Welcome, Guest. Please login or register.
Did you miss your activation email?

February 17, 2020, 08:03:56 PM

Login with username, password and session length

Recent Posts

Author Topic: Rebuilding Marshal Amp  (Read 14305 times)

Roly

  • Legendary
  • ******
  • Posts: 2184
  • Chip Points: 288
    • View Profile
    • Australian Valve Amps
Re: Rebuilding Marshal Amp
« Reply #15 on: April 04, 2015, 02:08:32 AM »
As a design, I can't see how you can go wrong with the Alembic/Fender Dual Showman circuit JMF posted above.   You know this works.  :dbtu:


A note about the power supply.

Your 60V trannny should still be quite usable with this doubler circuit, viz;

60VRMS

60 * 1.414 = 84.84 volts peak.

A lightly loaded multiplier like this outputs n-times Vpk, so this doubler will be 2 times Vpk.

84.84 * 2 = 169.68 volts

170V is not a lot but is reasonably respectable for this preamp, however to give its best I think it needs a bit more voltage, so I'd consider extending the multiplier by a stage to a tripler and you will get around;

84.84 * 3 = 254.52 volts

Around 250 which would be closer to the original.


HTH
If you say theory and practice don't agree you haven't applied enough theory.

CraftyZA

  • Regular
  • **
  • Posts: 13
  • Chip Points: 0
    • View Profile
Re: Rebuilding Marshal Amp
« Reply #16 on: April 04, 2015, 07:49:01 AM »
Thanks. Will do that tripler. Just a question. In south africa we have between 240 and 250v mains. Mine is sitting at about 251. So i need to aim for almost 1:1 ratio.


Sent from my iPad using Tapatalk

Roly

  • Legendary
  • ******
  • Posts: 2184
  • Chip Points: 288
    • View Profile
    • Australian Valve Amps
Re: Rebuilding Marshal Amp
« Reply #17 on: April 04, 2015, 08:19:54 AM »
Thanks. Will do that tripler. Just a question. In south africa we have between 240 and 250v mains. Mine is sitting at about 251. So i need to aim for almost 1:1 ratio.


We also have a 240/250V 50Hz system here in Australia.

But the multiplier runs from the nominal 60VAC coming out of your transformer secondary (a transformer you already have?).

If it has a suitable primary for your mains, you have no problem (apart from remembering to connect the safety ground to the transformer, chassis, &c).

Mains trannies are hardly ever specified by turns ratio, but by the RMS voltage on the primary and secondaries at rated load current.

An unregulated supply such as this is perfectly suitable for the application.  Leo didn't use regulators.   ;)
If you say theory and practice don't agree you haven't applied enough theory.

g1

  • Legendary
  • ******
  • Posts: 629
  • Chip Points: 73
    • View Profile
Re: Rebuilding Marshal Amp
« Reply #18 on: April 04, 2015, 02:59:50 PM »
But the multiplier runs from the nominal 60VAC coming out of your transformer secondary (a transformer you already have?).

If you have the existing transformer, just use it as JMF mentioned.  It has a 200VAC winding for the tube HV, as well as a heater winding.

Schematic attached:
« Last Edit: April 04, 2015, 03:01:07 PM by g1 »

Roly

  • Legendary
  • ******
  • Posts: 2184
  • Chip Points: 288
    • View Profile
    • Australian Valve Amps
Re: Rebuilding Marshal Amp
« Reply #19 on: April 05, 2015, 03:00:41 AM »
Certainly if you already have a tranny with a suitable high voltage secondary use that for a simpler rectifier/filter.

A 200VAC tranny will give you a peak DC voltage of;

200 * 1.414 = 282.8V  which will load down a bit - near enough.   :dbtu:
If you say theory and practice don't agree you haven't applied enough theory.

CraftyZA

  • Regular
  • **
  • Posts: 13
  • Chip Points: 0
    • View Profile
Re: Rebuilding Marshal Amp
« Reply #20 on: April 08, 2015, 08:06:38 AM »
But the multiplier runs from the nominal 60VAC coming out of your transformer secondary (a transformer you already have?).

If you have the existing transformer, just use it as JMF mentioned.  It has a 200VAC winding for the tube HV, as well as a heater winding.

Schematic attached:

Personally I prefer not to ground power supply to ground. Unless it's really necessary. It introduces all sorts of noise from the fridge and aircons from other rooms. The biggest culprit is a fridge. I've added a snubber to the plug on the fridge and that reduced noise by halve! It generates vast amounts of noise which becomes audible when grounding on the same plane where you central tap your power supply and audio grounds.
In my SRPP I have 2 grounding planes, linked with ceramic cap, and 2 diodes in opposite directions. Between the snubber on the fridge and this, very little interference gets through. Actually All I hear is valve humm i think.
 

Roly

  • Legendary
  • ******
  • Posts: 2184
  • Chip Points: 288
    • View Profile
    • Australian Valve Amps
Re: Rebuilding Marshal Amp
« Reply #21 on: April 08, 2015, 10:37:37 AM »
I'm sorry but I'm confused.

I don't see how we made the leap from high voltage supply transformer to supply safety grounding, some assumption in there that I'm missing.  Also given that almost every amplifier I deal with has a solid safety ground and no problems with mains-borne noise I'm wondering what is happening with the mains supply at your place.

"Ground lifting" with a resistor or anti-parallel diodes can certainly cure or reduce noise from some sources such as earth loops (grounded stomp supply?), but the problem is that you cure your sound problem at the increased risk of winding up dead if the ground lift fails, and I don't think that is a good trade off.

The normal (and highly effective) cure for mains-borne fridge pops and drill whine is a solid safety earth and a line filter (say recovered from a dead computer power supply).
If you say theory and practice don't agree you haven't applied enough theory.

CraftyZA

  • Regular
  • **
  • Posts: 13
  • Chip Points: 0
    • View Profile
Re: Rebuilding Marshal Amp
« Reply #22 on: April 13, 2015, 09:23:56 AM »
@J M Fahey, You don't by any chance have a pcb layout for that alembic? I've ordered the parts, And the Etching bath is ready. Or is it better to solder the components directly to each other?
Alternativly I can just do it on some G10, or even masonite board and use wires like the diagram I posted.

J M Fahey

  • SSGuitar Global Mod
  • Legendary
  • ****
  • Posts: 4142
  • Chip Points: 429
    • View Profile
Re: Rebuilding Marshal Amp
« Reply #23 on: April 14, 2015, 01:23:08 AM »
Sorry, never designed a PCB for it.
For one-off tube stuff I always use a piece of Pertinax (brown phenolic/paper board)  and plated eyelets.
Very fast to draw and make.
Similar to old Fenders.

Here's one way to make them:
http://www.forrestwhitesides.com/node/74

If you live in a relatively small city, order eyelets and raw boards as suggested above; I live in a larger city and get my Pertinax at some Industrial Electicity or Insulators supplier shop, it's standard stuff to custom mount swirtches, fuse holders, etc. , and get the eyelets for peanuts at some Shoe supplyhouse.

You may use a sheet of Formica for the board and get some assorted eyelets at the dollar store


Of course, avoid the aluminum ones which can not be soldered.

Best all around size is 1/8" or 3 to 3.5 mm .

CraftyZA

  • Regular
  • **
  • Posts: 13
  • Chip Points: 0
    • View Profile
Re: Rebuilding Marshal Amp
« Reply #24 on: April 14, 2015, 09:52:44 AM »
I stay in Centurion. Pretty sure I can source something like that.
Looking at the diagram in a bit more detail I discovered there is no R6, and C6 is used twice :-)

C6 is not a problem. I just labeled one C7 which seems to missing. So just a mislable issue
Am I missing something with R6?

CraftyZA

  • Regular
  • **
  • Posts: 13
  • Chip Points: 0
    • View Profile
Re: Rebuilding Marshal Amp
« Reply #25 on: May 06, 2015, 07:25:19 AM »
What should the voltage be on on the rest of the caps?
They only show the one on the b+ rail which is 350v

My components was delivered, but supplier gave little tiny ceramic caps for everything, and not the polly types. They are not waisted at least, i can use them on other projects as filters over diodes etc...
 I need to re-order the caps, but need to double check the voltages. The polorized caps in die diagram is obviously electrolytic alu caps. Trying to find radial versins for them. The main concern is over all those poly caps.

 

Roly

  • Legendary
  • ******
  • Posts: 2184
  • Chip Points: 288
    • View Profile
    • Australian Valve Amps
Re: Rebuilding Marshal Amp
« Reply #26 on: May 07, 2015, 03:28:21 PM »
Quote from: CraftyZA
supplier gave little tiny ceramic caps for everything

The less you specifiy, the more they will give you the cheapest thing to hand.  You really have to find your suppliers' specific part numbers for the caps you want (or spell it out on the order e.g. "0.1uF 100V polyester" - but it's less reliable because it depends on the parts picker knowing the difference between a poly and a poke in the eye, and they tend to mindlessly go by part numbers).

Quote from: CraftyZA
They only show the one on the b+ rail which is 350v

You need to be specific which circuit you are talking about.

It's a fair bet that the others will also need to be the same voltage rating because they will rise to the unloaded power supply voltage before the valves warm up, draw current, and pull the voltages down a bit.
If you say theory and practice don't agree you haven't applied enough theory.

CraftyZA

  • Regular
  • **
  • Posts: 13
  • Chip Points: 0
    • View Profile
Re: Rebuilding Marshal Amp
« Reply #27 on: May 08, 2015, 02:00:38 AM »
This Alemic circuit that was posted a few weeks ago.

I still need the parts the build the psu, and voltage doubler. Will get to that end of the month

Roly

  • Legendary
  • ******
  • Posts: 2184
  • Chip Points: 288
    • View Profile
    • Australian Valve Amps
Re: Rebuilding Marshal Amp
« Reply #28 on: May 08, 2015, 04:02:04 AM »
Quote from: CraftyZA
This Alemic circuit

Ah.  That actually says 300V not 350V which is why I was wondering.


Points arising from f2b.png;

- as the loaded supply is around 300V then the caps that are connected to this rail all need to be rated at 350V or better if they are to survive the peak HT voltage before the valves warm up.

- the anode loads R4 and R8 do not need to be anything like 2 watt rating, half-watt is more than sufficient.

- the tonestack is a classic Fender with identical values, so the mid control should be 10k linear not 250k as shown.  250k would pretty much disable the treble and bass controls.
If you say theory and practice don't agree you haven't applied enough theory.