Have had 2 of these on the bench today, first the F30. I was able to drop the 120K dropping resistor in the neg voltage supply down to 100k, and got some JJ el84s right at 21ma.
Of course the problem is that the same neg V feed also feeds the PI cathode negative voltage, which needs to be at 3V (per schematic). Before I modded the supply, the PI cathode was at 3.6V, and the mod to the dropping resistor to get it to 100K (which also feeds the bias supply) got it to 2.8VDC . I'm assuming that's ok, as it's closer to the target 3V than where it was, AND it got the el84s drawing proper current, but I just don't know enough about running a PI like this.
Is that cathode good at 2.8VDC? Is there a better way to bias this amp? I've got a Rocket 44 up next. Same deal.
Boogie DynaWatt biasing
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Smokebreak3
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Boogie DynaWatt biasing
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Stevem
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Re: Boogie DynaWatt biasing
I would think you’re ok where you are now.
2.8 volts is 8 % closer to the needed 3 volts needed while 3.6 volts is 19% from the needed 3 volts .
If there are no precision tolerance resistors in that local circuit just 10% ones then I can’t see why being within 10% of that needed 3 volts would be a issue.
Sorry, can’t help with your request for a better way to bias this amp, but more than likely someone here can.
2.8 volts is 8 % closer to the needed 3 volts needed while 3.6 volts is 19% from the needed 3 volts .
If there are no precision tolerance resistors in that local circuit just 10% ones then I can’t see why being within 10% of that needed 3 volts would be a issue.
Sorry, can’t help with your request for a better way to bias this amp, but more than likely someone here can.
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Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Re: Boogie DynaWatt biasing
That 15k is the tail resistor of the LTPI.
The voltage drop across the 15k determines the PI plate currents. Should be 15V to 18V (unfortunately the schematic voltages are not consistent).
Power tube bias could also be adjusted (within limits) by changing the 220k/2M2 voltage divider before the grids, though I don't think that's necessary.
The voltage drop across the 15k determines the PI plate currents. Should be 15V to 18V (unfortunately the schematic voltages are not consistent).
Power tube bias could also be adjusted (within limits) by changing the 220k/2M2 voltage divider before the grids, though I don't think that's necessary.
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Smokebreak3
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Re: Boogie DynaWatt biasing
Ok thanks guys. The F30 is fine for now
The Rocket 44 is another story. I actually had the luxury of having 2 in at the same time. It has the same silly dynawatt arrangement with the neg V supplies and PI setup as the F30.
This one hummed pretty badly even with good tubes/bias and new caps. The PT was putting out 450VDC on the poor EL84s, with the molex connectors to the board pretty brown.
Schematic shows 390V plates IIRC. So I checked the other one, and got the exact same readings...loaded/unloaded, even the AC off the secondaries was nearly identical. So the schematic is BS.
So yea, these repairs came with a disclaimer.
The Rocket 44 is another story. I actually had the luxury of having 2 in at the same time. It has the same silly dynawatt arrangement with the neg V supplies and PI setup as the F30.
This one hummed pretty badly even with good tubes/bias and new caps. The PT was putting out 450VDC on the poor EL84s, with the molex connectors to the board pretty brown.
Schematic shows 390V plates IIRC. So I checked the other one, and got the exact same readings...loaded/unloaded, even the AC off the secondaries was nearly identical. So the schematic is BS.
So yea, these repairs came with a disclaimer.
Re: Boogie DynaWatt biasing
Man, that is a very goofy circuit, and the patent is pretty weird too.
The owner's manual devotes 2-and-a-half pages as to why Randall Smith will not grant you a bias control, because the evil tube vendors have a conspiracy to sell you their defective tubes, but Mesa of course, have absolutely perfect tubes that they select to work in their amps... Which isn't conspiracy to sell you overpriced tubes.
The bias and tail of the PI are tied together to a -160V power node, through a big 120k resistor. There's negligible current in the bias node, but about 1.2ma in the PI tail. The bias voltage divider relies upon the PI tail to pull down the -160v, which is further divided to get to the depicted -11v on the EL84 grids.
If the total current through the PI goes up, the bias voltage will go more positive, and if the total current through the PI tail goes down, the bias voltage will go more negative.
It can be difficult to measure the tail voltage, so we can calculate it via the drop across the PI plate loads and thus, the current at the bottom of the tail.
I would also point out that the 1000uf/25v capacitor that filters the bias supply, will suffer significantly if the PI tube is removed or is not drawing current for any reason, such as during filament warm up...
I would expect that Mesa intended for the EL84 to run at relatively idle low current, owing to the 400v plate supply, but 333v screen supply.
Altering the bias circuit may have unintended consequences, given the goofy operating principle of the thing.
The owner's manual devotes 2-and-a-half pages as to why Randall Smith will not grant you a bias control, because the evil tube vendors have a conspiracy to sell you their defective tubes, but Mesa of course, have absolutely perfect tubes that they select to work in their amps... Which isn't conspiracy to sell you overpriced tubes.
The bias and tail of the PI are tied together to a -160V power node, through a big 120k resistor. There's negligible current in the bias node, but about 1.2ma in the PI tail. The bias voltage divider relies upon the PI tail to pull down the -160v, which is further divided to get to the depicted -11v on the EL84 grids.
If the total current through the PI goes up, the bias voltage will go more positive, and if the total current through the PI tail goes down, the bias voltage will go more negative.
It can be difficult to measure the tail voltage, so we can calculate it via the drop across the PI plate loads and thus, the current at the bottom of the tail.
I would also point out that the 1000uf/25v capacitor that filters the bias supply, will suffer significantly if the PI tube is removed or is not drawing current for any reason, such as during filament warm up...
I would expect that Mesa intended for the EL84 to run at relatively idle low current, owing to the 400v plate supply, but 333v screen supply.
Altering the bias circuit may have unintended consequences, given the goofy operating principle of the thing.