Prototypers wanted

[Pars]

Looks really awesome, Nate! It's great that you'll be testing for the heat around the PT wit those slots, too. I agree that it would be nice to get rid of the heat sink. I tried several dozen smallish holes in the same location as the heat sink, but no bottom or side venting - it didn't work and I put the heat sink back on. We were trying to keep from putting any holes in the bottom of the case and I got fed up with trying to drill through the sides with all the slots. (If you don't hit a slot dead-on, the hole is miserably screwed up.)

If you haven't done it yet, I'd definitely go with Doug's LED mod or use the higher CCS current with the resistor bias - I think Doug is calling that the WE connection? It's a different amp when you do that.

In any event, the plans are definitely to include a custom-machined case with the eventual kits. That should alleviate the issues for those of us who are tool-challenged.

HEY! I resemble that remark

Have done some listening with Blue LEDs in, but no comparison with going back to the Red LEDs, going to higher CCS current/resistor bias, etc. I did A/B it with my Gilmore and they are pretty close. The l'esp is a bit darker, I can hear the increased distortion at times, but not bothersome. I do prefer the Gilmore, but that might just be me. This was driving AKG K601s and using classical (Shosty and Stravinski). Using my Shure 840s, I listened to some Simple Minds... was fun; a bit of a bass hump with these (whether it was the phones or the amp ??) Didn't notice a bass hump at all with the AKGs.

[TomB]

Thanks guys!

Tom could you summarize the new recommended CCS settings?

Well, I haven't fiddled with the LED changes yet, but Doug says doing this does the same thing:

1. Lower the CCS resistor to ~56R. Those are the R4L and R4R resistors, as distinguished from the solitary "R4" resistor that's with the LM317 in the heater power supply section. Anyway, R4L and R4R are 75R. To get them to ~56R, just solder a 221R resistor in parallel with each one. That value was very convenient for me, because we commonly use it in some of Colin's diamond buffer designs.

2. After that, switch the cathode bias to the Western Electric connection (WE*) by switching all the jumpers - all 4 of them to the opposite pin.

* WE Connection - the jumpers connect the primary to the cathode with the bias done via resistor

[TomB]

HEY! I resemble that remark

Have done some listening with Blue LEDs in, but no comparison with going back to the Red LEDs, going to higher CCS current/resistor bias, etc. I did A/B it with my Gilmore and they are pretty close. The l'esp is a bit darker, I can hear the increased distortion at times, but not bothersome. I do prefer the Gilmore, but that might just be me. This was driving AKG K601s and using classical (Shosty and Stravinski). Using my Shure 840s, I listened to some Simple Minds... was fun; a bit of a bass hump with these (whether it was the phones or the amp ??) Didn't notice a bass hump at all with the AKGs.

Actually, Doug may be able to alleviate some of that increased distortion by changing to a higher voltage PT. It's possible that those AKG's are still pushing it more than it likes. As for the bass hump, did you opt for the lower value on the coupling caps? I'm using 3.8uf (instead of 4.7uf) and have heard that artificially produces a bass hump with a parafeed.

[Pars]

No, using 4.7uf Clarity caps... I may try going to the 56 ohm CCS and switching the jumpers to see what that sounds and measures like. Or go 65 ohms like Doug is using. I think I have some 499 ohm resistors around to put in parallel, could try that as well.

[dsavitsk]

Thanks guys!

Tom could you summarize the new recommended CCS settings?

Look at post 361.

[n_maher]

So as I mentioned before I was having some fit issues with the overall build. The result of that was that the bottom of the front panel looked like this.

That was unacceptable in my book and I wanted to see if I could at least make it better if not go away entirely. The solution, countersink the back of the front panel to allow the body of the pot to nest into it. So I mounted the panel in the vise on the mill, measured the pot body (just over 1/2" in diameter for the part that was binding on the front panel) and selected a 5/8" diameter end mill.

Unfortunately I wasn't able to completely cure the problem, the countersink would have been uncomfortably deep, so I decided to go with roughly half the panel depth as my max and see how it worked. The gap on the bottom is now less than half of what it was and is something I can live with. Moreover it was a proof of concept of something that I'd be wanting to try ever since I got the mill, namely that even after drilling and unmounting a panel I can remount, recenter and counter bore with good accuracy. This will be a nice bullet to have in the gun for future use.

[edit] Decided that I was being too much of a kitty cat and counter sunk the pot into the panel a bit more, fit is now really good. I also found two 1/4" male/female standoffs in the parts bin which work perfectly for supporting the pcb around the tubes so those went in too. Now I just have to solder in the two 221ohm resistors to lower the CCS resistor and I'll be done.

Edited October 17, 2010 by n_maher

[n_maher]

A final few parting shots, since I'm now done with this project.

Overall - for what it's worth the exposed surfaces all were finished by brushing with 0000 steel wool

(the mini-flexi is my new monitor stand for work)

Bottom - after the second go at counter sinking the panel is now flush. The dark line you're seeing is actually the concavity of the bottom of the chassis. I actually had to countersink the knob too, since there's so much exposed threaded shaft from the pot.

Innards - hopefully I have this setup correctly for resistor bias (please correct me if I don't)

Minor Quibble - in the final version you guys need to find a way to make it so that builders don't have to trim the heatsink

Otherwise I think that Doug has done a great job with the design of this amp. I'm enjoying it right now as I type this.

[Fitz]

Is anyone else getting audible buzzing from the power tranny?

[Pars]

^ Not me.

[dsavitsk]

Is anyone else getting audible buzzing from the power tranny?

Are you 100% sure it's the transformer and not the choke? Hammond stuff is notorious for buzzing, but I've never heard an Edcor do it. If you can eliminate the choke as the culprite (remove it and replace it with a resistor), then try the amp without tubes and see if it still buzzes. Also, did you install the jumpers from A to B, and from C to D?

[dsavitsk]

A final few parting shots, since I'm now done with this project.

Innards - hopefully I have this setup correctly for resistor bias (please correct me if I don't)

Looks nice (and looks correct). Any impressions of resistor versus LED?

[Fitz]

Are you 100% sure it's the transformer and not the choke? Hammond stuff is notorious for buzzing, but I've never heard an Edcor do it. If you can eliminate the choke as the culprite (remove it and replace it with a resistor), then try the amp without tubes and see if it still buzzes. Also, did you install the jumpers from A to B, and from C to D?

I'll see if I have a resistor to put in, but it's definitely loudest if I put my ear almost right against the power transformer, and much quieter if I move over an inch towards the choke. Jumpers are in place, no noticeable change in level without the tubes, and it works and sounds good other than the buzzing. If I only heard it up close I wouldn't be concerned, but I can hear it from a few feet away with the top off.

[dsavitsk]

I'll see if I have a resistor to put in, but it's definitely loudest if I put my ear almost right against the power transformer, and much quieter if I move over an inch towards the choke. Jumpers are in place, no noticeable change in level without the tubes, and it works and sounds good other than the buzzing. If I only heard it up close I wouldn't be concerned, but I can hear it from a few feet away with the top off.

Any chance you have a 120:120 isolation transformer around to use before this one? That would allow us to be sure it isn't DC on the power line.

At any rate, it sounds like it is likely a bad power transformer -- probably a gap in the lams. If that's what it turns out to be, I have another I can send you. If you can't get that one off the board, I can do that for you, too.

And be careful where you put your ear

[Fitz]

I'm pretty sure I don't have an isolation transformer around here, but you do bring up a good point about the mains, since the wiring in this house (and neighborhood) is old and unreliable. Haven't had any issues with other transformers on the same circuit, but who knows. I'll see if I can scare up an isolation transformer somehow and do a bit more troubleshooting.

[cetoole]

I'm pretty sure I don't have an isolation transformer around here, but you do bring up a good point about the mains, since the wiring in this house (and neighborhood) is old and unreliable. Haven't had any issues with other transformers on the same circuit, but who knows. I'll see if I can scare up an isolation transformer somehow and do a bit more troubleshooting.

Fudge one with a pair of equal transformers wired back-to-back.

[digger945]

Some power strips have one plug that is transformer isolated.

[Fitz]

Fudge one with a pair of equal transformers wired back-to-back.

Yeah, just gotta find two similar enough transformers around here. I have no idea where I stuck the box I had with miscellaneous transformers.

[Pars]

Did some non-critical listening to resistor bias tonight. 56 ohm ccs. So far, I like what I was hearing. Jumpers are kind of hard to get to, particularly the one tucked in by the parafeed cap. Might want to move those a bit on the final layout.

Edited October 19, 2010 by Pars

[n_maher]

Might want to move those a bit on the final layout.

Agreed, I had to use small needle-nose pliers to get the job done

Looks nice (and looks correct). Any impressions of resistor versus LED?

I didn't listen much before final assembly and what little I did do was non-critical to be sure. What I can say is that I'm not hearing any glaring faults so far, which is to say that I like it quite a bit. I need more time and I'm getting used to a new source concurrently so my listening impressions are going to be pretty limited in specificity.

[luvdunhill]

[dsavitsk]

Looks great, Marc.

I am thinking about changing the bias mechanism somewhat drastically. As has been discussed, the WE connection is not my favorite -- I think it sounds a little flat which is why I keep asking everyone about it. Additionally, I think the bias needs to be over ~2.5V in order to prevent high signal from causing grid current, but under 3.1V to keep the plate's voltage reasonable. However, LEDs in that range have a fairly high impedance, which drives up the plate impedance, which is a bad thing.

So, on a suggestion from Colin, I am thinking of using a PNP transistor in an emitter follower configuration as a bias element. This is in essence like the pass transistor in a shunt regulator. The idea is that when the base is biased up some amount, Vce (the voltage across the transistor) is the bias + 0.7V. Plus, the impedance is very low -- just a few ohms.

Here's what it looks like.

Notice that the bias is set by an LED, and the LED gets its biasing current from the resistor string that provides biasing current to the CCS LEDs, so the current is recycled.

If you want to implement this on your prototype boards, and I think you should as I think it sound a lot better (and it measures better), here's how:

- jumper the transformers to ground (LED connection).

- jumper for resistor bias.

- remove the bias resistors (R3L and R3R), and replace them with PNP transistors (BC560, etc.) with the emitter connected to the trace that connects to the tube's cathode and the collector grounded.

- desolder the ground side of R5L and R5R and stand them straight up.

- run a small jumper wire (per side) to connect the unconnected side of R5*, the transistor's base, and the anode of the biasing LED together

- For extra credit, connect the positive leg of an electrolytic cap (470u-1000u, 6.3V or higher) to the jumper wire with the negative lead to ground (the base of the transistor, or the cathode of the LED). It seemed to measure a tiny bit better doing this, though this is inconclusive.

The overall bias will be the LED's Vf + 0.7V. I used a 2V greed LED for ~2.7V of bias. If you have red LEDs in place, they are fine, too.

[luvdunhill]

The series mini LEDs in my photo sorta look like headphones!

[Nebby]

When I pulled my tubes out to get ready to put the new led's in I noticed that one of the tube pins decided to stay in the socket Any 6J6 tube should work, right? Nothing special to look for when buying a replacement?

[Pars]

- For extra credit, connect the positive leg of an electrolytic cap (470u-1000u, 6.3V or higher) to the jumper wire with the negative lead to ground (the collector of the transistor, or the cathode of the LED). It seemed to measure a tiny bit better doing this, though this is inconclusive.

FTFY

Good, something to do tonight since Fringe isn't on

[Fitz]

Definitely going to try the new bias setup when I'm working on mine over the weekend.