• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

6CM5 MKII

Hi guys, a newbie here
I have just built one of Grant Wills 6CM5 Stereo amp using the M1120 transformers (my only other build was Grant's Lamington). Grant was a great help when I was stumped by something. On first power up I got a horrible shriek but had read somewhere to reverse the OPT secondary leads. All was good and its sounds fantastic!. but:rolleyes:
I'm trying to get a better bass response and have just ordered some EDCOR CXPP25-MS-8K OPT's with a EDCOR XPWR039-120/240 mains tranny which outputs 110V x 2 @250mA and 6.3v x [email protected] I will use Grant's original design for B+ with a voltage doubler. I have also discovered that the 6N2P valves are almost identical to the 12AX7 apart from slightly lower gain and only 6V heaters. I have just ordered 8 from Russia (only $20!) and was wondering if any of you guru's can forsee any problems I may encounter i.e using 6N2P for PI, 250mA not enough to run everything etc . Why isn't everybody using these cheaper valves?
Any advise would be greatly appreciated ! :)
 

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The valves are not commonly used because their not plug and play compatible whit the 12AX7.

However. if you where to use a toggle switch it would be possible to build an amplifier that will accept both the tube types.

A problem you might encounter, is that the 6n2p heater consumes 300mA more current.

Take care.
V4lve.
 
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Hi guys,
If anyone's interested I have just about finished the amp with EDCOR transformers instead of the M1120's I used the 6N2P valves for pre-amp and phase inverter. (The mains transformer has 2 x 4A @ 6.3V to cope with the load.
I have wired 4 separate pots for adjusting the bias so I can balance mismatched valves(I'm using 6BQ6's).
My problem is, the bass response isn't that much better than using the M1120 100v line trannys.
When I play James Taylor its perfect but when I play RUSH I'm not getting that punching bass.
I have increased the values of the coupling caps, increased and decreased C2 and C6, changed NFB resistor from 10K to 5K to 20K.
I am playing it through 15in PA speakers. I have tried an old sansui amp to make sure its not the speakers fault and the bass is great.
Hope someone can help !:confused:
I have included the tagboard layout a friend did for me (beware there are some errors!)
 

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No punching bass means crap frequency response. The only way to be sure is to measure with a tone generator and scope across a dummy load. I had a similar problem with my Chinese Dynaclone VAA-70MKii. But the values of your coupling caps seem OK.

Incidentally, I did spot two errors: no top caps connected and ECC81 input tube. The missing anode connections are pretty fatal...
 
Thanks for the response.
The drawing doesn't show the op transformer wiring, I should add that!
I do have a tone gen, do I need to connect a dummy load or can I just use the speakers?, what am I looking for on the cro? When I put a 20hz tone into the speakers I can see the wave but can only just hear a faint rumble (plus lots of other noises possibly due to the power supply driving the T gen). I don't start to get any real tone till about 40hz?
Thanks in advance