2024 Monkeymatic Fiddy 1987

[xtian]

I'm reusing the DV Mark chassis and iron, along with the Marshall PCB from Andrea to build out a "plexi".

Space constraints mean I have to lay the preamp tubes down, and fab a riser for the power tubes.

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[xtian]

@frusciante89 : I sent you an email yesterday. Ran into trouble with this PCB. Hope you have your ears on!

[pdf64]

I'm not keen on the OT being so close to the input stages. A screening plate can help to keep things stable in that type of scenario so perhaps think about how one might be squeezed in before everything is set in stone.

[xtian]

Tube PCBs finally arrived from China. I 3D printed brackets to mount the preamp board with tubes horizontal so they fit under the OT. (I know, Pete, too close! But there are lots of "oh, well, whatever" moments trying to work inside this very non-standard chassis. On we go.

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[WhopperPlate]

Shielding plate between input and OT for sure will be desirable . I have dealt with oscillation issues working on customers amps with further proximity than this . Hate to see you have issues after all that work .

[xtian]

I'm almost ready to put in some tubes. But I'm stumped by the bias circuit, which is not working (max output is about –0.3vDC). All components test correctly out of circuit, and the diode and filter caps are oriented correctly. I lifted the anode of the 1N4007 diode D7, and I'm getting no significant DC at its free end. My HT circuit is working fine, putting out 410vDC at idle with no tubes and with light bulb limiter in line.

I get 153vAC at the left-hand orange wide (where I wrote "HT" in Sharpie on the board). Following the 220K/2W resistor, I meter 190vDC. But after the diode, nothing. And this is the second diode...they both measure 0.5v drop on my meter.

All voltages measured with respect to ground.

So WTH?

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[xtian]

I further simplified things by just clipping a 1N4007 into my meter probe, and touched it to the orange HT wire. With diode forward biased, my meter reads about 200vDC. But reverse the diode, and I get 0vDC.

Is this because I'm using a FWBR? The toroidal PT has no HT CT.

[xtian]

OK, no worries. I'm now using the PT's 46v-0 tap for the bias circuit.

Amp is working...but Volume controls are not working at all. Low signal is passing, and tone stack works.

More tomorrow, thanks for looking.

[xtian]

Volume problem was I simply forgot to connect the input to V2a. Sounding good now...and I only have 15mA per tube...need to adjust bias circuit.

[pdf64]

... Is this because I'm using a FWBR? The toroidal PT has no HT CT.

Yes, sorry I didn't notice the other pair of diodes hiding behind the orange wires
With a FWB rectifier, the HT winding sits at an 'offset' V DC level of about half the rectified voltage.
So there's no way to get a negative voltage out of that using a resistor dropper as per the 1987 / 2204.
Rather a blocking cap dropper can be used, to let the AC past but leave the offset voltage behind, as per some models in the JCM900 series eg C15 https://www.drtube.com/schematics/marsh ... -iss10.pdf
Merlin explains (scroll down to capacitor coupled bias supply) http://www.valvewizard.co.uk/bias.html

[xtian]

Pete, I do, indeed, see a little oscillation from the OT-near-preamp stages, mostly mitigated by moving the OT wires away. Sounding good, though!

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[WhopperPlate]

Pete, I do, indeed, see a little oscillation from the OT-near-preamp stages, mostly mitigated by moving the OT wires away. Sounding good, though!

What happens with the oscillations if you dime the presence and treble?

[xtian]

What happens with the oscillations if you dime the presence and treble?

White noise increases, but draw from wall remains steady at about 46 watts.

[cdemike]

I'd be interested to know how the power supply is acting! I did a bunch of digging before my last Marshall build and chickened out since I wasn't sure if I'd be able to get the same levels of power supply sag.