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Help to chose a practice amp (1W and less) to make
Help to chose a practice amp (1W and less) to make
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Old 16th September 2019, 11:22 PM   #1
PKI is offline PKI  United States
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Help to chose a practice amp (1W and less) to make
Default Help to chose a practice amp (1W and less) to make

Hi,

I am planing on making a small practice amp and got a little bit stuck between these designs:

1. AX84 FireFly based amp described here: Less than 1w practice guitar amp Tube Valve

2. Same OT section with an ax84 lead or crunch pre: https://www.ax84.com/static/coreprea..._Schematic.pdf
https://www.ax84.com/static/coreprea..._Schematic.pdf

3. Lead or simple preamp from ax84 with that Micro-champ OT:
Champ Micro
https://www.ax84.com/static/coreprea..._Schematic.pdf

Soundwise I'd like to have something marshall-like with mostly crunchy sound as I never use clean-clean itself.
I would assume that ax84 clean or lead would be what I wand. I can't say based on topology what would be better, but crunch preamp looks unnecessary complicated here. However, it looks like both of those preamps would require higher voltage that the firefly OT section needs, that would make PSU more complicated. Any way I could easily overcome that?

Another solution would be to repeat that Merlin's High-Gain+FireFly amp, but I am afraid as we all have different definitions for hi-gain :-) Any ideas how much gain it has compare to my desirable Marshall-like crunch?

That micro champ... I don't know, I think I just came across and do not see any advantage compare to FireFly one.

I would appreciate your suggestions? Thank you.
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Old 17th September 2019, 07:29 PM   #2
tristanc is offline tristanc  United Kingdom
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Originally Posted by PKI View Post
Another solution would be to repeat that Merlin's High-Gain+FireFly amp, but I am afraid as we all have different definitions for hi-gain :-) Any ideas how much gain it has compare to my desirable Marshall-like crunch?
You've reminded me that I need to take some sound clips. Will do that ASAP and post.

It is high gain enough that I can play along with Pumpkins / other grungy stuff pretty well. It's not 'chuggy', but that would be pedals anyhow. Merlin's description when I posted it on AX84 was something like 'wow, that's a lot of gain'. Around 40% on the gain is more 'crunch' I'd say.

With a bit more complexity you could try the October Studio (~2W) & graft on the Merlin HG or JCM 800 front ends: Index of /ax84/Mutineer_Studio

Index of /ax84/Mutineer

I was very happy with the Merlin HG + FF. But a proper PP output with LTPI gives a more 'grown-up' sound - hence the Mutineer Studio. Sounds 'bigger' with the same OT. The 'plexi' side is great for clean and crunch with pedals etc. The HG side is good - still some tweaking to do as I think I can get it better (especially the clean to mean thing).

Again, I'll try and take clips of each of my amps through the same cabinet so people can hear the difference.

Last edited by tristanc; 17th September 2019 at 07:33 PM.
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Old 17th September 2019, 07:35 PM   #3
PKI is offline PKI  United States
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Help to chose a practice amp (1W and less) to make
Thank you so much for your reply!

I have more mature SLO clone-ish, but it is 20W, so my idea was to make as low wattage as possible, as I am afraid that even 2W will be too much. What do you think 2W October studio sound more mature than FF at comparable volume (0.5W power)?
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Old 17th September 2019, 07:42 PM   #4
pelanj is offline pelanj  Czech Republic
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There was an interesting project called Moonlight, also around tge AX84 community when I followed it long time ago.

Also, try to google Weber Mass Pedal schematic or Hughes and Kettner Cream machine (or was it Blues machine?) for low power end stage inspiration.

Have you considered a power attenuator? Even a simple L-Pad like arrangement is effective. For home, in my custom built rack preamp, I have a 1W ECC82 power stage - which gets still attenuated to 1/10 W for playing at home. And with a low power amp, the attenuator is easy to build.
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Old 17th September 2019, 07:46 PM   #5
PKI is offline PKI  United States
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Help to chose a practice amp (1W and less) to make
I do have a webber attenuator and L-pad on a top so I can practice with my 20W amp, so I was thinking to make something small and portable for practicing and small jamming.
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Old 17th September 2019, 08:08 PM   #6
tristanc is offline tristanc  United Kingdom
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Originally Posted by PKI View Post
Thank you so much for your reply!

I have more mature SLO clone-ish, but it is 20W, so my idea was to make as low wattage as possible, as I am afraid that even 2W will be too much. What do you think 2W October studio sound more mature than FF at comparable volume (0.5W power)?
Ah, in that case you're covered - that's why I made both 18W and 2W versions to have a good starting point of a 'proper' sounding amp at both levels. 2W is still too loud. With the scale control though you can get some nice PA-style crunch from the LTPI at ~10mW output. It's sitting in my living room and I play along to backing tracks etc playing on the TV, so it's at conversation / not annoying neighbours level. I try to have the clean-to-mean thing happening so I can set and forget at the amp, use pickup selection and the vol/tone controls on the guitar to adjust from the couch without having to adjust the master volume every song. But you definitely notice it sounds better when you push this and the speaker starts to move slightly etc.

Yes, the Studio is more mature than the FF. The NFB and cancellation of 2nd harmonics(?) make it sound more like a 'proper' amp - lot's of time went in to voicing the power stage apparently: NFB calculation question - 4-4-0 / October Studio - The Amp Garage

But I've made a couple of tweaks here and there from the stock Studio to get round some of the perceived issues (blocking etc) based on Merlin's suggestions.

Read what Merlin says here on the FF output stage: The Valve Wizard

The Self-Split of the FF means it's really inefficient, giving the 0.5-1W. But still very loud.

I'd recommend my Mutineer Studio with the scale control (perhaps have it on a switch rather than a pot). Leave out the FX loop and reverb. Do both preamps, as I have, to cover clean and dirty. Or just do the HG and keep it even simpler. 5 knobs and a switch.

If you use LTSpice I can share models of both the FF and Studio builds to play with.
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Old 17th September 2019, 08:46 PM   #7
PKI is offline PKI  United States
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Help to chose a practice amp (1W and less) to make
I will gladly build your version! Especially as I have quite a few spare 6sn7 for my aikido.
I do not use sims for guitar amps I will just start with whatever version you suggest and then will tweak it by ear
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Old 17th September 2019, 09:16 PM   #8
tristanc is offline tristanc  United Kingdom
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Here's the latest schematic - I'm tweaking the bright cap C6 and R7/R8 for the interstage loss to try and get the touch sensitivity back to what I had with the FF. Also toying with reducing C3 to 1uF and/or leaving out Cx to reducing some wooly-ness with neck humbuckers if C6 is left out. This might be an outcome of playing at low volumes... Need to be careful.

Also, can use 12AX7s throughout - I think with the lower coupling caps stopping blocking and decreased voltage in the LTPI - the signal hitting the output tubes in my version is lower. I used bigger grid stoppers too. So the reduction in gain using the 12ay7 isn't that much.
Attached Files
File Type: pdf October-style-studio_v1.1.pdf (955.5 KB, 25 views)
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Old 17th September 2019, 09:56 PM   #9
PKI is offline PKI  United States
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Help to chose a practice amp (1W and less) to make
Thank you sir! Mind explaining what bright gain and dark gain are? :-) This should be enough for the amp right? 40-18106
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Old 18th September 2019, 07:18 AM   #10
tristanc is offline tristanc  United Kingdom
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Bright and dark gain controls are the jumpered front end of a plexi-type amp - on which the October is based. So the input signal to the October preamp goes through both 'channels' at the same time (one voiced brighter, the other 'normal' for 60 years ago) allowing you to mix the two to dial in a sound. In reality, I set the bright control first, then increase the dark from zero until just before it gets too flubby. Then I use my mid control to scoop it a bit. Great for jazzy cleans / bluesy crunch and pedals.

Yep, that OT looks good. Or the Hammond 125b or 125c I think. I used this one: Amp Maker 4W Push Pull Output transformer – Primary Windings for both this and my FF.
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