Amp tech wizzzzards--HELP ME SAVE A BASSMAN

Pens · Post by **Pens** » Tue Mar 06, 2007 12:14 am

Ok, so a friend of mine has a Bassman 135 head. It is now in my possession, I am trying to figure out how fuckered it is, and I need some help.

HISTORY:

He had it serviced, but only had one set of 6L6 tubes and a buddy borrowed them for his amp for a gig. When my friend went to put them back, one tube was put in wrong. The amp went boom, basically. So now I have it, trying to find out what is still good in it.

I can see burnt resistors on the tube sockets on several of the 6L6s.

What can I do to test if the transformers are ok? I think I'm a bit over my head on this.

I do not have a quad set of 6L6's to test, so any tests can't involve them. I just need to find a test I can do on the transformer itself to see if it is fried or not. I might buy the amp from him depending on what is still good. Most parts look fine, it is just the power section that looks crispy.

Anyone have any suggestions? I can provide any info you need, just ask.

DaveB · Post by **DaveB** » Tue Mar 06, 2007 4:44 am

How the hell do you put an octal tube in wrong? That's why they have a locator pin. On a fixed bias amp, I have no idea what resistors he burnt. Probably the the 470 ohm screen ones. If that carried on long enough, he might have taken the primary out of the output trans. check red to brown, then red to blue then blue to brown. Say the first two are 100 ohms, I think the third reading should read about 200. I'm not sure what ultralinear connection to the screens ( yellow striped) should read. I'm no electronics guy, but that shouild indicate an open or a short in the primary.

SCHEMATIC

filtercap · Post by **filtercap** » Tue Mar 06, 2007 5:29 am

If you just have two 6L6's, that's enough to test for sound with, should DaveB's transformer tests check out OK.

Replace the toasted screen resistors first, of course. Use resistors rated for 2 Watts to make further toastment less likely.

You might also check the speaker circuit to make sure there's some resistance there. If for some reason the speakers had gotten disconnected or shorted, that might have been cause for foom. With the speakers connected to the speaker jack (not the extension jack), test between the black and green/yellow leads on the output transformer. I'd expect a reading of slightly less than 4 ohms.

You can run the amp with just two 6L6 tubes, if you plug them into either the outer two power-tube sockets or the inner two. I have a 135W Twin and run it this way often.

Pens · Post by **Pens** » Tue Mar 06, 2007 5:32 am

Thanks for the response. I don't THINK the 1W resistors going to the output trans was fried, I recall them not looking burnt but I'm going to check them anyway. I'll check those connections you suggested.

Yeah, the tube went in wrong because a) they are old and loose and b)he sort of pushed a bit hard on it.

I'll check a few things out tomorrow and report what else I find. I didn't get much time to look at it today.

LimpDickCheney · Post by **LimpDickCheney** » Tue Mar 06, 2007 10:55 pm

Does anything work? Does the pilot light come on when you power it up? Any sound from the two other 6L6s? What resistors are fried? Try to highlight them on this layout... I realize it is the 2 6L6, so tell me on the ones shown what is burned.
layout

Pens · Post by **Pens** » Fri Mar 09, 2007 8:49 pm

Thanks for all of the suggestions. A couple of things:

One, this version has 4 6L6 power tubes. I do not have ANY of the power tubes, they were all fried and my friend used his spares in another amp that he is actually using now. He sold the cab, so I do not have a cab to hook this up to. Can I plug it in and fire it up w/o any tubes or speakers hooked up? To test the pilot light? I've never read this type of case before to know if this is safe to do.

Second, I measured the windings on the power transformer, and all windings SEEM ok on that, measuring between 2 and 8 ohms on each winding. I still have to measure on the output transformer.

LDC, on that diagram the resistors that are crispy is the 1k5 resistors across pins 1 and 5. Out of the 4 sockets 3 of them have that resistor burnt. The other resistor LOOKS ok, I have not measured them all yet due to lack of time. What other tests can I do?

filtercap · Post by **filtercap** » Fri Mar 09, 2007 10:51 pm

Looking at the schematic (DaveB's is the one) -- if your amp is on standby, I'd =cautiously= say you can switch it on with no power-amp tubes and no speakers. With even more caution, I think that with no power-amp tubes installed and thus no current on either side of the output transformer, you could have the amp off standby too. (Anybody?)

The pilot light will only really indicate that your tube heater circuit is working though, not the other segments of the power transformer.

Going by that 135W schematic again, if a 1.5K resistor on power tube pin 5 is fried to the point where it's no longer letting current through at all, that'll block the bias voltage from getting to the tube. Any tube you insert will likely run =very= hot and burn out in a short time.

LimpDickCheney · Post by **LimpDickCheney** » Sat Mar 10, 2007 12:24 am

You should be fine powering up without the power tubes. If the pilot light comes on you know that your power transformer primary and heater secondary windings are functioning. Check the fuse before you power up, a blown fuse will keep the amp from powering up. Looks like 520vdc plate voltage went to those bias resistors... toasty.

DaveB · Post by **DaveB** » Sat Mar 10, 2007 1:17 am

That's 520V loaded down with tubes. I don't know what kind of regulation the power trans has. With todays line voltage, don't be suprised to find it going over 600V to test. Make sure your meter is up to it. Alot of them only go to 600VDC. Just remember 600VDC @200ma can be pretty lethal. One hand (other in pocket), only use high voltage clamp on probes. Check the output trans first.

LimpDickCheney · Post by **LimpDickCheney** » Sat Mar 10, 2007 1:24 am

DaveB wrote:That's 520V loaded down with tubes. I don't know what kind of regulation the power trans has. With todays line voltage, don't be suprised to find it going over 600V to test. Make sure your meter is up to it. Alot of them only go to 600VDC. Just remember 600VDC @200ma can be pretty lethal. One hand (other in pocket), only use high voltage clamp on probes. Check the output trans first.

The plate voltage to bias resistor thing was my theory... I am not suggesting he put the power tube in wrong and test it. I guess I am suggesting he keep both hands in his pocket. You might start to think of a repair parts list. Ampwares has resistors and transformers (you need the prior, the latter remains to be seen). I get my tubes from thetubestore.com. Tube sockets might be needed... Ampwares. You can get tuner cleaner from Radioshack.

DaveB · Post by **DaveB** » Sat Mar 10, 2007 1:30 am

That was just a warning to Penpen. You can test for a while with no tubes. And Penpen, I think you already know. Don't touch the power supply caps

Pens · Post by **Pens** » Sat Mar 10, 2007 4:16 am

Ok, thanks guys. Yes, I understand the danger involved, I'll be keeping back from it when powering up.

I have tube sockets already in my parts bin, and those resistors too I think...they might be only 1/4 watt though so I may have to run up to Radio shack for them. I had already planned on replacing those parts, so that part is good....I'm surprised if there isn't more to do. If I can fix this thing with just the sockets and resistors I'll be incredibly happy. I sort of want to buy this thing from him, a point to point wired Bassman with 135 watts power and easily modified...I'll have to pay him more for it if the transformers aren't shot though.

Cool, I have my marching orders then. Thanks again guys.