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Post subject: Bassman LTD conversion to PTP
Posted: Fri Aug 30, 2013 12:10 am
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Ok guys I need a little help o this one.. I converted my Reissue '59 Bassman LTD to a mojotone point to point circuit.. Today I thought was complete in my build and somewhere I have missed something.. As i plugged in my Tele, set her down and turned on the amp.. Watched the pilot lamp come on and the filament in each tube glow normally.. Turned up the volume on the Tele and gave the her a strum at a 3 setting on all amp pots and......... Absolutely nothing...... I am wondering if I set up the input jacks incorrectly or perhaps built the power plug from the wall incorrectly... Currently I'm using the fixed bias through the Zener diode.. Safely doing the physical touch test the casing of the transformer in the chassis is warm to the touch and the transformer underneath the chassis is cold to the touch and I am thinking that this is an issue in itself but do not know how to troubleshoot this.. This build was fairly simple but I need a little troubleshooting help.. The following are the tube pin values that I recorded:

................V1...................V2...............V3........OP1........OP2......RECTIFIER
Pin 1.......147.9................4.8.............252.5......255........255...........0
Pin 2........1mv..............385.3mv.........28.6.......255........71.5........483
Pin 3........1.16................6.0mv..........45.5.......71.5........485..........0
Pin 4........71.5................71.5.............71.5.......485.........485........2.45
Pin 5........71.5................71.5.............71.5.......485.........485.........0
Pin 6.......143.8..............216.5...........252.5.......11.3......215.5.......2.45
Pin 7.......2.5mv...............4.8..............30.8.......485.........485..........0
Pin 8........1.16................1.57.............45.5.......71.5........71.5.......484
Pin 9........71.5................71.5.............71.5

Please help :roll:

Thanks!


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Post subject: Re: Bassman LTD conversion to PTP
Posted: Fri Aug 30, 2013 1:01 am
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You have a number of voltages that don't appear correct. Are these all DC voltages? Did you measure any AC (signal) voltages? Heaters should all be 6.3 volts AC.

V1: 2 and 7 should be about the same. 4, 5, and 9 should be 6.3 volts AC.

V2: 1 and 7 are way off and should be about the same as 1 and 6 of V1. 4, 5, and 9 should be 6.3 volts AC.

V3: 4, 5, and 9 should be 6.3 volts AC.

OP1: Should be no voltage at 1. Should be 6.3 volts AC between pins 2 and 7. 3 should be about full B+. 5 is way off. 8 should be zero (ground). 6 should be over 400 VDC.

OP2: Should be no voltage at 1. Should be 6.3 volts AC between pins 2 and 7. 5 is way off. 8 should be zero (ground). 6 should be over 400 VDC.

Rectifier: Should see 500+ volts AC between pins 4 and 6.

You have major problems and need to recheck all of your wiring and voltages.

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Post subject: Re: Bassman LTD conversion to PTP
Posted: Fri Aug 30, 2013 1:20 am
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Your right!! I did measure in DC... But even still, measuring again in AC I'm getting the following:

OP1 and 2 measured in AC on pin 1 and 8 read 0.82vAC.. OP1 and 2 read 24.7vAC at pin 2... Pin 6 on OP 1 and 2 reads 0.005vDC.. Pin 7 on OP 1 and 2 read 19.67vAC..

Rectifier pins 2 and 8 read 491.5.. Pins 4 and 6 measured in AC this time are both reading 372.5vAC..

V1: pin 2 and 7 read 1.706vDC.. 4 and 5 read 25vAC.. 9 reads 19.8vAC

V2: pin 1 and 7 read 4.8vAC.. Pins 4 and 5 read 25vAC.. 9 reads 19.8vAC..

Where did I get this wrong? How can I post a picture to my thread?


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Post subject: Re: Bassman LTD conversion to PTP
Posted: Fri Aug 30, 2013 3:01 am
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michaelbpuccio, the only critical AC voltage read on tubes, in the heater. All other voltages read are VDC. Compare your values to the Bassman circuit you followed, when you re-wired the amp to P2P. Which Bassman amp circuit did you follow?

To post photos, you need a Net site like Photobucket to upload your photos. Then, download these photos into the message box.


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Post subject: Re: Bassman LTD conversion to PTP
Posted: Fri Aug 30, 2013 6:31 am
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Used the 5F6A circuit that was supplied by mojotone.


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Post subject: Re: Bassman LTD conversion to PTP
Posted: Fri Aug 30, 2013 11:09 am
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Ok, Currently I'm am in the garage and working on this amp.. So far I found that my grounds on OP1 and OP2 pins 1&2 were not actually grounded.. Fixed that issue by grounding them directly to the chassis.

I also found that the .047mfd 600v CAP that comes off the standby switch was not grounded.. It's grounded now to the chassis..

I moved the 100ohm resistors from the pilot lamp to V1 pins 4/5 and 9 to chassis ground.. The wires coming off the lamp were wired to pins 2 and 7.. I turned on the amp and RECTIFIER glowed but the tubes did not.. I flip flopped the heater wires coming from the lamp to OP1 pins 7 & 2 then turned it on again and the tubes all glow, then turned the standby switch on.. Once it got warmed up it blew the fuse.. I tried again for sake of a possible bad/old fuse and replaced it with a new fuse, turned it on, all tubes glow, turned standby on and once again it blew the fuse once it warmed up.. I also noticed the blue glowing flame in the OP1 & 2 tubes when the standby was switched on and it goes away once it's turned off.. I'm almost there!! Might anyone know where I can look or have you seen anything like this? I truly think my issue here is with the way I have the power coming into the on and standby switches because I am not using the ground switch..


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Post subject: Re: Bassman LTD conversion to PTP
Posted: Fri Aug 30, 2013 12:34 pm
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Please post the schematic and/or wiring diagram that you are working with. I am not familiar with the MOJO Bassman kit. I have schematics for the original 1959 5F6A Bassman, Fender's first reissue of the 59 Bassman, the Bassman LTD, and the Weber 5F6A clone that I built. It would help to know what yours looks like.

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Post subject: Re: Bassman LTD conversion to PTP
Posted: Fri Aug 30, 2013 12:39 pm
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Mojotones schematic is located at this link:
http://www.mojotone.com/Amp%20Kit%20Sch ... 4&gc=clear

The wiring diagram is located at this link:
http://www.mojotone.com/Amp%20Kit%20Sch ... 4&gc=clear

I have reviewed the original bassman schematic as well as the weber.. But still I'm missing something......


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Post subject: Re: Bassman LTD conversion to PTP
Posted: Fri Aug 30, 2013 12:56 pm
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Hahaha cool!!! I figured out the posting pics on here.. Here is where I'm at....

Image


Image


Last edited by michaelbpuccio on Fri Aug 30, 2013 1:07 pm, edited 2 times in total.

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Post subject: Re: Bassman LTD conversion to PTP
Posted: Fri Aug 30, 2013 1:03 pm
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Rock Star
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Joined: Mon Apr 05, 2010 2:37 pm
Posts: 8708
Location: Natural Bridge, Virginia
michaelbpuccio wrote:
Ok, Currently I'm am in the garage and working on this amp.. So far I found that my grounds on OP1 and OP2 pins 1&2 were not actually grounded.. Fixed that issue by grounding them directly to the chassis.

I also found that the .047mfd 600v CAP that comes off the standby switch was not grounded.. It's grounded now to the chassis..

I moved the 100ohm resistors from the pilot lamp to V1 pins 4/5 and 9 to chassis ground.. The wires coming off the lamp were wired to pins 2 and 7.. I turned on the amp and RECTIFIER glowed but the tubes did not.. I flip flopped the heater wires coming from the lamp to OP1 pins 7 & 2 then turned it on again and the tubes all glow, then turned the standby switch on.. Once it got warmed up it blew the fuse.. I tried again for sake of a possible bad/old fuse and replaced it with a new fuse, turned it on, all tubes glow, turned standby on and once again it blew the fuse once it warmed up.. I also noticed the blue glowing flame in the OP1 & 2 tubes when the standby was switched on and it goes away once it's turned off.. I'm almost there!! Might anyone know where I can look or have you seen anything like this? I truly think my issue here is with the way I have the power coming into the on and standby switches because I am not using the ground switch..


Thanks for the schematic and wiring diagram.

Why did you move the two 100 ohm resistors? The purpose of those is to create a pseudo center tap for the heater power supply. You should measure 6.3 VAC across each resistor to ground and 6.3 VAC from each heater terminal on the tube sockets to ground. 2 and 7 are the heater terminals for the two power tubes. You need to check to be sure that the heaters of the preamp tubes and PI are connected properly. None of the heater terminals on the preamp tubes should be connected to ground. I hope you twisted the heater wires as you ran them from tube to tube, otherwise, you may have bad hum problems.

If the fuse is blowing when you turn the standby switch to play, you need to check for shorts in the high voltage power supply. When I built my Weber 5F6A, I accidently burned a hole through the coating on one of the power supply filter caps. With the amp in standby, all was fine. as soon as I switched to plat, the cap terminal shorted to the body of the cap and blew the fuse. Here is a photo of what I am talking about:

Image

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Post subject: Re: Bassman LTD conversion to PTP
Posted: Fri Aug 30, 2013 1:10 pm
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michaelbpuccio wrote:
Hahaha cool!!! I figured out the posting pics on here.. Here is where I'm at....

Image


Image


Hate to say this, but I see a lot of messy soldering. I would not be surprised if you had a short in there somewhere.

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Post subject: Re: Bassman LTD conversion to PTP
Posted: Fri Aug 30, 2013 1:13 pm
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Ok yeah I did not twist the heater wires... But that is due to not having enough wire in the kit.. Not a problem.. I can get some today.. Right not I'm just trying to get it working correctly...


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Post subject: Re: Bassman LTD conversion to PTP
Posted: Fri Aug 30, 2013 1:16 pm
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Reason for "messy soldering" on the OP1 2 and rect is because I thought that using the porcelain sockets is better than using the brown ones in the kit.. Is this not true? And in looking at everything I have not found a short anywhere...


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Post subject: Re: Bassman LTD conversion to PTP
Posted: Fri Aug 30, 2013 1:20 pm
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Is this really messy?

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Post subject: Re: Bassman LTD conversion to PTP
Posted: Fri Aug 30, 2013 1:25 pm
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