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drd c3g/300b - help needed

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hi all

finally wired up the remaining circuit of my breadboard. everything is wired p2p, even the 300b rectified filaments.

used my ipod as source and one of my totem model 1 as output.

powered up (not the first time, i did checks on voltage and dc on outputs before this) and played a track from bic runga's album.

it works!!! there is music... very delighted. really different sound from SS. i listened only for a short while as i know my resistor and regulator needs a larger heatsink (do not want to blow them).

measured the 300B anode, it is 605V(3V higher), c3g anode is 180V(10V higher)... i have to tweak the resistors a little.

first impressions from one speaker... warm and mellow. no harshness, but there is some humming which went away when i moved the input wire a little. i think the hum and buzz is due to incorrect termination of the input rca ground.

this is where i need advice on grounding. where to ground? before or after my switch? i'm going with a conventional single point ground, not sakuma's 2 point ground (due to chassis construction issues)

i am also having some problems which i will describe below.

my circuit is as follows:
An externally hosted image should be here but it was not working when we last tested it.


connected to ipod as source:
An externally hosted image should be here but it was not working when we last tested it.


description of circuit:
driver is triode strapped siemens c3g pentode post tube(http://www.jacmusic.com/nos/C3g-C3m-info.html) ac filaments, biased by 2 x 1.2V 2300mA NiMh at the cathode, output is 300b, dc filaments voltage regulated, rectifier is rca 83 mercury vapor tube.

psu part is 480-0-480 to rectifier to 4uF input cap to 10H choke, to 55uF GE oil cap to 5H choke then 55uF GE oil cap.

at the 480-0-480, there is a switch to open/close the 0v (ground). the same switch is used to turn on/off the primary 0-240v. this switch is a carling double pole triple throw, i am using it to switch primary 0-240v and also to switch the B+.

grounding:
my grounding for the psu caps are after this switch.
the 100k resistor from input signal to ground goes before this switch, but i terminated the input ground from the rca to after the switch (i think this should be moved to before the switch to reduce hum/buzz, any ideas?)

the cathode batteries are grounded after the switch, initially they were before the switch, but the OPT buzz with the powercord connected to mains even though the switch is off. removing the powercord stops the buzz. strange... what's the reason for this?

c3g filaments:
i added 0.5R to both pins of the c3g and got 6.8 something VAC with only the c3g tube in. then i replaced these with 1R and got 6.15VAC or so. thought i got it right. the moment i add other tubes in and switch on the filaments, it goes to 7.15VAC... not sure why.

murphy's AC heatsink?:
i tried to touch the heatsink to see if it is hot, but felt something... like little ants crawling on my fingertip... actually the heatsink is running 50 to 60 odd VAC when the filaments are on. there is no continuity from my lm317 nor my resistor that is mounted to the heatsink... but strangely... there is voltage. must be murphy's law. :p how can i isolate this? when i'm gonna build a chasis and tie the earth pin to it, will fireworks occur?

closeup of amp:
An externally hosted image should be here but it was not working when we last tested it.


c3g:
An externally hosted image should be here but it was not working when we last tested it.


heatsink just added last night:
An externally hosted image should be here but it was not working when we last tested it.


drd while on:
An externally hosted image should be here but it was not working when we last tested it.


83 while on. the blue glow is only visible from the top... bummer...
An externally hosted image should be here but it was not working when we last tested it.


day 2:
left it on for 5 to 10mins with the heatsink, no time to play with it yet. seems ok, no burning smell and no smoke.

i have replaced my tesla VR with 2 x 470R to get 940R(the required resistance from calculations), but this caused the c3g anode to go to 288VDC. will put in the tesla VR and try 900R tomorrow.

any ideas? especially regarding the c3g filaments? and thanks for reading this long post.

cheers
garbage
 
garbage said:
at the 480-0-480, there is a switch to open/close the 0v (ground). the same switch is used to turn on/off the primary 0-240v. this switch is a carling double pole triple throw, i am using it to switch primary 0-240v and also to switch the B+.

grounding:
my grounding for the psu caps are after this switch.
the 100k resistor from input signal to ground goes before this switch, but i terminated the input ground from the rca to after the switch (i think this should be moved to before the switch to reduce hum/buzz, any ideas?)

the cathode batteries are grounded after the switch, initially they were before the switch, but the OPT buzz with the powercord connected to mains even though the switch is off. removing the powercord stops the buzz. strange... what's the reason for this?

I read this over, and over, and.... well I'm still not entirely sure how you've got things connected. If you drew a schematic including PSU etc and your current grounding scheme it would help greatly.

garbage said:
c3g filaments:
i added 0.5R to both pins of the c3g and got 6.8 something VAC with only the c3g tube in. then i replaced these with 1R and got 6.15VAC or so. thought i got it right. the moment i add other tubes in and switch on the filaments, it goes to 7.15VAC... not sure why.

It is common for the unloaded voltage to be higher than the loaded voltage on the transformer (regulation and all of that...). As I understand this, you have added resistance in series with the C3G heater to reduce the voltage across the heater, yes? Now, when you add any other heaters in parallel on the same transformer winding the voltage rises? Weird. This may be a stupid question, but did you measure the voltage at the transformer secondary or across the C3G pins?

garbage said:
murphy's AC heatsink?:
i tried to touch the heatsink to see if it is hot, but felt something... like little ants crawling on my fingertip... actually the heatsink is running 50 to 60 odd VAC when the filaments are on. there is no continuity from my lm317 nor my resistor that is mounted to the heatsink... but strangely... there is voltage. must be murphy's law. :p how can i isolate this? when i'm gonna build a chasis and tie the earth pin to it, will fireworks occur?

You said you measured continuity, but continuity from which pin of the LM317 to the heatsink? The centre pin of the LM317 (Vout) is electrically connected to the case. So unless you have used mica and thermal grease or the like, the heatsink is at the same potential as the Vout of the LM317. You may have already known this, but I thought I should say this anyway just in case. I'm not sure what the 50 to 60 odd VAC is that you're measuring but it may be the 300B's drive signal as it appears at the cathode, yet there should be little to no AC voltage appering across the cathode resistors because of the ultrapath configuration... Isolate the chip if you haven't already and see if it goes away.
 
Re: Re: drd c3g/300b - help needed

thanks for the response audiousername.

audiousername said:

I read this over, and over, and.... well I'm still not entirely sure how you've got things connected. If you drew a schematic including PSU etc and your current grounding scheme it would help greatly.
quite confusing huh? i will post one tonight. :)

audiousername said:

It is common for the unloaded voltage to be higher than the loaded voltage on the transformer (regulation and all of that...). As I understand this, you have added resistance in series with the C3G heater to reduce the voltage across the heater, yes? Now, when you add any other heaters in parallel on the same transformer winding the voltage rises? Weird. This may be a stupid question, but did you measure the voltage at the transformer secondary or across the C3G pins?
the c3g heaters is dedicated, not shared with any other tubes. 300b has it's own 0-7vac which is then rectified and regulated, 83 has it's own 5vac. voltage is measured across the c3g filament pins (1 and 8) with tube in socket.

audiousername said:

You said you measured continuity, but continuity from which pin of the LM317 to the heatsink? The centre pin of the LM317 (Vout) is electrically connected to the case. So unless you have used mica and thermal grease or the like, the heatsink is at the same potential as the Vout of the LM317. You may have already known this, but I thought I should say this anyway just in case. I'm not sure what the 50 to 60 odd VAC is that you're measuring but it may be the 300B's drive signal as it appears at the cathode, yet there should be little to no AC voltage appering across the cathode resistors because of the ultrapath configuration... Isolate the chip if you haven't already and see if it goes away.
sorry, i meant i tested for continuity. there is none between the lm317 and heatsink and none between resistor and heatsink.
lm317 has a thermal pad between it and the heatsink, but the screw is not insulated. will check this out tonight if i have time. resistor has thermal grease applied.
 
psu layout

psu layout as follows:

An externally hosted image should be here but it was not working when we last tested it.


tied at G1: 100k resistor from signal to ground and 0 winding of 230-0.

tied to G2: battery bias for c3g, rca signal ground, psu caps ground and 300b cathode resistor.

does the above grounding look correct?

regarding the c3g voltage filaments, i made a mistake when taking measurement... i measured from the wrong end of the resistors... duh...

they are actually at 6.15vac. ;)

cheers
garbage
 
Re: Re: drd c3g/300b - help needed

pedroskova said:


Ouch...I thought the rated Va max for a 300-B is ~450vdc?

yes it is, and higher for some 300b variants.

i should have made myself clear. the anode of the 300B is at 606vdc, but anode to cathode voltage of the 300B is 342vdc. ;)
the 300b at the moment is running at 342v, -59v, 62mA. much different from my design of 350v, -74, 60mA. will have to tweak the resistors somewhat.
 
Garbage,

I don't see anything much obviously wrong with the PSU....

I assume you use the switch from the transformer CT to ground to allow the 83 time to warmup, yes?

You haven't drawn it, but you should ground the CT of the C3G filament heater winding (or one side if it doesn't have a CT). If it's already grounded - or referenced to some higher voltage leave it :D
 
audiousername said:
Garbage,

I don't see anything much obviously wrong with the PSU....

I assume you use the switch from the transformer CT to ground to allow the 83 time to warmup, yes?

You haven't drawn it, but you should ground the CT of the C3G filament heater winding (or one side if it doesn't have a CT). If it's already grounded - or referenced to some higher voltage leave it :D

thanks for looking. ;)

yes, the switch from transformer CT to ground is to allow for warmup manually.

ok, will try grounding the CT of c3g.

right now i am playing norah jones' come away with me cd with it. :)

very very slight hum(maybe bcos speaker is 87db) when i place my ear right next to the woofer.
i think this is expected, as i put the drd choke right in the middle of the 2 psu chokes. easier to breadboard.

in the actual construction, the drd choke will be mounted above the psu chokes at 90degrees.

cheers
garbage
 
some pics

updates on amp chassis building... below are pics of one mono side.

front and side panels 10mm aluminium.
bottom and back 2mm aluminium.
top 2mm copper.

side view:
An externally hosted image should be here but it was not working when we last tested it.


other side view:
An externally hosted image should be here but it was not working when we last tested it.


rear view:
An externally hosted image should be here but it was not working when we last tested it.


panels on:
An externally hosted image should be here but it was not working when we last tested it.


top view, waiting for 300b:
An externally hosted image should be here but it was not working when we last tested it.


cheers
garbage
 
hi all

thanks for the kind words regarding the chassis. this is my first attempt at building chassis from scratch (my first attempt at building tube stuff also)

did the other channel... and put in shuguang 300bsc tubes and played for an hour or so.

there is still the slight hum that i hear when i was doing my breadboard... seems it is not due to the drd choke in between the 2 psu chokes... not sure how i can reduce this hum.

the lm338 regulator for the 300b filaments seem to have been busted somehow... all of a sudden my 300b and 83 were glowing.. i rushed to switch off both the supply... not sure why both channels died at the same time. the lm338 is mounted to a large heatsink somemore...

now am using dc but not regulated on the 300b filaments... using them via dropping resistors of 1R each on pins 1 and 4 of the 300b to get abt 5.2v - 5.3v. but one channel only... cos no more 1R. need to get some on monday. can't wait to listen to stereo again.

when there was stereo, the soundstage is slightly forward, bass is tight, vocals sweet, highs with sparkle but not overly bright. driving my totem to levels loud enough for my smallish room.

here's a pic of the amp with the tubes all in, but both side panels opened bcos of troubleshooting.

An externally hosted image should be here but it was not working when we last tested it.


cheers
garbage
 
a couple more pics

after putting in the 1R resistors for the other channel's 300b filaments
An externally hosted image should be here but it was not working when we last tested it.


with an intent listener staring at the speakers
An externally hosted image should be here but it was not working when we last tested it.


night view, lights out
An externally hosted image should be here but it was not working when we last tested it.
 
Do you still have hum problems?

I see that you have two identical mono blocks, and assuming the hum problem remains in both, from breadboarding to final assembly, I can only think that after trying all your previous remedies, that it might be ground loop induced.

Where is the audio source coming from to your amps?

If your amps are earthed to chassis, and the chassis is the 0v of the circuit, and the source audio device (dac or preamp etc) is also built similarly and earthed, then the RCA leads complete a ground loop circuit which acts like a dirty great bit antenna, picking up all sorts of nasty noise, especially ac hum.

Try connecting an signal lead from the source to the amp but disconnecting the wire screening from ONE of the RCA plugs, or just leave the plug partially out of the socket so as the outer cylinder is not in contact.

Its a long shot mind you.
 
bobhayes said:

If your amps are earthed to chassis, and the chassis is the 0v of the circuit, and the source audio device (dac or preamp etc) is also built similarly and earthed, then the RCA leads complete a ground loop circuit which acts like a dirty great bit antenna, picking up all sorts of nasty noise, especially ac hum.

Try connecting an signal lead from the source to the amp but disconnecting the wire screening from ONE of the RCA plugs, or just leave the plug partially out of the socket so as the outer cylinder is not in contact.

Its a long shot mind you.

anything's worth a try...

my chassis is not at 0v of the circuit. seems like i have some problem when doing so.

i just tried to switch the amp on without a signal at one of them. it hums louder.
 
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This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.