Hi Everybody! I'm totally new to alot of this and some questions may come out really dumb but here goes!
I have this old guitar/instrument tube amp ... it's got 2 preamp tubes/2 output tubes, a power tran., and an output trans ... i'll have to post the tube types when i get back home but i do know they aren't the common ones like 12AX7, etc ... I replaced all the tubes with the correct ones but the thing hums and has very little output into the 8" jensen speaker ...
from the info i've gathered on the NET, it seems very likely that the CAPs need replacing. I looked inside the chassis and the date "october, 2X, 196X" <--- i put an X where i really couldn't make out the numbers .. the point is, it's old ... so on to the CAPS...
the caps that are in there: there are 2 blue ones that say 40uf 16V on them (can i replace those with 47uf 35V electrolytic?) and 3 Oil type caps that say .01 MFD +/- 10% 400 W.D.V.C - would i need to replace these or do they last long? what's a good match if i can't find the exact match ... the other cap is a cylinder type thing - a multi-cap is what it's called i guess ... i'll have to post again about those numbers .... so what should i do?
how can i test the transformers? the caps? should i replace the electrolytic CAPS first and see how the thing works? I would like to make some MODS to it too, like a standby switch, a 3-prong power cord, maybe a reverb tank, or anything else anyone may recommend ... the amp is probably less than 25 watts and i will use it mostly for a living room jam session guitar amp ... thanks in advance for all your help!!
kyle
I have this old guitar/instrument tube amp ... it's got 2 preamp tubes/2 output tubes, a power tran., and an output trans ... i'll have to post the tube types when i get back home but i do know they aren't the common ones like 12AX7, etc ... I replaced all the tubes with the correct ones but the thing hums and has very little output into the 8" jensen speaker ...
from the info i've gathered on the NET, it seems very likely that the CAPs need replacing. I looked inside the chassis and the date "october, 2X, 196X" <--- i put an X where i really couldn't make out the numbers .. the point is, it's old ... so on to the CAPS...
the caps that are in there: there are 2 blue ones that say 40uf 16V on them (can i replace those with 47uf 35V electrolytic?) and 3 Oil type caps that say .01 MFD +/- 10% 400 W.D.V.C - would i need to replace these or do they last long? what's a good match if i can't find the exact match ... the other cap is a cylinder type thing - a multi-cap is what it's called i guess ... i'll have to post again about those numbers .... so what should i do?
how can i test the transformers? the caps? should i replace the electrolytic CAPS first and see how the thing works? I would like to make some MODS to it too, like a standby switch, a 3-prong power cord, maybe a reverb tank, or anything else anyone may recommend ... the amp is probably less than 25 watts and i will use it mostly for a living room jam session guitar amp ... thanks in advance for all your help!!
kyle
Kyle,
Your cap questions:
1) Yes, the 47uF 35V will work in place of the 40uF 16V.
2) I would try some polypropylene ("orange drop") caps in place of the oil filled. Make sure you have the same or higher voltage rating.
3) New multicaps are available from Antique Electronic Supply (tubesandmore.com), or you can replace with a couple regular electrolytics.
I would also recommend investing in a multimeter (visit your local radio shack store) and checking the resistors. The values can drift quite a bit in 35 years. Also, I would go through the amp and re-flow the solder joints. Solder is strange stuff in that the chrystaline structure changes over time even at room temperature.
A word of caution: Before messing with the capacitors make sure they are discharged. The voltages in tube amps can kill! When you do get to the check-out phase of your project, clip the ground probe of your multimeter to the chassis, put your left hand in your back pocket and hold the hot probe in your right hand. Make sure you are not grounded. Enough preaching.
Good Luck.
Mike L.
Your cap questions:
1) Yes, the 47uF 35V will work in place of the 40uF 16V.
2) I would try some polypropylene ("orange drop") caps in place of the oil filled. Make sure you have the same or higher voltage rating.
3) New multicaps are available from Antique Electronic Supply (tubesandmore.com), or you can replace with a couple regular electrolytics.
I would also recommend investing in a multimeter (visit your local radio shack store) and checking the resistors. The values can drift quite a bit in 35 years. Also, I would go through the amp and re-flow the solder joints. Solder is strange stuff in that the chrystaline structure changes over time even at room temperature.
A word of caution: Before messing with the capacitors make sure they are discharged. The voltages in tube amps can kill! When you do get to the check-out phase of your project, clip the ground probe of your multimeter to the chassis, put your left hand in your back pocket and hold the hot probe in your right hand. Make sure you are not grounded. Enough preaching.
Good Luck.
Mike L.
What Mike said ^
Just add my $.o2.
Replace the high voltage electrolytics first. From the low OP and major hum this is the most likely cause. Those old oil coupling caps are prolly junk too but if you are on a budget the filter caps should be the first to be replaced. If you do this and the hum goes away and volume comes back great but keep an eye on your OP tubes! If they glow red then those oil filled caps are leaking DC onto the grids. This, if it is the case will kill your OP tubes real fast. Testing a transformer for simple shorts and opens needs a meter as well, a great thing to have if you really want to DIY🙂
A big second to what Mike said about keeping one hand in your pocket! I have been repairing amps for years and let me tell you 400VDC or 1000VAC can really hurt, even kill
You could get a cheap(as long as it has a meg ohm setting) analog meter for testing electrolytic Caps. You can discharge a cap by shorting them with a 1000 ohm 5 watt or simular resistor. After it is shorted(be sure it is totally drained) place meter(on high ohm scale) across it. The needle should first swing up and then back to almost infinate. Then reverse the leads and repeat. If the needle swings further after reversing the leads the cap can hold a charge. The needle should still come back to almost infinate Any other result indictates a bad cap.
A digital meter is far better for checking resisters and voltage. It is best IMO to have a good digital meter for most testing and save the analog just for testing electrolytic caps.
Do not try any Mods(except a new plug) until after it is working well. A standbye switch is easy enough but adding reverb would be a challenge for a beginner and may not be possible without a seperate power supply as you will need at least 2 or 3 tube stages for it and there is very little to go on as to weither your power transfo can handle the extra filament current. If it has extra channels you could make it a one channel amp and this way use the extra tubes, sockets and juice for your reverb. A somewhat complicated job and not something I would recommend for a beginner, sorry. Maybe add an FX loop and use out board reverb.
Good luck,
Adam
Just add my $.o2.
Replace the high voltage electrolytics first. From the low OP and major hum this is the most likely cause. Those old oil coupling caps are prolly junk too but if you are on a budget the filter caps should be the first to be replaced. If you do this and the hum goes away and volume comes back great but keep an eye on your OP tubes! If they glow red then those oil filled caps are leaking DC onto the grids. This, if it is the case will kill your OP tubes real fast. Testing a transformer for simple shorts and opens needs a meter as well, a great thing to have if you really want to DIY🙂
A big second to what Mike said about keeping one hand in your pocket! I have been repairing amps for years and let me tell you 400VDC or 1000VAC can really hurt, even kill
You could get a cheap(as long as it has a meg ohm setting) analog meter for testing electrolytic Caps. You can discharge a cap by shorting them with a 1000 ohm 5 watt or simular resistor. After it is shorted(be sure it is totally drained) place meter(on high ohm scale) across it. The needle should first swing up and then back to almost infinate. Then reverse the leads and repeat. If the needle swings further after reversing the leads the cap can hold a charge. The needle should still come back to almost infinate Any other result indictates a bad cap.
A digital meter is far better for checking resisters and voltage. It is best IMO to have a good digital meter for most testing and save the analog just for testing electrolytic caps.
Do not try any Mods(except a new plug) until after it is working well. A standbye switch is easy enough but adding reverb would be a challenge for a beginner and may not be possible without a seperate power supply as you will need at least 2 or 3 tube stages for it and there is very little to go on as to weither your power transfo can handle the extra filament current. If it has extra channels you could make it a one channel amp and this way use the extra tubes, sockets and juice for your reverb. A somewhat complicated job and not something I would recommend for a beginner, sorry. Maybe add an FX loop and use out board reverb.
Good luck,
Adam
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