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 NEW  Matt presents bias and operation data for the 6V6 tube in SE operation - 6V6 Single-Ended (SE) Ultra Linear (UL) Bias Optimization.

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PostPosted: 10 Mar 2011, 18:16 
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Joined: 04 Jun 2008, 20:59
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Location: Arizona, USA
Hi, The amp should be hum free. :| If you can't find the problem PM me for details on what to look for. The 1K should not blow. :( The current drain there is only about 3ma. If it is more then there is an error somewhere. 1/2 watt should be fine. In the Poddwatt to simplify the design and diy build I eliminated one of the one ohm resistors. You only need to really measure one cathode current as the CCS sets a total amount. Set the trimmer for 41 millivolts between the test points. If you want you can add the second resistor and actually do a precise balance, but my experience is that setting the one is quite close enough for all but the most demanding applications.

Good listening
Bruce

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PostPosted: 10 Mar 2011, 18:29 
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Joined: 03 Jan 2009, 10:44
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Thanks, Bruce! To clarify, then, I should simply measure between the two test points and adjust the pot to obtain 41millivolts?

At this point I'm almost inclined to start over with my power supply circuit. As you point out, there's clearly a problem which likely resides somewhere between the stool and the soldering iron.
Lofton


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PostPosted: 10 Mar 2011, 21:07 
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Hi, Correct on the measuring procedure. I believe I would start at the rectifiers and measure the B+ at each of the capacitors and at the upper anode of the SRPPs. PM me with the results. Over the past year I have managed to diagnose from afar a number of problems and yours should not be that complicated. Something is sucking up the B+. More than likely it is the cause of the hum too. My first thought is that one of the output tubes has failed. One other diyer had a bad tube (internal short) and had similar problems. Second question... you are using U/L mode ... correct?

Good listening
Bruce

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PostPosted: 11 Mar 2011, 13:50 
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Last night I extracted the power supply circuit and am going to re-do it. The original was not a good fit in the chassis and with all the messing to make it fit things just got ugly. Ugly to look at and probably ugly for sound.

When recreated I'll take measurements and let you know. I hate it when tubes are bad...guess I should order more spares. Thanks for the help, Bruce.


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PostPosted: 14 Mar 2011, 22:33 
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Hi, I know you have been around on the forum for a while, but I would like to share a thought to you and the other diyers who work with tube audio. Please no one take offense. :angel: When building a tube project I find it essential to use a chassis at least 50% bigger than would seem needed. There are several reasons, First, it lets the parts breathe. Cramped chassis lead to heating problems. Second except for RF circuits and really high performance audio circuits I have not found that it is of any benefit to cram parts in closely. True you should not excessively spread them out - so I allow enough room to move things around and on occasion replace parts with ones that might be physically larger. With a bit larger chassis I find that I don't have to compromise layouts as much. It is easier to put things where they make sense. Example, the input tube near the input jack or volume control and not have to worry that one or the other is too close to the power inlet. The larger chassis also makes it easier to trouble shoot the project. Things don't always work like you expected and the extra space makes it much easier to trace the circuit for faults. BTW using colored wires helps a lot too. So I hope these thoughts help someone out there to avoid some problems. 8-)

Good listening
Bruce

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PostPosted: 22 Mar 2011, 16:32 
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Joined: 22 Mar 2011, 16:30
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Hi There. I am interested in building my own 5751 SRPP / EL84 (6BQ5) Push-Pull Tube Amplifier
Is it possible to purchase a kit of all the parts and a power supply instead of trying to get parts from different stores?
Thanks

Dmitry


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PostPosted: 22 Mar 2011, 22:58 
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Hi, Yes. I just posted on another thread that I do not push my products on the forum :worried: as I feel it is rather tacky. See the recent postings on the KT120 OddBlock thread. However complete kits are available from Oddwatt Audio. You may wish to check the web site (oddwattaudio.com) for more information or contact it via email (sales@oddwattaudio.com). PM me if you wish more information.

Good listening
Bruce

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PostPosted: 30 Mar 2011, 13:35 
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Hello, Bruce at al.,
I've been out of the country while simultaneously scheduling some major back surgery...hence my silence. Anyway, your comments about chassis size is most appropriate. When I did the original layout I was not thinking, obviously. But this was not the problem, ultimately. My board fit into the chassis just fine...until I installed the IEC power input (in the back) and the power switch (in the front)...dead center and directly opposite one another. Being stubborn, I refused to ditch the "bad" chassis and order another, so I made a new board.

I got back from Ireland yesterday and will go for surgery on Tuesday. I'd like to get this amp working properly so I'll be able to listen to it while recuperating.

So here's my latest question: once I receive some replacement tubes, what is the best way to swap them in and out to identify the faulty one, without waiting for sparks and smoke? I suspect the answer lies in voltage measurements...but what is the proper sequence? If you folks want to ignore this apparent act of sheer laziness, please do so and I'll start reading books. But that's just too darned boring!
Lofton


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PostPosted: 31 Mar 2011, 15:37 
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Hi Glad to have you back, Surgery - ugh, been there done that, not much fun. On the question. The best way is to measure the current at the cathode of each tube (part of the reason for the 1 ohm resistors. You can put one in each cathode as I do in the big amps. Otherwise just measure across the existing one. When the power starts to flow, the reading for an EL84 (across the 1 ohm resistor) should be in the 40 millivolt range. If it is zero or twice that then you can figure out which tube is bad. The good tube will survive the test if you don't take too long (under a minute is OK). If the measurement is the 2X, then that tube is OK, unless it is a direct short. I would then remove that one and put in another. If it is also 2X then the other tube is not conducting and is bad. If the reading goes to the normal range (pretty much anything between 25 and 60 mv as you have not been able to set the balance yet) then the tube you removed is bad (shorted).

Hope this helps

Good listening
Bruce

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PostPosted: 27 Apr 2011, 19:06 
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Joined: 18 Jul 2010, 15:48
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Hi Bruce !

Three months ago, I have made SRPP EL84. :thumbsup:
This is a great amp with great sound. Now it is connected to a UL without NFB. I would like to try it in Triode connection, but I lose a lot of power. Can I try to connect in parallel two EL84?
Attachment:
20110418_213748_U-189.jpg

Dean.


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