Matamp blows output tubes, any help?

[spy]

Let me know your opinions on this.

I have a Matamp with 4xEL34 that for a second time fried one pair of the output tubes. It's an old Matamp close to an OR120 but I don't have a schematic in hand, also I don't know the exact model. The amp is fully recapped, has new diodes, new grid and stopper resistors and new bias supply circuit.

At low volumes it behaves just fine but when you crank it up and after a few rehearsals it blows the one side of the power section, let's say the push side. The first time I changed them, I let it on my bench with a dummy load at full power for many hours just to be sure everything is ok but now I have a pair of EL34s fried again! The sockets are clean with no carbon traces, I can't see any melted cable inside and all resistors measure and look fine. No weird smells inside and I measured the windings of the OPT both with a DMM the resistances and with a Variac the voltages, it seems fine.

[Steve Conner]

When you say "fried", "blown" what exactly is wrong with them? Melted screen grids? Gone gassy due to overheated plates? Bases arced and burnt up from excessive voltage?

Melted screen grids and arced bases tend to be caused by a too-high load impedance, maybe a bad speaker cable or a 4x12 with one blown driver. Overheating of the plates tends to be caused by a too-low load such as a short circuit, or an intermittent fault in the bias supply.

Parasitic oscillations can fry tubes in both ways.

[Leo_Gnardo]

SPY! Haven't seen you around these parts in a while. Orange 120, 80 & Matamp (as I'm sure you know) tend to run very high B+ voltage 525V or so, with the screen grid supply just a pinch lower. Maybe that was OK when a set of original Mullards would put up with it, but it seems the modern EL34's just don't last long with 500+ on the SG's. Could increase the value of your SG resistors. I've seen up to 2200 ohms. And/Or (horrors!) add a power resistor in the power supply string to drop the screen grids to something sane. I did this recently with a nice old Traynor YBA1A with KT88's and it worked a charm. @ 4000 to 6000 ohms 10W took the screen grids down to a point they weren't drawing excessive current.

Not a bad idea to have flyback suppressor rectifiers also, as suggested in the Trainwreck pages. Might consider 4 1N4007 in series or a 5000 PIV rectifier, due to the extra high supply voltage and expected proportionally larger flyback spikes.

FWIW I generally use Ruby EL34BHT. There's a "wing" of plate material added to help dissipate heat, as compared to their EL34BSTR. Might also consider trying KT77, as they're rated for higher voltages. I've used the JJ and one of these days will have to try out the Gold Lion. The '77s do sound different when overdriven, not everybody's cup of tea. If the amp is generally used below or just barely beyond clip level they'll probably do fine.

[spy]

I see arcs/flashes when I open the stand by switch so maybe their bases arced and burnt up. I need some further info on the other options, how do I know if the grids are melted? And gassy due to overheated plates? Sorry newbie questions.

Leo, I don't think it's due to high B+ voltage. The first set was a TungSol RI EL34 and the second EHX, I have used both in other Oranges and Matamps. But I'm intersted to use flyback suppressor rectifiers to protect the next set of tubes. Any schematic/link on this? Other ideas on protecting them? The HT fuse didn't protect two sets!

Also, I'm interested to know more about parasitic oscillations. How can I see them on my scope? These amps do have a cap on the anode of the PI's driver. Are there any other ways to eliminate them?

[Ted]

Arcs and flashes? Where from? Inside the valves? Or from the bases?



Parasitic oscillation in the output stage often manifests itself as hum - the output stage is working at full power, albeit at an ultrasonic frequency, and the power supply smoothing starts to struggle.
It can sound like a squelchy hiss (Yes I know. Talking about sound is like dancing about architecture). This is more often a sign of feedback from an earlier stage.

But really you need a scope. If it's oscillating at a low enough frequency you may be able to see it on the output transformer secondary. Take a look at the grids. I'm not going to advise you to look at the plates with a scope. But if you do, use a 10:1 probe with heat shrink on any exposed ground near the tip, great caution and possibly beta-blockers for shaking hands. Check scope range. Several times.

I've seen a few OR120s that ate output valves due to parasitics. Sometimes lead dress helps. More often, changing 35 year old supply caps fixes it. Matamps are not a straight copy of Oranges( and they are a lot newer), so this may not apply.

[spy]

Ted, inside the valves, left pair! Like flashes.

[Leo_Gnardo]

Arcs inside the tubes or at the base? You would probably see some carbon tracks on the tube bases, sockets, back of the sockets if it happened there. A popular spot is between pins 3 and 2. Does the arcing start on power up or when you send signal thru the amp? (edit: on power up. re read spy's inquiry. my bad.) If it's "with signal" there's a possibility of a damaged output transformer. Let's hope not. A measurement you can make easily is resistance of the primary windings. One probe on the B+/center tap, other on pin 3, plate. Some variation is expected, up to 20%. If one side of the winding is different from the other in a major way, say 10 ohms on one side and 28 on the other, we'd be looking at shorted windings in the primary and your output tubes would object to that. But likely only with signal applied.

Here's a link to a link that will get you all 29 Trainwreck pages. A lot of this was "secret stuff" 25-30 years ago, but much is now done by some manufacturers and also by techs who are trying to bulletproof amps for themselves & customers. Should be in everybody's library.

The Amp Garage :: View topic - Ken's amp modding pages

Somewhere in the middle of that you'll find advice on adding a string of rectifiers between output tube plates and ground. (Stripe end to the plate otherwise you'll be shorting the power supply to ground thru the OT primaries = bad, very bad.) The point is, when driving speakers, the cones can't exactly follow the signal, always a bit "late" because they have mass. When the cone w/ voice coil attached returns thru the magnet gap it becomes - a generator. It's sending a signal back thru the OT's secondary and shows up on the primary, multiplied by the turns ratio of the OT. And it can send spikes of 3000V onto the output tube plates. Tubes don't like that. The rectifier string quenches those spikes and reduces wear and tear on the output tubes, also reduces likelihood of arcing when driven hard. When I had a listen with and without the rectifiers, I could detect a small reduction in audible distortion when an amp's driven well past clipping. With the rect's in place the kind of distortion I like remains, and a certain annoying distortion clears up. Works for me! Maybe you too. Of course this makes no difference if there's no signal.

FWIW the RI TungSols and EH come out of the same factory (if I'm not mistook) so do Sovtek, RI Mullard, RI Gold Lion, tubes marked SED for US sale, and maybe some others. All marked and marketed by New Sensor. Some may be "improved" models, some may be identical with only the paint on the outside of the tube different. There's a lot of discussion & speculation about this, and it would be wonderful to know what's really what. Not just what the salesmen tell us. Which tubes are wearing what disguises? Who's behind that big red rubber nose? It's all like an impossible to understand spy novel to me - John Le Carre' anybody?

You could see parasitic oscillations on a scope. Generally don't appear until you're running a signal, a large one. Low frequency too - say 50 to 300 Hz. And many times don't appear when running the amp into resistors but do appear when speakers are the load. NOTE the following doesn't apply to ORANGE and probably Matamp because the output inverter/driver is done with a single triode, but does apply to many amps with the more common inverter/driver circuit: A disc cap 1000V rated, bridging the drive tube's plates anywhere from 47pF to 1000pF may help quench those parisitics. On some 60's-70's Fenders they ran a 0.02 uF disc from pin 5 - output tube control grid - to ground. Somehow still sounded plenty bright. On your Matamp, could try increasing the size of that disc cap you mentioned.

Overheated plates - I gather you didn't have the amp on long enough for plates to start going red, orange, yellow, white, meltdown so that's probably not a problem. Loss of bias voltage, or insufficient, can do this. Also having too much resistance in the control grid circuit - a fault common to some manufacturers. 2000 series Marshalls from '98 to '05 - guilty.

COULD start up your amp, just one output tube in socket. Add one at a time. If no arcing, then by adding another, it starts arcing, suspect the last tube you added.

Whew, enough typing. Gotta get another coffee.

[spy]

Wow, thanks Leo! The coffee is from me!

The arcs I see are from the "fried" tubes, if I put new ones the amp is working fine, right now until it fries again another pair after a few rehearsals at very loud settings.

I have checked the resistances on the OPT as you described also the voltages and ratio with a Variac and a lamp in series for shorts. It seems fine.

Now I understand what are these suppressor diodes, I have seen these in a lot of new amps. Thanks for the link and the Trainwreck secrets! I do have two Weber books I'll check if these pages are inside.

By the way, I love LeCarre!

[Leo_Gnardo]

I'd advise looking outside the amp for causes too. Speakers, speaker cables, speaker connectors. If anything's not 100%, that will make your output tubes unhappy when trying to deliver a large signal. Also that output ohms selector, I don't know what's on your amp but if it's one of those plug-in types make sure the contact plugs & receptacles are shiny bright clean and tight. If it's a switch, could be the contacts are burnt up. Seeing that a lot now in Marshalls & similar amps late 70's on. Momentary disconnection of speakers under high signal conditions = bad.

MusicMan put suppressor diodes on as far back as early-mid 70's. Peavey, not long after. Now you find 'em in Fender and other brands too. I'm guessing Ken Fisher picked up the idea possibly from the MusicMan but who knows, he may have done that earlier. He was wise enough and kind enough to pass it along.

- - - - - - - - -

Nice strong cuppa Chock-Full does it for me. After having been thru the expensive brands, always wind up back on Chock.

HA! Figured you might like LeCarre. Puts me right to sleep. If you can find it one of the scariest books I ever found is non-fiction, "The Super Poison" about the Seveso Italy dioxin event 10 July 1976, and all the buck passing after. With all the Italian surnames it gets confusing as any LeCarre, but it's about something real so that gets me in the right place. Brrrrr..... And to think they were making that stuff only a couple towns away - Clifton NJ - when I was growing up. Double brrrr.... If you wonder why hexachlorophene isn't much found in consumer products anymore, that's why.

[mac1amps]

There's also the [unfortunate] possibility that there's a short in the O.T. secondary, which would explain the continuous "taking out" of power tubes. Those kind of shorts WON'T show up by taking D.C. resistance readings. The biggest clue in that scenario would be a low output......that is, until it takes out ANOTHER power tube.

[spy]

Almost two months now and the amp is still kicking! Maybe the suppressor diodes do their work...

[Leo_Gnardo]

Almost two months now and the amp is still kicking! Maybe the suppressor diodes do their work...

Looks like that's the solution. Wonderful how a very modest investment in silicon rectifiers can save serious money in output tubes. Cheap insurance!

[Jazz P Bass]

Cheap insurance, yes.
The flip side of the coin is the failure of the diode (albeit, a rare occurrance) taking out the OT.