Rascal low voltage amp

[slacker]

Yeah that's right, when the switch is in the position shown in the schematic the second pot is engaged and increases the volume.

[marcos_s_p]

Hey Guys,

How are you on these stressing days? I hope you are all safe and well!

It's been a long time...too long. Sorry for not updating, but I had to focus on finishing my PhD last year which I finally did . But because of that I couldn't troubleshoot my amp yet.

So, how can I troubleshot it considering that right now I don't have a spare amp to plug an audio probe to it and then probe the built amp? I hope it was not too confusing lol. I already provided voltage readings previously, so hopefully that should give some clues to someone.

Do you have any ideas?

Many thanks!!
Marcos

[marcos_s_p]

Any ideas?

Thanks in advance!!

[Marcos - Munky]

So, how can I troubleshot it considering that right now I don't have a spare amp to plug an audio probe to it and then probe the built amp?

Build another amp just to use with the audio probe?

No, seriously. One LM386, any cap with value from 1nf to 100nf as the input cap, a 470uf as the output cap, any speaker you have on your parts bin (it could be any crappy one). Done.

Btw... I just saw you're using a 12AU7 as the output tube, and planning on running it at 24V. A 12AU7 will give you about 1W at 250V. At 24V, well... I'd bet your circuit is probably working, but the output is so low you can't hear anything at all.

A suggestion is to get rid of that 2N5088 and use a 555 smps to power it. So you can use a 12V power supply to power the heaters and the smps, and have about 200V-250V from the smps to power the amp.

[marcos_s_p]

Hey Marcos,

LOL I'm also from Brazil....but I live in NYC now.

Thanks for your reply. Well, I'm not sure if I have the components for the amp....I'll have to check....but I was wondering if there was another solution without building another thing.

About the power amp....I guess that's why there is a transistor there...to amplify the signal a bit before the valve. But this schematic was already tested and works with other people, so I guess it shouldn't be silent....there is definitively something going on. Maybe it has to do with the transistor that I used...I had to substitute from the original, could that be? Supposedly, it shouldn't... at least it was I was told by Slacker and other folks...

I appreciate any other opinions and ideas!

Thanks again,
M

[Marcos - Munky]

I guessed you're brazilian by the name lol. Beleza, xará?

The transistor is part of the preamp. The power amp it's made of the 2u2 cap, 1K resistor, V2 and output transformer. The preamp part should work ok as is, but I'm not sure on the power amp. Even if you have a strong signal from the preamp, I don't know if such low voltage is enough to give you any sound out of the power amp.

That said, I have a tranformer here I made early this week to use with a 12AU7 and didn't tested yet. I can quickly breadboard this output stage and power it with around 20V to see how much sound I can get out of it (if my transformer works lol). Just need to find a simple preamp that runs at around 20V.

As for the audio probe amp, I suggest you to build a simple signal generator like this one (https://www.eleccircuit.com/simple-signal-injector-by-transistor/) and a simple LM386 based little amp. Connect the signal generator to the input of your circuit, connect the audio probe directly to the input of the LM386 amp and use any crappy speaker you have. You'll have a complete audio probe testing tool that won't need any external amp nor anybody playing the guitar while you do the audio probe.

[Marcos - Munky]

So I breadboarded it, along with a simple preamp. Powering it with 20V, it was dead quiet. Then I powered the output stage with 150V and got a little buzz on the speaker. No guitar signal, because my preamp wasn't powered at all when I used 150V, but I could clearly hear a buzz. So probably the low voltage is getting you so low output you can't really heard it.

[marcos_s_p]

Wow, already?! lol thanks

That's interesting, but odd, no? I mean, considering that I built that in the specs that Slacker did and Sebsongs and for both it worked... not sure how to interpret that...

So, two things...is there a way for me to make it work as is? (more or less)

or, how can I mod it to work without much complications?

I'm not versed in High voltage stuff...hence I did the low voltage

Thanks,
Marcos

[Marcos - Munky]

I can't even imagine how they got sound out of an 12AU7 with that low voltage. Maybe for an headphone amp it's doable, but for a regular amp I don't have a clue. Lowering from about 250V to 200V already results in a big wattage drop.

For higher voltage stuff, yeah they can be dangerous, but there are some stuff that's more safe to work. A big 100W all tube amp could possibly kill you, but a little 1/2W amp is almost as safe as a 9V circuit. You can use this smps to power circuits with 2 or 3 12AX7/12AU7 tubes: https://postimg.cc/k6d2Bg6V. As long as you don't tough the voltage output when the circuit is powered, you're good to go.

You can, for example, build a low watt P1 (https://ax84.com/archive/ax84.com/p1.html). If you open the "options" file, you'll see at the bottom of the page a low wattage option, very similar to the output stage of the amp you're building (but the resistor is 470r and there's a cap). The only thing that differs a 6N1P used on that doc and a 12AU7 is the heater wiring. Open the schematic, pick everything from the input until after the master volume pot (actually, keep that resistor), replace the output stage with the output stage from "options" doc, use that smps I posted above for the high voltage, use a 12V power supply to power the smps and heaters and you'll have a very nice low power amp with a full tone stack.

If you want to give it a go, I have a verified layout for that smps and also a verified layout for the P1 with all pots onboard using a different power tube, but it should be easily to adapt to use a 12AU7.

[marcos_s_p]

Interesting, but I'm still wondering that there is something to explain what's happening - because it was done and worked before. I wonder if anyone else might know why or what?

Also, I guess I would have to modify quite a bit the whole thing by applying your ideas, no? Do you think there will be any advantages on your layout in comparison to the one I did?

Thanks again!

[Marcos - Munky]

The P1 is a different design from this one you're building. While P1 have a complete tonestack and a clean tone (that distorts a bit but don't get into mid/hi gain od), I believe this one may have a more jcm-like sound. You can use this design at high voltage if you remove the transistor stage and use 470uF and 22uF caps that can handle about 300V (you won't reach 300V, but it's better safe than sorry). But keep in mind the transistor stage is part of the sound, so it will sound a little bit different than the original purpose. And also you'll have to wire the heaters in a different way, but this is very easy to do.

If you want to try to make your amp work, the best thing is to contact somebody who built it sucessfuly. Slacker is one of them, and the last time he was online was on april 7, so he may be online again soon. But, as suggested, it's a great idea to have an audio probe, not just for this project but because it's a must have tool to debug non working audio circuits.

[marcos_s_p]

I see! And I agree about the audio probe...I have one....the thing is...I used to have a second amp...and now I don't And I don't have any spare parts to build a simple one.... lol

I contacted Slacker a few days ago, but haven't heard back yet. Let's see. Meanwhile if you guys have other ideas and suggestions I'm all ears!

Thanks again!!!

[duck_arse]

marcos_s_p - are you using some sort of computer to post your queries on this forum? and does your computer have a sound card, as we used to call them, of some description? most nearly all personal computers with sound capabillity these days will have a microphone input. feed your audio probe in and monitor on the comp speakers, if you are really desperate.