Having just read thru 47 pages of hints, I grabbed the information that related to tube related Amplifiers and collected it into one file. I thought this might also be useful for other:
MC30
Here is a good starter, MC30 bias upgrade for safety and reliability.
Replace the Bias filter capacitor with a 10 to 22 uF (I use 22uF) at 450Volt, then replace the bias diode with a
good silicon one, 1N4007 works well. Now that you have a new bias supply the voltage will be to much, to get it
back to original replace the 3.3K series resistor (one next to diode) with a 9.1K 1/2W resistor, measured bias is
exactly what it should be with these 3 changes, and the long life of your MC30 is one step further.
440V on the plate and -42V on the grid of the 6L6, power transformer type is of no consiquence, replace the
selenium and change the value of the resistor in series with it to set the bias, about 9.1K should get you close,
a value either way may be needed to get it right on. I find anywhere from -42 to -43 is good when plate is at
440V..
__________________
MC30 biasing and filament surge
Turn on surge is common, this is due to cold filiments getting their initial hit of voltage, if this bothers you
add an NTC resistor like the MC240 uses, thruthfully I would not care since it hasent caused the transformer to
fail for the past 40+ years.
My schematic is for the later MC30 drawing # MC-30-4 (#15329 and above, 6-15-56), R30 is the bias droping resistor
with C13 the - bias filter, the bias is called out at -42V with B+ at 440V, this is the most common version of the
MC30.
Watch out on the early version that uses the 2 extra .47uf capacitors and has the square bias rectifier on the
capacitor side of the board, it does not have a droping resistor and if you replace the selenium you will have to
add a resistor in the transformer feed side of about 8.2K.
Remember to much bias makes the amp run cooler but also decreases output power and raises distortion.
__________________
MC30 thru MC275 distortion
MC30 thru MC275 all, if you have increasing distortion as the volume is increased check the second stage, V2,
12AU7 Phase Inverter, pins 1 and 6 must be at the same voltage (265V), if not replace the .22 uF capacitor on pin
7. The grid bias is derived from a 2.2 M ohm resistor, if this capacitor as any leakage it will upset the bias
voltage resulting in the phase inverter outputs being unbalanced.
__________________
MC60 Bias update
When changing out old bias diode to a silicon (1N4007) change the 1.8K series resistor to 5.1K, check bias for -45
or slightly higher (-46) when B+ is at 435V on the KT88. If you have a fully meterd variac it should draw 1.2A at
120V.
Note: some very eary MC60 did not use a series resistor, you may have to experiment to come up with a good value
for the resistor.
__________________
Speaker terminal screws
If your screws on the speaker barrier strips are chewed up and you want to replace them watch out - if the new
screws are too long they will short to the chassis.
__________________
__________________
MC240 Bias update/mod
When replacing the selenium diode in the bias supply with a silicone diode (1N4007) use a 470 ohm resistor in
series with the transformer lead going to the diode, this should put the bias voltage on the button !! While you
are at it replace the bias filter, a 22uF 450V cap will stiffen it up and last forever.
__________________
Bias Voltage MC240
The banded end of the new diode should be tied to the transformer, this will give you a - voltage, it may need to
be tweaked with a resistor in series at this point to get the proper -46V (loaded) when the B+ is 430V at the
output tubes. When all is working as it should with the AC line at 120V the AC current should run 1.2 to 1.3 amps
when you have a set of good tubes installed.
__________________
Rebuilding Bias Supply on MC-40
I have 2 MC40's to do this to waiting on the shelf, the resistor will be in series with the diode and the lead
from the transformer. The value should be about 470 Ohms, you may have to adjust it slightly to get -52V on pin 5
of the 6L6 with the voltage on pin 3 at 470V. I use a variac to make sure the plate voltage on pin 3 is at 470
before I measure pin 5.
__________________
First thing to check on a MC240 is the mode switch, work it a few times to get the self wiping action to clear
oxidation off of the contacts.
__________________
" I opened up a 240 to see if the selenium rectifier had been changed and confusingly there has been a diode
soldered in but no resistor..."
A percentage of the 240s do not need the 470 ohm resistor in series with the diode, variables in the power
transformer and circuitry can change the voltage. It is best to measure to make sure 430V plate voltage and -46V
on the grid are the ideal with some variation up to -48V at the grid being acceptable.
__________________
MC240 Thermistor
CL-50 NTC, thermal resistor available from most supply houses, or from the factory parts department 800-538-6576.
The MC240 uses a voltage doubler type power supply, the NTC is there to limit the turn on surge to the supply,
makes the power transformers surge less, and doubler capacitors last longer.
__________________
Mcintosh Chrome
McIntosh chrome responds nicely to a good car wax, I use Turtle wax Carnuba paste, just don't rub real hard on the
lettering, and also gives the black paint a nice shine. If your chrome is getting tired with pits or hazing you
can use Wenol metal polish (
www.wenol.com), it can help revive the chrome to be as good as it can be. Whatever you
do use a clean soft rag to do it, keep using a clean section as the dirt it picks up can scratch the chrome
leaving swirls and hazing.
For chrome I use Wenol (
http://www.wenol.com/), it gets the rust and oxidation. For the glass I use Turtle Wax
Carnuba, can use it on painted surfaces just don't rub it a lot or you will end up with a beautiful gloss finish.
For general dirt I use clear Ammonia, is very gentile on lettering, both the Wenol and Turtle Wax can eat at
painted lettering if you rub to hard, the metal ID/information plates are safe.. Do not use Windex, or 409 type
cleaners as they attack lettering and painted surfaces.
__________________
MC225 hot 12BH7 and 22K, 1W resistors.
If your MC225 has cooked the two 22K, 1W resistors and the 4.7K, 1/2W resistor, replace the pair of .047uF bumble
bee capacitors feeding the grids of the 12BH7 at the same time you replace the resistors. When the .047uF caps get
leakage they will raise the grid voltage of the 12BH7, I have seen 96V on the grids, replaced the caps and had it
go down to 33V, close enough to get the tube running in spec. The second thing this will do is raise your output
power from a sad 22W to a happy 30W+ as it did for me on a customers 225 that was running all original McIntosh
tubes.
Mc225 Thermistor replacement?
In another thread I'm rebuilding the power supply of my MC 225, and a question or two has arisen. first, what does
everyone use to replace the thermistor with?
Secondly, when replacing the bias diode with a 1n4007, which way is the band oriented? The original has one wire
on one side, and two on the other.
What is the value of the bumble bee that is on the small power supply board?
I would use the G.E. CL-80, 47 ohm, 3 amp, NTC, gives a nice soft start.
The band is the + end, this is the end that goes toward the transformer R46-R47 junction.
The capacitor is a .01uF use 630V, C24.
__________________