Silvertone 1482 - Why so distorted?

[dbishopbliss]

> Could it be that the value is to high for my meter?

I can't see your meter from here. Does it have any specs?

A lot of popular-price DMMs only go to 1.99Meg. NOBODY (except tube-amp nuts) uses resistors that large. (Also in implies a test-current of 1uA, which is comparable to leakage in a low-price meter.)

Its a Radio Shack from the 90's. There is a Fluke 77-IV for $140 listed locally. Maybe I should invest.

[PRR]

> Its a Radio Shack from the 90's

Radio Shack stuff has long production runs and the Owner Manual may be on their site. You need a model number.

[1blueheron]

Heck, I made a cap with a 24x24 piece of window glass, two pieces of aluminum foil, and a bit of duct tape for a science fair project. (see attached pic). Anyhow, that perfect cap with perfect insulation between the two plates is said to have zero leakage.   

You've convinced me, but your Harry Potter impersonation in the pic doesn't appear to have your lady friend convinced...   Just joking, very good explanation and very helpful.

[sluckey]

I wish! That's Miss Mallory, my 12th grade homeroom teacher. Every guy in high school wished she would stand next to them! Heck, there were guys in my class older than she was! Lot of older drop outs came back to school in '66 when the draft rules changed! 

[dbishopbliss]

That's Miss Mallory, my 12th grade homeroom teacher...

I hear she was inspired by your project, stole your idea and went on to launch a successful company producing capacitors.

At least that is the story I'm going with.

[dbishopbliss]

I have the filament transformer installed now and it is an improvement. Things seem more lively than they did before.


While I was in there I swapped the bypass cap on the 6V6 cathode from 25uF to 72uF as recommended by jjasilli. To be honest I'm not sure I can really hear the difference. From what I gather this should extend the bass response but the entire amp is pretty bass-heavy right now so maybe that's not a good thing. I need to get one of those decade boxes so I can switch between caps easily.


I have a couple of questions that fall into "is that what that does" category...


I have been playing with both the microphone and the instrument channels. I am finding the microphone channel to have much higher gain than the instrument channel. The instrument channel sounds downright dull and dark comparatively. Looking at the schematic, there really isn't very much difference between them except that the instrument channel has a 68K resistor in series going to the grid. Admittedly, my resistors have drifted to 89K (if I recall correctly) so maybe this is a good explanation why things seem dull. Is the 68K resistor removing that much gain?


I also noticed that the Silvertone has these resistors BEFORE the 1M grid leak resistor. Fender and Marshall seem to have them after the grid leak resistor. Is this why jjasilli calls it a voltage divider as opposed to grid stopper?


Also,  jjasilli replaced his with 56K resistors. Any issue with having one resistor 68K and one 56K so I can have options?


Another question while I'm looking at the preamp. jjasilli installed a 10K grid stopper (after the 1M grid leak) on the mic channel... I'm guessing this will reduce the gain some but not as much as the 56/68K resistors but it will offers some protection from radio frequencies. 


I plan on replacing the input jacks on the amp with jacks that will ground when nothing is plugged in. Since all of these resistors are attached to the input jacks (including the pre-amp cathode resistors) I figure now is a good time to ask so I don't have to repeatedly solder and unsolder.


As always, I appreciate everyone's replies.

[sluckey]

I have been playing with both the microphone and the instrument channels. I am finding the microphone channel to have much higher gain than the instrument channel. The instrument channel sounds downright dull and dark comparatively.

I'm surprised. I don't see anything on that schematic to explain that. If the resistors measure the same between the channels then maybe one triode is hotter than the other.

The 68K resistors in the instrument channel do form a voltage divider with the 1M, so the instrument triode would only get 93.2% of your guitar signal, while the mic triode gets 100% of the guitar signal. But I doubt that would cause the mic channel to have "much higher gain", but it will have a little more gain.

For good flexibility I would put a grid stopper on the mic channel and put the 1M on the jack. Then use switchcraft 12A jacks wired like the standard Fender high/low jacks. This can be confusing if you've never wired these jacks. Hoffman has a good picture of exactly how to do this. I have an explanation for the ho/lo jacks in my amp scrapbook.

I would keep it simple and use three 68Ks and two 1Ms and three Switchcraft 12A jacks.

[jjasilli]

Some of the things in the last 2 Replies are subjective.  Other things could depend on the tubes used, their quality & condition.


To reiterate, the darkness of this amp starts at the input stages.  This is a prime example of the maxim that the input stage sets the tone for the whole amp.  I found that if you want a brighter tone for this amp you have to start there.  So, if you want a brighter tone, then the first step is to brighten the tone at the input stage(s).  Everything else is secondary or tertiary to that.

[dbishopbliss]

If the resistors measure the same between the channels then maybe one triode is hotter than the other.

I have swapped several tubes in that position. The original tube, a Sylvania 7025 and a Telefunken 12AX7. All seem to have the same effect. I found this grid stopper calculator. If I plug in the different values I get the following:


No grid stopper (0R): 0.97 (-0.26 dB)
68K grid stopper: 0.82 (-1.70 dB)
89K grid stopper: 0.77 (-2.27 dB)

Another thing that could be contributing... even though I have the volume pots set the same, it doesn't mean they measure they same. So perhaps one is actually turned up louder.

[dbishopbliss]

I just checked and the resistor I cannot read is a 2.2M going to ground. Could it be that the value is to high for my meter?

I cannot find a manual online but I did find a Radio Shack Catalog from 1994. According to the catalog the meter, Micronata 22-184, has a max resistance value of 2M. So there it could be that my meter won't read that resistor. Doesn't matter, I have one on the way. I could also try string two 1M resistors in series.

[dbishopbliss]

I have made the following changes so far:

• Replaced the input jacks with short jacks (12A) and wired more like a Fender with a Hi and Lo input on the instrument channel. I used 56K resistors for the instrument channel and a 10K grid stopper wired to the tube socket for the Mic channel. This changed the overall gain (for lack of a better term) for the three inputs so they all sound different.
• Added a 2.2uF cathode bypass cap to the Mic channel. Seemed to raise the gain, but tighten up the bass. Still a little tubby for me so I may play with a smaller bypass cap or change the coupling cap. Not sure what would be the "better" approach.
• Replaced the Instrument plate resistor with 100K, cathode resistor with 1.5K and 2.2uf bypass. This lowered the overall gain considerably but still easy to overdrive. Bass is much tighter than Mic channel.
• Added a 470K Phase Inverter Grid Stopper. This was the first change I made. Cleaned up the flubby bass a bit.

Things I tried but didn't like:

• 2.2uF bypass cap on the preamp stage of the PI. This raised the gain a bit. Already have plenty of gain so I took it out.
• 75uF bypass cap on the power tubes. This added considerable bass over the default 25uF. Playing with other values now. I'm leaning towards 10uF.

Now for the questions:

• I want to try adding the diode to the last filter stage. Jjasilli listed 1N4005, but I have a bunch of UF4007... can I use what I have?
• Is there a reason to use electrolytic cathode bypass caps in the power tube? I have some small film caps rated for 100V that will fit just fine.
• Thoughts on lowering the mic channel cathode bypass cap vs lowering the coupling cap?

Overall, this amp is really coming together. This is the first time that I have really played with swapping in values... playing, then swapping again. It has been enlightening.

[sluckey]

I want to try adding the diode to the last filter stage. Jjasilli listed 1N4005, but I have a bunch of UF4007... can I use what I have?

yes

Is there a reason to use electrolytic cathode bypass caps in the power tube? I have some small film caps rated for 100V that will fit just fine.

E-caps are typically used when you need a cap value over 1µF. But if you like the small value film caps then use them.

Thoughts on lowering the mic channel cathode bypass cap vs lowering the coupling cap?

Either will lower the bass. Try and see what sounds good to you.

[PRR]

> 75uF bypass cap on the power tubes. This added considerable bass over the default 25uF.

If I am understanding right, that isn't right. The cathode bypass on a push-pull power amp has little effect. Combined with "Why so distorted?", I wonder if one of your power tubes is not working.

[dbishopbliss]

> 75uF bypass cap on the power tubes. This added considerable bass over the default 25uF.

If I am understanding right, that isn't right. The cathode bypass on a push-pull power amp has little effect. Combined with "Why so distorted?", I wonder if one of your power tubes is not working.

The phase inverter grid stopper and the filament transformer seemed to fix the distorted issue. Since then, the thread has turned into more of a build log. I'm just using my ears... maybe I'm hearing what I want to hear but it really seemed to me that the bass was VERY full with a 75uF cap and less full with a 10uF cap. Still plenty there for me. I should probably try a traditional single coil guitar with it. I have my Stratotone and a Gretsch hollow body with Filtertrons. Both have plenty of bass.


Regardless... right now I'm pretty happy with the sound.

[dbishopbliss]

Now for a layout challenge...


I want to install some screen grid resistors on this amp. It seems like the common method of doing this is to install a 470R resistor between pins 6 and 4. The 1482 is wired point to point so this is going to be more challenging. The B+ wire is going to pin 4 on 6V6A (for lack of a better naming convention). Then there is a wire from Pin 4 going to the tremolo circuit, and another going to pin 4 on 6V6B. Because pin 6 on 6V6A is unused, I can just move all the wires from pin 4 to pin 6 then install the screen grid resistor as described.


However, pin 6 on 6V6B is already being used. I'm trying to think of how best to install the screen grid resistor. Options I can think of are:

• Drill a hole in the chassis and install a turret (insulated) and run the wire from pin 6 on 6V6A to the turret, then from the turret to pin 4 on 6V6B
• Connect the resistor to pin 6 on 6V6A and have it float over the other components. Extend the lead with wire to reach pin 4 on 6V6B
• Connect the resistor to pin 4 on 6V6B and have it flow over the other components. Extend the lead with wire to reach pin 6 on 6V6A

Any other ideas?


Or, should I not even bother with screen grid resistors.

[jjasilli]

Pins 1 & 6 are unused by the 6v6 tube.  Those lugs  can be used for any purpose, even unrelated to the tube.   If those lugs are already used,  you have a choice to make.   I have drilled and used terminal strips.

[sluckey]

Or, should I not even bother with screen grid resistors.

Bingo!

[dbishopbliss]

I thought grid screen resistors were a good thing to have on your power tubes. Can you explain why they are not needed.

[sluckey]

From a technical standpoint, I believe they are a good thing. However, a lot of cheap amps including your 1482 and it's bigger brother the 1484 didn't use them. I owned a brand new 1484 back in the '60s. I had it for about 5 years and never replaced a 6L6. But the little amp that really convinced me that small amps don't really need them is the famous Fender 5E3. My Fender Harvard and Ampeg J12 don't use them.

I don't know if the screen resistors were omitted because of bottom line $$$ or because it was part of the sound. Anyhow, when I built my 5E3 clone I didn't use any screen resistors.

But back to your amp... The reason I said "Bingo" was two fold. One, the original didn't have screen resistor. Two, there's no quick, easy, neat way to do it in that amp. If pins 1 or 6 had been vacant on both sockets, I would have said put'em on the sockets. Quick, easy, and neat. 
 

[dbishopbliss]

Ok... then I will just leave them out. I was thinking they should be there for safety reasons.


I think the only thing left for me to try (for now) is to adding the diode to the power supply and trying a different (smaller) bypass cap on the Mic channel.


I looked at the coupling cap value more closely... Originally, I was thought they were 0.1uF (like a 5E3) and I was thinking of making the Mic channel's smaller to attenuate some of the bass but then I realized it was actually 0.01uF. It isn't as small a plexi coupling cap, but I think it is small enough that going to 0.0047 would probably not be what I want. I think that the bypass cap will do what I want... although I was playing the amp quite a bit yesterday and maybe the bass is fine.

[dbishopbliss]

I just finished installing a 0.22uF bypass cap around the last cap (10uF) of the power supply. This seemed to be a very subtle difference. Thing seemed slightly more focused or clear for lack of a better way of describing it. Again, very subtle and hard to pin down... maybe I wanted to hear a difference, but I'm pretty sure it is there.


Then I added the diode after the last voltage dropping resistor in the power supply. This definitely made a difference. The attack is very fast and percussive now. I think I lost some of the gain, but I'm not sure why that would be. Maybe I just have the volume knob in a different place. Its getting late and my wife complains. :-) 


If anyone has a technical reason that can explain why these changes made the difference, please share. I'm liking the sound very much and I'm a little curious why I don't see these things in many circuits... maybe its because I mostly look at circuits from the 50's and 60's and Fender, Marshall and Vox just didn't do these things.


I think I'm done tweaking now. Just need to get the tolex and grill cloth on the new cabinets (yes plural) so I can start showing this amp off. I'm really enjoying the vibe.

[dbishopbliss]

I just realized... I haven't tried the tremolo yet... I will need to do that.

[jjasilli]

If your wife is complaining you must be on the right track!


0.22uF bypass cap around the last cap (10uF) of the power supply

the diode after the last voltage dropping resistor in the power supply.


I learned these PS mods from  "Inside Tube Amps", by Dan Torres.  The first mod is borrowed from hi-fi.  Real world caps are imperfect; the larger the cap, the more imperfection.  Large electrolytics are particularly "obnoxious".  A quality plastic bypass cap helps them do their job.  Typically this bypass cap should be 100X smaller than the electrolytic.  But I A/B different values while playing the guitar and settled on .22uF for this amp.  Attending to the 1st gain stage (last filter stage) gives the most bang for the buck.  In hi-fi tube amps, more filter stages might be bypassed.


The diode: When the Power Amp draws more current, causing voltage sag, it can suck current from the wall outlet, or from the preamp.  The diode "protects" the preamp, preserving attack in the preamp.  This compensates for droop in  the Power Amp, which I like.  I also like chokes which slow things down due to their hysteresis; and the diode compensates for that too, in the preamp. I consider these my standard PS mods: choke, diode, bypass cap.  I think tube rectification (or SS with inductive dropping R) & choke contribute to a blues or vintage rock tone. 

[dbishopbliss]

The diode: When the Power Amp draws more current, causing voltage sag, it can suck current from the wall outlet, or from the preamp.  The diode "protects" the preamp, preserving attack in the preamp.  This compensates for droop in  the Power Amp, which I like.  I also like chokes which slow things down due to their hysteresis; and the diode compensates for that too, in the preamp. I consider these my standard PS mods: choke, diode, bypass cap.  I think tube rectification (or SS with inductive dropping R) & choke contribute to a blues or vintage rock tone.

So what I am perceiving as "less gain" is really "preserving attack" which is why thing seem more clean. Might be interesting to put a switch for this... although I won't do it for this amp. Not a lot of space left.


Where did you mount the choke? Since I already have a filament transformer installed, I'm not sure I have room.


By the way... I took it to band practice yesterday. I was using it as a head because the existing cabinet rattles and the screws to mount the speaker are starting to pull through.


This little beast is pretty loud. I could not turn it up past 2, which is sort of a drag because I like the tones around 4. But, it took pedals really well. When I started it got really farty sounding with an overdrive pedal. It didn't have any of that yesterday.


Now I really want to continue with making a turret board layout and who knows... maybe I will make a head with some switches for the various options.

[jjasilli]

The choke is mounted near the PT.  The supplemental filament tranny is on the bottom wall. 


I often find that 12 - 20W is too loud, depending on size of venue, and vol drums & bass.  I built an attenuator, schematic below.

[sluckey]

Now I really want to continue with making a turret board layout and who knows... maybe I will make a head with some switches for the various options.

I made a layout a few years ago. If this looks interesting I will proofread the layout and email the Visio file to you.

[dbishopbliss]

I made a layout a few years ago. If this looks interesting I will proofread the layout and email the Visio file to you.

Of course you did. Just when I thought I might get to do something new. :-D

[shooter]

I will proofread

Just when I thought I might get to do something new

You can proof it for Steve 
course it's a waste of time but hey 

[sluckey]

I thought you used Visio, but I just looked at your board layout for that BandMaster. Now I'm not sure. Can you confirm? If no Visio then I'll just do a pdf. It will include a full size drill guide so making this board will be easy. Of course you may want to just use it as a guide and do your own layout. This layout is optimized for 1/2W metal film and Xicon 'chicklet' caps but I believe those pretty 1W carbon film resistors you used on the BandMaster will fit also. The board is 3-1/8 x 8.

So, which do you want, Visio file or PDF?

[dbishopbliss]

So, which do you want, Visio file or PDF?

I used to use Visio but now I'm on Mac and no longer have a license. So it will have to be a PDF. That said, I was just looking for a project to learn DIYLC as well as design my own board so not really necessary.


A number of years ago (wow, its been that long) I went through some lengthy discussions on this board about designing a low-wattage amp using a single 12BH7 for the power section. Maybe I will go back to those notes and take a stab at laying out that amp.

[dbishopbliss]

I finally got around to cleaning the pots on my Silvertone... had to buy some Deoxit. They were scratchy but useable. I didn't do a before and after listen so it may be psychological, but I swear there are more highs after I cleaned the pots. Could it be that some of the darkness I was complaining about was all due to dirty pots?

[sluckey]

Maybe you got some in your ears too?   

[Platefire]

One mod in my 1482 that I use a lot is on my mic channel I installed a Princeton 5F2 tone stack. The 1482 circuit is just naturally dark for what ever reason. This mod adds some nice controllable brightness. Platefire
My Schematic