Maggy 185 w/12AU7 1st stage...

[gadget73]

Does that jack actually short to ground, or just connect the 12ax7 grid to the 12au7 stage?

 

[knockbill]

No,,, we were discussing the line stage,,, the 12AU7 stage goes thru the line stage when the jack contacts are closed, the jacks aren't being used to take the line stage to ground, but to allow it to pass into the pre amp/PI (12AX7) stage... When they are insulated open, the 12AU7 feed doesn't go thru...
So if the line jacks are held open, the amp doesn't hum, or pass signal,,, altho a small signal gets thru at full volume... hope I didn't miss lead you too far!

Here's an original schem, sorry for the low res, I was never able to find a high res, unmodified schematic... The schematic of the 12AU7 stage used to replace the original phono stage is in the 1st post...

I appreciate the help, I may have misinterpreted the build/installation of this new 1st stage...

 

[knockbill]

Yep,,, glad its clear now... There has to be something wrong with the 12AU7 stage,, or the pots it was dead quiet before...
I could put the VC pots before the line stage, and abandon the 1st stage, but then its a 175 with a smaller PT.... there has to be a way to do this, as it originally had a passive tone stack and VC connected, thru the 7025...

 

[knockbill]

Haven't given up yet!
I took the pots out of the circuit, and it still hums and a little buzz... Also found with teh pots out, grounded plugs in the 12AU7 jacks still have a slight hum...

 

[knockbill]

Thanks for hanging in here!!!
I was actually thinking of adding more filter caps to this stage,,, I added a 68uF cap to the 3rd node (had 22uF originally) and a 4th node (22uF) when I originally (last winter) added the 12AU7 stage...Maybe this needs a larger cap? I'll try looking at it with a scope...

The values from the hand drawn schem are from Gordon's mods from the thread I linked (post 36) in the 1st post of this thread...

 

[knockbill]

Another miss cue... the original (7025) values were replaced with the values on the hand drawn schem...
470k is now 22K,,, 8200 is now 1k....

 

[knockbill]

Agreed,,, that's about all that's left... If the 22K plate resistors are raised to 50K, the supply voltage will go down even lower,,, Not sure I can raise the PS voltage enough to maintain 90V supply with this PT,,, the 7025 originally used didn't draw as much current...
But now I have some numbers to shoot for,,,
Thanks...

FWIW,,, I questioned the use of either a 12AU7 or a 12BH7 in this stage, as they have different current draws,,, most replies said they use them interchangeably, which didn't make sense, so I shelved the project...

 

[knockbill]

There was a 22k Rp there, I swapped in a 50K, and reworked the PS to get 90V, still hums ,, but I didn't increase the filter caps yet...
I'll try to get it working with this info,,, if not I'll return it to its original specs, and call it good...
Thanks for all the support...

 

[gadget73]

I'd consider maybe a 12ax7 as a cathode follower. It shouldn't draw much current, and you probably don't really need extra gain anyhow.

 

[knockbill]

Timely thought!! This is such a nice little amp, and I didn't have an accurate schem or plan,,, let alone the knowledge to redesign that 12AU7 stage, that I just put it back to stock.... Its dead silent and sounds good... The Voltages are almost spot on, to a little low... but that's OK... I tried it with its passive tone stack,,, but I'd really like to add a VC... I was thinking of just putting it ahead of the line stage, but then it would just be a 175...
If you have a working schematic with the CF,, post it up!!!! otherwise I'll just use it with its tone stack or a preamp....
Thanks again for your help...

 

[gadget73]

Try this. Its for a 6EU7, but a 12AX7 is functionally the same thing, just the pins are scrambled around a bit. This is directly from Dave's re-work of a Magnavox 9300. I can claim no credit for the design.

You can modify that 10K drop resistor as needed to get the plate voltage proper.

 

[knockbill]

Thanks,,, you posted that before, I think? I didn't see a VC on this one, that was my reason for the stage... Not sure I can get 320V out of this PS and have a couple stages of filtering,,, but if a volume pot will work here, I could try it...

 

[gadget73]

you should be fine with a volume pot between the input jack and the grid of the 12ax7. Just sub it for the two 470K resistors.

Should be able to take it directly off the choke. One version of this fed from a 340v source, the other from a 320v, depending on what particulars the rest of the amp was running at. I don't know that it makes a lot of difference. Schematic I found for the 175 shows a 345v source right off the choke.

 

[knockbill]

Thanks for the info,,, Maybe I'll build a mule instead of tearing this amp up anymore... the 185 has only a 305VDC Vp,, compared to the 345V the 175 has... Guess that bigger PT provides more current,, and voltage... When I tried the 12AU7 mod, I used a SS plug in converter to replace the 5U4, and got right at 345V out... So, its still possible to power that CF at the given voltages...

Shifting gears a little, I'm considering tying the VC pots to the line stage instead of cobbling in the CF, (altho I would like to try (hear) the difference)... Just have to figure how to connect them to that switch jack...

 

[knockbill]

Update,,, I decided to shelve the 12AU7 stage and rewire the amp to its original configuration, 12AX7/7025 first stage,, with addition of VC pots in front of the line stage... Also cloned Channel 2 to match Channel one, including removing the balance pot and FB... It seems to be happy, decent voltages, and sounds good with no hum at all... It works well with its passive tone stack, or the volume pots will control either input...
Only flaw is the chinsy Alpha pots,,, one of them cuts out near the hi end,,, but I'll find a pair of A-Bs to replace them with at some point... need some more 12AX7/7025s only have two for the line stage...

Still may mock up that CF, and try it on one channel as a comparison...

Thanks for all the ideas, info and support...

 

[frankiebull]

Only flaw is the chinsy Alpha pots,,, one of them cuts out near the hi end,,, but I'll find a pair of A-Bs to replace them with at some point...

Which Allen Bradley pot would you use?

 

[knockbill]

I have a bunch of the old JAN/industrial 2W pots,,, ran out of 500K tho... usually pick them up at hamfests/meets, Ohmite made them also... forgot to look for them at Kutztown!