NOTE: I can make no guarantees for the accuracy of following component values and ratings – it is a guide ONLY for those Marshall tech fans and experts to cross reference if they wish, from what I have seen in this Marshall alone. I have no prior knowledge or experience on this Marshall or any other model or age, except my 1972 Mercury 2060.
It looks like all original parts, but I have no way to verify this – it’s just a semi-educated guess from looking at solder joints and general condition etc. The current owner has had it for 30 years or more, so is likely original and unchanged. He bought it 2nd hand in 1978.
If you use this guide, check the values and ratings with a qualified technician against other schematics, if you are unsure what you are doing in attempting to re-create this circuit. This is a high voltage circuit – 400-600V – from looking at some schematics, so prior knowledge, care and attention is required more so than many other amp projects – a usual BE CARFUL!
I am logging this for historical interest and to try to clone this amp with modern high quality, quieter components as a project, while I have access to it.
I thought I would put up some better pics of the chassis interior for those who are logging history for these amps – sites such as:
This site was very helpful for me to get an idea what model I’m dealing with here. I think there was one question this site had about whether the
Version 5C: The very first JMP100s (Mid-Late 1967) model – which I assume this is – from this sites serial number range info – was made with a steel chassis, not aluminium – well this one is steel, and here is the rust:
White spirit got rid of the rust snow, and other grime, and gave the oxidised aluminium appearance to the transformer cases. I can now read the yellow caps values too – 50microFarads X 2 in each can.
The blue caps are 2 X 32microFarads per can. I think the Yellows are mains filters, strapped in series so they can handle the larger 400V-500V voltages the 100W required to get the power out. From these pics and my component list, you should be able to clone this amp, if you can source 100W transformers etc. I am going to try this at some point, as it should all fit in the same www.ampmaker.com chassis as the Maggie366 build.
You can see the rectifier diode unit on the left of the transformer above.
In conjunction with this circuit diagram and/or others, I will try to identify any component values that are different, but should still work with the parts list I made from this amp.
Just for my ease for writing I’m going to follow the picture list the way I drew it today – sorry if it’s a bit unclear – it was just for me to identify parts values, but with a schematic and the parts list you should be able to cross check and work out where things go. I will have to. I counted 45 board components alone:
- Diode – 1008 (equivalent?) number on it is maybe 19C01?
- 10uF, 650V?, electrolytic
- Bias Trim Pot, 27k?
- 10uF, 650V?, electrolytic
- 8k2 – These 2 8k2s are joined in series to make a 16k4 in place of the 20k/1W in the Unicord diagram. They are 3W rated also, in this amp
- 0.022uF (650V)
- 0.022uF, 400V
- 680pF, 160V
- 2200pF, 400V
- 560pF, 400V
- 0.022uF, 400V
- 0.68uF, 160V
- 320uF?, Electro, 160V?
Here is the pic I wrote on – some bits are hidden behind others – listed here:
27 – 560pF
28 – 100k
31 – 820R
44 – 8uF electro?
With this list, a schematic and the pics, I reckon it should be possible to mount the same as on the board, with about 50 turret connectors.
The rest of the OFF board chassis components you need are:
- 4 X 1k W21 – wire wound?
- 2 X 47k
- 2 X 1k5 screens of Valve 5 and 7 pins 5.
- 2 X mains toggle switches
- 2 X fuse holders and fuses – 1A DC and 2A mains
- 1 X 0.1uF, 400V
- 8 X 50uF (4 X 2 stacked Yellow can equivalents)
- 4 X 32uF (2 X 2 stacked Blue can equivalents)
- 1 X 20-80 Henry?? Choke – see site above or other schematics for other choke values used (1H?)
- 1 X 100W OT (centre-tapped primary; 16,8,4 ohm secondary) – suitable replacement for MAKE – Drake?
- 1 X 100W mains OT (1 primary, 3 X secondary’s)– suitable replacement for MAKE (see site above re Drake transformer info)
- 6 X switched input jacks ( 2 channels of Hi and Lo inputs = 4 , plus 2 x switched 4,8,16 0hm speaker outs)
- 1 X 0.005pF full bright volume cap
- 2 X 220V neons or 6.3V heater bulbs (may have to V divide a neon for 400-600V secondary?)
- 1 X 100k
- 4 X EL34 tubes
- 3 X 12AX7 tubes
- 3 X 1M pots – bass, vol1, vol2
- 1 X 27K middle; 1 X 500k treble, 1 X 5k9 presence
- 4 X rectifier diodes – 100W, 1A current capable – A10D10 equivalents = 1N4007 = 1A/1000V
- 2 X 1M (input channels)
Swapping 6L6 and EL34 Tubes …NOT So Fast There!
by Lane Sparber » Fri Feb 24, 2012 2:57 am
A LOT of people seem to want to swap out their 6L6GB tubes for EL34s and vice versa, and rarely if ever is any consideration given to whether or not the amp’s design topology will support this swap. I usually just recommend going with what the amplifier manufacturer specified for any given amp, but if you MUST experiment in this arena, here is a brief overview of the important points to consider:
Firstly (and most importantly), there’s heater current draw to worry about. A typical EL34 draws 1.6 amps per tube or so. A 6L6 draws just .9 amps per tube, so in this case, this swap from EL34 to 6L6 will put LESS stress on the amp’s heater winding/power transformer, which in turn causes less voltage sag. Going the OTHER way, however, COULD draw too much current from a power transformer not designed to source it and in a VERY shout time you’ll wind up cooking your PT. Rule of thumb: if you put EL34s in a 6L6 amp and the loaded heater voltage sags more than half a volt or so below what it was reading with the native 6L6 tubes, then I’d consider this swap unsafe.
There’s also a slight difference in the maximum voltage ratings on the plates of these respective tube types. For example, a standard EL34 is rated for 800vdc on the plates and 450-500 on the screens (depending on the manufacturer), while a 6L6 is rated for just 500 on the plates with the same screen ratings. Thus, if one were to safely consider going from an EL34 tube to a 6L6 tube, a quick check of the loaded plate voltage would be in order first to make sure that you don’t exceed the design specifications of the tube in question.
Next up, there are the screen resistors to think of. 1k resistors are most often used for EL34s, and 470R for 6L6 tubes. 1k WILL work for both tube types, but it might compress the sound too much for your liking on a 6L6. If I’m changing types on a client’s amp, I will almost always change out these resistors to the “proper” value for the tube type I’m installing.
Finally, there is the whole suppressor grid issue, which only applies to a VERY few guitar amps. A quick comparison of the pinouts for a 6L6 tube vs. an EL34 tube will show you that the suppressor grid in a 6L6 is TIED INTERNALLY to the cathode at pin 8. On an EL34, the supressor grid has it’s own pin (Pin1). So, if your amp was designed for EL34s and has a separate wire going to pin 1 of the tube socket, that means that you can’t just swap tube types without determining via the schematic if it’s safe. If pin 1 of each socket is unused, then simply tie it to pin 8 and you can then use either tube.
I hope this helps and explains a few things for you!
Here’s the actual component values in red, that are in this amp – I have left the original Unicord schematic values as a comparison: