Blues Jr. 1 Hot tube

[Magnut]

Hey folks,

I bought a used Blues Junior a few months ago. Latley the sound had started to cut out, get fuzzy, low volume.. So I put new tubes in thinking that it needed a little tlc.

It was fine for a few days. Now one of the power tubes glows much much brighter than it should and all of the problems are worse. I put the old tubes back in and still it's worse.

I took the back off and metered the caps that I could get to without much trouble and they were fine. The only mod I have done to it is the twin stack mod, which I don't think should affect it in any way.

Any ideas what may be causing this? Anything else I can check? I am prepared to remove the board and have a look at the back to see if there are any cold solder joints.

I am currently thinking that the bias circuit may have gone bad. ?? Thoughts?

Thanks in advance,
K C

[stokes]

If one tube is redplating,it is drawing too much current,check pin 2 for negative voltage on each power tube and see if you havent lost bias voltage to the tube that is redplating.You say "one tube is glowing much brighter than it should".If you are saying they are both glowing red,just one is brighter than the other,it could be that the amp is biased too hot for your tubes.Neither tube should have the plate glowing,just the filament.

[hasserl]

Also, try swapping the power tubes around, see if the problem follows the tube or stays with the socket.

[Magnut]

Today..

I put the tubes back in. They are glowing as they should be. Yesterday on one of them the metal piece inside got really hot. Today only the filament glows on both.

I put them in, swapped them around, tested, and the sound is all broken up. Sort of like a torn speaker, only sometimes the volume drops out. I did notice that as this whole problem started that it only broke up on a G cord or other G notes. I don't know if that means anything other than it was affected by that frequency.

I will have to look into checking pin 2. What should the voltage be? BTW the tube that glowed so red was the power tube on the end. I don't know if this is useful info.

[stokes]

If you are not experienced working on amps,it would be advisable to take it to someone who is.There are potentialy lethal voltages in an amp-even when it is unplugged and "off".The idea of swapping the tubes, as hasserl pointed out,is to see if the problem is with the tube itself or the amp.If the tube that red plates does it in either socket it is likely a problem with the tube.If the tube in one particular socket red plates,it is likely part of the circuit in that socket.The bias voltage at pin 2 should be in the area of -10vdc or so.What you are looking for here is to be sure both power tubes are getting negative voltage.

[Magnut]

Thanks Stokes for the safety reminder, I appreciate that. As an electrician I know how bad it can get. I am new to working on amps, but I am aware of the dangers even when turned off.

I believe the problem lies with the bias circuit. The socket that red plated the tube also makes the element in that tube glow a small amount brighter. or more so the other doesn't seem to glow as well as it should. The problem did not follow the tube.

Now I just need to figure out where to start trouble shooting it. I have the schematic. I will test the pin#2 and go from there. That's a good place to start. I am guessing that I can take these measurements with the power tubes out. Is that correct?

One of the reason I got this amp is that I am interested in amp circuitry and saw this as an opprotunity to do some modding and learning. I guess you need to be careful what you wish for.

EDIT* Now that I know some of the proper terms for this issue I have been able to do a better search. I am coming up with some things to try and understanding it better.

[stokes]

The "element" glow is unrelated to the red plating.By "element" I assume you are referring to the heater or filament.Yes the bias voltage can be checked with the tubes out,but it isnt a bad idea to check with the tubes installed as well.With no tubes it may appear to be okay,but under load the situation could change.If you find a problem with the bias voltage in that socket,just trace it backwards till you find where it is going off.

[tubeswell]

I am guessing that I can take these measurements with the power tubes out. Is that correct?

When you take the measurement with the tubes out, you will get an approximate plate (and screen) voltage measurement, that would change when the tube was inserted. Each tube forms a load, and if you take the tube out you take the load off, so the voltage at the plate pin (and also at the screen pin in the case of tetrode/pentode output tubes) will rise. Of course, there won't be any voltage on cathode pins with no tube in there, so taking the tube out won't help you much other than to determine whether the plate/screen voltage is approximately right. However, if you detect any DC voltage on the grid pin at idle, that probably indicates a problem with the prior coupling cap.

[Magnut]

Well I've finally got a bit of time to check this out. As I was testing the one tube started redplating.

I did not see any voltage on pin 2.

I also need to be sure I am checking the correct pin. Looking at the socket from the tube side with the flat at the bottom pin 2 is clockwise or counter(anti) clockwise? Dumb question I'm sure but I like to be certain.

Some pins appear to have upwards of 780v to ground. !!

[Magnut]

Using the schematic I have been going through some of the test points.

I have been getting some readings that are off by quite a bit. I want to be sure that some of the voltages listed are not typos. For example test point #2 I should be reading either 312 or 512 (hard to read) mvac. I am actually reading a full 300vac. So that makes me wonder if it should not be listed as a mili volt reading. There are a few instances like this. Either that or I am actually looking at the problem. ?

The schematic is a bit hard to read and I have good eyesight. Are there any cleaner versions out there? I am working with Rev C. I will do a search in the meantime. (I found one here but is not the Rev C I have)

I would like to keep going through the test points but I have heard of some typos in the past.

Thanks!

[Magnut]

Ok on pin 2 for both power tubes I am getting a negative voltage. I get -10.2vdc on the tube that does not redplate, and -12.6vdc on the one that does redplate.

Uggh! Ok so now I can start tracing the issue. It's a learning curve for me but I'm having fun.

I will keep digging. Any pointers appreciated.

[Magnut]

After much testing I've decided to replace the tube sockets. The problem is very intermitent. While the amp is on I can touch the tubes a little and make the problem come and go so thats what has made me decide to change the sockets. It will be good to get rid of those cheap ones anyway.

I will be going with a gold plated ceramic socket. At 2.75 each thats not bad. I will also go ahead and stiffen up the power supply cap with a 100uf cap and swap out the 22uf with some better ones of the same value. I've heard they can give some issues so as long as I'm in the neighborhood I'll swap em' out.

I'm going to put a trimpot in the bias circuit as well. Once again as long as I have the board out I might as well. The parts aren't expensive and it will give me peace of mind. I just hope the issues go away.

[Enzo]

You sure the sockets don;t just need to be resoldered? Try that, even if you plpan to change them. REsolder the power tube pins, see if it makes operation consistent. Even if you then replace them, at least you will know that the problem is not elsewhere in the amp.

[Magnut]

I had thought of doing that. The sockets seemed pretty flimsy. I wish they were chassis mount.

I tried checking the pin/socket tightness. There wasn't much room for working on it. So I decided as long as I had the iron hot I would pull those sockets out. I have to admit that I enjoy doing this kind of work and this gives me an excuse to noodle around a bit.

Perhaps before I receive the sockets I'll reach in there and resolder them. That would give me a little more time for trouble shooting if needed.

[Magnut]

This evening I put my fingers behind the sockets on the bottom of the board, wiggled the sockets a little and I could feel several pins moving on my finger. I had considered poor solder connections before and took a close look but never saw anything.

It was obvious now that there were some hairline cracks in the solder and that there was a poor connection and it also explains the intermittent condition. I should have taken this further the first time looking into it. doh*

Thanks Enzo for promting me to look again. I am playing again and quite happy. I will do the mods anyway for peace of mind but at least I know there aren't any other issues.

I learned quite a bit looking for this problem. I am relieved that it was a simple fix.

Thanks all!