New Build, Problems surface which stumped me.

[426Hemicuda1]

Hi, I'm trying my first JTM45 build, to the V2.1 instructions. I'm using the Classictone Magnetic components PT & OT, and the Mercury Magnetics Choke. After wiring in the PTP board, I began the voltage checks, as detailed on V2.1 Instructions pages 20 of 36, and everything was fine and checked out in steps A thru C, with one exception. I built an old fashioned voltage limiter/test lamp/cord, by placing an old ceramic 100 watt light bulb fitting on a 2 wire A/C line, with one line passing all the way thru to the plug, and the opposite wire cut and run thru the input & output of the 100 watt ceramic light bulb fitting. In theory, if you use this, it is supposed to eleminate any "rapid charging" of the caps & resistors, and also provide a "tell-tale" visual aid to determine if there is a short (by revelation of a very bright 100 watt bulb), or if it is working properly (by a visual weak & dim light look to that 100 w bulb), (if all is well), and a dead circuit (no light in 100 w bulb) if no light at all.
First result, wavery dim light that suddenly jumped to bright, then bulb blew instantly. 2nd result, blown bulb and a bright 100 watt light on the power switch, and no light with the stand-by switch on. Confusing, but I am tending to suspect the SPST carling switches, which may have gotten a bit too hot on solder in; OR, possible PT? Either way At step D is where I ran into trouble. New JJ GZ-34 tube which checks fine was used. I did NOT show 325 V on either side of the standby switch, and when I checked V4 & V5 @ pin 5 with bias adjusted fully clockwise (and later thru the range, to see if it changed voltage any); instead of -45 volts, I was getting a varied voltage of -111 to -114 volts. That had me shut down the project for now, until I can get further info. I did bypass step D and test the pre-amp and phase inverter tube sockets. They checked out fine with voltage.
So to me, it looks like a sole power situation, and some short somewhere, be it power switches or not. I also changed the lamp power from a 110/120 v, to 6.3V 15amp bulb. I disconnected all the A/C wires given in the instruction for the light, and re-pathed the wires to work only to power the primary SPST Carling on/off main power switch. I ran two wires from pin 2 & 7 of V4 (two 3.15v filament leads) to the lamp, to power a fender style 6.3V 15 amp bulb (cheaper). I'm tempted to wire the two leads for the power switches together to check everything else, and to verify for a possible short in the Carling switch. Strange thing, the fuses (Main & B+) have not blown (3 amp & 0.5 amp). Any advise is welcome. I suspect switches first, PT second. Am I way off base? TY

[426Hemicuda1]

Update. Received additional SPST switches today, removed the old ones, wired in the new power switch. Same results, limiter bulb power line shows bright 100W bulb, indicating short. More and more suspicious of Classictone PT. Re-checked all wiring, all PT wires are wired as they were instructed. Leary about blue wire going to Main 3@ fuse from PT and to SPST power switch. Given this is a rev. 3 (c) Classic tone, there is an indication that the colors of what "was or used to be" per their own admission on the PT specs, leads me to think the colors on the newer Classictone (Magnetic Components) PT are not as they should be? I guess my only solution is now to go thru and cut all PT wires one by one, (leaving a bit of color insulation on each so re-solder can easily be done), until the "short" is eliminated or light on limiter bulb power supply goes out. Any suggestions? PLease any input appreciated.

[Madditch]

Does it power on ok with out any tubes? If so check voltages on PT. Are you sure the limiter is wired correct? Try the light bulb limiter on a different amp. If those things check out try a different rectifier, preferably a SS plug if you have one.

[426Hemicuda1]

Thank you very much for your response. I have two GZ34 tubes, one Phillips, one JJ (brand new), it is the same with both. With help from the Triode tech, it seems that the correct part # PT & OT from the V2.1 instructions, made by Magnetic Components, went thru a color code "change" since the issue of the V2.1 instructions. Now that the color codes have been ran per the spec. sheets put out by Classictone, the brown & blue wires of the OT were supposed to be wired as black & red (As is the instructions for the Mercury Magnetics optional OT). Now that the wiring has been re-wired per the spec sheets for both the PT & OT, that problem has been solved, so to speak. The lightbulb was changed from a 120V to a 6.3V, and the former 120V wiring has been discontinued and cut out. Now, there are two feeder wires directly from pin 2 & 8 (3.15v each) filiment voltage pins, directly to the light socket, making the 6.3V. Light works properly now, when power switch is turned on. No more blown fuses, everything up to a certian point, seems to be powering up OK.
HOWEVER, the newest problem is shown below.
Per the V2.1 instructions on page I have successfully proceeded thru ITEM A, B, C & D instructions given on Page 20 with proper check-offs. Issues start on Item E, (checking the pre-amp tubes) there is a difference in the readings on V1, V2 & V3, in that the readings are substantially lower on pins 1 & 6. Instead of 180, 155, 205/180, 280, 195; I was seeing readings around 44 - 54. On pins 3 & 8, where it was 1.6, 1.0, etc. it was about 0.9. I did check the remaining pins (4, 5 & 9), and where it was supposed to be 3.15Vac, those readings were correct. I did manage to put the KT-66 tubes in, and they did light up correctly. Upon power up with all tubes in, plugged into a working speaker cabinet, when the power came up, you hear a sound I can only describe as being similar to the old Jimi Hendrix tune on his version of the national anthem.....you hear a rocket or artillery shell whistle starting with a high pitch, dropping to a low pitch (like an incoming arty shell) followed by a sound of a speaker short of EEEHHHHHH continual sound, until the power is discontinued. I changed out the speaker cord with a brand new, known, working, 1/4" plug guitar cord, and the sound still remains. So, to this point, I am unable to get to the point of biasing the power tubes. Would anyone have any new input that may be helpful? I thank you ALL in advance.

[426Hemicuda1]

Update on the extreme shrill artillery sound....swapping the black & red going to V4 & V5 on pin 3, from the Output transformer, eliminated the shrill arty sound. Now on power up, no shrill sound at all.
In as attempt to perform BIAS with multimeter on 200ma per page 22 instructions; PIN 8 for both V4 & V5 are close, and read only 38.1 & 38.3. Here is where it gets unusual. When trying to turn the bias pot counter-clockwise, turned from clockwise to counter-clockwise, there is NO change shown on the multimeter, while keeping the red lead on pin 8, on either V4 or V5. The bias pot seems to have NO influence or adjustment. Given it reads 38.1 to 38.3 on V4 & V5, this is less than the reccommended 40-45. I have proceeded to see if anything can be obtained as far as sound goes.
If Main Volume (far right) or Gain (2nd from right) are turned up over about 3, there is a high feedback sound, but I assume it is because it is facing the speaker cabinet. However, trying either guitar or 1/4" harmonica microphone plug-in, there is NO sound, from any of the 4 inputs, and this is after/during running all other control pots (Presence, Bass, Middle, Treble, Gain, Volume) from 1 - 10. I've tried switching the speaker out plugs, also. NADA. Re-checked all fuses, nothing is blown. If anyone has any input from here, I am ALL EARS, for I have reached an impass. Now, I am beginning to think the OT is blown, PT is working as is evidenced, but OT or choke??? Please advise.

[neikeel]

I doubt that you have blown your OT (as long as you have not hit the amp with a heavy load without the speaker/load connected you will be very unlucky).

Switched OT primary wires is very common and gives the banshee wail or horrible oscillation motorboating so correcting yours now is good.

Now it sounds as if you have three issues here.

First get a decent digital camera in good (preferably natural) light and post some pics of your amp, a straight ahead view of the whole internals, close ups of your OP selector switch and OP jacks, the wiring around the preamp can and the inputs. Mistakes that you cannot see will be picked up by eagle eyes here.

Have you done a voltage check as per the instructions with all of the valves/tubes out?http://metroamp.com/wiki/index.php/Jtm_ ... structions If these are all good then it is most likely there is a poor solder joint in the preamp signal chain (sometimes an underboard link falls out if you are new to this - why I suggest first time builder put their bus wire on top of the board). The screech you describe as you turn the volume up can be a poor preamp can connection or poorly/incorrectly routed NFB wire (usually purple in a kit) from the 27k NFB resistor to the presence pot.

It usually pays to add the 4k7 or 5k6 1/2 watt carbon film swamp resistors directly to pin 5 of the output socket to prevent parasitic oscillation with the grid wire hooked up to the end (keep wires short) which can also cause unwanted noise.http://metroamp.com/wiki/index.php/Outp ... _resistors

The bias thing sounds like you have grounded the wrong lug, check the wiring as per here:http://metroamp.com/wiki/index.php/Installing_Bias_Pot

It is when you look at the resources available here you appreciate the Metro Kits and site?

[arledgsc]

In as attempt to perform BIAS with multimeter on 200ma per page 22 instructions; PIN 8 for both V4 & V5 are close, and read only 38.1 & 38.3. Here is where it gets unusual. When trying to turn the bias pot counter-clockwise, turned from clockwise to counter-clockwise, there is NO change shown on the multimeter, while keeping the red lead on pin 8, on either V4 or V5.

Your meter needs to be in the DC voltage mode (millivolt scale preferred). You are not actually measuring current directly but the voltage across the 1 ohm resistor. By design with the 1 ohm resistor and using Ohm's law whatever is read on the meter directly converts to current. For example if you read 42mV across the 1 ohm resistor then in your head this equals 42mA of current. (Ohm's Law: V / R = I (current) or in our example 42mV / 1 ohm = 42mA of current).

[426Hemicuda1]

With a bit of help from all of you in here, and Chad @ Triode, I managed to discover and correct a couple of wiring discontinuities, based on the color codes from the latest revision of the PT & OT, which differed from the colors given. A swap of the black & red from the OT on the V4 & V5 pin 3, eliminated the artillery sound problem. I was able to get all voltage readings completed on the V2.1 instructions up to putting in the KT-66 tubes. All voltage readings were a slight bit higher than what the table showed, but usually within 10% - 15% of the table. Once KT66 tubes were installed, Plate voltage on Pin 3 & Pin 6 were 465 and holding constant, instead of the specified 400. The only issue still remaining, is the bias adjustment, or lack thereof of it. I had the PTP board farmed out and even after checking the wiring of it, it appears exactly as is shown on the V2.1 instructions. THe Bias still will NOT adjust or even flicker when the pot is moved or adjusted from one end to the other (about a 310° rotation). It remains at 34.7millivolts reading on the multi-meter, based off the pin 8 readings from V4 & V5. I did plug in a harmonica microphone into the input and managed to play thru it, (Volume on 3) and it sounds very good, clean sound, no hiss, no hum......but no bias adjust. I think this is the only remaining problem I have. I can only suspect the bias pot, but some of you may know different, and point towards something I have not thought of. BTW, I have not had a lot of good luck finding the exact replacement 25k bias pot I need, since the one on the board came from the assembler. Any help on pointing me to the correct part#, brand name, Mfg. and seller of it? ANY advice is appreciated, since I am SO close to getting this amp together and buttoned up. TY in advance.

[426Hemicuda1]

Per request, I've added voltage table of readings and picture.

GEDC0964.pdf

GEDC0964 is PTP board Picture

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My JTM45 Pin-Tube Socket Voltage Readings Chart.pdf

My JTM45 Pin-Tube Socket Voltage readings, PRE-KT66 & POST KT-66 install

(97.22 KiB) Downloaded 125 times

[neikeel]

......but no bias adjust. I think this is the only remaining problem I have. I can only suspect the bias pot, but some of you may know different, and point towards something I have not thought of. BTW, I have not had a lot of good luck finding the exact replacement 25k bias pot I need, since the one on the board came from the assembler. Any help on pointing me to the correct part#, brand name, Mfg. and seller of it? ANY advice is appreciated, since I am SO close to getting this amp together and buttoned up. TY in advance.

Likely incorrectly wired bias pot.

Show me a picture of the type of pot you have and how it is wired. Normally one lug to the bias resistor and two to the same ground as the bias caps - get this the wrong way round and rotating the pot will make no difference, although there are different types of pot so until we can see yours and the wiring cannot comment (btw 35mA is not far out!).

[426Hemicuda1]

Sorry for the fuzziness, my camera does not like the close-ups. The last picture is clear for overall, and you can see by the first picture (although fuzzy) the pot wiring for each lead is obvious & separated.
The two 10uf Caps (+ positive end/crimped end) are grounded & share a common ground. The right side 10uf cap is connected to the bias pot right hand lead with a wire on the topside of the PTP board (as can be seen); it also has a jumper wire underneath the board to the opposite (left) 10uf cap (+ side). This lead on the PTP board for the right 10uf Cap (on the bottom of the board) ALSO has a black wire (underneath the PTP board) going to the common ground buss wire between the presence and bass pot.

The rear bias pot lead pin (wiper of the pot) is soldered to a support turret, and not connected underneath to any other wire or turret connection, it seems to be a support only, with stand-alone feture. The common shared ground jumper (mentioned in the above sentence) between the two 10uF caps (+ positive side) go around & bypass this support turret, without touching it.

The opposite bias pot lead pin (left side in pictures) is connected to a 68K resistor running to the right side 10uf cap NEGATIVE end (uncrimped end). This is as is shown in the Metro V2.1 pictures.

Bias Pot Wiring.pdf

Bias Pot connections on PTP Board

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.

I removed the bias pot and discovered it is not defective, or I assume so. By that I mean I placed the multi-meter on a 200 ohm setting, and placed the black multi-meter lead to the front left pin, and the red multi-meter lead to the front right pin, and it showed no reading when turning the bias pot trim screw. However, when I connect the black multi-meter (or red) lead to the bias pot rear pin (Wiper Pin), and then connect the red multi-meter lead to either front pin and rotate the bias pot trim screw, then I do see the ohm readings adjust either lower or higher, depending on which way the screw is turned. Knowing the bias pot is working, I intend to re-install it. I know that the rear (wiper pin) must be connected or jumpered to one of the front pins, but to which one, I am totally unsure of. It's not in the instructions, which only mention connecting TWO bias pot leads, not three. This almost seems like I should rotate the 3 legged bias pot 120°, and have the bias pot wiper pin connected to either the positive side of the 10uF cap, or to the 68k resistor which is wired to the negative side of the 10uf cap. Once that one is connected, only ONE of the other to (formerly front) bias pot pins would be connected to the remaining end of the 10uf cap??????? I DON'T want to blow something up, so I am unsure which way to rotate the bias trim pot, or...... to re-solder it back as it was, and run a jumper wire from the wiper pin to one side or the other of the front pins.....I'm leaning towards this theory of thinking,....and to connect the jumper to the 68K resistor side???? PLEASE tell me if this makes sense? Please tell me if I am missing something? There is a second set of pictures attached. I've added a two pictures (#2 & #3) of the PTP board with the 3 pin bias trim pot removed, and the ink pen is pointing at this area. Pictures #1 & #4 are of the removed pot, so you can see the front two pins, and the rear wiper pin from the underside shot.

Bias Pot Wiring.pdf

Bias Pot connections on PTP Board

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. You should now have a good idea of how this bias trim pot is constructed, and the 3 pin trace lines it has.

[426Hemicuda1]

THANK YOU NEIKEEL - FIXED!!!! FINALLY!!! I went with my gut feeling with your advice, to rotate the 3 pin bias trim pot to where the wiper pin went to the 68K resistor which led back to the inside 10uF cap negative side. The rotation caused me to have to bend out one of the front pins to solder to the wire from the inside 10uF cap positive side, and place the remaining formerly front pin of the bias trim pot into the support turret that was connected to nothing. This made the pot adjustable. I had the rear of the amp facing me, so when I rotated the pot counter-clockwise (looking from the rear), the millivolt readings increased. My multi-meter had a 1.7 millivolt error of remaining above zero, so I subtracted that from the final setting of 47milli-volts, to give me a 45.2 milli-volt reading on V5 and a 45.0 reading on V4. Initial Bias is done!

Amazing, after all of my issues with color code changes from the manufacturer of the PT & OT, what ended up being the remaining issue was a mis-wired bias trim pot, on a PTP board that I had farmed out to the pros.....go figure. NOW, my amp is ready for the finish. I've got a slightly larger than normal small head cab design to place it in, since it does have the additional 7th control pot (Depth) sandwiched between the PRESENCE & BASS pots. I've already got the black crocodile tolex glued to the cab surround, along with the top grill & handle, with the gold piping. All that remains is to sand to size the front & back panels, add the tolex & gold piping and the logo to the front panel.

I wish to thank each and EVERYONE on here who gave advice (Neikeel & others), and Chad @ Triode for all the help. THANK YOU!!! I love this forum!!!
Pictures will follow, once it is compete.

Bias trim pot rotated - re-wired- re-soldered & working.pdf

Working Bias pot - rotated - re-wired

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[neikeel]

Glad to help