New to valve amps - given a 'project' - 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 17th May 2009, 10:19 PM   #1
diyAudio Member
 
Join Date: May 2009
Default New to valve amps - given a 'project'

Hello all. Complete valve noob here.

A friend gave me this amp that he found in a shed because he knew I wanted to build a valve amp for blues harp. It has been dated at around the mid '40s.

It was beyond help, so I (hopefully) methodically pulled it apart as I traced the circuit.

I have created a schematic of the circuit and it can be downloaded here. My intention was to rebuild it to the original schematic as my experience is limited. (I am an electrical teacher, but never fiddled with valves.)

It would be great if one (or some) of you could look it over and let me know if I've got it right, or what needs to be 'tweaked'.

The 2 resistors circled in green had no markings or colour code, but I measured them with a cheap ohmmeter and got around the 5k ohm mark - does that sound in the ballpark?

Thanks in advance for any help or advice.
  Reply With Quote
Old 18th May 2009, 02:55 AM   #2
diyAudio Member
 
Join Date: Apr 2004
Location: Monroe Township, NJ
The "mystery" component is a can style 'lytic and probably multi-section.

The phonograph employs a crystal (piezoelectric) cartridge and its O/P is measured in (sic) Volts.

If a restoration is attempted, EVERY capacitor needs replacement. Carbon composition resistors have to be checked for damage and drift outside of tolerance. An OEM tolerance of 20% would not be in the least bit shocking.

The circled resistors are cathode bias parts. Like JFETs, tubes (valves) are depletion mode devices.

FWIW, the power "iron", rectifiers, and 6V6s are what interest me most. They could go into a "flea" power, stereo, single ended triode (SET), amplifier. Should such a project be of interest, look here. Recycling "vintage" magnetics into new projects is a common enough practice.

BTW, you probably have the 6.3 VAC heater power winding wrong. "Normal" has the ends feeding the heaters and the CT grounded.
__________________
Eli D.
  Reply With Quote
Old 18th May 2009, 03:15 AM   #3
kevinkr is offline kevinkr  United States
diyAudio Moderator
 
kevinkr's Avatar
 
Join Date: Sep 2004
Location: Boston, Massachusetts
Blog Entries: 6
Default Think about possible hazards

The unit has been stored in a very wet environment as evidenced by the rust on the transformer end bells. In addition to Eli's suggestions I would look for evidence of compromised insulation in those transformers and once baked out (60C in an oven for about 12 hours) I would high pot the power transformer at minimum and it would not hurt to check the choke and OPT as well. I would use AC at maybe 1.5KV for the high pot test. Anything you build using this iron MUST have a functional safety ground - remember what this thing is going to be connected to. (A musical instrument) Electrocution in a worst case scenario is not impossible.

Were it me I would trash everything but the valves (possibly consider whether to strip and repaint the chassis) and start fresh for the above mentioned reasons.
__________________
"Extraordinary claims require extraordinary evidence." - Carl Sagan
  Reply With Quote
Old 18th May 2009, 04:01 PM   #4
diyAudio Member
 
Join Date: May 2009
Thanks for your reply guys.

Sorry, I haven't updated that webpage since first publishing it. My intention when directing you to it was to just let you have a look at the pics to get a feel for the beast I'm trying to awaken.I have since discovered that the mysterious component is a vibrator/oscillator for dual voltage use. It was removed before tracing the circuit.

Yes, the 6.3 volt taps seem reversed, but that's how it is. See the bottom right corner of the photo below...

Click the image to open in full size.

Just so you're not too worried about me electrocuting myself, I am a licenced electrician and am currently TEACHING electrical trade students at my local tech college. So if I DO electrocute myself, I've got NO excuses.

My request here is more to ask some experts to take a look at the schematic and tell me if I have traced a workable circuit. I'm certain I've made a few mistakes. My intention is to re-create the same circuit, using new resistors and capacitors. I don't have enough (read 'any') experience with valves/tubes to experiment with different variations.

I have found some circuits that utilise 3 x 6SJ7's in the preamp stage, and some with 2 x 6V6's in the power amp stage, but none tie them both together.

Thanks again for your input so far.

Muz
  Reply With Quote
Old 19th May 2009, 02:48 AM   #5
diyAudio Member
 
Join Date: Apr 2004
Location: Monroe Township, NJ
Muz,

Vibrators were used in automobile radios to chop the available low voltage DC into a square wave, which was stepped up and rectified to get the B+.

Thanks for the close up of the power trafo terminal setup. I see the ground bus. My hunch is that the 4 A./6.3 VAC winding is not center tapped. The red wire connected to the 4 A. lug is covering a marking up. What does that marking say?

A review of your initial post indicates you want to acquire a musical instrument amp. The 6SJ7 is a fine voltage amplifier. 2X 'SJ7s will provide plenty of gain for the low voltage I/P signal and tone stack losses. The 6X5 rectifiers draw 1.2 A. of heater current. 2X 6V6s draw 900 mA. of heater current. 2X 6SJ7s draw 600 mA. of heater current. So far, the total is 2.7 A. out of the rated 4 A. That leaves a very comfortable 1.3 A. for an additional small signal tube. I suggest you use a 6SN7, which draws 600 mA. of heater current, as a long tailed pair (LTP) phase splitter. Frankly, only the LTP and cathodyne topologies find favor, in this day and age.
__________________
Eli D.
  Reply With Quote
Old 19th May 2009, 11:15 PM   #6
kevinkr is offline kevinkr  United States
diyAudio Moderator
 
kevinkr's Avatar
 
Join Date: Sep 2004
Location: Boston, Massachusetts
Blog Entries: 6
I think Eli is onto something with his recommendation to use a 6SN7 based LTP.. I concur..

FWIW, and I am exempting you from this, but some of the worst electronic hack jobs I have had to deal with (and I'm dealing with one right now unfortunately) were perpetrated by licensed electricians who ought to have known better. Sounds like you are cut from a different cloth - just be careful and use a GFI if possible and proper safety ground.
__________________
"Extraordinary claims require extraordinary evidence." - Carl Sagan
  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
Valve amps are rubbish TEXAS TOMM Everything Else 7 6th October 2008 02:54 PM
Let's Talk Valve Amps and stuff fadboy Introductions 2 28th May 2008 08:46 PM
raccomandation with valve amps testarossa2k Tubes / Valves 4 7th May 2006 10:45 AM
heavy duty valve amps.... for PA use....? jc808 Tubes / Valves 29 1st January 2005 11:40 PM
Quad 22 Valve Amps WARREN Tubes / Valves 9 11th September 2004 10:39 AM


New To Site? Need Help?

All times are GMT. The time now is 05:04 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