Marshall JTM 45 Botched Clone Kit Let's Take a Look
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- Опубликовано: 10 окт 2023
- On the bench, we have Marshall JTM 45 clone kit. It's been badly assembled and the transformers look a bit suspect. We delve in to this mess of wiring to see if we can make a decent guitar amplifier head.
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I think the JTM45/5F6 is one of the simplest. It's entertaining to see how well people absolutely mess it up.
Very interesting tube selection. The hottest octal AF preamp tube is along the lines of a 5751. It'll be a fairly mild amp if you use the standard schematic resistor values for 12AX7's. Might have to up the plate resistors to 150k/220k and lower the cathode resistors to 680/510/470 or something to get any "Marshall" grind out of it. I don't mess around with octals a whole lot, some experimentation may be required.
That black turret board looks like a JTM45/5F6 circuit to me. The bias supply section layout is just turned sideways and the 10uf preamp filter was moved to that side of the board. Looks like one of those Ebay prebuilt thingies to me. Probably not a terrible way to go for beginners or if you're in a rush.
That exposed winding on the power transformer is just asking for some child(or adult child) to stick their hand in it. Magnet wire lacquer is very easy to get through. It looks simple enough to form a thin piece of aluminum/brass as a snug fitting cover.
Hello Matt, thank you. you are spot on with the cathode resitor on the 6SL7. The last 6SL7 amp I did I used 680R. what do you think to this long tail pair schematic in the link below? Bottom of the page.
www.aikenamps.com/index.php/designing-long-tail-pairs-the-load-line-approach
This is why although I have been tempted to, I have never built a valve amp kit. The bodge was strong in this one!
Hello Chris. Yes, it's a bit of mess is this one. I should be getting a round to sorting it out later in the year. Just been on you channel, looks good. Thanks for watching and take care.
I worked at a company that made ridiculously high-end audio analyzers. Metal oxides were used throughout the signal path because they allowed the signal to pass through without added noise, distortion, temperature fluctuations over time, etc. The overall effects of different resistors in an amp circuit are negligible. All you get from carbon film or comp is a slightly higher hiss level.
Hello Nick. That's very interesting. What a great job to have. I agree, about the carbon film and comps. The hype around Carbon Comps is plain daft. Thanks for watching and take care.
The nicer populated board looks very similar to a board they sell on eBay. That would definitely cut down on build time in making a Marshall clone...but now what fun would that be. Actually I wouldn't be afraid to use the one board in an amp build. I'm sure you will do this amp justice. Always nice to end my work week with watching your videos- keep up the good work, cheers 🎉
Hello Ron, thank you. Bit of a mess this one. But iam sure we can make something out of it. JTM18. Take care Ron
Hi my JTM45 that I built has GZ34 ECC83s/12AX7 and KT66 and the choke is 20H and if I recall the B+ was 435v the spare board you have looks like a JTM 45 I hope this helps you decide where you want to go with this chassis.
Hello John, thank you. That's good info. Thanks for watching and take care.
Too good love your work
Hello, thank you. Thanks for watching and take care.
There's a challenge Martyn.. I bet you're looking forward to getting to grips with it and the end result will no doubt be very interesting.
Hello Pete. Yes, it should be a good one. Thanks for watching and take care.
the pots im talking about its a standard little pot witha dust cover
I had seen them before when I thought about. They have them on some of the Selmer amps. Sometimes when filming you don't think the same. Take care.
You could use a 110v primary and with a 6 volt secondary transformer for a bias should go down that route just connect the 6 volt winding to the heater on the main transformer and you will get 110 volts + 1.414 for the bias.
Hello, some good ideas there. As these transformers will only manage 6V6 tubes on the output, I think I will wire this cathode bias. Thanks for supprting the channel. Take care.
The blue resistors look like metal oxide. I use them for high-power applications that have high wattage. I prefer carbon comp or carbon film for signal path.
Hello. They are. I will be removing them and replacing with Carbon Film. You are right Metal Oxide are not good for signal. Thanks for watching and take care.
The author of the book _Designing Tube Preamps for Guitar and Bass_ says "Metal oxide resistors are similar to metal film, but are usually avoided for audio because they can cause low-level distortion and are fairly noisy. They may still be found in the power supply however."
However, various contributors on the Antique Radios Forums, on the forum topic _Metal oxide vs Metal film resistors_ would seem to agree with the above.
Hello Mark. The book by Merlin Blencowe. I have that book. I knew I had read that somewhere. I will have a read on the forum too. Thanks for info. Take care Mark.
I'm thinking that output transformer with its 8K primary could have been from a higher-voltage EL34 amp.
The power requirements for those circuits are more work/expense, requiring either a special power transformer having separate high-voltage windings for plates vs. screens, or diode trickery to put half the plate voltage to the screen grids (ala Musicman, or my own Sovtek Mig 60).
Going with 6V6s is a good move here. It just means this output transformer is larger than strictly necessary but since it's already there, why not.
Hello Chris. It's the best way to go I think. The power transformer won't run 6l6 tubes with that little voltage. I gleaned a bit more info on this amp. It was never fired up. It was an on going project. It's obvious that it was never going to fire up. Like you say, An output transformer suitable for 6L6 tubes, a power transformer suitable for 6V6 tubes. The choke; well who knows where that came from. Also I never checked to see if that output transformer was ultra linear. It could be off a PA or even a HIFI amp. Thanks for watching and take care.
"Eugen" Roth, got some hits for a German TubeAmp site, but no schema.
Hello Brian. I couldn't find anything when I checked online. Looks like it will remain a mystery. Take care Brian.
Could you just turn it into an EL34, 50 watt plexi?
Hello Zack. The power transformer just isn't up to the task for EL34s either. They draw a bit more current than the 5881 tubes. It's 6V6 tubes for this one. Still I reckon it will still sound good, and smaller amps seem more popular these days. Take care Zack.
Looks like a JTM45 board I reckon.The power transformer looks like some russian types I`ve seen,will be an interesting project I`m sure.
I have to make that transformer safe before this can go on the road. Speak soon. Bit tied up at the moment with Mum. Take care.
KT66, yes you are right
Hello, thanks for confirming that. Take care.
Those are super tech caps. The black ones with the rabbit logo. I've used them. Supposedly a good.brand
Hello, thank you good info there. I will have a search on the net for some more info on those. Thanks for watching and take care.
Here's where I got mine.
this must be one of those cheap Amazon Chinese kits. such cheap parts , metal film resistors and cheap caps . man this is cheap alpha pedal pots and that wire looks el cheapo. cleaned up it will probably sound alright but how long will it last. that second board has all metal oxide resistors (1watts in the preamp section). might sound crisp!
Hello. The transformers don't belong to the kit. They look quite old. You are probably right about the rest. The second board is good but the resistors are rubbish. I think they are .5 watt as well in the pre-amp section as well being metal oxide. I will be using the second board and will be stripping all the resistors and replacing them. The sockets look cheap too so they will be going. It's a good point you make here and I will cover that in part two. I try to open things up for discussion on the channel and I welcome input from people on here. On here, there are some amps that I have repaired, and there some amps that we have repaired. Thank you and take care.
Geez, I reckon I could have done a far better job building that amp then that person did.
Hello Simon, yes a real mess. Lets hope it isn't that persons day job :-) Take care.