National guitar amp (?) "Sportsman" help needed - diyAudio
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Old 19th July 2014, 01:51 AM   #1
cancon is offline cancon  Canada
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Default National guitar amp (?) "Sportsman" help needed

Hi everyone. I'm just getting my bearings together so I don't have too much info to share except for some pictures and general info. So far I can't find a schematic for this amp, nor can I date it ('60s?). It arrived here with complaints of being noisy...excessive hum on one channel, hiss on the other & tremolo not working. However, I have since let the smoke out...

Power supply & output section: labeled 5U4 rect and 2x 7027 finals. Installed: Groove tubes 5AR4 and JJ 7027A outputs. The OT is mounted inside the amp cabinet and running two speakers in parallel (1 replacement, 1 original(?) Jensen) at 8ohms. The filters have been replaced, as well as the cathode bias bypass cap (a swollen looking 100uf cap...).

Preamp: 4x 12AX7, this unit is tethered to the power amp chassis with an 8-pin connector (6V heater supply for the preamp chassis, 120V switched supply for the power amp, B+ supply for preamp and ground). There is also a separate 2-pin cable for the audio signal.

I suspected the preamp unit for contributing to the noise complaints. It is entirely stock (coupling caps, etc...) and the 6V wires should really be twisted together.

I'd love to give you a run-down of voltages but as I said, I let the smoke out. First thing I did was to lay the amp face-down and remove the back panel. Then, I unplugged the 2-pin signal connection between the two chassis and turned on the amp (still face down). After a few seconds I could see smoke and fire through a hole in the power supply chassis where the transformer wires pass through. Upon closer inspection I see no evidence of this traumatic event (odor, charring...nothing). Furthermore, powering up the amp with all tubes pulled does not blow the main fuse or cause the transformer to bubble, make noise, heat up, etc...My guess is that the tubes shorted being on their sides? My tube tester shows no shorts on the 5AR4, and I can't test the 7027's as they aren't exactly 6L6's (internally pins 1 & 4 are shorted, as well as 5 & 6) and I think this is confusing my tester, which automatically points to shorts between the Plate and "SUP" (screen grid?) on both tubes. Or are both tubes really shot?

Lastly, quick measurements on the power transformer:
Primary: 2.6ohm
High voltage supply: 150ohms & 75ohms from each leg to grounded CT.
5V supply: <1ohm
6V supply: <1ohm

I could take more measurements between various legs and (what I assume to be) center-taps...but these are all stabs given that I don't have the schematic.

Sleep now. More digging tomorrow. Any tips are welcome
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Old 19th July 2014, 02:11 AM   #2
poynt99 is offline poynt99  Canada
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I think that is a mid-50's amp. Looks like there were several models of the "sportsman", and you may want to check "Valco" as well. The 1230T seems to be a popular model number. Is there a number buried inside?

Your best bet might be to just trace the circuit out. It's not that complicated I would think.

Last edited by poynt99; 19th July 2014 at 02:13 AM.
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Old 19th July 2014, 02:23 AM   #3
cancon is offline cancon  Canada
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No markings what-so-ever except a serial tag (?) T12362. The Valco "Sportsman" schematic online is not the correct diagram...but I do agree the brand of this amp is a toss-up.

I'm mostly concerned with the power supply - choosing a new power transformer if needed.

Last edited by cancon; 19th July 2014 at 02:28 AM.
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Old 19th July 2014, 02:58 AM   #4
poynt99 is offline poynt99  Canada
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Have a good look around inside the cabinet for a paper sticker. Also see if there is any sticker or stamp on the inside of the chassis itself.

What are the numbers stamped on the power transformer?
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Old 19th July 2014, 03:23 AM   #5
Enzo is offline Enzo  United States
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No problem the tubes on their sides.

As i read I was going to suggest unplugging the preamp to see if the PA worked on its own cleanly. But you did that.

Your tube tester can find real shorts, but tube testers rarely put real world conditions on a tube. In my view, a tube tester can tell you a tube is bad, but cannot really tell you it is good.

SUP wouldn't be screens, perhaps SUP for suppressor grid? Hard tellin'.

6L6 will often plug in and work where a 7027 belongs. The "shorting pins" are to 1 and 6, wwhich the 6L6 doesn't use. Look under the socket. SOME amps will take advantage of the 1-4 connection of a 7027 by running one thing to pin 1 and another to pin 4. But look, if everything was wired to pin 4 and on the other side pin 5 got all the wires, not 6, then stuff in a 6L6.


Look up "light bulb limiter" and make one and use it.

All or nothing is not the way to test. If the transformer powers up alone, that is a good sign. With the preamp umbilical unplugged, all three tubes removed, power it up. If the bulb stays dim, good. Try it without the bulb, holds OK? Measure the AC voltage coming out. Got 6v, got 5v, got something reasonable on the HV winding? If it makes voltages, doesn't get hot, and doesn't blow fuses or make your bulb bright, it is probably OK.

I can't quite see, are all the wires off the old can cap? The transformer works, now install just the rectifier, and power up on the bulb. When on the bulb, the voltages in your circuit will tend to be lower, so keep that in mind. Does anything bad happen? Of it holds, does the rectifier light up? Do you get B+ voltage on the filter caps?

BY the way, putting the amp on its face shouldn't hurt it, but some wires might move, so inspect carefully to make sure the sharp edge of a chassis didn't cut through some wire insulation anywhere.

I am looking at the power amp bottom photo. Looks like pin 6 is empty on both power tubes, but looks like the upper power tube has pins 1 and 4 wired. Am I right here?: looks like B+ from supply to pin 1 of that socket, then a wire from pin 4 over to pin 4 of the other socket. Pin 1 empty on the lower tube? Hard to see. Looks like the signal jack has two wires one going directly to the grid of each power tube. During tests, probably want to clip or tack solder something like s 220k resistor to ground for the grids. We don't want the grids unterminated. And looks to me like that power resistor lower left might be the power tube cathode resistor? Value printed on it.

Sorry, I wandered. With only the rectifier, power up and verify you have B+ on both plate and screen pins of each socket. With power off, verify pin 8 of each socket has resistance to ground of that cathode resistor, whatever it is.

If all that is OK, try just ONE power tube in one of the sockets. use the bulb. power up, what happens? Trouble or no? If that is no good, remove the tube and try the other in the same socket. Yes/no? Now try the other tube by itself. REgardless of the tube tester, but shuffling tubes like this we can determine if they work, and if the two sockets seem to work.

And always have a speaker connected.

Did we check the fuse rating?

If any of this goes bad, we can determine at that point what the issue is. If we can get the thing to operate like that, try touching the two input pins with something like a screwdriver and see if the thing amplifies some hum.
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Old 20th July 2014, 02:51 PM   #6
cancon is offline cancon  Canada
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Enzo, thank you so much for taking the time to present that detailed response.

To answer your questions:
- I found one charred wire (6V), but I don't think this was the cause of the fireworks. It was charred to the point where I could scrape away the insulation with my fingernail - but I was unable to see any bare spots before poking my way around. (pictures updated, check the link again).

- The old filter can is completely disconnected

- You were correct about the screen voltage being fed to pin 1 of the first tube, pin 4's being connected together, and pin 1 at the second tube n/c. So, no 6L6 substitutions for now. Gotta say I don't want to change a thing as this amp sounds (sounded?) very good.

- Signal jack is indeed connected directly to the input grids.

- Cathode resistor is 200ohms, both cathodes having a good connection

- 2A fuse on the primary is correctly installed

Today I built a light-bulb current limiter, popped in a 40W bulb and letter' rip as per your suggestion. I left the umbilical connected for now (to have switched power) but there are no issues at the moment. 5V, 6V and 350-350V are accounted for. The light powers up and then quickly dims to the low level where I need to kill the lights and pull the blinds to see it. When I removed the lightbulb and subbed in my tester to measure the current, I see about 150mA (transformer, indicator bulb and four 12AX7 heaters).


The problem starts when I install the rectifier: bulb turns on full, and then dims to about 50%. I get a B+ of about 350V. First & second power tube installed...I get no difference here, the bulb turns on brightly and then over the course of about 10 sec it dims a bit...I want to say 50-60%. During one test it fluctuated, got a bit brighter and then settled down but still illuminated as before.

So, I'm not sure how to read this device but I don't think it should be burning so brightly. I plugged in the rectifier alone with bulb out and meter in, the current started to approach 1A so I cut the power. I also subbed in a 5U4 and two things happened - bulb burned as bright as before with old rectifier, when i went to measure current (bulb out) I could see arcing at the top of the tube.

Few questions...
Could running the grids disconnected have caused this?
What was the point of the last person subbing in a 5AR4 instead of the intended 5U4?
Unrelated, is the 100uf cathode bypass high?
What type of PT secondary fuses should be installed in the future to prevent this from happening again?
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Old 20th July 2014, 03:07 PM   #7
kevinkr is offline kevinkr  United States
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Since the amp's power consumption when operating normally is considerably higher than 40W the bulb is going to glow fairly brightly and limit the voltage to the amp. You now need to change to a higher wattage lamp in your "light bulb limiter." I'd go to a 100W lamp next, measure voltages and currents as you do so and make note of the voltage across the primary - it will probably be well below 100V.

Note also that modern 5AR4 are fairly easy to damage and you may have an internal short that shows up when you apply full voltage to it. When in doubt replace. Note also that installing a pair of UF4007 (1N4007) in series with the rectifier plate leads makes for much better 5AR4 behavior long term. (Banded end to plate obviously)
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Old 20th July 2014, 05:33 PM   #8
cancon is offline cancon  Canada
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Back to the original 5AR4, with 100W bulb:

Primary: 112Vac
B+: 485Vdc

What is the advantage to a 5AR4 over a 5U4?

One more test (all with 100w bulb)...
The B+ is looking OK with rectifier installed, plus any of the 7027's in the right socket (the one that doesn't have a screen voltage because it depends on the other tube being installed to daisy chain it over). With either of the 7027's in the socket with the screen voltage supplied the voltage climbs up to about 450v and then drops to the 100v range, the bulb lighting up more as this happens. This happens with and without the 2nd 7027 installed.

Last edited by cancon; 20th July 2014 at 05:52 PM.
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Old 21st July 2014, 05:47 PM   #9
Enzo is offline Enzo  United States
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The one socket is wired so a 6L6 can be used. The other socket merely needs that wire moved from pin 1 to pin 4 and it too will take a 6L6.
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Old 22nd July 2014, 04:15 PM   #10
cancon is offline cancon  Canada
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So I'm still not sure what the 100W bulb illumination should be, but subbing in known good 5U4 and 6L6's causes the following: bulb lights up and then dims as B+ climbs to 400V range, once tubes heat up the voltage falls to 200V range and the bulb gets brighter. I shut it off at that point and can't confirm that this last observation is how the circuit settles out, as I don't want to damage these tubes. My instinct is telling me that this voltage drop is not normal and indicative of a problem. Could the transformer be showing good results with no load, but failing with a load?

Last edited by cancon; 22nd July 2014 at 04:18 PM.
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