First Boogie! First Post

[bp-plickner]

Hi Folks
My first post and my First Boogie.

I have been a fender guy for the last 40 years or so I acquired an early Mark II and have just started figuring it out. Interesting sound. Very different to what I am used to. I am considering removing the fetron stuff from the first 12ax7 and replacing with 1.5k bypass cathode resistors. I am thinking that this might bring this amp to life. I have done some research on this mod and would appreciate any help from folks here who have done this mod as well as any comments regarding the change in tone.

Input-No 1.5K By Pass Resistors?? Solid State devices to support Fetron?

It does have a bias adjust pot and 1 ohm resistors on the cathodes of the output tubes for easy bias checking, which is cool.

Bias Adjust Pot

Bias Test Resistors

Looks like an early MarkII

I have to say with my homemade fuzz face plugged in this amp is killer :shock:
It would be cool to get that fender chime also 8)

So if any one wants to share info regarding removing that Fetron stuf with this Newbie, that would be
awesome.

Thanks!

 

[Boogiebabies]

I don't see a relay, so it may very well be an oddball MK I or as mentioned an early IIA.
More likely a IIA with the bias pot and rear serial plate.

Anyhow, remove the 15K voltage divider off the 47K 2W B+ rail and remove the switch.
Replace the 1N5303/TCR5302/TCR5304 with 1.5K .5W resistors.
For Mark 1 specs, replace the two V1 plate resistors from 62K to 100K .5W, for IIA
specs replace them with 82K .5W. Of course you could use carbon comp for mojo, carbon film
or metal film depending on your preference. If your a purist and have to have originality, the
resistors used in that amp are German Beyschlag .5W which will require some hunting to procure.

Enjoy.

 

[Neptical]

Hey, BP...welcome aboard my friend. ..and congrats on the first Boogie purchase.
I can't really help you out on this one but I'm sure there will be folks dropping in to give ya some help. Are you ready for the Boogie addiction? :wink:

All the Best,
~Nep~

 

[dodger916]

I don't see a relay, so it may very well be an oddball MK I or as mentioned an early IIA.
More likely a IIA with the bias pot and rear serial plate.

Anyhow, remove the 15K voltage divider off the 47K 2W B+ rail and remove the switch.
Replace the 1N5303/TCR5302/TCR5304 with 1.5K .5W resistors.
For Mark 1 specs, replace the two V1 plate resistors from 62K to 100K .5W, for IIA
specs replace them with 82K .5W. Of course you could use carbon comp for mojo, carbon film
or metal film depending on your preference. If your a purist and have to have originality, the
resistors used in that amp are German Beyschlag .5W which will require some hunting to procure.

Enjoy.

Ed: My IIA+ had the bias pot too. Coincidence?

 

[bp-plickner]

Hey Neptical thanks for the welcome!

Hi Boogiebabies,
Good info, that should do the trick.
I have to get back into the amp and see if there is a diode and a cap on the input like the schematic that I didn't pick up in my photo.

Like you said carbon comps should do the trick.


Any opinions on the resulting tone?

 

[Boogiebabies]

I don't see a relay, so it may very well be an oddball MK I or as mentioned an early IIA.
More likely a IIA with the bias pot and rear serial plate.

Anyhow, remove the 15K voltage divider off the 47K 2W B+ rail and remove the switch.
Replace the 1N5303/TCR5302/TCR5304 with 1.5K .5W resistors.
For Mark 1 specs, replace the two V1 plate resistors from 62K to 100K .5W, for IIA
specs replace them with 82K .5W. Of course you could use carbon comp for mojo, carbon film
or metal film depending on your preference. If your a purist and have to have originality, the
resistors used in that amp are German Beyschlag .5W which will require some hunting to procure.

Enjoy.

Ed: My IIA+ had the bias pot too. Coincidence?

That's about the only thing it has in common now, if yours was not turned into a demon from hell !!!! :evil:

 

[Boogiebabies]

Hey Neptical thanks for the welcome!

Hi Boogiebabies,
Good info, that should do the trick.
I have to get back into the amp and see if there is a diode and a cap on the input like the schematic that I didn't pick up in my photo.

Like you said carbon comps should do the trick.


Any opinions on the resulting tone?

I don't think you'll see a cap or diodes. Just the 10K at the input jack and 1M off the input cable to the board.
It's easy to assume with the increase in gain from the cathodes Mesa was looking to prevent/filter noise at the input. The IIA was the
last MK series to have a resistor at the V1 grid.

The improvement in tone should be considerable with a proper cathode circuit. If you lose too much gain you may want
to consider dropping the B+ rail resistor to 33K or even 6.8K like the IIB. The whole reason for the fetron was poor availability
of preamp tubes at the time. The IIA is only seeing 250V on the B+ opposed to 408 on the IIB. With the removal of the TCR's your cathode
voltage will drop too.

 

[bp-plickner]

I got the 1.5K cathode resistors in place along with a 10k dropping resistor on B+ power rail after I pulled the Fetron Switch. I left the 63K resistors on the plates of V1 and I am seeing about 270 Volts there. There was a diode and a cap on the grid of V1 that I removed when I attached the input to it. I left the 10k resistor on the input jack.

What a difference
Good Bye Honk Hello Harmonics.

I changed out all of the preamp tubes also before the mod which showed promise and better tone. But after the changes it is Sounding like a real tone machine.

Thanks For Your Help

Peace

 

[boogieslayer]

Are you ready for the Boogie addiction? :wink:

All the Best,
~Nep~

Welcome ! hope ya stick around a while, theres so much information here its ridiculous and so many great folks ready to help with any problem you come across its silly

The Boogie Addiction is a Killer , i'd never trade it for anything !

 

[talltxguy]

Welcome to the Boogie Board.

I purchased my first Boogie last year and still remember the excitement upon hearing that Studio Preamp blasting through a 50/50!

This is a great board for information, and Boogies are a great brand for tone.

 

[tonefreak25]

Welcome to the Boogie Board.

I purchased my first Boogie last year and still remember the excitement upon hearing that Studio Preamp blasting through a 50/50!

This is a great board for information, and Boogies are a great brand for tone.

I can second this emotion!! Except using a 2:50 instead. Welcome to the fun.

 

[bp-plickner]

Thanks Folks.
I replaced the 63K plate resistors this morning with 100k resistors on V1 which dropped the plate voltages to about 240 volts
which sounds a little different.

I have to play it some now and stop screwing around trying to make sound like a 1959 3-10 fender bandmaster :lol:

Right on about tons of good info and cool people here!

Peace