Teisco 10 New problem!

[docz]

Hello, after enjoying my amp since I fixed it with a lot - and I mean a lot - of great help from you guys something happened. I was practising, then suddenly the volume dropped down real low. Upon further investigating I found that turning the volume pot has no effect - other than on 0 then it cuts off. Other than that the volume was fixed. The reverb effect is actually louder than the input signal! Tremolo works, reverb works, tone works. If tone is set to mid-high and reverb is on near full a nasty feedback starts to build up.

Inside I measure 295V plate voltage on the power tube, and 10V cathode voltage. The 200Ohm resistor I installed in my restoration project will draw about 0.67mA (I'm not sure if we are talking mA or A here excuse me and please correct me if I am mistaken )through the tube correct? Isn't that a tad hot for a EL84? Reading the datasheet I can see that it ideally wants 250V and 0.48mA in SE mode, should I fix this?
(Wov... I did not think 6 months ago that I would ever be capable of saying that..)

Anyway back to the problem...

I tried substituting the preamp tubes, no effect. Investigating inside I found no broken solder joinst, weird voltages, broken components or wires. There is continuity between all points in the circuit.
Removing the effect tube removed all sound. The same happened when removing the preamp tube.

I'm guessing that the first gain stage of the preamp circuit has cut out somehow, and all we are hearing is the final gain stage and the effect gain (What was that called again buffer?) But then again my experience with circuitry is isolated to this amp and a couple of stomp boxes I've built so this could probably be anything.

Any ideas folks?
All thoughts greatly appreciated.

Schematic for the amp can be found in my restoration thread here:


https://tubeamparchive.com/viewtopic.ph ... highlight=

Cheers!

DocZ

[Structo]

Did you try different tubes?

First thing to try on a tube amp since the tubes wear out.

[docz]

I didn't try different tubes, I guess I'll order a new pair. I thought these tubes would last a long time?

But isn't it weird that both tubes would wear out at the same time?

DocZ

[Structo]

Yes I realize that the 37189 tube is not a common one to have around.


I wonder if a socket is acting up.

When you pull a tube out it should be a pretty snug fit requiring you to wiggle it in a circle while pulling it out.
And when you insert a tube it should feel like a lot of resistance.
If not then you may have a loose socket that either needs to be re-tensioned or replaced.

I didn't go back and read the multi-page thread about your amp so I can't remember what all you replaced.

It could be one of the other components that either died or a bad solder joint.
Could be a bad pot as well.

Did you bang your hand on top of the amp when it was acting up?
I had a bad tube that I didn't think was bad because I could tap on the cabinet and have it make music again. Finally I replaced the tube and that was the problem.
Tube amps can be temperamental, especially old ones.

Have you tried moving things around with a chop stick while strumming the strings? Tap on the various components and move wires a little.
Sometimes you get lucky and find the bad part.
It is common for a cap to be microphonic when you tap on those so don't get alarmed if you hear that.

[docz]

Yes I tried to chopstick while playing, didn't give me any clues.

It doesn't use a 37189, it uses an EL84. Probably been replaced in the past.

To remove the tubes I have to wiggle them like you say, and when inserting them the rectifier and power tube offer the most resistance. The preamp/effect tube don't. I have some sockets, maybe it is time to change them?

DocZ

[Structo]

I would try to re-tension them first.

Discharge the caps first then take a small tool like a jewelers screw driver or dental pick and make the pin sockets slightly smaller by squeezing one side of it.
They are shaped sort of like a C so you just make the inside diameter smaller.

Did you check out the pot and on/ off switch to make sure they are functioning?

[docz]

Sorry to bump my old thread, but I thought I'd post it here since I allready started here.

I haven't had time to work on my amp for a while, I have been busy on the road for quite some time now.

Anyhow...

I went over the amp again and checked all pots, switches, I even swapped all the tubes out for a set of fresh ones.

Then I tried to fire her up, and now there is no light, no sound and no voltage to be meassured on the secondary windings. I checked the fuse, it is ok. I guess the transformer is cooked.

So now I'm on a mission to find a replacement transformer, that woun't cost me an arm and leg. I haven't found anyone here in my country that fits the specs. It is 260-0-260V@60mA. I found this one:

http://www.hammondmfg.com/pdf/5c0028.pdf

I'm not sure if this is any good, but that is the closest I can find.

Unless any of you guys have any tips, maybe I can revive my old transformer?

Cheers!

DocZ

[Phil_S]

Hello Doc. Sorry to hear of your problem. I think that Hammond should work nicely. If this were mine, I'd want to know what caused the problem. It could be just age. I wouldn't want to install a new one with the same result.

[Cliff Schecht]

Remove the PT and Ohm it out to make sure that the primary (or whatever) is actually blown. If it reads open or a dead short then it's bad for sure. I have a transformer that is close but just to small (250-0-250@40mA) or I'd offer it to you.

[Hellhammer]

Did you check the fuse with a meter? They may look ok but still be blown at times.

[docz]

Sorry to revive my old thread, but I finally found out what was wrong with my amp. I let it sit under my desk for a while, I had some health issues again and Christmas took most of my time so I haven't had time to work on it.

I did re-measured the fuse and tested it to make sure it was not gone.

I took out the transformer and measured it once more, and it was dead, I got infinite resistance on the primary winding, so my guess was that is was broken.

So earlier this month I ordered a new transformer and took the amp out again from under my desk.

While waiting for the new transformer, I noticed by accident that one of the wires going from the coil to the solder lug looked like it was coming loose. When I picked up the transformer to inspect it broke loose. Seems like corrosion has weakened the wire and it had broken free. So I tagged a temporary wire to it, and tagged that to the solder lug. Now when I measured with my DMM it showed resistance again!. So I clipped some mains power to the primary, and connected the pilot light to the other winding. And there it lit up like a Christmas tree! I measured the secondary and all voltages are back to normal.

Now my question is this: The wire that broke free is a tiny brown thingy that was just long enough to fit to the lug, it has broken off at the bend where it was wrapped around the lug - is there a safe way I can fix this? Or should I just use the new transformer and toss this one? (Even though it seems to be working..) Ideas are welcome!

DocZ

[docz]

Here are some pictures.

I was thinking I could try to strip that old insulation off, and solder a bus-wire to the stub, and use some shrink-tubing or electrical tape - but I am afraid that might be unsafe, any suggestions?

Regards,

DocZ

[Firestorm]

It doesn't look like you have enough lead length to do a "proper" splice. What I might try to do is to carefully strip the remaining insulation off the broken lead and clean the metal to get it good and shiny. Get a bare wire of the same gauge and put it alongside the broken one, overlapping it. Wrap the two leads with a spiral of buss wire to keep them physically joined and solder everything together. Heat shrink the assembly and then strip the insulation off a larger gauge wire and slip that bit of insulation over the heat shrink to double insulate everything. Hopefully, there's nothing else amiss with the transformer; but then you have another on the way.

[docz]

Thank you for your reply, that was something along the lines of what I was thinking, I'll do that.

I think this was all that was wrong with it, since when I clipped the broken lead to the lug, and hooked it up to the wall everything seemed fine.

I think that this may have been all that was wrong with the amp from the get go... oh well... you live you learn

If this works, I think I'll try to build a new amp from scratch with the new one - been wanting to that for a while!

DocZ