Bues Jr. in for repair - flabby low end

[JazzGuitarGimp]

Hi All,

I have a Blues Junior on my bench for repair. Had a blown fuse so I pulled the EL84's and tested them on my tester. No shorts or gas, but on of them was well into the REPLACE section of the meter. Also, I noticed signs of overheating on the plates of both tubes. I put the 84's back in and slowly brought the amp up on the variac and before I got up to 120V, both 84's were red plating. This amp has non-variable fixed bias, so I assumed I'd be adjusting one of the resistors on the bias divider network. I ordered a new matched pair of EH EL84's from CE Distribution. Before I put the new tubes in, I measured the DCR of both halves of the OT Primary winding - one came in at 94.9R and the other at 94.4R. Also, I did a bit of research on the net and found that Fender biased this amp so far into Class A that many of these amps have these 12W tubes idling at 13W. So I clipped in a 100K pot across the 33K (R37) resistor so I could cool the tubes down a bit - then I would just solder a fixed resistor of the value the pot winds up at across R37. Seems like a good plan… However, the first thing I noticed is that the two tubes do not bias the same - matched pair??? I figured if I had about 2.2VDC across the OT primary DCR of 94.9, that would get me close to 8W at idle. Well, I've got one tube at 2.5V and the other at 2.2V. I swapped the tubes to see if the issue is with the tubes or the amp, and it's the tubes: the one that biases at 2.5V is always 2.5V, regardless of which socket it's in. The resistor in || with R37 wound up being 43K, and I've got 345VDC at the CT of the OT.

Okay, now my question: With the MV all the way up and the guitar volume all the way up, I am getting flabby low end (open low E string) after the gain control gets the amp into saturation. Here's the weird part, if I take the gain all the way up, the flab goes away. Is this the way this amp is supposed to respond / sound?

Many thanks,
Lou

[xtian]

Okay, now my question

No, not normal, obviously. Did you eliminate the cab/speaker from the variables by playing thru an external, trusted cab?

Next, I'd scope the output waveform and see if it matches what you're hearing as you dial the gain up.

[billyz]

Change c6 to either .oo1 or I have used 500pf. Tightens the low end no flab.

[groovtubin]

u have a bad oscillation imho.........i`ve seen it cause massive saturation B4 but nuthin else! Bad filter caps, grounds, check lead dress too

jim@Omegaamps.com

[JazzGuitarGimp]

Hi Guys,

Thanks for the pointers. I'll be back on this job tomorrow, and I will report back with my findings.

Thanks again,
Lou

[martin manning]

Some Blues Jr.'s are known to have oscillation problems, even more so with the back panel removed: http://billmaudio.com/wp/?page_id=115

[JazzGuitarGimp]

Some Blues Jr.'s are known to have oscillation problems, even more so with the back panel removed: http://billmaudio.com/wp/?page_id=115

Yes, In my internet search for more information, I came across a discussion that mentioned the yellow-board version having oscillation issues with the back removed, but the green-board version (which is what I've got here) seems not to be affected. Nevertheless, I will scope the speaker output for oscillations.

[JazzGuitarGimp]

Some Blues Jr.'s are known to have oscillation problems, even more so with the back panel removed: http://billmaudio.com/wp/?page_id=115

I forgot and used the browser's back button...

[JazzGuitarGimp]

Okay, I finally got back to this repair yesterday. I started by unplugging the internal speaker and connecting the amp to an external EVM12L half-open back extension cab: same flab in the low end. I then connected my scope across the speaker to look for signs of oscillation, and I looked with the volume controls set to numerous positions, both with and without an input signal: no signs of oscillation. So I called the owner, and told him what was going on, and asked him to come over and have a listen - I figured if he is happy with the way the amp sounds, then there is no good reason to continue chasing ghosts, and incurring more repair $. He said the amp sounds like it always has, and was happy with the repair. I really wasn't looking forward to pulling the board just to downsize a coupling cap or two. So, all's well that ends well. Thanks everyone, for your suggestions and support.