|
Audio Asylum Thread Printer Get a view of an entire thread on one page |
For Sale Ads |
69.141.96.215
In Reply to: RE: A million questions. Before 'n Before posted by 1973shovel on September 17, 2015 at 16:25:49
I've uploaded a schematic for that sort of phono preamp. Yes, the B+ is half wave rectified. It's pretty easy to switch to 4X UF4007s, for full wave bridge B+ rectification.
Those GE style setups rely on the inductance of the cartridge's coils for part of the RIAA EQ. IT will tell you how to fix things up, including the 47 Kohm load modern carts. require.
FWIW, I suggest that, instead of grounding the CT of the filament winding, you bias it off B+ to 60 or so V. Another hum/noise control tweak is using Sovtek 12AX7LPS tubes, which are genuine 7025 equivalent that contain a spiral wound/hum bucking heater.
Eli D.
Follow Ups:
I only did a quick glance yesterday when I opened it up to take the picture. My first thought was wondering if I could fit four UF4007s in there.
I'm going to have to do some homework on your suggested bias scheme, and how to impliment something like that. As for the Sovteks, I'm currently using them in my Vorhis, a modded PAS, courtesy of Jim McShane, and I'm very happy with them.
This is one of those "someday" projects I'll get to after I retire, but I always like to acquire as much information as I can, when I can, and keep it squirreled away.
Thanks again for the schematic and the feedback.
"You won't come back from Fletcher-Munson curve"-Jan and Dean
The output coupling cap is .1 uf; not the .01 on the scheme.
In some versions of these units, the Center Tap of the filament is sometimes connected to ground internally. An internal connection, inside the tranny, makes it tough to change the filament supply topology. Worse, I have seen this on some chrome power tranny versions as well as non-chrome power tranny versions that were available. So, check your units before purchase or rebuilding.
While the Sovtek 12AX7LPS might have a "genuine, 7025 type spiral wound filament," so do many, many vintage 12AX7s. Unfortunately, to prove this, you would have to break the older tubes apart since the spiral winding technique might not be noticeable looking through the glass. Most tubes "look" like they have a V shaped filament because we usually only see the V shaped white filament leads under the tube plates' structure.
Also, the Sovteks are wider than vintage 12AX7 tubes. Thus, Sovtek 12AX7 glass actually contacts the tube shields on the chrome cuties. This last fact is sad. The manufacturer definitely seems to have had an ulterior motive for the purchaser to require new sockets and shields. Remember planned obsolescence ? This is an example of "required changes to accommodate the replacement." That said, I like the sound of the LPS (Long Plate Spiral) as well.
Post a Followup:
FAQ |
Post a Message! |
Forgot Password? |
|
||||||||||||||
|
This post is made possible by the generous support of people like you and our sponsors: