Robrob's Micro Deluxe - advise on voltages

johanbarnhard

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Hi, I'm new to this forum and new to builing amps. I have just built the 5E3M , https://robrobinette.com/Deluxe_Micro.htm. Pretty much as designed, but with some mods:
- bright switch to switch off the bright cap
- Line out and added a LTR-110 audio transformer to that to make a balanced output.
- NFB switch
- fx loop
I use a hammond 369JX power transformer : 250-0-250 with solid state rectifier.

I pretty much followed the amp startup procedure, although I didn't use a light bulb current limiter (dare to live..). I choose the fuse conservately (200 mAmps). And it worked after some poking and solder improvements..

I measured these voltages:

B+ : 345 VDC
B+1 : 336 VDC
B+2 : 260 VDC

V1 is a JJ 12AY7 (for polical reason's :))
Pin 1: plate voltage A triode: 128 V
Pin 3: Cathode: 1,98 V
Pint 6: plate voltage, B triode: 130 V
pin 8: Cathode: 1,94 V

V2: JJ 12au7
plate voltage: Pin1 and 6 315 V
cathode: 13,38

My Questions:
1. Do these voltages look ok?
2. I have som hum/buzz that goes louder when I turn volume and master up. it's 50 hz. I suspect the heater wires. (this is my first build and first serious soldering project..)

The sound is ok I think, but my guitar skills are poor. I have the idea that when I first played this amp that the distortion kicked in earlier and now it seems to be a little more civilized.

Hope that someone can at least say something on the voltages?

regards,
Johan
 
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King Fan

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Welcome. I don't know this amp, but I'm sure you saw that while trying different development setups, Rob says he ended up liking "B+1 of 390V, B+2 of 304, 150 and 146V on the V1 plates, 2.3 and 2.4V on the V1 cathodes; 375V on the V2 plate and 11.8V on the V2 cathode." Your 375 isn't miles off from 390. How does it sound? What value output cathode resistor are you running? And FWIW, what's your AC heater voltage (pin to pin)?
 

2L man

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Bias/cathode and anode voltages look fine. Rob has designed the power amp to use four times less OT load (50k each triode) what is typical for 12AU7 (12k each triode). Output power comes low and distortion comes higher but obviously that is the goal for these micro amps. In theory shallow and low loading loadline stress power tubes but perhaps these triodes can handle it?

Edit: Obviously here 25k OT produce less distortion when also 6k OT power is ~0,8W. Using 6k OT max power comes ~1,8W.
 

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johanbarnhard

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Thanks for both replies. I realise that my B+1 is lower then designed by Rob. I have: 342 VDC. My heater wires are on 6,2 VAC.

I use the hammond 369JX which comes with a lot of primary wires. I used the 230VAC wiring schematic. But our current power net is delivering 223 Volts.
I therefore now tested with the 220VAC wiring schematic.

That leads to:
B+1: 358 VDC
and 6.5 VAC between the heater wires

This sounds better!

Better for the amp I think, but maybe to hot for the heaters? (and iife time of the tubes?)

I use output cathode resistor on V2: 820 Ohms

Regarding 2L man's response: I need some time to understand this info :)

Cheeers,
Johan
 

2L man

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Don't worry if the filament is 0,2V too high, it won't cause any harm!

When you open the attachment I posted and follow the loadline to the right it go over 500V because loudspeaker/OT load is very small. There comes a possibility for tube suddenly get destroyed! Mechanism there is when Bias current store energy to OT and when tube conduct less, every other half wave form, this energy turns to voltage which show on power tube plate.

On SE amp it is possible to see almost double the B+1, still quite smooth sine wave peak to peak signal. When power stage is driven to distortion even higher voltage spikes appear.
 

robrob

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Johan, your voltages look fine. See my amp troubleshooting page for hum hunting.
 

2L man

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Thank you! I'll chase the hum. And meanwhile build a cabinet and after that I can post some pictures.
Definitely begin detaching wire from 820R/25uF and "power amp ground lug in chassis" (on layout it is black wire) and solder it between 2nd and 3rd filter cap negatives. This simple mod make pre amp current return to power supply thru this wire (instead thru input jack and chassis). It should take less than five minutes.
 

johanbarnhard

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2L man, its wired like this:
07A27A51-92A9-41BE-B1E3-BA0CF0DBFB86.jpeg


i’m not sure what changes you suggest?
Thanks!
Johan
 




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