magnavox 8301 revamp, need's a second look - 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 23rd December 2012, 06:08 AM   #1
trigger is offline trigger  United States
diyAudio Member
 
trigger's Avatar
 
Join Date: Dec 2009
Location: michigan
Default magnavox 8301 revamp, need's a second look

I aquired a 8301,

Click the image to open in full size.
(not this one, but similar, pic just for reference)

I removed all the EQ and got it ready to play. I checked the voltages and was curious if i could get some second eyes on the circuitry on the inside before installing the eye candy to the outside.

here is the 8301 with recently removed EQ

Click the image to open in full size.

I did a check on the voltages-and don't know what they mean- so, here is a schem that I pulled off this sight, i wrote the test run results in red. i am not certain that all of this shem is exact to the amp that I am working on; rather a reference.
In red are the voltages from amp while playing music at a low level.
Click the image to open in full size.

In short, dose every thing look ok?

I plan to split the cathode resistors, and add a vol, new fittings, plug, fuse, and power light.

did i miss any thing? i have new Russian factory tubes, any precautions before installing them?

My thanks go out to those who posted such good posts about this amp and others like it!

the amp sounded quite good during the test run, the tubes installed were untested and looked mis-matched. after swapping the 6bq5 from right to left one was tube was louder. i practice safe work- i do not really know what iam doing here besides doing it safely-any help is much appreciated.

-i made a duplicate thread on audiokarma, well for some reason.. i thought i was posting here at diyaudio, and was not, sorry to be a bother.-
  Reply With Quote
Old 23rd December 2012, 07:10 AM   #2
diyAudio Member
 
azazello's Avatar
 
Join Date: Aug 2008
Location: Bulgaria
I think, R6 and R13 must be about 100-120 kohm to increase plate voltage to 135 v. Every EL84 must have separate cathode resistor about 400 ohm and separate cap about 1000 uF.
  Reply With Quote
Old 23rd December 2012, 08:32 AM   #3
diyAudio Member
 
mr2racer's Avatar
 
Join Date: Jul 2009
I'm kinda of an amateur but I have built two EL84 SE amps recently so I would like to make suggestions. What Russian tubes do you have? I think the 6P14P are the best to use. I would replace all the power supply caps. In fact all the caps. I'm not a fan of carbon composition resistors. They stray off value in time and old ones often have absorbed moisture. If you feel up to it look at the RH84 circuit, do a web search. Its simple to build and could be done in that chassis quite easily.

RH 84 - Tube Audio ...... RH DESIGN That old wire isn't the greatest either.

Most EL84 cathode resistors I've seen are in the 250-270ohm range. You have to watch out for modern EL84's as they aren't as robust as the old ones. Look for 6P14P or EL84M. Matched pairs are on Ebay for cheap. Avoid 6P15P It can be a pain.

I would suggest new tube sockets as well. You can tinker with it or gut it and start over. If you have high efficiency speakers this amp can be made to sound really good.

I've been helped loads by the guys here. So I hope this helps!
  Reply With Quote
Old 23rd December 2012, 09:22 AM   #4
diyAudio Member
 
artosalo's Avatar
 
Join Date: Feb 2010
The anode voltage of 6EU7 is far too low. Remove R7 and R14.
Then the voltage level should rise to normal readings.

Edit: After using my brains a bit, I noticed that it is fully opposite.
Check that these resistors are there. Otherwise the anode voltage may be as you have measured.

Last edited by artosalo; 23rd December 2012 at 09:35 AM.
  Reply With Quote
Old 23rd December 2012, 02:23 PM   #5
yoaudio is offline yoaudio  United States
diyAudio Member
 
yoaudio's Avatar
 
Join Date: Oct 2010
Location: Northern Michigan
Audiokarma has a good thread on this amp.

Magnavox Updated 86-01 - AudioKarma.org Home Audio Stereo Discussion Forums

Here's the schematic I used and it sounds good.

Click the image to open in full size.

The finished product, a nice amp for the office.

Click the image to open in full size.
  Reply With Quote
Old 23rd December 2012, 03:38 PM   #6
trigger is offline trigger  United States
diyAudio Member
 
trigger's Avatar
 
Join Date: Dec 2009
Location: michigan
I will implement the input resistors and 100k pot as shown in the updated 8601, thank you for sharing. so, besides the; split cathode resistors, R8andR15 being changed to 20k, and the vol. I have an updated 8601?

i am a bit confused about increasing the plate voltage to 135v, what should i be doing here, removing or deceasing or nothing..? should i try the new 6eu7 and check?

thank you all much for the help, i cant wait yo listen to the amp. ive got a few speaker options; original vintage magnavox, vintage jensen p10r's, sansui200, other vintage salvage speakers, and a pair of 4'' full range fostex knock-off coming from china now, also i the new russan tubes i have to use are N709 knock-offs.
  Reply With Quote
Old 23rd December 2012, 03:51 PM   #7
yoaudio is offline yoaudio  United States
diyAudio Member
 
yoaudio's Avatar
 
Join Date: Oct 2010
Location: Northern Michigan
Quote:
Originally Posted by trigger View Post
I will implement the input resistors and 100k pot as shown in the updated 8601, thank you for sharing. so, besides the; split cathode resistors, R8andR15 being changed to 20k.

R8andR15 between 10k and 20k. I had a 20k but settled on 12k.
If you are ever in Cadillac you can hear this amp.

Last edited by yoaudio; 23rd December 2012 at 03:54 PM.
  Reply With Quote
Old 23rd December 2012, 04:27 PM   #8
diyAudio Member
 
artosalo's Avatar
 
Join Date: Feb 2010
Quote:
Originally Posted by trigger View Post

i am a bit confused about increasing the plate voltage to 135v, what should i be doing here, removing or deceasing or nothing..? should i try the new 6eu7 and check?

Check first that you really have R7 and R14 there.
These are a sort of fixed bias resistors and form together with cathode resistors R10 and R17 the necessary bias voltage (some 1.4 V) to the cathode of 6EU7 .
You must get the anode voltage above 100 V, otherwise 6EU7 is distorting a lot.

If you want to avoid the use of R7 and R14, then the whole bias arrangement of 6EU7 must be redesigned.
Therefore the easiest way is to check that your actual circuit is according to the circuit diagram.
  Reply With Quote
Old 23rd December 2012, 05:19 PM   #9
trigger is offline trigger  United States
diyAudio Member
 
trigger's Avatar
 
Join Date: Dec 2009
Location: michigan
R7 and R14 are 150k original, I tested them while installed and they measured 135kish i will replace them with newer 180k, perhaps this is the issue.
the help is much appreciated, its been a fun project so far.

i plan to replace all outdated components in February along with refinishing chassis and encasing in wood vinear , really i am trying get it running until then, getting every thing sorted out.
  Reply With Quote
Old 23rd December 2012, 06:53 PM   #10
diyAudio Member
 
indianajo's Avatar
 
Join Date: Jan 2010
Location: Jeffersonville, Indiana USA
I would measure the resistors before shotgun replacing them. Most of the ones in my 1964-67 Hammond organs, and the 1961 dynakit equipment, were fine, even though the dynakit amps lived in humid Houston with no air conditioning for some years. One organ was two blocks from the Ohio river at Madison, IN. Europeans have a lot more trouble with low resistance carbon comp resistors IMHO, than we do here. Some resistors over 1 meghom go low resistance in Hammond B/C/D/A100's but I haven't found any in my equipment that didn't look overheated. Maybe the US Navy kept the Allen Bradley and Sprague resistor plants putting out their best effort.
__________________
Dynakit ST70, ST120, PAS2,Hammond H182(2 ea),H112,A100,10-82TC,Peavey CS800S,1.3K, SP2-XT's, T-300 HF Proj's, Steinway console, Herald RA88a mixer, Wurlitzer 4500, 4300

Last edited by indianajo; 23rd December 2012 at 06:57 PM.
  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
Yamaha A-1 Revamp bluesmoke Solid State 17 10th November 2012 06:02 PM
Kinshaw Perception Pre Amp needs revamp ! RCruz Solid State 16 22nd January 2010 10:00 AM
Half Chang to fit my need's ? lykkedk Full Range 32 14th January 2008 04:24 PM
fet amp design revamp unclejed613 Solid State 2 14th January 2007 08:26 AM
A Krell Revamp CheerfulYC Solid State 7 18th February 2004 01:39 AM


New To Site? Need Help?

All times are GMT. The time now is 04:38 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