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PostPosted: 30 May 2010, 12:05 
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Joined: 14 May 2010, 15:09
Posts: 30
Location: Toronto, Ontario, Canada
So i just finished up my EL84 SIPP OddWatt amp.

It sounds awesome! no hum!....

Ill post up some pics later after i get them off my camera...i took a lot of pics

I do have a few issues however..

For starters, the balance between each channel is not quite right, but i attribute this to a faulty dual gang input pot, i tested it with my meter and each gang doesn't read the same resistance except at either extreme of the travel. Its not that each gang is log or lin, they are both log, but one reads improper from about 10%-90% travel, hardly changing at all, then it will jump at the extremes to the proper value....so i think its faulty...I'm in the process of getting a new one.

Second problem..more serious...since i don't have a UL output tranny, i used a DPDT switch to change between pentode and triode strapped configs. As per gofar99's advice, i used a 1k resistor from the shield to the primary winding of the OPT or to the center tap. Now only on 1 channel, and on one of the tubes, the 1K resistor gets very very very hot, and burns up....this however doesn't prevent the amp from playing music....

Am i doing something wrong? i checked the wiring for each channel and they are the same...so why is only 1 resistor burning up...and why doesn't it prevent music from being played.

Last issue for now...tube bias...so in another post i made the newb mistake about how i correctly bias the tubes, but gofar's design...i think...is nearly like a fixed bias, using the LM317 as a const. current device. Now i do realize that no two tubes are exactly the same, so there is an adjustment pot built in as well as 1 ohm resistor as a test point...my question is, the 1ohm resistor is only on one side of the pot...how can i ensure that equal current is going to each tube??? because doesn't the pot act like a voltage divider? shouldn't i need another 1ohm resistor on the other side to the other tube and use both as a test point?

Anyways, see this thread for my past issues: viewtopic.php?f=9&t=1232
and i got my plan from this page http://diyaudioprojects.com/Tubes/EL84- ... -Tube-Amp/ - in the circuit diagram it is the inserted 1K resistor off pin 9 on the bottom EL84 tube (the one grounded through the 220ohm resistor)


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PostPosted: 31 May 2010, 08:39 
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Joined: 14 May 2010, 15:09
Posts: 30
Location: Toronto, Ontario, Canada
So, no one has any answers to my questions? Well, i guess some trial and error is in order then! In the future i plan to upgrade some of the caps in the driver stage, as i used some pretty crummy caps the first time around...just in case i destroyed everything!

Here are some select pics.

Image

Image

Image

Image

And here is my horrendous wiring job....though as i said, there is no hum! (not the finished product BTW)
Image

Here it is on my shelf with my new JSB Image B-15 speakers.
Image


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PostPosted: 31 May 2010, 09:01 
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Joined: 24 Feb 2009, 03:24
Posts: 562
Location: USA
Congratulation!
I will make the same!
Look...I think must be like this....
http://diyaudioprojects.com/Tubes/KT88- ... -Tube-Amp/

_________________
Hi End is back proportional to the number of composite parts!
Projects: OTL 6AS7 Gen, Electric, SEs 2A3 RCA, 300B JJ, 6S4S, 4P1L, EL11 Telefunken, 6AS7 RCA, 6S33S, 6S41S, 6S19P, PP 6005 Gen. Ellectric , headphone ampl. OTL Loftin White 6AS7 RCA....SE E84L& E80CC Siemens&Tel-n.
http://azazello-sound.blogspot.com/


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PostPosted: 31 May 2010, 09:51 
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Joined: 14 May 2010, 15:09
Posts: 30
Location: Toronto, Ontario, Canada
Yea, I'm going to wire in a new 1ohm resistor tonight. I sat back and thought about it, and i realized that since its a constant current input, and if i know the current going to one side, i know the current on the other side (simple math)...doh!!!

Even taking into account the variance in resistance values, knowing the approx. input and output of one side, i should be able to get it reasonably close with the current design....but as i have 3 more 1ohm 1% resistors lyin around, might as well insert one more into each channel and get it right on!

Of course, none of this solves why the 1 resistor keeps overheating! I don't understand it, i mean, if it occurred on both channels i could suspect there being an issue...but its only 1 channel....MIGHT be related to the pot issue, but the pot issue affects the other channel!....

i might disconnect the wires and put my meter in place and do some current measurements, see if there are any significant differences...


On a side note...I'm already thinking of the next one!!!! I think i might tackle the OddWatt 225, but go for some more power, and use the Edcor 50W trannies.....but this is pretty far away....need some money first...

I am amazed at the power and clarity of this amp though...it surpassed even my wildest expectations! it sounds sooo good, and the bass is nice and tight!, even with my cheap china tubes...I'm going to put in an order for some Electro Harmonix EL84's and see if there is any improvement, and maybe upgrade some caps...


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PostPosted: 31 May 2010, 10:52 
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Joined: 28 Oct 2009, 02:34
Posts: 501
Location: Utrecht Nederland
Hi there,

I got a one ohm resistor on each side of the poti, as well as test points, accessible from the top of my amp, to your question. But you are right, the sum of the current for both tubes is double than what you rad across the 1 ohm resistor, the rest is simple math. Nonetheless, i like to be able to check both sides, since I like to roll tubes a lot. Be careful with disconnecting the LM317 while in operation, it will break right away. I use 2 watt resistors and mine don't get hot. The 25 watt Edcor opts are heavily underrated, no need to buy bigger ones, really, I just bread boarded a Class AB amp with these 25 watt Edcor opts, and those opts will put in my test setup almost 30 volts into 8 ohm resistive load, that is 30 times 30 divided through speaker impedance, lets say 112,5 watts, respectively. (Now, this is not usable power, of course, heavily distorted, but maybe 75 to 80 watts Class AB, they will do without problem, I am serious.) The opts I used for this test were Edcor's CXPP25-8-5K. My OddBlock has now Edcor's CXPP25-MS-8K, which are really the first choice for that project.

About your overheating 1 ohm resistor, maybe a bad LM317, sometimes that happens and they allow way too much current, happened to me, blew up a brand new power transformer. Do you have another one? Otherwise exchange right with left and see what happens, than the other resistor must blow, is the LM317 bad. I hope I could help a bit. I know the OddBlock rather well, fell free to drop me a pm and ask anything you like.

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Greetings from Holland, Ben

My DIY Audio Projects:
- Single-Ended 12B4A tube amp with ECC802S driver
- DIY Push-Pull KT88 tube amp (OddWatt amp from scratch)
- 832 / GU32 tube push-pull amplifier project


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PostPosted: 31 May 2010, 11:12 
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Joined: 14 May 2010, 15:09
Posts: 30
Location: Toronto, Ontario, Canada
Bennybuis, sorry i should have been more clear, its not the 1 ohm resistor that is overheating. If you look at this schematic http://www.rockonband.com/6F5POW.jpg (similar but not the same as mine) the resistor i am talking about is the one in series from pin9 of the EL84's that gets connected to the lead from pin7 which goes to the output transformer. As per gofar99 (Bruce) i used a 1k ohm resistor on all leads from pin9 to pin 7(to OPT) or to the center tap of the OPT. (triode strapped or pentode mode)

I use a DPDT switch to change between modes.

Only on one channel, and on one of the tubes does one of the 1kohm resistors get hot...and i mean scald my finger, smoke coming off hot... i tried 2 different 1Kohm resistors as well....

It only happens in one of the modes...cant remember which right now...but if no one has a good answer im just going to replace all the current defective parts (pot), properly bias the tubes, and buy some more 1K resistors and try it again, noting which mode it gets hot in. Also, I'm using 1/2 watt resistors, but if all 4 were burning up i would assume that was the problem...but its just this one....


Last edited by MTron on 31 May 2010, 11:42, edited 1 time in total.

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PostPosted: 31 May 2010, 11:28 
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Joined: 28 Oct 2009, 02:34
Posts: 501
Location: Utrecht Nederland
Hi ,

now I understand you, sorry for being stupid sometimes :?

But to your problem why the resistor keeps smoking, I don't know either. Gonna look at the schematic for some longer and try to figure out what could be wrong to cause that. I let you know if I get an idea. Success, it will be good at the end, I promise. :)

_________________
Greetings from Holland, Ben

My DIY Audio Projects:
- Single-Ended 12B4A tube amp with ECC802S driver
- DIY Push-Pull KT88 tube amp (OddWatt amp from scratch)
- 832 / GU32 tube push-pull amplifier project


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PostPosted: 31 May 2010, 12:00 
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Joined: 24 Feb 2009, 03:24
Posts: 562
Location: USA
Mtron:.....Only on one channel, and on one of the tubes does one of the 1kohm resistors get hot...and i mean scald my finger, smoke coming off hot... i tried 2 different 1Kohm resistors as well....


You can try to change the tube....

_________________
Hi End is back proportional to the number of composite parts!
Projects: OTL 6AS7 Gen, Electric, SEs 2A3 RCA, 300B JJ, 6S4S, 4P1L, EL11 Telefunken, 6AS7 RCA, 6S33S, 6S41S, 6S19P, PP 6005 Gen. Ellectric , headphone ampl. OTL Loftin White 6AS7 RCA....SE E84L& E80CC Siemens&Tel-n.
http://azazello-sound.blogspot.com/


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PostPosted: 31 May 2010, 13:56 
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Joined: 14 May 2010, 15:09
Posts: 30
Location: Toronto, Ontario, Canada
hrmm, good idea....ill try that

Though, im also in the process of getting some Electro-Harmonix EL84's to replace the El-cheapo china tubes i have.

Quick question, there is a 1K input resistor just after the volume pot....what would the effect be if i dropped its value to about 900ohms....would there be less attenuation of the input signal, resulting in a louder signal for any given position of the volume knob?


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PostPosted: 31 May 2010, 15:57 
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Joined: 24 Feb 2009, 03:24
Posts: 562
Location: USA
MTron wrote:
Quick question, there is a 1K input resistor just after the volume pot....what would the effect be if i dropped its value to about 900ohms....would there be less attenuation of the input signal, resulting in a louder signal for any given position of the volume knob?

1 kohm or 900 ohm, result is the same....this is resistor against oscillations....

_________________
Hi End is back proportional to the number of composite parts!
Projects: OTL 6AS7 Gen, Electric, SEs 2A3 RCA, 300B JJ, 6S4S, 4P1L, EL11 Telefunken, 6AS7 RCA, 6S33S, 6S41S, 6S19P, PP 6005 Gen. Ellectric , headphone ampl. OTL Loftin White 6AS7 RCA....SE E84L& E80CC Siemens&Tel-n.
http://azazello-sound.blogspot.com/


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