Bass Valvecaster Build and Schematic

[lake_land]

Bass player here - my first post and I'm working on my first build... a tube overdrive with two 12AU7 tubes - based on the "Matsumin Valvecaster" for which there's a lot of information floating around. I've added an op amp boost out front, a presence pot (sort of), Treble and Bass pots using RC filters, and Dry and Wet pots. I built this on a breadboard and everything works and it sounds good to me, but before I make it on veroboard and mount it in an enclosure I'm wondering if anyone has any comments on the schematic - am I making any stupid mistakes here? Thanks!!

[iainpunk]

what kind of opamp are you planning to use?
there is no DC path between the output and the inverting input!!!!
this results in "latching", where the opamp dives to full positive or negative.
make sure that there is no capacitor between the out and the inverting side.

it might be a good idea to put a transistor buffer before and after the tone control, because of the relatively high resistance it sees on the output of the tube and the low resistance on the input of the mixer.
while we are on the topic of the mixer, i see that you have two volume controls, isn't that a bit redundant??

i took the liberty to draw up something for you

iain

[swamphorn]

Be prepared to write "crackle okay" on the panel under the gain controls. Then again, it looks like the canonical Valvecaster has the gain control wired up like this anyways. Aside from iainpunk's observation of the op amp (it could work anyway since there's a DC path to ground through the Dry pot and, to a lesser extent, the grid; I wouldn't rely on it though), I would rework the tone stack and mixer section. As it stands now you have five different volume controls. I'd get rid of the Volume pot (since the Wet pot does that anyways) and then I'd change the Treble/Bass tone controls to a Big-Muff-style tone control (in other words, mix the low and high pass legs with a single pot). If you have a dual op amp I'd also recommend an inverting summer for the output stage which will better isolate the wet and dry paths and provide a lower output impedance.

[lake_land]

Wow, thank you both for your excellent and very helpful replies! To answer your / ask a few more questions:

I am currently using an RC4558 dual op amp, so I will definitely use the other half of it to do the inverting summer for the output stage.

The second tube can be bypassed using a switch and the control labelled "volume" is so that there isn't a huge volume jump when adding that tube into the mix. I guess "volume" is a pretty misleading label for that pot though, so my bad.

- swamphorn: You're right that the gain knobs crackle a lot: is there a way to reduce that?

- iainpunk: Will any kind of NPN transistor to isolate the tone control? I have 2N5088s on hand... Also, in the spirit of learning: can you tell me more about the rationale/effects of removing the 1uF caps at the end of each tube stage and adding a 220nF cap between the tone controls and the ground?

Thank you again! Really looking forward to making the modifications tonight 

[iainpunk]

it could work anyway since there's a DC path to ground through the Dry pot and, to a lesser extent, the grid; I wouldn't rely on it though

no it won't, it's not about the DC bias on the two inputs, the + side is biased Vcc/2 and the - side is biased at GND, thats always going to output the maximum output voltage of the opamp

1{Will any kind of NPN transistor to isolate the tone control? I have 2N5088s on hand... }
2{Also, in the spirit of learning: can you tell me more about the rationale/effects of removing the 1uF caps at the end of each tube stage and adding a 220nF cap between the tone controls and the ground?}

1} while any npn transistor would work, i recommend using normal silicon transistors, i'd advice against darlington pairs because of the (DC) voltage drop that occurs. 5088's are alright.
2}its purely to maintain the DC bias for the next buffer. transistor buffers are lovely and simple but they need to have a high enough Bias voltage (DC offset) to work. to achieve this, i changed the tone stack to pass DC. 220nf is supposed to be alright, but using a 1uF is better, given it is a foil (non polarised) cap, an Electrolytic (polarised) cap can be less than ideal because of the relatively high equivalent series resistance, lowering the depth of the filters.

im glad to hear you are exited

iain

[rankot]

Are you sure you have built the Presence control like this? I'm pretty sure it will work only as some kind of volume pot.

[iainpunk]

Are you sure you have built the Presence control like this? I'm pretty sure it will work only as some kind of volume pot.

wow, i didn't even notice it when i first looked over it (that's why you shouldn't go on the internet after the 2nd whiskey), pretty sure it's basically a sketchy volume control. changed it to a feedback filter the capacitor is dropped to 1nf but due to the miller effect, it could be a bit to large of a value... i also drew up something against the crackle

markt the new things in blue

[lake_land]

iain did you mean to attach an image to your last post?

My thinking with that negative feedback bit was from this post (see image)



I watched some videos about it and it seems that feeding some of the output back into the cathode creates negative interference and can lessen the intensity of the effect. My thinking was to combine this with a filter so that just some of the high frequencies would be affected - i.e. let low freqs go to ground and let high freqs into the cathode. Whoops, yeah the schematic I first posted does not achieve that. But maybe the below does? ... but sounds like I need to read a bit about the miller effect...



Thank you all again for your help. Tonight's going to be a fun night!

[iainpunk]

iain did you mean to attach an image to your last post?

yeah...


dont try to break your head over the miller effect to much, it basically that components seem larger in negative feedback loops.
if you do what you posted last, there won't be any gain coming from the tubes because there is no current path to ground.
im not a huge fan of negative feedback going to the cathode, because its not as effective as negative feedback to the grid, and when you use a variable resistor/pot, you can always dial it back but never make it more

edit 1 } also, if there is a 10k pot and a 100k resistor in series, the 10k pot won't do much.

edit 2 } the buffer at the end isn't right, the collector should go to vcc and the emitter should go to the mixer.
also, the mixer opamp isn't biassed at all. the + side should go to a 6V bias node and the inputs need to be at 6Vdc as well.

[iainpunk]

i edited the mixer and the last buffer a bit, i tried to put your most recent bits that were correct into the modifications i drew up to the best of my ability (MS Paint is an art form)


ill search my old notebook for my own dual "valvecaster" design i build years ago, it sounded good, i ran it on 36V and used a E80CC tube instead of the 12au7 for the 2nd and 3rd triode, it had a filter mixer before the last gain stage, mixing the compressed low end with clean mid and high end, was very useful on bass to get a pretty clean tone with lots of low end sustain

[anotherjim]

I'd advise changing the bias method for the opamp. As it is, 1/2 of any ripple/noise on the +12v supply is going get amplified by the first preamp and then reamplified by all that follows. If an unregulated supply was used, the heater current will ensure plenty of supply ripple!
A proper voltage divider with bypass cap should be made for the opamps like in most pedals.
I would also suggest having some RC filtering in the +12v feeds anyway.
Feed the heaters and any indicator LED directly from the +12v input.
Then insert a modestly sized R (470R?) and a filter cap to ground (470uF?) in the +12v feed to everything else. Also feed a voltage divider (10k+10k) from the filter for the opamp bias and stick a 10uF bypass cap on that. Feed the bias to the first amp +in via a 1M resistor. Reduce the signal input cap from 1uF to 100nF. Connect the bias directly to the +in of the output mixer and pot returns. You might add an input polarity protection diode. Reverse voltage won't bother much except the opamp which will be fried if that happens.

Since the following circuits have many stages that don't have much noise rejection, I think reducing noise on the input is worth pursuing.

The TL072 has lower noise in a high input impedance circuit  (Engl and others use them on the input to their tube amps) so it makes a better first stage. Whatever opamp, stick a 100nF ceramic across its supply pins. Insert 100R in series with the output to keep it stable against cable capacitance and, as it's for bass, increase the output cap to 2.2uF to ensure it keeps all the low end even if it's plugged into a 10k mixer line input.

The input opamp will have x11 gain which means that a hot input that can hit 1v peaks will exceed the input range of the opamp and I don't think you want to hear it clip either. Also, the gain setting resistors can go down by 1/10th to reduce resistor noise. So 4.7k and 27k will give about x6 gain.

[lake_land]

iain: oooo *pacepalm* - novice mistake on that second buffer and the mixer. Thanks for pointing that out. I think I've got them fixed in this schematic. I wired up the Presence knob like you suggested (tho with a 100K pot, I don't have 1M right now). Is the idea in this case just to send bass frequencies back to the grid as opposed to canceling treble freqs via the cathode? It seems to work that way and sounds nice - much more of a "bass boost" kind of pot now... at least to my ears. Wow - these transistor buffers make a huge difference! The attack of each note is much more consistent. Your old valvecaster sounds cool! I'd love to see your schematic. When you say "compressed low end" do you mean you had a compressor circuit in there as well?

anotherjim: Thanks so much for your tips! I spent most of the morning thinking and reading about them and adding them to the schematic. I think I've got the opamp power supply set up as you suggested, both with a dedicated voltage divider/bypass cap and decoupling caps on either side of the power pins. I've also added (i) a small resistor and cap in series that the +12V feeds see, (ii) an output resistor and bigger output cap, and (iii) modified gain setting resistors on the input opamp as you suggest. Based on the attached schematic, did I understand those suggestions correctly?

Also feed a voltage divider (10k+10k) from the filter for the opamp bias and stick a 10uF bypass cap on that. Feed the bias to the first amp +in via a 1M resistor.

I didn't quite understand what you mean with that part ^. Are you recommending further dividing the +6V with a '10k+10k voltage divider/10uF bypass cap' then using that (+3V?) to further bias the +in of the first amp via a 1M resistor? Or are you describing how to construct the +6V supply for the opamp?

Thank you all again for your help. My breadboard is getting much more full, but also sounding much better too!

Luke

[anotherjim]

I mean this...

In this example, Rin is only 1k. We use 1M for guitar. Don't use the scheme with those 2M2 resistors for audio. The above scheme gives the 6v bias you need and cleans it up. The 10k resistor divider value isn't that critical - some go lower or a lot higher.

[iainpunk]

iain: oooo *pacepalm* - novice mistake on that second buffer and the mixer. Thanks for pointing that out. I think I've got them fixed in this schematic. I wired up the Presence knob like you suggested (tho with a 100K pot, I don't have 1M right now). Is the idea in this case just to send bass frequencies back to the grid as opposed to canceling treble freqs via the cathode? It seems to work that way and sounds nice - much more of a "bass boost" kind of pot now... at least to my ears.

no, the signal coming out of the tube is "upside down" compared to what comes in, if you add a upside down signal with a normal one, they cancel out each other. if you have a inverting gain stage and you mix the output into the input, there will be almost no sound (depending on the mix ratio of course), this is called negative feedback, it also makes amplifiers more linear.
what we do here is canceling out only the high frequencies with the negative feedback, while the low frequencies aren't affected because they aren't fed back by the capacitor and resistor network.

Wow - these transistor buffers make a huge difference! The attack of each note is much more consistent. Your old valvecaster sounds cool! I'd love to see your schematic. When you say "compressed low end" do you mean you had a compressor circuit in there as well?

no what i meant is that the amplitude of the notes become quite consistent if they are clipping, essentially compressing the signal. if you then filter out the higher harmonics and replace them with clean midrange and treble you have lots of compressed "woof" and still natural, dynamic sounding "kleng". the frequency where i seperate them was variable. im sorry but i can't seem to find my old notebook, but i'll draw what i remember when i have the time. i can't retrace the build because i cannibalised it for parts, i recently used the last pot from that build in a fuzz pedal.

edit: i forgot to say: Welcome to the forum

iain

[iainpunk]

here, an edited schematic, make sure that the red power line is separated from the main power line to reduce noise.

edit} there were also some resistors which only make stuff worse (crackle in one of the mix pots, i suspect)

[lake_land]

Awesome - thank you both for your help! I really appreciate it. This is a great learning project.

I set up the input opamp last night and it sounds great - very clean and a huge volume boost. Will set up the mixer portion of the opamp tonight sans voltage divider.

- I can see how DC comes from the second buffer and into the mixer to provide bias for the input to the second opamp, but will this be close to a ~6v bias? Or is that not so important?

- Also, how should I think about the 470k resistors to ground before each input to each grid? What is their function?

most recent schematic attached...



cheers,

luke

[swamphorn]

The 470 kΩ resistors to ground is a grid leak bias resistor. A small amount of current flows through the resistor into the grid which brings it to a slightly negative voltage with respect to ground. The bias of the op amp will approximately be the bias point of valves 1b/2b (depending on the selected gain level) minus the voltage drops of the transistor buffers. I would recommend AC coupling the Wet path and connecting the grounded points of the Wet and Dry pots, as well as the non-inverting input of the op amp, to the half-supply point used to bias the input op amp. If you do this you might also want to increase the bypass capacitor for your half-supply voltage divider to ensure a low AC impedance.

As an aside, in what program do you draw your schematics? Your schematics have a very professional appearance.

[iainpunk]

I can see how DC comes from the second buffer and into the mixer to provide bias for the input to the second opamp, but will this be close to a ~6v bias? Or is that not so important?

tubes have a relatively small headroom compared with other active devices, and especially the negative cycle side. the signal is significantly smaller than the headroom opamp and buffers have. there are 2 transistor buffers, which each take off 0,6V of DC, so there would be around 4,8V of bias if the tube is biassed at exactly 6V. there is no way that the negative wave is bigger than the 4,8V of negative headroom, while the headroom is even greater on the positive side.

i think your schematic is correct this time around, but i can't be sure because i have had a few beers tonight.

cheers,
Iain

[lake_land]

Thank you both for your advice ! I'm pretty sure I understand what you are saying.

swamphorn: the changes you suggest are essentially this, right? [changes marked in orange]



Iain: I take your point to be: there is plenty of DC getting to the opamp through the buffers so, even if we don't know exactly how much, it's enough to get the job done.

I'll finish wiring up this whole thing tonight and let you know if there are any issues. Thanks again!

Luke


EDIT: Swamphorn: I've been using microsoft powerpoint to make these schematics. It has tools to draw shapes and lines. When I draw a set of lines like for a resistor, I group them together and can then duplicate the whole bunch to save drawing time. It's kind of a slow process, but I think it looks nice and I have access to powerpoint. I think Beavis Audio uses Microsoft Visio, their schematics look awesome, but Visio costs extra mooney...

[iainpunk]

swamphorn: the changes you suggest are essentially this, right? [changes marked in orange]

almost, the orange wire should be connected to the other side of the 1M resistor and should have its own 1M resistor in its path. i can't say which method of bias is superior, both methods have worked for me in the past. the methods of biassing have many unofficial names, but i call swampthorn's method "the individual stage biassing method" and the method i suggested "the continuously biassed signal method". both work fine in most cases