Another Magnavox 185 / 175 conversion

[baddog07]

I’ve done some of the mods I’ve found on this forum. Love this place by the way.
1. Separated the bias to the power tubes and removed it from the hum balance pot.
2. Removed the balance pot from the circuit.
3. Changed the NFB loop to a 100r resotore in series with a 47pf silver mica cap across a 4.7k resistor.

I still have the extra 12ax7 at the input installed in the circuit. My question is why do people remove this tube? What’s the pros and cons of this tube being in the circuit ?

Thanks
Keith

 

[baddog07]

Pics of the chassis

 

[baddog07]

Still need to add a power cord, fuse, mod the filter caps, replace couple caps and so on. Lol

 

[knockbill]

I left the 7025/phono stage in my 185,,, Reasons... I still have the console(s),,, it is basically a two stage input amp, I use the 7025 phono stage for lower input Voltage sources, and the 12AX7 stage for CDP type inputs... No reason to remove it, pull the tube if you want an extra tenth of a volt on the heaters... For the power cord, I used a 2wire polarized cord, got hum with a 3wire cord grounded to the chassis... I started a recent thread here about installing input level/volume pots, if you aren't using a pre amp...
Have fun!

Edit,,, pic looks like yours doesn't have the 12AX7 input jacks? Stock AX7 input shorting jacks are between the 12AX7s in pic,,,

 

[cademan]

It's both 12AX7's that need to stay in place. Both are the AF amp/phase inverters for both the left & right channels.

It's the 7025 which can be removed as it just gives an extra gain stage for a ceramic cartridge. Since yours does not have the extra RCA input jacks between the 12AX7 tubes like John's, you are going to have to know where to input your signal to both tubes.

If you use the inputs to the 7025 tube, you might have way too much gain when using a line level device such as a CD player and what not.

Without the 7025 tube, the model #AMP185 effectively becomes a model #AMP175.

 

[knockbill]

Yep,,, if you want to have use of both stages, you can add a pair of jacks in the provided holes, wired to the 47K grid resistors (or remove the resistors and use pots) of the 12AX7s... They will need to be shorting jacks or use shorting plugs to reduce hum, if you want to bypass the phono stage... There are quite a few different 185/175 models out there, with minor differences,,, I haven't noticed a 185 chassis without the extra jacks, before your pics... 175s also have higher plate/screen voltage...
Sam's schematics show some of the differences between models...

 

[baddog07]

Thanks for the info and reply . I think I follow what you guys are saying. If I add rca jacks at the input of the 12ax7’s of the phase inverters (47k /470k network ) it will be a 175 amp. Now if I want to keep the phono input in tact I’d have to install shorting plugs when not in use.
I have been using the amp with my phone ( amazon music ) into the phono inputs. Seems to have plenty of bass . Going to install the rca jacks at the input of the phase inverters and see what sounds better to me.

I have another question for you guys. Someone mentioned adding screen resistors to the power tubes. Any idea how to do this and what value cap and resistors to use?

You guys rock...

 

[baddog07]

Pics of my progress.

 

[baddog07]

I left the 7025/phono stage in my 185,,, Reasons... I still have the console(s),,, it is basically a two stage input amp, I use the 7025 phono stage for lower input Voltage sources, and the 12AX7 stage for CDP type inputs... No reason to remove it, pull the tube if you want an extra tenth of a volt on the heaters... For the power cord, I used a 2wire polarized cord, got hum with a 3wire cord grounded to the chassis... I started a recent thread here about installing input level/volume pots, if you aren't using a pre amp...
Have fun!

Edit,,, pic looks like yours doesn't have the 12AX7 input jacks? Stock AX7 input shorting jacks are between the 12AX7s in pic,,,

Your amp looks cool. Nice work.
I was thinking of adding a pilot light to mine and might do it at some point. Also might add a dual gang volume pot too at the input of the phono stage.
Mine has a little hum with the grounded power cord but it also had the hum with the two prong cord so I’m going to have to dig into that issue. New filter caps might help with that issue. Rolling some different tubes ( rca, ge , etc.) in the 7025 spot till I found the quietest tube helped with the hum a bit too.

 

[baddog07]

More work done.

 

[baddog07]

Made the phono input match the Paraphase input. 47k / 470k network at the input of the phono.

 

[baddog07]

My music room.