Rascal low voltage amp

[marcos_s_p]

Hey guys, any other thoughts? Depending on your input I'll adapt the schematic before I start

Many thanks again!

[marcos_s_p]

Hey guys!

I did another option for the wiring based on a more point-to-point ish soldering. It looks a bit more messy, I guess...and it might be a bit more difficult to solder. Not sure though. Anyhow here it is.
https://drive.google.com/open?id=1oO3mspXBVNoIdqK2jyC0WtuXOJaqhmuA

What do you guys think? Is there any advantages into using the 1st version vs the second?

version 1:
https://drive.google.com/open?id=0B3RBntEXGMeNYkZaWWVXeW9FQnc

Also, Should I be careful with the placement of any component that I might have missed out?

Many thanks again!!

[marcos_s_p]

Hey guys,

me again! lol I had a rough year so unfortunately I had to put aside this project. However now I have everything and started build even the small cab! I have 2 questions though!

1- As I move a lot and and I have a set of two small speakers laying around I though of using those for starters (in the future I could replace if that's the case). However each one is rated at 0.8watt, which gives me roughly 1.5watt total. Would that be enough or the amp will blow them up? They are from a tiny practice amp that I had....

2- The grounds should be wired through the chassis and connected to the negative of my power supply, right? It is a ~30V supply from an old scanner that slacker said it should work fine

Thanks again for your patience and help!!
Best,
Marcos

[thomasha]

Would that be enough or the amp will blow them up?

I don't think so, the 12au7 will give you 1W on the best conditions, like 200V at the plates, in PP. Without checking the loadline I would say this amplifier gives around 0.25w to 0.5w?
The 12AT7 is stronger, some companies claim it can give 5W in PP...
Since you are running this at 30V it won't be a problem. Unless the speaker company also overspecified it's wattage...

2- The grounds should be wired through the chassis and connected to the negative of my power supply, right? It is a ~30V supply from an old scanner that slacker said it should work fine

Ground goes to negative side of the 30v PS, just like in a pedal. Normally in tube amps I would connect the circuit ground to the chassis through 2 points. One point is the first filter capacitor ground, and the second is at the in jack. So you have the shortest return for the power section and it is far away from the preamp section. Please, correct me if I'm wrong.

[marcos_s_p]

Hi Thomasha,

Thank you for your answer. About the ground, I think you have much more knowledge than I do on the subject. So, for curiosity, why shouldn't I ground near the preamp? But also, isn't this circuit so small that everything is close to the preamp? lol However I guess that applies more to amps that have a power transformer which would lead to ground loops that would interfere with the output transformer, etc, no? In my case, there isn't as it is in the power supply, outside the chassis.... so. I believe this interference wouldn´t happen, no?

In my wiring layout I envisioned 4 ground points just because of easier positioning for wires and components... Do you care to take a look and let me know if you would change it?

Many thanks again!!!
Marcos

[thomasha]

There is a very good, and very short PDF from Merlin about grounding:

http://www.valvewizard.co.uk/Grounding.pdf

There you will find some information.

Even in smaller builds you still want the shortest path to the ground to avoid any loops. The chassis should not be the ground of the circuit, just connected to it. A star ground is always the best way to goo, try to reduce your 4 points. Is the layout the one from january?

[marcos_s_p]

Hmm, I see.

Here is the last layout: https://drive.google.com/file/d/1IGQzmuudnmOJICtt4Gm90WCK45JyYB_x/view?usp=drivesdk

I guess I could keep the input jack and then interconnect the others to the one where the eletrolitic cap following the input from the jack, no? And only connect to the chassis there. Right?

Thanks for the help!!!

[thomasha]

I guess I could keep the input jack and then interconnect the others to the one where the eletrolitic cap following the input from the jack, no? And only connect to the chassis there. Right?

Yes, that should do.

[marcos_s_p]

Many many thanks!!

I do have one last question. There are 2 volume pots on it, and one is separated via a SPST footswitch. I need to bring the signal from the chassis to a stompbox where the switch will be. What cable would be appropriate? I guess I should use something that would shield the signal to not bring noise. But for that I would have two options. Using two mono guitar cables side by side and having the signal going in one and coming back in the other and connecting the shield to the ground. Or a stereo cable. But for the stereo there will be one signal coming and another going within the same shield which will create electromagnetic waves...not sure if that would be good. What do you think??

Thanks once again!!
Marcos

[thomasha]

Wait, what?
You want to use the second one as a footswitch boost?
Is it the gain pot, (after the first stage) or a master volume?

I would use the stereo cable, because it's easier.
Another thing is, that when you use shielded cable for the gain pot you're trying to avoid that some stronger signal in the end of the chain couples to your cable creating some kind of feedback or noise.

Since both signal will be of the same magnitude, I don't think it will be a problem.

[marcos_s_p]

The idea is to have 2 set of settings to emulate two channels without having to add another pre-amp. So basically I can use to boost or to switch between a cleaner and dirtier sound with the footswitch . This is the grain/volume... There isn't a master on this amp... So the changes come from this. In this case from those as one is right after the other and is our not activated by the switch .

For the stereo... Wouldn't it be a problem that there will be signal in two directions in the same shield? Shouldn't they be shielded separately?

Thanks again!

[thomasha]

For the stereo... Wouldn't it be a problem that there will be signal in two directions in the same shield? Shouldn't they be shielded separately?

As I said before, I think that since both signals have a similar intensity one won't interfere with the other in a significant way. The problems we have in amps is high signal levels, as seen in later stages, creating a positive feedback with previous stages and resulting in squealing.

Here, there is no gain stage between the signals, in this case, if some signal is picked from the other line it won't create an ever increasing signal and squeal.

I was thinking that you could switch the grounds at the pots, but this way they would be kind of interactive. One pot would be grounded and be the acting voltage divider, while the other would be a resistance in parallel with the upper side of the voltage divider.
This will not work unfortunately.
If one gain is maxed ( the non-grounded) it would short the upper side of the second pot. Maybe with some changes... I'm missing something ...

[PRR]

> a set of two small speakers ... each one is rated at 0.8watt, which gives me roughly 1.5watt total. Would that be enough or the amp will blow them up

Are you still planning a tube at 24 Volts?

At such low voltages you will hardly have a few tenths of a Watt. You are fine.

[marcos_s_p]

Thomasha, about the stereo...I understand. And that's good as it's more practical.
About the pots, not sure if I understand the advantage. Anyhow, you said it didn't work... So it's better to keep as it is for now, no?

PRR, it will be fun at around 30V. Not much of a difference than 24V. I guess I'm fine .

Thank you for your help
.

[marcos_s_p]

I'm halfway through it. Already built the chassis, soldered most of the components, and cut the wood for the cabinet

However,  thomasha you got me thinking. As I explained, the idea of the 2nd vol pot was to have a boost after....or to switch between cleaner and dirtier. Looking at the schematic, I'm not sure if that is the case.... is it? I'm still learning electronics...lol.... so I'm not completely sure how those 2 pots wired that way would behave....

Also, just to be sure....I should wire to the switch with the stereo...and the shield I just connect to the ground, right?

Many many thanks!!
Marcos

[marcos_s_p]

Hey guys,

So, I have all built up...but noticed a silly mistake.... the 2 speakers I have that I am recycling are 4Ohms 0.8watt each. So, for the OT that I am using which is the fender 022921 from the reverbs... ideally I should use a 4ohm speaker, right? So, of course I could use one of them with 0.8watt.....But this might be a bit borderline to the amp watts....

Anyhow, could I use them wired in series giving me 8ohms? Or even in parallel to 2Ohms..... Would my OT and amp be able to handle that.....or the speakers? What do you guys think is the best option here?

Thanks!!

[PRR]

Tube at 30V, you are likely well below 0.1 Watts. Also no possible "mis-load" will hurt the tube. Hook it up any/all ways and rock out. You might wake the baby. You probably won't call the cops in the next town over.

[marcos_s_p]

Thanks PRR,

Any advantage in hooking the 2 together? If so, better in parallel or series?

Thanks

[PRR]

2 speakers is always better than one. Series or parallel, no clue, try both.

[marcos_s_p]

Hey guys,

I need some help... I finished it and plugged it...... but it is silent dead....no hum....no sound either.... I tried the speakers in parallel and series, nothing as well.... the valves light up, but very dim.

Any ideas? The only big difference from the schematic is that I had to use the BC549 transistor instead of the 2n5089...

Any help would be appreciated!

Thanks,
Marcos