Amp Tech Thread / Ask a tech Q

[dunwichamps]

I have UL/Pent switches on my amps using Kt88s with dynaco style iron. UL is a smidge less power but a different feel when you get it going. I like the option of switching, of course you gotta design it into the amp.

[Hemisaurus]

I have UL/Pent switches on my amps using Kt88s with dynaco style iron. UL is a smidge less power but a different feel when you get it going. I like the option of switching, of course you gotta design it into the amp.

A switch and a couple of resistors, not rocket-science.

[VOLVO)))]

why does radioshack not carry the cap I need? Sigh.

[dunwichamps]

I have UL/Pent switches on my amps using Kt88s with dynaco style iron. UL is a smidge less power but a different feel when you get it going. I like the option of switching, of course you gotta design it into the amp.

A switch and a couple of resistors, not rocket-science.

well physically just 1 switch, nothing else but the feedback changes a lot when you use it. Another thing to consider if your start with a UL amp it may not have separate screen supply caps ala a Model T so to make it operate in pentode you need to design the extra caps into the power supply or its not going to work. 

[Hemisaurus]

why does radioshack not carry the cap I need? Sigh.

http://www.radioshack.com/product/index.jsp?productId=2103610&filterName=Brand&filterValue=RadioShack

    W PALM BEACH CROSS CTY PL
01-8946 | 2.02 miles*
See a map
   Store Hours:
   Mon-Sat 9 am - 9 pm
Sunday 10 am - 7 pm

   
   561-684-1131
CROSS COUNTY PLAZA
4348 OKEECHOBEE BVD H-100
W PALM BEACH, FL 33409
         
   In stock In stock

[VOLVO)))]

Locally.

[Hemisaurus]

Oops your right, that's the other FL dudes zip.

Says it's in stock in all the Tallahassee Ratshacks though. At a pinch you can do two 4.7 uF's in parallel long as they are both 50V, do you need 50V, what's the supply voltage of that circuit? Maybe a 35V or even 16V might cover you. I was just going by the original part.

[VOLVO)))]

So, I cant access the sunn forum at all anymore... anyone got a silverface concert bass schematic and a proper transistor bias procedure?

[VOLVO)))]

The 140 is fixed, at least. Thanks, Herb.

[ryansummit]

i found an extension cab on craigslist and the guy wants to know asap if i'm gonna buy it
and i know this question was asked yesterday and i'm gonna be the big douche to ask it again
but is 2:47 am and i just got texted by the guy(crackhead?) and my search finger is tired

the speaker out on my head is 120w-4ohm
the peavey 2x12 in fantastic condition for cheap is 120w-4ohm
am i ok or should i wait for somethin with more watts or more ohms
thanks

[Hemisaurus]

The cab matches the head, so that's OK, assuming that the cab is rated 120W RMS, or continuous, and not Peak.

It does mean, you won't be able to use it with another cab though, without rewiring the cab or building a special box.

[ryansummit]

shit im sorry
its a combo 4x10
with speaker out 120w
so not just going through 2x12
i think its 200w altogether
thanks for quick answer
ima grab it

[Cursed71]

Let us know what it turns out to be

So, I have a thought on this Laney Klipp that's blowing fuses.  With the voltage having been changed for use in the states, would the size of the fuse also have to change?  I have the amp at a tech right now and he's running it through the paces, but the fuse thing seems logical since its the a/ c fuse that keeps going.

[dunwichamps]

yes the fuse needs to change when from 220/240 to 120 VAC

[Hemisaurus]

and, to be all technical, generally times two, so 1.5A for 240V means 3A for 120V

[Cursed71]

Yup.  That was it.  That, and just cleaning up some wiring that was really close together ( lots of electrical tape from previous "overhaul") and a rebias.  Used it with another amp at a show this weekend with zero problems.  Thanks ya'll!

[VOLVO)))]

SO, would I know if I lost a pair of tubes in my Model T? I was just playing it, and it dumped some volume on me and started getting a little dirtier (I wouldn't really know, I had a maxed out Caitlinbread SCOD in front of it.) It seems loud as fuck, still, but after the little ~tick~ it doesn't seem AS loud, maybe I'm having the placebo effect?

[Mr. Foxen]

If something's a bit wrong, get it sorted before something becomes a lot wrong.

[Hemisaurus]

Like the man said, you lose a pair of tubes, your expected output impedance is now double, so you are melting your output xfmr the more you run it.

The obvious way to figure it would be to pull a pair of the output tubes, double the load, or flick the switch down one, try it out, then swap in the other pair. Or just put the tubes in an amp that runs only a pair.

[VOLVO)))]

Yep, dead tubes. The SCOD did it. So, I've got a quad of old, huge 6550s. If I drop them in, and flick the power, that's effectively a tube tester, right? If they all fire, I should be good for launch?

[VOLVO)))]

So, I finally got around to taking that Concert Bass to someone who is qualified. Got it back, filter caps were bad, but he said he didn't even suspect them due to what the scope read, then he just replaced them anyways, and it fixed the horrible noise issue. He said it was loud, but... it isn't. At all. Not as loud as it should be, at any rate. He said there's something wonky in the preamp, probably a bad cap. He pretty much told me to replace all of the electrolytics, but he didn't want to do it because I can do it myself.

What would the volume issue possibly be, if the poweramp runs perfectly flat on the scope? Preamp not sending enough signal?

[Hemisaurus]

If it's not the power, it's the pre.

[VOLVO)))]

So basically, just replace the caps and it should be loud as it should be?

What if i brute all of them and it STILL isnt loud? What's the next level?


I just hooked it up to the beta pre. Its loud.

[Hemisaurus]

Next level would be to trace it with a scope and see where you lose signal.

[VOLVO)))]

Proper. He said if it gets to that, he would scope instruct me.

One more, biasing the T, no different than doing the proper math for the proper mA? I hope?