Vox AC10C1 is a hybrid amp?

[radiocaster]

The JCM-1 has some diodes, but there's a switch to add/take them out.

By the way, I looked at the JCM-1 schematic again, and there's also a transistor somewhere (which you can't switch out).

Also, link to AC10C1 schematic, I don't think it's been posted yet :
http://music-electronics-forum.com/attachments/44726d1504209765-vox-ac10c1-sch.pdf

 

[printer2]

By the way, I looked at the JCM-1 schematic again, and there's also a transistor somewhere (which you can't switch out).

Also, link to AC10C1 schematic, I don't think it's been posted yet :
http://music-electronics-forum.com/attachments/44726d1504209765-vox-ac10c1-sch.pdf

The Marshall uses the LND150 as the cathode follower (using a mosfet it is called the source follower).

 

[mnutz]

Kinda brings up

Normally I would not comment on a thread like this, nobody's minds will be changed. But I took a look at the schematic and I am impressed at what Vox has done. Mind you I have done something similar with one less tube to another classic circuit. First off I will say that they did the amp a good deed by using the LND150 as the MOSFET in this amp. Many have said it has a sound similar to a triode and sounds pleasant even with some mild clipping.

Not that it is driven into clipping on the first stage, depends on the signal strength from the guitar. But I would say it spends most of its time playing clean. Not to say it does not amplify the signal, it does and it does so in a position where most tube amps pass the signal cleanly on to the rest of the amp. The first triode in an amp provides a clean signal out (unless you use a booster or other pedal to hit the tube harder than just a guitar). Grab your favorite scope and take look. Back to the circuit, the LND150 amplifies the signal (just as the first stage of a 57 Bassman) then passes it through the volume control for the channel and the hits the second stage.

Now this second stage can be hit hard enough to do some damage to the signal. This second stage has two triodes, both halves of a 12AX7, the gain stage and the cathode follower. Much of the Bassman's tone comes from this stage (along with Marshalls and Voxs). In a perfect world the cathode follower should just isolate the tone stake from the gain stage. But when Fender first designed the amp we did not have a perfect world. Heck, he thought the amp should amplify bass not guitar, what was he thinking? That said the cathode follower does not do its job perfectly, thankfully not as this error gives us some nice tone. The LND150 would work better in this position, it would actually do its job and pass along a clean signal (well at least what the triode before gave it). But that would give us less flavor to our steak. So Vox had the choice in saving a tube and socket, put a triode in the first stage where it does not have a lot of effect on the sound (But it is the most important position some cry! BS.) and make some purists happy. Or put it in as a cathode follower in a position where much of the magic is done. Marshall knew this and made some high priced amps and put the cathode follower in behind a whole lot of gain stages to keep the amp from getting to sterile.

After the tone stack Vox put a couple of opamps to drive the reverb tank. They dropped the signal from the tone stack by 1/11 then boosted the signal to the tank by two. Seems a little odd, why not just cut the signal by 5 and then run the op amp at unity (a gain of one)? I have no idea, this is not your standard op amp used in a guitar amp, I am guessing they had good reason. The next opamp is just used as a buffer, no gain from it. Can't complain about a transistor sound, it passes the signal as neutral as can be. But the gain heeds to be boosted before it gets to the phase inverter. And they again used the best sounding MOSFET for the job, the LND150. With the signal back up to normal strength the PI (phase inverter) can do its thing as if there were no reverb section.

Now for the people who feel cheated that there is a bunch of semiconductors in the signal path you could actually take the signal after the tone stack and feed it into the PI and bypass the reverb section. The reverb section really just looks like an effects out and return with the loop having a reverb tank stuck in it. You still have the first stage MOSFET sticking in your crotch bothering you but you could also jumper around it and you would have an all tube path. Not much grit though as we are short a gain stage. At full volume you would have a clean , all tube machine. Could always put a clean boost pedal in front of it though, to get that tube sound.

I know, a lot of words, but I don't plan on commenting again and I didn't want any misunderstanding on what I think or don't think about the design of the amp. OK, I would have run the opamps at a higher voltage that Vox would have, might have reduced the hiss a little.

And another thing, the diodes around the LND150 are only there to protect the device from high voltages, not used as clippers.

Thanks very much for this explanation!

So I wonder then, why is the AC4c1-12 different, and does not contain any FETs?

I'm talking just about the newest, currently available AC4c1-12 that is part of the same custom series as the new AC10 and 15.

Vox claims "legendary top boost tone at lower volume".

 

[bluescube]

AC4

image removed

 

[mnutz]

Here is the AC4c1-12 schematic.

 

[printer2]

So I wonder then, why is the AC4c1-12 different, and does not contain any FETs?

They have four triodes available, no need for a phase inverter as it is a single ended amp (one output tube). With the cathode follower (which gives no gain) they have three triodes to use as gain stages.

The type of phase inverter they used on the other amp uses two triodes and the gain is split between the two, together they are only giving the amount of gain as a single triode. They also use the cathode follower in the circuit so that mean they effectively only have two stages worth of gain using (the stage preceding the cathode follower and the gain of the two triodes in the phase inverter). To add another gage of gain they could have added another 12AX7 (more heater current and more room needed to put in the tube, and of course added cost) or add a mosfet to give us our missing stage of gain. And they put it in the front of the amp where it is not used to clip the signal. Just for the ones that are not familiar with looking at the schematics, the LND150 biases up like a tube and runs at tube voltages rather than your standard fets or mosfets that can usually only take under 40 volts.

 

[Silverface]

And they put it in the front of the amp where it is not used to clip the signal. Just for the ones that are not familiar with looking at the schematics, the LND150 biases up like a tube and runs at tube voltages rather than your standard fets or mosfets that can usually only take under 40 volts.

I just grabbed a short clip to reference -

That paragraph is a superb description of both amps' signal processing, and the quoted part regarding the LDN150 is a very important point.

Yet I'm sure someone will jump on it and say "See? It's a solid-state device used for *gain*!! It's a hybrid! Lions and tigers and bears - oh, my!"

 

[jorbjorb]

AC4 does not sound good at any volume to my ears. Sounds like a mosquito. The AC10 sounded great when I last played it.

 

[3-Chord-Genius]

The type of phase inverter they used on the other amp uses two triodes and the gain is split between the two, together they are only giving the amount of gain as a single triode. They also use the cathode follower in the circuit so that mean they effectively only have two stages worth of gain using (the stage preceding the cathode follower and the gain of the two triodes in the phase inverter). To add another gage of gain they could have added another 12AX7 (more heater current and more room needed to put in the tube, and of course added cost) or add a mosfet to give us our missing stage of gain. And they put it in the front of the amp where it is not used to clip the signal. Just for the ones that are not familiar with looking at the schematics, the LND150 biases up like a tube and runs at tube voltages rather than your standard fets or mosfets that can usually only take under 40 volts.

At first I thought this thread was absurd (as illustrated by my numerous posts above pointing this out), but thanks to people like printer2, who can explain this stuff accurately and in a way most of us can understand, this thread has turned out to be very useful in confirming what many of us already knew: the Vox AC10C1 is no less a tube amp than any tube amp ever produced.

 

[studio1087]

I'm too dumb to get all this. I know this about my AC10c1.....

• 12AX7 dual triode vacuum tubes in the preamp stage

• EL84 pentode tubes in the power stage

I love the amp. I know the reverb is digital. The reverb in the Blackstar Club 20 head that I gigged was digital. Both amps sound fantastic.

I sold my AC15c1 to buy the AC10. The 15 was like lifting a small pig and it sounded really harsh. The 10 sounds prettier clean and the clean jangle comes right through with a TS-808 or my JHS Angry Charlie.

Most everything under $1000 has a PCB (and I don't have a problem with PCB's).

Sometimes I think that amp snobs are worse that wine snobs. I'm sure there are people who would tell me that my Dr. Z amps are horrible.

Can't we all just get along?

 

[markal]

Can't we all just get along?

I think we have ample evidence at this point to answer that question with a resounding "NO"!!! But at least this fight is pointless and harmless, unlike many other things that choose to fight over.

I'm with you. The AC10c1 sounds great. I still want one.

 

[3-Chord-Genius]

I'm too dumb to get all this. I know this about my AC10c1.....

• 12AX7 dual triode vacuum tubes in the preamp stage

• EL84 pentode tubes in the power stage

I love the amp. I know the reverb is digital. The reverb in the Blackstar Club 20 head that I gigged was digital. Both amps sound fantastic.

I sold my AC15c1 to buy the AC10. The 15 was like lifting a small pig and it sounded really harsh. The 10 sounds prettier clean and the clean jangle comes right through with a TS-808 or my JHS Angry Charlie.

Most everything under $1000 has a PCB (and I don't have a problem with PCB's).

Sometimes I think that amp snobs are worse that wine snobs. I'm sure there are people who would tell me that my Dr. Z amps are horrible.

Can't we all just get along?

I also sold my AC15C1 and purchased an AC10C1. I was okay with the tone of the AC15, but the thing weighed too much. The AC10 tone comes really close to the AC15 (and in some ways, I suppose, surpasses it) and is light enough to carry around without dislocating a hip.

 

[Staggerlee666]

Just curious, how would you go about jumpering around the LND150 to get that pure tube path on the preamp stage?

Normally I would not comment on a thread like this, nobody's minds will be changed. But I took a look at the schematic and I am impressed at what Vox has done. Mind you I have done something similar with one less tube to another classic circuit. First off I will say that they did the amp a good deed by using the LND150 as the MOSFET in this amp. Many have said it has a sound similar to a triode and sounds pleasant even with some mild clipping.

Not that it is driven into clipping on the first stage, depends on the signal strength from the guitar. But I would say it spends most of its time playing clean. Not to say it does not amplify the signal, it does and it does so in a position where most tube amps pass the signal cleanly on to the rest of the amp. The first triode in an amp provides a clean signal out (unless you use a booster or other pedal to hit the tube harder than just a guitar). Grab your favorite scope and take look. Back to the circuit, the LND150 amplifies the signal (just as the first stage of a 57 Bassman) then passes it through the volume control for the channel and the hits the second stage.

Now this second stage can be hit hard enough to do some damage to the signal. This second stage has two triodes, both halves of a 12AX7, the gain stage and the cathode follower. Much of the Bassman's tone comes from this stage (along with Marshalls and Voxs). In a perfect world the cathode follower should just isolate the tone stake from the gain stage. But when Fender first designed the amp we did not have a perfect world. Heck, he thought the amp should amplify bass not guitar, what was he thinking? That said the cathode follower does not do its job perfectly, thankfully not as this error gives us some nice tone. The LND150 would work better in this position, it would actually do its job and pass along a clean signal (well at least what the triode before gave it). But that would give us less flavor to our steak. So Vox had the choice in saving a tube and socket, put a triode in the first stage where it does not have a lot of effect on the sound (But it is the most important position some cry! BS.) and make some purists happy. Or put it in as a cathode follower in a position where much of the magic is done. Marshall knew this and made some high priced amps and put the cathode follower in behind a whole lot of gain stages to keep the amp from getting to sterile.

After the tone stack Vox put a couple of opamps to drive the reverb tank. They dropped the signal from the tone stack by 1/11 then boosted the signal to the tank by two. Seems a little odd, why not just cut the signal by 5 and then run the op amp at unity (a gain of one)? I have no idea, this is not your standard op amp used in a guitar amp, I am guessing they had good reason. The next opamp is just used as a buffer, no gain from it. Can't complain about a transistor sound, it passes the signal as neutral as can be. But the gain heeds to be boosted before it gets to the phase inverter. And they again used the best sounding MOSFET for the job, the LND150. With the signal back up to normal strength the PI (phase inverter) can do its thing as if there were no reverb section.

Now for the people who feel cheated that there is a bunch of semiconductors in the signal path you could actually take the signal after the tone stack and feed it into the PI and bypass the reverb section. The reverb section really just looks like an effects out and return with the loop having a reverb tank stuck in it. You still have the first stage MOSFET sticking in your crotch bothering you but you could also jumper around it and you would have an all tube path. Not much grit though as we are short a gain stage. At full volume you would have a clean , all tube machine. Could always put a clean boost pedal in front of it though, to get that tube sound.

I know, a lot of words, but I don't plan on commenting again and I didn't want any misunderstanding on what I think or don't think about the design of the amp. OK, I would have run the opamps at a higher voltage that Vox would have, might have reduced the hiss a little.

And another thing, the diodes around the LND150 are only there to protect the device from high voltages, not used as clippers.

 

[jorbjorb]

You guys need to hit the gym.

 

[3-Chord-Genius]

You guys need to hit the gym.

Monthly fees cost too much. I have a weight bench in the basement and protein powder in the kitchen.

 

[jorbjorb]

Monthly fees cost too much. I have a weight bench in the basement and protein powder in the kitchen.

That's not too bad. I'm sure you could lift an AC15C1 lol.

 

[printer2]

Just curious, how would you go about jumpering around the LND150 to get that pure tube path on the preamp stage?

Take the signal from VR3, the treble control wiper and feed it into C55, the 22nF capacitor at the phase inverter. Naturally you will have to break the current connections. Would have to cut the traces on the circuit board. The treble one is easy to find, the C55 is left of the V3 triode. Mind you, anyone that has to ask how to do it might not be the one I would like to do it. Would be interesting if you found a pot with a push-pull switch on it for the treble control to switch between the amp with reverb and without.

On the input side I would not bother but I would think the easiest method would be to pop out C59 100nF, C67 470pF, and use the holes (or are they surface mount, the pads in that case) to run a wire between the two points. You still have the input volume control attached that basically functions as the one on your guitar. Just leave it turned up to 10, otherwise pop it out and connect the input signal to the top of the pot, put a 500k to 1M in the wiper hole to the ground pin of the pot.

Actually now thinking about it, you could avoid cutting the traces for taking out the reverb section by removing R85 470k, and C55 22nF, and jumping a wire across the appropriate holes (or pads). Won't be able to switch in the reverb as easily this way (maybe add the removed components dead bug style, again not something for everyone) but the mod is fully reversible. That is unless you muck up the job.


Hope I got it right, if anyone wants to look it over and comment if I missed something I wouldn't mind. I have guitar building more on my mind rather than amps. I am surprised you guys sucked me in to add a few more posts.

 

[studio1087]

You guys need to hit the gym.

I'm 53 and on October 10 I'm getting a new left shoulder.

I just had cataract surgery. I have back issues. I am a cartoon for hitting 50 and stuff fell apart on me.

An AC15C1 is 24 x 11 x 18and they weigh 50 pounds.
A Deluxe Reverb is almost the same size and they weigh 42 pounds.
An AC10C1 is 20 x 8 x 16 and they weigh 27 pounds.

The AC10C1 is old fart tested and approved.

(image removed)

I like to turn amps up but not have my volume bleed into drum mics or vocal mics. The AC10 or my hopped up Blues Junior are my friends for hitting my desired volume and if I have to load and unload and repeat. I admire you crazy youngsters.

 

[RLee77]

It is precisely the V1 position that is the most important in an amplifier, which takes the weak signal of the guitar pickups and not only the amplification, and characterizes the sound base of the whole circuit.

Nah, the v1 gain stage is actually pure voltage gain, very linear (clean), and no clipping (under realistic conditions), and probably contributes the least to the tonal character of the amp. It's the best candidate for a mosfet, IMO.

 

[jorbjorb]

I'm 53 and on October 10 I'm getting a new left shoulder.

I just had cataract surgery. I have back issues. I am a cartoon for hitting 50 and stuff fell apart on me.

An AC15C1 is 24 x 11 x 18and they weigh 50 pounds.
A Deluxe Reverb is almost the same size and they weigh 42 pounds.
An AC10C1 is 20 x 8 x 16 and they weigh 27 pounds.

The AC10C1 is old fart tested and approved.

(image removed)

I like to turn amps up but not have my volume bleed into drum mics or vocal mics. The AC10 or my hopped up Blues Junior are my friends for hitting my desired volume and if I have to load and unload and repeat. I admire you crazy youngsters.

"old fart tested" should be a marketing line used by VOX lol. I like it!