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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: 27 Nov 2011, 11:50 
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gofar99 wrote:
The only thing I would comment on is that I would have put the power transformer at right angles to the outputs. That is usually the best way to really minimize hum transfer between them.


That's originally what I thought too. I did the headphone trick that I've previously used on guitar amp builds (wire up PT to the mains and connect headphones to the OT secondary) and it was quieter with them the same way around. All I can think is that because the OTs are not directly infront of the PT, the magnetic field is "curving round" at that point. Thanks for the advice... all advice is good :up:


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PostPosted: 27 Nov 2011, 14:49 
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Cool, The Edcors must have better shielding than I give them credit for.

Good listening
Bruce

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PostPosted: 21 Dec 2011, 21:09 
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gofar99 wrote:
Cool, The Edcors must have better shielding than I give them credit for.

Hi,

I decided to take the plunge so I ordered today a pair of GXPP10-8-8K. I know it's 8K vs 10K but if I decide to use other tubes, 8K is a more common OPT value.

This Poddwatt makes me wonder by its simplicity and great review, it says the output power is 5-7W but an EL84 is SE is about 4-5W so not much a difference between PP vs SE.
I'm really looking forward to hear the difference. Not to open a can of worm but is the Poddwatt which is a Class A UL PP very different sound wise vs a simple Class A SE ? Power output is roughly the same. I have a few items to work at home so I will try to work on it soon..anyway I should receive these OPT in 5 weeks..

Thanks,
Isophon

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DIY SS Amp : LM3875 GC | TA2024 | TA2020 (AMP-6) | TA2022 | Amp Camp Amp | JLH
DIY Tube Amp : RH84 SE | 6V6 SET | 13FD7 SET (Chinook) | 807 UL SE | 6BQ5 Class A PP UL | 6LR8 SE
DIY Spkrs : Cyburgs Needle | MLTL (FE206E) | CSS EL70 BR | Open Baffle (Alpha 15A + FE206En) (in process...)
DIY Pre-Amp : MooseFET.
To come : MLTL (CSS enabled EL70) + more tube amp...


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PostPosted: 21 Dec 2011, 22:08 
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Hi, Ugh, Are Fords different from Chevys? Yes. In general terms the types and levels of distortion are different. There are books on that and I don't know that I can sum it up easily here. A SE amp will sound somewhat different from a push-pull one. U/L is a method of improving the sound of either type and while the sound may be improved the basic characteristics will remain.

For power output though I may be able to give you some insight. Let's start here: the poddwatt is a class A U/L push pull amp. One thing to keep in mind at the start is that power ratings are determined in several ways and may not be what they seem. (reminds me of the 60's horsepower ratings of autos) If you look in a tube manual like the one from RCA (great reference BTW) the power ratings often show a % of distortion. For an EL84 SE pentode design, it is true you can get about 4.5-5 watts. Unfortunately at a rather high percent of distortion. Way beyond what I would consider acceptable. Now if you take two EL84s in push pull class A you can get about twice that power at a significantly lower distortion level. Still a lot higher than I like. At this point many designers use various things to fix the distortion - most often lots of negative feedback. I have seen numerous designs that use in excess of 20 db of NFB. Ugh, IMHO bandaids. A direct consequence of this is more gain is needed and thus more stages of amplification and with more parts,.... the cycle repeats. Moving on, a consequence of the U/L configuration is it will reduce the power output by something in the 30% range from a pure pentode design. So in the Poddwatt, I traded off output power for better linearity and lower distortion. Other not so obvious features further reduce the distortion. Also it definitely seems that within reason fewer components results in a more pristine sound. The end result is 5-6 watts (depends a bit on tube brand) at well under 1% distortion at usual listening levels. In the larger amps,the distortion at one watt is at the limits of my test equipment at 0.05%. I don't personally know of any SE designs that can do that. There might be some, but I have never seen one, certainly none that can also deliver 15 watts at less than 1%. A few other things relate to the low distortion level, one is the choice of a 10K output transformer. Using the 8K one will give you a little more output but at a slightly higher distortion level. Trade-offs again, whoever came up with "there is no free lunch" could have had tubes in mind. All my power amp designs trade off output for sound quality, I figure that if you really need more power, then probably one of the bigger amps in the family is in order. There are now ones from 5-7 watts to 45 watts all class A U/L push-pull. From the smallest to largest they sound a lot alike. Low distortion, wide frequency response, low noise. Now all this rant, might seem self centered and I try not to go that route. :| An area I share with all other designs and designers is the desire to reproduce something as close to the musical original as can be done. :sing: There are lots of folks that really like one type or another I have no beef with them. Unfortunately, I hear things in other amps that I don't like and designed them out of mine. Forgive me for being long winded here, but the questions implied the need for a more than brief answer and to some extent reach into the heart of audio reproduction in general.

Good listening
Bruce

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PostPosted: 22 Dec 2011, 21:12 
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Hi Bruce,

Thanks a lot for your support. Much appreciated :thumbsup:

Best regards,
Isophon

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DIY SS Amp : LM3875 GC | TA2024 | TA2020 (AMP-6) | TA2022 | Amp Camp Amp | JLH
DIY Tube Amp : RH84 SE | 6V6 SET | 13FD7 SET (Chinook) | 807 UL SE | 6BQ5 Class A PP UL | 6LR8 SE
DIY Spkrs : Cyburgs Needle | MLTL (FE206E) | CSS EL70 BR | Open Baffle (Alpha 15A + FE206En) (in process...)
DIY Pre-Amp : MooseFET.
To come : MLTL (CSS enabled EL70) + more tube amp...


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PostPosted: 15 Jan 2012, 19:05 
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Hi Bruce...
So after the success of building my fathers poddwatt, I'm now turning to building my own. Courtesy of ebay I've just won an old Armstrong 221 stereo integrated ecl86 push pull amp (1 x ecc83 and 2 x ecl86 per side) and thought this would be a good donor (I would have just given the old amp an overhaul but it's designed for very low sensitivities (approx 80mV input signals) and I want mine for a cd player. I also love the sound of my fathers poddwatt!
A little bit of info about the donor amp...

The HT is 140v before rectification (the original supply used a "delon voltage doubler" to get reasonable voltages).
I intend to replace the ECL86's with EL84's.

So, I have a couple of questions (if that's ok)...

1) Using an identical power supply to that on the amps original schematic (up to resistor R51on the original schematic - see attached) would give me an HT of 290V DC. Is this suitable for the oddwatt, or would it be best to increase the value of R51 to obtain about 210v (as in the original poddwatt). I know it's pushing the EL84's close to their design limits, but if the voltage was left at 290V, what values would I need to change in the to keep all the valves biased at the right dissipation?
2)The OT's are not ultra linear. For best results should I connect the screens directly to the OT primary CT, connect to the CT using a 1k resistor or connect to the anodes using a 1k resistor?
3) Last question!!! The heater tap is 6.3v. For best results, should I use a voltage doubler to get 12.6v dc or would I be best to stick to 6.3v ac?

Thank you in advance for your time... let the fun begin!

Image


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PostPosted: 17 Jan 2012, 15:27 
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Hi, The first question is .... is the power transformer able to handle the needed current? If so, then the voltage doubler will work OK. It is not my favorite way to get higher voltage DC, as the regulation is not as good as a more conventional supply and the ripple will be at line frequency (50 or 60 HZ depending on your AC mains) and not at twice the frequency like with a bridge or standard supply. It makes it a little harder to filter. Based on practical experience with doublers, you will probably get right about 280-290 VDC as shown. Even though I seldom use them, I would consider replacing R51 with a 1-2 henry choke (rated at about 400 or more ma). This will bring the DCV B+ down to more reasonable levels. To use the EL84s as pentodes I would use a 1K /1watt resistor from each screen to the far side of R50 in the schematic. You may need additional capacitance there depending on what actually exists now. You can use the 6 VAC heaters but you need to remember that they need to be elevated above ground by about 60-80 VDC like in the standard Poddwatt design. I would attach the DC to the center tap of the 6 V winding and not use the resistors that are there now.

I'm glad the original amp performed well for you. The Poddwatt was kind of a lucky accident. I wanted to design a simple integrated stereo amp and what happened is history.

Good listening
Bruce

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PostPosted: 25 Jan 2012, 13:56 
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Hi Everyone, A possible source of concern with some resistors has come up. :| If you use the increased bias on the output tubes (via a 2.7-3.3 M resistor from the B+ like in the latest circuit there have been failures of one brand of resistors. The resistors are rated at 350 volts and 1 watt and are being operated at 250 V (no way to get more as that is the unloaded B+ available) and about 20ua. Still, we have had at least 3 fail (open). The manufacturer is looking into it as they believe as we do that there is no way it can occur (surprise). The symptom of the amp when the resistor fails is that it will not balance properly. One output tube will hog the current. Sound is rather poor as well. I'll let you know what turns up. For any new builds I believe I would use two resistors in series (a 1.2 and a 1.5M) in place of the single resistors.

Good listening
Bruce

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PostPosted: 25 Jan 2012, 21:36 
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i noticed a few pages back about case/enclosure size, what is an optimal size for an enclosure to keep heat to a minimum without being too large?


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PostPosted: 26 Jan 2012, 10:17 
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Hi, For a typical Poddwatt I use an 8X12X2.5 inch chassis. The output trannies in the corners, power one in the middle (at right angles to the others) and build the power supply in the middle of the chassis. Driver tubes on the outsides and EL84s in the middle either parallel to the front of the chassis (looks best this way) or one behind the other. The LM317s can go inside the chassis as long as there are a some (about 12 1/4th inch) vent holes above and below their location. More holes are better. The LM317s will dissipate about 700mw each. I also recommend that all things like capacitors be rated at 105C and not the more common 85C. Just a precaution.

Good listening
Bruce

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