[tomb]

Quote:

Originally Posted by amphead /img/forum/go_quote.gif Is that a custom volume knob, or do you have a part number

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[amphead]

Quote:

Originally Posted by bperboy /img/forum/go_quote.gif The main benefits of the redesigned Millett is that the buffer and the e12 circuit are built in on the board, as well as the power supply regulation. Thats what makes it a great little amp; only one board needed for all the different features!

It's very hard not to get involved with the MAX build, so I will be watching/posting in this thread often! Aaaaawsome!!!
Thanx for the knob info!

 

[TimmyMac]

Is it possible to elevate the tube sockets above the board, say at the level of the top deck of your enclosure? The thought of all that tubey goodness popping out the top of the enclosure is enticing...

 

[vixr]

something like this might work

 

[bperboy]

You could do that, but then you'll run into problems with biasing. You'll need wires running from the top panel down to the board, and those wires will prevent you from opening up the top panel to bias the tubes and buffers. This is all assuming that you're using the Hammond case though. The way I have mine set up, I just mounted the sockets on the board, and then run without a top cover, that way I don't have to worry about issues with stripping out the panel screws and overheating with the MOSFETs. Plus, it just looks cool being able to see all the parts directly. If you're using a different case, you can be creative and think of a way to set up a bracket or something with openings for biasing and such. If you end up still wanting to do this, you'd probably have to drill holes in the top cover for the trimpots and test points, something that I think will make the top look fairly unclean, so far as the design goes.

 

[vixr]

Quote:

Originally Posted by bperboy /img/forum/go_quote.gif ...If you end up still wanting to do this, you'd probably have to drill holes in the top cover for the trimpots and test points, something that I think will make the top look fairly unclean, so far as the design goes.

I think you're right, the top plate was installed on my SOHA for about a day, I took it off to tinker with the new tubes I bought and its been off ever since

 

[tomb]

Positioned correctly, it's possible to have enough of the tube sticking out to actually grasp it for plugging in and unplugging. Also drilling strategic holes in the top for the trimmer screws has been done on a number of occasions. I am uncertain if this works well without a transparent top, but I think it would if one has fairly bright tube lights, or even better - air circulation holes in the sides as well as the top.

There are also tip jacks that can be installed in the rear plate that allow your DMM probes to simply plug in. Depending on which slot the board is in, you can run all the test point wires underneath. Both strategies may allow complete adjustment and tube removal without taking the top off.

Caution! this image is not to scale!

 

[bperboy]

Tomb, thats a very nice strategy for getting good clean test points and biasing. I think your solution is much more elegant, but mine easier and maybe more practical. If I had the money, I'd have FrontPanelExpress make up that design, and then the problem would be solved.

 

[splaz]

Well not really max related but wow, the tubes I got for it really are NOS.

I rubbed my thumb on the label and it pretty much disintegrated, it was faded already... maybe like a good wine they improve with age.

 

[tomb]

Quote:

Originally Posted by bperboy /img/forum/go_quote.gif Tomb, thats a very nice strategy for getting good clean test points and biasing. I think your solution is much more elegant, but mine easier and maybe more practical. If I had the money, I'd have FrontPanelExpress make up that design, and then the problem would be solved.

Yep - I basically posted that to stimulate some ideas about what's possible. To be honest, I don't have any of mine covered, either. The tip jacks work really well, though - I use them all the time even on my old Milletts - the test points were in some dangerous spots on the old ones.

There comes a time, though, when you stop fiddling with the tubes and settle on what you want. Then forays into the innards of the case are much less frequent.

Quote:

Originally Posted by splaz Well not really max related but wow, the tubes I got for it really are NOS. I rubbed my thumb on the label and it pretty much disintegrated, it was faded already... maybe like a good wine they improve with age.

Yes - some of the lettering will rub right off as if it's just so much dust. I think they probably expected it, though - the important designations seem to always be etched into the glass.

 

[NeilR]

A couple of comments on Tomb's tip jack biasing scheme... because I'm muddling through some kind of solution I can live with but I'm not there yet.

1) I did that for my old Millett's tube bias but not the dDB bias points. The DrewD boards have 2 pads on 100 mil spacing on each bias point. I used 2 pin Milex KK plugs and sockets, using just one pin. I don't think you can get a one pin molex plug (needed for the Max)although I think one could be rigged up with some butchering and parts substitutions.

2) You only need 5 bias ports. The board provides for bias points (TB1 and TB2) between the emitter and resistor on both sides of the complimentary pair. The only reason to measure both is on initial startup just to make sure they are close. If the circuit is built right and working, they will be close and they will stay close. You run a port to either TB1 or TB2.

3) I don't think Tomb's rear panel layout will work when using 1.5" heat sinks, as I did, because the board needs to go in the bottom rail. I doubt you could squeeze pin jacks under the board with it in the bottom rail. I haven't dug out the couple of pin jacks I should have in my bin to see if they will fit on top; I suspect they will with some placement care.

My main problem with pin jacks here is that I don't want the rear panel permanently attached to the board, as it would be if wires were just soldered to the pin jack and associated bias pad. I've thought about doing some dort of in-line plug and jack with Moles-type connectors, but haven;t taken it any further. the idea being some way to disconnect the bias port wires, either directly at the pad or some inline arrangement. If anyone has any slick ideas I'd like to hear about it

 

[tomb]

It's true that the second set of tip jacks are not critical for the other DB test points, but some of this was an exercise in seeing how everything would fit in a worst case situation - as is often the goal with CAD design work. Certainly, if you locate the board on the bottom slot - due to 1-1/2" heat sinks - this won't work.

Actually, only 4 tip jacks would be needed over any length of time, perhaps just 3. Those would be the tube bias jacks, gnd, and perhaps V+. Once the DB's are set, as long as you confirm supply voltage, they shouldn't vary that much. I have seen voltages alter enough to change all of your settings at different locations though (like work vs. home) - so the V+ might be useful.

Having three or four soldered leads hanging off the board is not the best scenario, as Neil says, but again, once buttoned up there's very little reason for getting into the innards again. It's also an impossible scenario to sell the amp to a neophyte without such adjustment features and tip jacks.

There are single post terminal blocks in existence - at least I've seen some. It could be someone cut a 2-pole in half, but it's been done.

EDIT: Molex can get expensive unless you combine pins - that could work, though. Another thought is some male/female fully-insulated faston connectors - or those inline bullet connectors used in car wiring. With a little effort, you can probably get the nose of one of those bullet connectors soldered directly onto the end of the tip jack. That way, you wouldn't have any loose connectors rattling around on the bottom of the case. Just a suggestion - I may try it.

 

[NeilR]

Hi Tom,

I only mentioned the 5 jacks to clarify the minimum needs for buffer biasing.

I originally wasn't going to bother with biasing ports, more or less for the reasons you mentioned. However, it is becoming more of an issue for me simply because closing up the case radically alters the buffer bias. This is true of the BJT build as well as my MOSFET build, and really it's more important with the BJT build because that circuit is PTC and the bias rises with a closed case. I discovered that when playing with my Steinchen buffers. I'll bet most people running those buffers in closed cases (even with some ventilation) are running them MUCH hotter than they think based on their open case bias measurements.

Even with the MOSFETS, which should reduce bias when the case is closed, the bias levels are so high that I think it is a good idea to be able to easily measure them with the case closed so that any estimates are confirmed. Just MHO. The more I work with the build, the less confident I am in predicting the closed case bias.

I can run mini clips to my test points and out the tube holes, but that forces me to remove the tubes, remove the panels, remove the cover, install the clips, reinstall the cover, reinstall the panels, warm it up for an hour or so, take the measurement, turn it off, let it cool down, remove the tubes, remove the panel screws, remove the cover, remove the clips, reinstall the panels, reinstall the cover, reinstall the tubes, and make sure I didn't swap tubes! Having gone through that process a few times in order to give some accurate biasing advice, it's starting to get old

Just wanted to throw all that out so builders can think about all the nagging issues related to bias ports and biasing strategy. It got me thinking.

Regards,
Neil

 

[bperboy]

Regards to heat Neil, thats a very good idea, especially coming from you!

What was it we call you now? JFET Conflagorous?!

 

[amphead]

Quote:

Originally Posted by cetoole /img/forum/go_quote.gif Just thought I would post a quick pic of the reason I am still up tonight. Just having real trouble turning Opeth's Deliverance CD off. Had a bit of trouble with way too much gain and getting the volume knob too high with my KSC75's earlier tonight, and even with my HD580s, I cant even make it a quarter turn on the volume knob. This is with DuMont 12ae6a tubes and the cathode resistors unbypassed.

This might be dumb, but I was wondering what would happen, if the volume pot had a 50k resistor added in series, on the high side, to act as a voltage divider and reduce the maximum volume range?