Mini guitar amp from a Webcor tape amp, is it possible? - diyAudio
Go Back   Home > Forums > Amplifiers > Tubes / Valves

Tubes / Valves All about our sweet vacuum tubes :) Threads about Musical Instrument Amps of all kinds should be in the Instruments & Amps forum

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 27th July 2006, 06:45 AM   #1
diyAudio Member
 
Join Date: Jul 2006
Default Mini guitar amp from a Webcor tape amp, is it possible?

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.
Attached Images
File Type: jpg guitars 015.jpg (59.3 KB, 207 views)
  Reply With Quote
Old 27th July 2006, 12:45 PM   #2
Sherman is offline Sherman  United States
diyAudio Member
 
Join Date: Feb 2004
Location: Chicago area
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.
__________________
--Sherman
  Reply With Quote
Old 27th July 2006, 03:35 PM   #3
sreten is offline sreten  United Kingdom
diyAudio Member
 
Join Date: Nov 2003
Location: Brighton UK
Hi,

I'll also say you should not spend anytime electrically connected
to any equipment that does not have an isolation transformer.
Plugging your guitar into it I regard as being very connected.

Very dangerous ....................

/sreten.
  Reply With Quote
Old 27th July 2006, 06:25 PM   #4
diyAudio Member
 
Join Date: Jul 2006
Thank you for your replies.

I am being careful in handling this thing. My Grandpa advised me as to what kind of voltage all things tube can store.

Is it possible to add a 3-prong plug for safety? does it need a fuse in it? if so, what rating would you recommed?

Thanks.

- Jorge
  Reply With Quote
Old 27th July 2006, 09:46 PM   #5
diyAudio Member
 
Join Date: Apr 2003
Location: Portland,Oregon
Blog Entries: 4
Send a message via AIM to DigitalJunkie
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.
  Reply With Quote
Old 27th July 2006, 10:05 PM   #6
Sherman is offline Sherman  United States
diyAudio Member
 
Join Date: Feb 2004
Location: Chicago area
Quote:
Originally posted by StiffHand
Thank you for your replies.

I am being careful in handling this thing. My Grandpa advised me as to what kind of voltage all things tube can store.

Is it possible to add a 3-prong plug for safety? does it need a fuse in it? if so, what rating would you recommed?

Thanks.

- Jorge

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.
__________________
--Sherman
  Reply With Quote
Old 27th July 2006, 11:08 PM   #7
diyAudio Member
 
Join Date: Apr 2004
Location: Monroe Township, NJ
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
__________________
Eli D.
  Reply With Quote
Old 28th July 2006, 01:43 AM   #8
diyAudio Member
 
Join Date: Jul 2006
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
  Reply With Quote
Old 28th July 2006, 07:45 AM   #9
diyAudio Member
 
Join Date: Jul 2001
Location: The Netherlands
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..
  Reply With Quote
Old 28th July 2006, 01:31 PM   #10
Sherman is offline Sherman  United States
diyAudio Member
 
Join Date: Feb 2004
Location: Chicago area
Quote:
Originally posted by StiffHand
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.

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.
__________________
--Sherman
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
4x12 Guitar cab using guitar and bass speakers help andrewskaterrr Instruments and Amps 15 23rd April 2009 11:43 PM
the Very Mini Magico Mini zei Multi-Way 7 11th February 2008 08:10 PM
sparking Output transformer In Webcor 14x240 Amp inuyasha44320 Tubes / Valves 2 6th November 2006 04:33 PM
Webcor Amplifier Speaker Charlie C Tubes / Valves 2 11th June 2005 04:53 PM


New To Site? Need Help?

All times are GMT. The time now is 09:42 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2