TSL100- V6 not firing up!

[fdesalvo]

I received some new KT77s/12ax7s for the amp and was going to install them after rehearsal last night. After taking the rear grill off of the head, I noticed that the the stock EL34 (V6) was not firing. It looks like it's got no heater. I swapped the tubes around and no tube will light up in that socket.

Out of curiosity, I threw a meter on the bias points. The recommended range for this amp is 80-90mv/pair. V7/8 were drawing 57mv while good ol' V5/6 drew 114mv. Why would the one tube working in that pair draw twice the current/voltage?

I removed the chassis from the box and could clearly see where V6's socket connects to the board. I did not see anything obvious- no solder joints appeared to be cold; no fuse open; no components burned or distorted.

The tubes are known good and I swapped them around and they work in every position other than V6. What could it be?

I've already printed the schematics and have identified the heater/filament circuit for the poweramp. Each power tube has it's own 6.3a fuse. V6's filament fuse is not blown, so I am at a loss. I will troubleshoot the rest of the heater supply tonight and update you all.

[Enzo]

First, you do realize, don't you, that on those Marshalls with the little three pins with trimmer controls sticking out the rear, the pin and trimmer on the left actually controlls the two tubes on the right, and the pin and trimmer on the right controls the two tubes on the left? it is sorta backwards. SO if one tube is out, the pin on the other end will be the one with lower current. COuld that be your issue?

As to V6 itself, if no tube lights there, it is a case of missing heater supply. Pull the tube and stick your meter probes down pins 2 and 7. If the 6VAC is not there, check continuity to the next tube. It has to be something like:

cracked solder
bad solder on a jumper wire
bad fuse clip
bad solder on the V6 socket
loose pin on socket
socket pin broken between solder end and the end that grips the tube pin
cracked circuit board

[fdesalvo]

Hi, Enzo! Long time, man. I'm aware of the bias points/pots and what they do. And I believe it's the heater supply, too. I wil let you know whats up, brothah! Thanks for taking the time to help me.

First, you do realize, don't you, that on those Marshalls with the little three pins with trimmer controls sticking out the rear, the pin and trimmer on the left actually controlls the two tubes on the right, and the pin and trimmer on the right controls the two tubes on the left? it is sorta backwards. SO if one tube is out, the pin on the other end will be the one with lower current. COuld that be your issue?

As to V6 itself, if no tube lights there, it is a case of missing heater supply. Pull the tube and stick your meter probes down pins 2 and 7. If the 6VAC is not there, check continuity to the next tube. It has to be something like:

cracked solder
bad solder on a jumper wire
bad fuse clip
bad solder on the V6 socket
loose pin on socket
socket pin broken between solder end and the end that grips the tube pin
cracked circuit board

[fdesalvo]

V5-8 are showing the same Vdc~7.1
I retensioned the pins in V6- no change

The only thing I've left to do is touch up the solder joints on V6 to rule that out. Physically, the joints look no different than those on the adjacent sockets.

I'm at my wits end at this point.

[Enzo]

I repeat, pull the dark tube and stick your meter probes down the socket holes. Is there 6VAC on those pins???

If there is not, then power off, pull the next tube also and measure continuity pin to pin for pins 2 and 7 between the sockets. I'd have to expect one or the other path was broken.

When you checked the heater fuse for that tube, did you actually pull the fuse from the clip and measure it with a meter? Or did you just look and visually inspect it? If you measured it while still in the clip, you were measuring the parallel current path through the tubes and PT winding - that will fool the meter. Pull teh fuse from the clip and test it.

If the tube doesn't light, either it is a bad tube, or there is no 6 volts getting to it. If all the other tubes light, then the 6v supply must be OK. That leaves the "wiring" and the socket.

[fdesalvo]

Enzo,

1. The tubes are good- the dark one will light up in the other positions.
2. I did not pull the fuse, I tested it in its clip. But visually, it's perfect.
3. Pins 2 & 7 on the suspect socket did read the same as the other sockets, strangely enough...

Maybe my meter is crazy? Tonight, I will remove F2 and test it. I've been testing the amp on standby because I do not have a load to plug into. That shouldn't throw off the reading on the heaters, right??

I repeat, pull the dark tube and stick your meter probes down the socket holes. Is there 6VAC on those pins???

If there is not, then power off, pull the next tube also and measure continuity pin to pin for pins 2 and 7 between the sockets. I'd have to expect one or the other path was broken.

When you checked the heater fuse for that tube, did you actually pull the fuse from the clip and measure it with a meter? Or did you just look and visually inspect it? If you measured it while still in the clip, you were measuring the parallel current path through the tubes and PT winding - that will fool the meter. Pull teh fuse from the clip and test it.

If the tube doesn't light, either it is a bad tube, or there is no 6 volts getting to it. If all the other tubes light, then the 6v supply must be OK. That leaves the "wiring" and the socket.

[fdesalvo]

I removed the circuit board and there are no cold solder joints, no blown fuses, or open resistors anywhere. The heater pins are not borken in this socket.

After reassembley, I measured the VAC on pins 2/7 of each power tube and they all show the same reading. Why is V6 not lighting up, then??

Can anyone point me into another direction? If it's not the heaters, what could cause this socket to not light the tube?

[Enzo]

OK< so we tested the fuse OUT of its clip then?

When you measure the 6VAC at those pins are you measuring on the solder side or at the socket pins from the outside?

Look down into the socket pins - have either pin 2 or pin 7 spread? SO it no longer makes good contact with the tube pin?

With power off, did you do the test I described of pulling V6 AND the one next to it and measuring continuity between pin 2 on one socket to pin 2 on the other, then the same test for pins 7?

Here is an experiment: Put the red meter probe on one of the heater pins, and lay the black probe down not touching anything. WHat AC volts do you get? I actually had a repair once on something with a 6VAC light string. Theyu would not com eon, yet, ther was 6VAC coming from the transformer wires. Turned out it was fooling my meter. Just like touching the prober with your finger makes a couple volts reading, the discontinuous 6VAC winding was still making as reading in my meter. THAT is why we need to make continuity tests.

You want to get REAL hard core? Pull V6, and power off. Stick a free Molex pin or something down pin2 and another down pin 7 on teh socket. This will give you something sticking up to clip to. Now clip test wires to those pins. Leave the wires set on the tabletop. Measure the 6V betweeen them. Is it there? If so, we continue. Clip those wires to the pins 2 and 7 of the tube and let the tube lay on the tabletop. Only its heaters are connected now. Power up. Does the tube light? WHile it is running or trying to run, measure the voltage between pins 2 and 7. is it still there? Or did it collapse? Or if you prefer, you can connect those clips to a speaker instead of the tube. 6.3VAC across a 8 ohm speaker is about 5 watts - it will be loud but the speaker will be OK.

[fdesalvo]

Solved...

My DMM was giving me false readings. It was the filament fuse (F2), which was at fault, though it looked perfect. I've never seen a blown fuse that didn't look obviously burned. Looking at it very colsely revealed that the filament inside of it only appeared crooked. The other filaments within the heater fuses are perfectly stright from end-to-end.

I figured this out by hooking a 9v battery to a KT77s heater pins with my alligator clips and it ignited. With the suspect fuse in line, the heater would not light. Bingo.

I don't understand what's up with my meter- it wouldn't even read the voltage cross a 9v battery. Now I need to find a 6.3v fuse...

Thanks, Enzo, I've learned alot from you in the past and I appreciate your help.

[Rick Erickson]

[it is sorta backwards.]

Sorta??? It's completely bass-ackwards and rather stupid to boot. I'd say it's "totally British" but I wouldn't want to offend any of our fine friends on the other side of the Ocean... When I find one like this, and as I recall, they aren't all backwards: I "fix" it by reversing the jumper connector inside the amp.

[fdesalvo]

^Yeah, this one's not backwards, though I wouldn't say the same about some of the engineering that went into this thing...when I was removing the circuit board, the process felt the same as dismantling my pc .