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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: 24 Mar 2017, 05:46 
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I'm here again about the subwoofer output. Just to confirm that increasing the resistors' values up to 20k to get more output level drives in problems.
Actually the sub's volume pot mutes the sub's speaker in the last third of its course albeit the main speakers seem to work fine.
This strage behavior doesn't happen with project resistors' values.


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PostPosted: 24 Mar 2017, 08:28 
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Location: Arizona, USA
Hi. Yes it will.

Good listening
Bruce

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PostPosted: 26 Mar 2017, 19:20 
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eshazen wrote:
All good now! What a great amp!
I think it sounds a little cleaner with the NFB connected.
I really need to find some better speakers... maybe a set of Pensils or something
like that is my next project!


Can you post a picture of the wiring now that you are complete and have the feedback connected?

After rewiring my amp like your schematic it now works! but the sound is a little muddy, with some unwanted reverb I still need to set the bias

P.S. It took me forever to realize that I didn't have the B+ connected to the the 5751's


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PostPosted: 26 Mar 2017, 20:36 
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Hi, If the sound is "mushy" then there is some sort of problem. Perhaps a bad or weak tube. BTW these amps should never be run for more than a very short time without being balanced. A few minutes is OK. The reason is that the amount of current allowed for both tubes will flow through the pair regardless on how they are balanced. If one is nearly set at zero current the other will have to handle it all. This will cause excess dissipation in that tube and will lead to failure if not corrected. BTW the sound will not be very good either. The design is based on a maximum +/-30% load on the tubes. With a single tube consuming most of the current this will be exceeded. This is a wide margin. Especially considering that most that are once balanced will stay within 2-4% over a six month period. A mismatch of up to 10% is OK and will not be audibIe. Above that you may encounter asymetrical response and distortion. I do recommend that you check the balance about every 6 months and anytime the amp starts to sound unusual.

I personally prefer to keep the NFB on. It has little effect on the sound (some folks like it with and some without) and is there primarily for stability against above band instability. In theory it is possible to (under really unusual circumstances) get some ringing at about 75K HZ without it.

Good listening
Bruce

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PostPosted: 26 Mar 2017, 21:09 
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I fixed it, I had my Left feedback connected to my Right channel. Once I swapped that it sounds MUCH better. Keeping the wiring straight was a lot harder than I thought is would be. It's going to be awhile before I try to build an electronics project from scratch again.

I have the BIAS set so the one channel is a 41.6mv & 41.6mv And the other channel is at 43.0 and 43.1mv

My Poddwatt finally sounds good! My only comment is that the bass seems a little low when compared to my K502. I need to set up a fast way to switch between Amps to do a comparison. I am pretty tone Deaf but everything seems much clearer on the Poddwatt. I THINK I can hear more detail in the music I am listening to.

I might have to add a subwoofer. I have been learning to play bass guitar and I like exta bass now because I am paying attention to it more than I used to.

Thanks again for all of your help and Support BRUCE!


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PostPosted: 27 Mar 2017, 09:05 
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Hi If built correctly it will have more bass than a K502. The K502, K-12 and K16s all have very "muddy" bass and that may deceive you. (I have a K12 and K16 around here somewhere and have modded them extensively to get better sound, but they don't even come close to the Poddwatt) If the Poddwatt sounds shrill or lacks bass, try the following. With some sound going into it...if you disconnect the NFB does the sound go up or down. It should go up if the phase is correct on the transformers. If it goes down it is backwards. and depending on your build there are a few fairly easy ways to fix it. There is only a very little amount of NFB and the amps can work well with none, quite well with it on, and even pretty good with it backwards. It has fooled a number of builders into thinking it did not sound as good as it was supposed to when all that was the problem was the phase of the NFB. Let us know what you find.

Good listening
Bruce

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PostPosted: 29 Mar 2017, 16:11 
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Hi Everyone, There is a modification you can use on this amp that may be of interest to those of you that don't like the 25R pot in the cathodes. BTW that arrangement is used in all the commercial ones of all sizes as it is trouble free and does the job well. Anyhow there are two ways to balance the tubes, one with pots. This adjusts the cathode to ground voltage on each tube (one goes up as one goes down). That in turn alters the voltage between the grids and cathodes as they are essentially at ground potential. Thus the current is controlled. An alternative way (shown in the attachment) is to have the cathodes at the same potential and vary the grids. In a design with a CCS in the cathode circuit you really can apply a lower voltage (apply a negative voltage) to the grids as it will cause the CCS to shut down and actually if it gets below about 3 volts will do some funny non-linear things. So the answer is to add some small amount of positive voltage to the grids. The CCS will take care of the change. Be aware that what ever is added to the grids will increase the cathode voltage by a like amount. This will cause increased dissipation in the CCS. So the modification is to apply a small positive voltage to each grid via a divider network and adjusting pot. The attachment shows how this can be done. The circuit values are typical but may need to be adjusted for your particular application. It will work with any tubes that when you add the extra voltage will not cause the CCS to pass its ratings. So this can not be used on the KT120 amps, KT88 amps and others that have over 30 volts (approximately) across the LM317/317HV. A side benefit of the modification is that you can get about and extra 1/2 watt from a pair of EL84s in the Poddwatt circuit. Not audible really, but a freebee anyhow.
Attachment:
Mini Mono Block Balance Revision 2016.jpg


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PostPosted: 02 Apr 2017, 11:58 
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There was at least one request, so here are new photos of my amp in it's current (final?) configuration. Feedback is on the red/black twisted pairs running from speaker terminals back down to the feedback network on the turret board bottom center of the chassis.
The orange wires near the speaker terminals are the (unused) 4 ohm transformer outputs.

I also added a nice 6.3V incandescent pilot light from AES

Image

See http://ohm.bu.edu/~hazen/TubeAmp/ my page for larger photos.

Image Image

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PostPosted: 02 Apr 2017, 16:20 
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Hi, looks nice. Good job on the turret boards. My only thought is I like to keep the driver tube further away from the power trannie. In this amp it is not critical, but in low noise things it is good for a few db.

Good listening
Bruce

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PostPosted: 16 May 2017, 16:06 
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Joined: 25 Feb 2012, 09:33
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Location: Wakefield, West Riding of Yorkshire
Hi everyone,
I'm looking to build a poddwatt, and about to start sourcing parts (I'm in the UK). For the power tranny I can get a 190-0-190.
Would there be any adjustments to be made for the extra 10v?
Apologies if a similar question has been asked, but it;s a pretty long thread!


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