Rascal low voltage amp

[bluebunny]

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

Did you account for the different CBE vs. EBC pinouts?

[marcos_s_p]

Hey bluebunny,

Thanks for your answer! Yes I did. In this case it's a no-brainer as it's only a 180 turn of the component.  Could it be that the BC549 is not strong enough to drive the valve?

Another think I keep thinking. In this schematic there is an extra vol pot that was not in the original layout. Could it be the problem? It is wired with a connection from the ground (pin1) of vol pot 1 directly on the pin 3 of 2nd pot and through a switch to the middle pin. I wired the switch with a stereo cable and the shield I connected to ground in the input jack.

Any other ideas??

[bluebunny]

Thanks for your answer! Yes I did. In this case it's a no-brainer as it's only a 180 turn of the component.  Could it be that the BC549 is not strong enough to drive the valve?

Worth asking - been there, done that.       I think the BC549 should be a reasonable sub: specs look to me to be in the same ballpark as the 2N5089.

[marcos_s_p]

Considering that, it shouldn't be the issue right? I could exchange the transistor... But I only have 2n5088... Which is lower in gain than the 89... :/

Aside of that, I have no idea where to start debugging nor how ..... Lol I'm not the best in electronics yet...

[marcos_s_p]

Any ideas? :\

[PRR]

Let's re-post the link to schematic-
https://www.diystompboxes.com/smfforum/index.php?topic=119475.msg1114788#msg1114788

> I have no idea where to start debugging

You have been here enough. You know the drill. VOLTAGE READINGS!!
https://www.diystompboxes.com/smfforum/index.php?topic=29816.0

Voltages are key here because the tubes are run FAR from "normal" conditions. They will also show if your BJT is backward or busted. At least we will have *something* to chew on.

[marcos_s_p]

PRR, I know and thanks, but as it is not a pedal should I give the readings of the transistor and each pin of the valves? Right?

How should I set the pots? Min, Max and middle?

As there is no ICs, or other stuff I guess this should be it... Should I take any extra care as I am dealing with valves?

Thanks!

[thomasha]

Hi, readings at the valves help, both side of the output transformer (high voltage side).

- Test if the transformer is OK, if you have the 30V on both sides (plates and B+ node). Are the speakers good?

- Start with the output stage, check cathodes and anodes for voltages, grid should be at 0V but not grounded (volume must be set higher, I always set it in the middle just in case I wired it backwards). Tap at the grid pin, there should be something coming out. Even if the preamp is not working the power stage should be easy to fix.

From there work the audio path to the front of the amp. Checking the bias of each component, tube or transistor.

[marcos_s_p]

So, here it goes.

Project: Rascal mk1 from slacker
Schematic: https://drive.google.com/open?id=0B45V-V1ycMU9dnhmcVhtNXlWbTg
Layout: https://drive.google.com/open?id=1mCSB_WdmFk3OU46ZZ6xBH6R44bC6aPom
Picture of the chassis (a little messy I know): https://photos.app.goo.gl/i9AJVd4WSagEzkjr5

V+ from the power supply: 34.6V

Transistor BC549
C: 29.8
B: 23.3
E: 24.2

V1, Preamp - 12AT7
Pin1 - Anode: 22.9
2 - Grid: -0.6
3 - Cathode : 0
4 - Heater : 25.9
5 - Heater : 12.6
6 - Anode : 23.2
7 - Grid : -0.5
8 - Cathode : 0
9 - Heater center : 19.5 (pin not connected to anything)

V2, Power - 12AU7
Pin1 - Anode: 29.5 - To Output transfomer
2 - Grid: - 0
3 - Cathode : 0
4 - Heater : 12.7
5 - Heater : 0
6 - Anode : 29.5
7 - Grid : 0
8 - Cathode : 0
9 - Heater center : 6 (pin not connected to anything)

B+ to output transformer 33.2
Speakers are wired in parallel (but I tried in series). They should be fine as they were working in the previous amp that I'm recycling.

I hope that helps and thank you in advance!
Marcos

[PRR]

Your heater voltages are correct, though they can't really be wired as the schematic note says.

> Transistor BC549
> C: 29.8
> B: 23.3
> E: 24.2

We expect B to be 0.6V higher (more positive) than E. You show a 0.9V difference the wrong way. I'm tempted to say you read the legs wrong, but the difference is far larger than expected.

OTOH, you could not have 24.2V at the top of the 3k3 resistor(?) unless the transistor were working with normal gain.

Are you really dropping 5 volts across the output transformer primary? What part are you using there? (5V would be small for a 300V amp but looms large on a 35V amp.)

[marcos_s_p]

Your heater voltages are correct, though they can't really be wired as the schematic note says.

Well, I wired those as per Slacker instructions and he double checked it. Also this was all done by slacker and it was built and confirmed by him and sebsongs. So, I'm not sure if I follow... What is the issue? Could this be an alternative way of wiring them?

> Transistor BC549
> C: 29.8
> B: 23.3
> E: 24.2

We expect B to be 0.6V higher (more positive) than E. You show a 0.9V difference the wrong way. I'm tempted to say you read the legs wrong, but the difference is far larger than expected.

I doble checked and it's definitely that way. The only other option would be EBC instead of CBE, but I'm pretty sure it's not....

Are you really dropping 5 volts across the output transformer primary? What part are you using there? (5V would be small for a 300V amp but looms large on a 35V amp.)

Using a Fender 0221921 used for reverbs...

What do you suggest then?

PS: My multi-meter is quite simple, so it might not be the most precise in terms of .0xxx (I have a better one but I need to fix its leads)

Thanks again!
Marcos

[slacker]

Your heater voltages are correct, though they can't really be wired as the schematic note says.

It's been a while since we discussed it but I think he's running the heaters in series with each other and then a series resistor to drop the rest of the voltage from the supply.

[marcos_s_p]

BTW, the voltage after the diode to OT is 33.2... Still 3.4 V drop from the 1ry in the OT.

I'll try to fix my other multimeter that is better and re-do the readings, but It might take a while :/

About the heaters here is what we discussed: https://drive.google.com/open?id=0B45V-V1ycMU9dFAzblBEdWhJZm8

You can also see in the layout

[PRR]

> running the heaters in series

I was just wondering because the schematic I was looking at showed 12V to both. But the reported 25.9 - 12.6 - 12.7 - 0 is a fine series-string. The 3% difference from book-value is insignificant.

> it is silent dead....no hum....no sound either....

Utterly silent? Then I would suspect a bad connection from OT to speaker. Or a bad wire, OT, or 12AU7. (You say the speakers work on other systems.) A live 12AU7 and OT "should" make *some* hiss or buzz. Very faint, but some.

[marcos_s_p]

So the heaters are fine as it is?

Utterly silent? Then I would suspect a bad connection from OT to speaker. Or a bad wire, OT, or 12AU7. (You say the speakers work on other systems.) A live 12AU7 and OT "should" make *some* hiss or buzz. Very faint, but some.

If I put my ear glued to the speakers I can listen a very, but very faint hiss. But, plugging a guitar cable (that usually results in some noise) or adjusting the pots doesn't change anything nor make any noise. If I don´t put my ear there it's dead silent....

Valves and OT are brand new... could it be that the valves were not warm enough? I left quite a while on and tried again.... several min...almost 30m...

What about the other readings?

Thanks again 

[slacker]

So the heaters are fine as it is?

Yeah the heaters are fine.

You could make an audio probe http://diy-fever.com/misc/audio-probe/ to try and track down where the problem is. Start with the volume turned down low and have the volume low on the amp you're using to listen with, some of the signals should be loud. Start at the emitter of the transistor and if you have signal there the problem is with the power amp section. If you don't get signal on the emitter work back towards the input, you should have sound on both the anodes of V1, see if you can find where the signal goes missing.

[marcos_s_p]

I'll make the prob. It's been a while that I was planning to. As soon as I have news I'll let you know. It should take a some time though, maybe in the weekend .

Should I avoid probing anywhere specifically in the circuit? Or should I have any special cares?

Thanks

[marcos_s_p]

Hi Guys,

It might be a dumb question, but as I'm still learning here it goes. Would that be the signal path? Or am I doing something very wrong....lol?



I'll likely check this over the weekend

Many thanks!
Marcos

[slacker]

Yeah, that's it. To hear anything after the volume pot the pot will have to be turned up a bit, sorry if that's obvious.

[marcos_s_p]

Thanks Ian

About pots, no worries about the comment...better to say than not in my opinion! However I do have a question about the behaviour of those volume pots... So, basically, if the switch is of only the V1 is controlling the volume and in the moment that the switch is on, V2 will add up to the signal of V1, right? Or am I missing something?

Thanks again for your patience!