Banggood "Tube Amp 2" Kit

[tiv48]

Bit strange the link shows the same amp from a different supplier complete with a circuit .
The circuit shows the valve is connected
see http://www.ebay.co.uk/itm/Class-A-6J5-Va...SwbPxXPubZ
Rob T

[Nick]

Looks like a "Little Dot I+" or a "Little Bear P2-1" clone minus the onboard PSU.

I think the schematic as posted has problems too.

[Murphyv310]

I was wondering about the circuit.
On low HT like this circuit I've found the decay time to be very long indeed when removing the filament supply, best would be be to pull one of the valves out and see if audio in that channel disappears.
If not surely Bangood or whatever supplier should be informed.

[Nick]

Indeed - the delay can be a minute or more.

The use of the NE5532 (or similar) in this mode is not unusual - if you follow any of the HiFi (not Audiophool) fora (e.g. HeadFi), you will see similar circuits for headphone drivers - as Mark correctly states, the first opamp does voltage gain, the second voltage follower (plus the 2 x 47R resistors) improves (doubles) current drive and allows for 32ohm headphones, and the global feedback keeps the overall gain under control - you can piggy-back extra opamps to further increase available output current (about 40mA per opamp). The 5532 is good in that the outputs are short-circuit proof to ground or either supply rail.

[ken]

MHello All,
I have just packed the whole lot away but from what Trevor says perhaps disconnecting the heater supply was not the best way of checking to see if the valves did anything. I'll get it out again and try pulling one of the valves and see if I lose one channel. I could not read the valve part of the first circuit dig. The image did not render well here although the rest was OK. I'm replying on a phone. When I get a chance I'll look at the link posted by Rob in the hope the circuit diagram is better. Thanks for all the replies. It's given me some hope that the valves might do something and I haven't been duped after all.

Regards,

Ken.

[ken]

I've just got back to this today. I connected it up, ran it for about 5 mins, then removed one valve. The results surprised me a little. When I pulled the valve both the rhs and lhs channels dropped out then after may be 5- 10 secs one side came back but not the other. Now if one valve is pulled the other loses its heater supply so how can the side that keeps going still it continued to work. When I plugged the valve I had removed back in both channels returned. I perhaps should have pulled both valves to see what happened but I did not think of it at the time. Can I also ask those that have looked at the circuit diagrams provide by Andy and Rob if they can see the connections to the valves or the valves themselves clearly. I can't but I'm running an old laptop on linux using opera as a browser so the problem might be there.

Regards,

Ken.

[radioman]

Hi Ken,
As has already been noted, there are issues with the circuit diagram - the valve symbol isn't present and the +15V and -15V legends near C3/C6 are wrong as well, so if I get time I'll have a go at redrawing/modifying it so that it's easier to see what's going on.
I'm pretty sure the valve is just acting as a cathode follower as has already been said.
When you remove one valve there will be a large negative-going pulse across the 4.7k cathode resistor (which is connected to -15V) and this is pulling current through the op-amp coupling capacitor CD11/12. These are shown as electrolytics, and the bigger they are the longer they will take to charge.
This will cause the op-amp output to swing to one of the supply rails for a time, hence the muting of one channel.
Not sure how the signal returns though or whether this is the best way to understand the circuit !
Does the signal disappear again after a minute or two ?


Andy

[Murphyv310]

Hi.
I'd be inclined to remove both valves and test it with them out, in fact when you power it on with signal and phones connected there should be the usual delay for the heaters to warm up.

[ken]

Hi Andy,
Thanks for the reply and the explanation. I'm glad its not just me that has difficulty with the rendering of the circuit diagram. If you do have time to redraw it that would certainly help.

Does the signal disappear again after a minute or two ?

The signal did not disappear again in the time I had one valve out of circuit. I was going to switch off before I put the valve back but thought it was unlikely to be damaged, at these voltages, if  I pushed it back into the base . I'm sure I had it out more than a couple of minutes. I'll try it again tomorrow for a bit longer and see if it drops out again.

Regards,

Ken.

[radioman]

Well, I didn't redraw the circuit (maybe should have... ) but I've 'Photoshopped' the image and come up with this.
   
Hope you find it helpful.

Andy