SRX-Plus Build Thread

[tomislavkufrin]

i have some spare digital attenuator boards, and i was thinking to make one for SRX+ (i have made 10k one for SS Dynalo). is 10k ok for the digital attenuator in this case, or should i go for higher value? 

[JimL]

The original SRX schematic shows a 250k to 500k attenuator, but that is most likely due to it being designed to be driven by a tube source. As long as your source can drive 10k, which almost everything these days can, a lower impedance attenuator should be fine.

[Blueman2]

I am using a 10K Alps quad pot on both of my SRX+ builds with great results.  

[tomislavkufrin]

looking forward to trying this amp with sr-009. now i have 727, and its like looking at a really pretty girl with no tits, ass and personality.

[tomislavkufrin]

ok, one more srx-plus is alive and kicking.

i have managed to zero everything, but i have some annoying hum issues on left channel which i am pretty sure i did not have when i first tried the amp. even with the hum it works really nice with sr-009's ability to embed music directly into your eyes whether they are open or closed. for digital attenuator i have borrowed an idea that i like on rme adi-2 dac. volume steps are changing depending on the rotation speed of the encoder: 0.5db, 1db or 2db. maybe for tube amp this is not really needed as you would reach the volume level necessary to hear music right about the time the tubes are nice and warm.  

now i feel disappointed in myself that i did not impulse buy all the parts for the carbon so i have something to do until the situation with the corona virus calms down a little bit.. 

[Blueman2]

@tomislavkufrin,

Congratulations!! My SRX-Plus remains one of my favorite builds, even with my Carbon and Blue Hawaii.  There is beauty in simplicity, and the SRX-Plus is all of that.  As for the hum, I too have had issues with hum in the past, but was always able to get rid of it by 1) using different tubes and 2) elevating the filament voltages, and 3) fine tuning the trim pot settings. 

As for elevating the filaments, see my old post earlier in this thread:

Quote

I do have both filaments elevated.  12AT7s are at 60V (voltage divider off B+ rail) , and 6SN7s are at -340V (tied directly to B- rail).    I did this to eliminate hum.  I do of course have have 2 separate 6.3V windings (I modified my board to use 6.3V for 12AT7s) so they each are elevated to their own levels.

Definitely do not give up and accept the hum.  I have 2 SRX+ builds, and both had hum at first but after doing some tube swapping and endless fine tuning of the trim pots, both are hum free.  

Enjoy!!

Edited March 17, 2020 by Blueman2

[mwl168]

1 hour ago, tomislavkufrin said:

...but i have some annoying hum issues on left channel which i am pretty sure i did not have when i first tried the amp...

I have had hum/buzz issues with my SRX Plus occasionally. I have always been able to trace the issue down and resolve it. Agree with Blueman2 that you should be able to have a noise-free SRX Plus.

Here are 4 reasons that cause hum\buzz in my experience:

1. position/arrangement of the filament supply wires

2. dirty pin(s) on the tubes 

3. noisy tubes

4. poorly matched input tubes

2 and 3 can be easily identified by swapping the tubes around.

Good luck and enjoy the amp! 

 

 

[jose]

We've all had issues with hum on the SRX-P. 

A couple of weeks ago I changed the CV4024 for a pair of GE 5751 in the second stage and I had a Hum, always on the same channel.... Just I swap them out for a other pair and the problem is over.

Interestingly this pair of GE 5751 in the Megatron are totally silent. 

 

Edited March 17, 2020 by jose

[tomislavkufrin]

it looks like i have found a main suspect, i played around swapping tubes and i managed to get the hum on the other channel. the noise level is also a bit better in this configuration. i'll order some new tubes. 

 

1 hour ago, Blueman2 said:

@tomislavkufrin,

Congratulations!! My SRX-Plus remains one of my favorite builds, even with my Carbon and Blue Hawaii.  There is beauty in simplicity, and the SRX-Plus is all of that...

definitely. i have a thing for simple stuff that work well. i think it is a very good place to start. to stop also. 

[Blueman2]

And welcome, @tomislavkufrin, to the elite group of 10.  I updated the first post to include you in the list of 10 builds so far for the 'simple-is-beautiful' SRX-Plus.

JimL
congo5
MLA
Sorrodje
mwl168
luca
jose
blueman2 
Juggernaut1101
tomislavkufrin

 

Edited March 17, 2020 by Blueman2

[mwl168]

I am particularly intrigued by Blueman2's comment on elevating the filament supplies to eliminate hum.

Similar to Jose's experience, I feel my SRX Plus seems to be more susceptible to hum than my other tube amps. I read a bit into this issue and center-tapped filament supply windings and filament supply elevation were two recommendations I gathered.

@Blueman2: care to elaborate your thoughts/experience on this topic? Thanks in advance!  

[Blueman2]

1 hour ago, mwl168 said:

@Blueman2: care to elaborate your thoughts/experience on this topic? Thanks in advance!  

MLA showed me a great article here: http://www.valvewizard.co.uk/heater.html

Basically, there can be leakage between the Heater and the Cathode.  Since the heater is usually driven by AC at 60hz, some of this hum can come into the signal path.  Elevating the heater to a voltage near the Cathode can reduce the leakage and the hum.  If you have an isolated transformer for the heater, then it will allow the heater to eventually elevate itself as voltage on the Cathode drives to heater to 'float' to near its own voltage.  But this still results in hum in some cases.  So forcing the heater to a voltage that is near to the cathode can help.  

Here is the way I constructed my divider:

Quote

I put the output stage to B- (-370V for my build).  For the input stage, I made a voltage divider using 470K and 100K resistors across ground and B+, which gave me 65V for the input stage.  Tweaking the 500K pots then removed what hum remained. 

And, as stated before in this thread, do NOT use the same transformer to power both the Input and Output stage tubes.  You must you 2 totally independent windings, one for input stage, other for output stage heaters.    

Edited March 17, 2020 by Blueman2

[mwl168]

Thanks! 

Interestingly, I found that same valvewizard article MLA recommended during my search and found it to be very educational. 

 

11 minutes ago, Blueman2 said:

Here is the way I constructed my divider:

Did you miss appending a pic?

 

12 minutes ago, Blueman2 said:

And, as stated before in this thread, do NOT use the same transformer to power both the Input and Output stage tubes.  You must you 2 totally independent windings, one for input stage, other for output stage.    

Hmm, I am using two separate secondary windings from the same and only transformer to supply the filaments - one for the output tubes (6.3v) and one for the input tubes (12.6v). Are you recommending to use two independent windings or two independent transformers? 

[tomislavkufrin]

1 hour ago, mwl168 said:

Thanks! 

Interestingly, I found that same valvewizard article MLA recommended during my search and found it to be very educational. 

 

Did you miss appending a pic?

 

Hmm, I am using two separate secondary windings from the same and only transformer to supply the filaments - one for the output tubes (6.3v) and one for the input tubes (12.6v). Are you recommending to use two independent windings or two independent transformers? 

http://deewm.com/Webcache/The_Valve_Wizard/Heater_Filament_Supplies/Heater_Filament_Supplies.html 

something like this(lower pic)? it is basically same like valve wizard, but with artificial centre tap. 

also what i found regarding separate trafo/windings.. what i understand, only separate windings are needed, not two transformers. somebody please correct me if i am wrong. 

 

On 1/12/2016 at 4:33 AM, JimL said:

Correct on the filaments.  The 12AT7 filaments sit about 60v above ground, the 6SN7GTA filaments sit about -315 volts, so they MUST be separate windings.  This is to prevent the filament-cathode voltage limits from being exceeded which would lead to filament-cathode leakage and early tube mortality.  Note that Kevin Gilmore's board (it's his design, as my original used 6.3 volts for both) uses 6.3 volts for the 6SN7GTA and 12.6 volt connections for the 12AT7s, so you'll need two separate filament voltages as well.  This prevents a builder from thinking he can get away with a single filament winding for input and output tubes, because they are two different voltages.  However, it also means that the transformer has to supply both voltages - or you need a separate filament transformer for one of them.

 

Yes, you can keep the +/-350 volt outputs.  I actually was going to use those voltages but my transformer didn't quite get there so I dropped the voltages to +/- 325 volts.

 

The -20v output goes to the input stage tail constant current sources - the resistors are chosen so that the combined current to both of them drops the voltage from -325 (or -350) to around -20 volts.  Actually for -350 volts you need a about 10k higher resistance to get to -20 volts.  -15v would do just as well, in which case you could simply tie the 6SN7GTA filaments to -350 volts, or just let them float.  The +15 volt isn't needed, unless you want to use it to power a high voltage delay circuit.

 

[Blueman2]

 

1 hour ago, mwl168 said:

Hmm, I am using two separate secondary windings from the same and only transformer to supply the filaments - one for the output tubes (6.3v) and one for the input tubes (12.6v). Are you recommending to use two independent windings or two independent transformers? 

Two independent windings.  And since Input uses 12.6V and Output uses 6.3V heaters for this SRX-Plus board layout, there is really no way to screw that up.  BUT, I modified the input tubes wiring to be able to use 6.3V (rather than 12.6V) since I had a trafo with 2x6.3V windings already.  I just wanted to make sure that if someone else did this, that they would not try to get away with just one 6.3V winding for both Input and Output.  That would be very bad due to different cathode voltages.

Edited March 17, 2020 by Blueman2

[Blueman2]

5 hours ago, tomislavkufrin said:

also what i found regarding separate trafo/windings.. what i understand, only separate windings are needed, not two transformers. somebody please correct me if i am wrong. 

Correct, you just need separate windings, not necessarily separate trafos.  

And the lower diagram would work fine.  For me, with 370V B+ rail, I have R1=470K and R2=100K.   That gave me 65V for filament elevation.   I did not use C1, but that could only help.  

[tomislavkufrin]

i used 500k and 100k what i had spare for 60V elevation. this is only for input tubes, output ones i left floating. C1 i need to buy first, after that i will try.

now the hum is minimal. still present but not annoying. 

edit: it seems the hum disappeared after the tubes warmed up a little bit . or at least it is at the level of my pc fan.

[jose]

If the valves are the "correct" my SRX´P is completely silent. However now that we have time, I think the next update will be to pass the heating strands to 12.6vDC

 

[Blueman2]

5 hours ago, tomislavkufrin said:

i used 500k and 100k what i had spare for 60V elevation. this is only for input tubes, output ones i left floating. C1 i need to buy first, after that i will try.

now the hum is minimal. still present but not annoying. 

edit: it seems the hum disappeared after the tubes warmed up a little bit . or at least it is at the level of my pc fan.

Just for the heck of it, try tying Output to B- rail using 100K resistor and see if that does anything.  I think leaving them floating is probably OK since it is more likely that input tubes are source of hum.  But what the heck, worth a try.  

And as @jose and @mwl168 mention, you might want to still try cleaning your tub connectors or switching tubes around to see how that impacts things.  And readjust the trim pots to null everything out.  

All this talk about hum in the SRX-Plus really makes me feel nostalgic about my first build!!  

Edited March 18, 2020 by Blueman2

[jose]

2 hours ago, Blueman2 said:

All this talk about hum in the SRX-Plus really makes me feel nostalgic about my first build!!  

Remedy for nostalgic: 

[Blueman2]

Or remedy for boredom.  I do have 3 weeks of 'shelter in place' here in the SF Bay Area.  Hmmmm.  

Edited March 18, 2020 by Blueman2

[jose]

Welcome to the club. 

I have been since last Friday in home (I work in a multinational) and here in Spain we have been officially since Monday... and it seems that we have at least two or three more weeks.

[tomislavkufrin]

6 hours ago, Blueman2 said:

Just for the heck of it, try tying Output to B- rail using 100K resistor and see if that does anything.  I think leaving them floating is probably OK since it is more likely that input tubes are source of hum.  But what the heck, worth a try.  

And as @jose and @mwl168 mention, you might want to still try cleaning your tub connectors or switching tubes around to see how that impacts things.  And readjust the trim pots to null everything out.  

All this talk about hum in the SRX-Plus really makes me feel nostalgic about my first build!!  

for the moment i will leave it and enjoy the sound. hum is almost completely gone. if i make all the modifications immediately i will have nothing to do in the following weeks. also, my girlfriend will kill me if i open the chassis one more time to tweak something. i will try go get some higher quality/new tubes also. 

[Blueman2]

@tomislavkufrin, how about some pics?

[tomislavkufrin]

11 hours ago, Blueman2 said:

@tomislavkufrin, how about some pics?

not the prettiest build, but i'm learning. goal was to make it work, and worry about the looks later. metal works was something new for me, you need to have good tools. these are my first two amps i have ever built. lower one is SS dynalo. 

edit: looks like i can upload maximum one picture(500 kb).