Sorry for the lack of progress on the 6L6 SE-UL. I really wanted to get the chassis finished off this week but several factors have conspired to keep me from working on it.
The first factor is paint. I’ve been looking for another brand of spray paint to use and think I have found one. However, there’s a catch. The manufacturer makes their paint in four sheens: Matte, Satin, Semi-Gloss, and Gloss. But it seems that the very particular solvents used for their semi-gloss are in short supply right now. As such, so is their semi-gloss paint. If the situation doesn’t improve in a couple of weeks, I’ve decided that I’ll be going with a gloss finish on the metal as well. It will work out ok, it’s just not what I had intended at first.
The second factor is some other commitments. The heat here finally broke so I’ve been helping my daughter with her house build. We are to the point of installing standing seam metal roofing. Something that’s hard to do when the temperatures are in the mid 90s °F (≈35°C). I’ve only done shingle and shake roofs before so every aspect of this is a learning experience. Especially, since this is a hidden fastener system, there’s a lot of things you need to do in just the right order. So the roof is going slowly but we are making progress.
The third factor in my amp delay is that the fruits and vegetables are coming in here. Fresh fruits and vegetables means canning season is upon us. We just picked about 40# of blueberries some of which are being pickled for holiday meals and some have already been made into jam. And I’ve just brought in about 25# of cucumbers which will be made into dill pickles and bread-and-butter pickles; about 30 pints in all. And in just a week or two, the first of the sweet corn will be in and that means corn relish. Probably about 24 pints. When the apples come in I’ll also be putting up jars of pie filling. The problem with canning is you have to do it when the produce comes in, so it kind of trumps everything else. This spring, when the first asparagus was only $1 a bunch, I put up about 15 jars of pickled asparagus. We eat on this stock of stuff all through out the year. It really adds nice variety through the winter months and some of it, like the pickles and asparagus, make great snack food.
My hope is that I’ll be able to find the paint I need and get the amplifier chassis finished up. As soon as the chassis is done, the amp should go together in rather short order.
As always, questions and comments are welcome.
Hello Matt,
I guess you had a fun summer with hikes & lots of outdoors !
I have built your 6L6 design: lots of power, great sound stage & transparency ! Thanks for a wonderful design once again.
I had a few questions:
– I do get 401V-402V as B+ for the 6L6 and 350V-351V as B+’ for the 6SL7
– However, with the 470 Ω resistor (measured individually at 472Ω & 473Ω), I could not reach a 32V bias point, It was hovering around 30,4V So I switched to 500 Ω, now It reaches 31.8V (right) & 32.6V (left). Would you say this is acceptable ?
– On the 6SL7 the bias sets at 2.31V (right) & 2.35V (left). Is it in the ballpark ? Or should it be adjusted ?
– When I push the volume up It sometimes “clips” or gets “saturated”, I get a bit of distortion. What could that be due to ? (NB my OPTs are 5K-30W SE Toroids rated at 150mA with UL tap at 43%)
Thanks in advance for your comments.
Christophe
There is some variability between tubes with respect to currents and bias points. The change in cathode bias resistance from 470Ω to 500Ω is only about 6%. This is really an inconsequential change. Your 6SL7 bias points are fine. The load line design is -2.41v (based on GE plate characteristics published Dec 1956) so your are very close.
As for overdrive, it depends on your source. The power stage is biased at ≈32v and the gain stage has an amplification of ≈42v/v. The power stage will always be the first to go into overdrive. This occurs at an input voltage to the gain stage of ≈0.76v peak or ≈0.54v RMS (32v / 42v/v ≈ 0.76v). The volume control has a gain of approximately 0.1v/v at the 50% or 12 o’clock position. So at the midrange setting the input required to go into overdrive would be approximately 5.4 RMS. However at full volume, the input required to go into overdrive is only 0.54v RMS. If your source is 1.0v RMS (0.0dBv) then the amp will go into overdrive somewhere in the upper half range.
Short answer is I think your amp is fine. If you have a “hot” source just keep the volume down a little bit. The hottest source I have is my 27″ iMac. The audio card on this computer will output over 3.0V peak (≈2.1v RMS). But my Cambridge AXC35 CD player peaks out about 6dB lower at 1.5v peak (≈1.1v RMS). It’s good to take a careful look at your sources and know their characteristics.
Does this help.
I will check the voltage of my sources (CD & DAC) and if they are “too hot”, I will keep them reigned in ! Thanks for your guidance. As for the tubes I used: I have been “tube rolling” with TungSol 6L6 and JJ & I feel the TungSol sound a bit better on this amp. As a 6SL7, I use a Reflektor 1579 tube (special version of 6N9S), and for the rectifier I use a 5U4G “coke bottle” with a slightly higher primary voltage on the mains transformer: looks good ! Again thanks for the design & the assistance !
I meant SECONDARY voltage on the mains transformer…
I am not sure what you found, but I still think black chassis paint is an option for the project because it is simple and durable.
Apples and cucumbers are coming in here in PA. So, I can identify with a busy August getting food prepped and stored.
Like Jeff, I don’t have a garden anymore because of the deer. They will actually eat the flowers next to the house anymore.
I am glad you can enjoy the season, and I am looking forward to the seeing pictures of the next step in the amp build.
I think that gloss black would look very nice with the wood chassis.
I’m certainly enjoying my build of your design. It sounded great when I first fired it up and seems to sound even better now. A very rich, full sound.
We have deer. Almost no produce coming off my property. We used to grow lots of veggies, but deer have literally exploded in my area and all plants are in danger. Easily a couple orders of magnitude population growth in last 5-7 years. I’m not exaggerating.
I’m envious.of those who can have plants.
Well, the blueberries and cucumbers for pickles came from farms near us. But I do have tomatoes, plums, and cherries, and a few blueberries on site here.
I envy you your deer population. I grew up in Michigan and Fall always meant deer season and venison steaks, jerky, sausage, and ground venison (with about 15% beef fat added in). Where I am now the deer population is rather muted and they are Black Tail Deer which are small and don’t provide very much meat. I really miss those big Midwest whitetails.