn't it?  
I checked a few of the components for drift, and the first power resistor was 1K5, from 1K. I'll replace that before powering up again.  
Thanks for your tips RG!  
7/4/2000 12:57 PM

"I checked the diodes and one is ok, the other is bad (it drops 0.133V both ways). This would effectively short the secondary of the PT and poof the fuse wouldn't it?"
Ah-yep. That'll do it!

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