Building Marshall clone from scratch

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

slashfan wrote:Ok!! I still don't get 70% of the stuff written above, I'm still learning. Okay, now my doubts. Try to answer simply. Still a newbie here.
1) Are there different sockets for different types of tubes? If yes then which ones have common sockets?
2) I have a schematic diagram of a JCM 800 combo which is a 50 watter. How can I re-design the circuit to make it around 20 watts?
3) What are gain stages.
4) What function does a choke do?
5) What does the amount of gain depend upon?
6) How can I choose a proper output/matching transformer?
7) Does the number of output tube have anything to do with the amount of gain, because the SL 5C has only 1 output tube and still delivers lots of gain.
8) What dies biasing an amp do? What is cold bias?
1. Yes, download the tube data sheets to find out which you need.
2. Well you can graft on a 20w output stage onto a Marshall preamp but it might not sound like a Marshall any more because the output stage is all part of the sound.
3. Any stage that amplifies the signal, usually, but not always, each tube in the schematic, dual triodes can be 2 gain stages. (Some exceptions are cathode followers (no voltage gain) and phase inverters,)
4. Simply put it passes low frequencies and blocks higher ones dependant upon value, but there is quite a bit more to them than that.
5. Several things, the 'gm' of the tube, the type of circuit and component values.
6. You need some experience at designing tube amps, or copy an existing design.
7. No, the output tube is usually an impedance converter that takes a voltage signal from the preamp stages and boosts the available current (hence power) to drive the speaker.
8. Not easy to explain to someone who knows nothing about tube amps. Look up class A, and AB power amp theory. Biasing is necessary in push pull amps to minimise unwanted* distortion.

*As opposed to wanted distortion that you are trying to replicate.

All the above is very simplified. Designing tube amps is not something you learn from reading posts on the net.

From these questions I would suggest that if you are hell bent on building a tube amp you should start with a kit ,or at least an existing design, rather than trying to design your own low power Marshall. It will probably be way cheaper, quicker, and less frustrating because you need a certain level of knowledge of how they operate before you can even start to design your own.

I am sure other FSB members will clarify these points (and correct me if/where I am wrong).
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Post by slashfan »

Yeah by now I've realized that I didn't even scratch the surface of what I am about to do, but as I said earlier the DIY kits which cost $700-$800 will cost around $1200 + shipping which is then $200-$300. So, I am first gonna try to build the Marshall Combo for that I'll find a components list somewhere. Ok, another doubt, suppose I have two transformer one specifically made for amplifiers and one I found locally but has similar specs. Will it affect the tone if I use the local one?
And can anyone suggest a book I should read to gain more knowledge on this stuff?
Thanks for all the help guys!

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slashfan wrote:So, I am first gonna try to build the Marshall Combo for that I'll find a components list somewhere. Ok, another doubt, suppose I have two transformer one specifically made for amplifiers and one I found locally but has similar specs. Will it affect the tone if I use the local one?
And can anyone suggest a book I should read to gain more knowledge on this stuff?
I'm sure the component list will be available. Sinner (a member here) did some Marshall and Vox amp build projects a couple of years ago but his website is down. You could try and PM him and see if he has the data.

It is a bit difficult to predict if the locally sourced OP transformer will sound like a genuine one, it depends how close it is to the original.

As for books, sorry but I can't help you there, I learned all I know by modifying and fixing tube amps (and TVs, radios, etc) and asking the old tube guys I worked with at the time. I made a lot of mistakes as back then there was no internet to Google every time you had a problem.

Best of luck with whatever you decide to build.
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Post by ivan H »

Ok, personally I don't think you are ready to build an amp but to help you learn I'll answer your questions.
1) you will need 9 pin minature, or noval sockets (x4) & 8 pin octals (x2).
2)how many inputs does the jcm800 schematic show? You need the schematic of a 2204 / 2203, or master volume amp. It is modified by adding a 3rd gain stage almost identical to the 2nd stage (I described the mod earlier).
3)the gain stages I mentioned are triode stages & supporting circuitry in the pre amp. 1 triode =1 gain stage. They amplify voltage.
4) a choke (in this application) keeps fluctuations, or ripple from the power tube's anodes, or plates, out of the HT supply to the rest of the amp by offer low resistance to DC & high impedance to AC. Man, look things up if you want to learn, you could google that,,, damn.
5)the "gain" of a stage depends on how hot or cold the stage is biased, load resistance, interstate attenuation etc, in other words, a lot. But this is "design" which you don't need to do to build to a schematic which I have told you where to obtain &/or described how to build using a 2204 / 2203 schematic.
6) for a 50 watter you need a 784-139 Marshall transformer that you can buy as a spare part from Marshall, though it will be will be wound on a plastic bobbin. Get from Classictone, Heyboer or Marstran & it'll be period correct & wound on a paper former.
7) No, gain, or voltage amplification is a function of the pre amp. Power tubes amplify "current."
8) Biasing a tube sets the quiescent, or no signal operating point of the tube. The cold biased tube I mentioned is the 2nd pre amp stage in a 2204 or 2203. It has a 10k cathode resistor which biases the tube "towards cut off" rather than at the mid point of the characteristic curve as is normal aimed for. A cold biased pre amp stages "clips" the negative going half of the wave form.
You also asked how you can turn a 50 watter into a 20 watter (question 2); you could encorporate a "pentode/triode" switching arrangement into the power amp that will give you like 50/15 watt operation, though triode operation (so called) doesn't sound like pentode operation.
Hope this helps. Cheers

Edit; the "locally made" transformer you asked about later, is it the output transformer? If so, yes it will affect not only tone, but also how the power amp "feels" or reacts. Power transformers also affect how the amp "feels", or reacts, sag etc. I would use the correct transformer. I'm in Australia & usually buy through Heyboer or "mostly" Marstran & ship to Australia. You gotta pay for quality. Cheers

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Post by ivan H »

If you want a "layout" for this type of amp, google "Ceriatone AFD 35 Layout"). This will give you a complete basic "build picture". I think it also incorporates a Post Phase Inverter Master Volume (labelled & aka a PPIMV). This will let you turn a 50 watter down to apartment bedroom level (they change tone a little the more they're turned down, but work well), & when turned right up are completely out of circuit. The AFD35 also lets you footswitch (relay) between a stockish master volume circuit & their version of the SIR #36. Again, no one knows for sure what #36 was completely. I don't think even Levi remembers as he's never built another & gives conflicting info when he has answered requests. Caswell on the other hand has continued building & developing his #39 mod & been very helpful when people have asked. Posted on several forums, name timcas. Cheers

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Post by ivan H »

Just to clarify the answer to your question 3, a "gain stage" is any of the pre amp tubes that are amplifying voltage. In a jcm 800, whether it is a 4 input type (1987 or 1959) or a master volume type (2203 or 2204), the gain stages will be the first 3 triodes. The 4th triode is a cathode follower & the 5th & 6th make up the phase inverter. Sorry I didn't answer it correctly first time around, I was still drinking my 1st coffee for the day. Cheers

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Post by slashfan »

Okay.. thanks guys... the reason i ask this questions to you guys is because i google those terms which i dont understand from your posts... other than that... does cathode follower serve any other purpose other than amplifying current? Why is the grid current taken as its input... where does the output go to next... is it powered directly by the PT...can anyone tell me what a cathode bypass cap is?? is it a capacitor used in parallel with a resistor to add more gain! does it have any other function...and most importantly how will i know if i m ready to build an amp?

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Post by ivan H »

The cathode follower is acting as an impedance buffer to drive the tone stack. Check out valve wizard's site, he has a lot of good info there for you to learn from. A cathode bypass cap is a capacitor placed in parallel with a cathode resistor. It thus allows AC to bypass the resistor, getting rid of degenerative feedback (sometimes only at certain frequency's, depending on capacitor value) & increasing gain. It can be used for tone shaping in this manner, ie, marshall's 2k7/0.68uf 1st stage cathode that boosts upper mids & highs as apposed to fender's 1k5/25uf 1st stage cathode that gives a flat responce. Building from a complete layout, like the Ceriatone AFD35 I told you about requires only soldering & observation skills as it gives a complete picture of all circuitry, wiring etc. I recommended you take this route. Cheers

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Post by slashfan »

Thanks and yeah, I was also thinking about going for Ceriatone's #35 AFD but sourcing components myself rather than buying it.

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Post by ivan H »

Do a Google Images search for "Ceriatone AFD #35 50 watt layout" & you'll see what I mean. This is a very complete layout & the amp does feature a post phase inverter master volume (a dual pot labelled PPIMV) so will go from thumpingly loud down to apartment bedroom level no problem. Cheers

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Post by slashfan »

Is this the layout? I've read about the PPIMV and so. e forums say that it cuts out high frequencies amd but are also out the circuit when they are turned 'ON'. So if I'm gonna use the amp in bedroom levels will the tone change if I use the same settings but turn the master volume all the way up?
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Post by ivan H »

Hi, yes that is the layout. As to the PPIMV, no, they don't cut high frequencies. Wot they do do is progressively lessen the negative feedback as they are turned down. The presence control is in the negative feedback loop, so the more you turn the PPIMV down, the less the presence works. Obviously if the presence is turned up it makes ur tone brighter, so this is the only way it would affect highs. Also, running the PPIMV on a low setting can make the pre amp sound a little fizzy, but not badly. I have PPIMV's on the two 50 Watters I play with (I use one 4x12 & switch between the two amps with a head master switcher. One amp is a '68 spec 1987 clone & the other a 50 watt with Caswell #39/39+ mod). I use the PPIMV's to "balance" the volume of the 2 amps, depending on the venue, though its usually never below 7 on the dial. These settings don't really affect NFB, though I made it variable on the #39 amp anyway. Even using a 4x12 cabinet, the PPIMV's will turn them down to bedroom level that is useable. If there is anything you don't understand about the layout just yell out. The power transformer needed is a 1202-118. Hope this helps. Cheers

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Post by slashfan »

Hey guys, had my exams going on so got no time to respond or even work on the amp. So, I've gathered some more bunch of stupid questions (there is a new and revised edition of the layout which is a 2015 one rather than the 2014 one which is posted).
1) What does SB in the 'HT Fuse 0.5 A SB' stand for?
2) For the mains fuse do I use the 2A SLO BLO 240V for a 240V wall outlet or do O use both of the mains fuse?
3) What does OT-CT in the Output Transformer mean? (Is it Centre Tap)
4) What are bias test jacks?
5) Where are the terminals of the OT secondaries needed to connect the impedance selector?
6) Just underneath the filament indicator there is a component named 7805, what is it?
7) About shielding it is written here 'Hot Shield = 1 inch(with glue wire) what does that mean?
8) What gauge of wire do I use for wiring?
Thanks, Cheers!!!

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Post by deltafred »

Without seeing the schematic these are the things I can answer.

1. SL - Slow Blow
3. CT - Centre Tap
6. 7805 is a 5v 1A regulator
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Post by ivan H »

Ok, I'll try to answer in order. 1) SB 0.5A = 500mA slow blow fuse. (2) for the mains fuse you use a 2A fast blow fuse if your wall supply is 240VAC (or 220). If its like 110 or 120 VAC you need a 3A fuse. This is the fuse for the incoming AC supply voltage. The HT (high tension, aka B+) fuse is the 500mA slow blow & is the DC voltage fuse. It come directly after the rectifier & needs to be a slow blow type as the filter cap's inrush current (as they charge) will cause a fast blow type to blow. (3) yes, OT CT = output transformer's primary winding center tap. (4) the bias test point are the banana plug sockets connected to the power tube cathodes & one connected to ground. They facilitate external reading of the voltage drop across the 1 ohm bias sensing resistors, the reading in mV corresponding to that tubes total quiescent current draw. For what its worth I never fit these to my builds & consider it an inaccurate method. (5)follow the layout. (6) the 7805 regulator keeps voltage to 5VDC. The unused 5VAC winding on the PT (for tube rectifier filament) is made use of to derive a supply the channel switching relay. (7) ok, Hot Shield. This is a piece of shielded wire (coax) where the outter braid is connected to the anode pin of the tube socket (1st triode). Using the coax's capacitance, this is a form of negative feedback at higher frequencies to help control squeal & also a bit of tone shaping, giving a cocked wah effect. Where is says glue it, I use double walled heatshrink to insulate the outter braid from the chassis, then a dab of hot glue to sucure. I start with the outter braid much longer & trim it using trial & more trial to find the right lenght. I use Belden RG174 shielded cable for this as its DC rated a good bit higher than the anode voltage here. Cheers

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Post by slashfan »

Thanks for the replies.
1) But I can't seem the find the OT secondaries needed for connecting the impedance selector.
2) What type of calacitor do I use in the turret board? (electrolyic or the normal ones)
3) Do I need to buy the things which hold the tubes in place? (not talking about the sockets)
4) If I buy the resistors, capacitors and all other things from a local store will it's quality alter the sound?
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Post by slashfan »

Oh yeah one more thing. If I don't use the bias test jacks then do I just connect the terminals 8 of the tubes together?

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Post by deltafred »

slashfan wrote:1) But I can't seem the find the OT secondaries needed for connecting the impedance selector.
2) What type of calacitor do I use in the turret board? (electrolyic or the normal ones)
3) Do I need to buy the things which hold the tubes in place? (not talking about the sockets)
4) If I buy the resistors, capacitors and all other things from a local store will it's quality alter the sound?
1. If you buy the correct output transformer for this project (Plexi 50 OT) then it will have colour coded leads that match those of the diagram you have posted. If you source one locally then you are on your own.

2. The capacitors will be specified in the BOM *Bill Of Materials) for this amp, find that and stick as close to it as possible (see below)..

3. If the "things which hold the tubes in place" are spring retainers then no you don't need them unless you are going to move the amp about a lot (gigging and touring). If it is going to sit in one place then tube retainers are not required. If they are screening cans then you would be advised to use them to keep hum, oscillations and interference to a minimum.

4. Get the BOM and see what is specified. If you cannot buy the parts of the necessary spec or greater (type/wattage/voltage rating etc) locally then I would suggest that you get them mail order*. If 1/2 watt resistors are specified and you can get 1/4w or 1w then go for the 1w. Same with capacitors, go for a higher voltage rating rather than a lower one.

Why would you want to miss the bias test points off for the sake of 2 resistors and 2 sockets? They are very useful to have. If you do miss them off connect terminals 8 to ground.

* When I was first starting out building effects and amps I got hold of a manufacturers datasheet for a 100w solid state amp. At the time everyone was using tube amps and SS was the new thing so I decided to build one. I bought everything locally and a lot of parts were substitutions (back in the 1960s so no internet).

I spent weeks building this thing and set it up to the instructions in the datasheet and everything seemed ok. I connect suitable speaker cabinets to it and switched on, still ok. I plugged a guitar into It and cautiously increased the volume. It lasted about 10 seconds before it died.

I had the correct value components but the spec was sadly lacking on a lot of them, particularly the output transistors and their heatsinks.

I would suggest that you look at every tube amp kit you can find and see what parts they use before blindly substituting what you can buy locally. Ask here if you are uncertain.
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Post by slashfan »

Thank you, I learned a lot with the help of you guys and also a bit of researching from my side. Talking about this BOM it's the only amp model whose website page doesn't have a working BOM link. I'll still try to find one. If possible can you expand of the part of Bias Test Jack and tell me what's going on in there?
Again Thanks!! Cheers...

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Post by deltafred »

The BOM for any 50w amp with the same output tubes should be very similar, there are usually only minor differences in component specs.

The bias jack is for setting the bias voltage (to give the required standing current to eliminate crossover distortion), This sets the standing current at (with no input signal) and defines whether the amp is biased hot or cold. Get it too hot and the output tubes will 'red plate' and be damaged in a very short time, too cold and it will start to get crossover distortion.

Measuring the voltage across a 1 ohm resistor will give you 1 volt per amp, so if you want a 20mA quiescent current you put a voltmeter on the test points and adjust the bias voltage until it reads 20mV. (If the valves are mismatched you will probably get 2 different voltages.)

An alternative method is to make up a tube plug and socket with all pins directly connected except pin 8 which has a 1 ohm resistor in series with flying leads which you connect to your voltmeter. Much easier to include them at build time.
Politics is the art of so plucking the goose as to obtain the most feathers with the least squawking. - R.G. 2011
Jeez, she's an ugly bastard, she makes my socks hurt. I hope it's no ones missus here. - Ice-9 2012

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