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Discussion in 'Amp Central Station' started by Brewbaker, Apr 29, 2005.
What are the classic symptoms?
Slightly under-biased sounds cold.
Way under-biased tends to sound harsh and gravelly.
But how far under are we talking and with what tubes?
Playing around with the bias on my 68 Pro Reverb with GE 6L6GCs, I found that from 25mA to 35mA doesn't change the sound in any way that matters. With a plate voltage of 450V, I'm running them at 26mA, which could be considered cold. Sounds great.
If it sounds good to you, then that setting will extend the life of the power tubes..You will be happy. When biased too cold, you will hear the dreaded SS type of harshness. The hotter you set them the sooner the tubes will breakup. Red plates equal too much current draw and extremely short tube life...like minutes?
A technical correction...
It sounds backwards, but under-biased means the tubes are running too hot. You may see red-plating. Over-biased means the tubes are running too cold and in the crossover-distortion zone. Under-biased amps sound mushy and blow fuses. Over-biased amps sound harsh.
More specific info...
The reason I posted this question is that my SFPR is sounding very cracklely and not smooth--harsh and fizzy distortion--when it starts to overdrive--say around 6-7. It has the Stokes and Paul C mod, fresh JJ 6V6s and new JJ preamp tubes. I've tried both a 12AX7 and a 12AT7 in the PI position but no real difference. Before opening it up--yet again I thought I'd get some background on bias targets for these tubes. It has an adjustable bias pot and I have a Weber BiasRite--so I'm ready to go. What range should I shoot for?
My brown Princeton came OEM with a bias current of 16mA @ 395V B+. I added a pot, and set it for 23mA @ 360V B+.
Really couldn't tell that much difference in tone.
I know what the Stokes mod is, and what it's all about..but not sure about the other mod. Seems like it's something to do with the Phase Inverter.
Princetons sound best (IMO) when you don't mess with them.
I have a silver face princeton non verb, that Ilike alot.
first , static plate dissipation. A 6v6gt max dissipation is 12watts.
watts , is a voltage , current relationship, such as , volt x amps(ma) = watts.
My princeton , with a 5ar4 , at 425v x .025 amps(25ma)= 10.625 watts.
the princeton has a very marginal PT, 10.5 watts is all I can get out of mine. The plate voltage goes down very quickly , when you try to bias it hotter.
I liked the way it sounds biased with the plate voltage, between 420and 425volts.
I tried to bias in the 28ma range and the plate voltage droped resuting in tone like you are describing.
The margial PT is the reason the voltage dropes so fast.
but I love the sound of my princeton, the only mod I`m tempted to do is the stokes mod, but I haven`t done it yet.
hope this helps
Racehorce, you're just backwards! That's a slightly-different approach to biasing, as opposed to the current-draw method. You've zeroed in on the voltage range that sounds best to you, which is a lot easier to tweak than "50-70% of maximum plate dissipation" that everybody and their dog uses. If you wind up in the 50-70% range, perfect!
I understand the voltage current relationship. Static plate dissipation is vlotsx amps = watts of static plate dissipation.
A point well take is that you do want to to hit in the 50-70% range of plate dissipation, generally.
My specific instance was a princeton. the power supply is marginal, I couldn`t get mine above 10.5 watts because of the power supply design.
the current to voltage ratio is adjustable ,@ more current thruogh the tube , less plate voltage.
the higher plate voltage sounds better in this application
the static plate dissipation stays the same , unless in this application you add filtering or a bigger transformer.
but then its not a prince ton any more
Racehorse, I hope you didn't misunderstand me and think I was poo-pooing your method or disagreeing with it, because I certainly wasn't. I think it's a good approach, provided your tubes aren't biased too hot. I wonder how consistent it is. I can't imagine it would be any less so than the current draw method. It really all comes down to the individual tubes, so you would have to compensate a little from pair to pair, theoretically...
I think a lot of folks get hung up on this bias 'current' deal.
Generally, most amps we use are designed and biased for Class AB1 operation. The output sections are not designed with the idle current as the primary consideration..but the relationship between the signal that drives the power tube grids from the Phase Inverter and the signal at the output tube plates.
The B+ is then worked to keep the plate dissipation at the level of Class of operation. Consider that in Class AB1, the tubes will dissipate max power before it reached max volume...and the 70% 'rule' gives the tubes their safety margin.
The most reasonable thing to do with setting idle currents these days..since older amps have higher B+ that they used to...is do your best to keep the tubes running at 70%.
If your B+ has gone up too far, consider swapping in GTA's, or use tubes (like JJ's) that have extended plate power factors.
How do I check the biasing my tubes? What should it be for a Bassman RI?
According to the schematic at Mr. GearHead, the bias voltage is set to -54VDC.
With 490V B+, I'd expect to see about 30 - 35mA per tube....knowing how conservative Fender is with tube operation.
The HRDlx runs at 30mA @ 425V B+, and the HRDville runs 30mA @ 480V B+ .......for comparison - both, 6L6GC amps.