Lane's Corner - One Tech's Blog

Discussion in 'The Workbench' started by Lane Sparber, Dec 20, 2010.

  1. Lane Sparber

    Lane Sparber Well-Known Member VIP Member

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    2203/2204 Squeal Remedy

    Does your 2203/2204 squeal like a banshee when the volumes, treble and presence controls are almost all the way up? Does it do this even with new tubes and caps? What I do in this situation is replace both grid wires on V1 with shielded cable and that usually clears it up. Here's how I do it (slightly modified from an earlier post - I just wanted it to be here in my sticky thread so it would be easier to find!): :)

    Remove ALL wires entirely going to pins 7 and 2 on V1 and set them aside.

    The new shielded cable you're going to install from the "HI" input jack to pin 7 on V1 gets it's shield connected at the jack's ground. I use heat shrink on the other end to keep the shield from accidentally shorting anything out. Then, solder a 68k resistor between pin 7 and the hot of your shielded cable on that end. Keep the leads as short as possible.

    The other cable is even easier. It's ground gets soldered right at the volume pot. The other end gets the same heat shrink treatment and the hot goes right to pin 2 on V1.

    Keep both of these runs as short as possible and you should find that squeal greatly diminished, if not gone entirely.

    Good luck!

    -Lane
     
    Last edited: Jan 8, 2012
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  2. Lane Sparber

    Lane Sparber Well-Known Member VIP Member

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    One of (if not THE) most often asked question around most D.I.Y. forums is "How do I safely drain my filter caps?" Well, there are several methods you can use. Some of them are safe, and some...less so. I'll discuss a few of them here.

    First, I'd like to dispel a myth which states that if you unplug the amp while it's powered up and leave the STANDBY on, it will discharge the filter caps on it's own. This is only PARTIALLY true. If your amp's high voltage rail does NOT have a "totem pole" stack (more on this below), it will only drain the caps until the tubes become cool enough to stop conducting, and this DOES leave a residual charge in the filter caps. Thus, you should ALWAYS check with your meter to make ABSOLUTELY sure that your caps are drained before doing ANY work on the amp.

    So, continuing on, there are basically two ways that the filter caps can be wired into an amp's circuit. In some stages, two caps can be wired in SERIES (commonly referred to as a "totem-pole" stack) for a specific filtration section, while most stages just use one single cap on it's own. Keep in mind that both methods can be (and often ARE) used in the same amp. Some examples of amps with a totem pole stack would include almost all Fender Silverface and Blackface amps above 50 watts, and Marshall DSL and TSL amplifiers. Examples of amps WITHOUT a totem-pole stack would include other Marshall amps, such as the 1987, 2204, etc. The main difference between amps that use a totem-pole filter stack and those that don't for our purposes here is that amps incorporating a totem-pole stack into their design will, in theory, COMPLETELY DRAIN THE CAPS THEMSELVES. Why is this? Well, when we place two caps in series, we need to stabilize the voltage across them so that one does not pull any more voltage than it's brother. This is done by placing a resistor in parallel with each cap so that they are in balance with respect to the voltages that each one "sees." These resistors also serve a secondary purpose - they drain all of the voltage out of the cap once the power supply is turned off. This is why these resistors are also commonly referred to as "bleeder resistors." These resistors don't just drain the caps they're connected to - they will usually drain EVERY cap in the amp's HT circrcuit - even filter caps in different stages that they're not directly connected to. This being said, it's impossible to know what's inside YOUR amp without actually dismantling it or studying the schematic. It's worth noting that even if your amp DOES have bleeder resistors, you should STILL check across the caps with your meter (after the amp's been powered down for a few minutes with the standby switch "ON") to make sure that there's no residual voltage there before doing any work on it.

    So...you've got the amp apart and need to drain the caps. How to do it? Well, here's ONE method I do NOT endorse. In this video, amp tech Cesar Diaz uses a screwdriver to directly short the caps to ground. This is NOT advisable because it's dangerous and can scar the chassis (and/or your screwdriver). It also creates a huge spark. I.M.H.O., it's all kinds of bad:

    [ame=http://www.youtube.com/watch?v=hedZTu7S4HE]diaz mtn - YouTube[/ame]

    My favorite cap-draining method is to take a simple jumper cable with alligator clips at each end and attach one end to the chassis. The other end goes to the pin 1 (or pin 6) connection at any preamp tube socket that uses a 12AX7 or any of it's relatives. Then just wait 1-2 minutes for the caps to completely drain and you're good to go. Leave the "standby" switch ON for this method. Be sure you use wire that's rated for at LEAST 600v and is 22awg or thicker in gauge. So...how does THIS method work? Pins 1 and 6 are the plate connections, and shorting them to the chassis shunts any voltage left in the caps to ground through the plate load resistors already in the amp! Still, this method assumes that the plate loaders are in good repair, so as ALWAYS, you'll need to check across each cap with your meter before proceeding further to make sure they're drained. In fact, I usually leave the jumper in place whilst working on the amp so that dielectric absorption (the phenomenon of the caps recharging themselves due to atmospheric conditions) doesn't rear it's ugly head and bite you! :think: Just remember to remove the jumper before powering up or you might get a NASTY surprise!! :mrgreen: Once again, I can't overstate that you should STILL check the caps with your meter before proceeding!!

    The final method I'd like to discuss is to take a powerful resistor with jumper clips and directly short each cap to ground. This method is SIMILAR to the method Cesar Diaz used above, with the notable difference being that instead of directly shorting across the caps, we are using a high-wattage resistor drain the caps. This is MUCH safer and yields a less nasty spark. For this method, we just need to determine what value (and what wattage) of resistor will get the job done safely and effectively, and we can figure this out using ohm's law.

    We know that 500v is the most voltage that can be in any single filter cap in a typical Marshall, Fender and most other amps. To determine which resistor would work to discharge a fully loaded filter cap (which is the "worst case" scenario), first we need to determine the current that would flow through that resistor using ohm's law. Lets start with an 11k, 10 watt resistor as an example...just to see what would happen:

    I=E/R or Current=Voltage/Resistance, so:

    I=500v/11,000 ohms = .045454545 amps.

    Now, we plug that into THIS formula:

    P = I^2 x R or Wattage=Current Squared times Resistance. Here we go:

    P = .045454545 (squared) x 11,000 ohms = 22.72 watts. Thus, we can see that, at only 10 watts, This resistor is not going to be able to safely handle discharging a fully loaded standard issue filter cap.

    I myself use a 100k 5 watt resistor for this purpose, which is MORE than adequate for this task. How do I know this? Let's plug it into the same formula:

    I=500v/100,000 ohms = .005 amps, and so:

    P = .005 (squared) x 100,000 ohms = 2.5 watts.

    I soldered two spare test leads I had to the ends of this resistor and covered the whole mess with heat-shrink to make it look nice and increase durability. Here's a picture of my apparatus. I call him "Mr. Bleeder:"

    [​IMG]

    You can find a 5 watt 100k wire-wound power resistor here for a whopping 36 cents:

    280-PRM5-100K-RC Xicon | Mouser

    I hope that through the above methods you can figure out how to safely drain your filter caps AND determine which method works best for both you and the amp you're working on. Working on amps IS a lot of fun, but safety must come first and the filter caps should be checked before doing ANY work on an amp.

    Please stay safe, folks!

    -Lane
     
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  3. Micky

    Micky Well-Known Member VIP Member

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    GREAT post, as always!
     
  4. Adwex

    Adwex New Member

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    Yes, great post. Glad to see you again Lane.
     
  5. thrawn86

    thrawn86 Well-Known Member

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    Good to see you Lane! Always enjoy your posts.
     
  6. core

    core New Member

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    Hi Lane! Great to see this post going so well :) I'm coming in way late and it may have been answered. Stupid question, but I've always been curious, what exactly is going on when you power down an amp? As you keep playing through the amp you hear the guitar fade out for several seconds even though there is no power to the amp. My first thought is that it's draining any excess current left in the tubes but I've always been curious to exactly what is happening.

    Cheers!
     
  7. Lane Sparber

    Lane Sparber Well-Known Member VIP Member

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    Honestly, it varies from amp to amp. Basically the "fade-out" you're hearing is, in fact, the caps draining until the tube filaments cool off to the point that they (the tubes) are not conducting anymore. Tubes don't actually store current, so that part of your statement was a bit off of the mark, to be honest. Once the filaments have cooled to the point that no more electrons are boiling off of the cathode and making the jump to the plate, the caps "hold on" to any stored electricity they have left at that point, assuming there are no "totem-pole" resistors in the circuit, like I mentioned earlier.

    Excellent question, Core! :wave:

    -Lane
     
  8. core

    core New Member

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    Thanks Lane, great explanation! Ahh, I remember now about the tubes not storing current, they pass current. That's why they are called valves doh! :)

    Not to take the place of actually using any of the draining caps method when working on them I'd assume just to be safe.
     
  9. All4Tone

    All4Tone New Member

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    Lane, this type of post adds so much value to this forum. The work you put into posting this info for the rest of us here is very much appreciated and you may have even saved someone from toasting themselves or their amp..

    Cheers!!
     
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  10. bvoris

    bvoris Well-Known Member

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    Hey Lane I just got through your entire blog. Thanks for keeping this up. There is a lot of really good info for everyone here. Thanks again also for the Mr. Bleeder LED version. Your recommendation I am sure has kept me from frying my sack while working on the few amps I have had the privilege to work on.
     
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  11. Lane Sparber

    Lane Sparber Well-Known Member VIP Member

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  12. Kunnz

    Kunnz Active Member

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    LANE

    Your back!

    havn't seen you about in a while, I hope all is going good with you, you F-----N genius

    :thumb:
     
  13. Lane Sparber

    Lane Sparber Well-Known Member VIP Member

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    LOL!!

    Thanks, Kunnz. High praise indeed, but I assure you - genius I ain't! :D

    I'm mostly "retired" from forum life, but I do like to stop by occasionally.

    -Lane
     
  14. Lane Sparber

    Lane Sparber Well-Known Member VIP Member

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    Measuring Plate Voltage

    I'm moving this post here (and making some edits), as people seem to be asking about it recently:

    To measure the loaded plate voltage, pull the chassis from the amp and fully power it up - BOTH power and standby switches ON. Make sure ALL tubes are installed. Make SURE a properly matched speaker load IS connected. Turn all knobs to zero. Set your DMM to read D.C. voltage, and if it's not auto ranging, set it to the 500v (or higher) setting. Place the red lead of your DMM on PIN 3 or any power tube socket (for 6L6, EL34 and 6V6 tubes), and the black lead goes to the chassis (ground). You should get a voltage reading somewhere in the 400s, generally.

    NOTE: If the power tubes are removed from the amp, the plate voltage will RISE, as the power tubes provide a resistive load to the HT supply. Also, with all power tubes removed from the amp, the output transformer is out of the circuit, so you technically do NOT need a speaker load hooked up to the amp in this case. That being said, always having the proper load connected to the amp when working on it is an EXCELLENT habit to get into. :thumb:

    WARNING!!!! See my sig!

    -Lane
     
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  15. thrawn86

    thrawn86 Well-Known Member

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    Good to see you again, Lane. :wave:
     
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  16. Lane Sparber

    Lane Sparber Well-Known Member VIP Member

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    Repairing Vintage Blackface and Silverface “Champ” and “Vibro Champ” Amplifiers

    The Blackface and Silverface “Champ” and “Vibro Champ” amplifiers present several unique repair and restoration challenges. Having worked on several, here are my findings – and what I do to correct the modern problems associated with these vintage amps.

    First, the heater string needs to be re-done to minimize the noise it can induce into the signal path. Stock, one leg of the heater string references ground directly. This is FAR from ideal in terms of noise cancellation. I remove all of the heater ground references at the tube sockets, and completely re-wire the heater string using the standard “twisted pair.” I then attach the standard two 100 ohm false center tap resistors. However, instead of grounding these resistors directly, I run the resistors’ junction to the power tube cathode. This will elevate the heater string by about 20-35 volts with NO extra parts, and in tandem with the twisted pair, it makes the heater string all but silent. This is critical in “class A” single ended amps such as this.

    Next, we need to look at the B+ rail voltages. Here’s where it gets a bit funky. In almost ALL of the Champs/Vibro Champs I’ve seen, the B+ is very, VERY high. I’ve seen it get as high as 420 vdc! This seems to be the case for all but the VERY earliest examples of these amps. The Champs’ power transformers mirrored the Princetons, and when the Princetons went with a push-pull design incorporating more voltage, the Champ’s PTs followed suit. To over simplify, the “AA764” circuit had the lower B+ on the schematic, and the “AB764” circuit reflected the higher B+ voltages.

    At any rate, by the Silver Face era, ALL of these amps were blasting their 6V6 tubes with very high voltages. When you consider this phenomenon in tandem with the higher wall voltages we see these days, it can cause some very real reliability issues – especially considering the lower quality of current production tubes. 420vdc is a LOT on these 6V6s! In addition, this higher voltage can cause the power tube to exhibit an extremely harsh, un-musical distortion characteristic when pushed due to the EXTREMELY asymmetrical clipping on the top half of the waveform. The problem is that Fender didn’t update the cathode circuit on the power tube when it went with the higher voltage PT. There are two ways to correct this. If the amp has ALREADY been recapped, you can simply replace the 470 ohm resistor and 25uf/25 volt cap with an 820 ohm or 1k 2 watt (or better) resistor and change the cap to a 50uf/50volt cap. The reason for the difference in the cap specs is due to the fact that we are shifting the bias point with this resistor change, so in order to maintain a similar frequency shelving point, we need to shift the cap up. Also, since this new resistor will cause the voltage at the cathode to rise to 35-40 volts, we need a cap that can handle that higher voltage level. If the amp has NOT been recapped, I solve this high voltage issue a different way: I add an additional PI filter BEFORE the first pre-existing filter stage. The benefits of this method are two-fold. First, it removes exponentially more ripple from the DC B+ supply than the stock configuration does. Secondly, it allows us to drop the B+ to much more manageable levels for longer tube life using the correct value decoupling resistor. I’ve found that a 5 watt 1.5k or 2.2k resistor in this position will bring the B+ down to a much saner 340-360 target voltage. If you’re replacing the cap can with discreet capacitors, this is a relatively easy mod. If you want to replace the existing cap can with a new FP type from CE, they DO make a 20uf-20uf-20uf-20uf model that’s perfect for this application. If you choose this method, there is NO NEED to swap component values on the 6V6 cathode – it will fall right into spec without additional modification. I shoot for a target current draw of 35mA-43mA or so at the power tube.

    While we’re inside the amp, we should also examine are the existing filter capacitor values. At about the same time these amps were switched to the higher voltage, Fender began installing a 40uf-20uf-20uf filter cap can as well. A quick look at the datasheets for the 5Y3 rectifier tube tells us that the 5Y3 wants to see a 20uf capacitor on the output. Fender nearly doubled this. I always go with a 20uf on replacement. That being said, I’ve rarely, if ever, seen a blown rectifier tube in these beasts.

    If the amp doesn’t have one already, it never hurts to install a 1k/5w screen resistor on the power tube socket. This will prolong tube life with current production tubes significantly.

    Finally, be advised that with the AA764 and AB764 circuits, there’s a “quirk” with the “vibrato” footswitch control. While this switch WILL ground out the LFO circuit and stop the actual oscillation, there is STILL DC being sent from the vibrato circuit to the second stage cathode. This amount of DC voltage IS STILL MODULATED BY THE INTENSITY CONTROL! The effect of this is that when the oscillator is switched off, the “intensity” control will act as a secondary volume control and cut the amp’s volume to varying degrees depending on where it’s set. I usually just tell my clients that rather than re-design the LFO circuit, it’s better and cheaper to just disconnect the footswitch and turn both the “speed” and “intensity” knobs to zero when the tremolo function is not required. I haven’t had a complaint yet.

    In conclusion, these are GREAT amps for recording, practice, and even some live situations when maintained correctly. I hope these hard-earned pointers and tips will be useful to you during your next repair!
     
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  17. MartyStrat54

    MartyStrat54 Well-Known Member

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  18. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    Good read and nice bits of information. :)
     
  19. Micky

    Micky Well-Known Member VIP Member

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    When I re-worked my Princeton Reverb, I just couldn't wrap my head around the tremolo footswitch, and the strange way it worked. I didn't even think to check that it might bypass to the second stage.

    Thanks for the info Lane.
     
  20. Lane Sparber

    Lane Sparber Well-Known Member VIP Member

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    Micky - your Princeton Reverb's vibrato footswitch operates in the EXACT same manner as the Vibro Champ - which is to say that it simply grounds out the positive feedback loop in the first triode of your "vibrato" tube. In the most basic terms, the three caps between the plate and grid of this triode form a positive feedback loop that reverses phase every time it goes through one of those caps. This is what generates the oscillation used in the vibrato circuit. Your switch just grounds out one node of this loop, effectively shutting it down. Where the two circuits DIFFER is that in your amp, the vibrato varies the bias voltage to your POWER tubes - sending them into cutoff repeatedly at a rate controlled by said oscillator. In the Vibro Champ, it controls the bias to a PREAMP triode.

    Check out Merlin Blencowe's article. As usual, he's a fountain of excellent info!

    The Valve Wizard- Tremolo Oscillator

    Hope this helps!

    -Lane
     
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