Carvin Bass Amp tube question

[DrNomis_44]

A tube/valve will only start to ‘compress’ the sound once it reaches its clean gain limit and starts to distort slightly. From that point on, you’ll be getting an increase in compression at the same rate as harmonic distortion increases. Unlike a compressor which kicks in on the whole signal once it crosses the threshold and only returns to non-compression mode after the signal level drops below the threshold for a period related to the release time of the compressor (almost always a preset value on non-studio compressors and any compressor with three knobs or less), a tube will only reduce the peak signal value above the clean limit. Doing this will alter the signal wave shape, so introducing distortion and adding harmonics. So it’s a very different type of compression to a normal compressor, and hand-in-hand with extra harmonics and an initial ‘warmer’ sound, before audible distortion kicks in and you know the tube is definitely overdriving and adding grit.

Apparently, when the tube is being lightly overdriven, and you're getting that warm sound out of it, the distortion consists of mainly even harmonics due to asymmetric soft-clipping, I remember doing some experimenting with some 12AX7 tubes, a variable HT supply, a signal-generator and an oscilloscope, I built a simple triode amplifier stage on a breadboard and fed a sine wave signal into it while monitoring the output on the oscilloscope, I noticed that below a certain signal-level the output signal was pretty much a clean sine-wave, as I increased the input signal level, the bottom half of the output signal became more rounded and compressed, whereas the top half became elongated and peaky, then, when I increased the input signal further, the top half of the waveform started to clip sharply while the bottom half remained gently rounded, according to what I can remember, I might see if I can do a thread in the amplifier section one day about tube/valve distortion.

[Simon Barden]

Don't forget that was a single stage triode, whereas the 12AX7 here is configured more like a power amp with two triodes, each one presumably doing half the input wave, so the overall result will be more symmetrical.

[DrNomis_44]

Don't forget that was a single stage triode, whereas the 12AX7 here is configured more like a power amp with two triodes, each one presumably doing half the input wave, so the overall result will be more symmetrical.

Good point, might be a good idea doing a set of scope screenshots for both a single triode-stage as well as some screenshots for two triode-stages in series as you would find in a guitar amp, we could then compare the results with each other.

[JimC]

Looking at Talkbass it seems as if the general consensus is that the Bx500 tube is not in a gain stage and has at best a minimal effect on the sound.

[fender3x]

Looking at Talkbass it seems as if the general consensus is that the Bx500 tube is not in a gain stage and has at best a minimal effect on the sound.

Add on more to the consensus ;-)

Sent from my ONEPLUS A3000 using Tapatalk

[dave.king1]

A tube/valve will only start to ‘compress’ the sound once it reaches its clean gain limit and starts to distort slightly. From that point on, you’ll be getting an increase in compression at the same rate as harmonic distortion increases. Unlike a compressor which kicks in on the whole signal once it crosses the threshold and only returns to non-compression mode after the signal level drops below the threshold for a period related to the release time of the compressor (almost always a preset value on non-studio compressors and any compressor with three knobs or less), a tube will only reduce the peak signal value above the clean limit. Doing this will alter the signal wave shape, so introducing distortion and adding harmonics. So it’s a very different type of compression to a normal compressor, and hand-in-hand with extra harmonics and an initial ‘warmer’ sound, before audible distortion kicks in and you know the tube is definitely overdriving and adding grit.

I'm hearing you but my tiny mind is still struggling to understand why the signal chain was apparently complete regardless of switch position with the valve removed.

Anyway, now watching with interest

[JimC]

my tiny mind is still struggling to understand why the signal chain was apparently complete regardless of switch position

My tiny mind has been struggling to understand the circuit diagram, but I seem to see a signal path that is parallel to the triodes, so if I read it correctly there would indeed be signal with the tube/valve removed. But the circuit is at least two levels of complexity above what I am capable of comprehending, and I have certainly failed to understand what that part of the circuit does.

[DrNomis_44]

I just downloaded a pdf of the Carvin BX500 Revision F schematic, I'm not sure if it is the exact revision you have, but it should be close enough, anyway, just been studying the tube/valve circuit section, and the first thing I've noticed is that both of the Triodes in the 12AX7 tube (V1A and V1B) are wired-up with the plate/anodes going directly to the +HT supply, which I'm guessing is maybe at anywhere from +125V to +150V, given that there's a 22uF/160V cap (C8) going to ground that's acting as a supply bypass cap.

Okay, so if the plate/anode of both of the 12AX7 Triode stages (V1A and V1B) are going directly to the +HT supply via no resistors, that means each Triode stage is configured as what's called a "Common Anode/Plate Amplifier" or "Cathode Follower" stage, now a Triode configured as a Common Anode Amplifier, or Cathode Follower stage, produces a voltage-gain of slightly less than one, but, it also provides a low output impedance (which just means that the output can supply more current to several inputs without loading-effects degrading the signal).

I had another look at the Carvin BX500 Rev. F schematic to see where the 12AX7 tube/valve (V1) was getting it's input from, and it looks to me like the input signal is coming from the master volume pot, the signal first gets sent to a diode-clipper network which is made up of two blue Leds (D52 and D53), and what looks like a bridge-rectifier formed by 5 other diodes (D55, D57, D58, D59, and D60) which I'm assuming are standard power diodes, I've seen a similar diode-clipper network used in a few Marshall amplifiers (the Marshall Silver Jubilee, the JCM800, and the JCM900), and also a tube/valve pedal using two 12AX7 tube/valves (the Mesa/Boogie V-Twin), that diode-clipper network is most commonly used to generate distortion by clipping the signal more or less symmetrically, I think that in the case of the Carvin BX500, that 12AX7 tube/valve (V1) is being used to buffer, or provide a low output impedance for the diode-clipper network according to my analysis of the Carvin BX500 Rev. F schematic, since both triode sections of the 12AX7 tube/valve aren't contributing much voltage-gain I don't think it actually has much effect on the tone, other than maybe introducing a subtle brightness or maybe adding a very slight amount of the so-called "Tube/Valve Warmth", also, since the 12AX7 (V1) isn't contributing much gain you should be able to use any 12AX7, ECC83, 7025, or any of the other 9-pin equivalents of the 12AX7 twin-triode tube/valve and they all should work normally, providing that they all use the same pin connections.

There's also a Fet (Field-Effect Transistor) Q102, which is used to switch in/out the signal from the 12AX7 tube/valve.

I should also add that since there are two separate paths that the signal can take, starting from pin 7 of the op amp IC A7.B, that means that if the 12AX7 tube/valve (V1) is removed from it's socket, the signal still has an alternative path it can take to the input of the power amp section, which means that whether the 12AX7 tube/valve (V1) is in it's socket or not, there won't be any difference in the sound other than no clipping/distortion from the diode-clipper network.


That is,unless my analysis of the Carvin BX500 Rev. F circuit is incorrect, which I hope not, but if it is then I apologize in advance.

[Simon Barden]

I'm hearing you but my tiny mind is still struggling to understand why the signal chain was apparently complete regardless of switch position with the valve removed.

Because it wasn't.

With the valve removed and the switch in the valve position, there was no sound. With the valve removed and the switch in the valve bypass position there was sound. So the switch worked, the two paths worked (provided the valve was installed) but no sonic difference meant the valve was operating cleanly at the volumes Fender3x was using.

Jim, the circuit is in parallel but there are transistor switches being used to switch the signal path. SO one transistor is turned on and the other turned off (and vice versa) to bring the valve circuit in and turn off the bypass path, or else to open up the bypass path and cut off the valve path.

[JimC]

Yeah, I saw the transistor switches, it's the fine detail of what was happening round the valves where I thought I could trace some sort of alternate path. But I couldn't make sense of it.
Signal comes in through h9bb and connects to grid of v1a and also grid of v1b via R260, but in the schematic I'm looking at R260 is marked 0 jmp which I assume to be a link with 0 ohms. I then trace the same connection through R240, which is marked n/u which I assumed is not used ie no component just a track in this variant. I then trace through R261, which is another link, and on to h9ba. But thinking again maybe n/u means absent, ie open circuit, in which case it makes more sense.