babyhead

Help with a tube amp-
« on: March 22, 2008, 03:05:18 PM »
Hey Folks-

I have taken on a project involving a Silvertone 1483. There is a choke that is parallel to the terminals of the speaker that has been disconnected. It has almost no DCR. If there is a short in the choke, I really dont want to put it in parallel with the output, do I?

Schematic:
http://www.schematicheaven.com/bargainbin/silvertone1483.pdf

Old thread on another BBS:
http://archive.ampage.org/threads/4/gagd/262855/Silvertone_1483-1.html#262855

It makes sense that it is filtering the low end. I just cant imagine putting a 1z resistor across the output across the speaker. I have found some hammond chokes with 1z DCR but Im not sure of their function.

Is it that the DCR significant because we are dealing with AC?

This is my first foray into guitar amps... staying careful

Thanks-

BH


shabtek

go for it
« Reply #1 on: March 22, 2008, 03:55:22 PM »
if the thing works without the choke, connect the choke and see what it does--er, how it sound
a tube output stage would rather be shorted than open-circuit
the choke may have low dcr but yer not putin dc thru it
"really fine players do not use stomp boxes or master volume, they match the amp to the room and turn it up to 11.  Stevie Ray, BB King, Albert King, Duane Allman, Dicky Betts, Louis Armstrong"
   -CJ

bcarso

Help with a tube amp-
« Reply #2 on: March 22, 2008, 04:08:10 PM »
You need to see if it has a reasonable amount of inductance and especially decent Q to see if there is something wrong with it.  Shorts in chokes are not unknown but I'd think pretty rare in this application.  It might be there to reduce low frequencies and hence proportionally boost highs.  Rather a brute force approach, and counts on the highish output impedance of the amp to establish the rolloff, but so it goes. Maybe the "sound in the iron" was deemed desirable. :roll:

At low enough frequencies the choke is approaching a short all right, but maybe the amp highpasses enough by then to avoid calamity.

Is the driver original?  Maybe, if it was replaced at some point by a "better" driver the choke was no longer needed.

Odd that the schematic gives DCRs for some windings of the transformers but not for the choke.  Note BTW that the notation for the 5Y3 filament winding belongs upstairs with the other half of the HV.  You wouldn't get much heating with a 70 ohm 5V winding!

As shabtek advises, I'd hook it in and see what it sounds like, briefly.  If possible look at the line current draw and sweep the thing with an oscillator.  If the power consumed is way high or rises dramatically at low frequencies I'd shut it down and remove the connection.

babyhead

Help with a tube amp-
« Reply #3 on: March 22, 2008, 08:19:10 PM »
Success!!!


Thank you for your input. The amp has some other issues, other than being built by a drunk spider. I may build a turret style replica, gut this one and drop the rebuild. Good times.

Thanks again.

PRR

Help with a tube amp-
« Reply #4 on: March 22, 2008, 10:41:34 PM »
> There is a choke that is parallel to the terminals of the speaker

Measure the choke's AC impedance. I am sure it is over 8 ohms at 400Hz.

That makes almost no sense. If they wanted less bass, they cudda used a cheaper OT.

The way it makes sense: they wound up with a too-cheap speaker. The cone slapped on low notes. Given a choice of a better speaker, or a new-design cheaper OT, they took path 3 and tossed a choke in there. Maybe they had a truckload of the too-good OTs and slappy speakers before they noticed the problem, and a fairly lame choke was the cheapest way to market.

Since I'm sure the original speaker is past its peak freshness date, and it has been or will be re-speakered, leave the choke out. Select a speaker suitable for a 20 Watt guitar amp; much easier to find clearly specified gitar speakers today than in 1960.

BTW, there is an old RCA(?) paper showing a saturating choke used to reduce cone-slap. Iron V/F saturation line is similar to a speaker's slap-line, and flux-choking is less offensive than paper slap. But the design is too painful for most designers, and since those early days a better speaker (or lamer bass) has generally been cheaper than another iron and winding.

> built by a drunk spider.

Visible drunkeness in low-pay workers was usually instant dismissal. One drunk can do more damage more quickly than he/she is worth. However the factories didn't much care what race, religion, or species the workers were... if sober spiders could do the job, that was fine.

The general mess is due to extreme penny-pinching. The 5-tube radios developed haywire assembly to a fine art, and these techniques were used in most low-price gitar amps. BTW, you find the same style in a 1946 Fender.... it was later that Leo realized that guitar amps must be repaired, and switched to the classic cardboard tagstrip.

The plan shows a "death cap" on the 2-wire power cord. I hope you have gone modern with a 3-wire cord.

Look in at www.EL34world.com forum. There is a guy there knows too much about those Silvertones.

> Note BTW that the notation for the 5Y3 filament winding belongs upstairs

This was re-drawn by Sams (either for FotoFacts or for Darr's book). Most Sams schematics have mistakes.

babyhead

Help with a tube amp-
« Reply #5 on: March 23, 2008, 02:42:01 PM »
Thanks PRR!

Yes. The Death Cap is of no danger.

The speaker was a Jensen 15" 50 watt with a pretty mushy cone so I recommended a new 100 watt version. It did clear up the splat but my guy is looking for "his sound back." Linearity is much easier... we have yet to really open the amp up... maybe today. I just installed a humbucker on my upright bass. :twisted:

Thanks again-


 

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