TUBE TECH MP1A noise

I have this microphone preamp lying on my workbench. The users says it is too noisy and asked me to see what could be done. Initial measurements show that the heating voltage on the tubes is too low. The voltage on the ECC83 input tubes is 9.5V (pins 4 and 5). Since the circuit that supplies the heaters looks fine, I'd be interested to learn if anyone has any experience with this preamp. In my experience, increased noise may be the result of insufficient supply but I'm not really sure if this is the case here. I'd also appreciate it if someone could share the original service manual.

Regards,
Milan

Milan,

I have an MP-1A that arrived on my bench today with a dead channel. I checked the filament voltages on both channels. It turned out that they are all different, because the 3 filaments for each channel are all in series. There is a dropping resistor at one end fo the string that drops about 4 volts. Two filaments in each channel measured 10.5v and one about 8.8v. It seems like this is perhaps a designed voltage.

There is no available service manual. It uses the Phantom 48V for heaters, all put in series. But the mounted ECC-type tubes don't really like series heating - they'll change quite a bit over time, overheating some, and underheating othet tubes.

Try changing all tubes, and see id that dosen't help.

The current "JJ" tubes are rather good!

Jakob E.

You can wire the heaters as hum cancelling using this configuration.....

..it's 48V DC..

Jakob,

Kindly do not cloud the issue with facts!

:green:

Keef

Hi guys:

Thank you all for your replies. I'm going to check the tubes first and try to heat them with nominal voltage. If the noise level doesn't improve , I'll replace the tubes. The design of the whole thing is rather disappointing, though. :sad:

Regards,
Milan

You have a 4u7/35V electrolytic in the feedback path around the input tube. Imo the weakest part of this design. Replace this with new high-quality electrolytic - or better - with a Wima MKS02 4u7/50V polyester.

Jakob E.

you should probably take some real nice pictures of
how they have their heaters hooked up :grin:

gear pron is always nice. *nudge*


g.

you should probably take some real nice pictures

Click to expand...

Do you mean something like:



:wink:

Jakob E.

You can wire the heaters as hum cancelling using this configuration.....

Click to expand...

..it's 48V DC..

Click to expand...

:green: You guys are gentle.... :green:

Confusion: I think the McCurdy AU300 uses a similar configuration..... in this case 36v DC. Why, other than hum cancellation?

Radiotron Designer's Handbook 4th Ed Page 786:
All methods employing rectified and filtered heater supply have a ripple component which may be large enough to cause audible hum.

Click to expand...

Nobody?............

low current means lower stray field?

Dunno.... looking at the photo of the board it doesn't appear to be wired to get phase cancellation, does it........

For the McCurdy they simply state that the higher voltage was chosen

"rather than 12v in order to keep the associated components to a minimum size for a given regulation and wattage

Click to expand...

I assume this would increase the life of the capacitors ... but it seems hard on the valves...... maybe there's more variation in modern stock...