Building Marshall clone from scratch

Tube or solid-state, this section goes to eleven!
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ivan H
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Post by ivan H »

Hi, no matter where you get your OT, the wiring schematic & color code will be available for it, do your research into this before ordering it. You can see on the layout which capacitor are electrolytics & which aren't. I wouldn't use the bias test points or 1ohm bias sensing resistors, it is inaccurate. The power tube cathodes (& suppressor grids) tie to ground, directly at the power tube socket (a mounting bolt). When it comes time to bias power tubes, use this method. With amp off & drained, measure & note the DC resistance of each half of the OT primary (CT to each plate pin on tube sockets). Now, with bias supply set to the largest negative voltage (to start with, adjust as needed), power the amp up & let stabilize for 10 minutes, the measure & note the DC voltage drop across each half of the OT primary (CT to plate pins). Now divide the voltage drop of each half by the previously measured resistance of the corresponding half. This is a much more accurate method as you are reading the voltage drop across a much larger resistance & it is reading plate current only, not plate & screen current. No you do not need power tube retainers. Use resistors rated for 350VDC. Yes signal cap "type" does affect tone. A good scource for components is valvestorm.com. Robert is a very helpful guy, ships internationally & has a good range of stock for builds like this. Hope this helps. Cheers

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deltafred
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Post by deltafred »

Thanks Ivan H. I stand corrected on the biasing,
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ivan H
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Post by ivan H »

Hi, using one ohm biasing resistors is ok. For people new to servicing their amp they have the advantage of actually biasing the tubes a little cooler than most people think they really are due to the inclusion of screen current. That is assuming the sensing resistors are all matched to one ohm. I always check screen current anyway, which is good practise (same method, voltage drop across screen resistors). I've been using the St Petersberg Winged C EL34's in my 2 gigging amps for years (& still have a few new sets),& their screen current is usually around 8mA per tube, which gives an idea of how much cooler a tube could be biased. The reality is though, that we should bias for tone, not to a % of plate dissipation. I like to set bias to 50%, note bias voltage, set bias again to 70%, note bias voltage, then find the sweet spot between, (its often in the high 60's % area, like 66~68%, but not always). If you can get good tone & responce at a cooler bias the tubes aren't working as hard & will last longer. Cheers

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slashfan
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Post by slashfan »

Okay, thank you for explaining, helped me a lot. I have some more doubts
1) What does BP (written in BIASING potentiometer or whatever it is) mean?
2) Can I use the highest watt of resister used in the whole circuit rather than buy different ones?
3) Can anyone explain to me the different connections in the OT?
4) I recently met a person who knew another person who can make a custom transformer so I'm thinking to get him to make those transformer (only my opinion need more opinions on this) because the total cost of the three transformers goes upto $243 which is too expensive. So I need some info on what the materials used are like the material used to wound the primary and secondaries etc.

Thank You. Cheers!!!

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slashfan
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Post by slashfan »

Can I get some replies?
One last question. In the layout, the 3rd terminals of the output tubes have 'OT An' written on it. I figured out that OT might be referred to as Output Transformer but can't seem to figure out the other word.
Thanks!!!

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lead2203
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Post by lead2203 »

slashfan wrote:Can I get some replies?
One last question. In the layout, the 3rd terminals of the output tubes have 'OT An' written on it. I figured out that OT might be referred to as Output Transformer but can't seem to figure out the other word.
Thanks!!!
You would really be better off buying a kit. There are some good ones out there. Just look around online for some. You can find cheap ones ..and those can come out sounding very good. There is a Marshall jtm 45 kit from Ampmart for around $350 from China...it's not bad for the price. You shouldn't start with a hi-gain amp for your first build. Build this stock first to get a better understanding of a basic Marshall style amplifier..then start modding it with the knowledge you have gained.You just have too many questions..that you can find the answers for yourself online...Google the questions and you will find answers. There are also much better forums out there then this for amp questions and building.

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