Mc 60

[Little Nellie]

A few weeks ago, I got myself a (one) MC60. One of those amp I just got to have. I have not done much to it other than watching the tube glow without anything hook up. Since I do not have a MC 75 (yet), just wondering how does the MC 60 sound compare to the MC 75? It has a pair of GE 6550A installed and with Tele 12AX7, AU7 and a GE 12BH7. I search thru some of the old post and see some member has MC 60. Any insight to this great machine?

Silas

 

[CarlV]

I found that the Tung Sol 6550 floats my boat with mine. I don't have a comment on sound comparisons. I haven't a/b'd with a 75, to do so you need both identically restored IMO as they are just too old not to be, as well as a fire hazard. From there then you have that neither will have the exact factory
fresh signature as well, but it will be better restored than the old original as the worn capacitors lose sound quality as well.
One of mine is in my banner.

Carl

 

[Little Nellie]

Carl,

Thanks for the comment. You mention fire hazard, can you tell me more? Do you think all the old filter caps will pop and cause a fire?

thanks

Silas

 

[CarlV]

Yes the power supply side has electrolytic capacitors and a selenium rectifier that go with age and can cause major damage read transformer, and even fire if bad things happen when you are not right there. 40 years seems to be the point these things need attention. Don't get freaked out, just don't leave them on unattended until addressed. Search for "selenium" for circuit specifics.

Carl

 

[Little Nellie]

Thanks for the tip. I did a search and found Terry's hint on replacing the selenium rect for the MC30. Great, I will do that on my MC30. Have you done the replacment for the MC60? I would assume a resister will need to be change to have the correct bias voltage, right? If yes, do you know the value?

thanks

Silas

 

[CarlV]

Yes I have and no I don't remember what value. Terry should be able to answer that once he gets settled back in from his vacation.

Carl

 

[dewickt]

Working vacation, I was being paid all the way...
I used a 5.1K in front of the new diode (lead from transformer), the objective is to get -45 volts bias on pin5 when you have 435 volts on pin 3 of the output 6550 (KT88). I use a variac to set the 435V before I measure the bias -45, you may need to tweak the value but -45 to -47 is a safe range.

 

[Little Nellie]

Terry,

thanks for the info. Let me make sure I get this correct, you are replacing R26 (1.6K) with a 5.1K. Adjust the value of the new R26 for bias is at -45.

BTW, looks like I need to do the same treatment to my MC225 and MC240 also. Can you give me a starting value of the resistor for those? Can I assume to use IN4007 for all of them?

Thanks for your help.

Silas

 

[dewickt]

I don't have the same schematic as you are using, the early version had no resistor in series with the diode. You have what you will need to do itcorrectly, -45 bias when the plate voltage is 435. Check the hints and kinks stickey as I have posted most of the values that I use for most of the amps.

 

[Little Nellie]

Terry, thanks.. I found your old post. Lots of great info.

I found a bunch of 1N4005 (600v) diodes laying around, I noticed you call for 1N4007 (1000v), since the voltage is about 45 volts, I should be able to use the 1N4005, right?

Thanks for all your help.

Silas

 

[dewickt]

I use the 1N4007 due to I don't have to think about how the bias voltage is derived, sometimes it is taken at a high voltage point and sometimes the bias supply has its own low voltage winding, MC30 typically derives bias from the HV AC at the rectifier and can hit -450V at turn on. The schematic I have for the MC60 shows a lower voltage tap for the bias, a 600 PIV diode would be safe here, if in doubt put 2 in series and adjust the resistor value to make up for the added drop.