Double and triple check.

[marcoloco961]

My Express build has been put on standby by some backordered wire and some 1 Meg pots. I have managed to come up with enough scrap wire to get the PS board ready minus the bias wire. The impedence switch, input and output jacks are all put together.And the heater wires run.

I have re-checked my work several times, but it always seems something will slip past. If you gentlemen would be so kind as to give a few pictures a quick double check for me, I would appreciate it.

Feel free to offer any critisisms on anything I may have screwed up. I do not want to melt my trannys! For some reason I do not seem to mix well with the power tranny's.....LOL

Also, do pin 1 + 8 on the power tubes have to be run to ground if bias test points are used or is the ground thru the test point resistor and ground?

[marcoloco961]

Anyone? Por Favor....................

[marcoloco961]

Here is a finished pic or two. I was hoping for a double check from someone before I plugged it in, but oh well..........................I am going to go for it this afternoon. Being my first build I am very nervous about frying some expensive parts, but I guess it's all part of the learning process.

[M Fowler]

marco,

I have been reviewing and did not see any obvious problems and with the new finished pictures I don't see any problems. Once fired up you may need to do some shortening of wires or placement of the wires which is normal. You may also find that using a shielded wire from V1 pin 2 to wiper on volume pot might be necessary. But all that stuff takes place after first fire up.

Mark

[John_P_WI]

Being my first build I am very nervous about frying some expensive parts, but I guess it's all part of the learning process.

Don't worry about frying stuff, it can be replaced. Take care of yourself!

Seriously, use a lightbulb limiter or variac on the startup. Well worth the step to do this.

John

[marcoloco961]

Thanks guys, I have made a current limiter and am getting ready for "the moment of truth". I have re-checked everything at least 4 times. I did find 2 or 3 cold solder joints in the process. Damn I'm nervous. Going to go get my proverbial 10 ft pole and flip the switch.

[Structo]

+1 on the light bulb current limiter.
Very simple to make and can save the PT, Tubes, etc.

Use a 40 watt light bulb for startup.
With NO tubes installed connect the amp to the bulb limiter.

When you flip the power switch and or standby switch on the amp the light bulb will flash brightly for a second or two when the filter caps charge, then it should go down to a dull glow.

If the light stays bright, you have a short or other fault causing high current draw by the amp.
Turn off the amp quickly and troubleshoot your wiring.

If the bulb returns to a normal low glow, turn off the amp.
Now install the tubes.
Change the light bulb to a 60 or 75 watt bulb.
ANYTIME YOU HAVE POWER TUBES IN THE AMP, CONNECT A SPEAKER LOAD!

Turn it on again and the bulb will flash again when the caps charge.
Then once again it should go down to a glow.
If it stays bright this time you have a problem with the tube wiring.

If it checks out, you can take the limiter out of circuit and try the amp.

[marcoloco961]

Time to trouble shoot. Upon initial power up the bulb lit dimly and stayed dim. It never did start bright. I left the power on for 3 or 4 seconds and switched it off. I could smell something getting warm. I could not tell for sure if it was actually something getting hot or just my imagination so I flipped the switch again for about 8 or 9 seconds. All looked good and then right before I was going to shut it down and hook up my meter, the light went from dim to bright all at once. I shut it down immediately, checked filter caps for DC voltage and there was none present. Started feeling around for hot components and found the MOV to be smokin. Took out the PS board and tested PS Tranny. Still good thank god! Checked continuity on primary and secondary of OT. looks good. I did have a speaker cabinet connected at power up with no tubes. I read to do it that way somewhere, upon further inspection I see it is only needed when tubes are installed. I did use a 90 watt bulb. Is that a problem in itself? Also upon inspection of the jumpers under the 9.1 K resistors, it look like they might have been pushed out when I soldered the wires from the filter caps in from the top. Possibly breaking the connection in the voltage divider circuit. As the filter caps had no DC voltage at all, would it be safe to assume that this could have caused the problem? I assume the MOV is trash and needs replaced, any other parts you would inspect to start with?

[M Fowler]

I would double check that wiring on the on/off switch, no reason for that to have gotten hot like that unless the switch is wired wrong.

Mark

[marcoloco961]

God do I feel stupid . What was it they say about small simple (stupid) mistakes.

Upon further investigation, I realized that I had ordered a MOV with a clamping voltage of 140 and a maximum voltage of 43V Ac. The parts bom shows a 130 or 140 max with a clamping voltage of like 400V AC. I apologize for wasting your time. Be easy on me. This is my first build.
I will go get the proper MOV tomorrow and try again. Thanks a bunch for the help. This is a fun project, but a little on the overwelming side for me right now. It helps to have someone to offer advise and opinions. Keep up the good work guys.

[M Fowler]

The MOV is there as an added measure of protection and is not necessary but even so there has to be another reason for the amp to blow fuses.

Mark

[marcoloco961]

It's not blown a fuse yet. I was putting 120 volts AC through a MOV with a max. AC voltage of 43 volts. That is why the MOV kept smoking out. I ordered the wrong part.

[Structo]

Take the MOV out of the circuit and try it again.
I know that part is part of Ken's Trainwreck design but most amps don't have those.

I just plug my amps into a power conditioner unit that prevents spikes and surges.

I would use a least a 60 watt bulb.

The lower the wattage bulb the lower the current to the amp.

I usually start with a 25 watt bulb which cuts the current way down to prevent any damage to the PT.

If I don't have anything wrong when I power up without tubes and the 25 watt bulb I'll sometimes go with a 75 watt bulb and power up with the tubes in.
If it looks good there, I take the limiter out of the circuit and see if the amp will make guitar noise.

[Firestorm]

The MOV is just there to absorb spikes from the power line. It's a "sacrificial" component because after it gets hit a few times, it will stop working. But the amp will still work as Structo says. The fact that you found no residual voltage on the filter caps (which you should do even with the bleeder resistor) makes it seem that you should take Mr. Fowler's suggestion and recheck your switch wiring. Even if some connections to the dropping resistors came loose, you should still see voltage at the main filters.

[marcoloco961]

Replacing the MOV with the properly rated part solved that problem.

All tests went well until the final power up. At this point my DC voltages at the B+ stages were all about 20 Volts low. Power at the wall socket was 120 Volts exactly. They were as follows:

B+1 = 396
B+2 = 382
B+3 = 305
B+4 = 288
B+5 = 273

I did use the 3 watt, 9.1 K dropping resistors as opposed to the 2 watt ones. Would this make a difference??

The rest of the readings were slightly low also. They were
V1a Plate = 202, cath = 1.8, heater 6.6, V1b Plate = 180, cath = 1.3
V2 Plate = 257, cath = 2.7
V3a plate = 200, grid = 23, cath = 36, V3b plate = 199, grid = 23, cath = 36
V4 plate = 395, screen = 381, bias = -30
V5 plate = 395, screen = 377, bias = -30

I have 42mA on V5 and 45mA on V4. I also have an issue in the tone stack somewhere. With the treble, mid and bass pots all the way CCW the volume knob goes from nothing to a slight sound and then bac to nothing. As soon as you turn up any of the 3 tone knobs to 2 the volume knobs seems to function properly.

I am going to go over the pre-amp and tone stack section and check for errors. Does anyone have an idea of where I should start looking for the problems?