Megatron

THREAD

2014

400V Line (included 300V line) = total 61mA/ch

-400V Line = total 54mA/ch

used 10R current limit resistors

So about 65ma/ch limit

measured .517v across 10R

on 400v rails that might be enough

Used STN9360 for the CCS

bias  2ma first stage, 200ua second stage, 27ma outputs 

Used Two AS-1T175 for 350vac 450v zeners and 390kR =410v rails

300v rail around 280v with 16k dropping resistor

the two 1T175's have a total of 4) 6.3v winding's, plus one 25va with two 6.3v

and a 30va 12-0-12 for front end

(used to be 6-0-6)

 

 

 

 

 Feb 2020

Took amp apart to update a few things and make it easier to work on.

Added a ground loop breaker from the FirstWatt F5 Turbo PS, 35A bridge and CL60 Thermistor.

added a DC to DC buck module for fan @7.5vdc

added XLR balanced inputs

changed the 2 channel RK27 pot to a Alpha quad

added a HV timer delay and DPDT relay

reworked amp board traces to change front end heaters to 12v 

(removed ground on each CT, cut a feeder trace at other ends and grounded it)

changed front end transformer to 30va 12-0-12

 

increased current through front end as detailed in later part of thread.

CCS resistors from 2.2k to 1.5k

and tail resistors from 3.3k to 2k

12ax7 plate voltage should be 240v

I measured 236v 

and 136v on the 12au7 plate

 

changed the 7815 to a 7812 and put it under on a plate for cooling

 front tube heaters draw 700ma total @12vdc 

timer/relay and fan draws 120ma

 after 20 min of load it measured 36C with FLIR thermal camera

rated for 2.2A @ 25C

should be OK?

measurements/changes

R19    parallel 39k and 15k =10.8k     B+ now 290v

12AX cathode resistors   15k parallel to 2k = 1765R

1765R@ 1.795v   1ma bias

12AX plate voltage is 239v

warmup amp draw= .7A

running= 1.5A

-----------------------------------------------------------------------------------------------------------------

removed redundant second stage  330k plate resistors
B+ up 5.5v to 295.5v
12AX plates down 30v to 210v
Bias down .15 to .86ma
12AU plates at 175v 

amp sounds darker suspect low plate voltage is cause

 

-----------------------------------------------------------

  raised cathode resistors from 1765 to 2k

plates now at 225v

amp brighter and more gain

 

raised cathode resistors from 2k to 2.2k

12AX plates 240v 

DONE