pix of Custom built guitar amps/cabs

[tubelectron]

hey look damn good to me, don't see why you'd be hesitant to post those.

Yes, I agree... And that "Matchless" evocation look is pretty good also ! I never ventured in the Matchless look but you're giving me ideas, vigilante397... 

it's a Marshall TMB 18W clone with reverb.

Is it a circuit from Ceriatone or you modified / created it ?

A+!

[vigilante397]

Is it a circuit from Ceriatone or you modified / created it ?

I mostly followed the Ceriatone layout (love Ceriatone stuff), but the reverb was added based on a schematic I found on the TDPRI forum, some guy there built an 18W w/ tremolo and added reverb and posted his schematic. He used 6V6's for power though, which isn't to my taste, so this is EL84's.

[tubelectron]

Is it a circuit from Ceriatone or you modified / created it ?

I mostly followed the Ceriatone layout (love Ceriatone stuff), but the reverb was added based on a schematic I found on the TDPRI forum, some guy there built an 18W w/ tremolo and added reverb and posted his schematic. He used 6V6's for power though, which isn't to my taste, so this is EL84's.

OK. Thanks ! 

A+!

[thomasha]

I had never seen the vox escort before and tubelectron opened that door. So I got one and wanted to build something like the ac4 inside it.

I did some minor changes in the circuits, as reducing the gain of the EF86, as suggested by Merlin, to avoid problems.

The tremolo uses the 12au7 instead of the 12ax7. Since the gain is much lower, I had to add two extra CR stages to make it oscillate at the same rate.

The output stage is a way lower than the EL84, because I used the EL95. Nothing wrong there, it's a small box, I don't need all that power at home. Just had to change some values here and there.

So here are the pictures:








Just need to fix that grill cloth, it's made of a different material (wool or cotton?) .The one I have is brown and looks like made of nylon.

[thermionix]

Awesome.  EF86 Vox is best Vox.

[amptramp]

Great way to get around the limitation of the usual three R-C phase shift oscillator.  The 12AU7 has a µ of 20 and the normal three-section equal R / equal C phase shift oscillator requires a gain of 29.

[rankot]

I did some minor changes in the circuits, as reducing the gain of the EF86, as suggested by Merlin, to avoid problems.

What exactly?

[thomasha]

I followed this suggestions:
http://www.valvewizard.co.uk/pentode.html

but use some parts available, that means:

Anode resistor is 100k
Screen resistor is 470k
Screen capacitor 100nF
Cathode resistor 1.5k

I also made a quick video, just to show the tremolo speed settings:

I start from the lowest setting and go up until the effect stops. There is some mic distortion from the camera at some parts.

[tubelectron]

Yes ! Great job thomasha : I immediately steal your pictures 

Excellent idea that tremolo, and moreover it sounds great... When I made my VOX AC3 Top Boost, I hesitated but finally went simpler for fear of lacking room : you demonstrate that I could do it. Congratulations !

Yours :


Mine :



Why not rebuilding mine with a trem ? Would you post (or send me in PM) your compete schematic, thomasha ? So I could see what you've done...

Thanks !

[thomasha]

Hi!
just made a paint job on the original schematic...here you have it:



For a bigger image just copy the link and remove the last l before the dot

If you want to use a 12AX7 in the tremolo you can use the original tremolo section, with only 3 CR Stages.

[tubelectron]

Hi!
just made a paint job on the original schematic...here you have it:



For a bigger image just copy the link and remove the last l before the dot

If you want to use a 12AX7 in the tremolo you can use the original tremolo section, with only 3 CR Stages.

Perfect  ! Thanks Thomasha 

In my AC3 Top Boost, I had planned a hole for an additional 12Axxx tube : I'll see if I can enter you trem circuit stuff "compressed" on a little separate board, without changing my original circuit (except the rotary pentode/triode switch).

The trem would then modulate the 2nd preamp triode, à la Vibro-Champ. The rotary Batt/Mains selector would become the trem speed pot...

It's a true compactess challenge !   

I think you spent some required skill and time ton enter all the components in your chassis, since your wiring is even more compact than mine 

A+!

[Marcos - Munky]

Finished this one yesterday. PCL84 tube amp, powered by that 555 smps.


Little sound demo recorded on my smartphone. Used just the guitar, no effects or equalization.
https://soundcloud.com/marcao_cfh/pcl84

[amptramp]

A 12AX7 with a three-section R-C filter will definitely oscillate but it would not carry enough cathode current to modulate the input stage.  A 12AT7 has a µ of 60 and can carry the same current as a 12AU7.  It also has the advantage of not being an audiophool part that commands a high price.  A three-section equal R, equal C oscillator requires a gain of 29.

[rankot]

Finished this one yesterday. PCL84 tube amp, powered by that 555 smps.


Little sound demo recorded on my smartphone. Used just the guitar, no effects or equalization.
https://soundcloud.com/marcao_cfh/pcl84

Sounds quite good! Is there any amp noise you have, besides this ambient noise that can be heard on recording?

How much power do you get out of it, since you'are using SMSP for powering?

[Marcos - Munky]

There's no noise. Well, there was an oscillation at higher volume settings, which was solved by adding 2 ceramic caps to ground.

I still didn't measured the output power. I'm planning on doing it today.

Edited: IF I did the measure correctly, it's about 0.28W at about 140V. I'm still not sure if I did it correctly. It's loud enough for me to worry about using it plugged on a 4x12 while living in a flat.

[Marcos - Munky]

Another PCL84 amp. It's the same circuit as the previous one, but with a power switch and a green led added. This time, I used a 9V 300mA transformer as the output transformer (had to do the air gap).

[rankot]

How did you know you can use that transformer for OT? You did some calculations, or measurements?

[Marcos - Munky]

I did both things, a physical measurement and some calculations.

The core of this one is 1 cm x 1.5 cm, which results in a 1.5 cm² area. The equation for the core area is Sg = 10 * (P/f)^1/2, where P is power andf is the lowest frequency. Some books use 7.5 instead of 10 as the constant, but 10 is a better number since you'll get a bigger core area, and it's better to use a overdimensioned transformer than an underdimensioned one.

Solving that equation for Sg = 1.5 cm² and f = 60 Hz, I got 1.35 W as the output power. We don't know anything on the windings, so it's not a good idea to use this full number. And since my previous PCL84 amp gave me about 1/4 W, it's safe to use this "1.35 W" as the output transformer.

For the impedance, the equation is windings ratio = V1/V2 = (Z1/Z2)^2. This is a 220 V to 9 V transformer, which gives a 24.4 winding ratio. For a 8 ohm speaker (Z2), that gives Z1 = 4K8. The PCL84 asks for a 10K transformer, so it'll result in a impedance mismatch and lower output power (about half of the power). Since I used a 6K transformer before, I just gave a go on the 4K8 one and it worked pretty well. But if you want to use the 10K impedance, you can use a 6 V transformer and a 8 ohm speaker. Use the 220 V side as the primary and the lower voltage as the secondary. Don't know how bad is that mismatch in a bigger amp.

Then you have to rearrange the lams because a single ended amp asks for a transformer with an air gap. Basically, you put all the E lams, then add a layer of paper (photo paper or something like that) and then the I lams. Then superglue them together or something like that .

[rankot]

Thanks, good explanation! I can buy some cheap power transformers and use them as OTF for small SE tube amps.

[Marcos - Munky]

Yeah, for low power SE tube amps you can do it. The sound quality will be lower than a good OTF, but in my opinion it'll probably not too much noticeable. There's some work to disassemble the lams and assemble them with an air gap, but once you figure out how to do it you can do it easily. I'm hitting the corners with a ball hammer until the lams move a little bit and breaks the glue/varnish, then using some pliers to pull the lams. The first one always get destroyed