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Magnavox 8301 Mods

Tuquala

Tuquala

Active Member
I recently picked up a Magnavox 8301-10 amplifier, and plan on modifying per a thread started by cubby01 about the same amp. I plan on driving it with a Yamaha RX-V3000 from the pre-outs and hooked up to Forte 1's.

Some questions I have are pertaining to ideas and comments from cubby01's thread:

I plan on removing the tone control and loudness, and putting in a volume control according to diag 3. Any negatives to having a vol control vs none??

Adding a 470k resistor (or ??) from grid to ground on the 6EU7. Will this be right for a solid state preamp such as in the Yamaha?? or would a different value be recommended??

Since this is a 8301-10, would changing the R9 & R16 resistors from 15k to 47k be beneficial? I am assuming that there would be an increase in gain according to Tom Bevis.

And I would also assume that removing C6 & C9 would do the same for gain??

Lastly, I have seen comments on changing the 6EU7 to a 12AX7 with the associated wiring changes. Would this be worth the time?? I am guessing that that would increase the output power of this little amp? Or would there be more internal or wiring changes? Would the power supply be up to that?

Thanks for any comments and help ahead of time!!


Gerry
 
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C6 and C9 provide some bass boost (actually high frequency attenuation) - you don't want it - just clip them out. Leave the other resistors alone unless you need more gain. The bass control only works with a ceramic phono cartridge - it will have no effect. Treble control is cut only. You can remove the "loudness" boost by disconnecting the PEC wires at the taps on the control. Or leave it if you like... If you remove ALL the controls, add a resistor on each input grid - 100K, 470K, doesn't matter as your preamp is much lower.

The change to 12AX7 is only to use a more common tube. Compare the price for a Mullard 6EU7 vs. 12AX7 and you'll forget about that idea...
 
Tom. sorry about the misspelling of your last name!! Thank you for your help.

By "ALL" controls, do you mean volume also?? And, that resistor would be connected grid to chassis ground on both channels, correct??

Finally, (sorry to sound illiterate), define PEC for me!!!


Gerry
 
Well, if you have a preamp, do you NEED another volume control? And yes, grid to ground resistors are needed if controls are removed - volume control provides the ground path now.

PEC: Centralab's trade name for Packaged Electronic Circuit i.e. square flat brown thing with wires... it's a half dozen resistors and caps in a flat ceramic case.
 
I think I want to put in the vol control, in case I want to use the amp with a line out source such as a cd/dvd player. I originally was using the vol control on my C/M 29C connected direct from a blu-ray player.

Last ? (!!!), on the 470k resistor: from the grids on the 6EU7 to exactly where?? Orig statement was to ground, then to the volume wiper. Isn't the wiper the center tap??? Or would you just go to the ground side of the volume control??
 
I recently went through one of these, I'm posting the "original" schem with all the
bass/treble/balance/cartridge stuff out, an what I did + suggested and volume pot
installed for what your after on the "updated" one. (I have other plans for mine). :naughty:

This is not a "mod" but more just streamlined the original circuit, there is better. ;)
 

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Thanks, Kegger!! Appreciate seeing it on paper, makes sense.

I see a few resistor changes and voltages have changed. Also, no balance control. what are the effects on the power and/or sound??
 
The balance pot was in the feedback loop, not where you'd want one.
The other changes are just to optimize the original amps circuit layout.
(and the "extra voltage" does squeeze a bit more power from it)

For my power supply I used a 3 section can cap to replace the old one
with 3 sections of 47uf each, then the stage after the 100ohm resistor
I paralleled another 100uf cap underneath, better filtering and "reserve".

Power supply caps don't need to be exact as I have them on the schem
just keep that first one between 33 an 47uf, same with the 3rd one, the
second one is your outputs reservoir and where I add in a bit more there.

--------

Those 10ohm resistors added to each output tube cathode is so you can
measure and match each output tubes current. Just measure across each
one and look for roughly .31v on each, which is 31ma (close to that is fine).
 
Kegger, thanks for the extra info, very helpful! Here is my in-between plan (til I chase more components): Remove the balance control, new volume control, add the grid resistors, change R9 & R16 to 47K, remove C6 & C9, and add some capacitance to the power supply side. Any thoughts or negatives to this??

A bit later, after using this as modded for awhile and building a "case" for it, I plan on going into the other resistor changes to bring the voltages up as you have done.

As an aside, I picked up a beautiful Fisher Diplomat console yesterday. Pics later! It's a '66 model, walnut, and only changes are a BSR turntable instead of the Dual 1010 it originally had. Not sure what I am going to do with it, that's a future decision!!!
 
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Only thing I'd say is a "bad" idea there is upping the R9 & R16 to 47K, you'll have to
little feedback, the amp will be "fuzzy" sounding and to sensitive, I've played with a
8601 an now this 8301, an you don't want to decrease the feedback on these amps.

The original 6.8k then 6.8k in series with each other is a pretty decent place to be at.
(could replace them both by using a single 10-25k resistor in there place, an "listen")
 
Thanks again for the insight. I will redo without changing those 2 resistors and see how it sounds with the other mods.
 
Well, it's alive!! Basic modifications done per the help given here. Temporarily hooked up in garage to a couple of 8 ohm 6 X 9 Ampex boxes and a CD changer, and it sounds pretty good!! Lots of volume, should do alright with my Forte's. Put in 2 Ruskie EL84 Mullard reissues, since one of the maggie 6BQ5's was broken. Probably will need some break-in time and a nice wooden case before it comes into the house!!

Cute little devil, and sounds pretty good for what it is!! I'll shoot some pics tomorrow and post.

Thanks again to Tom and Kegger for their input.


Gerry
 
Here are a couple pics of it at this time. Gotta think about what to do for cabinetry. Maybe just a couple of board sides and a piece across the front behind the knobs.

Does have a low hum which is not amplified by the volume control. Something to research in the future. Since the Ampex speakers are pretty low on bass, the hum may show up more prominently in the house with the Forte's.

Tanks agin!!!
 

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Kegger said:
Power supply caps don't need to be exact as I have them on the schem just keep that first one between 33 an 47uf, same with the 3rd one, the second one is your outputs reservoir and where I add in a bit more there.

--------
Click to expand...

I had a question about your recommendations on filter cap upsizing. I understand that the capacity of the first cap is limited by the the rectifier. I also get the benefit of increasing the second cap - as you say for a reservoir for the power pentodes. But why do you recommend limiting the capacitance of the third? Would additional capacitance not benefit the AF stage, for instance by reducing ripple so as to make the AF stage more quiet? Is there a down-side to additional capacitance there?

Thanks

Dave

 
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There isn't really downside to upping the last cap, an I don't recommend "limiting"
it, but just as you say if you up the second as I suggested then the third is more
of being redundant, the supply is pretty much clean enough by the time it gets to
the third stage that it just doesn't "need" a lot there.

Won't hurt anything, and if you've got really sensitive speakers you might want to
increase it a bit, I'd probably just go a bit bigger still in that second stage though.
 
OK, that makes a lot of sense. In some of these smaller chassis, it can be difficult to find a place to stuff additional electrolytics. One might as well put the capacitance where it will do the most good. It works for both stages in the second position.
 
You got it exactly, a lot of people fail to realize the supplies are in series with each
other an what you do in the earlier stages trickles down to the later stages as well.:thmbsp:
 
Mods with original tuner controls?

Tom Bavis said:
C6 and C9 provide some bass boost (actually high frequency attenuation) - you don't want it - just clip them out. Leave the other resistors alone unless you need more gain. The bass control only works with a ceramic phono cartridge - it will have no effect. Treble control is cut only. You can remove the "loudness" boost by disconnecting the PEC wires at the taps on the control. Or leave it if you like... If you remove ALL the controls, add a resistor on each input grid - 100K, 470K, doesn't matter as your preamp is much lower.
Click to expand...

I'm doing something similar on a restore/upgrade of a '62 Maggie cabinet (8601-10 amp, 5930 tuner). Been comparing Kegger and Mike's mods for the 8601 (8301 looks pretty much identical to the 8610 + 59xx volume/tone circuit) though most are based on making the amp standalone. I'm trying to come up with a recipe using the existing tuner, volume, bass/treble controls, adding modern inputs (iPod, CD, MM cart) and speakers.

If I plug in a line-level input right at the input switch (before the bass/treble controls) I see what you describe -- extremely bassy sound and non-working bass control, plus it's extremely muffled sounding unless the volume is all the way up. Am confused why that would only affect the phono input (since the ceramic is just a straight line in), but not the FM -- only because it is stereo? I see no specific phono/RIAA circuitry anywhere, unless it's tucked into somewhere else that's non-obvious.

Keeping the original controls, should I still snip out the 0.01 caps on the input (not present in the original 8301) and the 0.1 caps in the feedback loops? That would leave 39.8K in the feedback loop -- should I still drop that there as well? I'd still like to be able to use the AM/FM, so would I need to add anything back into that side to compensate?

Otherwise, seems unanimous to up the coupling caps between the 6EU7/EL84 to 0.1uf-0.22uf, as well as significantly upping the filter cap that feeds the output transformers to 100uf+, and also the cathode bypass cap to same.

I really appreciate any suggestions for how to do this!

HS

(adding tone control schematics and stock 8301/8601 for quick reference)
 

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Kegger said:
I recently went through one of these, I'm posting the "original" schem with all the
bass/treble/balance/cartridge stuff out, an what I did + suggested and volume pot
installed for what your after on the "modded" one. (I have other plans for mine). :naughty:
Click to expand...

Could I use the original Magnavox volume control (3.3Meg)? What (if any) mods should I make to this schematic?

(Having a hard time finding a replacement volume pot at 100k or so with the right shaft length that will fit the original knob)
 
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