Amp Tech Thread / Ask a tech Q

[justinhedrick]

i could do that too.

[dunwichamps]

then you just get a simple 1 jack in, 2 jacks out splitter.

[VOLVO)))]

http://sunn.ampage.org/sdp/index.php/topic,5502.0.html


Here ya go!

Use a plastic project box, remember that.

[dunwichamps]

or you can use insulated jacks. Cliff Marshall Style Jacks are isolated so you could use them with a Hammond chassis.

i guess i was overthinking this problem. oops

you could even through in a switch to change from series to parallel

[VOLVO)))]

They sell those at the Shack? 

[dunwichamps]

They sell those at the Shack?  

No but I can get you wholesale prices, shipped to your door

If you need a deal on something, let me know I can pass along my prices to you directly
(tubes, pots, caps, transformers, ICs, Semiconductors...)

[Hemisaurus]

The big bugger is we can't see the prices, until we get an account, sigh

[dunwichamps]

The big bugger is we can't see the prices, until we get an account, sigh

Just ask, ill let u know

email me for info, dunwichamps@gmail.com

[VOLVO)))]

oooo... Kickass...! 

[SoupKitchen]

So I saw this on the Bay; probably too much money, no?

http://www.ebay.com/itm/200-watt-tube-amp-chassis-rebuild-parts-/380372753328?pt=LH_DefaultDomain_0&hash=item588ffb9bb0

I've built several amps, from Champ copies and tweed Deluxes to a Metroamp kit for a Marshall 1987. I've got an itch to put together a KT88 head with a modified Marshall preamp. Nothing too complicated. Slightly off topic: is there really any difference in assembling a 50W amp and a 200W amp (besides component differences, obviously). I mean as far as assembly techniques, testing, etc. There seems to be a lot of fear, but the way I see it, 400 volts DC kills you just as dead as 700 volts DC. Proper protection is proper protection, right?

[Hemisaurus]

No huge difference, just number and type of tubes. Might not be that large a plate voltage, I don't think KT88's do 700V.

Things to watch for are make sure the power supply you design is adequate, and that the driver circuit is appropriate for the tubes. Watch for the attenuation value if you add an NFB loop, have fun.

Have a look at other quad KT88 designs, see what they use for phase splitter / driver.

[SoupKitchen]

Hmm, looking at several driver designs (Marshall Major, Marshall VBA400, Hiwatt 200, Sound City LB200, Sunn 2000S), it seems that many designs don't change all that much. It's only when you get to more than four tubes that driver designs change. The Sunn, for example, has the same driver (6AN8) as the Sunn 200S (2x KT88). Maybe I'm not looking close enough. Also the Hiwatts have that complicated LTP design with DC bias from a cathode follower.

[dunwichamps]

Just use an LTPI any old marshall uses. You should optimize the driver tube selection, 12ax7 make not be the best here, maybe a 12at7 or 12au7 if your looking for the headroom in the driver stage

I have built a version of the Sunn driver which uses an EF86 and a 12au7 as 6an8s are not in production anymore. Its pretty good but nothing special in terms of 50W versus 100W versus 200W

Kt88s can handle 700V on the plate but screens cannot exceed 600V (http://lib.store.yahoo.net/lib/thetubestore/JJ-KT88.pdf) Make sure you have a large dropping resistor after the choke (10W or 5W resistor) to bring down the voltage below the limit.

[VOLVO)))]

If I had a grand, I'd do the Marshall 1959 kit

[dunwichamps]

If I had a grand, I'd do the Marshall 1959 kit

Which brand? Ceriatone or Weber?

[Hemisaurus]

Like Duwich said, that new Weber kit is only $515


$515
Replicates the signature roar of hard rock bands. Single channel 100 watt head with full Tone Stack, Presence, and MV.

There's not a lot of difference between MV and non-MV.

Hey Dunwich, have you got another name, or should me continue to call you Dunwich?

[VOLVO)))]

Ceriatone, Nick.

[dunwichamps]

You can call me Dunwich or Nick, some people know my name some people dont its kool i respond to either

Ceriaton makes a nice kit but the shipping is nuts. Weber is more budget and I would recommend upgrades for even a moderate level builds (Tubes and Pots) which will bring up the price to ~650 (plus ship) A must upgrade is Jacks for weber kits.

The 8cm100 kit could be made into a SL type but the input jack setup is not the same as an SL but if you dont mind that then its all good.

In general upgrades for a Weber kit are:

(Tubes, Pots, Switches, Jacks, Impedance Selector).

Weber iron is pretty good. I have never had any problems, there PTs run cold and there OTs are good for the price.

[Hemisaurus]

I had thought about a Weber Kit being a cheaper way to get the chassis, xfmrs, headcase altogether than separates, until this Crate fell into my lap. I'll be gutting it, probably next year the speed things move right now.

I had thought of doing a Sunn Model T with just the Both jack on something, I don't know if anyone ever uses the other four

I guess I'll stick with Dunwich, though I seem to keep dropping the n and making you Du'Wich which may be your French name?

I think a nice 100W build would be some generic 100W'r from whoever, and then build it to taste, and at some later point upgrade the OP xfmr to the MC one which is like a $70-80 upgrade if memory serves.

That eylet board on your Metro amp looks nice, is that the Weber one?

[dunwichamps]

I had thought about a Weber Kit being a cheaper way to get the chassis, xfmrs, headcase altogether than separates, until this Crate fell into my lap. I'll be gutting it, probably next year the speed things move right now.

I had thought of doing a Sunn Model T with just the Both jack on something, I don't know if anyone ever uses the other four

I guess I'll stick with Dunwich, though I seem to keep dropping the n and making you Du'Wich which may be your French name?

I think a nice 100W build would be some generic 100W'r from whoever, and then build it to taste, and at some later point upgrade the OP xfmr to the MC one which is like a $70-80 upgrade if memory serves.

That eylet board on your Metro amp looks nice, is that the Weber one?

A Model T could be built out of the SL kit, since a Model T is very close to that circuit.

Metro eyelet board is Weber but i will not be using it for future Metros it will be a custom turret

[HeavyEar]

I've got an 70's tube amp that's been in storage but only for the last 2 years. Can I just fire that baby up and not worry about it? It's was serviced prior to storage,  electrolytic's in spec, biased etc. I don't have a variac or "ramp-up" power device - is that something I should consider? Thanks Tube wizards...

[Hemisaurus]

If it has been completely recapped when it was last serviced, you should be OK, modern electrolytics tend to be a little hardier. If it's got some older caps in there, then you might want to get it re-reserviced.

Cap Jobs - Do I need one? How often? Why?
What's a cap job? A technician may recommend you have a "cap job". This means that he will replace every single electrolytic capactor in the amp, from the power supply right down to the cathode bypass caps.
This is because electrolytic (polarized) capacitors have an inherent wear-out mechanism and will eventually die even if you don't play death/metal/country/barbershop through them every day - in fact they may wear out sooner if you leave it sitting in the attic. Here's why.

A capacitor is essentially two conductive plates separated by an insulator. The bigger the plate area and the thinner the insulator, the higher the "capacitance" is. Electrolytic capacitors get a very thin insulator by "growing" an insulating layer of aluminum oxide on the outside of a rolled up piece of aluminum foil.

The oxide layer is "formed" at manufacture by feeding the aluminum foil a very small and carefully controlled amount of current. The current causes a chemical reaction between the foil and the water solution (electrolyte! ... hey... is that where they got the name?? yep.) which makes an oxide layer grow. As the layer grows, they use higher and higher voltages to force the same small current through the layer, which gets thicker and more resistive with time. When they have to use the full rated voltage to get the forming current through, the cap is fully "formed" and ready to ship.

If the capacitor is used regularly, has voltage applied to it, and does not get too hot, the oxide film lasts up to a few decades. If the capacitor is not used much, or gets too hot, the oxide film slowly un-forms, the leakage current goes up, and it will eventually short.

Electrolytic caps are designed to last ten years. It is a tribute to the quality of manufacture that they often last three, sometimes four times that.

Old amps, particularly if they have not been used regularly need to have every electrolytic cap replaced. This cap job may be needed every ten or so years.

Non-electrolytic caps do not have this wear out mechanism, and do not need replaced for this reason. Modern capacitors can in some circumstances be much better than old ones, and you can sometimes get a clearer, more sparkly tone by changing the non-electrolytic caps - assuming that is something you want to do.

Do new caps need to be formed?
There's a lot of controvery on "reforming" replacement caps. Here are a few answers.
Manufacturers of caps design their caps for a ten year working life, and a five year shelf life. That means that the stresses and heat of working in equipment will leave the vast majority of caps functioning OK after ten years of normal operation. After that, it's gravy to the buyer.
They also design them to work OK after sitting on a shelf unused for five years, meaning that the cap should not fail if it's put into operation at rated voltage after sitting unused for five years. As noted above, the caps do slowly un-form without regular use.
If the electrolytic caps you use to fix your amp are over five years old as determined by the date code on them, you ought to at least worry about forming them, and if they're over ten years old (like NOS multisection cans), definitely re-form them. Other than that, put them in and turn it on.
How do I "re-form" electrolytic caps?
You'll hear folks talk about "bringing an amp up slowly on a variac"; this can work but is not particularly good for your tubes. A better way is this:
Pull out all the tubes.
if your amp has a tube rectifier, solder in temporarily some high voltage silicon diodes across the tube lugs to be a rectifier that does not depend on the filament voltages. If your amp has silicon diodes, you can skip this.
open up the wire that goes from the rectifier tube (or solid state diodes) to the first power supply filter stage and solder in series with the wire a temporary 100K 2- 5W resistor. This resistor will limit the current that can flow into the caps and the amount of voltage that is applied to them to safe values that will cause the insulating layer to re-form.
clip your voltmeter across the resistor
button it up. Turn it on (no tubes in it, remember). Watch the voltmeter.
when the voltmeter reading drops to less than 20-30VDC, your caps are formed.
open it back up and pull out those diodes and resistor, putting it back in original shape.
The forming could take hours to days.

[HeavyEar]

Thanks for the informative reply, Hemisaurus!

[Hemisaurus]

Well that's from the GeoFex tube maintenance FAQ, it just saves me typing it all out myself

[Hemisaurus]

When working on tube amps, remember