hello all,
has any one tried this in an amp? IXYS depletion mosfets are now available for this, so i would like to know from those who tried and what the results were...
has any one tried this in an amp? IXYS depletion mosfets are now available for this, so i would like to know from those who tried and what the results were...
Hi Tony,
Sure, they work great. Nice on the output tube if you have a preference for direct coupling and don't have an aversion to cathode bypass capacitors.
Also very useful for experiments, especially with direct coupled circuits. Using a cathode CCS with the output allows the cathode to find its voltage at the current you want even though the grid voltage is being held by the driving stage .
Together with an adjustable voltage (CCS or Gyrator) on the plate of the driver you can take the operating point for the output tube anywhere you want for a test drive. Also great for trying different inductive loads as the CCS will eat the plate to cathode voltage differences caused by differing inductor DCRs.
Also works for limiting DC imbalance in a PP OT. Not needed in most cases I guess but works well with old regulator tubes that have poorly matched and sometimes wandering sections. One in series with each cathode and the cathodes capacitor coupled.
'Hope this is the sort of feedback you're looking for .. . . or were you looking for a 'liked/didn't like the sound' kind of answer?
Sure, they work great. Nice on the output tube if you have a preference for direct coupling and don't have an aversion to cathode bypass capacitors.
Also very useful for experiments, especially with direct coupled circuits. Using a cathode CCS with the output allows the cathode to find its voltage at the current you want even though the grid voltage is being held by the driving stage .
Together with an adjustable voltage (CCS or Gyrator) on the plate of the driver you can take the operating point for the output tube anywhere you want for a test drive. Also great for trying different inductive loads as the CCS will eat the plate to cathode voltage differences caused by differing inductor DCRs.
Also works for limiting DC imbalance in a PP OT. Not needed in most cases I guess but works well with old regulator tubes that have poorly matched and sometimes wandering sections. One in series with each cathode and the cathodes capacitor coupled.
'Hope this is the sort of feedback you're looking for .. . . or were you looking for a 'liked/didn't like the sound' kind of answer?
thank you, just the answer i was looking for.....
reason i asked is that i want to make a set amp using the 2A3 and the 6550 in the same chassis, of course not a the same time....
i have an opt made with 3.5k/5k taps, my reasoning is that if i can peg the cathode currents to say 60mA, then things can be easier..
crazy idea? yes, but why not?
I believe you would need a large value bypass capacitor. I flirted with the idea in spice, but never in practice. A CCS on the plate and a resistor on the cathode is rock solid with the bonus of a much smaller bypass cap than more traditional passive or reactive plate loaded stages. A CCS in the cathode does allow more room for "slop" in a DC coupled amplifier, but it seems, at least to me, more work than reward. A gyrator loaded stage driving a CCS loaded stage would be more practical, in my opinion.
i will be using cap coupling, the voltage amp i am using is a 6p15p running at slightly over 10 ma. cathode current, i will use a g2 regulator for the 6p15p...
i understand that with a 60ma CCS at the cathodes, then either of the 2a3 or the 6550 gets is grid bias automatically...
i understand that with a 60ma CCS at the cathodes, then either of the 2a3 or the 6550 gets is grid bias automatically...
I did that, but did not like it. When the amp approached clipping bias voltage went away. It caused clipping more audible and less graceful. With 300B I would go in the opposite direction, with a fixed bias and G2 drive.
anyway, i will implement that amp to have both options, CCS or cathode resistor cum bypass caps....
I have successfully implemented what you are inquiring about. Irons out unmatched output tubes, but of course, the bias will be unequal, so asymmetrical driving needed to get full power on either tube - then the balance controls need to be adjusted. But nobody drives these low-powered amps to full power.
The CCS in the driver stage has the advantage of giving lots of linear volts to the final tube. I even went one further and paralleled the two sections of the 6SN7s. I hear a big difference in tone with the CCS in the plate of the combined 6SN7s. But the bottom line is that the amp sounds just great (at least to my ears).
Attachments
If an amp sounds 'bad' when clipping, do one of three things:
Turn the volume down
Build a more powerful amp
Play your guitar on the clipping amp.
I don't think 1 dB less power out of an amp has ever killed anyone (a 10 Watt amplifier that only put out 7.9 Watt).
But stranger things have happened.
But an amp with fixed G1 bias, and driving signal to G2 does sound very interesting.
But 300B does not have G2, so a beam power tube could fit the bill.
Turn the volume down
Build a more powerful amp
Play your guitar on the clipping amp.
I don't think 1 dB less power out of an amp has ever killed anyone (a 10 Watt amplifier that only put out 7.9 Watt).
But stranger things have happened.
But an amp with fixed G1 bias, and driving signal to G2 does sound very interesting.
But 300B does not have G2, so a beam power tube could fit the bill.
Last edited: