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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: 06 May 2009, 21:48 
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Joined: 06 Jun 2008, 14:03
Posts: 272
Location: Tulsa, Oklahoma
Bruce the ipod amp sounds interesting, Is it possible to share more info on what it will be?


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PostPosted: 11 May 2009, 15:31 
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Joined: 26 Oct 2008, 13:48
Posts: 70
Location: Central California coast
Thanks for the input on the SMPS :up: interesting way to get a filament supply.

Next question - in my quest to build one of these jewels :)

I noticed the EL84 uses only one grid stopper on the grounded screen power tube and it is 47 ohm. Whereas both the KT77 and the KT88 use 220 on each of the screens. So how do you determine if you need grid stoppers, if only one is needed, and what value it/they should be??

Thanks
Mike


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PostPosted: 12 May 2009, 15:08 
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Joined: 04 Jun 2008, 20:59
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Location: Arizona, USA
Hi, Great questions, actually two of them. In answer to the first one, why only one grid stopper. I didn't think any was needed for the second tube. Live and learn. Fortunately the lack of the second one didn't seem to cause any problems. Now for why and how to determine the values. The easy answer is stability. Many circuits will oscillate or "ring" when a high frequency signal is applied. The grid resistor acts like a filter when combined with the inter electrode capacitance of the tube. Values can range from 0 to around 20k for typical projects. Basically it limits the high frequency of the stage. I generally go for the least value that will work. This is sometimes hard to predict mathematically. So, from a practical point of view I use between 220 and 1000 ohms with power tubes that seem to be fairly stable and have modest to high values of inter electrode capacitance. For a wide band high gain preamp tube I would consider 5-10K. If the response (when measured) suffers, then you can change it. I have not found cad programs to be all that accurate in predicting the values. I would be glad to hear from others who have experience in this area.

Good listening
Bruce

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PostPosted: 13 May 2009, 11:00 
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Joined: 26 Oct 2008, 13:48
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Location: Central California coast
Thanks Bruce. :up: In the mean time I studied up on grid stoppers and concluded it was for stability. Now I can finish that part of the design. I've heard carbon comp resistors are good in this application, but I am prone to metal film for everything possible. :2c: Comments anyone?

Mike 8>)

If there's no sound in a vacuum, where'd the music come from? :lightbuilb:


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PostPosted: 13 May 2009, 12:04 
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Location: Winnipeg, CANADA
Hi Mike,

It really comes down to personal preference on which resistor to use. I personally like the sound of carbon comp, but they actually introduce more noise.

Cheers

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PostPosted: 18 May 2009, 00:53 
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Joined: 17 May 2009, 23:48
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gofar99 wrote:
With the transformer you have and a SS bridge rectifier, you would get about 360 volts B+ with a capacitive input filter and about 300 with a choke input. To keep within the current range desired I would use 6L6GCs on the higher voltage with an idle current of 90 ma per pair. With the lower voltage, 6V6s would be suitable at about 80 ma per pair. Some brands would handle a bit more, but I would rather err on the safe side. For an output transformer, one in the range of 8-10k would be my choice. Lower values can be used, but there is a sacrifice in output power. The 6SL7 is a good choice for the SRPP driver. The B+ to it seems to be best in the 210-250 volt range. A little more is OK, but you need to be sensitive to the heater cathode rating of the tubes. With a SS supply it is possible to have the full B+ across the tube before it is warmed up. I like to include a 30 second (more is fine) delay before applying the B+. Additionally it may not be obvious but the resistor values and capacitance values in the last two sections of the power supply filters were chosen so that they charge up just in time for the tube to warm up. The B+ thus does not exceed the H-C rating at any time. The values would need to be adjusted for other tubes and current drains. Once charged up the filters need to remain at the correct B+ when the circuit is working. For the 6SL7 this is in the 1.5-1.7ma range.

Hi Bruce,

I have just finished building the first of a pair of oddblocks, and fired it up at the weekend. First of all, I want to say that the sound is excellent, and everything worked a treat with all measurements as expected, first time. As I currently live in Malaysia I have had trouble sourcing many of the exact parts, and some of the inferior parts I've ended up with (like the coupling caps) are due to be replaced in due course.

As I couldn't get the exact values for the filter caps at 500v, I originally had lower voltage caps in series, but eventually decided that was a bodge and replaced them with the closest matches I could find at 500v. The 100uF are replaced with 120uF, and the 50uF are replaced with 82uF (the smallest 500v available). I didn't really think much of it, but then bumped into this post, where you explain the importance of the choice of the cap values in the last two stages of the HT filter.

Being a newbie at all this (this is my first project since school), I am not sure in what way the B+ might be affected by these larger filter caps. Presumably the transformer will be very slightly more stressed, but apart from that?

From my measurements the 240v seemed stable (dead on 240v in fact), and the B+ never went beyond your specifications, but I am asking now so I know whether I should be ordering the exact value caps from overseas for my second channel, or continue with the approximated values.

Really looking forward to listening to these in stereo...

Cheers,


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PostPosted: 03 Jun 2009, 21:23 
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Location: Arizona, USA
Hi, I apologize for not responding sooner, but things are moving fast here. First for all you folks who might wish to get kits, they are now available. See the http://oddwattaudio.com site.

I'm glad your build was successful. What did you finally settle on for tubes and output transformers? The amps are deceptively simple and provide excellent sound. There are a number of parts that can be subbed. The filters as noticed in the previous post can be adjusted. Bigger is OK, Smaller can cause some increases in the hum and noise levels. The B+ can also be quite a bit higher on the output tubes. In fact I use 475 on mine at the present time. The output power is slightly greater but other wise everything is the same as at the lower B+ values. The factors to watch for are the dissipation of the power tubes. I try to keep the dissipation below 80% of the rating. This helps tubes last longer and is less stress on the other parts of the amps. More of a concern is that the SRPP driver does not go over 250 B+. At the same time, the B+ bias applied to the heaters should not exceed 1/3 of the B+ applied to the SRPP top anode. The heater to cathode ratings of the tube will be exceeded and the tube will fail. A slight change in the circuit that is mostly tube dependent is that if you use the 5751 in place of the 6SL7/12SL7 then it is beneficial to change the feedback capacitor from 1000pf to 470pf. This really only affects the sound above 15kHz.. Apparently the 6SL7 and 12SL7 deliver a better high frequency drive to the outputs than does the NOS Phillips 5751s. The effect is that the change increases the response at 20kHz by about 1 db. Testing does not indicate that it has any effect on the amps stability or other performance characteristics.

Good listening
Bruce

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PostPosted: 04 Jun 2009, 12:11 
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Joined: 14 Oct 2008, 17:35
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Location: Dartmouth, Nova Scotia
I would just like to comment on how nicely set up your site is for the Oddwatt amplifiers. Good price for the quality you get with the kit, and the site is very user friendly, simple and organized. :up: :up:

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PostPosted: 04 Jun 2009, 19:36 
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Location: Arizona, USA
Hi, Thanks, the site still has some work to do. I expect it will be around next Wednesday before it is complete. I will pass the comments to the webmaster (another contributor to the diyaudioprojects site and forum).

The next projects and in turn kits ought to be fun as well - a lower power amp similar to the original Oddwatt projects with EL84s, a line stage preamp, and a headphone amp. All the projects will get posted on the diyaudioprojects site for folks to build on their own if they want.

Good listening
Bruce

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PostPosted: 04 Jun 2009, 21:14 
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Joined: 17 May 2009, 23:48
Posts: 34
Thanks for the extra info Bruce.

The first monoblock was built with JJ KT88's and a NOS 12SL7, but I couldn't find appropriate transformers locally, so bit the bullet and ordered from Edcor US (shipping at about 40% of their price). I've just ordered my second set of transformers (ETA 4-5 weeks), and as the cost has been shooting up anyway, decided to go for broke and switch to the Gold Lions. I found a US site with good prices and very reasonable shipping, and they also have the same JJ can caps you used, so I've order those too. Other caps to go in the rebuild are Obliggato Premium Film from HK (0.33 and 2.2uF), which look interesting, and Obliggato Film in Oil for the 10uF. Have you had any experience with these caps? They seem to get good write-ups.

For the feedback cap I still have a simple 50v polyester and at about 10 cents it feels a little out of place next to the gold obliggatos! I notice from your project pics that you have a much bigger cap there, but can't tell what type it is. I assume that as the NFB is essentially not heard, the quality of this cap can be ignored?

Cheers,


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