Mercury Magnetics: Studio Pro Upgrade Kit For AC4TV

[Headstark]

Installed the Merc Mag upgrade yesterday and R-19 is "Slowly roasting o'er an open fire".

Dim Bulb Tester did not even light up with a 75 watt bulb, but after several minutes, R-19 which is a 3.9k begins smoking.

It read 3.940k on a Fluke, but I was wondering if a metal film resistor can check out okay and go faulty like when energized.

Has any one installed this mod, and might you have voltages across R-19 and R-19 in respect to ground?

Going to re-check the build yet again, replace the burnt toast, and I would not mind knowing the correct voltages to put to a properly supplied resistor.

Thank you,

headsta(r)ck

[Masco]

You don't give any details of the mod. Is it anything other than replacing iron?

Is this the board in your ac4? : http://i217.photobucket.com/albums/cc21 ... AC4009.jpg

R-19 is a 22k carbon film. The only 3k9R in the amp is in the cathode circuit of the 2nd stage preamp 12ax7.

[Cantplay]

It changes both trafo, rebiases the tubes, and changes power supply from CRC to CLC.

John

[xtian]

power supply from CRC to CLC

Huh? CRC = cap - resistor - cap?

CLC = ?

[Cantplay]

They give you a choke to replace the resistor.

John

[xtian]

They give you a choke to replace the resistor.

John

I inferred that, but what does CLC stand for?

[Colossal]

In the parlance, L = inductance
A choke is an inductor, therefore a CLC filter is Cap - Choke - Cap

[JazzGuitarGimp]

What Colossal said. This is why, in schematics that have chokes, the reference designator starts with L. So, L1, L2 L3, etc.

[headstack]

Figured out what was going on with this thing.

C-17, which is one of the original parts that stays was the incorrect value.

Was supposed to be a 10u and happened to be a 220u so it was loading down the circuit.

The amp sounded pretty nasty from day 1 and the tone control did very little, but I figured it was just a cheap bedroom amp and expected nothing much from it.

The tone control is still weird, is trebly when anti clockwise, and as you dial it in is gets slightly darker.

Kind of like a bad low pass filter

My guess is, something still left on the board is funky from the high current situation.

Have to give it another going through.

Are we allowed to post readily available commercial schematics here, or does that violate TOS?

I could isolate the pertinent landscape so as not to divulge the entire "Super Secret Serkit" if that works.

Thank you for the time and insight.

[rock_mumbles]

I did some modding on an AC4TV of my brother's ... I focused on getting rid of the fizzy tone ...

- 10uf cap on v1a cathode (C16 - 1uf stock)
- pull the v1b cathode bypass cap (C11 - that was the 10uf I used in v1a)
- pull the 'tail' cathode cap on v1b (C12)
- jumper C12 to remove the DC grid bias on v1b
- reduce the value of the second coupling cap (C18) to 22nf (100nf stock)
- increase the tone stack cap C4 (220pf) to a more standard 500pf value
- decrease the value of the EL84 cathode bypass cap to 50uf (220uf stock)

[headstack]

I did some modding on an AC4TV of my brother's ... I focused on getting rid of the fizzy tone ...

- 10uf cap on v1a cathode (C16 - 1uf stock)
- pull the v1b cathode bypass cap (C11 - that was the 10uf I used in v1a)
- pull the 'tail' cathode cap on v1b (C12)
- jumper C12 to remove the DC grid bias on v1b
- reduce the value of the second coupling cap (C18) to 22nf (100nf stock)
- increase the tone stack cap C4 (220pf) to a more standard 500pf value
- decrease the value of the EL84 cathode bypass cap to 50uf (220uf stock)

the Merc Mag "Upgrade" removes nearly all the original components, and builds a very different circuit.

C16 is now 22uf
C11 is gone
C12 is gone
C18 is 680pf
C4 has a 20k carbon comp resistor in the upper hole headed in series with a 4.7nf chicklet going to the upper hole of R7
There is a jumper across C3
The EL84 bypass cap is now a 47uf

When I was stripping the board, it had me thinking "Why not keep this stock and just build a new turret board to load in here?"

Overall it sounds way better!

Loads of headroom, good chime, nice harmonic content, but the tone stack is basically kaput atm.

[Mark]

Here is some food for thought.

http://www.dougcircuits.com/AC4V3.jpg

[Masco]

I did some modding on an AC4TV of my brother's ... I focused on getting rid of the fizzy tone ...

- 10uf cap on v1a cathode (C16 - 1uf stock)
- pull the v1b cathode bypass cap (C11 - that was the 10uf I used in v1a)
- pull the 'tail' cathode cap on v1b (C12)
- jumper C12 to remove the DC grid bias on v1b
- reduce the value of the second coupling cap (C18) to 22nf (100nf stock)
- increase the tone stack cap C4 (220pf) to a more standard 500pf value
- decrease the value of the EL84 cathode bypass cap to 50uf (220uf stock)

the Merc Mag "Upgrade" removes nearly all the original components, and builds a very different circuit.

C16 is now 22uf
C11 is gone
C12 is gone
C18 is 680pf
C4 has a 20k carbon comp resistor in the upper hole headed in series with a 4.7nf chicklet going to the upper hole of R7
There is a jumper across C3
The EL84 bypass cap is now a 47uf

When I was stripping the board, it had me thinking "Why not keep this stock and just build a new turret board to load in here?"

Overall it sounds way better!

Loads of headroom, good chime, nice harmonic content, but the tone stack is basically kaput atm.

Wow! I guess just replacing the iron wasn't convincing enough for MM.

[rock_mumbles]

...

Wow! I guess just replacing the iron wasn't convincing enough for MM.

The stock circuit is fine clean at low volumes but pushed into distortion it's horrible with the DC biased second preamp triode, fizzy, fuzzy horrid ...

...
the Merc Mag "Upgrade" removes nearly all the original components, and builds a very different circuit.
...
When I was stripping the board, it had me thinking "Why not keep this stock and just build a new turret board to load in here?"

Overall it sounds way better!

Loads of headroom, good chime, nice harmonic content, but the tone stack is basically kaput atm.

The tone control stock is basically a Marshall 18Watt Tremolo tone control with a wrong value of treble cap it should have a 500pf treble cap not 220pf, the 18W control works pretty well.
The modified control is a treble cut control sort of like the control in a Spitfire ... although I don't understand where the added resistor goes ???

[Masco]

...

Wow! I guess just replacing the iron wasn't convincing enough for MM.

The stock circuit is fine clean at low volumes but pushed into distortion it's horrible with the DC biased second preamp triode, fizzy, fuzzy horrid ...

Kind of makes me wonder what a modified one would sound like without the MM iron.