Maplin Millenium 4-20

[Sunwontrise]

Hi folks,

I bought a old tube amp, based on the Maplin Millenium 4-20. Basically a Mullard 5-20 clone

It has big output transformers and a massive chrome chassis, so I think it's worth spending some time and money on it.

It has a powerful filter section, with 3x 200uf 500/600v filter caps.

I changed every electrolytic capacitor and the amp is working again. No hum, but a light overdrive on one channel using louder volumes. Tube swapping doesn't solve the problem - so maybe I need new sockets and maybe test some filter caps and resistors.

I found a page with some modifications of the Maplin, so I was wondering what these modifications do to the sound of the amplifier - maybe someone could tell me

thanks, and cheers,

Felix

 

[Alan4411]

I cannot see your picture / image?

This?

 

[TheGimp]

.5W resistors are only rated for 300V working voltage. If you are running higher B+ I would recommend 1W minimum.

 

[DF96]

What do you mean by a "light overdrive"? Do you mean early clipping?

The mods on that diagram are a mixture of good ideas and bad ideas. Good ideas:
1. remove bypass capacitors

Bad ideas:
1. remove 10M input resistor - this protects against failure of the volume pot. Even better, keep the resistor and add a coupling cap - this will lengthen the 'quiet' life of the pot.
2. remove the input low pass filter
3. swap the LTP to ECC81/12AT7 - this unbalances it so it then needs unbalanced anode resistors to push it back
4. reduce grid stopper values
5. fiddle with loop compensation components - this could be bad or good depending on the exact layout, OPT type etc.

 

[Sunwontrise]

Yes, I mean this shematic

Okay, so I wont went through the mods.

With overdrive I mean a distorted sound out of my speakers with rising volume level.

My speakers include a 15" Monacor SPA-115PA and a Celestion CDX-1430 horn driver - passive crossover network 8 Ohm overall impedance.

Changing tubes does not solve the problem - so I think it has something to do with the electric components.

Cheers,

Felix

 

[Alan4411]

If one side works OK, but you have distortion on the other, start with simple voltage checks. Compare the bad to the good side.
You have replace the electrolytics, but have you checked the coupling caps are in good order and not leaky electrically?

 

[DF96]

Yes, compare DC voltages from good side to bad side.