blowing fuses

[UNCLEDUB]

GIbsonman: I am getting sound out of the amp but I can't turn it up or down, none of the tone knobs do anything at all and the bright switch doesn't do anything. I have checked all conections with the continuity meter and everything is in order.

I have gone through the schem. checking it with the layout and I am learning a lot. This whole experience has taught me more than any book or guide ever could. Not that I didn't learn anything from the other two but nothing compares to getting your hands dirty.

KellyBass: I guess it's time to start from the begining again and recheck it all. There has to be something out of wack. I just have to find it. I have a little poker stick that I checked over the conections with while the amp was running and everything seemed solid. Ground connections on the pots all checked out.

Maybe a bad cap somewhere? How could I test that? I am also going to retest all of the resistors while i'm at it. and try different preamp tubes.

[passfan]

How much sound are you getting? If it's not a roar you have a preamp tube section not on and yes they will pass a little bit when they're not working, go figure. Check your grounds, your cathode grounds , especially your ground for your tonestack, it sounds like it's floating (if it loses it's ground it stops working). Is your pot ground buss connected to the chassis ?

[UNCLEDUB]

Ok, so I retinned all my joints just to make sure everything was good. I changed preamp tubes to see if that was the issue. Nothing yet...

I pulled the bright switch out and went over all the conections and i'm about to put that back in.

The grounds have been checked and rechecked as that was what I suspected as well. The grounding buss isn't connected to the chassis but it is grounded through the pots. I can add a short wire to connect it to the chassis if you guys think that would help with stability.

I'm going to try to bias the amp to see if that will bring the voltages closer to what they should be but I know that wouldn't fix the pot problem.

Again I am really greatful for all of the help.

[Gibsonman63]

I would ground the bus bar. To me it sounds like you are getting signal, but are unable to attenuate with the pots, which indicates no path to ground. If you have some jumpers, you can easily test. Also, if you grounded the pot lugs that go to ground to the pot instead of the bus bar, try jumpers from the lug to the bus bar in those places as well.

On my build, I was not sucessful in soldering to the back of the pots, so I soldered everything down to the bus bar and then grounded it by the input jack. I gave up on soldering to the pots, because I felt like I was applying too much heat for too long and the solder wasn't sticking anyway.

[passfan]

I don't even use a pot ground anymore and yes, he needs to ground that buss directly to the chassis. I adopted the ground style out of the Astroid 60 schematic and have'nt had a ground issue since, and I use it on every build no matter what style of amp.

[UNCLEDUB]

Hey guys, I'll get on grounding the buss asap.

I took the time to disconect all of the pots and bright switch and resolder them with new wires. I feel like I did a cleaner job this time.

I also biased the tubes.
from pin 3 across a 1 ohm resistor.
V5 read 39.6mA
V4 read 34.1mA

I used the higher of the two readings and multiplied that times the plate voltage of 401 to get 15.87W of disapation.

I also noticed when taking voltage readings off of the phase inverter tube when I touch pin 2 or pin 7 with the DMM the hiss completely goes away. while touching pin 6 of V1 is really noisy.

Now that I have biased the amp I'm going to go take new voltage readings and ground that buss wire directly to the chassis. I really hope it is a simple grounding issue. I have checked that all the grounds have continuity from the earliest point on the chain and they all checked out but maybe I have missed something somewhere. I'll take new pics of the pots and preamp section if you guys think that would help.

Thanks for all of the help

[passfan]

Are you checking for continuity between them or are you reading it all the way back to the main transformer grounding point ?

[UNCLEDUB]

I'm checking between them.

I added a ground wire from the buss wire directly to the chassis. This didn't fix the problem.

I took voltage readings from everything again.

Mains: 119.0
Sec: 611
B1: 395
B2: 283
B3: 303
B4: 286
B5: 269
Bias: -33 (this is taken from the bias point on the preamp board)
The bias voltage on pin 5 of the tubes is different being -27V.
Heaters: 5.5 measured at every tube.
V5: plate 392
screen 379
V4: same as V5
V3: pin 1: 188
pin 2: 11
pin 3: 34
pin 6: 187
pin 7: 10
pin 8: 34
V2: pin 1: 236
pin 3: 2.97
V1: pin 1: 197
pin 3: 1.7
pin 6: 172
pin 8: 1.37
I'm going to go over the power side of the amp again to see what could be pulling the heaters down.

[gearhead]

Looking at V1, make sure that pins 1 and 6 are connected to the right spots on the preamp board.

[KellyBass]

Did you say you double checked and measured all your resistor values with a meter? And checked them against the layout?...100ohm instead of 100k? Very easy to do.

[passfan]

How about a high resolution full on gut shot at the best setting your camera has. There are some places we just can't see.

[UNCLEDUB]

Ok, so after checking it a million times it took gearhead to point it out to me. I don't know how I kept missing that but I guess thats the difference between a rookie and a pro. Again I am really greatful for all of the help. I'm about to go tidy some things up now that it seems to be working properly. do a final voltage check and then get to work on the cab.

Has anyone tried tone tubbys with a trainwreck. They are a little extra warm so i'm thinking they may tame the brightness a little. The friend I am building the amp for is really wanting to try tone tubbys as well. Personally I just want to match the amp with the right speakers.

Thanks again guys for the help.

[UNCLEDUB]

Ok, so the pots work but the heaters are still running low. Any idea on what could cause this. They are only putting out 5.5V.

[M Fowler]

5.5 is good go with it.

[passfan]

Well CHA-CHINGG, ride em cowboy.