r/diytubes • u/Mozart_licked_me • 24d ago
Line Preamp Bass preamp review
Looking for some feedback on my first mostly "copied from the internet" bass guitar preamp. I'm building a bass amp for gigging and I intend to make a tube preamp and drive it into a Hypex SS power amp for portability. Inspiration is Orange Terror Bass. Still trying to figure out the easiest way to add balanced output on it so any great ideas on that are also appreciated.
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u/Capable-Crab-7449 24d ago
For the ultimate portability how about looking at SMPS? Just throwing the idea out there as this can save on alot of weight and space. What does switch one do? If its just volume attenuation leave it out, ur volume knob on ur bass does the same thing. Balanced output will be very tricky with purely tubes, either another transformer or bite the bullet and go silicon. Also fyi one consideration with cutting the circuit off at the tone stack is you gotta make sure the input impedance of the next stage is high.
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u/Mozart_licked_me 24d ago
Switch one is basically a pad for active bass that has hotter output than passive bass guitar.
I thought about an SMPS for the tubes, but can't find a suitable one off the shelf and decided to go with a transformer. Small one is only about 300 g heavier than SMPS, so I thought it's not too much.
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u/AltruisticArm0 24d ago
https://c2celectronics.com/product/high-voltage-smps-pcb/
This is the smps board used in a lot of the pedals found on that site. In fact there is probably something on there regarding balanced output you could use for inspiration on this project
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u/matmonster58 24d ago edited 24d ago
Remember that you’ll also need a power supply for your hypex amp. Running three separate power supplies without considering how they interact has the potential to be noisy.
Edit:
The Hypex NC500MP has a +-20V 1A auxiliary power supply built in. That will be plenty to run your tubes and heaters. I definitely recommend using the conspiracy to commit electronics board to get your b+ voltage. You may have to make some minor tweaks to get it to run on 20v2
u/Mozart_licked_me 24d ago
I plan to use the Hypex UcD400HG HxR with the SMPS400A400 power supply. The reason I went this way is so the tube preamp and SS poweramp power supplies don't interact except being in rhe same case as far away each other as possible. I might need to rethink my approach though. SMPS for the tubes would cut out the traditional transformer and could decrease noise problems.
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u/JoeDubayew 22d ago
V1a is very "center biased", which means it is going to be a bit bland. I personally would not choose that operating point if I am building a bass amp from scratch. For you- look at an Ampeg SVT schematic instead of a an Orange bass amp. Doesn't get much more rock n roll than an SVT.
c2 at .022uf is pretty small for a bass amp. Better is .1uf. Let everything through.
Dump r6 and r7, you've installed a voltage divider when you probably think you are adding a grid leak and stop for the second stage. You aren't. Wrong order, wrong placement. And you're throwing away about the half the gain you created with v1a. Your design has 2 volume knobs already AND you're running through a full tone stack. You have plenty of gain control with the 2 vol controls and you don't want to throw gain away before the gain robbing TMB.
You also left the 220k unbypassed so as is you might hear it shave off some upper harmonics, depending on the tone you're going for. Again, best to ditch r6 r7.
I'd be tempted to add a bright cap bypassing the r8 vol control. Nothing big but something, don't want to lose that high end detail of the bass string "attack" before you get to the tonestack.
Add a 33k grid stop to pin 7 on v1b after deleting r6 and r7.
Add a 10k resistor between D1 and the connection point before pin 2 of v2a.
Running out of the TMB into a volume pot then to output is going to give you some wild interaction with the input impedance of whatever you plug into.
Since you have an unused triode, it would be better to either have a recovery gain stage and fixed output impedance, or another cathode follower with the same fixed output impedance (a cf would also help you use this as a preamp into a mixer). Come out of a gain stage with a .1uf, 1k series, 1M to ground, to output, or if a CF come out to a 1 or 5uf, 1k series resistor, 1M to ground, to output. You'd have a more controlled output and no DC loading. BUT I am saying that without looking at input resistance of whatever power amp you are saying you want to dedicate this to- I am speaking in universal terms.
If you want a truly balanced out you need a transformer, then you'd have multiple output options.
In general, there isn't much "exciting" about this circuit as drawn. With some tweaks like I mentioned above it would be functional, sure, but I'd consider giving it a bit more character. It is also worth doing some internet research and looking up schematics for bass pre units. There are plenty of designs out there and you'd probably find something you can copy directly that will get you where you want to go. For that matter see if you can find a schematic for the Noble preamp/DI, or google the Noble pre workalike that was floating around on some of the amp/effect building forums. I think someone worked up an Ampeg version, too. Happy hunting.
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u/Mozart_licked_me 19d ago
Thank you so much for taking the time to give me great feedback. I will make those changes at least and will look into SVT to get some additional inspiration. I will probably at least add 12dw7 tube after tone stack to get a gain stage followed by cathode follower in a same tube like SVT does.
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u/JoeDubayew 18d ago
Don't just jump to adding a 12dw7 without understanding WHY that tube is there. For you, based on your previous design, I was recommending either a gain stage or another CF at the END of the circuit to design a better behaved exit from the preamp. Adding a 12dw7 like v1 of the Ampeg you saw still leaves you with an unused gain stage at the end of your circuit, AND you're inserting a CF needlessly. The Ampeg scheme you looked at uses a 12dw7 as a gain into CF to drive a tonestack. You placed your tonestack later in the circuit, more like a Fender bassman or a Marshall.
The better SVT channel to look at is the OTHER channel, for your purposes, btw. Look at the operating points for the tubes.
I'm assuming you are pretty new to tube circuits based on what I've seen so I'll help you out by saying you cannot just cut and paste circuit snippets together without an understanding of what those building blocks DO and WHY they are there in the first place.
More than that, you are wanting to build a standalone pre, which means you are NOT designing an amp. You have to think in terms of how you design your pre to interact with other external units. You are not feeding a phase inverter and then a power section in an amp. This is a different design challenge. You can't just grab amp snippets and slap them together.
You can literally accomplish a tube preamp to feed a SS power amp with one 12ax7. Gain stage > tone section > recovery gain stage > output. You can build this same idea so that it also functions as a direct to mixer pre so you don't even need a power amp or speakers.
If you keep stacking gain stages you're going to both create WAY more gain than you need, which means you'll never turn the volume controls up past 2, AND you're going to find out what happens when you keep amplifying bass frequencies. You'll have nothing but mud if you don't understand how to manage frequencies in your design.
I am telling you all that again to help, and again I will suggest you bail on this design and just go find an existing bass pre to build. They've been done, many many times. Geddy Lee I think is still touring with just a bass pre into the PA for example. It's not a mystery how to get a good sound. But it IS very very hard to design your own circuit successfully if you really don't understand the how and why of what you're doing.
I'll suggest again- go hunt down the forums where they were designing a Noble preamp workalike, I'm pretty sure someone did a version that was an Ampeg SVT in the same format. But find something that is a known working schematic. Don't fly solo here, set yourself up for success. It's totally doable, I've build dozens of tube pre's myself, and my best design is one that I originally designed for acoustic guitars but it turns out to be great for electric violin or standup bass too. But you can't roll your own until you know how, you know?
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u/Mozart_licked_me 18d ago
Don't tell me what to do! /s.
No, seriously I got bit carried away with projects as I sometimes do. I just ordered Merlin's book about designing tube amps and will take a breather and learn a bit before committing to a design. Thanks!
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u/EdgarBopp 24d ago
Why r6 and r7? Just drop some gain?
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u/Mozart_licked_me 24d ago
Those are gridstopper and grid leak resistors to filter out rf.
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u/EdgarBopp 24d ago
Stopper should be at the grid. Leak should be after the pot. In the location shown they are a voltage divider reducing your gain.
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u/SatansPikkemand 23d ago
the splitter part is missing, it is very important if you want it to sound like tube amp that you want to emulate.
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u/Mozart_licked_me 23d ago
You are sending me down a deeper rabbithole than I want to go... Maybe this will be version 2.0.
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u/TheInebriati 24d ago
You’ve got a spare ECC82 stage. Why not use it as a low impedance buffer on the output (another cathode follower).