Phase Inverter Grid Stopper for Silvertone 1482 Help

[dbishopbliss]

I'm working on my Silvertone 1482. In general, I like the sounds I'm getting. Still trying to figure out replacement speaker, etc.


I've read through this thread, but I don't think I want to change the amp to be more "Fendery" but I have noticed that it gets really flubby sounding when I plug in a high-output P90 loaded guitar. Also it farts out with an overdrive pedal that has any hair to it.


I found Rob Robinette's page on adding a Phase Inverter Grid Stopper Resistor to a 5E3. I think this may be the solution I'm looking for but I'm having trouble following the point to point wiring.

I've included a link to the layout diagram in the other thread. Can someone tell me where I should install the resistor? Thanks for your help.

[mresistor]

See the green wire going to the 12AX7A PI pin 7 - lift either end of that wire and insert the 470k resistor .  The wire is coming from R13 . I would attach the resistor to the tube socket.

[2deaf]

Pin 2 is the PI grid.

[jjasilli]

Note that you say 1482, but post a 1472 layout.  This should be clarified.


If hi input signal is causing an issue, a reasonable inference is that the problem is at the amps's input stage.  This is a worthwhile thing to verify or rule out.  This could be done with a 'scope; a dedicated listening amp; or running the input stage's output into another amp.  Doug's Library Page shoes how to do this safely.  You could also plot the loadline of the input stage to see how the tube will behave under the input signal voltage in question.


That said, it might be simpler to just try a blocking resistor > PI.  I'm not a fan of the shotgun approach, but it works sometimes.

[mresistor]

Pin 2 is the PI grid.

Pin7 is the first grid of the PI.


I believe the 1482 is similar, but that was a good catch JJ. He probably doesn't have a layout of the 1482 so used the 1472 instead.

[sluckey]

Pin 2 is the PI grid.

Pin7 is the first grid of the PI.

Pins 1, 2, and 3 are the PI on that 1472 layout as well as the 1472 schematic. Pin 6, 7, and 8 are just a gain stage prior to the actual PI.

Here's a schematic that I trust for the 1482. It also shows pin 2 as the PI grid...

     http://el34world.com/Forum/index.php?action=dlattach;topic=21255.0;attach=63899

[mresistor]

OK   let me try again..     to the op   


I would lift the wire on Pin 2 of the PI 12AX7 and the other end at the standoff where the wire terminates and then insert the 470k resistor on the tube socket and connect the other lead to the terminal standoff, using some shrink tubing to insulate the bare leads of the resistor. 


Sorry for the misunderstanding.

[sluckey]

 

[dbishopbliss]

The layout of the 1482 and the 1472 pretty much identical from what I have read (see the thread linked in the original post). I have compared my amp to the diagram and it matches except for a few of the resistor/capacitor values. None of the schematics I have seen have pin designators on them. Unfortunately, my Mac doesn't seem to be able to open the diagram that sluckey linked to.

I just wrote the following at the same time mresistor posted before I submitted

So, if I have this right I can replace the wire going from pin 2 to the terminal strip with a 470K resistor. Or I may try using the 1M 'Sweet Pot' he suggests.

Seems like i got it right. Thanks for the help.

By the way, I am really surprised by how quiet this amp is. Don't get me wrong, its not silent but considering it doesn't have shorting jacks or twisted heater wiring its pretty darn quiet. I was thinking of cleaning up the the heater wiring... should I bother?

[shooter]

its pretty darn quiet.

should I bother?

Don't

 

leave good, good, till it ain't

[jjasilli]

My 2 cents:  If it ain't broke don't fix it.  There is an issue with overdrive, so I'd focus on that first.


BTW: my 1482 was noisy, so I upgraded the heater supply to twisted pairs, and grounded the input jacks.  It's still a bit noisy with single coil PU's; only noticeable in between songs.  Note: the open V-shape chassis is not the best shielding. 

[dbishopbliss]

I will leave the heater supply for now.


Another question that I should know but somehow always forget... I'm replacing the filter caps so I'm also replacing the resistors in the power supply. According to the schematic, the voltage is 350V going in to a 2.2K resistor, then 330V going into a 27K resistor and 260V on the other side.


I think I recall people saying they use 3 watt or 5 watt resistors here but my calculations seem to suggest that a 1/2 watt resistor would be more than adequate. That makes me think that maybe my calculations are off. Here's my math:


First resistor: 20V / 2200R = 0.009A and 0.009 * 20V = 0.18W
Second resistor: 70V / 27000R = 0.0026A and 0.0026 * 70V = 0.18W


While I'm at it... I have read people using a 10W for the cathode resistor but again it seems like overkill.


16.8V / 270R = 0.06A and 0.06 * 16.8V = 1W


Wouldn't a 5W be good enough?


Thanks again.

[sluckey]

Your math is fine to calculate the actual power dissipated by the resistor. It's a good rule to double that number and pick the next higher common power rating. So 1/2 watt resistors would normally be OK for the B+ droppers. The main reason that people like to use 2 watt or 3 watt metal oxide is because they are flame proof. It's not uncommon for a filter cap to become very leaky or even shorted. When this happens the upstream resistor(s) will be subjected to much higher current flow and may even burn up. The flame proof resistors will still heat up but at least they will not start a fire.

3W metal oxide or 5W wire wound are sufficient for your cathode resistor. This resistor is also subject to high current if the tube shorts so high wattage is desirable.

[dbishopbliss]

Use big flame proof resistors to prevent fires. Got it. :-)


Here's another question... Are electrolytic capacitors used for a similar reason?  I noticed a 10uF/500V electrolytic cap is like $9 but you can buy a 10uF/630V metalized polypropylene for less than $6.  The PPE caps also seem to come in many more values so I could get closer to the original values in the multi-cap (5uF, 10uF, 20uF).


By the way, would there be an advantage to increasing those filter caps? They seem lower than most amps.

[sluckey]

It's a cheap amp. I'd be putting Illinois caps in it. 1/3 the cost of those F&Ts.

You can increase the size of those caps for better filtering but it might change the tone/feel a bit. I would probably use 22, 10, 10 IC caps.

[dbishopbliss]

I don't see a 10uF Illinois option on Doug's site. I ordered all F&T because the 22uF was only 27 cents more than the Illinois. I could always buy from Mouser but I like to give him the business for providing this forum.

[sluckey]

I wonder why the F&T 10µF is nearly 3X the cost of the F&T 22µF.

[jjasilli]

By the way, would there be an advantage to increasing those filter caps? They seem lower than most amps.

You earlier stated that you like the stock tone of the amp.  In that case, you should keep your rebuild as close to stock components as possible.  So-called bargain basement amps have their own distinctive tone.  As sluckey says, "upgrading" the type or value of components is likely to alter the amp's tone.


I bought my 1482 after reading a Tonequest article in which Ry Cooder extolled its virtues.  But when I got my amp from an eBay seller, I didn't like its tone.  So I did extensive mods. It still retains some bargain basement flavor.

[PRR]

The running power may be 1/2W, but the start-up surge can be considerably higher, dozens of Watts. This leads to the case where the amp is fine for 100 or 1,000 starts, and then a resistor blows "for no reason".

When you have high and unknown surge-stress, and weight/cost are not real issues, it can be wise to go generous.