Designing a single stage with a PSU
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knightstreet
- Posts: 3
- Joined: Wed Oct 08, 2025 9:19 pm
Designing a single stage with a PSU
I'm attempting to design a single preamp stage for a guitar amp. I have a JTM45 PSU and the kit to get a rectifier up and running (GZ34 based). My question is how do I step the voltage down to the required input voltage to the anode with the required smoothing for the initial stage of the amplifier. No one as far as I can tell in the literature tells you how to get a basic PRACTICAL setup with a single AC supply and a single stage for the purposes of (intelligently and safely) poking about with the circuit and making basic modifications to gain a greater understanding of the individual stages of a guitar amp. If anyone couldgive me a picture of a circuit diagram with an explanation it would be greatly helpful to me.
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knightstreet
- Posts: 3
- Joined: Wed Oct 08, 2025 9:19 pm
Re: Designing a single stage with a PSU
Thanks sluckey, I have the relevant books (Morgan Jones, Merlin Blencowe) and have seen similar diagrams.
The diagram assumes you have designed all the stages and all the currents are being drawn which accounts for the 6mA accross the 4.7k to give a 28v drop accross the first resistor and each drop can be calculated thus down the chain.
What if I just want to design and analyse the input stage (far left on the diagram) and have it supplied from 400v (post reservoir) for example but have my 343v supplying the tube at the input stage with all the subsequent filtering. Can I just place dummy load resistors to ground for each stage such that it draws the relevant current (accounting for power in the current drawn)? I just want to analyse a single tube circuit then add another and so on until I have a full understanding of each stage in isolation. It's the equivalent of building an engine for a car rather than the full car I guess with the expectation that the rest of the car will affect the performance but at least you know how the engine works....
The diagram assumes you have designed all the stages and all the currents are being drawn which accounts for the 6mA accross the 4.7k to give a 28v drop accross the first resistor and each drop can be calculated thus down the chain.
What if I just want to design and analyse the input stage (far left on the diagram) and have it supplied from 400v (post reservoir) for example but have my 343v supplying the tube at the input stage with all the subsequent filtering. Can I just place dummy load resistors to ground for each stage such that it draws the relevant current (accounting for power in the current drawn)? I just want to analyse a single tube circuit then add another and so on until I have a full understanding of each stage in isolation. It's the equivalent of building an engine for a car rather than the full car I guess with the expectation that the rest of the car will affect the performance but at least you know how the engine works....
Re: Designing a single stage with a PSU
Is your ultimate goal to have a full working amp using this power supply? If so I think using the dummy load resistors would be a good idea. Then you could replace each resistor with a tube circuit as you progess. Just be sure the dummy resistors are of adequate power handling capability and voltage rating.
Glenn
I solder better than I play.
I solder better than I play.
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knightstreet
- Posts: 3
- Joined: Wed Oct 08, 2025 9:19 pm
Re: Designing a single stage with a PSU
Yes Glenn, that's the ultimate aim. I just needed verification that this was sound thinking so thank you for answering. I'm an inherent self-doubter so any type of reply that confirms/challenges my questioins are a great help so thank you both for the input. From the diagram I expect the Power stage to be 7.6 W and 21k so nearest equivalent would be 22k 10W, is this the correct calculation? The other stages look much less power hungry and I calculated them as 371k 0.3W, 352k 0.3W and 343k 0.3W respectively. So I'd choose the nearest equivalent and 0.5W resistors should do is this in the right area?
Re: Designing a single stage with a PSU
Not having a schematic of the amp hand I can't answer about the calculations being right or wrong.
On the power handling, as a hobbiest with no engineering experience, I prefer to use resistors that are, at a MINIMUM, at least twice the expected power handling capability. I'd use 1 watt resistors and 15w or 20w for the power stage. In my experience resistors running near their rated power level can get very hot because they shed less heat than the higher rated ones. Depending on the resistors composition and quality the amount of resistance can vary, sometimes significantly, with temperature. This in conjunction with the fact I'm only building a single amp vs. manufacturing many (read $$$) just "lets me sleep better".
I'm happy to help but would like to stress that I am a hobbiest, not an engineer. While I do have a considerable amount of electronics repair experience I do defer to the highly skilled engineers that frequent this forum. Hopefully one of them will lend an opinion and either correct me or reassure us both-whatever the case may be.
On the power handling, as a hobbiest with no engineering experience, I prefer to use resistors that are, at a MINIMUM, at least twice the expected power handling capability. I'd use 1 watt resistors and 15w or 20w for the power stage. In my experience resistors running near their rated power level can get very hot because they shed less heat than the higher rated ones. Depending on the resistors composition and quality the amount of resistance can vary, sometimes significantly, with temperature. This in conjunction with the fact I'm only building a single amp vs. manufacturing many (read $$$) just "lets me sleep better".
I'm happy to help but would like to stress that I am a hobbiest, not an engineer. While I do have a considerable amount of electronics repair experience I do defer to the highly skilled engineers that frequent this forum. Hopefully one of them will lend an opinion and either correct me or reassure us both-whatever the case may be.
Glenn
I solder better than I play.
I solder better than I play.
Re: Designing a single stage with a PSU
Hello --
Good advice on increasing the resistor power ratings. Use 1W for each of the small signal stages. Recommend 25W or larger for the initial power stage and at least 5W for the screens assuming that fits in your topology.
Unclear what this project is intended to achieve. Any amp is more than the sum of parts. And even still, with AC coupling through the signal chain the stages are substantially independent and can be poked/prodded with a scope or some version a tracing signal tap if there's faith in one's ears.
Best ..
Good advice on increasing the resistor power ratings. Use 1W for each of the small signal stages. Recommend 25W or larger for the initial power stage and at least 5W for the screens assuming that fits in your topology.
Unclear what this project is intended to achieve. Any amp is more than the sum of parts. And even still, with AC coupling through the signal chain the stages are substantially independent and can be poked/prodded with a scope or some version a tracing signal tap if there's faith in one's ears.
Best ..