Music Man RD-50 Driver Transistor Conundrum

[Technomaniac]

Hi. I have in my workshop for repair a Music Man RD-50 Amplifier. It came in making an indescribable noise, still present with everything turned anticlockwise.
I have narrowed it down to the output stage, have eliminated the power supplies and the op-amp. The noisy signal is present at the bases of the JE1692 devices. They are mounted to the heatsinks with plastic screws, and over time, plastic screws become loose. They were loose. So they could have been overheated.
But, the noise has, after several test runs, proven to be an intermittent. It could or might not be manifest at switchon, and appear after some minutes.
Or, it could be present from cold, and disappear after a few minutes.

SO, my present theory is that being loose on the heatsinks may have damaged one or both of the JE1692s.

Now, I would next like to replace these devices to see what happens.

But, this is not so simple a task.

I have the schematic, and it shows the JE1692s as junction transistors. Both in the schematic drawing, and below it, where it shows the pin configuration.

Also I have been looking at forums, and public opinion is that these devices ARE in fact, junction transistors. The originals are NLA, but several people have replaced them with junction transistors and the amps have worked.

To add to that, in every Music Man amplifier that I can find data for, the devices are shown as normal transistors.

But I have reason to believe that they are in fact JFETS.

I can find NO DATA on line or in my vast collection of data manuals.

When I test both of them they check like JFETS, NOT like normal transistors.

Unfortunately, the pins seem to be in an unusual configuration. I would have to cross the legs of any potential replacements I might have on hand, and I don't like doing that.
Is there anyone who has the device listed in a data book and who can quote the ratings and lead configuration confidently?

Thanks,
Bruce.

[Dave H]

Welcome, I checked the schematic and they look like npn bipolar transistors to me. Enzo suggested MJE15030 is an old ampage thread.

[Enzo]

Yes, why would you think they were JFETs?

"See what happens" is not a very effective way to troubleshoot.

Is this like yours:
http://bmamps.com/Schematics/Musicma...cman_rd50a.pdf

JE1692 is a house number for whatever transistor they really are under the skin. You won't find them or a data sheet, because it isn't a "type". You can compare this amp to the Peavey VTX series amps.

Every TO220 I have ever seen has the same pinout.

So the noise remains with the op amp removed?

You MUST use the main speaker jack first, it has the shunt contacts. But when you do, the signal is coming through the external jack cutout contact Is it dirty?

Are the 6.8 ohm resistors healthy? And is the voltage drop across them within spec?

Convinced it is the output? Pull ONE power tube. is the remaining one noisy? Now put the pulled one back and pull the other. Did either tube when alone sit quiet, or was the noise always there regardless?

Power supply stays clean? Did you include that +30 on the grids?

[g1]

It's definitely a bi-polar, not Fet, see attached service note.
Also, I'm not 100% sure about which model, but seem to recall an IC driving the transistors that must be the EXACT type.

MM Service Bulletin 7m.pdf

P.S. If they are not reading like ordinary transistors they are either defective or the wrong type.

[J M Fahey]

I remember long ago seeing one schematic which carried the commercial equivalent of the mystery one, so it´s not a guess but an officialoly scnctioned one.

Just quoting from memory, it was some 70/80V CE which is not too much by any means, I´d be happier with 100VCE transistors, since IF power tube grid is at +30V and a 6L6 at cutoff (what would be Vp in Fets) can easily need -60V Vgk or more (remember -52 V was Fender *standard* bias and not cutoff by any means)

And current capability was 6 or 8A, apparently way overkill here, but I remember the suggested transistor had HUGE gain at such a low current, yet didn´t need to be a Darlington.

Personally I toyed with the idea of making it a Darlington for ease of drive but stepped back because in the future no one else would be able to repair it.

The Math behind it is that a 6L6 at full power may pass some 250 mA peak, maybe more in AB2 conditions, with positive grid, so each transistor which may drive 2 power tubes in parallel, may have to supply 500mA or more.

Since a typical Op Amp can supply 5mA without difficulty, but not much more, reason is that transistor Hfe/Beta is minimum 500/5=100X at , say, 600mA , not an easy task.
That´s why I don´t much trust MJE150xx , the optimzation which makes them high voltage at the same time lowers current gain Hfe , every design is a compromise, you gain here, you lose there.

Yes, in some cases they must work, specially driving a single pair of tubes; not so sure with 4 big bottles.

And I bet there is some "undocumented feature" of 1458 which allows them to drive heavier loads than specified in the datasheet, I always see them specified by Marshall as reverb drivers, here as non darlington power transistors, sometimes as headphones drivers.

Remember they are a very dated design, almost unused today, also that amps chock full of TL072 , which by datasheet have the same current capability, reserve 1458 for the tasks I mkentioned.

Of course this will not come out in simulation, but in the real world.

[Enzo]

Juan, here is the data sheet for 2N6292, which the MM factory says is the real part:
2N6292: 7.0 A, 70 V NPN Bipolar Power Transistor

It doesn't claim gain that high.

The op amp output is cap isolated.

[oc disorder]

from

http://www.pacair.com/mmamps3/sites/default/files/docs/MM%20Service%20Bulletins%20All.pdf

(Via ocr)
SERVICE BULLETIN #7
MUSIC MAN Driver TRANSlSTORS
JE1692 Transistor Specification,
The JE1692 transistor is a type 2N6292 transistor
for a minimum gain of 70 at 2 amps of collector
current. Any T0220 transistor meeting or exceeding
the following specs should work as a replacement.
The majority of off the shelf 2N6292 transistors
will also work as most will meet the gain spec
without testing. Furthermore any T0220 transistor
meeting or exceeding the following specs will work
as a replacement. (It is recommended to use these
in pairs.)


2N6292 Type NPN - ICMax 7Amp - VCEO 70V - ft_Min 4mhz - Hfe Min70

UPDATE TO JE 1692 TRANSISTOR SPECIFICATION
2N6488 SHOULD BE USED AS THE BEST REPLACEMENT FOR THE JE 1692.
THIS IS THE DRIVER TRANSISTOR THAT IS USED IN THE BASS
AMPS HD-75 AND HD-150 AND IS A SUPERIOR PART.

2N6488 NPN 15AMP 80V


and original page



from Intro Page "The Music Man Reference"

[J M Fahey]

Well, they confirm what I said.
In old transistors gain dropped a lot with increased current.

Unfortunately there is not a graph or curve showing JE1692 Hfe (current gain) at lower values , I remember seeing an old one, belonging to some official replacement (that´s where I got the commercial number from), which might have been 2N6292 or not, but which I´m certain showed quite higher Hfe at the lower currents involved and justified the use of a 7/8A device.
Or maybe it was a 12A device, the point is that it showed very high Hfe (for a large non Darlington bipolar that is) at lower current.

Notice that datasheets show 2 values confirming my memories , at some "Recommended Maximum" level , let´s call it RM (which depends on device type), and Absolute Maximum, let´s call it AM (which is the same for all devices):

From Enzo´s recommended datasheet (about 2N6292):

DC Current Gain Specified to 7.0 Amperes
hFE = 30-150 @ IC
hFE = 3.0 Adc 2N6111, 2N6288
hFE = 2.3 (Min) @ IC = 7.0 Adc - All Devices

which I read as:
* 2N6292 is the full specs device (the datasheet carries its name)
* 2N6111 and 6288 are inferior performance ones.

so:
* all had terrible Hfe at 7A (2.3)
*inferior quality 2N6111 and 6288 had Hfe spread between 30 and 150 at 3A which makes me think:
* superior quality 2N6292 hovered around the upper end while the others were closer to the lower one; average of all is still an average.

How do I know that? ...
well, the datasheet itself says so, not in words but in numbers: while 2N6292 , as stated on the "headline" , is a 70VCE part :
Collector-Emitter Sustaining Voltage

VCEO(sus) = 30 Vdc (Min) 2N6111, 2N6288
VCEO(sus) = 50 Vdc (Min) - 2N6109
VCEO(sus) = 70 Vdc (Min) - 2N6107, 2N6292

in the early days when process was not as tightly controlled as today, somewhat lower quality batches were not junked but assigned a different model number and sold, lots of buyers did not need max spec but somewhat less.
I can easily think that those labelled and sold as 2N6292 where the cream of the crop and so probably also had the highest Hfe, still within the tolerance band of course.

EDIT: thanks OCD, your post confirms what I said and more, plus it leads me to the datasheet I remembered ;
what does your post say?:

SERVICE BULLETIN #7
MUSIC MAN Driver TRANSlSTORS
JE1692 Transistor Specification,
The JE1692 transistor is a type 2N6292 transistor
for a minimum gain of 70 at 2 amps of collector
current.

which reads: IF Hfe is 70 at 2A , it will be significantly higher at 0.5 0r 0.6 A

How do Iknow?
well, I learnt long ago how transistors work, but not to rely on my word (of course), let´s see the datasheet of a transistor recommended by MM as replacement:

UPDATE TO JE 1692 TRANSISTOR SPECIFICATION
2N6488 SHOULD BE USED AS THE BEST REPLACEMENT FOR THE JE 1692.
THIS IS THE DRIVER TRANSISTOR THAT IS USED IN THE BASS
AMPS HD-75 AND HD-150 AND IS A SUPERIOR PART.

I opened the 2N6488 datasheet and Thanks God it has a detailed Hfe vs collector current graph , just what I remember reading looooong ago:

DC Current Gain Specified to 15 Amperes-- hFE = 20-150 @ IC = 5.0 Adc; hFE= 5.0 (Min) @ IC = 15 Adc

Notice the brutal Hfe variation with current; no plain "numbers" at lower currents but they show a graph which clears everything:


how to red the graph:
a) although datasheet shows a spread of values, the graph shows an average one , which IS realistic, some devices will have higher specs, some will have lower.

If you pick a random part, Hfe may be anywhere between 30 and 150, so to play it safe you "should" design with 30, the minimum value ... but Fender suggests to select and that throws randomness through the window.

They say to use parts with at least Hfe 70 @ a high current of 2A ...but that means that Hfe will be quite higher ar 0.5A , which I calculated earlier coul be peak tube current .

How much higher Hfe?

Well, graph shape keeps reasonably constant, so by checking values at a certain Hfe, we can calculate values at another by seeing how much should we "move" curves or scale:

* first go up, red line, from the 2A point until it touches the 25C Hfe curve, then red line to left until the vertical scale, I read over 90 there, can we agree on Hfe 92?
Which should be read as: ON says that "the average transistor will have Hfe 92 at 2A current"
Which can also be read as: "50% of the transistors will have Hfe larger than 92, 50% will have less than 92"

* now read on the same 25C Hfe curve, what will be the average Hfe at 0.5A , which is NOT mentioned in the datasheet as a "number" but can be read from the graph:
go up, Red line, from 0.5A until it again touches the 25C average Hfe curve, then left red line until Hfe scale: we read clearly above Hfe 150 ... can we agree on 160?

So we see that the average transistor suggested by MM easily surpasses the required 100X Hfe I calculated earlier.

Refreshing how I did it: it´s the ratio between max plate current for 2 tubes per side (some 500mA) and driver Op amp current capability (5 mA) so NEEDED Hfe is 100X or better.

Ok, nice design, but MM wants to use more than 50% of the transistors they buy, so they relax specs a bit, they do not spec Hfe 92 @2A as shown in graph but easier to meet Hfe >70 at 2A.

Of course such a weaker transistor will also have less Hfe @ 0.5A ... will it still be enough to be driven by a humble OpAmp? ... 70´s technology to boot, no 30mA capable NE5532 way back then.

Ok, then let´s again draw the red line starting from 2A ... but we now stop somewhat earlier at Hfe 70 height (lower blue line).
Notice the distance needed to reach the "average" 25C Hfe line, then on the 0.5A red line, we also stop the same distance under the higher red line and draw a second, higher, blue line to the left, and read the new Hfe value: STILL above needed 100 Hfe ... do we agree on Hfe 120X ?

By the way, what I said earlier in 10X less words, only now the graphic design method is fully shown.

By the way, what old style Engineers used when Digital computers were primitive and unreachable or even non existent , everything was done using graphs and slide rules.

FWIW that´s *why* old datasheets displayed so many graphs , curves and tables; I bet future ones will just include the SPICE model and nothing else ... what for?

Nobody uses the graphs any more and most simulate, so ......

Not kidding and you can quote me on saying this

[J M Fahey]

Juan, here is the data sheet for 2N6292, which the MM factory says is the real part:
2N6292: 7.0 A, 70 V NPN Bipolar Power Transistor

It doesn't claim gain that high.

The op amp output is cap isolated.

Oh yes it does
Only it does not show the 500mA one but the 2A one, not the same.
Look at my other post to see how I calculate it

As of the cap coupling, too sleepy now (04:45 AM here) , but just think of this:
You have 2 100W into 4 ohm amplifiers, one capacitor coupled (think Acoustic 124), the other direct coupled (think Peavey Bandit) .... how much peak current will each deliver to the load, just at clipping?
Will the capacitor coupled one deliver less? ...... or more ? ..... or the same?
Does the coupling capacitor have any influence on peak current when delivering same power into same load?

[Dave H]

Does the coupling capacitor have any influence on peak current when delivering same power into same load?

The peak current to the load will be the same. If it's capacitor coupled the op-amp doesn't have to supply the base current needed to sink the idle plate + screen current as that is provided by the 75k collector to base resistor. Anyway even with 80V on the cathode the 75k can only supply 1mA so the op-amp has to supply the other 4mA (assuming 5mA peak base current)

[Enzo]

So without going to the same length, how do those same calculation stack up when applied to the MJE15030 part?

http://www.onsemi.com/pub_link/Colla...MJE15028-D.PDF

The hfe chart shows over 100 for small signals.

I dwell on it because I have used the MJE part for years on these without diminished performance that I can detect. In their VERY similar Classic VTX amps, Peavey uses the MJE15030 in their amps.

[nickb]

In this particular amp each transistor drive a single 6L6. The peak current will therefore be < 250mA. Taking the MJE15030 worst case HFE as 75 ( half of the average at 250mA) the base current will be < 3.3mA. The driver is a TL072 and the datasheet guarantees 5mA with a 10Vpp output. I'm pretty sure you'll get more than that at a lower swing - you don't need too much as the load is 680 ohms. This even ignores the help from the base-collector resistor.

Here's my take. Putting it all together for two 6L6's (e.g. PV Heritage VTX) you'd have to be pretty unlucky to actually need 6.6mA and the op-amp can cope in any case.

Juan does a great job of pointing out the big issue.

[J M Fahey]

So without going to the same length, how do those same calculation stack up when applied to the MJE15030 part?

http://www.onsemi.com/pub_link/Colla...MJE15028-D.PDF

The hfe chart shows over 100 for small signals.

I dwell on it because I have used the MJE part for years on these without diminished performance that I can detect. In their VERY similar Classic VTX amps, Peavey uses the MJE15030 in their amps.

And you are right.
In this case, the MJE15030 , a more modern and improved part than what was avaiable in then70´s DOES fit the needs and then some.

Lots of Math can be substituted by proper graphs.
As you know I am a great fan of graphical calculation and do NOT simulate

Here only Math needed is to estimate what current gain is needed to drive each power tube to, say, 250mA ... value pulled from the 6L6 datasheet graphs by the way

So for 60/70W amps, we need 250/5=50 ... almost any transistor can do that, even TIP41

For 100/130W amps: 500/5=100

As you found, graph shows that an average MJE15030 has Hfe of 150 at 0.5A current ... cool and that matches your personal experience and Peavey suggestion.


I also guess that since average is so good, *most* MJE15030 fit the bill, and practically all in the 2 tube amps.

My point in general is that the "published number", as shown by themselves is incredibly pessimistic:

ON CHARACTERISTICS (Note 1)
DC Current Gain hFE
(IC = 0.1 Adc, VCE = 2.0 Vdc) 40
(IC = 2.0 Adc, VCE = 2.0 Vdc) 40
(IC = 3.0 Adc, VCE = 2.0 Vdc) 40
(IC = 4.0 Adc, VCE = 2.0 Vdc) 20

and is contradicted by their own "typical/average" graphs .... I **guess** they want to avoid recalls, customer complaints, Lawyer pressure and all that jazz so they **guarantee** a low standard any device can pass.

That the actual product is way better is the cherry on the cake

One small side note: for what we use, the best graph is the one showing plain Hfe at different currents; the "small signal" one is better suited to gain-bandwidth calculations, in fact one scale is Hfe, the other is frequency, from 500kHz to 10MHz ... we fly much lower than that

In a nutshell: rest assured that the Real World matches your experience, I´m happy that my calculations often tend to be realistic

[g1]

For the record, here is the service note that mentions the driver IC needing to be 1458 in certain models, so there must be something to that.
"NOTE: Use only a 1458 op amp to drive the output section."