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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: 05 Mar 2012, 16:46 
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Joined: 25 Oct 2011, 17:46
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HI Bruce,
I know that the poddwatt sounds at its best with 5751's, but what alterations would I need to make to bias a 12ax7 correctly?

I have four ECL86's kicking about which are basically a 9w el84 and half a 12ax7 in one tube and fancied using them.

Best regards,

Paul


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PostPosted: 05 Mar 2012, 18:13 
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Joined: 04 Jun 2008, 20:59
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Location: Arizona, USA
Hi, The best combination with the voltages used in this amp would be upper cathode resistor of 1200 ohms, lower one split at 1000 and 220 ohms so that the NFB can be applied to the junction. It will not be quite as low distortion as the 5751 in the circuit, but still well below 1% at 1watt for most of the range. At very low frequencies it will probably get to 2% at 1 watt at say 40 HZ on down. The 5751s are under 0.5% all the way to 20 HZ. The distortion is likely to not be audible. You could fiddle with the NFB values and increase the amount of NFB and improve the distortion, but IMHO you lose a lot of the sound characteristic of the amps. :( I believe the really clean sound is largely because of the really low levels of NFB (just enough to be sure the amp is stable under weird loads and not for curing distortion). Another issue with your tubes may be the lower dissipation. You might want to reduce the current to 35ma per tube instead of the 42 in the amps. In the present circuit they are dissipating just about 9 watts (anode and screen combined). This may be too much for your tubes. Output transformers in the 8K to 10K range should be fine though. Let us know how it works out.

Good listening
Bruce

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PostPosted: 05 Mar 2012, 18:27 
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Joined: 25 Oct 2011, 17:46
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Thanks for the advice. It's going to be a slow project but I'll let you know how I get on.


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PostPosted: 23 Mar 2012, 10:36 
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Joined: 16 Mar 2012, 11:15
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Location: UK
This is going to be my first amp build and I'm looking forward to it massively. I have sourced all of the parts on-line apart from the valves which I acquired from work. I've tested them in the valve tester at work and found them to be a good match so I'm happy with that. I'm in the UK and am having trouble sourcing the power transformer. I don't mind paying for the Edcor transformers but the UK shipping is very high.

Has anyone in the UK made this amp with other transformers? I don't mind having separate 180V and 12V units.

Also the unit will be fused at the wall but what rating of fuse is recommended on the input to the the unit? I want a separate fuse on the unit which will be a slow blow but I haven't worked out the current draw from the amp and hoped that someone had already done so.

I'm using some very old Russian tank parts for the switch and volume knows which should give the amp a nice retro look which I'm fond of. I'll be sure to post pictures when it's completed. Massive thanks for any advice.


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PostPosted: 23 Mar 2012, 13:31 
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Location: Arizona, USA
Hi, For 120 Volt mains it is a 2 or 3 amp, for 240 I use a 1.5 or 2. Power consumption when operating is quite close to 150 watts. But the input surge to the transformer and filters is significantly more. My experience is that in tube gear any fault is likely to be a rather large one ( :eek: ) and will easily take out the fuse.

Good listening
Bruce

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PostPosted: 23 Mar 2012, 15:34 
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Heed wrote:
Has anyone in the UK made this amp with other transformers? I don't mind having separate 180V and 12V units.


I searched for a long time. I found nothing cheaper. The shipping for a UK sourced transformer may be cheaper but the transformer itself was far cheaper from the states.

Paul


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PostPosted: 23 Mar 2012, 18:17 
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gofar99 wrote:
Hi, For 120 Volt mains it is a 2 or 3 amp, for 240 I use a 1.5 or 2.


Big thanks Bruce. As you say, any fault will quickly blow a fuse. I'll go for a 2A slow blow I think.

paul525715 wrote:
I searched for a long time. I found nothing cheaper. The shipping for a UK sourced transformer may be cheaper but the transformer itself was far cheaper from the states.
Paul


Yep, I've spent an hour or so tonight searching. Looks like I'll get all 3 from Edcor as I love their covers. I considered getting them sent to a friends place in the USA then getting him to send them over but if Edcor will mail them as a low cost gift then I'll just get them sent direct. Thanks for the reply. The transformers are the last thing I need to start my build in earnest. I'll post some images when I'm done.

Here's my chassis. It's a very heavy duty roasting tin I found by chance today. I'm going to have it sat on top of a slice of oak board. I'll trim the lip from the baking tray and have an indented lip routered into the oak so that the tin sits flush. The metallic coating on it looks great. It's 330x230mm in UK money or 13' by 9' in old money.

Image


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PostPosted: 26 Mar 2012, 17:21 
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Location: UK
Last question (hopefully) before I start my build. Does anyone have a veroboard layout for the PSU circuit? I can make one up myself but I'd like to have a known working reference as I've never turned a circuit diagram into an actual circuit!

Also, solen capacitors. In 15 years of electronics I've never heard of them. Is Solen a brand or a specific type of component? I'm assuming that they can't be replaced with normal caps. I'm struggling to find solens in the UK anywhere apart from eBay which doesn't have the right values for sale.


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PostPosted: 26 Mar 2012, 17:42 
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Location: Arizona, USA
Hi, The best way to layout a board (the ones in this amp are pretty much insensitive to layout) is to start with where the power comes in and work part by part to the ends. Much of the actual active circuit components can be wired directly to the tube sockets although I find that a few "lug strips" placed nearby are helpful. In the original Poddwatt I ran a "buss" ground made of 12 gauge bare copper wire directly over the center of the tube sockets. They were all in a row and the buss was about 1 inch above the sockets. That made it easy to make the several ground connections. A single wire from the center of the buss went directly to the grounds on the input and output jacks. Also a single ground from the power supply section went to the buss. This arrangement prevented ground loops (multiple paths to the same places) that will induce hum. One of my early projects (5751 /EL84 push-pull amplifier from October 2009 on this site) shows this type of layout. The amp in that project is the forerunner of the Poddwatt. Solen is a brand of capacitor and is made in France. Reasonable priced. Other brands are fine as well. Typical ones are WIMA, Sprague, and I'm sure there others in your area. In locations where I used Solen capacitors you need only use one that is a "poly" type. They can be polyester, polystyrene, mylar, teflon, etc. Not electrolytic types. The reason is that poly types are far better at passing higher frequencies and more linear in general than electrolytics.

Good listening
Bruce

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PostPosted: 26 Mar 2012, 17:50 
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Heed wrote:
Last question (hopefully) before I start my build. Does anyone have a veroboard layout for the PSU circuit? I can make one up myself but I'd like to have a known working reference as I've never turned a circuit diagram into an actual circuit!


If you feel you have to use veroboard, remove every other copper strip to avoid arcing or use breadboard like in the original pictures of the poddwatt.

For the PSU I ended up going point to point for connection of the caps (I used multicap cans to save on space) and I used tag strip for the diodes and LM317s.

Image

Heed wrote:
Also, solen capacitors. In 15 years of electronics I've never heard of them. Is Solen a brand or a specific type of component? I'm assuming that they can't be replaced with normal caps. I'm struggling to find solens in the UK anywhere apart from eBay which doesn't have the right values for sale.


In the UK Maplins sell an alternative which are good quality, but as with everything Maplins sell, they're not cheap. They're also rated at 630v but that makes no difference (apart from the fact that they're a bit bigger). The Maplins order codes are KR78K and VM93B. If you want to stick with Solen, Banzai Music in Germany sell them and they're competitively priced (but again, they are 630v ones).

I hope this helps,

Paul


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