Other Amps

Blondish amp build

76

Ha! I knew it :P

Yeah, those high impedance circuits can do a lot of strange things with a few pF of parasitic coupling

Glad you solved it.

I had the same kind of experience with the Presence pot I had on the SuperSix soundwise. Too little feedback makes the amp too dark and boomy. Depends on speakers though. A stiff and bright speaker could demand another NFB value. That's probably why a AC30 with Blue Celestions work. bright speakers with no feedback at all.8-)

The blackface layout NFB settings was the best sounding to my ears.

For frontplates you might call Duralraphics Link...

77

Cool, thanks for the link MaBeRy.

I see another potmeter coming into my amp in the very near future.8-) And I may give that blackface NFB a whirl as well at some point.

It's been a very helpful lesson this one. I'd only thought about minimize spacing when I layed out that PI board. Completely forgot to consider the circuit impedances and crosstalk. I learned something here. Thanks again for your help! :D

78

Told you to get the right thingymajig connected to the appropriate doohickey, didn't I???

I admire the heck out of you folks who can do this!

79

Yes strummerson, you were right all along! I should have listened, then none of this would have happend.8-)

80

BTW, the tulips are doing nicely. Always a spectacular sight around this time of year!

They do indeed :D

As promised, here are some near my employers site just after the dunes of Petten. Taken yesterday evening just before band rehearsal. Enjoy8-)

81

Wow, absolutely stunning MaBeRy! :D

82

M,

Sorry to be late to the party, but, duuuude! Looking awesome! Looking forward to sound clips.

83

No prob M, glad you found your way here. The more the merrier!:D

If I wasn't hit by a rouge flu, I'd have some clips up already.:| hopefully tomorrow, or monday after bandpractice!

84

Update! Videos! :D :D

The amp is more or less done. I took some time yesterday to put in the zeners that bring down the B+ now to a reasonable 440V. Low enough to run a set of Tung Sol 5881s. In the preamp I'm running a Tung Sol ecc803s and the PI is -for now- a simple Sovtek 12ax7. Also moved the sag pot to the front panel, but I'm not using it much at the moment. The amp looses some of it's punch if I turn that up all the way. The jury is still out on that.

The biggest headache of the whole project turned out to be fixing the thing in the chassis. The nut that is behind the rediculously large PT is a nightmare to reach. It took me well over 3 hours (!!!) to find a way to get it fixed.

With the cab I'm using (2x12; Eminence Red Fang & Eminence Swamp Thang), the amp has a very nice low-mids presence. Highs are never piercing, always well defined. Creamy and beefy are the terms that spring to mind.

And... Wow this thing is LOUD.

I took it out to bandpractice yesterday and I had to roll back the guitar master volume to pretty much 25% to make it blend well with the rest of the group. The amp's volume was at about 3.

And now, without further ado: here are two quick clips I did after bandpractice yesterday. Sorry for the sloppy playing.

Sandu

Summertime

85

Michiel it sounds awesome! Love it!

Also i didnt have time to finish of the Logo, but i guess i have a reason now! Ill get onto it ASAP!

86

Thanks Jayce! Glad you like it and I'm very curious how the logo is going to turn out.:P

Here's some more pics.

The array of (12V/5W) zeners to bring down the B+ voltage This way, the zeners are pressed against the chassis for additional cooling. I mounted this arrangement with a couple of small dabs of thermal paste. The chassis got pretty warm after 2h playing last night. I had to cut a corner with the single cap: I ran out of HF caps to suppress the diode junction noise, so I just used a single cap across all 4 diodes. The amp is dead quiet though.

Zeners mounted between the PT and the front panel

Chassis, before cramming it into the cab

Front panel controls

88

That sounds like a great amp Michiel. Good work!

90

How did i miss this ... What a project, very nice!

Great thread, although i don't understand most of it! :D

92

Sounds great M!!

Is that the Viking?

I'm also totally stealing Summertime's arrangement.

First amp is built! I'm so jealous.

93

Hey M, I'm stoked you like the summertime arrangement. It's one of the first jazz songs I learned and (apart from speed) I've always played it like this. Pass on the jazz, brother! :D If you need 'em I'll put the chords on (virtual) paper for you.

BTW, it's the Gent. Strung up with TI swing 12s (flatwound). I don't remember if I did this time, but usually I use the 'mild' roll off setting on the tone-switch Next time I'll bring out the Viking.8-)

94

Yes. Bring out the Viking!

Sounds like a line from a historical action flick.

And I'd LOVE to see the charts for your Summertime as well!

Sounds great by the way. Maybe next year we can meet up half way, say in Greece or Montenegro?

95

Cool, Greece is a definite maybe! :)

My company has offices in Athens and Patra that I visit a couple times per year. I'm pretty sure we can work something out when I'm there again.:)

96

Greece jam!! That would be so cool.

M, you have to check out Kenny Burrell's Midnight Blue and I have a cool transcript of Miles' Milestones that use that m7-m7-maj7 line. I love that. I sat down and stole your licks a bit ago, thanks again.

Perhaps I better start another M jazz thread though so I don't hijack your wicked amp thread.

Looking forward to shushed Viking licks!

Also, was that a voltage divider you added to bring your B+ down? Just tap off a few extra resistors?

97

Haha, I figured you'd get my licks off of the vid.:) And I' be interested in your milestones version. I'll chrck out midnight blue as well. BTW, me playing Sandu is your doing.;) It stuck and I found a way to play it 'my way' (albeit ... Sloppy;-)).

And back on subject:

No, that's not a voltage divider. It's a chain of zener diodes. A zener diode works like a normal diode, ie. Current can flow in one direction, but not the other. The only difference is, in 'reverse', a zener diode will start conducting again if the reverse voltage is high enough. And when it does, it will give a very constant and reliable voltage drop. You'll have to know how much current is going through it though: max current * voltage drop = rated power of the zener. They do get pretty hot though, so you'll need to think about a way to cool them.

A voltage divider is a crude device (IMHO) that only works well on small signals in 'high impedance' circuits. For it to work well, the current flowing through both resisors of the voltage divider should be the same. When you connect a current drawing power tube to the middle node of the voltage divider, the tube can pull so much current through only the upper resistor, that the concept of the divider does not work pridictably anymore. As a rule of thumb, the current drawn from the center node should be at least 10-100 times less the current flowing through both resistors in the voltage divider.

A quick example:

A 6L6 draws peak currents of 200mA. The current in the voltage divider would have to be in the order of 2A (scared now) to 20A (ducking for cover).

This voltage divider would be in the HT path, with the full 500V at the top and the bottom tied to ground.

This means that the total IDLE power consumed by that voltage divider is between 500x2 and 500x20 = 1k to 10k Watts. And that is without doing anything.

... Even that massive Fender 75 PT would not be able to sustain that kind of power.

So, in this case a voltage divider would have been a very unpractical.

Class dismissed.;-)

98

I hope thats not going to be on the test... 8-o

But whens the tube bias trem going in? :D

99

Awwww, now I have to learn about zener diodes?? And I thought I wouldn't have to know anything past 1951. :P

Thanks for the quick lesson, M. OK, not that I'll totally get it, but what's the math on your zeners? And if I understand, you picked a certain amount to put in "backward" and that gave an even and lower B+. But, how does this work with idle vs. totally rocking out?

Lastly, how would you have done it if it were 1951? (LOL, not that expect you to spend your morning typing stuff out for me...) :D

100

Jayce: the bias trem might be something I'll experiment with in the future. For now no plans for onboard trem. (although I'd love to get some hands-on with your liquor rocker :D).

M: exactly. The zeners will give a constant specified voltage drop, regardless of the current flowing through them (unlike a resistor). And yes, they go into the circuit backwards!

The math on the zener is: estimate the peak current your amp will draw on the HT (~250mA I estimate for mine). Multiply that with the rated voltage drop of your zener, e.g. 12V for mine. The bare minimum wattage for my zeners would be 0.250 * 12 = 3W, or they will fry.

Now, the (loaded) B+ of my PT is 490V. I want to bring that down to the 440V region to run a set of 5881s at max capacity. (if I run them at 490V, they'll burn to a cinder). So.... I'd need a voltage drop of 490-440 = 50V.

For that I could use a single zener with a 50V drop, but I'd have to use one that is rated for 50 * 0.25 = 12.5 watts. That's going to get really hot. Those watts have to go somewhere and that somewhere is heat.

So instead I used 12W zeners at a lower power rating to make the cooling a little more manageble. And to get a 50 volt drop, I need 50/12 = ~4 zeners. In my case I found that the voltage actually dropped to 439V. So it looks like the drop across these zeners is actually a little bit more than the specified 12V. Makes me happy tho :D


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