Blues Deville distortion thoughts??

[Prairie Dawg]

I've got a friend's Blues Deville here, one of the American made ones from the middle nineties, and it's got a distortion that's kinda hard to describe so here's what I know.

The guy started noticing this and went and got a set of Winged Cs and preamp tubes and installed them with no joy. So he brought it over and I've just now been able to take some time and see what ails it. I checked the bias and at about 440v on the plates the tubes were biased about 32 ma. I've subbed power tubes and preamp tubes with no change. I just got done checking it with another speaker enclosure-well, actually, it's my Weber 5E3P with a pair of Jensen P12R reissues and they're good speakers.

That didn't produce any notably different results, so I do not think he's gone and blown the speakers on the Deville. I'm going to do a little reverse checking but that's what I'm thinking.

When you get the volume up about 40 per cent the tone's loose and flabby, with a kind of growly distortion that sounds like blown speakers. Driving the same speakers with my 5E3P they sound nice and tight.

I've been using the normal channel, by the way.

So I think the problem's internal, and seeing as it is a fifteen year old amp that's never been opened up I'm kinda suspicious of the electrolytic capacitors.

Anything else I ought to be looking at? Because I'll be opening it up for a closer look. Weaknesses? Dodgy parts? Other experiences?

Thanks, all.

[SatelliteAmps]

Cold solder joints. It's the second thing I check for in those amps, when looking inside.

[phsyconoodler]

Check where the tube sockets mount to the pc board.

[Prairie Dawg]

Cold solder joints. It's the second thing I check for in those amps, when looking inside.

What would be the first?

[TweeDLX]

I've got a friend's Blues Deville here, ...
Anything else I ought to be looking at? Because I'll be opening it up for a closer look. Weaknesses? Dodgy parts? Other experiences?

Thanks, all.
Check the plastic input jacks while you're in there. Big +1 on the solder joints.

Mike

[SatelliteAmps]

Exactly what these guys said, with the addition of all the electrolytic caps, large resistors (i.e. bias supply), diodes, and then everything else in the amp.

[jay42]

What would be the first?Burn marks. There's no better indicator than heat and fire.

[SatelliteAmps]

Tubes are always first.

[Prairie Dawg]

Check the plastic input jacks while you're in there. Big +1 on the solder joints.

Mike

Looks like I got my marching orders. Film at 11.

Do those jacks do anything special or can I just swap them out for some good Switchcrafts?

[SatelliteAmps]

If I remember correctly, they are switching jacks, PC mount style. Swapping for Switchcrafts might be a pain, because they might not fit, and they might not switch the same way.

[hdahs143]

Not sure about the Blues Deville, but all in the Hot Rod series had a habit of dropping one of the plate resistors on the phase inverter tube. This kills half of your signal and makes the amp sound like arse. Check to make sure you have operating voltages on pins 1 & 6 of all your preamp tube sockets.

Good Luck!

[Prairie Dawg]

Not sure about the Blues Deville, but all in the Hot Rod series had a habit of dropping one of the plate resistors on the phase inverter tube. This kills half of your signal and makes the amp sound like arse. Check to make sure you have operating voltages on pins 1 & 6 of all your preamp tube sockets.

Good Luck!

I got sidetracked this morning because the fan went out on my computer power supply last night and I had to play Dr. Scrounge. But I've got a lot of likely suspects plus a couple Ritchie F. threw at me.

I figure I'll get everything cleared away and clear my head of bad thoughts before I go fishing in Lake Deville.

Interesting electricity story. I've got an ancient variac-it's a Superior Adjust-a-Volt and I've got it rigged with a pair of alligator clips and banana plugs for voltage monitoring. Mostly used for testing transformer ratios. So it's got an on-off switch, it's definitely off, and the leads always dangle over the edge of my bench. So I'm diving down below the bench to unplug my soldering iron for the computer fan deal and my ear brushes against these two alligator clips-POW!

Be careful there, guys and gals.

[Prairie Dawg]

I've got 'er open on the bench and the middle capacitor looks suspiciously like it's been making stinky smells. Whether that will fix it is another question but it's definitely getting new power supply capacitors.

http://img.photobucket.com/albums/v435/rluedeman/DSCN2824.jpg

[phsyconoodler]

There is one more filter cap further to the right that is troublesome on HRD amps.
Can't hurt to change them all.

[Prairie Dawg]

Just as an update I have replaced all 6 of the power supply electrolytics and I have also replaced the bias supply electrolytics and first and second preamp tube cathode electrolytics. I also modified the amp for adjustable bias.

During this process I've been researching all known problem areas on this amp and trying to check everything. I did find something that is very interesting.

On the phase inverter tube there are three plate load resistors. One is 100k, the other two are in a split load arrangement with a 91k and a 9.1k resistor on one side and a 100k on the other side.

What's interesting about it is that the 91k resistor that's installed is actually a 200k resistor and appears as if it is open-it passes no voltage at all. These are R50, R51, and R52 on the schematic, and R51 is the culprit. It's off value and *appears* to be open. I'm going to pull one end and check it again.

Would this be a source of the nasty distortion that brought this amp to my door, Adam?

Below is the Dawg-a-tron adjustable bias conversion. I got the pot and knob out of my parts museum. The resistor that lived there was 27k and this is a 50k pot adjusted to 27k.




http://img.photobucket.com/albums/v435/rluedeman/BDVbiascontr.jpg

[jay42]

hmm...I wouldn't leave that knob attached...especially if the pot turns easily. If its action is very stiff, it's less of a concern. If the pot turns very easily, I would torque paint it with nail polish, once you've set it.

Replace R51 with the 91k value.

[Prairie Dawg]

hmm...I wouldn't leave that knob attached...especially if the pot turns easily. If its action is very stiff, it's less of a concern. If the pot turns very easily, I would torque paint it with nail polish, once you've set it.

Replace R51 with the 91k value.

Not a bad idea about the knob. Things that can be maladjusted will be maladjusted. Gotta go score some nail polish tomorrow.

As a point of information, if you have an old 35mm camera and the black parts are brassing or have flakes out of the finish, black nail polish is just the ticket to touch them up. I always knew Goths were good for something.

Now to find a 91k resistor....hmmmmmmmm

[TweeDLX]

Not a bad idea about the knob. Things that can be maladjusted will be maladjusted. Gotta go score some nail polish tomorrow.

As a point of information, if you have an old 35mm camera and the black parts are brassing or have flakes out of the finish, black nail polish is just the ticket to touch them up. I always knew Goths were good for something.

Now to find a 91k resistor....hmmmmmmmm
PDawg,
Sounds like you may have found the culprit, but...Here's a link about changing the stock jacks to Switchcraft if you ever need it: http://studentweb.eku.edu/justin_holton/inputjack.html .

Mike

[hdahs143]

If that resistor is open, you will not have operating voltage on pin 1 of the phase inverter tube. Instead of lifting an end, just do a voltage drop check across the resistor. If it drops all of the applied voltage, it is bad. This is the situation I described in an earlier post.

When you replace it, replace all 3 with 1 watt resistors.

Let us know how it works out.

[Prairie Dawg]

As a point of information my color vision is not as acute as it once was. What I was looking at was, I thought, a 200k resistor but it was, in fact a 100k and in the correct location. However it is completely open which would have whacked the PI.

What I did was a search of the literature and I've been systematically working my way through it and checking everything that's ever been identified as a problem area with these amps. Your post was what led me to start checking these resistances, for which I thank you. It's as close to a smoking gun as I've found yet.

I've got a few more things to check and replace if needed. I'm fixin' to toss all the 100k plate load resistors on the theory that they were all from the same batch.

http://img.photobucket.com/albums/v435/rluedeman/Open100k.jpg

Here's a good one.

http://img.photobucket.com/albums/v435/rluedeman/Good100k.jpg

[Prairie Dawg]

Just as an update I fired this amp up today and it sounds great-loud, clean, and tight. I don't want to give it back. Before, it was flabby and farty sounding.

The culprit was an open 100k plate load resistor, R52 on the schematic. It looked fine but on removal it had a crack on the underside. I've resoldered all the dodgy looking connections to the board (a lot of them) and replaced all the 100k plate load resistors on the theory that they came out of the same lot.

Thanks to everyone for all the good observations and pointers. That no doubt saved me a lot of troubleshooting time to go to the known problem areas.