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PostPosted: 21 Jul 2018, 19:37 
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Joined: 29 Jun 2016, 15:59
Posts: 60
Bummer.....
Finally finished building my 2nd OddBlock (KT88s). Started checking it. B+, SRPP and filament voltages look good. But, all of the LM317 current (184 mA) is going to only one of the KT88s.
It turns out that one of them is faulty (I swapped it with a known good one).
Hopefully Antique Electronics can do something for me even thought I bought the tubes months ago.

Question, would this have damaged the good tube since it was getting all of the current? I basically tested it for about 15 mins,

Danny


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PostPosted: 21 Jul 2018, 21:08 
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Joined: 04 Jun 2008, 20:59
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Location: Arizona, USA
Hi, Probably not as they can take quite a beating ...for a while. If it balanced when you put in different tube then probably all is OK. The biggest issue is getting another one that will match the one you have. If it was a matched pair then they should be marked for gm either on the box or the tube base. Then just request another one like or at least close to it. If they were not matched or marked then it is sort of pot luck. The values for individual tubes run as much as 20-25% different. The amps are designed for a max of 10% mismatch. The reasons for this are related to stability and the nearly zero amount of negative feed back. If the one you get will not balance with any of the others. Swap them around for matches in both amps if need be. If it won't match then let me know as there are a few workarounds. I don't like to use them unless really needed as they can add non-linear behavior at high power levels. They are OK at modest levels that most folks would use and the non-linear actions are not audible most of the time.

BTW: AES is a good company to deal with. Both my company and I use their commercial side for most of our tubes.

Good listening
Bruce

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PostPosted: 22 Jul 2018, 09:16 
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Joined: 18 Jul 2018, 13:16
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hello bruce
g2 el84 each at + 230v.
power around 4w.
voltage at 1R every 42mV and that is 42mA each el 84.
it seems alright.
tested different lamps and all seem to be fine, I do not know.
output transformers are new to el84 PP


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PostPosted: 22 Jul 2018, 15:07 
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Location: Arizona, USA
Hi, The values are in the right range. I don't know how you were measuring the output ...but based on my amps (that I use daily) the output should be in the 5-6 watt range. As class A U/L amps they are inefficient compared to AB ones. A true AB pentode mode one could deliver about 15-18 watts. The Oddwatts are designed with the emphasis on quality of the sound not quantity. Considering that loudness is a logarithmic function and not linear the higher output of AB amps would not be that much louder. The EL84 version Oddwatts are best suited for speakers with sensitivities in the 91 db/watt and up range. They do work OK with lower sensitivity speakers if the room is small or you don't listen to really loud music. I use mine on Martin Logan Electrostatics that are about 91 db/w and they are just fine. My room is about 12X17X 9 high (in feet). The need for more power is handled by there being other amps in the family up to the KT120 version that can deliver nearly 50 watts. It can power all but the most power hungry speakers. In a typical (an admittedly vague term) most listening is done at less than 1 watt power levels. I figure on my ESLs that the average power level is actually in the 100 milliwatt range.

BTW: just in case the question crossed your mind..the amps can not easily be changed to class AB. It would take a revision of the input section and IMO lose most of the sonic benefits of the design.

Good listening
Bruce

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PostPosted: 22 Jul 2018, 21:43 
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Joined: 03 Feb 2018, 00:47
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Location: Brisbane, Australia
Hi Guys, after a little playing, my Stereo Oddblock Octal with EL34's sounds good. A small amount of hum from the speakers with no input, but when playing you don't hear it.
My main concern is my input filter capacitors are 100uF rated at 500V. After warming up, the voltage on them is 499V. My mains is 240V nominal, but runs more like 248. Note my power transformer is Edcor XPWR117. I can add a resistor, but that just wastes more energy as heat. I am tending towards replacing the first filter caps with 600V types. Is that the right thing to do? Maybe I should just change to KT88's to draw more current... ;)

One other question, I have an input selector on the front, and RCA connectors on the back. I have triple shielded coax in between. I assume that I connect the shields (ground) to the ground point nearest the driver tubes. Is that correct. I have seen people leave them unconnected at one end, but that means connecting equipment would have to rely on the safety earth connection and thus prone to problems.

Thanks,
Glenn.


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PostPosted: 23 Jul 2018, 00:08 
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Joined: 18 Jul 2018, 13:16
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Hello.
I measured the power classically on a load and used the 1khz generator, I watched the oscilloscope when the amplitude would begin to round out. Involvement around 4.5v at 4ohm.
I will still try negative feedback if the sound does not change.
maybe I'm going to raise the anode voltage to about 280vtc I will watch the cathode el84 current.


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PostPosted: 23 Jul 2018, 04:49 
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Joined: 24 Oct 2010, 07:05
Posts: 283
Danny wrote:
...all of the LM317 current (184 mA) is going to only one of the KT88s.
Check also the grid-to-ground resistors, it is possible that a cold joint or broken part (resistor) could led to the behaviour. (proven by myself :blush: )


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PostPosted: 23 Jul 2018, 06:48 
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Joined: 03 Feb 2018, 00:47
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Location: Brisbane, Australia
Hum is gone. Just need to solve the excess voltage issue. Currently using variac to keep it down.


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PostPosted: 23 Jul 2018, 16:06 
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Location: Arizona, USA
Hi gsf23, The output is almost exactly 5 watts, voltage squared divided by the impedance (4.5 X 4.5 / 4= 5.06). If you raise the B+, you will not get much more output. Perhaps a watt. Not likely to be audible. The consequence though may be short tube life. They are dissipating around 9 watts each now. Max combined anode and screen is around 13 (not 12 anode+2 screen as they are not additive) depending on tube brand. At 280 B+ minus the 10-15 on the cathodes would give you 265 times 0.042 = 11 watts. Within spec but considering there is unlikely to be significant audible increase why bother. It would be simpler to increase the current to 56 ma per tube. Still in spec, but like with the higher voltage, not much gain in sound level, again why bother. To do this though, just replace the 15 ohm resistor with a 11 ohm one. Measure the value to be sure it is right as you are getting close to max dissipation with it. Unfortunately if the volume now is not enough for your tastes then the amp is probably unsuitable for your needs. Poddwatts are designed to be used with efficient speakers. Power hungry ones need bigger amps.

Good listening
Bruce

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PostPosted: 24 Jul 2018, 18:24 
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Joined: 03 Feb 2018, 00:47
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Location: Brisbane, Australia
I have Simmons Audio research speakers which are "bookshelf style". They are 8 ohm 88dB, 65Hz - 20KHz. I am underwhelmed by the bass response from my JJ EL34 Oddwatt. Could this just be that these speakers are not man enough? My amp has 0.33uF coupling caps and the spec is 0.22uF. This should make it better bass-wise not worse? Is there anything I should check on the amp itself?

Edit: I really don't think it is the amp BTW, Bruce's design is awesome and I have pretty much built to his design.

Thanks,
Glenn.


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