70's Bassman tube problems

[Marblelee]

One of the 6L6s in this amp seems to be going out. I say seems because it could be something else. when you first turn it on the right power tube lights up as normal but the left one stays dim. when you play the guitar the left one brightens up bit. When played for more than an hour the left tube completely goes out (you can tell because the tone gets weak and dirty). You turn the amp off let it cool turn back on and the tube works again (still dim though).

The history is this: the amp has been coupled with instrument cable instead of speaker cable and played for sometime like that. And the amp is 4 ohms and the 4x12 cab is 8. My friend just changed the tubes but did not bias them (that I know of). I'm wondering if the tube is going bad or is the amp is experiencing bigger problems?

It has also had a blackface mod.

[MWJB]

"right power tube lights up as normal", let's just verify what "normal" is. You should see yellow points of light at the top & bottom of the tube, even in a perfectly working amp these may not appear to be even, from one tube to another, depending on the physical orientation of the tubes internals. There may also be varying amounts of a gaseous blue/purple glow that will react to your playing. The big grey or black metal plates inside the tube should not be glowing at all (red or orange incandescance).

The first step to finding out whether the amp has bigger problems or not, is to verify the bias, and voltages if necessary, of the tubes you have installed and try some known good tubes at an appropriate bias current (30-35mA).

Hooking up the cab with instrument cable was not a good idea, but if the cable was changed before you noticed any degredation in tone you may have got away with it. If the centre conductor had blown open the amp would have shutdown pretty quickly, most likely blowing the power tubes and potentially damaging the OT. If the dialetric failed, shorting tip to ground, the amp would have stopped making a sound but if you caught it in good time you may have escaped any serious damage.

[Marblelee]

thanks for the reply. the left power tube is very dim and it eventually goes completely out.

There was some degradation of sound that is when I found out that there was an instrument cable connecting the amp to the cab.

Also can you tell me how bad it is to plug a 4 ohm amp into an 8 ohm cab?

And can anyone direct to any info source on biasing silverface fenders?

and is there a chance that the tube is just bad.

I'll look into to all the stuff you told me to. I have seen what tubes should look like though. and the left one is barely on. is it possible that the phase inverter tube is going bad?

[Tom Phillips]

If one power tube really is going “completely out” and you can’t see any glow inside the tube in a darkened room then it could be as simple as a loose tube socket contact or bad solder joint in the heater supply. You can try swapping the two power tubes and determine if the problem follows the power tube or stays with the left one.
A fix could be re-tensioning the tube socket contacts, inspect the heater wiring and repair as necessary etc. Anyway, these are the first things I would check.
Regards,
Tom

[stokes]

Using an 8ohm cab with the 4ohm output wont harm anything,you may or may not notice a slight loss in power,but it wont be a drastic loss.Using a guitar cable for a speaker is not good.The problem with this is that the small "hot" lead in the guitar cable cant take the current of the output and can burn out causing you to have no load on the output.If the wire didnt burn and cause the output to open up you likely didnt do any damage from this.If the glow you are describing is indeed the heater and not the plate,I would check as Tom described.If it is the plate glowing there could be a bigger problem.Again I would check as Tom said by switching the tubes and seeing if the problem follows the tube or stays at that socket.

[Mars Amp Repair]

Also if the intermittent (assuming it is the heater) moves with the one tube, it could be the connections from the wires in the tube to its octal plug on the tube itself.
Acutally, you just turn the tube upside down & inspect the tips of the tube pins for bad solder joints. I have found some brand new tubes with this issue.

If that is the case you simply resolder the ends, then clean up any excess solder on the sides of the pins with solder wick.

glen

[Amp Kat]

When the pins inside the socket open up it looses contact even though other pins in the socket are closed enough to hold the tube in tight. Sometimes you can push on the tube from the bottom and see it light up then when you release it, the heaters go out again. You can bend the pins back in from each side from the top of the socket to try to make a smaller circle. Bending them in as a triangle can be bad as they can crimp inside and when you put the tube back in it crushes in and causes it to get uneven. Changing the socket is the way to go if that is indeed the problem.

[Marblelee]

when I opened it up two of the pins on the heating element were touching. So I moved them apart from each other. I then put the amp back together and replaced the tubes; the one that had been in that position originally was still dim but then I switched them the other worked fine in the left socket. The only problem is now is that it damaged the originally placed left tube. Got to go buy a new pair. Thanks for your help. I hope this will be the end of this problem

[Tom Phillips]

I never heard of a tube heater going dim because of damage. They either lite up or they don't. However, there are differences in how bright the heaters look in various tubes due to the physical contruction.

[Marblelee]

I can see anything else that could be the problem. But 2 of the pins on the socket were touching and the tubes did stop lighting up at all when I switched them.

[Tom Phillips]

I just meant that maybe the tube itself is OK now that you fixed the touching pin problem.