Guitar Amp build suggestions

[Potato Cakes]

Hello, everyone,

I am looking at building a pair of guitar amps for recording use and after looking over a number of schematics the two I've landed on are the Dr. Z Mini and the Matchless Spitfire 15. The Spitfire has everything I could want in an amp: volume, tone, and master controls. But it has too much power for what I need so I started looking at how to utilize bits from the two designs to make something more suited to what I am after. So without turning this into a gear consumer discussion, I have some questions for those more experienced guitar amp people here in our very fine community:

Is there a great benefit to tube rectification over using diodes?

For a dual EL84 output design, will simply removing one of those tubes with the associated components and then connect that end of the transformer to ground be all that is needed to lower the output wattage of the amp? I also noticed that the Dr. Z schematic has a 5k/2W resistor going to the grid of the power tube and didn't know if that should be incorporated as well for lower power output.

Should I use the second 12AX7 tube preamp stage or no? I have pedals so I don't particularly need the amp to do all of the breaking up, but some from the amplifier might be nice. I would like the amp to handle transients nicely, so maybe the answer is yes.

Is an effects send/return desirable when using a pedal board? I've only used effects as a front end to an amp and I do not need this build to be multichannel.

What are some recommended sources for guitar combo cabinet enclosures? Mojotone has some pretty good ones that are made of birch, but I didn't know if there are some other companies making a quality enclosure which others here could recommend.

Lastly, I could use some speaker recommendations. I do not have any real experience with different models of speakers and I only have so much tolerance reading through gear head descriptions that are doused in adjectives like creamy, lush, sparkly, twangy, etc. I am looking for something in a 10" that does well with distorted, overdriven signals with well defined low mids but has pretty good sensitivity so as to not be a one trick pony.

Thanks!

Paul

 

[Rocinante]

While I have yet to build cabinets I have built several successful preamps and amps.
Elliot Sounds (ESP) has a simple but great sounding amp on his page. I built a dual channel preamp (relay switchable) and amp, and it really is a great versatile solid state amp.
As I am sure you have found online, the Alembic is popular and that's because it's a great tube preamp. I built a dual channel followed with a solid state amp, and added a relay switchable distortion circuit based on a big muff that can be applied to either channel.
Both a great.
There is a many Soldano pcb's out there which if I ever find the time...
At any rate rewarding stuff.

 

[john12ax7]

Tube rectifier vs diode is a preference / feel thing, the rectifier will have more sag and looser feel.

Usually a dual EL84 will be push pull,  you can't just remove one tube in that case. (You may come across a single ended amp with tubes in parallel,  in which case you could remove a tube and adjust the impedance and bias if need be.  But these are rare).

The grid resistors are usually to prevent blocking distortion and stop the grid from conducting. They should not be removed.

How many pre-amp tubes is again a personal preference.  I like at least having the option of getting some overdrive from the amp.

Given your desired goal don't bother with an effects loop.

For cabinet makers I've seen small builders post on places like ebay so that's worth a look.  Local carpenters can be an option too.

I prefer 12" speakers but you can look on youtube there are some good comparison videos that give you a good idea of the different speaker tonal differences.

One thing to ask yourself is do you want less power simply for volume reasons, or do you prefer the sound of a single EL84 single ended amp?

 

[Squeaky]

Everything John12AX7 is spot on.

I would add that the Spitfire circuitry looks a bit Vox-like to me so you might want to take a closer look at that family of amps. Consider the Vox AC10 and I used to have a Blues Jnr (2 x EL84s I think) which was a pretty good little amp at about 10W. The Fender 5F1 is a great 4W amp but not for all guitars in my experience. It can be tweaked though. 

You could possibly try and build a low wattage version of a Spitfire by combining the input (V1, eq, volume) of the Spitfire with the output (PI onwards) of something like the So-Low Watt (ECL84 output)?

https://el34world.com/Forum/index.php?topic=13438.msg127145#msg127145

The Matchless Lightening has switchable coupling caps (at output of V1) as do some of the more modern Vox amps. This is a pretty useful tone control.

I would probably go with a lower wattage amp with less gain stages and when you want tube breakup you just turn the amp up! (I am a fan of the Vox family of amps).

Tube rectification is a taste thing... as is the use of under-powered transformers. Depends on style of guitar playing as well.

If I recall correctly, one of the main things about the selection of the type of ply used in high quality guitar amp speaker cabinet construction is trying to avoid materials that have voids (effectively air pockets) in the laminated sheets. A really dead (dense) material is something like MDF, which doesn't resonate much at all and is therefore not generally preferred for guitar amp cabinets. Used in bass amp cabs though I think.

I would probably go with a little 10" AlNiCo speaker and build a separate head and cabinet (open-backed). Again, a bit of a personal choice thing.

 

[Potato Cakes]

I meant to post the schematics for those following along at home. Here is the Dr. Z Mini

 

[Potato Cakes]

Here is the Spitfire.

 

[Potato Cakes]

Tube rectifier vs diode is a preference / feel thing, the rectifier will have more sag and looser feel.

Usually a dual EL84 will be push pull,  you can't just remove one tube in that case. (You may come across a single ended amp with tubes in parallel,  in which case you could remove a tube and adjust the impedance and bias if need be.  But these are rare).

The grid resistors are usually to prevent blocking distortion and stop the grid from conducting. They should not be removed.

How many pre-amp tubes is again a personal preference.  I like at least having the option of getting some overdrive from the amp.

Given your desired goal don't bother with an effects loop.

For cabinet makers I've seen small builders post on places like ebay so that's worth a look.  Local carpenters can be an option too.

I prefer 12" speakers but you can look on youtube there are some good comparison videos that give you a good idea of the different speaker tonal differences.

One thing to ask yourself is do you want less power simply for volume reasons, or do you prefer the sound of a single EL84 single ended amp?

Thanks for the response.

The desire for less output power is for recording purposes. I have had several guys I've worked with record their amps on stun but then wonder why the recording of it sounds thin. When a microphone capsule/diaphragm starts to vibrate/cavitate it effectively starts to high pass itself. A similar thing happens with woofers in speaker cabinets. So I'd like to keep the output lower but still be able to open up the volume on the amp if some breakup is desired. Also, these amps are always going to be in a controlled environment, so anything pass 5W or so is completely unnecessary.

I do not know if I like the sound of push/pull over single ended guitar amps. My experience with tubes is building and modding studio gear. When I mentioned losing one of the EL84s in the Spitfire amp it was solely for output level and I did not consider the consequences to audio characteristics. I would be fine using a push/pull scheme if I knew the benefits but I would still have to figure out how to lower the output, whether it be at the cathode resistor/capacitor of V3/V4 or the changing the value of the resistor before those tubes' grid connection. Or maybe it's as simple as adding a T-Pad, L-pad, or shunt resistor at the speaker output. I think if I had a simple, elegant solution to lower the amp wattage I would just stick with Spitfire schematic and change the rectifier to be diodes.

I have found some cabinet options that I think will be great. I'm looking for 10" as I find them to be a little more responsive/tighter when used with guitars. But I'm not against exploring 12" options. My focus right now is the electronics part.

Thanks!

Paul

 

[Potato Cakes]

Everything John12AX7 is spot on.

I would add that the Spitfire circuitry looks a bit Vox-like to me so you might want to take a closer look at that family of amps. Consider the Vox AC10 and I used to have a Blues Jnr (2 x EL84s I think) which was a pretty good little amp at about 10W. The Fender 5F1 is a great 4W amp but not for all guitars in my experience. It can be tweaked though. 

You could possibly try and build a low wattage version of a Spitfire by combining the input (V1, eq, volume) of the Spitfire with the output (PI onwards) of something like the So-Low Watt (ECL84 output)?

https://el34world.com/Forum/index.php?topic=13438.msg127145#msg127145

The Matchless Lightening has switchable coupling caps (at output of V1) as do some of the more modern Vox amps. This is a pretty useful tone control.

I would probably go with a lower wattage amp with less gain stages and when you want tube breakup you just turn the amp up! (I am a fan of the Vox family of amps).

Tube rectification is a taste thing... as is the use of under-powered transformers. Depends on style of guitar playing as well.

If I recall correctly, one of the main things about the selection of the type of ply used in high quality guitar amp speaker cabinet construction is trying to avoid materials that have voids (effectively air pockets) in the laminated sheets. A really dead (dense) material is something like MDF, which doesn't resonate much at all and is therefore not generally preferred for guitar amp cabinets. Used in bass amp cabs though I think.

I would probably go with a little 10" AlNiCo speaker and build a separate head and cabinet (open-backed). Again, a bit of a personal choice thing.

Thanks for the link for the So-Low watt amp. Changing the EL84 section to values of the So-Low amp with the front end of the Spitfire is what I was thinking. I just didn't know what values to use where for that section. I take it I don't need the gain stage as shown after the effects return for the So-Low as I am not going to be using an effects loop, correct?

Thanks!

Paul

 

[Whoops]

so anything pass 5W or so is completely unnecessary.

I really don't know what sound or needs you are pursuing but the problem with tube amps rated at 5w or 10w is that you don't have much headroom so you almost don't have any clean sound at all, just overdrive.
They can be fine for overdrive purposes but be aware that they are not flexible or versatily for recording.

An amp like the Fender Deluxe reverb, rated at 22watts can be used in a studio for clean sounds or for drive sounds, it also takes well other overdrive/distortion pedals, personally that would be my choice it I wanted a low watts amp.

 

[abbey road d enfer]

When a microphone capsule/diaphragm starts to vibrate/cavitate it effectively starts to high pass itself.

I can only disagree with this assertion. The phenomenon you describe would happen with SPL of 150+dB.

A similar thing happens with woofers in speaker cabinets.

The mechanics of an electrodynamic loudspeaker and the power involved are so different such a comparison is inappropriate.
Thousands of recordings done with a Deluxe Reverb or an AC30 are proof enough that they are perfectky adequate.

I would be fine using a push/pull scheme if I knew the benefits but I would still have to figure out how to lower the output, whether it be at the cathode resistor/capacitor of V3/V4 or the changing the value of the resistor before those tubes' grid connection.

The best way to do it is decrease the B+ voltage, which simply involves using a power xfmr secondary rated at a lesser voltage. Decreasing the voltage by 30% halves the power.

Or maybe it's as simple as adding a T-Pad, L-pad, or shunt resistor at the speaker output.

That is another possibility, although most power soaks tend to also modify the sound.

I think if I had a simple, elegant solution to lower the amp wattage I would just stick with Spitfire schematic and change the rectifier to be diodes.

Doing that tends to increase the power...

 

[john12ax7]

That is another possibility, although most power soaks tend to also modify the sound.

I was going to suggest trying a power attenuator. The reactive load type can be quite transparent.  If you treat it like an additional speaker cab and hook up in parallel it's a simple way to get half power.

 

[Potato Cakes]

I can only disagree with this assertion. The phenomenon you describe would happen with SPL of 150+dB.
The mechanics of an electrodynamic loudspeaker and the power involved are so different such a comparison is inappropriate.
Thousands of recordings done with a Deluxe Reverb or an AC30 are proof enough that they are perfectky adequate.
The best way to do it is decrease the B+ voltage, which simply involves using a power xfmr secondary rated at a lesser voltage. Decreasing the voltage by 30% halves the power.
That is another possibility, although most power soaks tend to also modify the sound.
Doing that tends to increase the power...

I can only disagree with this assertion. The phenomenon you describe would happen with SPL of 150+dB.
The mechanics of an electrodynamic loudspeaker and the power involved are so different such a comparison is inappropriate.
Thousands of recordings done with a Deluxe Reverb or an AC30 are proof enough that they are perfectky adequate.
The best way to do it is decrease the B+ voltage, which simply involves using a power xfmr secondary rated at a lesser voltage. Decreasing the voltage by 30% halves the power.
That is another possibility, although most power soaks tend to also modify the sound.
Doing that tends to increase the power...

I too would tend to believe the SPL rating for microphones and that dynamic mics meant for stage should reproduce the sound of the amp as it is heard with ear within reason, but I've encountered too many times on tour and in the studio where the guitar player cranked his amp to get the desired break up, but the recorded sound was quite thin by comparison. The cavitating transducer is not as accurate of a comparison as I looking for but it was best way I can explain what I have experienced with this discrepancy of the sound of the source and reproduction. For me, amps at a lower level have recorded better and sounded very big, but I admit that haven't spent as much time as I would like with experimenting with that relationship between amp volume and recorded results. Hopefully after building these I will get to do just that.

For the amp characteristics, I must reiterate that I have not spent much time with different amps to be come familiar with each circuit's characteristics to make a confident decision about what to do. I know that I have liked how the Matchless and Dr. Z have sounded with various players over the other options, so that's why I gravitate towards those designs.  ​

Thanks for the tip with the B+ voltage. This was going to be my next question. I believe that this will be the best way to achieve what I think I want. I agree with power soaks affecting tone. I was hoping to avoid this solution.

I greatly appreciate everyone's input, especially with this project where I am having to resort to more speculating and guessing than I would like. And as always I am very thankful for the advice and corrections I receive when I work on projects that are a step into the unknown for me.

Thanks!

Paul

 

[john12ax7]

There have been some huge guitar tones recorded with small amps,  definitely some merit to that approach.

But I'm also curious as to what mics you are using.  For something like an SM57 I think it sounds better as the SPL increases.

 

[Tubetec]

Ive got good results even with small(5w) single ended tube amps for clean sounds, they dont cancel hum the same way as push /pull amps though so more psu smoothing might be appropriate . Of course if you do increase the volume single ended amps clip asymetrically , so not really what most guitarists expect . Small push pull amps ,say with a pair of EL84's can certainly sound bigger than they are , but you dont have the same hassle of bleed into adjacent mics from a 50 or 100 watter cranked up .  In general the better distortion charachteristics from tube amps is in the output stage ,a single stage preamplification is probably good enough if you run humbuckers but for single coils the extra gain is usefull to have , depends to some degree on phase splitter and if you have feedback from the transformer secondary or not .

 

[Squeaky]

Low powered combos are great for the studio. An exception might be recording at the metal end of the spectrum. You probably want a bigger and close-backed cabinet for some of that sort of stuff? I have generally taken the approach of converting combo amps into heads to mix and match with different cabinets.

Further to my previous post, you would keep everything downstream of the 0.0047uF cap on the so-low (the follow V3 and V4 are two sides of a phase inverter) and replace everything upstream with the Spitfire circuit from (and including) the 0.01uF cap. The 0.01uF (Spitfire) 0.0047uF (So-Low) is the join point. It should work if you get the B+ voltages right. You might want to tinker with the value of the coupling cap to the phase inverter (0.01uF Spitfire/0.0047uf so-low).

I want to reiterate that what I suggested was an example, of what you might consider doing, that I thought of in a minute or two (i.e. it was not rigorously thought out). Given that the B+ to V1 of the Spitfire is quite high (329V) you will probably have to think carefully about the power supply (I think the ECL84s want about 250V - 260V), delta V is about 80V. This is a problem with trying to get the tone of bigger amps in smaller packages.

If it was me, I would probably go with the traditional Vox AC30 V1 voltages (about 290V) and components, given that the cathode and plate components are so similar in value (between the Spitfire and AC30). Less of a delta V using 290V. You could easily sub in the Spitfire volume and tone arrangements after V1 followed by the So-Low output. However, it will probably end up sounding different to the Spitfire. How much, I don't know.

There are probably plenty of other 5W output circuits out there (other than the So-Low) worth looking at.

I recently decided I wanted to build a low power push-pull amp for about a 5W output. The most time consuming aspects (at least for me) to work out were the phase inverter voltages and resistors, and the B+ power supply. There are useful calculators and explanations online about how to use tube specification graphs. Even so, I suspect I will be tinkering with voltage dropping resistors, grid resistors and coupling caps when I do get round to building it. I also suspect I may melt some output tubes. I went with valve rectification and choke filtering.

Point is, if you have never built a guitar amp before then it may be best to start with something simple, with a well established reputation (published schematics, components and layouts, cabinets and chassis for purchase), as opposed to jumping straight into building a hybrid (designing your own layout, no readily available face plate, punching and cutting all metal work all yourself, building your own cabinet, not to mention tolex gluing). Otherwise you may invest too much of your time tinkering.

Another option, there is a lot of great work done on other forums in respect of building low wattage amps that you might want to look into further.

Regards,

Nick.

 

[Spencerleehorton]

I've just built a fender champ 5w and with adjustable NFB it's very flexible for jazz clean to full on plexi at only 5 watt.
Used with a treble booster or a distortion I  can get a VH type tone and a Hendrix style sound, sounds great double mic with sm57 and a ribbon mic.
I'm also making a 50 watt plexi and a friedman BE-50.
I already have a vox ac-10 and a egnator.
Not that impressed with the egnator and will probably move that on at some point.
I'm using the heads through creambacks.
With all these amps I should have all bases covered!!

 

[Potato Cakes]

I think I'm going to stick with the Spitfire schematic as is and just mess with lowering the B+ voltage. Now I've got to spend the next month or so gathering parts. I'm going to do more reading on speaker options and try to extract objective information from the subjective descriptions.

Thanks!

Paul

 

[Squeaky]

...try to extract objective information from the subjective descriptions.

Thanks!

Paul

Good luck removing all the creamy/glassy/shimmery/chimey/warm descriptors! And don't forget about "cone cry".

 

[Matador]

A great sweet spot for volume for recording is indeed a pair of EL84's, running AB1 into the recommended 8K transformer, with a supply voltage of 250V (and screens running around 215V).  If you need more power, you can boost up to 300V, and run close to 15W.  I haven't found any more power was needed for close miking in relatively small spaces.  My second favorite combo is a pair of 7581A's running into a 6.6K OPT, with a B+ about 350V, which is good for about 25W.  The same tube pair running 400V into 3.5K can get close to 50W.

From there, it's about how much gain you want in front of the phase splitter.  A few stages run modestly can make a great pedal platform.

 

[Spencerleehorton]

I think the spitfire is a good choice, very vox like and with my ac-10 since I recapped and changed a few off resistors it sings like an angel and records very well.