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Thread: national glenwood model 90

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    national glenwood model 90

    Hello,
    A friend dropped off the amp because it was blowing fuses, always.
    I agreed to check it out and have opened it up and see that all the cap cans have been removed and replaced by new caps which seem close to what would be done in a valco amp.
    This amp used a 6973 tube for the reverb and someone has rewired it to use a 6BQ5.
    The tube complement is as follows: The amp dates from '65 with 2-12" jensens.
    2-6L6's and a 5U4 in a power section
    3-6EU7's and 2-12AX7's and 1-6973 which is used for reverb (someone changed this to use 6BQ5 and replaced the original reverb transformer with a fender style and added a fender style tank)

    Plugged into the wall this amp blows a 3 amp fuse instantly.
    If I plug it into my current limiter, the lamp glows slightly but not brightly as with a dead short.
    I can connect a speaker load to the output and play a guitar through it and it sounds okay, both the reverb and tremolo work.

    I can pull the rectifier and power tubes and it will still blow the fuse.

    Can anyone help me try and trouble shoot this amp, I've not been able to find a schematic anywhere. The closest I've found is the Valco 510-80, the transformer numbers are the same but the national glenwood has reverb.

    Thanks for any help.
    Dave

  2. #2
    Senior Member Enzo's Avatar
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    So someone replaced the original caps, that doesn't mean none of them are bad anyway. OT failures are rare, but disconnect the B+ side of the OT primary, still blows?
    Education is what you're left with after you have forgotten what you have learned.

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    Hi Enzo,
    Thanks for the reply.

    I disconnected the 3 primary leads on the OT and turned the amp on without the rectifier and power tubes and it blew another 3 amp fuse.
    When connected to the current limiter the amp works, even the reverb and tremolo but the minute I plug into regular power the fuse blows.

    I checked the PT and got the following readings:
    2.2 ohms across the primarys
    55 ohms from one primary to chassis
    53.4 ohms from the other primary to chassis

    I checked the secondary windings and got the following readings:
    119 ohms across the secondarys
    58 ohms from one secondary to the ct
    61 ohms from the other secondary to the ct

    any other ideas?

    I checked the capacitors and they measured within spec (out of circuit)

    With the amp plugged into the current limiter the amp works, even the reverb and tremolo function. When plugged into house current the fuse blows immediately when turned on.

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    I've done some more investigation on my problem and have discovered that unloaded the heater secondaries measure 6.7 volts across the wires. When the amp is in standby with all the tubes installed the heater secondary measures 5.2 volts. Isolating the the power section from the preamp section the heater secondary measures 5.8 volts. I've retensioned and cleaned all the tube sockets, reflowed solder on all heater pins and connections and inspected the wires for breaks or issues but came up empty.
    Does anyone have any advice on where to go from here??
    thanks,
    Dave

  5. #5
    Senior Member Enzo's Avatar
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    You can't test caps for leakage with a meter. Your filters might be OK at a couple volts from your meter, but they might leak like a screen door at 200v or more.
    Education is what you're left with after you have forgotten what you have learned.

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    Senior Member mozwell's Avatar
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    Not familiar with this amp at all (and i might be totally on the wrong track), but is 55 ohm from one primary wire to ground possibly the issue here, i would have thought the primary should be floating & have no direct connection to earth.
    Does this amp have a "death cap", if so, the general view is this should be removed & the amp wired for a 3 pin mains lead.
    Lots of info on this site for how to do this i believe.

    If you disconnect ALL the PT secondaries, does the amp still blow a fuse when it is powered up

    It might be worth while taking an hour or so & tracing the amp, & posting the hand drawn schematic here.
    Is the bias cap installed correctly (positive to 0V)

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    Quote Originally Posted by Enzo View Post
    You can't test caps for leakage with a meter. Your filters might be OK at a couple volts from your meter, but they might leak like a screen door at 200v or more.
    I have pulled one leg on all the filter caps, one at a time and clipped in a know good cap in all locations. No change in my issue.

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    Quote Originally Posted by mozwell View Post
    Not familiar with this amp at all (and i might be totally on the wrong track), but is 55 ohm from one primary wire to ground possibly the issue here, i would have thought the primary should be floating & have no direct connection to earth.
    Does this amp have a "death cap", if so, the general view is this should be removed & the amp wired for a 3 pin mains lead.
    Lots of info on this site for how to do this i believe.

    If you disconnect ALL the PT secondaries, does the amp still blow a fuse when it is powered up

    It might be worth while taking an hour or so & tracing the amp, & posting the hand drawn schematic here.
    Is the bias cap installed correctly (positive to 0V)
    Hi Mozwell,
    the bias cap is installed correctly. the amp is really complex with tons of caps and resistors and beyond my ability to draw a schematic. It is similar to the Valco 510-80 except it also has reverb.
    I have removed the death cap and the reverse ground wiring as it is not necessary with a grounded power supply.
    I disconnected all the secondaries and plugged the PT into a plug strip with a 15 amp breaker and it didn't blow nor did it get hot.
    The ohm reading I took were across the primaries and from one primary to centertap which was connected to the chassis.
    I've tried pulling all the preamp tubes one at a time to see if the heater voltage might jump up, but I only get a slight rise as I pull one after the other of the preamp tubes.
    Thanks for you help.
    Dave

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    Here is a link to the schematic for the Valco 510-80
    http://www.cornelvis.com/schematics/...lco_510-80.pdf

  10. #10
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    If it blows with the rectifier tube removed, then it would seem to be a heater winding issue.
    It sounds like you already suspect that due to the low voltage reading.
    Suggest you calculate the total heater current for all the tubes, then put an appropriate resistor across the heater winding to simulate the load. If fuse blows when delivering rated heater current, then the PT, or at least the heater winding, is bad.
    You might get away with adding an external heater transformer.
    Quote Originally Posted by Justin Thomas View Post
    In my ideal world, I'm not too loud - your room is too small!

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    Quote Originally Posted by g1 View Post
    If it blows with the rectifier tube removed, then it would seem to be a heater winding issue.
    It sounds like you already suspect that due to the low voltage reading.
    Suggest you calculate the total heater current for all the tubes, then put an appropriate resistor across the heater winding to simulate the load. If fuse blows when delivering rated heater current, then the PT, or at least the heater winding, is bad.
    You might get away with adding an external heater transformer.
    I disconnected all the wires except for the mains and the pt still blows the fuse so I think my PT is toast.

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    Quote Originally Posted by davohilts View Post
    I disconnected all the wires except for the mains and the pt still blows the fuse so I think my PT is toast.
    Something changed?

    Quote Originally Posted by davohilts View Post
    I disconnected all the secondaries and plugged the PT into a plug strip with a 15 amp breaker and it didn't blow nor did it get hot.
    Quote Originally Posted by Justin Thomas View Post
    In my ideal world, I'm not too loud - your room is too small!

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    So, what convinced me that I had a shorted PT was that I disconnected all the secondaries from the amp but left the primaries connected. I then plugged the amp into the wall outlet and turned on the switch. The fuse blew so I figured okay the PT is toast. I disconnected it completely from the amp and removed it completely.
    I then considered that maybe the switch could have a short in it. I didn't have a double pole switch so this morning I hooked the PT primaries to my variac and installed a fuse holder in line with the hot side leading from the variac to the PT primary. I then turned the variac on and slowly brought the voltage up to 120 volts. When I got to 120 volts the fuse didn't blow, this matched my test with the 15 amp breaker on the plug strip but with a 4 amp slow blow fuse. Maybe my PT isn't toast! I picked up a new double pole single throw on/off switch and wired it into the amp then installed jumpers from the switch to PT primaries and installed a lamp across the 6.3 v winding on the secondary side. I then turned on the amp and the fuse didn't blow and the lamp came on. Looking better, I thought.
    So I reinstall the PT and reconnect all the secondaries plug the amp back into the variac and turn on the switch, slowly bring it up to 120 volts and everything works. I then unplug the amp from the variac and plug it into the wall outlet and turn it on again. The amp powers up for about 10 seconds and then the fuse blows again.
    I'm at my wits end, I know I'm missing something but I can't seem to find out what the issue is.
    While the amp was plugged into the variac at 120 volts, I plugged a guitar into it and played through it for about 3 minutes and it sounded good. I thought I'd solved the problem but no, when plugged directly into the wall outlet it blew the fuse.
    Help!
    I'm really frustrated.

  14. #14
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    Earlier on you noted that there was some sort of continuity from some winding to the chassis, which would be true if the center tap was connected to ground.

    If the transformer is sitting out free from the chassis, check it for any continuity from any winding to the core lamination.

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    Hi bill 52, I've reinstalled the PT in the chassis and have it plugged into my variac at 120v. One difference with the new switch installed is I now have 6.4 volts across the heaters, not the 5.2 volts as before. The amp works perfectly as long as its plugged into the variac set at 120 volts, why would plugging it into a wall outlet cause it to blow a fuse and not when plugged into a variac?

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    Quote Originally Posted by davohilts View Post
    ...why would plugging it into a wall outlet cause it to blow a fuse and not when plugged into a variac?
    I don't know why.

    You stated earlier that the line reverse wiring and death cap have been removed and I assume that you have a 3-wire ac cord connected. How is it wired? Is your variac an isolation type transformer?

    And you never cleared up the questions regarding the continuity from winding to chassis.

  17. #17
    Old Timer olddawg's Avatar
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    Just for test. Disconnect your primary again and connect a fused suicide cord to the primary. (2 wire cord) Then plug it into the wall outlet and see if it works. Or you might try a ground lift adapter first. I suspect you have something leaking into the chassis ground. Btw, what is your line voltage there? 120vac? I had an amp do this once after someone installed a 3 prong power cord. I reversed the primarily wires and it worked fine. I made damn sure there was no leakage to the chassis before I released it.

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    I have drawn up a sketch of the power amp and its interface with the preamp thru the on/off switch and standby switch. There is an 8 pin socket in the preamp that accepts a male plug from the power section. Also the power tubes pin 5 connect to the preamp through the use of a 2 pin socket which has a plug and wires that are connected in the preamp. The standby switch also ties into one of these connections in the preamp.
    I have a 3 wire grounded power supply in the amp and my variac is an old one and I don't know if it an isolation type but I assume it is. That would explain why I don't blow fuses when plugged in it and do blow fuses when plugged into a wall outlet.
    I haven't resolved the question about the PT wiring continuity.
    To do this should I remove the chassis and do continuity tests? Which wires of the PT should I test for continuity?
    thanks again for the help.
    Dave
    nationalamp001.pdf

  19. #19
    Old Timer olddawg's Avatar
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    Try a ground lift adapter first.

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    Quote Originally Posted by olddawg View Post
    Try a ground lift adapter first.
    so you suggest i put on an adaptor to the grounded cord and plug it into the wall outlet to see if it doesn't blow the fuse?
    just checking before I do it.

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    well, as I grabbed the cord I realized that Olddawg nailed it. I'm embarrassed to say that the ground lift adapter was ready for me because it was installed in the old variac's 2 prong outlet. And guess what, the variac didn't have anything to do with not blowing the fuse, it was the ground lift adapter. So it looks like something is grounding where it shouldn't and I suspect the standby switch wiring. I didn't really understand what was going on with the standby switch wiring but figured it must be proper, but now i'm not so sure.
    It is in my sketch of the circuit, does anyone see a problem there? As you can probably tell electronics is not my strong suit but I'm trying to learn little by little.
    Thanks for opening my eyes!
    Dave

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    From what I see in post #18, on page 1, it looks like you have a leg of the AC from the wall grounded right after the power switch? It's like the Standby is basically a second power switch, one side of which is grounded? Is that right?

    Justin
    "When receiving a shock I emit a strange loud high pitched girlish squeak." - Alex R -
    "Sort of like not checking for toilet paper before taking a dump. ." - Chuck H -
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    Quote Originally Posted by Justin Thomas View Post
    From what I see in post #18, on page 1, it looks like you have a leg of the AC from the wall grounded right after the power switch? It's like the Standby is basically a second power switch, one side of which is grounded? Is that right?

    Justin
    yes, there are two lamps that turn on and off with the power and standby switches, there are shielded cables running from the standby and power switches back to the lamps in front then back to the switches. One leg goes to ground. I don't understand how it works but the leg going to ground seems wrong. I don't know enough about electronics to figure out how to properly wire the standby.

  24. #24
    Old Timer olddawg's Avatar
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    Your AC mains has a hot and a neutral. Have you simply tried reversing them?

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    Quote Originally Posted by olddawg View Post
    Your AC mains has a hot and a neutral. Have you simply tried reversing them?
    nope, but I will now. Thanks.

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    Quote Originally Posted by davohilts View Post
    nope, but I will now. Thanks.
    I switched the hot and neutral AC mains and get the same result. I have gone over the amp looking for anywhere I may have a wire touching the chassis or any components touching each other and can't find anything.
    So I jumped from the switch on the preamp chassis to the primary connections on the power amp chassis. I unplugged the 8 pin connector on the preamp chassis which connects the power amp and preamp and added a ground wire connecting the two chassis. I then plugged the amp into the wall outlet without the ground lift adapter and the amp powered up and didn't blow a fuse.
    This tells me that the power amp and switching for power on and standby are correct without going to ground anywhere.
    That leaves me with the preamp chassis to troubleshoot for a ground issue. Could one of the 10 or so coupling caps be causing this issue. They are of course original. Paper/wax type and I'm hesitant to just start changing out components.
    Does anyone have any suggestions as to how to go about troubleshooting this?
    Thanks!
    Dave

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    Old Timer olddawg's Avatar
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    Usually this happens as a result of retrofitting a 3 prong cord to an old amp that had a 2 wire cord and a death cap. I would draw out a schematic and make sure the preamp doesn't have a hot chassis.

  28. #28
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    If unsure how to wire the Standby switch, bypass it completely. It's a convenience, not a necessity. I don't think it matters which way you have the plug. Looking at your drawing, there's a dead short of AC mains to ground as soon as the power switch is closed.. As I see it, anyway. If later on you want to add a Standby, there are safer and easier ways to do it.

    Also see a ground that you drew AFTER the Standby? I think even if it was AFTER Standby, your amp would pop the fuse when you switched from Standby to Play.

    Any way to post a picture of that switch?

    Justin
    "When receiving a shock I emit a strange loud high pitched girlish squeak." - Alex R -
    "Sort of like not checking for toilet paper before taking a dump. ." - Chuck H -
    "All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -

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    I will work on a schematic as best I can. Upon further investigation I've run across something I don't understand.
    The power section and switching schematic I uploaded shows the layout of the power amp and how the switches connect to the lamps and the 8 pin socket and plug and 2 pin socket and plug that connect the two sections.
    With the grounded power cord plugged into the wall outlet and the 8 pin plug disconnected from the preamp I have jumpered between the switch to the primary side of the PT, connecting the hot and neutral to the two primary wires of the PT. I also ran a ground wire jumper connecting the two sections.
    The amp now comes on and doesn't blow the fuse so I thought I'd take some voltage measurements.
    The AC secondaries on the PT going to the rectifier tube measure 365VAC. The rectifier is putting out 400VDC and I also get this reading on pin 3 of the 8 pin plug which I have unplugged from the preamp section. Pin 5 which is supposed to be carrying 120VAC from the mains to the primary side of the PT measures 444VAC and pin 6 again should be 120VAC but measures 324VAC. The voltage difference between pins 5 & 6 is 120VAC.
    Pins 7 and 8 carry the heater supply to the preamp and they measure 3.3VAC each.
    Somewhere something is really messed up but I don't know where or why.
    Does anyone have any idea what the issue is? How is such high voltage getting into the primary side of the PT??
    Thanks for any help on this.
    Dave

  30. #30
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    Looking at the drawing that you posted, if the standby switch is in fact wired like that you are probably lucky to be alive and still posting here.

    STOP SCREWING AROUND WITH THIS! Fix it correctly or send it to someone that is qualified to do it. You are messing around with ac line voltages that can kill you and your attitude seems to be that if this doesn't fix it, let's try this and if that doesn't fix it we'll try this.

    There is only one correct way to wire the primary circuit of this amp. If you don't want to die, then make it right and then move on to fixing the rest of the amp. Listen to what everybody here has been trying to tell you.

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    Hi 52 Bill,
    I really appreciate what people are trying to tell me, I guess I'm just not understanding what is being said. I am using extreme caution when ever I do anything inside the amp and rightfully so. What I am doing is trying to understand the correct way to wire the primary circuit. Is there a schematic that you could point me to that could help?
    Thanks and I'm sorry if I have been annoying you, that is not what I am trying to do.
    Dave

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    Dave:
    You're not annoying me, you are frightening me. Start by answering this question.

    Do you know the three wire ac cord color codes?

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    52 Bill,
    Yes, at least I think I do. Most are black for hot, white for neutral, green for ground.
    Dave

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    Okay, so far so good. The hot should connect to the power switch and to the line fuse.

    You apparently have a double pole switch for the power, so you have both hot and neutral switched, which is fine. The hot lead then goes to the fuse or do you have the fuse connected first?

    After the switch and the fuse, the hot and the neutral both connect directly to the power pilot light. This light turns on whenever the main ac switch is turned on.

    The hot and neutral also connect to pins 5 & 6 of the connector cable. This cable sends the 120vac to the primary winding of the power transformer that is located in the power amp chassis. The original schematic also shows two caps connected to the primary winding, one to ground and one across the winding. I think that you stated that you have removed the cap to ground. Is this correct? What about the cap across the line?

    The standby switch should do two things. One half of the switch is used to turn on and off the standby light and the other half grounds out the audio signal that feeds the driver stage. Your drawing shows the hot ac line connecting to the standby switch ground connection. Is this in fact true? If it is, then disconnect that before doing anything else.

    The standby light needs to connect across the hot and neutral lines when the switch is turned on. This is just a simple series switched circuit.

    Once you are done there should be no continuity between either hot or neutral wires and the metal chassis. Test this with your meter set to read resistance.
    DrGonz78 likes this.

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    Quote Originally Posted by 52 Bill View Post
    Okay, so far so good. The hot should connect to the power switch and to the line fuse.

    After the switch and the fuse, the hot and the neutral both connect directly to the power pilot light. This light turns on whenever the main ac switch is turned on.

    The hot and neutral also connect to pins 5 & 6 of the connector cable. This cable sends the 120vac to the primary winding of the power transformer that is located in the power amp chassis. The original schematic also shows two caps connected to the primary winding, one to ground and one across the winding. I think that you stated that you have removed the cap to ground. Is this correct? What about the cap across the line?

    The standby switch should do two things. One half of the switch is used to turn on and off the standby light and the other half grounds out the audio signal that feeds the driver stage. Your drawing shows the hot ac line connecting to the standby switch ground connection. Is this in fact true? If it is, then disconnect that before doing anything else.

    The standby light needs to connect across the hot and neutral lines when the switch is turned on. This is just a simple series switched circuit.

    Once you are done there should be no continuity between either hot or neutral wires and the metal chassis. Test this with your meter set to read resistance.
    The way it is laid out the hot goes to the switch, then the unprotected side of the fuse. From the protected side of the fuse the hot feeds the pilot light, one pole of the double pole standby switch and the #6 pin feeding the power section.

    You are correct. The neutral feeds the power switch, then goes on to the #5 pin feeding the power supply. There is also a shielded cable running from pin 5 to the pilot light.

    I removed both caps, the one across the primary winding and the one to ground. I still have them, should I have left the .02 ceramic disc cap across the switched side of hot and neutral?

    What I meant to show was that the double pole standby switch has the hot connected to one switchside with the feed to the lamp on the other switchside there is a connection to ground with the switched feed to one of the 2 pin plugs feeding the #5 pin of one power tube. When the switch is in the off position it the standby light is on and grounds the #5 pin of one power tube. In the on position it turns off the standby light and ungrounds the power tube 5 pin.

    I have tested continuity and have none except for the grounded side of the double pole standby switch that grounds the power tube pin5 when in standby mode.

    52 Bill, I really appreciate you taking the time to get my head on straight.
    thanks,
    Dave
    Last edited by davohilts; 10-08-2015 at 09:30 PM.

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