Is the Blue Robbie starved-plate?

[R-AP.SCI]

No, it is not.

[Dizzy77]

Originally Posted by R-AP.SCI

No, it is not.

Do you mean it is not starved plate or it is not high voltage?

[R-AP.SCI]

It is not high voltage.

[Dizzy77]

Thanks for clearing that up for me. Surprising that when people criticize the unit they don't bring this up don't you think? Ah well, bizarre design for the price it seems.

[Jazz Noise]

Originally Posted by Dizzy77

Thanks for clearing that up for me. Surprising that when people criticize the unit they don't bring this up don't you think? Ah well, bizarre design for the price it seems.

"Starved Plate" is pretty vague at times so it's not a fair criticism of a unit. A 300V power rail is a sign of cost, but not quality. Be aware that some modern circuits will use charge-pumps to get a the desired power rails from low voltage transformers, so the input voltages don't neccesarily mean anything.

What Should Be The Cut Off For Calling a Circuit "Starved Plate"

This thread might give you some good clues into why a lower plate voltage might change the sound - it's to do with linearity, really, but it gets more complex as more factors of the tube must be taken into account. I'm not quite as ontop of Toobz as I'd like to be, but there's some great info on diy-audio on Delta-Mu, plate voltages and output impedances if you're inclined!

[Jolly_Rogers]

Hmmm, i'm confused. Does Blue not use high-voltage to save money or are they getting the sound the way the want without using high-voltage? In other words, would it be better if it were high voltage?

[tINY]

There are 12Axx pre-amp tubes that get decent linearity at some low voltages (12v grounded Cathode). It should definitely add some distortion to the signal, which is the whole point of the tube in this case....



-tINY

[Jolly_Rogers]

Originally Posted by tINY

There are 12Axx pre-amp tubes that get decent linearity at some low voltages (12v grounded Cathode). It should definitely add some distortion to the signal, which is the whole point of the tube in this case....



-tINY

Are you saying if it were high voltage then it would have more distortion? And since Blue probably want a clean sounding mic-pre they use low voltage?

[R-AP.SCI]

I believe that @tINY was making an overall statement of fact that there are some tubes designed to be linear at low voltages. Further evidence of his statement not referring to the Robbie specifically (but just to tubes in general) is the fact that the Robbie uses a 6dj8/6922 which is not part of the 12axx family. Hence his following sentence of "It should definitely add some distortion to the signal..." THAT was a direct reference to the Robbie and its tube being used at low voltages.

The question of "why" would require and only result in speculation on our part. One could theorize that in order to save space and cut cost they used lower voltages (as in general higher voltages require larger parts....), or maybe they simply wanted it to sound that way and the savings came as a fortunate side effect....or BOTH, or any wide variety of conclusions can be arrived on....(yes, cost IS the major determining factor in many companies, so one COULD say this is the most likely reason. But that would still require a bunch of presumption as to it being THE primary reason...long story short, we most likely will never know with any true authority).

There are also many other design considerations to be had (things such as the input and output -as far as I know- are solid state and run on the same dual rails as the tube stage which implies certain performance goals that MIGHT be more easily achievable with a lower plate voltage). One has to take into consideration the bias conditions, not to mention interstage coupling and the conditions within which it was designed to work in order to achieve their performance goals....and a myriad of other things. Thus without a schematic to be analyzed and studied it wouldn't be wise to start presuming why they did what they did or how it performs (bench wise at least) in order to say it would be "better" with greater plate voltage. Taken as an isolated fact, without context, in general yes, the tube stage would achieve a greater amount of linearity in an absolute sense...but that doesn't mean very much, nor does it state that with higher voltages to the plate in this particular design would be of a great help.

I hope that helps.

[Jolly_Rogers]

Originally Posted by R-AP.SCI

I believe that @tINY was making an overall statement of fact that there are some tubes designed to be linear at low voltages. Further evidence of his statement not referring to the Robbie specifically (but just to tubes in general) is the fact that the Robbie uses a 6dj8/6922 which is not part of the 12axx family. Hence his following sentence of "It should definitely add some distortion to the signal..." THAT was a direct reference to the Robbie and its tube being used at low voltages.

The question of "why" would require and only result in speculation on our part. One could theorize that in order to save space and cut cost they used lower voltages (as in general higher voltages require larger parts....), or maybe they simply wanted it to sound that way and the savings came as a fortunate side effect....or BOTH, or any wide variety of conclusions can be arrived on....(yes, cost IS the major determining factor in many companies, so one COULD say this is the most likely reason. But that would still require a bunch of presumption as to it being THE primary reason...long story short, we most likely will never know with any true authority).

There are also many other design considerations to be had (things such as the input and output -as far as I know- are solid state and run on the same dual rails as the tube stage which implies certain performance goals that MIGHT be more easily achievable with a lower plate voltage). One has to take into consideration the bias conditions, not to mention interstage coupling and the conditions within which it was designed to work in order to achieve their performance goals....and a myriad of other things. Thus without a schematic to be analyzed and studied it wouldn't be wise to start presuming why they did what they did or how it performs (bench wise at least) in order to say it would be "better" with greater plate voltage. Taken as an isolated fact, without context, in general yes, the tube stage would achieve a greater amount of linearity in an absolute sense...but that doesn't mean very much, nor does it state that with higher voltages to the plate in this particular design would be of a great help.

I hope that helps.

It does, very interesting.

[drtechno]

Its the headroom spec that gets me. 34db of headroom out of a 90V triode? full wave bridge rectified 120VAC is approx 180V. most likely a cascode like design (like the popularity of TCJ Akido pre amp) if thy went this direction.

the only thing is gain. 33 (6dj8) compared to 100 (12ax7). but that is halved in cascode designs.

A direct coupled grounded cathode with a cathode follower would work (with a little help using a ccs) that would be a gain of like 33 but would be able to drive low impedance output by use of the cathode follower.

btw 180V would somewhere 70% of maximum rating of a 6922. the headroom characteristic is more from this as this is about twice the standard class A plate voltage (if they are using any of the grounded cathode designs). Help from a voltage doubler/tripler could raise the voltage higher, but I haven't got any of the 6922's to run reliably past 450V.

But who has one of those thingies to measure the plates?

Besides, starving the plate actually compresses the signal slighly and induces fuzz. That was the things the old tube guitar pedals are based off of.