Pete Millett Low-Mu Triode Preamp

[wirewiggler]

Ok,
this one have driven me nuts, I have been having a hum problem I can't seem to resolve. I am seeing about 3 mv at the output and looking at the scope it would appear I have a high freq oscillation. I have tried (as per Pete's instruction ) isolated heater supplies, external power supply, changing grid resisters on the el34 to no avail.
here is a link to the build article http://www.pmillett.com/lowmu_preamp.htm . I built it pretty faithfully to the original (other than turning it 180 degrees ). I would think if it were oscillation moving my hand around the tubes would alter it but it does not. Anyone have any suggestions?

Thanks
Bill

 

[audiowize]

Try plate stoppers?

 

[audiousername]

Where are R3, R5, R9 and R10 physically located?

 

[dsavitsk]

I tried to build a version of this a long time ago and could never get it quiet. I used lots of different 6080's and while they were all noisy in different ways, they were all too noisy to use. I eventually gave up on the project.

 

[SY]

Where are R3, R5, R9 and R10 physically located?

My thought, too. They should be smack up against their respective grid pins.

 

[wirewiggler]

They are tied to the pins, I have varied them up and down. If I wanted to eliminate the tubes and check the B+ what value resister would I substitute? I think the oscillation is in the EL34 current source but would like to confirm it. My trouble shooting skills are weak so this is a good learning opportunity.

Thanks
for your help
Bill

 

[jan.didden]

Maybe a good moment to buy a simple oscilloscope?

Jan didden

 

[N1ESE]

Maybe a good moment to buy a simple oscilloscope?

Maybe a good time to re-read his first post where you'll see that he IS looking at it with a scope?

 

[Gordy]

I suggest to start by ensuring that the power supply is functioning as required.
Disconnect the audio circuit and load the supply with resistors representing max load. Adjust supply as required to achieve target ripple. (Consider replacing choke with resistors as a short term measure).
Once you are happy with the supply then reconnect to the preamplifier circuit.
Focus on one amp channel only. Remove the vacuum tubes from the other channel. (Get one channel fixed then replicate the fix in the other channel).
Disconnect the load from the preamplifier (i.e. disconnect any power amp connected).
Step through the circuit one item at a time to identify at which point the fault is introduced.


I have seen a similar problem when there was inappropriate earthing arrangements; the preamplifer audio ground was connected directly to chassis ground which then connected to mains ground which then connected to the power amp chassis ground which then connected to the power amp audio ground which then connected back to the preamplifer ground via the 'ground' conductor of the un-balanced interconnection cable. The sum total was a big ground loop and lots of hummmmmmmmm...

 

[SY]

I, you want to check the CCS, disconnect the 6AS7s and sub in a dummy load resistor. In the original circuit, stage current is 50mA and the plate voltage on the 6AS7 is 125V. Using Ohm's Law, R = V/I = 125/50 =2k5. Power is P = IV = 0.05A x 125V = 6.25W. Use a 2.2-2.7k 10W resistor and see if the voltage across it is 50mA times the resistance.

 

[jan.didden]

Maybe a good time to re-read his first post where you'll see that he IS looking at it with a scope?

Right. Missed that...

Jan Didden

 

[kmaier]

I looked at Pete Millet's site and read the specs he had for his. According to the specs, the noise figure was a -55dBV. According to most standards, 0dBV references 1VRMS level. 55dB down from 1VRMS would be about 1.78 millivolts RMS. It would not appear to be that quiet for a preamp.... unless I missed something

Regards, KM

 

[wirewiggler]

Thanks for the pointers, Sy you jogged my memory from basic electronics 30 years ago. It was my first attempt at a tube pre and I was hoping using Pete's arrangement with the copper base with turrets I would not have an issue with ground loops. It is very micophonic as well - I could live with that. I will pick up a pair of resisters to sub next Monday. I also need to pick some new probes for my scope. I was reading something about if you are checking high voltage rails with a scope you should float the ground on the scope for safety any comments ?
Jan after I get this project completed - fix preamp, convert 300b set amps to DC heaters, and sit back and enjoy new Austin 166 folded horns. I plan to pick up a DCX2496 and put all your mods on it . I plan to drive a Aleph mini, Aleph 30,and a UCD180 into Sigfried Linkwitz's Pluto. I hope to bring it to burning amp next year.

thanks for your guys help.

Bill

 

[SY]

No, you absolutely don't want to float your scope grounds. The scope itself should be grounded via the power cord. The probe's ground clip goes to signal ground in the circuit. Use a 10x probe to minimize loading (both resistive and capacitive).