Single tube guitar amp, adding a transformer to a formerly transformer-less design - diyAudio
Go Back   Home > Forums > Live Sound > Instruments and Amps

Instruments and Amps Everything that makes music, Especially including instrument amps.

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 1st March 2013, 10:31 PM   #1
diyAudio Member
 
Join Date: Mar 2013
Location: United States
Default Single tube guitar amp, adding a transformer to a formerly transformer-less design

Building a small practice amp from a reclaimed phono circuit.

Formerly used a dreaded selenium for 24v, I'm planning on using a transformer laying around for the heater but wanted to know if i need to consider any other factors. Added a filter cap and a fuse to the ac in. Formerly a Motorola Suitcase Phonograph, now housed in a dead ss marshall practice amp chassis (already had a 24v trans! ).

Considering a pre-amp to get some more volume and gain, i know it's not gonna be loud, just looking for unique tone and a fun amp using a single cheap valve.

I can post the schematic (original and proposed amp both) if needed.
  Reply With Quote
Old 1st March 2013, 10:32 PM   #2
AJT is offline AJT  Philippines
diyAudio Moderator
 
AJT's Avatar
 
Join Date: May 2003
Location: Palatiw, Pasig City
be sure to use isolation transformer, circuits that work directly from the wall are not allowed here....
__________________
the best advertisement for a good audio design is the number of diy'ers wanting to build it after all the years....never the say so of so called gurus....
  Reply With Quote
Old 1st March 2013, 10:37 PM   #3
diyAudio Member
 
Join Date: Mar 2013
Location: United States
Quote:
Originally Posted by Tony View Post
be sure to use isolation transformer, circuits that work directly from the wall are not allowed here....
As I will be holding the hot (guitar) end of this, I plan on doing so. What is the proper way to isolate such a circuit, just throw an iso-former on the mains? I'm using a grounded chassis with a 3-prong fyi.
  Reply With Quote
Old 1st March 2013, 10:40 PM   #4
AJT is offline AJT  Philippines
diyAudio Moderator
 
AJT's Avatar
 
Join Date: May 2003
Location: Palatiw, Pasig City
just make sure your circuit uses a power transformer to completely isolate the mains from the rest of the circuit...
__________________
the best advertisement for a good audio design is the number of diy'ers wanting to build it after all the years....never the say so of so called gurus....
  Reply With Quote
Old 2nd March 2013, 03:25 AM   #5
diyAudio Member
 
Tom Bavis's Avatar
 
Join Date: Jun 2002
Location: Macedon NY
Mouser and Allied (among others) sell a 50VA isolation transformer for about $15. Once you have a transformer you can ground the chassis for safety.
  Reply With Quote
Old 2nd March 2013, 09:26 AM   #6
DF96 is online now DF96  England
diyAudio Member
 
Join Date: May 2007
Instruments and Amps?
  Reply With Quote
Old 2nd March 2013, 04:54 PM   #7
diyAudio Member
 
Join Date: Mar 2013
Location: United States
Chassis is grounded. Looks like the reason they went with this tube is it used 110 for the plate, and a 25v heater. Looks like i'll need a 2nd transformer too as in addition to the selenium hack, they also used the phono motor as a step-down.

Am I correct in my assumption I can use 2 transfromers of the same value back to back (110vac->12vdc->110dc) to get isolated line voltage for the plate?

Yes, chassis is grounded already, just sorting out what needs to fit an where. Going to try to keep all the xformers and the 25L6GT inside the cage as it's not too hot and I'm adding a ventilation grate and some internal shielding. The chassis is an old 6" driver combo amp.
  Reply With Quote
Old 2nd March 2013, 06:28 PM   #8
diyAudio Member
 
Join Date: Mar 2013
Location: United States
Also, I pondered throwing this in the Insturments/Amps section, but it's really more relevant to the particular issues of tubes and whatnot. Consider it a 200ma line level amp if you must.
  Reply With Quote
Old 2nd March 2013, 07:04 PM   #9
kevinkr is offline kevinkr  United States
diyAudio Moderator
 
kevinkr's Avatar
 
Join Date: Sep 2004
Location: Boston, Massachusetts
Blog Entries: 6
All musical instrument and amp threads belong in Instruments & Amps so that is where it goes forthwith.. This is a one tube guitar amplifier, and almost all threads there are about tubed guitar amps. Following forum guidelines is greatly appreciated..
__________________
"Extraordinary claims require extraordinary evidence." - Carl Sagan
  Reply With Quote
Old 2nd March 2013, 08:34 PM   #10
diyAudio Member
 
Join Date: Apr 2004
Location: Monroe Township, NJ
MT,

Instrument amps are not my thing, but this thread has stuff of interest to me.

Start with the nnL6 data sheet and facts emerge. 1st, you will be quite OK if you bridge rectify "120" VAC to obtain the B+. 2nd, the power pentode needs several volts of drive signal.

As a 50 VA N-68X from Mouser costs $11.20, why bother with back to back filament trafos? Full wave bridge rectify the O/P of a N-68X, filter, and you've got your B+ rail. Either 4X UF4007s or 2X UF4007s and a 117Z6 (for sag) make up the bridge.

Several stages of voltage gain are needed between the guitar pickup and the power pentode. The phonograph "donor" used a crystal cartridge whose O/P was a few volts.
__________________
Eli D.

Last edited by Eli Duttman; 2nd March 2013 at 08:36 PM. Reason: typo
  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
guitar amp transformer upgrades cyclecamper Instruments and Amps 63 16th November 2012 08:12 PM
Adding heater transformer to my 6b4g amp. help a noob please. murphythecat8 Tubes / Valves 10 22nd October 2012 02:30 PM
use symmetrical transformer for single supply amp? tsiros Power Supplies 32 8th September 2012 07:34 AM
question about output transformer for guitar amp hpupo Tubes / Valves 6 15th April 2011 01:53 PM
isolation transformer won't supply filiments - 1968 Harmony H400A tube guitar amp tonebrulee Tubes / Valves 7 9th April 2008 05:33 PM


New To Site? Need Help?

All times are GMT. The time now is 04:17 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