Hi All,
I just want to thank all the experienced posters here for sharing their kind advice in this forum.
I have build a few end stages, usually with small modifications to known schematics. This is the first time I built a phono stage, and it really sounds great.
Zero hum thanks to star grounding and a nicely regulated DC heater supply. I do have a little RF hash, but it is very minor and easily fixed with a few matched low value resistors on the grids.
I experimented with some cold cathode voltage regulators (OD3A's have a wonderful glow) but found it to be more dynamic and life-like without them. Maybe it was due to the different mains transformer. I tried Hammond iron with an expensive full wave rectifier and OD3A's, but this version uses Lundahl iron with a 6BY5GA in a hybrid bridge.
I still have to do a few small things, like build a cover for the (admittedly ugly) Lundahl transformer. Right now I am using budget China tubes but they will be probably be replaced with something matched of higher pedigree.
Again, I would just like to thank all experienced posters for their input and advice in this forum. I don't post much, but do read the forum frequently and have learned a lot over the years. Thank you.
Kind regards
Ian
I just want to thank all the experienced posters here for sharing their kind advice in this forum.
I have build a few end stages, usually with small modifications to known schematics. This is the first time I built a phono stage, and it really sounds great.
Zero hum thanks to star grounding and a nicely regulated DC heater supply. I do have a little RF hash, but it is very minor and easily fixed with a few matched low value resistors on the grids.
I experimented with some cold cathode voltage regulators (OD3A's have a wonderful glow) but found it to be more dynamic and life-like without them. Maybe it was due to the different mains transformer. I tried Hammond iron with an expensive full wave rectifier and OD3A's, but this version uses Lundahl iron with a 6BY5GA in a hybrid bridge.
I still have to do a few small things, like build a cover for the (admittedly ugly) Lundahl transformer. Right now I am using budget China tubes but they will be probably be replaced with something matched of higher pedigree.
Again, I would just like to thank all experienced posters for their input and advice in this forum. I don't post much, but do read the forum frequently and have learned a lot over the years. Thank you.
Kind regards
Ian
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Looks very nice mate, well done 🙂
I need to do something similar, do you have a schematic for this build?
I need to do something similar, do you have a schematic for this build?
I used a modified schematic similar to that of the EAR834p... not so complex really.
If you look up this 'romy cat' circuit online you will find he uses air tuning caps in the RIAA correction. The sound is very good with these, but I am using Amtrans AMCH instead.
The power supply for mine is also slightly different, using a Lund LL1649 transformer I happened to have. I added 2x 100 ohm wirewound resistors in series to the HT secondaries to reduce current inrush to the rectifier. It is a hybrid FW rectifier circuit with a 'virtual' center tap made of a 6BY5GA and two CREE silicon diodes. The added serial resistance smooths the response curve nicely. The 15H chokes are Hammond 159M, and most capacitors are motor run type.
I have also made a separate circuite to de-charge the caps when it is in the off position.
I breadboarded a few different schematics before deciding to put this one into my chassis. One deciding factor is that I have a lot of ecc88 and ecc83 type (various brands). So schematics I experimented with utilized these valves. There are a lot of great designs out there. I am sometimes astonished at circuits that are branded 'new' that actually have been done ages ago...
Ian
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Very nice job, I did however notice that that is a Lundahl power transformer with exposed contacts - this is a bit of a safety hazard, and you should install a protective cover over the transformer to prevent potential shock and electrocution.
Yes, I noted this in my first post. The Lundahls sound great, but building covers for them is a bit of a pain.
Yeah well, mine has a star ground and regulated DC heater supply and it HUMS. No hum and just a little RF hash (just like you) if no input is connected but when I plug in any kind of source (turntable, guitar, mp3 player) I get a loud hum. Anyway, that's my problem (still don't know what's going on). I just wanted to ask you if you like the sound and if the gain isn't too much for your cartridge. Mine has 6.5 mV output voltage and I have too much gain. I'm thinking of adding a voltage divider at the output. Not much of a difference between yours and mine. My RIAA cap is 100pf not 110pf.
So's electrocution.. Seriously though you can probably get a suitable housing on line, or contact the vendor you bought the transformer from - they should be able to help out. You'll know exactly what I mean when you inadvertently brush against one of those pins.. 😱 I've done it too many times.. 😀
One of these might work to cover an exposed Xformer:
http:/www.goodcomponent.com
Click this link:
Shuguang,Shuguang treasure,Fullmusic,Guiguang tubes,amplifier chassis,tube sockets,capacitor,resistors,coil meters,toggle switches,tag boards,transformer covers,various parts for tube guitar amps diy <meta name="keywords" content="tube sockets,shugua
Scroll down to:
Transformer>>>Tranformer Protect Cover>>>Page 1>>>Page 2
GDTC-03 $10.50
Inner: 126*96*110 (H)
Outer: 130*100*116 (H)
“or”
GDTC-08 $14.61
Inner: 127*127*130 mm
Outer:134*134*136 mm (H)
Other sizes on offer as well. Silver or Black or Silver/Black. I’ve not bought any yet but they look good and are priced Hong Kong (Shenzhen City) cheap. There's other cool looking stuff as well.
Donald.
http:/www.goodcomponent.com
Click this link:
Shuguang,Shuguang treasure,Fullmusic,Guiguang tubes,amplifier chassis,tube sockets,capacitor,resistors,coil meters,toggle switches,tag boards,transformer covers,various parts for tube guitar amps diy <meta name="keywords" content="tube sockets,shugua
Scroll down to:
Transformer>>>Tranformer Protect Cover>>>Page 1>>>Page 2
GDTC-03 $10.50
Inner: 126*96*110 (H)
Outer: 130*100*116 (H)
“or”
GDTC-08 $14.61
Inner: 127*127*130 mm
Outer:134*134*136 mm (H)
Other sizes on offer as well. Silver or Black or Silver/Black. I’ve not bought any yet but they look good and are priced Hong Kong (Shenzhen City) cheap. There's other cool looking stuff as well.
Donald.
I know.
Do you know what they want for a Lundahl transformer cover? These Lundahls MUST always have a cover and they are not sized like any other transformer... something like 105 x 90 x 80 mm...
I can make my own with some folded aluminium and epoxy for a LOT less. I will paint it black soon.
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I also use a matched pair that measure 102 pf, since AMCH are not available in 110pf. Tried bypassing with different value 8uf silver mica's but the sound was worse. There is something better though... try bypassing with an OD3A tube 🙂 capacitance is about 5 pf and sounds really not bad (tip!) This change might become permanant...
Sorry for your hum... did you put the PS onboard or separate? Separate PS is always better, but I was just being lazy here. She who rules the living room said it could not exceed a certain size... If you look close at how I did mine, the ground line for the PS goes star to the first Motor-run cap, which goes through the hybrid 'bridge' straight to ground. The chassis's only connection to ground is directly at the post.
I do this with all my single ended amps, so it made complete sense to do it with a pre-amp.
hmmmmm.... maybe try a smaller 1st coupling capacitor?
Thanks for the tips. My trouble is the first tube, if I bypass it the preamp is noiseless (too bad this is a phono preamp 🙂). It is picking up noise and is also microphonic and it shouldn't be. I'm using tube shields. I have to relocate it and see if this fixes my problem. My PT is on the chassis but not so close to the tubes as yours.
I have that tube. First thing is to fix the hum and then after some time (I have a hate/hate relationship with this preamp right now) I may start thinking about mods.
I have that tube. First thing is to fix the hum and then after some time (I have a hate/hate relationship with this preamp right now) I may start thinking about mods.
Classic symptoms of a ground loop. How about a schematic, with the ground connections exactly as built?
Sheldon
A PT can radiate a lot of EMI and a TT cartridge can pick it up from several feet away. I had a TT sitting close to an amp and had hum I couldn't track down to cables or anything. I moved the TT to rearrange things and the hum dissappeared if I moved it more than 2 feet away. The TT was grounded and all that to the amp just like it was supposed to be but the cartridge was picking up the EMI coming off the amp PT. Hope this helps.
20
Hi Sheldon and 20to20, really appreciate your willingness to help me out with this problem but I have tried every possible grounding scheme and read every article on the web about ground loops (OK, I'm exaggerating a bit). The TT isn't the problem. I'm clueless and tired of trying to fix this (two weeks scratching my head already), besides I don't want to hijack soulmerchant's thread.
Well, I have a star ground, what else can I do? Shielded wire to the input tube, ground lifted from chassis via 10 ohms resistor. Earth wire from ac cord wired in and out makes no difference. Giving up, it will be much easier for me to start all over again with a different chassis and different layout.
Well, I have a star ground, what else can I do? Shielded wire to the input tube, ground lifted from chassis via 10 ohms resistor. Earth wire from ac cord wired in and out makes no difference. Giving up, it will be much easier for me to start all over again with a different chassis and different layout.
It's interesting that you feel you have way too much gain for 6.5mv input. Have you "quadruple" checked the first tube input components to make sure they are setting the correct input impedance? If the input impedance is too high you can overdrive it with next to nothing for a signal.
20
Unfortunately, I have "quadruple" checked everything including myself. 🙂 The phono stage has X300 gain so it overdrives my amp with the pot at max. setting. Nothing strange here. The chassis has eddy currents I can't get rid of, I know because my hand feels it. I don't know if this is the source of my problems but those currents are there the moment I plug in the AC cord OR any interconnect cable. Doesn't matter if the PT is on or off the chassis. Wired in or wired out. The chassis just needs a cable plugged in with some electricity in it to feel 'funny'. I have built many tube gear and this is the first time I'm really clueless about what's going on. I really don't want to bother anyone anymore with my problems, be glad it hasn't happened to you!
Too much gain is not so good. I find this solution nice since I can avoid a line stage. And I have a few other reasons to try this 😉
You could reduce gain somewhat with a 12au7 (ecc82) in the third valve position. just make sure to change the cathode resistor value to around 68K ohm.
Its definitely very strange to get eddy current on the chassis like you are experiencing. Maybe you could post some photos of your work?
Also, there are other (VERY GOOD) phono circuits with lower gain... One I also really like is Broskie's Unicorn. For C1 and C2 I used 0.22uf ERO MKT1813's that were in my box. It is the phono stage I most like for listening to classical music late at night...
I keep thinking it must be something in your PS. Is it a 100Hz hum you have? If you can, please post a photo.
Ian
You could reduce gain somewhat with a 12au7 (ecc82) in the third valve position. just make sure to change the cathode resistor value to around 68K ohm.
Its definitely very strange to get eddy current on the chassis like you are experiencing. Maybe you could post some photos of your work?
Also, there are other (VERY GOOD) phono circuits with lower gain... One I also really like is Broskie's Unicorn. For C1 and C2 I used 0.22uf ERO MKT1813's that were in my box. It is the phono stage I most like for listening to classical music late at night...
I keep thinking it must be something in your PS. Is it a 100Hz hum you have? If you can, please post a photo.
Ian
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