# Diagnosing Bad Tube in Guitar Amp

Thread Starter

#### Jojo_B

Joined Nov 17, 2009
17
I have owned a Fender Hot Rod Deluxe 40W tube amp for about 9 years now, and I believe some of the tubes are on their way out. The problem is the sound is distorted and not being amplified at all, regardless of what position the volume knob is in (I can still hear my guitar output, but faintly).

This amp contains 8 tubes all together, 6 low-level (12AX7) and 2 high power (6L6). According to a schematic I found, all the tubes have test points with expected values for certain conditions (Note 4 in the schematic). So instead of my original thought to buy all new tubes, is measuring these test points a good indication of which tubes may have gone out? Do the high power tubes typically fail first? Seems like I need to be careful with measuring the output of the high power tubes. Anyone have any other suggestions?

The link for a schematic is here:
http://www.blueguitar.org/new/schem/fender/hotrod_deluxe.pdf
(I didn't want to post the schematic here, I am not sure what the forum rules are for posting possible copyrighted schematics)

Thanks.

Last edited:

#### S_lannan

Joined Jun 20, 2007
246
try using the pre amp out and power amp in sockets to isolate where the bad tubes are.

You should be able to plug a guitar directly into the power amp in and get a fairly loud clean sound. Ideally you can patch in the pre amp of another amp into this.

Try patching the pre-amp out of this amp into another guitar amps power amp input, see if the pre-amp functions properly etc.

These tests don't expose you to any safety-hazards.

Nine years is a fair life for a set of tubes, I take it you're not playing gigs every week.

#### THE_RB

Joined Feb 11, 2008
5,438
I would replace the 2 output tubes straight off! Then try swapping the 12AX7 tubes around and see if it makes a difference, if so then one or more 12AX7 are on their way out, they are not that expensive anyway it may be best just to replace them too.

After 9 years (especially LOW use) there are probably caps that need replacing too.

#### Duane P Wetick

Joined Apr 23, 2009
426
I used to build guitar amps many moons ago, and the push-pull 6L6 design was one of my favorites. If you are getting no or minimal output, by all means, replace both 6L6's as they do the heavy lifting (amplification).

Cheers, DPW [ Everything has limitations...and I hate limitations.]

#### flat5

Joined Nov 13, 2008
403
If/when you replace the 6L6s, replace them as a pair. You want the tubes to be "matched".
Inspect them for two minutes with power applied but don't play. Look for the plates becoming orange. Not the filament - the device that heats up the cathode. If the plate starts to glow orange, turn off the amp. Replace the grid coupling caps to the 6L6s.
Google will help you understand these terms.
Good luck!

#### beenthere

Joined Apr 20, 2004
15,819
Blue glows in tubes (gassy), and noise caused by tapping the tube, meaning loose elements causing them to be microphonic are things to investigate.

Thread Starter

#### Jojo_B

Joined Nov 17, 2009
17
Yes, I am definatly a "low" use player, I actually bought the amp used 9 years ago, so I'm not too sure how old these tubes really are. I went ahead and ordered a matching set of 6L6GC Groove Tubes, and I'll let you know how it turns out.

#### flat5

Joined Nov 13, 2008
403
Looking at the schematic I see there is no protection for the output transformer if you try to play with no speaker connected. Don't ever do this.
It could very well cause the problem you are having by destroying the output transformer.
You can add some protection by soldering a 47 ohm 2 watt resistor at the 8 ohm speaker socket. Connect it to ground and to where the green transformer wire goes. This way the transformer always has some load but not so much that you could ever hear the difference if it was removed.

#### Mike33

Joined Feb 4, 2005
349
+1 The_RB. I have a HRDx, it's been pretty dependable (my main gigging platform, not the best amp in the world but a workhorse). I would suspect power tubes. The 12AX7's last much, much longer, altho a decade....there are 3 12AX7's in the preamp, giving 6 triode sections. Go ahead and change those 6L6's with a matched pair. If the problem persists, it is likely related to either the OT as mentioned, or power tube resistors (plate resistors, I believe, were an issue with some of those amps).

Let us know how this turns out; I am curious since i own the same amp! If you're electronically-handy, you could repair resistors yourself should that turn out to be the issue...you would see the burn marks on the PCB if you take off the many screws in the back cover. Be aware that there are voltages in there, even turned off, that could theoretically kill you, though!!!! Don't put fingers in there...caps would have to be properly drained for that.

#### hgmjr

Joined Jan 28, 2005
9,029
Take a close look at the physical conditions of the capacitors. Depending on how much abuse the amp sustained before you got it, the caps could have aged to the point of becoming leaky.

hgmjr

#### bertus

Joined Apr 5, 2008
20,330
Hello,

Also take a look at the solder joints around the tubes.
Those could go bad due to the heat of the tubes.

Greetings,
Bertus

#### THE_RB

Joined Feb 11, 2008
5,438
Fenders are notorious for the caps. If they hadn't been done in 9 years I would probably just spend the extra $15 or so and replace the electros. #### bloguetronica Joined Apr 27, 2007 1,372 After 9 years (especially LOW use) there are probably caps that need replacing too. I hate to contradict other members, but "if it ain't broken, don't fix it". Recapping is only needed for equipment that wasn't used for years in a row. Equipment that is used from time to time will probably won't need recapping. If you electrolytic capacitors don't show any signs of age (bulging, venting or any sort of brown leakage), and if they are from a reputable brand, they are probably fine. If fact, they can be more healthier than any cap inside an equipment that is used on a regular basis. Old caps are better than newer ones in terms of quality, since they are not affected by "capacitor plague" and they do not follow RoHS or any other "greenish" regulation. Oh, and if you need to do some soldering, stick to the old tin-lead rosin core solder. Don't use any of the lead-free stuff, since you might end with a damaged joint, PCB or components over years of use due to phenomena such as cratering or copper dissolution, not to mention tin whiskers and tin plague (the last starts to occur below 13ºC, on an exponential rate, and will convert your joints to powder). Last edited by a moderator: #### hgmjr Joined Jan 28, 2005 9,029 Fenders are notorious for the caps. If they hadn't been done in 9 years I would probably just spend the extra$15 or so and replace the electros.
I agree with THE_RB. A careful examination of the capacitors for signs of bulging would be a good use of your time. Depending on the storage conditions of the unit when not in use, the caps may have begun to dry out. If they have never been replaced they probably need replacing anyway. Replacing capacitors in the amp is an inexpensive an s simple undertaking, allbeit dangerous if you don't make sure the caps are discharged before you go poking around inside the unit. The amp will be ready for another 5 to 10 years of service. It may also prevent you from having to buy a replacement tube you did not need in the first place.

hgmjr

#### flat5

Joined Nov 13, 2008
403
When I repaired Fender amps in the 1970s I found the grid coupling caps to the output tubes OFTEN had too much leakage. Maybe the caps are much better these days but I would be weary.

Last edited:
Thread Starter

#### Jojo_B

Joined Nov 17, 2009
17
I agree with THE_RB. A careful examination of the capacitors for signs of bulging would be a good use of your time. Depending on the storage conditions of the unit when not in use, the caps may have begun to dry out. If they have never been replaced they probably need replacing anyway.
hgmjr
Are these the grid coupling capacitors (highlighted in red in the attached picture, referenced by http://www.blueguitar.org/new/schem/...rod_deluxe.pdf)? If so, I took a look at them...I have seen no swelling (they're not electrolytic, so I don't really know what this is going to look like) or burning in or around the caps (I tried to take a picture, but my camera is not cooperating with fine detail)...It may be more trouble than it's worth to add new caps because there are a bunch of bundled connectors (so I can remove the board) that are set and soldered, that I'd have to remove and re-solder when done.

Last edited:

#### bloguetronica

Joined Apr 27, 2007
1,372
If so, I took a look at them...I have seen no swelling (these are ceramic, so I don't really know what this is going to look like) or burning in or around the caps (I tried to take a picture, but my camera is not cooperating with fine detail)...
Ceramic capacitors won't bulge or swell or leak like the electrolytic caps, nor they will dry out. Those caps are probably fine and will last for ages. On the other hand, electrolytic caps will last about 20 years, if they are good enough. However, I fail to see a nine year old electrolytic cap having problems. By replacing those, you might run into problems later, specially if the replacements are of worse quality than the original ones. And this is more frequent to happen in our days, with the implementation of the RoHS directive.

#### hgmjr

Joined Jan 28, 2005
9,029
Are these the grid coupling capacitors (highlighted in red in the attached picture, referenced by http://www.blueguitar.org/new/schem/...rod_deluxe.pdf)? If so, I took a look at them...I have seen no swelling (they're not electrolytic, so I don't really know what this is going to look like) or burning in or around the caps (I tried to take a picture, but my camera is not cooperating with fine detail)...It may be more trouble than it's worth to add new caps because there are a bunch of bundled connectors (so I can remove the board) that are set and soldered, that I'd have to remove and re-solder when done.
View attachment 15582
If you can send close up jpegs of the amp with the cover off so that we can see the guts maybe we can tell for ourselves what condition the amp is in.

hgmjr

#### flat5

Joined Nov 13, 2008
403
Capacitors very in quality. Some are made to last for many years. Some are used by a manufacturer because the part is expected to last the warranty period. The Fender amps made in the 50', 60's did not have great coupling caps.
Caps have improved in general and Fender may use much better caps "now". Like I said, if the new tubes are glowing orange in the plates replace the coupling caps if you decide the resistors are ok. I suppose you could cut the cap's lead going to the grid to verify the cap is the problem. At this point you could also measure with a VTVM to see if the cap is leaky.

Edit:
You MAY see damage when inspecting electrolytic caps but likely not for other types.

Last edited:

#### bloguetronica

Joined Apr 27, 2007
1,372
The only electrolytic caps I see are the ones in the preamp stage. Those could be dry.

Capacitors very in quality. Some are made to last for many years. Some are used by a manufacturer because the part is expected to last the warranty period.
Off-topic: Unfortunately that is how it is. And hobbyists suffer likewise, because the market is flooded with poor quality capacitors, not to mention other components as well.

Similar threads