Greetings fellows. I am new to the forum and new to tubes as well.
My Grandfather was a radio technician back in the day and used to build tube radios, phonographs and TVs for Universal and Sylvania. Unfortunatley I only learned a few basic things from him before his passing many, many years ago.
What brings me to the forum is that I am a hobbyist guitar player and recently bought a single tube 25L6GT Webcor amp for a few bucks and was planning to make a 1 or 2 watt guitar practice amp.
Here is brief description and issues amp:
First, amp does not work, I get hum and some crackle. I tried a new GE tube and still does not. My guess is that the electrolyitc capacitor is dry. (capacitor reads 50 MFD 150V) I have not been able to find one to purchase.
There is a tag that reads Amplifier No. 14X351-2 . No power transformer, single tube 25L6GT. 3" speaker that will be replaced. One volume/on/off pot and another I assume is tone.
There is a red wire and a yellow wire that I am assuming are the input but not sure. Another couple of wires are not connected but these are very thin, I assume they are for a led or light.
Can the 25L6GT tube be replaced for a more guitar compatible tube without modification to the circuit? can I use a "more powerful" tube? (if that is even a question)
I am attaching a picture of the actual amp.
Any help would be appreciated.
- Jorge
P.S. My main amp is a Peavey Classic 30 that uses 3 12AX7 and 4 EL84 tubes. I have JJ/Tesla tubes in it. I am really happy with the sound and power.
My Grandfather was a radio technician back in the day and used to build tube radios, phonographs and TVs for Universal and Sylvania. Unfortunatley I only learned a few basic things from him before his passing many, many years ago.
What brings me to the forum is that I am a hobbyist guitar player and recently bought a single tube 25L6GT Webcor amp for a few bucks and was planning to make a 1 or 2 watt guitar practice amp.
Here is brief description and issues amp:
First, amp does not work, I get hum and some crackle. I tried a new GE tube and still does not. My guess is that the electrolyitc capacitor is dry. (capacitor reads 50 MFD 150V) I have not been able to find one to purchase.
There is a tag that reads Amplifier No. 14X351-2 . No power transformer, single tube 25L6GT. 3" speaker that will be replaced. One volume/on/off pot and another I assume is tone.
There is a red wire and a yellow wire that I am assuming are the input but not sure. Another couple of wires are not connected but these are very thin, I assume they are for a led or light.
Can the 25L6GT tube be replaced for a more guitar compatible tube without modification to the circuit? can I use a "more powerful" tube? (if that is even a question)
I am attaching a picture of the actual amp.
Any help would be appreciated.
- Jorge
P.S. My main amp is a Peavey Classic 30 that uses 3 12AX7 and 4 EL84 tubes. I have JJ/Tesla tubes in it. I am really happy with the sound and power.
Attachments
Jorge,
First- welcome to the forum!
Second- be very careful! No power transformer means power is taken directly from the mains and therefore it is possible for the chassis to become 'live' which is a bad thing.
Please take the time to read the safety thread on this forum. Lots of good info there. Tubes generally use high voltage (this amp probably uses between 110 and 200V which is actually low in the world of tubes).
I'm not familiar with this amp but I can try to answer a few of your questions.
1- Replace the electrolytic. It is a common value and rating. By the way you can replace it with a higher voltage rated cap. If you can't find a 50uF/150V cap you can use a 50uF/250V in its place. Stick with the original uF rating (within reason, a 47uF cap would work just as well).
2- Check all the resistors as the older carbon comp resistors can 'drift' in value. It wouldn't hurt to replace them as well. Use quality metal film resistors.
3- The 25L6GT is a beam tetrode tube. It has a 25V heater (thus the '25' in its name). Not an extremely common arrangement these days in either hifi or guitar amps. But tubes are still available as you found out.
The VT201C is a replacement. Changing the tube to something else would require essentially rebuilding the amp.
In class A (which is the case with this amp) it is good for about 3 1/2 watts peak with around 200V on the plate and about 2 watts with 110 volts on the plate. That could make a very good practice amp with the right speaker. Check out Antique Electronic Supply or Angela Instruments for suitable speakers.
4- I am not an 'expert' and am not familiar with this amp. In particular I wonder about the input stage, where is it? Is it sensitive enough for guitar? Others above my salary grade can probably answer that.
Be careful, have fun and look at this as a learning experience. Don't be discouraged if things don't work the first time.
First- welcome to the forum!
Second- be very careful! No power transformer means power is taken directly from the mains and therefore it is possible for the chassis to become 'live' which is a bad thing.
Please take the time to read the safety thread on this forum. Lots of good info there. Tubes generally use high voltage (this amp probably uses between 110 and 200V which is actually low in the world of tubes).
I'm not familiar with this amp but I can try to answer a few of your questions.
1- Replace the electrolytic. It is a common value and rating. By the way you can replace it with a higher voltage rated cap. If you can't find a 50uF/150V cap you can use a 50uF/250V in its place. Stick with the original uF rating (within reason, a 47uF cap would work just as well).
2- Check all the resistors as the older carbon comp resistors can 'drift' in value. It wouldn't hurt to replace them as well. Use quality metal film resistors.
3- The 25L6GT is a beam tetrode tube. It has a 25V heater (thus the '25' in its name). Not an extremely common arrangement these days in either hifi or guitar amps. But tubes are still available as you found out.
The VT201C is a replacement. Changing the tube to something else would require essentially rebuilding the amp.
In class A (which is the case with this amp) it is good for about 3 1/2 watts peak with around 200V on the plate and about 2 watts with 110 volts on the plate. That could make a very good practice amp with the right speaker. Check out Antique Electronic Supply or Angela Instruments for suitable speakers.
4- I am not an 'expert' and am not familiar with this amp. In particular I wonder about the input stage, where is it? Is it sensitive enough for guitar? Others above my salary grade can probably answer that.
Be careful, have fun and look at this as a learning experience. Don't be discouraged if things don't work the first time.
/sreten.An isolation transformer would be a VERY wise idea..(Manditory,even.)
Especially if you're going to be using it as a guitar amp.
Don't even THINK about plugging your guitar into it untill the amp is isolated from the line.
After that is added,a 3 prong cord,and a small fuse (kinda depends on the size of the isolation transformer,but 2-3A should be plenty) would be a good idea.
Especially if you're going to be using it as a guitar amp.Don't even THINK about plugging your guitar into it untill the amp is isolated from the line.
After that is added,a 3 prong cord,and a small fuse (kinda depends on the size of the isolation transformer,but 2-3A should be plenty) would be a good idea.
StiffHand said:
You can add a three prong plug and a fuse but that doesn't solve the problem. An isolation transformer separates the amp from the mains and is far cheaper than a funeral.
When you plug your guitar into an amp with no power transformer then you are electrically connected to the amp. If the chassis of the amp is live due to a failure of a component or even just plugging the plug in the wrong way round and you then touch anything that is grounded you get fried.
Jorge,
I hope you paid a very few bucks for that POS. As has already been stated, that thing is DANGEROUS. Please notice the POOR quality phenolic wafer tube socket. That thing under the O/P trafo that looks like stacked metal squares is a Selenium rectifier and it's a toxic time bomb. Also, you didn't acquire the preamp electronics needed to raise a LOW voltage signal enough to drive the 25L6.
IMO, you should save all 25L6s you have along with the O/P trafo. Sadly, all the rest is trash.
Look here at another thread on this "board", where getting respectable performance out of the very similar 50L6 is discussed.
The 25L6 draws 300 mA. of heater current. That can be safely obtained from a Radio Shack Catalog # 273-1366 transformer.
The preamp circuitry you use will vary in complexity depending on your choice of options. If you forego a tone stack, a 12DW7 should be enough to drive the 25L6.
edit: fixed typos
I hope you paid a very few bucks for that POS. As has already been stated, that thing is DANGEROUS. Please notice the POOR quality phenolic wafer tube socket. That thing under the O/P trafo that looks like stacked metal squares is a Selenium rectifier and it's a toxic time bomb. Also, you didn't acquire the preamp electronics needed to raise a LOW voltage signal enough to drive the 25L6.
IMO, you should save all 25L6s you have along with the O/P trafo. Sadly, all the rest is trash.
Look here at another thread on this "board", where getting respectable performance out of the very similar 50L6 is discussed.
The 25L6 draws 300 mA. of heater current. That can be safely obtained from a Radio Shack Catalog # 273-1366 transformer.
The preamp circuitry you use will vary in complexity depending on your choice of options. If you forego a tone stack, a 12DW7 should be enough to drive the 25L6.
edit: fixed typos
Thanks guys, I will probably just leave it alone then. I do have a healthy respect for electricity. Don't want to fry myself or any of my guitars.
I like tinkering with things but I want to keep my soul inside my body for a long time. I will focus on the guitar I am modifying (a lot less dangerous hobby).
I may just buy a small vintage guitar amp in working order and have it looked at by a pro here in Nashville before I plug up.
Thanks again.
- Jorge
I like tinkering with things but I want to keep my soul inside my body for a long time. I will focus on the guitar I am modifying (a lot less dangerous hobby).
I may just buy a small vintage guitar amp in working order and have it looked at by a pro here in Nashville before I plug up.
Thanks again.
- Jorge
Sorry, but I think I agree on letting this one go because of savety reasons, espacially if you plan to use it for a guitar amp...
But don't stop, go look for a small old table radio.. maybe sometimes they use a Single ended EL84 or alike for output stage.. these kind of radio's make escelent guitar amps..
But don't stop, go look for a small old table radio.. maybe sometimes they use a Single ended EL84 or alike for output stage.. these kind of radio's make escelent guitar amps..
StiffHand said:
Jorge,
If you would like to have a decent tube practice amp for your guitars check local antique stores and flea markets for old console record players.
I found one for $25, pulled the amp and speakers from the lousy cabinet and built a new cabinet to install them in. The particle board I used for the cabinet was actually wood grained printed shelving from Menard's that cost me another $8.
All I had to do was mount a quarter-inch input jack, and change the two wire plug to a three prong plug with the earth (green) wire attached to the metal cabinet. It works great.
It seems the input level of an old record player (probably ceramic cartridge?) is close enough to that of a guitar that no additional preamp was needed.
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