Help with Maco 750

[Cricketsman05]

I just bought a Maco 750 without tubes. The schematic provided with the box shows 9 tubes but the box only has eight sockets.

I was told by the seller that the box used 8 8950's but was last run using 2 8950's driving 6 2057's.

The schematic shows 1 preamp 8950, driving 2 6BL6's ,driving 6 6BL6's.

It sure would be nice if I could figure out what is going on.

Anyone have a schematic or owners manual for an 8 tube Maco 750?

Thanks

 

[AudioShockwav]

8950`s and M-2057`s are basically the same tube, the M-2057 was just a bit more robust.
Both tubes above require 12/13 volts on the heaters, the 6LB6`s use 6 volts on the heaters.
I have never owned one of these units, but if you look at the picture
at CBTricks
http://cbtricks.com/Amp/maco/maco_750b/index.htm
I only see 8 holes ( sockets ) in there unless the predriver is UNDER the deck.
If you look at the schematic
http://cbtricks.com/Amp/maco/maco_750b/graphics/maco_750b_redraw_sch.pdf

Shows a single 8950 pre-driver, 2 6lb6`s used as driver tubes, and 6, 6LB6`s used as finals
It looks to me like Maco did away with the Pre-driver, and just did not note it on the Schematic.
The M 2057 are so rare that I doubt you will find any for sale, and if you do, they will want gold for them, I have seen them sell for $100 each NOS
Even 8950`s are going for big money...good NOS ( new old stock ) sweep tubes are getting harder and harder to find.
6LB6`s are a little more common.
Search the web for prices, and there is Flea-bay, it is a crap shoot with tubes from there, butI have scored some good tubes there in the past.

( Read the Warning Below about working on your amp before anything )

Look here:

http://www.nj7p.org/Tube4.php?tube=6LB6
For the 6LB6
And here:
http://www.pmillett.com/tubedata/8950.pdf
For the 8950

This is the pinouts for a 6LB6/8950, pin # 1 and # 12 are the heater
pins, check the voltage at the socket`s with your meter, if you have 6 volts on the heaters for the finals and drivers, it is wired for 6LB6`s, if there is 12 volts there, someone mod`ed it for 8950`s
WARNING!
! BE DAMMNED CAREFUL WORKING IN THAT AMP WITH IT PLUGGED IN AND POWERED UP !!
If you are not experienced working with high voltage in tube amps, get someone to do it for you, that amp can kill you.
Really.
Even when powered off,and unplugged, the high voltage caps can hold a lethal charge.

73
Jeff

 

[Cricketsman05]

Hi Jeff and thank you for the response. Today I put 8 8950's into the box. It would not key up. Switching between hi/low and standby and operate provided me with several flash overs and pops. Now three of the six finals are not lighting up at all. Still no key up. The two drivers and three of the finals are lit. The Maco isn't looking to good right now. I did find one cold soilder joint. Didn't make any difference on anything that I could tell. I am really at a loss as to where to start. There is some dust that needs to be cleaned up and then I guess I will start trying to check some voltages.

 

[Cricketsman05]

I have the amp where it will work some what. When I key up I now have a relay chattering. I can manually engage the relay and the amp works as it should I think. If I start out with 2.5 dk it chatters up to 5.5 dk. After 5.5 dk it doesn't do any thing.