Valves stripped bare…. - 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 6th January 2005, 06:26 PM   #1
diyAudio Member
 
scott wurcer's Avatar
 
Join Date: Jan 2004
Location: cambridge ma
Default Valves stripped bare….

I had an ST70 in need of new caps lying around my workshop so I decided to do a holiday vacation quickie on it. Thanks to planet10’s link to EDCORP I picked up a couple of their 10k/150 interstages for about $15 and some 500V/100uF caps off of
eBay for $3, the rest was from the junk box. I stripped it down to almost nothing but metal and glass, trioded the outputs with 100 Ohms, and mounted the new trannies on a nice piece of mahogany where the circuit card was. My line stage is a Jung AD823/AD815 composite amp so it has no problem with the 150 Ohm input, I just moved my attenuator to before the line stage and lowered the resistance in the bias network to 10k.

I checked all the voltages to prevent the magic smoke and fired it up. First impressions were very favorable. A quick frequency response (w/speaker load) showed only a little bumpiness due to imperfect damping and at a comfortable level (my office is small) the 1kHz distortion was .025% thirds and .02% seconds. The most surprising thing was how quiet it became, hum too (even though I worried about the unshielded trannies you need to put your ear up to the speaker).

Now for the questions; I got 427V plate supply and ran the outputs at the original 100mA. Not being a valve guy, is there a better operating point? Most of the stuff I found in the archive was for SE amps. Any other comments?
  Reply With Quote
Old 6th January 2005, 06:35 PM   #2
diyAudio Member
 
Tube_Dude's Avatar
 
Join Date: Mar 2002
Location: Aveiro-Portugal
Default Re: Valves stripped bare….

Quote:
Originally posted by scott wurcer


Now for the questions; I got 427V plate supply and ran the outputs at the original 100mA. Not being a valve guy, is there a better operating point? Most of the stuff I found in the archive was for SE amps. Any other comments?
Hi Scott

As the EL 34 have a maximal power dissipation of 25 W . If the 100 mA are the current of two of them , than we have 50 mA in each.

With 427 rails ,that make ~ 21 W. A still conservative way to use them.

Regards
__________________
Jorge
  Reply With Quote
Old 6th January 2005, 06:59 PM   #3
SY is offline SY  United States
diyAudio Moderator
 
SY's Avatar
 
Join Date: Oct 2002
Location: Chicagoland
Blog Entries: 1
You can tune the idle current to minimize 3rd, but you're already at a pretty low level. Nice little amp, eh? Now it's time to regulate that B+...
__________________
You might be screaming "No, no, no" and all they hear is "Who wants cake?" Let me tell you something: They all do. They all want cake.- Wilford Brimley
  Reply With Quote
Old 6th January 2005, 11:56 PM   #4
diyAudio Member
 
Join Date: Sep 2002
Location: Memphis, TN
Hi Scott,

How did you use the Edcors?

Regards,
Danny
  Reply With Quote
Old 7th January 2005, 02:27 PM   #5
diyAudio Member
 
scott wurcer's Avatar
 
Join Date: Jan 2004
Location: cambridge ma
Quote:
Originally posted by Ciscokid
Hi Scott,

How did you use the Edcors?

Regards,
Danny
I guess I left out my original purpose. I wanted to try the ST70 as a no feedback PP triode amp. I bought what were actually preamp output transformers (10k/150) and used them backwards as step-ups directly coupled to the grid bias resistors, the added benefit of no capacitors in the signal path. My SS preamp has no trouble with the 150 Ohm input impedance that the amp now has. So what I have is an 8.16 to 1 step-up directly coupled to the EL34's and the output transformer and nothing else in the signal path. The sensitivity is a lot lower than normal but that particular line stage puts out 20V p-p into even 50 Ohms easily.

BTW the transformers were obviously made to order, they were wrapped in plastic before the Q-dope had cured. Now I know why its called dope, I almost passed out when I opened the box. I tried using a 20k load hoping to get a slightly higher input impedance but the highs started to roll off so I stuck with 10k. I just split it into 2 - 5k sections and fed the -32V bias into the center tap.
  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
Lite DAC 38 PCM1704 bare PCB's Alfetta87 Swap Meet 2 19th April 2009 05:48 PM
Insulated vs. bare metal stators I_Forgot Planars & Exotics 26 10th March 2007 11:47 PM
Bare SDI card? Htguy Digital Source 0 11th June 2005 01:22 AM
ANOTHER Torroid question......but bare with me.... JCoffey Chip Amps 18 1st November 2003 03:45 PM


New To Site? Need Help?

All times are GMT. The time now is 03:47 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