SRX-Plus Build Thread

[JimL]

Good show.

[jose]

OK, I´m bit frustrated. I can't match the ecc81 on 0 volt during much time.

My amp pass all steps and volts are correct, including offset and balanced. I have a strong noise on both channels like hum and when I apply 2 volt on the input I have distorsion on both channels too.

So I don´t know what can I do. I´m using 3 ECC81 quad and I have the same result.

 

[MLA]

Don't know which PSU and trafo setup you have, but on my build I had to elevate the heaters to get it hum free. If you haven't already, I suggest you try that. 

[jose]

Thanks for your comments.

I use two trafos, one of them with the BH PSU (+ -355 volt and -15 volt) and for the heaters I use 12.6v @ 5Amp / 6.3v @ 5Amp. For the ECC81 I have a 2 Ohm resistor before (for getting off a bit the voltage). So I can try it again without this resistor.

[MLA]

I think the two ohm resistor is fine; what I was referring to is referencing the heater supplies to other DC levels. i think JimL had a nice post on this earlier in this or the other SRX+ thread; I'll dig it out when I get home. 

[MLA]

Found it; resistor values and hookup is in PSU schematic:

Essentially you set up two voltage dividers, one between B+ and ground, the other between B- and ground, and then you connect these to the input and output filament supplies respectively.

More info on heater elevation and hum: http://www.valvewizard.co.uk/heater.html. I did the artificial center tap thing they describe on my build to  connect the reference DC voltages.

 

 

 

Edited January 14, 2017 by MLA

[jose]

Thanks MLA. I had knew this good article

I´m working on the ground wires and inputs and I think that I have controlled !!!! 

I had at the input (unbalanced/rca) and a wire between G and cool channels conect and I started again the verifications. My surprise is that now (with the inputs connected) I have managed to adjust the voltage between plates perfectly and the voltage between plates and ground without problems.   The latter is quite misleading. Problems with the ground?

The sound is nice... I'm listening "Don Giovanni" now perfectly but I can hear a weak hum without music and with a complete silence on the room.

I need to simplify the wiring of the temporary assembly to find out what's going on.

Edited January 14, 2017 by jose

[JimL]

I've been too busy to hook up my amp after changing the PS until just before the weekend, and discovered that it was much noisier than before even though the PS was quieter, so after going through everything I tried replacing the 10M90S in the input tail current source with the DN2540 that was there initially.  Less noise though still not back to the best.  So I no longer recommend using the 10M90S in the input current source.  Sorry about that.  Haven't had any hum problems, though.

Edited January 14, 2017 by JimL

[MLA]

Cool, very nice that it's going the right way! And yes, since the circuit is inherently balanced, for unbalanced input you need to tie the negative input to ground, same as Dynalo. 

A prime hum suspect would be the routing of the wiring to the filament supplies. I remember on a tight KGDT build that the difference between hum and no hum was moving the filament cables one centimeter to the left. Literally.

Good luck!

   

[jose]

Thanks MLA.

No problem Jim. I'll use the 10M90 for a time... until I start mounting the amp boxes I guess.

[mwl168]

I had hum problem with my SRX Plus initially too. I traced the hum problem to two factors - the wiring, especially the AC filament supply, and the contact between the pins of the 12AT7's and the tube sockets. 

I rearranged and tidied up all the wiring and cleaned the pins on all the 12AT7's and my SRX Plus has been hum-free.   

 

[jose]

I´m sure that my Hum problem is a wiring issue. This is not annoying but I know it is there. On the other hand, is tricky have so many wire in a little space but now I´m working on this part.

For cleaning pins I'm thinking about use isopropyl alcohol and a aluminum wool.

[jose]

At the moment and once removed the hum noise (coming from the pre-valves ), I have not noticed that the ampli makes noises or strange things.

I'm listening Bach,(Goldberg Variations/Glenn Gould)   

Thanks for your help guys

[MLA]

Congrats! Enjoy

[JimL]

Great!  Glad that you got it sorted out.  Enjoy! 

Edited January 26, 2017 by JimL

[jose]

Thanks!

I have had many problems due to the pre-valves.  so I changed some ECC81/CV4024 quads... but now it´s work OK.

[jose]

I'm working on two chassis but I have a stupid question; is a soft start recommended?

[JimL]

7 hours ago, jose said:

I'm working on two chassis but I have a stupid question; is a soft start recommended?

I did on mine, but probably doesn't make much difference, if any.

[MLA]

I don't have one in mine, and it does fine so far...

Edited February 20, 2017 by MLA

[mwl168]

I don't have one on mine either although I do have a thermistor on the AC mains.

[jose]

 

Still in progress...

Edit, sorry, I would like post a picture but Picassa/google photos has problems

Edited March 4, 2017 by jose
Picassa web Disabled

[jose]

[nopants]

where'd you get the case? looks great as always.

[jose]

With top

 

 

 

Thanks. I buy my boxes in MODU and then I do the mechanised ones myself with the help of a drill, Dremel and a lot of patience. That's why my boxes have simple designs and are so similar.

 

I admire the cases that make other members much more elaborate than mine but with my tools and skills I can't do much more.

 

 

 

[Blueman2]

Well, after almost 2 years off, I am back into building this project.  I was able to build a stable KGBH PSU (actually 2 of them) with -20VDC added for the SRX-Plus.  We are about 1/2 thru building the amp boards (1 for me, 1 for my son).  Looking forward to reporting back as we make progress.