Magnavox 6V6 amp upgrade - 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 24th April 2013, 09:51 PM   #1
diyAudio Member
 
Join Date: Mar 2013
Location: Central Texas
Default Magnavox 6V6 amp upgrade

Howdy again, folks:
I've had this Magnavox PP stereo 6V6 amp for a while. It's actually working right now but all I did was to clean and paint the chassis, rewire the power and provide the missing cathode path because of their use of a remote Bal pot (these amps were used in consoles).
I wanted to upgrade this guy to an extent. The ground rules: no iron changes, same 6V6 finals, no extensive chassis mechanical changes.
I also wanted to use some of a batch of 6DT8 dual triodes on hand (a kissing cousin to the 12AT7) and wanted to use an LTP for the PI.
The paper design is complete and attached.
The original version uses 2 ea 6EU7 tubes but there is a hole for a third 9pin. Based on how high up I have to turn my preamp to get significant output, I wanted a bit more gain. So the empty hole is being pressed into service and a third 6DT8 used to boost the overall gain by a modest factor of 9, easily changed one way or the other via the cathode gain setting (but not bias setting) resistor currently at 3.3K.
I don't want to misrepresent anything, this is finished on paper only, I have not put iron to solder yet.
Currently, I do have a scope and signal generators but no means of directly measuring THD nor IMD so I'll have to be happy with simple listening tests. The current (stock) unit has been in constant operation for about 6 months now so its sound is well known.
Someday I'll be able to get a 16b Pico scope and do full blown FFT and other measurements
But, for now, the listening test will have to do.
Some parts will be kept but not many. The power supply remains stock, as I said all the iron is reused and the 6V6 tubes.
While the current amp sounds surprisingly good, some of the cost saving techniques used really grate on me. Especially, the use of a common cathode resistor for all FOUR finals! Talk about inviting runaway if any of them get a little hot or cold, either way.
My implementation uses a single resistor per channel. That actually could be improved by using individual cathode resistors and individual bypass caps but, I'll start with the common pairs.
OK, that's it for now, I hope to start the actual conversion soon, will let y'all know.
Thanks!
Rene
Attached Images
File Type: png Magna 6V6-PP-LTP.PNG (89.2 KB, 136 views)
  Reply With Quote
Old 25th April 2013, 01:32 AM   #2
diyAudio Member
 
Join Date: Apr 2004
Location: Monroe Township, NJ
You're close to "classic" Mullard 5-20 topology. Why not go all the way?

A CCS in the LTP's tail is excellent, but a 10M45S is simpler than the BJT stuff shown.

You don't need "gobs" of gain from the I/P voltage amplifier. Pick a nice, linear, medium μ type. The 6FQ7 and 12BH7 are (IMO) prime candidates, for the job.
Attached Images
File Type: jpg Mullard 5-20 Schematic.jpg (380.3 KB, 121 views)
__________________
Eli D.
  Reply With Quote
Old 25th April 2013, 02:44 PM   #3
diyAudio Member
 
Join Date: Mar 2013
Location: Central Texas
Much appreciated, Eli. One of my goals is to not get any new iron for this guy. I have a different, from the ground up, design which will become my eventual daily use amplifier. This one is a temporary one and it's as much an exercise in just mildly improving the "Maggie" as it is in learning a few things along the way. One of them is that if biased correctly and used kind of conservatively, the 6DT8 could do just fine. I'll find out. Certainly it is a good tube for a lower impedance driver than, say, 12AX7. Again, we'll see. Of course you are right about the CCS but I have the parts on hand, I am very comfortable designing my own current sinks and I believe this one will give me enough performance under the circumstances of original iron, etc.
The next design will be a blank sheet start, actually it's no longer a blank sheet because I have already done a lot of preliminary work, but not based on anything I already have on hand.
There is one limitation to the 10M45S and that is its BW. Its dynamic impedance does drop significantly at the higher frequencies so that needs to be considered. My simple, lower performance circuit, and others which are more sophisticated using cascaded JFETs and/or BJTs, are potentially flatter to a higher frequency.
Thanks for the circuit, though. It just might come in handy for my next, "dream" amplifier!
Rene
  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
Magnavox amp M Gregg Tubes / Valves 8 2nd April 2012 07:15 PM
converting a magnavox pp 6v6 in triode mode tubo Tubes / Valves 10 18th May 2011 02:07 PM
magnavox 185aa with 4-6v6 tim614 Tubes / Valves 3 31st July 2008 12:07 AM
Magnavox 175 Stereo Tube Amp (PP 6V6) GordonW Swap Meet 2 13th September 2007 12:21 AM
Magnavox CDB480 upgrade mmerig Digital Source 0 24th April 2005 01:28 PM


New To Site? Need Help?

All times are GMT. The time now is 12:22 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