Mesa Boogie Rocket 4Forty

[lowell]

It's there, and is used for the bipolar low v supply. I thought the same thing though when I saw that and that is indeed my plan B.

[lowell]

G1 I misinterpreted your suggestion

How about lifting the coupling caps from the PI.

I had those lifted already with no effect. I was thinking you meant the cap INTO the PI grid.

[lowell]

This is baffling. 358vAC to the diode cathode, -22v on the anode... sorry if I misposted earlier. The -8v is at the power tube grids. And that is with the anode lifter and using a sub cap! Any objections on rewiring the thing to a more traditional arrangement? LTP and use the 70vac tap?

[g1]

My objection is that it worked before and should be fixed for the benefit of all who run into the same problem.
Just a reminder that the cap shown is 25V so we don't want to lift anything that will cause that to be exceeded. (maybe it got toasted already?)
You have the PI coupling caps disconnected and the PI removed, yet something is still pulling down the voltage. Maybe a leaky board or tube socket? Are the sockets board mounted?

[lowell]

I worked on it till 3am ish. With the diode anode lifted AND the 120k lifted (the end of each which is connected together) I found that the bias stayed around -8v with the PI installed and standby in ON position. This is the best I'd seen it thus far. The solder pad of those points I lifted were reading about 1.6M to ground. I couldnt see why that should be...but even so I wouldn't think that's significant enough load to pull down the bias.

So...I kept those lifted and changed the 120k to 82k. This puts the grids around -20v.

My plate voltage at -20v on the grids is 440v. Bias is just over 70% at 21ma per tube.

I'm happy with it. It works.

[kHz Tube Amps]

Hi Lowell, If you can find the time I would really like to get a copy of the Rocket 44 schematic. Thank you very much! Rich

[Jazz P Bass]

Here it is: Rocket 44.zip

[teemuk]

This amp has something odd going on with the schematic, typical for boogie and likely to be incorrect. The phase inverter cathode connects to the bias supply via a 15k resistor. I see NO CATHODE RESISTOR in the schematic. It just say E on the cathode, and E is also the end of that 15k from the bias supply.

There is a cathode resistor. It is that 15K resistor that connects the cathodes to the negative voltage supply.

BTW, "bias supply" is only the one titled "F". The only common thing with PI supply and bias supply is that they both connect to same negative voltage source. Voltage dropping resistors, that isolate the two circuits from each other rather effectively, also generate proper voltage drop in combination with the voltage divider formed by 120K series resistor, series resistor hooking to either PI or bias circuit, and load presented by the aforementioned circuit.

Very weird. I get theoretically why they do it, for more PI headroom, but still just...different.

Exactly. It's only "different".

Different in comparison to the "generic" LTP circuit, which is "self-biased". This one is "fixed-biased" circuit. That's all.

[kHz Tube Amps]

Mesa Boogie Rocket 44

Here it is: [ATTACH]47691[/ATTACH]

Hey there JPB, Thanks for the schematics! Rich

[lowell]

I have another one of these on my bench. Guess what the issue is. Yep, the same. This is not the same amp I worked on before, it still has the stock 120k resistor in the bias supply. I'm still baffled as to why I had to modify the bias supply to fix this amp. Let's see what I find.

[lowell]

Lifted the anode of the bias diode, removed the 120k dropping resistor, floated the junction of the two, ran a floating wire to the 33k/15k junction. Fixed. Conductive board.

[Jazz P Bass]

Lowell.

Welcome back!

[lowell]

Thank you sir! I've been on a real estate investing adventure for the last 1.5 yrs so haven't been doing as much amp work. Still love audio electronics though! And this forum is incredible... love it.