Silvertone 1482 mods?

[JohnnyCrash]

I generally like leaving old amps as-is, but I'd like to get a tad more flexibility out of this newly acquired amp... so I'd like to roll some thoughts around :)

I'm starting a thread in the Amp Forum about a speaker swap, so for now, let's leave the speaker out of this.

It's similar in many ways to an old tweed Deluxe. Here is the schematic:
http://www.schematicheaven.com/barga...ertone1482.pdf

BRIGHTEN THINGS UP
The circuit is creamier on top due to a lack of preamp bypass caps. I'm guessing this is to drag down gain a bit since it's already sorta gainy (meagerly tonestacked 12AX7 preamp as opposed to a 5E3 with a 12AY7).

1. Here's what I'm thinking - perhaps adding a bypass cap to the Mic channel (it's more open and gainy as you can see by it's lack of input grid resistor and only having a 1 Meg grid load resistor). This will give me a very gainy channel with a bit of brightness - I am worried about RF noise though.

2. Next I was thinking of leaving one of the Instrument channel's 68k inputs as-is and making the other one a 34k. Actually, maybe a lot smaller than 34k, IDK (it might mean more RF noise).

As you can see by the schematic (gut photos coming soon) the inputs are not switching types and both are identical 68k grids with shared 1 meg load resistor. In a typical Fender input you have a Low input jack switching to make a 68k grid with a 68k load and the High channel to make a 34k grid (paralleled 64k's) with a 1 meg load.

I'd like to make sure one input on the Instrument channel is 100% stock, but I doubt a 34k will be as drastic of a difference as a 68k... unless of course, I were to mess with the 1 meg loading resistor and put in a Fender style setup with switching jacks - which I'd prefer not do, it's not noisey when not plugged into and I'd like to keep the standard jacks in there (plus they're rusty so I don't wanna even try removing them right now hehe).

This would give me a stock Instrument input and two different input sounds, but I don't know how much brighter a 34k will even sound (plus it'll be a little gainier) - I just tried something like this on a Champ clone I built (68k, 34k, both with a 1 Meg load resistor).

3. Another option would be to add a bright cap over one channel's Volume pot. Cranked it would go back to being creamy in the treble, but at least at cleaner volumes it'd be brighter.

4. Perhaps a better option would be:
A - Mic channel add a bypass cap.
B - One input on the Inst channel a cap bypassing the input grid resistor.
C - Leaving the last input alone.

My only concern is RF noise on the Mic input (now gainier) and on the bypassed Inst input (the cap on the input could go against what the 68k resistor is in there to do, reduce RF).

GAIN
The overall gain at max volume is very cool, but not super saturated. I love the sound as-is, and have Marshall clones and a 5E3 clone for gainier stuff anyway, but adding gain is an option I'm open to (the Mic bypass cap would do that). My main concern is leaving at least one input stock and brightening up things on other inputs.

So, what do you guys think? I'm open to other options as well. I'm the resident dumbass and I need help from you smarter guys here (I like the new Shock Brothers DIY forum, I will always have help available to me now!).

[muchxs]

J, Harmonica players love that boxy cabinet and that cheap speaker. I don't. The 1482 is a killer amp but it's stifled by that cabinet. That amp absolutely rips, stock but plugged into an extension speaker cabinet. Bypass the stock speaker in the stock cab, listen to the amp through something else.

It's too bad the chassis is so unusual, it makes it difficult to re-package it in something bigger and deeper. Someone here did just that.

[JohnnyCrash]

I actually like the stiff, punky, almost frustrated sound the shallow cardboard cab gives.

As usual it's hard to tell if it's the cab, the speaker, or the circuit (or parts, that OT looks to small to sound that stiff, but it wouldn't be the first time looks have surprised me).

[celeste]

Johnny,

There are better ways to deal with RF at the input then series resistance. Series inductance for one. Try a ferite bead over the hot wire from the jack. You can harvest a nice selection of beads out of old computer power supplies.

[muchxs]

Originally Posted by JohnnyCrash

I actually like the stiff, punky, almost frustrated sound the shallow cardboard cab gives.

As usual it's hard to tell if it's the cab, the speaker, or the circuit (or parts, that OT looks to small to sound that stiff, but it wouldn't be the first time looks have surprised me).

Speaker and cabinet both choke that amp.

Some musicians get all misty-eyed about a particular vintage tone. The 1482 is the epitomy of boxy sounding IMO. The harp guys like the 1482 because the cab and speaker act almost as a band pass filter, the cab chokes the amp and it's not as prone to feedback. Harmonica players are in a never ending battle against feedback.

The quick way to find out if you like preamp changes is to go for it. Clip in a bypass cap and see if you like it, or tack it into the circuit with a soldering iron. Same with your grid blockers. There is an audible difference between a 68k grid blocker and a 33k grid blocker. Go as low as 10k or even go straight in, no grid blocker. It's a matter of finding a balance between noise and tone. If the noise becomes objectionable use a short length of shielded cable to your grid.

[celeste]

Johnny,

A few things you can try to brighten it up. A cathode that is not fully bypassed will give you more gain in the higher frequencies. That will brighten you up if you can stand the increase in gain. Something between .1 and .68Uf may be usefull here.

A smaller coupling cap will drop lower frequencies and that will brighten you up. Something as low as 1000Pf may be the ticket

Larger grid stoppers interact with miller effect to brighten an amp up. Free fre to go up to 470K here.

I am not a fan of bright caps around pots because they change more then just volume as you turn the control.

Something that would brighten the whole amp up would be faster caps like PP in the power supply

[DrewB]

Originally Posted by celeste

A cathode that is not fully bypassed will give you more gain in the higher frequencies. That will brighten you up if you can stand the increase in gain. Something between .1 and .68Uf may be useful here.

Can you elaborate on "not fully-bypassed?" I have a 5F1 clone (no cathode-bypass cap) that sounds dull with the volume control set below 7, so cleans with any sparkle at all are impossible. Granted, I usually run the volume between 10 or 11, anyway; that's the sweet spot. I increased the gain of the gain stage by lowering the cathode resistance to ~800 ohms, which increased the amp's distortion potential; I'd like to introduce some sparkle at lower volume settings. I also don't like what bright caps on a volume control do at higher volume settings; the effect is audible and undesirable. I don't want to go to smaller coupling caps for fear of losing the girth that the amp has with the .02 uF caps.

Originally Posted by celeste

Something that would brighten the whole amp up would be faster caps like PP in the power supply

What are PP caps?

[celeste]

The bypass cap forms a high pass filter with the cathode impedance. If the corner frequency of the filter is above the lowest frequency you want to pass, the cathode is siad to not be fully bypassed. Trying to calculate the corner frequency is like chasing your tail because it changes with the tubes op point. Try returning to the original cathode resistor and "clip leading" in caps between .1 and .68uf. Any size really, that is just a suggestion on where to start.

PP is polyproplene. They do not have the rise in ESR that electrolitics have and so are closer to an ideal cap at higher frequencies.
They are bigger and more expensive, but are one more tool in the tone tool bucket

[JohnnyCrash]

Originally Posted by DrewB

Can you elaborate on "not fully-bypassed?" I have a 5F1 clone (no cathode-bypass cap) that sounds dull with the volume control set below 7, so cleans with any sparkle at all are impossible. Granted, I usually run the volume between 10 or 11, anyway; that's the sweet spot. I increased the gain of the gain stage by lowering the cathode resistance to ~800 ohms, which increased the amp's distortion potential; I'd like to introduce some sparkle at lower volume settings. I also don't like what bright caps on a volume control do at higher volume settings; the effect is audible and undesirable. I don't want to go to smaller coupling caps for fear of losing the girth that the amp has with the .02 uF caps.



What are PP caps?

A bright cap across the volume pot should be pretty much bypassed once you start getting into overdriven volumes. Bypass caps on the other hand are audible at all volume levels.

I agree on the coupling caps. Keep em big for midrange. For treble a different apporach has been helpful for me - bypass caps or a bright cap across the volume pot (bypass increase gain, but shift the upper midrange - bright caps are ineffective once overdrive starts kicking in so are better for lower volumes/clean settings).

Originally Posted by celeste

The bypass cap forms a high pass filter with the cathode impedance. If the corner frequency of the filter is above the lowest frequency you want to pass, the cathode is siad to not be fully bypassed. Trying to calculate the corner frequency is like chasing your tail because it changes with the tubes op point. Try returning to the original cathode resistor and "clip leading" in caps between .1 and .68uf. Any size really, that is just a suggestion on where to start.

PP is polyproplene. They do not have the rise in ESR that electrolitics have and so are closer to an ideal cap at higher frequencies.
They are bigger and more expensive, but are one more tool in the tone tool bucket

Polypro in the power section? You certainly do not mean in the DC filteration part of the circuit, right? Otherwise, the only power couplers in something as simple as a 1482 are the two 0.01uF ones to the 6V6's. Anything smaller there would make for a very ratty amp (especially given the shallow cab and brittle speaker).

PP for filteration is not something ordinary, nor generally recommended (for a few reasons).

If I'm wrong, I'd like to know more. There are only the two or three ways I mentioned above that I know of in increasing brightness in such a primitive circuit (and some tweak with gain, or change depending on volume) so whatever tips you have I'm very open to.

[celeste]

Originally Posted by JohnnyCrash

Polypro in the power section? You certainly do not mean in the DC filteration part of the circuit, right? Otherwise, the only power couplers in something as simple as a 1482 are the two 0.01uF ones to the 6V6's. Anything smaller there would make for a very ratty amp (especially given the shallow cab and brittle speaker).

PP for filteration is not something ordinary, nor generally recommended (for a few reasons).

If I'm wrong, I'd like to know more. There are only the two or three ways I mentioned above that I know of in increasing brightness in such a primitive circuit (and some tweak with gain, or change depending on volume) so whatever tips you have I'm very open to.

Yes I mean the electrolitics that filter the DC from the 6X4. 20, 10 and 5uf. The only draw back to PP threre is size and cost. They can handle much higher ripple then an electro so are more suited to filtering high ripple DC then electros. They do not have the rise in ESR with frequency that electros have so can dump their charge faster. If you think of an ampflication stage as just a power supply modulator, then you see the need to have a supply as fast as the amplification stage.

Below a few hundred Hz, it is pretty much a wash, but by 1k a PP likely has an ESR 2-300 ohms lower then your electro (likely 500 ohms for those old caps) and as frequency rises it gets worse. Often times the difference is enough to do away with a bright cap. They do a much better job of filtering SS rectifier noise because of the reduced ESR at elevated frequencies.

They are a size issue. Solen are likely the easiest to find, for new builds, I hunt ebay for surplus motor run caps.

Other then size and cost, I can not imagin why someone would not use PP filter caps.

[JohnnyCrash]

I put in a 10k Instrument input and a 0.68uF cathode bypass cap on the Mic channel.

I left the other Inst channel at 68k.

More gain and a bit brighter on both, but it seems the speaker is still holding down a lot of treble... either that or the Tone controls (which I haven't checked yet).

I like the changes just fine, but I'm also going to take another look at the Tone controls. Sounds great and now I have a bit more flexibility.

[ThermionicScott]

An alternative to using PP caps in the power supply is to parallel your first capacitor with one that is 1/10th of the capacitance. The smaller one will take care of the higher-frequency noise the bigger one won't get due to ESR. If cost is a consideration (when isn't it?), Mouser has high-voltage 1 and 2 uF caps for pretty cheap, too.

- Scott

[celeste]

Originally Posted by Scott S

An alternative to using PP caps in the power supply is to parallel your first capacitor with one that is 1/10th of the capacitance. The smaller one will take care of the higher-frequency noise the bigger one won't get due to ESR. If cost is a consideration (when isn't it?), Mouser has high-voltage 1 and 2 uF caps for pretty cheap, too.

- Scott

You really need to do the math to see where the capacitive reactance of the bypass caps starts to rise. I stopped using them in favor of just starting with good caps because I usually ended up with a notch centered about 1K of little filtering because the ESR of the electro was up so much and the bypass cap reactance had not dropped enough yet. It is an iterative tail chasing situation. You need to know where ESR starts to rise, then you need a bypas cap who's reactance is low enough, to compensate for the rise in ESR, into the load impedance of the following circuit.