• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

magnavox 8301 revamp, need's a second look

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I aquired a 8301,

An externally hosted image should be here but it was not working when we last tested it.

(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

8299523392_4101ca0a7d_c.jpg


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.
An externally hosted image should be here but it was not working when we last tested it.


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.-
 
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!
 
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.
 
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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.
 
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.
 
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.
 
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.
 
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hello all,

i ve got most everything straightened out, one question with the plate voltage of 6eu7 anode;
i have the voltage to 125v, now when a signal is playing it drops to 50v, is this normal?

i have split up the cathode resistors, and installed new PIO coupling caps, every thing almost to spec besides the power supply, the amp sounds very nice.
 
yes when i plug a rca cable into the amp the amp drops from 125v, at 6eu7 anode, to 50v.

with no signal(music) or with signal..

i really appreciated the help, where do i start, possibly a bad connection? the amp has no fussy or static sounds when i nudge or tap the components, and sounds clear.
 
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According to the schematic you showed, there is no DC-blocking capacitor at the input.
It is obvious that your music source is feeding DC-voltage to the amplifier, i.e. to the grid of the 6EU7 and the tube begins to conduct due this extra "bias".
Try to add capacitors just before volume pots. Suitable size is above 100 nF.
 
Yes. .047 or .1 is better. Make sure your .1 are rated 50 v or above. Low grade (not NPO or CPO) ceramic caps distort a little, but the higher the voltage the better. You can replace them when you develop a design and put an order together worth paying $8 freight for.
 
hello, i am now working on "beefing up"the power supply. at this time i am not removing the old electrolyte cap. I want to just add a cap or two to beef things up, for more available voltage.

i have new; 120uf 400v and a 180uf 400v capacitors--would they be useful--can they be implemented?-- i saw a few others that added a cap from ground to old cap. it appeared it was 50uf 400v..... i am curious if the extra parts i have could be useful.
 
...I want to just add a cap or two to beef things up, for more available voltage.

i have new; 120uf 400v and a 180uf 400v capacitors--would they be useful--can they be implemented?--

According to the schematic you showed, there already seems to be 285 V for output tubes. It is near the maximum for EL84.
Secondly the addition of capacitance do not significantly increase the voltage.
But if you have hum, then extra capacitance sounds sensible.
I would add the capacitor after 100 ohms resistor.
 
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