national glenwood model 90

[davohilts]

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

[Enzo]

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?

[davohilts]

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.

[davohilts]

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

[Enzo]

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.

[mozwell]

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)

[davohilts]

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.

[davohilts]

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

[davohilts]

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

[g1]

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.

[davohilts]

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.

[g1]

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?

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.

[davohilts]

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.

[52 Bill]

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.

[davohilts]

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?