But, I can't adjust the voltage at pin 1 on the chip to 6v - it remains at a constant 11.04v. Pin 7 is adjusted to 6.00v.
In that case that channel cannot be producing sound as well.
what's the voltages on pin 1 and 8 of the tube ? Does pin-8 adjust with the volpot or is it at a constant + Ub ? (around 12V ?)
check the opamp: pin 1 and 3 should have the same voltage. pin 5 and 7 should have the same voltage. If so connect pin 3 and 5 and see if both channels have the same voltages on 1 and 7 If so adjust the output to 6V (both pots will affect the setting) If that's O.K. the opamp section is fine and something is not right in the tube section. It appears as though possibly the cathode resistor (or traces) might be open circuit.
Another problem I have is that the volume control (20k log) is no good (NOT A RESULT OF FRANS' MODS - IT WAS LIKE THIS BEFORE). As soon as I switch on I have too much volume - it starts loud and only gets louder! I have to use the volume control on my iPod to adjust it to comfortable levels.
It looks like the volpot 'ground' or common is not connected directly to the - (ground) check with a multimeter (resistance setting) with the amp off. set volpot in the middle setting. 2 of the pins should be connected directly to - battery (0 Ohm or very close)
Last Edit: Sept 23, 2012 9:47:50 GMT by solderdude
There is no obvious damage to the traces that I can see. Perhaps I damaged something when I swapped out the lin pot. It was a PITA to get out!
If the bottom left pin as described earlier is open circuit and should be connected to ground can't I simply run a wire from that pin to a convenient ground point somewhere on the PCB?
Simply solder the 2 adjacent pins (ground) together
That might fix the tube issues as well as the grid is grounded through the vol pot and obviously missing now. Also check if there is continuity between the middle pins of the potmeter from each channel go to both grids (pin2 and pin7)
Undo the straps on the tube socket and see if it works. If still 11V on one out check the traces towards the tube.
Last Edit: Sept 23, 2012 14:31:00 GMT by solderdude
Well, I’ve had this little unit running for a while now so I though I’d report back.
After implementing Frans’ modification it now runs cool to the touch – that is, the case is around body temperature – i.e. neither hot nor cold to the touch. Of course, you can still poke your finger through the valve ‘window’ and touch the valve which is obviously hot. However, compared to the standard version it is a vast improvement. In the standard version the case gets worryingly hot and the unit emits a disturbing ‘hot PCB/components smell’ - maybe this is normal and would reduce as the unit gets older/runs in? As for running time, I now get around 5.5 hours which is more than enough for me as a portable. Charging time is around 1.5 hours.
In terms of performance, like Miguel, I can’t really remember the pre-mod sound quality too well but it seems slightly less strained and certainly less distorted at higher volume levels; and it certainly goes very loud. But you know what, apart from the heat issue, I was quite happy with it in standard trim. It’s still not silent with no music playing – with a slight hiss present, but I don’t find that distracting when portable.
Overall, I’m very happy with it; it’s very musical and energetic – with more apparent depth than my Neco portable. If I needed a more ‘accurate’ sound and/or longer duration I’d plump for the Neco. But, for the cost and having a little fiddle about under the bonnet, for me it was worth a punt and I really enjoy listening to it.
With winter on the horizon, if you buy one and make sure the heater resistor is 20 ohms you’ll have a rather fun portable amplifier with a built-in pocket warmer.
A bit of a problem has surfaced! I now hear a bit of crackling noise from the left channel which was not there before. I am thinking of opening up the amp to verify my soldering but if it is due to a poor solder joint wouldn't the noise he heard on both channels?
The left channel noise continues but it seems to go away after 30 minutes of use. It could be the tube acting up! The volume pot continues to be noisy when going up or down, thought of replacing but it can get expensive to do so in an amp that started so inexpensive.
The vendor sent me a new charger since the one he had shipped originally with my amp was not the correct one. The new charger has an led that goes from red to green when the battery is fully charged.
The amp has about 15-18 hours of use and the bass is very nice: deep, tight, and powerful. The top end is a tad recessed but very smooth.
Had just ordered this amp few days ago. Still waiting for the delivery.
However, would like to mod either using the 5w 20 ohms resistor or using the TSR 1-2465 regulator. May try the 20 ohms resistor first as it pretty simple and cheaper.
Before I decided to go for the TSR, below are some questions. 1) Will the 20 ohms resistor get very hot or just warm? 2) Will the "switching" of the TSR 1-2465 regulator affect the sound as compare to normal 20 ohms resistor? 3) Will this "switching" damage the tube in the long run? 4) If using the TSR 1-2465, could I eliminate all accompanied components and only using the TSR itself with a diode on the output of the TSR?
Please advise if possible as I am new to these electronics stuff. Thanks and appreciated your time.
Please see reply 51 on page 3. Frans used those extra components around the regulator for sound technical reasons. Personally, I can't see how, other than perhaps a small reduction in heat, this regulator will result in any audible improvement,especially if filter components are removed, and when performed by somebody with limited experience may result in damage.
Understand that the extra components around the TSR could be for technical sounding reasons. However, the spec sheet stated as below...
"The new TSR-1 series step-down switching regulators are drop-in replacement for inefficient 78xx linear regulators. A high efficiency up to 96 % allows full load operation up to +60 °C ambient temperature without the need of any heat-sink or forced cooling. The TSR-1 switching regulators provide other significant features over linear regulators, i.e. better output accuracy (±2 %), lower standby current of 2 mA and no requirement of external capacitors. The high efficiency and low standby power consumption makes these regulators an ideal solution for many battery powered applications".
It also stated that * Only For input voltage higher than 32 VDC an input capcaitor 22 µF / 50 V is required. For our case here, the input voltage only about 12v and thus I presume capacitors are not really necessary. Really interesting.... Thanks.
Hi I did read the data sheets before replying. Frans would have used the capacitors at the output of the regulator to further reduce noise and ripple. However, there is nothing stopping you from leaving those added components out.Frans would have been been trying to obtain the best highest possible technical performance from this simple circuit, including the best achievable signal to noise ratio. Alex