Amp Sounds Like Tremolo When Master's Up, Power Tube Strobes Blue

[justahuman]

AMP: Used Mesa Boogie Solo 50 Single Rectifier (just got in mail from Guitar Center)

PREAMP TUBES: V1, V2, V3 - JJ ECC83S; V4, V5 - no name Chinese 12AX7A
POWER TUBES: JJ 6L6GC, Tube Depot-15

FUSE RECOMMENDED: 2.5A Slo-blo
FUSE INSTALLED: 3A 250V

ISSUE: Sounds like tremolo whenever master volume is high. Left power tube pulses blue light in sync with sound (video below shows this happen as soon as the master reaches a certain threshold). Occurs on both channels, even when individual channel volumes are at zero. Right power tube looks fine, but its socket appears to have some scorching (could have occurred before old owner replaced tubes and sold to GC).

ATTEMPTS TO FIX: Cleaned jacks, straightened preamp pins & reseated, cleaned power tube pins and reseated, patch cable in effects loop, double checked cab impedance w/ multimeter, tried diff. guitars and cables. I have a brand new pair of JJ 6L6GC tubes, but don't want to try them if the issue might take them out, too. Any ideas?

 

[Pete Farrington]

Motorboating.
A bad HT cap or connection thereto.

 

[Michael Smith]

Motorboating.
A bad HT cap or connection thereto.

Is that D/C ripple we are hearing?

 

[Pete Farrington]

Is that D/C ripple we are hearing?

It could well be.

Are both output valves actually conducting anode current? The right one may be dead, maybe due to a blown screen grid resistor.

 

[Michael Smith]

I wonder if Guitar Center tests used amps before putting them up for sale? Do they offer any warranty on used equipment?

 

[justahuman]

Forgot to mention I hit the low E string in the video before I turned up the master volume, then turned the master down once the motorboating started; it's not generating that pitch by itself, nor is the volume swelling beyond the obvious tremolo effect.

Not sure about the right power tube's anode; I'm not comfortable probing around in the chassis. Are there any obvious visual indicators that I can look for? They both SEEM to be fully lighting up, but I'm not sure beyond that.

I've bought numerous amps from GC and more often than not there is an undisclosed problem. Not sure how much of that is GC apathy and how much is rough handling during shipping. The manager at my local GC claimed his location is one of the few that actually check used amps before buying. They've always accepted my returns, but I've had to climb a phone ladder a number of times to get my shipping refunded. You can buy an extended warranty on used equipment, bit I think you have to go through one of their in-house "techs," which I don't trust. I saw one guy trying to pry the volume knob off of a new Gibson w/ a flathead screwdriver!

Thanks to both of you for your suggestions! I'm going to call a tech on Monday and now I'll be able to better communicate the problem. I want to keep the amp, so hopefully GC will foot the repair bill.

 

[Michael Smith]

Good idea to get a tech involved @justahuman If you are not comfortable with probing around for voltages. Let us know how this goes.

 

[Lowerleftcoast]

I wonder if Guitar Center tests used amps before putting them up for sale?

I tried to sell a Silverface Deluxe Reverb to a local GC. Actually, I wanted to trade it for a vintage Gibson amp plus a few shekels. The Gibson looked in sad shape. Thick dust on the chassis and tubes. Two prong cord, etc. The employee was checking the DR and sending pictures to their vintage appraisers. I asked about the Gibson and he told me the equipment they buy has to function or they will not buy it. He assured me the Gibson worked.
He said they did not do trades but I could buy the Gibson and they would pay me $400 for the DR. (At the time, GC had a few Silverface DR listed in *Used Gear* for $1400.)

I would have taken $400 and the Gibson amp. There was no way I was selling the DR for $400.

 

[Jon Snell]

Have you had the main smoothing capacitors checked?
If they are out of specification and low capacity or even the wrong values, that will cause instability and when the 6L6s draw current, make the other supplies reduce in voltage in turn causing the 6L6s to reduce the current drain. That in turn causes the HT voltage to recover and off we go again.
If this only happens at high volume levels, to be fair, when it was tested, may have been overlooked.
Check it without anything in the effects sockets ... one never knows!

Beam Tetrodes, Pentodes and large Triodes can glow blue when being driven hard. This is normal.

 

[Pete Farrington]

Not sure about the right power tube's anode; I'm not comfortable probing around in the chassis. Are there any obvious visual indicators that I can look for?

You’ve got a multimeter?
If so, you could check that the the screen grid resistor isn’t blown from outside the chassis, no need to remove it from the cab.
Just remove the output valves, power up, set the meter to measure high V DC (500V range), touch the black probe tip to the chassis shiny metal. With the red probe, at the output valve sockets, count 4 anticlockwise from key and insert the probe tip. The left socket should be around 400 - 500V, how about the right?

If you’re not good to take readings inside an operational valve amp chassis, it’s useful to have a bias probe that enables anode or cathode current of the output valves to be checked.

 

[Dacious]

I'd be sending that amp back. That is U/S and if you persist it'll fry something. The scorched tube socket is a big red flag especially as that tube's likely the culprit because something caused it. As someone said, probably a blown screen grid or something due to over current but there could be other issues and Mesas can get expensive just in bench time real fast.

 

[Pete Farrington]

Yes, that socket needs replacing, it looks like there’s been arcing between anode and heater circuits.
This may be the schematic https://el34world.com/charts/Schematics/files/Mesa_boogie/Boogie_solo50_rectoverb.pdf
Such arcing would normally have caused further damage, eg to the heater balancing resistors or the DC supply for V1 heater.

 

[ChrisDowning]

I tried to sell a Silverface Deluxe Reverb to a local GC. Actually, I wanted to trade it for a vintage Gibson amp plus a few shekels. The Gibson looked in sad shape. Thick dust on the chassis and tubes. Two prong cord, etc. The employee was checking the DR and sending pictures to their vintage appraisers. I asked about the Gibson and he told me the equipment they buy has to function or they will not buy it. He assured me the Gibson worked.
He said they did not do trades but I could buy the Gibson and they would pay me $400 for the DR. (At the time, GC had a few Silverface DR listed in *Used Gear* for $1400.)

I would have taken $400 and the Gibson amp. There was no way I was selling the DR for $400.

Well, most shops work on 60% so they want to give you $400 as they then sell for $1000 - which then sounds better - for them and a s/h customer. And the answer, if we don't like those potatoes, is to sell it ourselves on eBay or Reverb for say 15%. I.e. my Takamine bought in 1999 for £1050 would be new now at £2300. But take it to a shop and they are thinking maybe they can get say £1200 as something like this from Takamine new would be £2300 (which surprised me.) So sell for £1200 they'll want to buy for -60% = £750 for me. On eBay I might get £950 minus something like 13.5% cost - so = £822 minus postage. But you might consider the time you put in at eBay and the postage faff just not worth about £50. And there's the dilemma.

In the UK we have a 30-day returns law that would allow you to take this amp back as bought and get your money back. Which in turn makes shops way more careful that they get it right straight off.

 

[justahuman]

I pulled the power tubes & was about to follow Pete's instructions on checking the voltages across the screen grid resistors when I noticed the left socket has arcing between pins 1 & 2! According to the schematic and 6l6GC pinouts, these shouldn't even be connected, right? Decided I'm going to leave this for a professional. We'll see what GC and my tech say on Monday.

LEFT SOCKET

RIGHT SOCKET

 

[Michael Smith]

I think that arcing is between pin 2 and 3.

 

[Lowerleftcoast]

I noticed the left socket has arcing between pins 1 & 2! According to the schematic and 6l6GC pinouts, these shouldn't even be connected, right?

That is only true in some amplifiers.

In this Mesa Boogie Solo 50 pins 1 and 8 are connected because it allows the use of either 6L6 or EL34. The 6L6 tube screen grid is internally connected to the cathode (pin8). The EL34 screen grid is connected to pin1 so, if EL34 tubes are used pin1 and pin8 must be connected externally. The Mesa Boogie Solo 50 has a switch to change the bias so a quick change between 6L6 and EL34 tubes can be made.

Many Fender amps use pin1 of the socket to hold a control grid stopper resistor. These amps can not swap in EL34 tubes unless they are modified to accept EL34 tubes.

 

[Pete Farrington]

The EL34 screen grid is connected to pin1

Pin 1 is the suppressor grid g3, usually connected to the cathode.
Pin 4 is the screen grid g2, needs to be at high voltage.

 

[jrblue]

that tube's likely the culprit because something caused it

Wisdom. It's likely not the tube; it's something affecting the tube. And yup, good luck tracking that down in a Mesa, unless it ends up being some common, well-known fault. I love Mesa stuff but never, ever use it anymore because I don't need or even want the trillion added features, and I don't want equipment with as many connections as the Bayeaux Tapestry has threads. I would absolutely send it back rather than getting myself committed to finagling some uncertain repair process.

 

[Lowerleftcoast]

Pin 1 is the suppressor grid g3, usually connected to the cathode.

Thanks for the clarification/correction. I had a lazy description in my post.

 

[Burning Fingers]

I had a Bassman reissue doing a similar thing yesterday...amp was humming and motorboating insanely and 1 power tube was cooked beyond belief while the other power tube was only 3/4 cooked.
1 of the power tube sockets had a little bit of a scorch mark.

The original tubes were Groove tube branded.

It was caused by the 3 power filter caps on the pcb board .. all branded IC ..they had all gone open circuit.
The caps in the cap cage were fine and looked like they had been replaced recently.

Replaced the 3 dead caps and put in new JJ power tubes and now the amp is as quiet as a mouse until you play a note or chord...no hum or redplating just the classic Bassman sound.

Edit: the owner now tells me a mate of his had dropped the amp and the problems started after that.
That explains the distortion of the structure inside the totally cooked power tube but not the 3 caps failing all at once..or maybe it was 1 at a time failing as replacing just 1 cap cured the motor boating even though the other 2 were still open circuit.. I replaced those of course.