Amp Tech Thread / Ask a tech Q

[Hemisaurus]

Anything that hasn't been turned on in a while, could be burning off a layer of dust OR if you're overloading a particular circuit, you could be burning the wire in the walls. Lots of electricians use wire that's undersized for the breaker they put on a circuit. Ie putting a 20A breaker on a circuit with wire that should only carry 15A.

If the drywall catches fire, that'll be a clue

[liquidsmoke]

I was boosting the signal and experimenting with stuff. I'll be more careful next time. I don't think I seriously melted anything though. That EH boost adds way too much gain by the way. Eventually I'll probably just break down and spend real money on a serious clean boost but we don't need to get into that topic again right now.

[Hemisaurus]

I offered to build you a passive volume

[Pissy]

The easiest way, I've seen done in high power amps, is separate jacks, one for each impedance tap.



You can buy impedance selector switches, Nick could get you one from cedist.com.
http://www.cedist.com/search/node/impedance%20select%20switch

The Hammonds they refer to in the explanation of that rotary switch is what I have I believe. There is a feedback wire going back to the power section that makes that multi jack method difficult.  The rotary switch was what I was after though. Thanks.

[Hemisaurus]

Why does the feedback wire make it difficult? They usually just come of the 16 ohm tap regardless, or is the Garnet different?

http://www.opentip.com/Musical-Instruments/Switch-Rotary-Impedance-Selector-p-1621668.html

I should add the Hammond is not for the feedback, but for the fact that rather than using simple taps and a common ground, the use separate coils, and link them in parallel or series depending on the impedance needed, is this how the Garnet works?

http://www.hammondmfg.com/1608.htm and scroll down.

[Pissy]

It's on the last page of this PDF. The 10k reach around is what I'm talking about.

http://www.webphix.com/schematic%20heaven/www.schematicheaven.com/bargainbin/garnet_b260d_bto_bass.pdf

[Hemisaurus]

The way it's drawn, it looks like there are no taps, just two parallel jacks, with the feedback resistor. The one jack is shorted, so that if you don't connect a speaker you drive into a short rather than open circuit.

What I was attempting to say, badly, was if you have regular taps, you would take each tap out to a separate jack, and leave the feedback resistor connected to whichever tap it normally is connected to. Even if your speaker is not connected to that particular tap, it doesn't matter, it's still active for the feedback loop.

Whichever way you are most comfortable with, is the way to go though

[Pissy]

That makes perfect sense!!

I had attempted to do exactly what you're talking about (there are two parallel speaker jacks) then noticed that resistor going back there and figured that I needed to jump the new tap connection back to it effectively shorting the two taps together, which wouldn't work obviously.

But you're right, it's active anyway regardless of a load connection.

Thanks, for pulling me back from over thinking.

Incidentally, I'm not sure why the Garnet schematics never showed the different taps, but if you open one up, the other taps are tied off to a tab strip.

In the world of those Canadian amp heads, they say that for a stock Hammond 3 tap Xfmr, the yellow is 16, green w/Yellow is 8 and green is 4.

It's different for a two tap Xfmr.

[liquidsmoke]

I offered to build you a passive volume

How much? I want it. How many db?

[Hemisaurus]

I offered to build you a passive volume

How much? I want it. How many db?

[liquidsmoke]

I forgot, seriously. But the EH was only about $40 or so. I'm sending it back.

I want a true clean boost but don't want to spend big bucks.

[moose23]

If you boost the signal you will have more gain, it's fairly simple maths. Still it's probably worth trying the MXR Micro Amp for a flat clean boost even if it's just for comparison purposes.

[showdown]

Builder/Tech Q: I have an old Simms-Watts 200w tube PA head that's pretty shot, but the transformers are good. Can I clone a Hiwatt DR up to and including the PI and connect it to the 4 x KT88 output section of the PA amp? The power and bias supply will of course be completely rebuilt with new caps and diodes.
I've built tube gear before so I know about high voltages, but I haven't designed anything so far...

[dunwichamps]

yes you could do the DR pre to pi but the DR201s have split rail power supply design which would be different here if the Simms Watt has a single rail supply. However, dont worry about this, should be fine.

[VOLVO)))]

http://www.ampix.org/albums/userpics/10075/Acoustic_140.pdf

So, that blue cap is explodededed, and has no values. With my minimal schematic knowledge, it is... C106? .01?

Someone check my work? plz?

[Hemisaurus]

Unlikley, it looks like an electrolytic, and .1 is a pretty low value for that.

Seeing as it says 10-50 on it, I'm going to go with C111 the 10uf 50V cap.

We are talking about the metallic blue one at the bottom of the first pic?

[VOLVO)))]

Dah, the 'sploded one. Is there any method to reading these and tracing? Why cant all caps have ratings on them...

Would the shack have that?

[VOLVO)))]

Oh wow. Or I could have assumed it was the one in the logical place... right where it is on the board. Im dumb.

[Hemisaurus]

Yeah, back in the day, they didn't label caps consistently, some are default in uF some in nF, some maybe even in pF.

A fair guess for an electrolytic, which is one of those small 'can' type whether it has a lead at each end, or two leads at one end, is it's going to be > 1uF, so any number on it is going to probably be in uF so 10-50DC would be 10uF 50V DC.

Another clue is that electrolytics are polarised (they have a positive and negative), so as there are only a few caps on that schematic with a little + on them, that narrows your choices down more.

[VOLVO)))]

-taking notes-

[Mr. Foxen]

Builder/Tech Q: I have an old Simms-Watts 200w tube PA head that's pretty shot, but the transformers are good. Can I clone a Hiwatt DR up to and including the PI and connect it to the 4 x KT88 output section of the PA amp? The power and bias supply will of course be completely rebuilt with new caps and diodes.
I've built tube gear before so I know about high voltages, but I haven't designed anything so far...

yes you could do the DR pre to pi but the DR201s have split rail power supply design which would be different here if the Simms Watt has a single rail supply. However, dont worry about this, should be fine.

Simms Watts 200s ran in ultralinear, so the output section is pretty different, and according to Chambo, the output transformers weren't very good and barely made the 200w. Also some of the Simms 200 watters were 100v line out only, which makes them pretty fucking useless.

[Hemisaurus]

You can get 100V line to 4/8 ohm transformers, even up to and beyond 200W, they run about $60-80 here, you can just wire it on to the back of the O/P.

You don't need to run it in ultralinear mode, you can do it either way as well.

[showdown]

I was thinking of having switchable ultralinear, it should be a little tighter in UL if I want to play bass through it. Mine has the normal 8 & 16 ohm outputs (well, 7.5 and 15 ohms, but close enough).

I've read that the transformers are pretty good, but I can't tell who made them, there are no labels or stampings on them. And even if it can't fully make 200 watts, it should still be plenty loud

[dunwichamps]

running it UL is fine, okay nice linear output with little thd as it was designed for. however, this is possibly not what you want if your going for a normal setup run in pentode/tetrode operation.

[Hemisaurus]

My favourite sounding amp is ultralinear, and I use it for guitar and bass.

It also has 60 year old tubes, not NOS, just the original tubes, which helps