AquaTarkus... I'm not so sure any more that much overdrive sound is coming from V1 itself. Maybe it IS clean and just drives V2 hard. I have a couple of tubes coming that will help me with this -- a 12DW7 and a one of a kind ECC823 tube from JJ that is a 12DW7 only backwards; high gain and low gain triodes are connected to exchanged pins so that the low gain triode will appear in the circuit where the 12DW7 would put its high gain triode.
If my bench wasn't in storage, I could just listen to the signal at different points along the signal path and settle this for sure. But it ain't, and I can't.
If my bench wasn't in storage, I could just listen to the signal at different points along the signal path and settle this for sure. But it ain't, and I can't.
Agreed, the solid-state part will definitely sound sterile, but keep in mind that:
(a) There are subsequent valve stages in the preamp (as well as the power amp?), which will add the usual lovely valvey distortion, and
(b) The evidence suggests that a 12AY7 in this particular circuit, used the way Fretts is using it, is already producing inaudibly low levels of distortion.
With bass guitar as the source, I find even 10% second harmonic distortion is quite hard to hear...probably because the few measured guitar harmonic spectra I've seen show most guitars spit out far more 2nd, 3rd, and 4th harmonic than the actual fundamental frequency!
With regular six-string guitar, I find I can often hear intermodulation distortion when there's 10% second harmonic distortion, even if I can't hear the harmonic distortion itself. But with bass guitar, depending on the type of music, it may be quite rare to play two notes at one time ("double stops"). So IMD usually isn't a factor, unless you play like Victor Wooten!
The only way to be sure is to have Fretts try it out for himself.
-Gnobuddy
Edit: I see Fretts posted while I was typing this up.
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What if very clean does not sound sterile? What if it is another characteristic that gives us our desired sound?
And speaking of, how do we know it is the distortion that fretts is hearing with a 12AX7 that he does not hear with a 12AY7? Just because the difference is easy to determine from the spec sheets does not mean it is our answer.
I have actually been considering two things, voltage gain, and distortion. Not just distortion.
I have no 100% guarantee that these are the only relevant factors, but what are the other candidates you recommend we consider? I don't believe in magic and mojo, and I don't think you do, either. So what does that leave for us to consider?
From my point of view, audio gain stages are more or less defined by just a handful of parameters: gain factor, frequency response, nonlinearity/THD. (Things like input and output impedance, maximum headroom, too, but I can't see those being factors here.)
So let's look at the list of suspects:
1) The frequency response with either 12AX7 or 12AY7 is much wider than the frequencies created by a bass guitar, so we can eliminate frequency response as a possible factor.
2) Voltage gain is higher in both stages with the 12AX7, as verified by the loadlines I drew. We know we need to cut the 12AX7's gain down if we want gain and volume knobs to end up in roughly the same place.
(I think they should end up in roughly the same place to keep the "feel" of the amp the same, i.e. no hair-trigger gain knob, etc. )
2) Fretts says lowering the gain control between V1a and V1b (with a 12AX7 installed) isn't duplicating what he likes from a 12AY7. That means matching the signal level out of V1b still leaves the amp sounding different. The rest of the amp is seeing the same signal as before and can be eliminated.
If the same signal voltage at V1b anode still sounds different, that points to differing amounts of THD in the signal.
3) The 12AY7 loadlines show squeaky-clean behaviour compared to a 12AX7 in the same circuit. This too suggests that lower THD is what Fretts is hearing (and liking).
Which is why I didn't propose that maximum anode current, or Vgk for cutoff with 100 V on the anode, or other random spec sheet parameters might be the cause.
As I said earlier, I certainly won't know for sure if I'm on the right track until Fretts confirms or refutes my suggestions. But I believe I'm looking at the most likely causes of the differences he's hearing. If you have a different opinion, by all means please do share it! We can always use extra brainpower!
-Gnobuddy
Tonight I will be experimenting with two versions of the tube that has dissimilar triode sections. One has a 12AX7-spec triode in the first position and a 12AU7-spec triode in the second position. The other tube has them reversed. In one case, I can hear a high-gain triode feeding a low gain triode, with a 'gain' pot in between. In the other case, I can hear a low-gain triode going into a high gain triode with a 'gain' control in between. This should tell me a lot about who is doing what in the lineup.
That sounds like an excellent test to me. I look forward to hearing your results.
-Gnobuddy
Followup: I hated both of them. Both stages need to have lower gain. It seems if either of them are high-gain, I'm back to the raspy, grating type of overdrive that I'm trying to get away from. This was a detour and a dead-end.
What if what I really want is just a different amp entirely? This amp is really close, and so is its big brother the BB15 which I tried today at the store and liked, that's why I'm thinking it can be reigned in and improved. In my opinion, based on experience working in a music store and playing live in bands, there is the sound that sells the piece, and the sound you really need in a band; they are not the same thing. I think the amp as built is designed to wow them in the music store, not be an optimal live performance or recording amp. The sound I am after is several degrees "down" from the design Fender settled on, but still hotter than their traditional clean sound. My position is they took it too far past the magic spot and I want to bring it back to that spot that they COULD have stopped at but didn't.
What if what I really want is just a different amp entirely? This amp is really close, and so is its big brother the BB15 which I tried today at the store and liked, that's why I'm thinking it can be reigned in and improved. In my opinion, based on experience working in a music store and playing live in bands, there is the sound that sells the piece, and the sound you really need in a band; they are not the same thing. I think the amp as built is designed to wow them in the music store, not be an optimal live performance or recording amp. The sound I am after is several degrees "down" from the design Fender settled on, but still hotter than their traditional clean sound. My position is they took it too far past the magic spot and I want to bring it back to that spot that they COULD have stopped at but didn't.
I have worked a lot with 12AX7s as an amp designer and it's important to understand that the operating point plays a very important part in overdrive tone. A simple rule of thumb is that if you bias warm enough for the anode to idle at half of the preamp rail voltage, you will get a warm, thick, vintage-style crunch. Speaking very generally, you would get this with a 100k anode load, with 680 ohms on the cathode. There are other combinations which also work.
The reason this is that the tube will actually operate asymmetrically, and go into soft limiting on the downswing, and never hit 0V. So you get asymmetrical clipping, with a rounder soft edge on the clipped side. Of course, if you drive the tube even harder, it will also get into some square-edged hard clipping on the up swing. The combination of clipping modes also sounds good, if you like a Marshall sound. To avoid that, use attenuation resistors between the stages, as well as possibly using the lower gain triode as the first stage.
I haven't tried this with 12AY7s, but i expect they will probably bias differently from 12AX7s (perhaps warmer?) I've tried 12AT7s, but found they can sound muddy when overdriven with humbuckers.
Just passing this on, without knowing if it will give you the sounds you are looking for.
That's very useful info, thank you Aqua Tarkus.
Isn't the Vox AC30 actually very warm biased AB rather than true Class-A? I've read a lot to that effect over the years. In any case, that warm fat Vox overdrive sound is one of two reasons to love an AC amp. I have a "JMI" replica of a '62 AC15 and it does exactly that.
At this point, I have taken it as far as I can go without opening the chassis and changing some parts. I have tried every tube of interest in there and only the AY7 hits the spot. This is the original plan anyway, just didn't know where to start. I have a much better understanding since I started the thread, thanks to you and you and you. Plus the intense reading I have been doing the past few weeks. Sections I used to gloss over because they looked too dense and discussed to many variables at once are now of sharp interest, I have been going over those repeatedly until they are starting to sink in.
Once I can get the gain structure sorted out, chapter two of this adventure is going to be to massage the high end response. The way it is designed, the treble and midrange control are so overlapped that I have little control over the high end without excessively affecting the amount of drive. In this amp, with all tone controls at zero, the output is zero. They act like three volume controls. I do like the arrangement, just not crazy about the frequencies the knobs affect. The approach I have landed on is to set the gain and volume to maximum, then bring up the bass control a small amount to get some girth and body to the tone, then bring up the middle control almost to max, this gives me the majority of the gain, and the treble control stays at zero, it's useless in this scenario. Even set like this, there is more treble than I would like but there is nowhere left to go without killing the volume and gain by reducing the middle control.
I'm sure I could lose some high end because even when set differently to a bold glassy clean sound, a la Wind Cries Mary, I only use a third to a half of what is available from the treble control.
Isn't the Vox AC30 actually very warm biased AB rather than true Class-A? I've read a lot to that effect over the years. In any case, that warm fat Vox overdrive sound is one of two reasons to love an AC amp. I have a "JMI" replica of a '62 AC15 and it does exactly that.
At this point, I have taken it as far as I can go without opening the chassis and changing some parts. I have tried every tube of interest in there and only the AY7 hits the spot. This is the original plan anyway, just didn't know where to start. I have a much better understanding since I started the thread, thanks to you and you and you. Plus the intense reading I have been doing the past few weeks. Sections I used to gloss over because they looked too dense and discussed to many variables at once are now of sharp interest, I have been going over those repeatedly until they are starting to sink in.
Once I can get the gain structure sorted out, chapter two of this adventure is going to be to massage the high end response. The way it is designed, the treble and midrange control are so overlapped that I have little control over the high end without excessively affecting the amount of drive. In this amp, with all tone controls at zero, the output is zero. They act like three volume controls. I do like the arrangement, just not crazy about the frequencies the knobs affect. The approach I have landed on is to set the gain and volume to maximum, then bring up the bass control a small amount to get some girth and body to the tone, then bring up the middle control almost to max, this gives me the majority of the gain, and the treble control stays at zero, it's useless in this scenario. Even set like this, there is more treble than I would like but there is nowhere left to go without killing the volume and gain by reducing the middle control.
I'm sure I could lose some high end because even when set differently to a bold glassy clean sound, a la Wind Cries Mary, I only use a third to a half of what is available from the treble control.
Yes, that's correct, the Vox AC30 is warm-biased Class AB, although misleadingly marketed as "Class A".
See Is the Vox AC-30 Really Class A?
The Last Word on Class A
Same goes for the Marshall 18W and most Matchless amps. Their power stage overdrive sounds wonderful. However, resitively-loaded triode preamp stages work in a quite different way to power stages.
You can experiment with tone stack values by using Duncan's Tone Stack Calculator - TSC I personally like a Marshall tone stack variant you will find online as the "jalapeno" tone stack.
See Is the Vox AC-30 Really Class A?
The Last Word on Class A
Same goes for the Marshall 18W and most Matchless amps. Their power stage overdrive sounds wonderful. However, resitively-loaded triode preamp stages work in a quite different way to power stages.
You can experiment with tone stack values by using Duncan's Tone Stack Calculator - TSC I personally like a Marshall tone stack variant you will find online as the "jalapeno" tone stack.
Sorry to hear that.
To ask the obvious, have you tried simply lowering the output level from your bass guitar? Particularly if you have an active bass, you might need to lower the volume on the bass quite a bit. Maybe that will be enough to keep the input stage from overdriving, and if you then also dial down the gain control between V1a and V1b...
And that is a question to take very seriously! When I was younger and sillier I put a lot of time, hard work, and money into modifying my car to handle better. The car was an old GM classic with the handling and braking qualities of a rollerskating hippo.
I would have been much better off selling it, and buying a car that had been designed to handle better from the start!
Back to bass amplification, this is what I've been playing my bass through lately: ART Tube MP/C | Sweetwater
My settings: Lowcut ON. +20 dB gain switch ON. Compressor speed AUTO. Turn up the gain knob until the two yellow "tube drive" LEDs are flashing, set the compressor threshold so the (-3 dB) "gain reduction" LED lights, set the output level to make your PA happy, and there you are!
(Using the Lowcut ON setting sounds counter-intuitive for bass, but I'm finding it makes the bass sound more articulate, sit much better in the mix, and sound less muddy. And I'm playing a 5-string with the usual low B string!)
I find the same ART TUBE MP/C also sounds great as a mic preamp with vocals, and does a really nice sparkly almost-clean with my semi-hollow electric guitar. And it does a great job of taming singers suffering from the "whisper and shout" singing style, too. Really nice little box.
-Gnobuddy
There is probably a small misunderstanding -- although the entire amp line is called "Bassbreaker", not one of them is intended for bass at all. The tweed Bassman ended up being the de facto first choice for guitar players when they were being made new, then Marshall copied it more or less verbatim for their first few amplifiers, also intended for guitar players. Their Model 1962 JTM45 2x12 combo amp got nicknamed "Bluesbreaker" because of Eric Clapton's use of it during his stint with John Mayall's Bluesbreakers. Bassbreaker is a mashup of the two recognized names for the whole DNA line of those amps. I used to play a little bass as a fill-in or a trade-off when the bass player wanted to play something on a different instrument, but I'm a guitar player through and through. I mentioned playing some bass through the amp as a way to loosen up a tight, new speaker.
Having said all that, I have tried lowering the guitar's volume and it does help a bit but it also dramatically changes the overall tone, beside the fact that it is very difficult to hit the right amount of "turn-down" on the fly when you have about .025 seconds to do it and go back to playing. These are all good suggestions, no doubt about that. I'm still focused on getting the amp to behave as I want it to.
Having said all that, I have tried lowering the guitar's volume and it does help a bit but it also dramatically changes the overall tone, beside the fact that it is very difficult to hit the right amount of "turn-down" on the fly when you have about .025 seconds to do it and go back to playing. These are all good suggestions, no doubt about that. I'm still focused on getting the amp to behave as I want it to.
Thank you for the correction. Got my head on straight now. I think.
Guitar is the only instrument I'm actually competent on, but I play a little bass, and tinker with keyboards too at home.
Every time I put some effort into getting better on the bass or keyboards, I also find that somehow I got better on the guitar too. I took a few drumming lessons once, and my rhythm guitar playing got better. So did my bass playing.
I think every instrument teaches you something slightly different about music. Drums are great at teaching you about rhythm. Bass is great at teaching you about chord progressions. Keyboards are fantastic for learning music theory and chord-tone soloing. And then it all starts to come together on your main instrument (guitar for me).
How did the tone change? Did it get duller? The usual "tone suck" effect caused by cable capacitance?
If so, what about putting a pot, a "bright cap", and a foot-switch in a little stomp box enclosure? Set the level at your leisure with the pot, leave it there, stomp on the switch when you want to bypass it and have the full guitar signal (for solos or whatever).
The bright cap value can be tweaked to give you the tone you want when the pot is turning down the guitar level.
If you can get the tone you want without messing around inside your amp, life will be easier, and you won't void your warranty. That would be a nice outcome, no?
-Gnobuddy
A nice outcome indeed. You know, great minds must think alike because I made such a box, an "un-boost" box, when I used to have a reissue Bluesbreaker combo! And it does seem that whenever you play an 'away' instrument, you play better when you come back to your 'home' instrument.
Rolling down the guitar volume was the old Tone Suck effect, yes.
The only amp I have heard recently where I wouldn't change a thing was at an amp show a few weeks ago, it was a Red Iron amp based mostly on a Trainwreck Rocket that had the most perfectly crafted tone. Things like that spoil me and make it hard to accept tone that is Almost There. I see no alternative other than buying that amp or seeing what I can do to the little Bassbreaker to bring it into the light.
I guess it's time for me to fish or cut bait - my next move is going to have to be, spill its guts and make a few changes and see what I think of it!
Rolling down the guitar volume was the old Tone Suck effect, yes.
The only amp I have heard recently where I wouldn't change a thing was at an amp show a few weeks ago, it was a Red Iron amp based mostly on a Trainwreck Rocket that had the most perfectly crafted tone. Things like that spoil me and make it hard to accept tone that is Almost There. I see no alternative other than buying that amp or seeing what I can do to the little Bassbreaker to bring it into the light.
I guess it's time for me to fish or cut bait - my next move is going to have to be, spill its guts and make a few changes and see what I think of it!
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do you already have a "bright cap" and resistor (in series) bypassing your volume pot in the guitar?
thoglette - Thanks, good point. To go into it a little more, it's not exactly dullness, it's just sort of whimpiness. There's still some top end there but the guitar just sounds weak, lacking both top and bottom. And a hard to pin down 'pushiness' too.
I'm pretty sure I put the bypass cap in this time. I take this guitar apart every year or so either because it needs maintenance or I want to try something out. It's been a while and I don't 100% remember anymore, but since this one is the main live playing guitar I normally would have put it in. It's a Strat type and a pain to open up, so I never do unless the guitar's going down for maintenance.
I'm pretty sure I put the bypass cap in this time. I take this guitar apart every year or so either because it needs maintenance or I want to try something out. It's been a while and I don't 100% remember anymore, but since this one is the main live playing guitar I normally would have put it in. It's a Strat type and a pain to open up, so I never do unless the guitar's going down for maintenance.
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