Building a Major

[Realeigo]

Heya

I just finished building my first tube amp, a JCM 800 2204 and it plays like a dream.
and now i want to build a second amp!
I was thinking of building a Marshall Major 200w amp because John frusciante plays one too. I read they are a bit of a troublesome amps, blowing up tubes etc, but have new freshly build majors the same kind of problems?

That aside.
I have found this schematic (http://www.drtube.com/schematics/marshall/1967u.gif) of a Major. And did some research.
And ofcourse some questions came up. So here they come.
Ill start at the power stage.
- At the rectifier. Are those caps in front of it 0,22? It's a bit blurry there.
- Still at the rectifier, wich diodes are used? and how many? (8x A10010?)
- Bias stage (or however its called..) Are those 2 diodes 10D8?, the 3k pot is that the bias potmeter?
- Where do i get 375uF caps? I cant find them in the intenet shops near me. (TAD, Verelec, Tubetown, Musikding) Or can i maybe put 2 caps parralel?
- Screen Resistors: Can i use 1k instead of 250ohm. Same problem as the 375 caps, cant find them at the internet shops. But if i do does it effect to the sound?
- Anything else i need to know?

That's it, sorry for my bad english but i hope you understand.

[Bruce / Mission Amps]

Heya
That aside.
I have found this schematic (http://www.drtube.com/schematics/marshall/1967u.gif) of a Major. And did some research.
And ofcourse some questions came up. So here they come.
Ill start at the power stage.
1. -At the rectifier. Are those caps in front of it 0,22? It's a bit blurry there.
2. -Still at the rectifier, wich diodes are used? and how many? (8x A10010?)
3. - Bias stage (or however its called..) Are those 2 diodes 10D8?, the 3k pot is that the bias potmeter?
4. - Where do i get 375uF caps? I cant find them in the intenet shops near me. (TAD, Verelec, Tubetown, Musikding) Or can i maybe put 2 caps parralel?
5. - Screen Resistors: Can i use 1k instead of 250ohm. Same problem as the 375 caps, cant find them at the internet shops. But if i do does it effect to the sound?
5. - Anything else i need to know?

That's it, sorry for my bad english but i hope you understand.

1. Yes .22uF, use plastic ones....22uF@630v
2. Use inexpensive 1N4007 diodes, with a minimum of two to three in each leg is good enough.
3. Use the same 1N4007 diodes for the bias supply.
4. Depending on your actual high voltage DC, you can use any combination of 350v caps in series to get close to this, two 375uF caps in series is under 200uF so a pair of 100uF@500v caps in parallel would give you about the same thing. Or, four 100uF 350v caps in series parallel combination would too.
Check this Ebay auction # 270158985219
Look up JEA Capacitors Ebay sales too.
5. These amps, with their huge power supply needed to supply current to four KT88s, can be very dangerous... be extremely careful with it!

[Realeigo]

1. Yes .22uF, use plastic ones....22uF@630v
2. Use inexpensive 1N4007 diodes, with a minimum of two to three in each leg is good enough.
3. Use the same 1N4007 diodes for the bias supply.
4. Depending on your actual high voltage DC, you can use any combination of 350v caps in series to get close to this, two 375uF caps in series is under 200uF so a pair of 100uF@500v caps in parallel would give you about the same thing. Or, four 100uF 350v caps in series parallel combination would too.
Check this Ebay auction # 270158985219
Look up JEA Capacitors Ebay sales too.
5. These amps, with their huge power supply needed to supply current to four KT88s, can be very dangerous... be extremely careful with it!


1: Can i use Mallory 150's? It says 630v.
2,3: Ok In parralel right? For the rectifier.
4: Where does the 0v wire go now? Because the schematic says its between the + and the - of the two caps.

And then some more questions :D
Can i use 1k5w for screen resistors?
what type is the 25k/5w resistor
Are the 270 at the phase inverter not 270k? Because in a JCM 800 theyre 1m, so 270 is pretty low.

[Realeigo]

250uf + 500uf in series is 375uf right?

If so, i could use a 500uf + 250 uf rated @500v JJ caps
:D

[tubeboy]

250uf + 500uf in series is 375uf right?

If so, i could use a 500uf + 250 uf rated @500v JJ caps
:D

Capacitors in series are solved by the formula

C = 1 / ((1/c1) + (1/c2) + (1/cN))

c= 1 / ((1/.0005) + (1/.00025))

c= 1 / 2000 + 4000

c= 167uF

[Realeigo]

My bad... 500+250 parralel in series with 500+250? Thats almost excactly 375...

[Steve Conner]

The sections of those multi-section capacitors can only be connected in parallel, not series, because they share a common negative terminal.

Electrolytics have a tolerance of -20 to +50% on the capacitance! So don't agonize over getting the value right: your carefully constructed "375uF" cap could end up anywhere between 300 and 562uF! Just replace the 375uF parts with 470uF and the 200s with 220.

For the 250 ohm screen resistors, you probably want to use 220 ohms. They are lower than usual because this is an ultra-linear amp. See how they are connected to taps on the output transformer.

On a more general note, the problem you're having is because these things were all designed before preferred component values became standardized:

http://www.radio-electronics.com/info/data/resistor/resistor_standard_values.php

Electrolytic caps usually only come in the E6 series of values because their tolerances are so loose.