old Marshall 30w transistor schematic please

[Gix]

where i can get this schem?
no fet emulation and others please

thanks in advance

[vanhansen]

Is this what you're looking for?  If not, you might want to provide some more info.  I'm just taking a guess based on the "Marshall 30 watt transistor".

5010 Master Lead, 30W 1x12" combo

5203, 30W 1x12" combo

[Gix]

5010  Thanks!

[col]

I have most of the parts to build one of these but can't find any info on the transformer. Does anyone know what voltage it uses? I realise that there will be a +, earth and -ve and need to source a tranformer that will deliver the correct voltage and amps.

Col

[R.G.]

I have most of the parts to build one of these but can't find any info on the transformer. Does anyone know what voltage it uses? I realise that there will be a +, earth and -ve and need to source a tranformer that will deliver the correct voltage and amps.

I can get you into the ball park.

The thing purportedly makes 30W into a single speaker, which is 4 ohms. To get 30W into 4 ohms, we calculate: power P = v^2/R, so P*R = V^2, or V = SQRT(P*r). Then V = SQT(30*4) = 10.95. That's the RMS voltage on the speaker. We need to know peak to get at the power supply. Vpeak = Vrms*1.414, or Vpeak = 15.5V. Add to that a volt for output transistor saturation, and you get 16.5V peak. That peak is either way, so it's +/- 16.5V. This seems to be reasonable, as the preamp uses a 1458 opamp, which can work with up to +/- 18Vdc without dying, and there are no regulators in the circuit to drop voltage to the 1458, only R23 and R24, 2.7K, which would drop off a volt or two.

So you need +/-16.5Vdc. The rectifiers eat up a diode drop on each side; you need 17V peak on each side of the transformer output. That's 17/1.414 = 12Vrms on each half of the transformer, or 24Vac center tapped. Kewl - even a standard value!

As to current, it will need to do 30W reasonably continuously. The efficiency of an AB circuit is about 78.6%. so we're sending 30W to the load and that means we need DC power of 30W/0.786 = 38W from our +/-16.5V supply. The average current is then 38W/33V = 1.15A.

To figure transformer RMS current, the RMS current supplied by a transformer into a capacitor filter in the full wave bridge is about 1.6 to 1.8 times the average. That's 1.84 to 2.07A.

So you need a 24V centertapped secondary, which is rated at 2A. An you're in major luck, as that's a very standard value.

[Steben]

Why would you build this one?
What's so good about a naked opamp clipping SS amp? This is no better than a 100$ practice amp.

[col]

I thought about this as I already have most of the parts and I have never built an amp before other than a Ruby. I did a layout for the pre-amp and tone shaping section on stripboard and just wanted to try it. I don't think it would get much use as I always use my Laney valve heads for gigs and I already have a 30w combo for home use. If you have any better suggestions for something that can be manufactured from easily available parts between 30 and 60w that would be cool. I have seen the schematic for a 65w power amp using an IC which I have one of but can't remember the number. I must build it sometime and try it.

[Steben]

TDAxxxx... and LM38xxxx... IC's are very nice though. You know little brother LM386 allready, yet it has a big family. Power sections in IC's are far more easy to deal with. I agree that a preamp like this one can be useful, yet I doubt the complexity is that needed.
Remember that on the road from Tubes to FET's to diodes to IC's to transistors the complexity rises getting a good tone.

[pinkjimiphoton]

nekro nekro nekro bump...

RG was close, it needs a 28v center tapped transformer, 1 amp, like the hammond 166j28. it will fit the mounting holes, but this is the stock replacement for these suckahs!

[Rob Strand]

RG was close, it needs a 28v center tapped transformer, 1 amp, like the hammond 166j28. it will fit the mounting holes, but this is the stock replacement for these suckahs!

If you measure a transformer with no load the voltage will the higher than the rated voltage, somewhere around 10% to 13% for 50VA but it is higher than that smaller tx's and lower for larger tx's.   The rated voltage is at full load.

The regulation is (as used by many manufacturers these days),
24V tx: reg% = 100% * (28 - 24)/28 = 14%
25V tx: reg% = 100% *(28 - 25)/28 = 11%

A 30Wamp might use a 60VA transformer  (skimped say 45VA).

So the "rated" voltage could well be 24V to 25V.

[pinkjimiphoton]

yeah, i been restoring and repairing these for folks. the 12 watt, 30 watt all are basically the same amp, other than minor changes in the power supply. the hammond is a good replacement, and fits great. aesthetically ya want it to fit the holes usually
and its easier to just KNOW a good part number when you're searching

edit: it matches the original voltages in the original transformers, as i have several of these in my collection in various versions. that's why i said 28v, i think it even specs that on some of the schematics.

believe it or not, the main dif between the 12,20 and 30 watt models is the speaker impedance. 12 @ 16r, 20 @8r, 30 @ 4r

[Rob Strand]

RG was close, it needs a 28v center tapped transformer, 1 amp, like the hammond 166j28. it will fit the mounting holes, but this is the stock replacement for these suckahs!

The transformer in those amps appears to be on the edge as I've see number of posts regarding dead transformers.

That Hammond model you suggested could be somewhat better than the original.  The specs for that transformer (and many other Hammond transformers) quote"
"We use Class B insulation (130 degrees, C) for extra protection - UL listed as a Class A
(105 degree, C) design"

The class B winding means it will handle a little more abuse than the original tx, probably all it needs to prevent it popping.

What's probably going on is the original tx is rated correctly but it is mounted in one of those low-profile chassis where the heat can't get out.  As a result the transformer fries or the thermal fuse opens when the amp is pushed.   A few months back anotherjim posted a thread on a Peavey amp with a low-profile chassis shutting down when someone sat on it.   (Of course you need different tx VA rating for different amp models.  For a 30W amp you need about 50VA, which is what RG recommended.)

A loose end changing transformers is the fuse rating.  It is possible the original tx has a one-shot thermal fuse (I dont' actually know) which pops when it overheats.   I don't think those Hammonds will have a thermal fuse.

[amptramp]

I think you may need higher voltage on the transformer to accommodate the drop in the power filter capacitor.  2200 µF seems like a really tiny number for a 1.15 amp drain.  Using the equation:

CV = IT

where C is capacitance in Farads
V is voltage drop between alternations of the line voltage
I is supplied DC current in amps
T is time from one alternation to the other

C = 2200 µF
I = 1.15
T = 0.008333 seconds for 60 Hz and 0.01 seconds for 50 Hz

V = IT/C

which is 4.356 for 60 Hz
or 5.227 for 50 Hz

This is why you needed a 28 volt transformer where higher filtering would have permitted a 24 volt transformer.  The voltage drops in the calculation above could be reduced by using a larger filter cap.  It should be over 10,000 µF.  The op amp, emitter follower and differential stages all have some power supply rejection but for full power, the output stage needs the voltage R.G. calculated to avoid hum modulation.

[pinkjimiphoton]

RG was close, it needs a 28v center tapped transformer, 1 amp, like the hammond 166j28. it will fit the mounting holes, but this is the stock replacement for these suckahs!

The transformer in those amps appears to be on the edge as I've see number of posts regarding dead transformers.

That Hammond model you suggested could be somewhat better than the original.  The specs for that transformer (and many other Hammond transformers) quote"
"We use Class B insulation (130 degrees, C) for extra protection - UL listed as a Class A
(105 degree, C) design"

The class B winding means it will handle a little more abuse than the original tx, probably all it needs to prevent it popping.

What's probably going on is the original tx is rated correctly but it is mounted in one of those low-profile chassis where the heat can't get out.  As a result the transformer fries or the thermal fuse opens when the amp is pushed.   A few months back anotherjim posted a thread on a Peavey amp with a low-profile chassis shutting down when someone sat on it.   (Of course you need different tx VA rating for different amp models.  For a 30W amp you need about 50VA, which is what RG recommended.)

A loose end changing transformers is the fuse rating.  It is possible the original tx has a one-shot thermal fuse (I dont' actually know) which pops when it overheats.   I don't think those Hammonds will have a thermal fuse.

there's no fuse in the originals. its funny, as the power supply and amplifier are virtually identical between the 12, 20,30 and to some extent the 50 watt models.
so far in the repairs i've made, i've had two dead transformers, both the secondary had popped on one side, usually the side that goes to the + of the diode bridge.
the filter caps are fairly small.... stock is 2200uf. i've been putting 4700u in, even 6600uf.... in the 12 watt reverb i did, i used three 2200 in parallel to get 6600 and the amp runs clean and cool with very little intermodulation and no crackles etc.
i ended up replacing all the resistors in the preamp with metal film to get rid of noise, too, which helped.
the 28vac works out to, after all the rectification and filtering to about 14.4-15.1 v on both the rails.
BUT some of them use a 9.1v zener in the driver section that drops the negative rail to about 10 volts. these seem to have a warmer and more bass heavy tone...
these are really weird little amps!

that hammond will work in the 12, 20, or 30 watt models. just be sure to ground the center tap to the chassis ground as well as the board ground. it will work without a chassis ground but it will be noisy and kinda dangerous... i was getting about 12-15vac on the chassis without it. the two red taps can go to either side of the secondary, doesn't matter.

the amp fuse is specc'd at 500ma, btw. it doesn't seem to draw very much.

hey rob, up for a challenge? i can post the top and bottom of the boards and the part values, if you are up to the epic/herculean/rockstar task of making a schematic, which does NOT exist for this model.
apparently i have the earlier version, maybe the earliest... its marked jm78-2 on the bottom of the pcb.

on the ssguitar amp forum, someone traced the preamp... but it doesn't match mine, or the other two "official" schematics.

crazy!

i meant no dis to rg, of course... just saying. in the working amps i have, they all run about the same 28v

[pinkjimiphoton]

thanks for the explanation, ron

that helps me figure out how to "improve" these... cuz you're right, the hum can modulate the signal so badly you can get literal harmonization on some notes lol...i had one that sounded like you were playing in 5ths!

bigger caps definitely help. i'll try going bigger and seeing if it helps. there's a distinct sonic improvement with 6600u than the 2200 that are stock.

the series of amps all use the same PT. according to marshall tech support, 28vac center tapped. it also matches the readings i took from the amps i've worked on. i have the 12 watt stack, the 12 watt reverb, the 30 watt non reverb, a DIFFERENT 30 watt non reverb <what i'm working on now, just a pcb someone had pulled> a 30 watt reverb, two 1x12 50 watt reverb combos, two of the 50 watt 2x12 combos, and two 50 watt headpieces.
i really dig this series of amps, and hard

[pinkjimiphoton]

pics of the board. will upload a bom later. attempting to draw the circuit out, taking a bunch of notes.

but hopefully this'll help some other poor suckah on theyze one-way trip someday

[pinkjimiphoton]

[Rob Strand]

I'm in the process of shifting PC's ATM so I can't do any tracing.

As far as I can tell, your board *is* a 30W unit.   It has two TO-3 output transistors and
heatsinks with fins on all four sides.

What I  can tell you is the Marshall schematics you find on-line are 'sort of' correct.   I don't know what the hell goes on with their schematics but here's the general way forward using schematics and boards from that era.
- Find the schematic which matches the board as close a possible.  (not straight forward)

You will then find:
- the schematic mostly matches
- the parts designators on the PCB don' t match-up with the schematic!
- when the schematic doesn't match you will find minor mods to the circuit
- in some cases the shape of the circuit matches the schematic but the part values are different.

All-in-all a big mess.

At least two people have put-up revised schematics for the 12W versions.


So start with Solid-state, 30W, no reverb, which is this schematic,



You can see it sort of matches this part of the circuit,

https://postimg.cc/9R2GGxj1

The top half of the board is the power amp you can see the 10 ohm, 39 ohm and 1k5 ohm resistor.   Then notice
the designators of the 1k5's not match up with the schematic.  You can also see C21 on PCB (100uF 25V) doesn't
match with C22 on the schematic.

The lower half of the board is the preamp:

R1 is 220k on the schematic and 1M on the PCB.
Next you have the 2x68ks on the PCB so some small change there.
The other parts around IC1 match-up with the schematic.

I think you get the general idea.  If you take that schematic you can kind of fumble through the PCB.

One difference is your PCB is marked JM78-2 whereas the schematic implies the PCB is JMP27A.

FWIW, the 50W models have a different transformer (larger, higher VA) and probably a higher voltage.
The TX would be likely be 70VA to 100VA.   The back panel on the amp is marked 72VA but you
can't read too much into that.

[PRR]

.... 2200 µF seems like a really tiny number for a 1.15 amp drain.... 4.356[V] for 60 Hz ...  ...should be over 10,000 µF.  .... hum modulation.

10,000uFd is a starter for Hi-Fi.

Popular-price stage amps have never run that much. 2,200uFd is reasonable for a 8 Ohm loading. 15% ripple is commercially acceptable (it won't drive-away customers).

Also even a mad shredder can't hold 1.15 Amps steady. Not even half of that for a whole second.

[pinkjimiphoton]

here's the BOM from the board i have,
hope it helps someone. still working on trying to draw up the schematic, wonky eyes suck!

5010 early version BOM
board jm78-2

vr1 gain b22k
vr2 volume al00k
vr3 treble b220k
vr4 mids b22k
vr5 bass a1m
vr6 pres b22k

ic 1 mc1458e

q1 bc184
q2 bc182
q3 bc182
q4 bc212
q5 bc184

q6 mj3001 darlington
q7 mj2501 darlington

zd1 9.1v <4739>
u
db1 br62

r= red box cap
t= tantalum
e= electro
c= ceramic

c1 47n@250 r
c2 2.2u@35 t
c3 2.2u@35 t
c4 2.2n c
c5 100n c
c6 2.2u@35 t
c7 2.2n c
c8 220n@100 r
c9 4.7n c
c10 220p c
c11 47n@250 r
c12 47n@250 r
c13 22n@400 r
c14 2.2u@35 t
c15 100u@25 e
c16 100u@25 e
c17 220n@100 r
c18 22u@25 e, axial
c19 2.2n c
c20 220p c
c21 100u@25 e, axial
c22 non existant
c23 2200u@25 e, radial
c24 2200u@25 e, radial

all resistors 5% cc  1/4 watt unless specified

r1 1m
r2 68k
r3 68k
r4 10k
r5 100r
r6 22k
r7 10k
r8 470r
r9 non existant
r10 22k
r11 68k
r12 10k
r13 33k
r14 4.7k
r15 2.7k
r16 2.7k
r17 10r
r18 6r 1 watt
r19 270r 3 watt
r20 470r
r21 10k
r22 3.9k
r23 4.7k
r24 680r
r25 10r
r26 2.2k
r27 1.5k
r28 1.5k
r29 1.5k
r30 10r
r31 39r