70's Italian amp "Sound Big 10" schematic?

[JvR]

Hello everyone, this is my first post here after searching and finding information on here for many years. I have found this amp, Sound Big 10, locally and although it works, I would like to see if there's something to be done about the ticking tremolo. I haven't been able to find the schematic of this one though. The bigger "Big 25" I found, but this is definitely different because it has fewer tubes. It has 2 ECL82's and a 12AX7.

Obviously the brand name in relation to tube amps gives way too many hits on Google. I would be vary happy if someone has it!

[PRR]

Welcome!

> The bigger "Big 25" I found, but this is definitely different because it has fewer tubes.

Show/link that. Tube-count can be misleading.

[JvR]

Sure, Here's the schematic of the Big 25 and the guts of the Big 10. Fewer tubes and different tubes. The Big 10 has ECL82's (6BM8) and one 12ax7.
An amp with a similar tube combination is the Watkins Westminster, although that one has a rectifier tube and the Sound has 2 diodes. The tremolo on that amp is unusual as it seems to modulate the plate voltage of V1.
I have read that a lot of the Italian amps from that time were copies of British amps, so who knows. It's very hard for me to trace the circuitry from the 'PCB'. It can be done of course but I thought I would first try to find the schematic.

[PRR]

> Fewer tubes and different tubes. The Big 10 has ECL82's (6BM8) and one 12ax7.

12AX7 "is" ECC83, isn't it?

ECL82/6BM8 is a power pentode and a high-Mu triode almost like a 12AX7 (maybe closer to 12AT7).

But you say the "10" has just one 12AX7/ECC83? And has tremolo?? You are right, that's not enough.

[kagliostro]

Ciao JvR

There was a forum in Italy (il Riparatore) that had a section devoted to old amps and it was a good source of info about old Italian products

Unfortunately the forum is closed since a pair of years and I've no way to contact those guys

I think you must redraw the schematic by yourself

The PCB seems a single plate and so the job will be easier

You can try to put the PCB (traces face) on a scanner then print the image and using a pencil drawn the components on the printed PCB, when you have done that it will be easier to draw your schematic

If you encounter difficulties to draw the schematic, post the image of the PCB you printed and completed with components and we will try to help you

Franco

[Esquirefreak]

Just from looking at the PCB and tubes, this looks like a Hagström 39. And if I remember correctly, that was, to some degree, based on some Philips hifi amp. Compare the schematic of the Hagström to your amp, maybe?

/Max

[kagliostro]

The only schematic I find similar to the description is the Hagstrom-39



I find it on Doug's database but I'm not able to find photo of the PCB of that amp

Franco

[JvR]

Good tip about scanning the PCB, Franco! Thanks.

I've managed to solve the ticking vibrato issue by adding a small cap to ground from the grid of the oscillator triode. The amp works fine after replacing the electrolytic capacitors. At this point I have no immediate need for a schematic anymore. Thanks for the replies!

Ciao JvR

There was a forum in Italy (il Riparatore) that had a section devoted to old amps and it was a good source of info about old Italian products

Unfortunately the forum is closed since a pair of years and I've no way to contact those guys

I think you must redraw the schematic by yourself

The PCB seems a single plate and so the job will be easier

You can try to put the PCB (traces face) on a scanner then print the image and using a pencil drawn the components on the printed PCB, when you have done that it will be easier to draw your schematic

If you encounter difficulties to draw the schematic, post the image of the PCB you printed and completed with components and we will try to help you

Franco

[Firetone]

I believe that these amps where imported to Scandinavia by Hagstrom, under the name DNS (it is supposed to stand for Denmark, Norway, Sweden).
I think that this is what they called the DNS Studio 20, and if that's correct the schematic can be found here:
http://ljudbojen.com/download/file.php?id=19894

[JvR]

Interesting! The values of the components I checked all match with this schematic. Except I don’t see the negative feedback in my amp. I do feel like I hear it though. It’s a very tame amp. Thanks a lot for this!

I believe that these amps where imported to Scandinavia by Hagstrom, under the name DNS (it is supposed to stand for Denmark, Norway, Sweden).
I think that this is what they called the DNS Studio 20, and if that's correct the schematic can be found here:
http://ljudbojen.com/download/file.php?id=19894

[PRR]

> I don’t see the negative feedback in my amp.

It can't be very much NFB, and might be omitted if the speaker were mellow. Maybe so little difference that it came down to $0.24 of resistor cost- at some point in the life of any audio company, they start leaving-out stuff like that. (Note that they already omitted the cathodyne bias resistor, a time-honored cheap-trick.)

[JvR]

Yes I see that. And what about the plate resistor of the first stage? Am I wrong or is it not in the schematic? There’s a 100k resistor after the 4.7nf cap that’s connected to the plate. That can’t be right. Or?

> I don’t see the negative feedback in my amp.

It can't be very much NFB, and might be omitted if the speaker were mellow. Maybe so little difference that it came down to $0.24 of resistor cost- at some point in the life of any audio company, they start leaving-out stuff like that. (Note that they already omitted the cathodyne bias resistor, a time-honored cheap-trick.)

[sluckey]

And what about the plate resistor of the first stage? Am I wrong or is it not in the schematic?

That's a schematic error. As drawn the amp would make zero musical sound.

[PRR]

> There’s a 100k resistor after the 4.7nf cap that’s connected to the plate.

That resistor is part of the tonestack, so the impedance does not go to 0.9kp capacitive in midband.

(Never saw "kp" before, must be the week between "p" being approved and "n" getting in.)

You guys are right that there must be a plate resistor and, maybe coincidence, 100K is very likely.

Pick on this.

[sluckey]

I thought the "kp" (kilo-pico) was funny. I like it better than nano.   

[kagliostro]

Thanks for sharing the correct schematic PRR

Franco

[EL34]

Ok, I grabbed the imageI'll upload it with the same name so it replaces the other one

[isaac_teller]

file:///C:/Users/zafim/Downloads/Sound%20Big%2025%20RT%20Schaltplan%20(4).pdf


Hi. Just noticed that there is no capacitor between the driver triode of the reverb tank and the input of the tank. Wouldn't this arrangement fry the transducer?

[kagliostro]

Seems you linked something that Is on your PC

and so not available

https://el34world.com/Forum/index.php?action=dlattach;topic=31496.0;attach=112661

Franco

[isaac_teller]

Post #3. Sound big 25 schaltplan.