Hey all, I have this nice Magnavox 138BB tube amp that I use as a Guitar amp (stock) and driven with a Joyo American Sound pedal/preamp. Works ok, but not very loud...so I added a LND150 Gain stage in front of it. Now plenty loud but has quite a bit of hum...sounds like 60hz... I like this setup....pushes the 6V6 nicely.
The 220 ohm source resistor leaves me with around 125vdc at drain (1/2 B+), so I think the LND150 is biased correctly....but would like to reduce the hum... any advice? Thought of reducing the R2 from 1M to 470k....
I am "stealing" B+ from the two high side of the 470k anode resistors in maggie amp.
Without the LND150 circuit, no hum at all...
The 220 ohm source resistor leaves me with around 125vdc at drain (1/2 B+), so I think the LND150 is biased correctly....but would like to reduce the hum... any advice? Thought of reducing the R2 from 1M to 470k....
I am "stealing" B+ from the two high side of the 470k anode resistors in maggie amp.
Without the LND150 circuit, no hum at all...
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I've no experience using those high voltage depletion mode FET's but the gain looks to be sky high with 100k and a bypassed 220 ohm. As I'm sure you know, the hum is coming from something external and 60 Hz suggests stray pickup rather than supply ripple which would be 120Hz. If it is 60 Hz it will be a deep pure tone. I suspect the actual gain in that configuration will be device dependent and depend on the transconductance of the FET.
Lowering R2 just lowers the input impedance which achieves little once you connect a source to it although I suppose it depends to some extent what you are connecting.
Obvious question has to be whether you need all that gain. I would try rehashing the values of the 100k and 220 ohm to give lower gain and also not directly bypass the source resistor.
Lowering R2 just lowers the input impedance which achieves little once you connect a source to it although I suppose it depends to some extent what you are connecting.
Obvious question has to be whether you need all that gain. I would try rehashing the values of the 100k and 220 ohm to give lower gain and also not directly bypass the source resistor.
Thanks Mooly! The site suggests these operating points for a simple boost...
http://www.hawestv.com/amp_projects/amp_solid_tube/fetpreamp_200v1.htm
http://www.hawestv.com/amp_projects/amp_solid_tube/fetpreamp_200v1.htm
The operating points (100k and 330 ohm in the link) set the drain voltage and the gain with a bypassed source resistor is sky high. The 1uF in your diagram gives a gain boost at higher frequencies, the cap is far to low in value to maintain gain at LF.
R2 does not alter the gain of the FET stage, all it does is form an input attenuator with R1 and the combined resistance of the two added together becomes the input impedance. That might load the source to much depending on what that is.
The gain with no bypass cap (if the FET could deliver) would be 100k/220 which is around '450' and with a suitable bypass cap it goes much higher still so its no wonder you hear hum and noise.
R2 does not alter the gain of the FET stage, all it does is form an input attenuator with R1 and the combined resistance of the two added together becomes the input impedance. That might load the source to much depending on what that is.
The gain with no bypass cap (if the FET could deliver) would be 100k/220 which is around '450' and with a suitable bypass cap it goes much higher still so its no wonder you hear hum and noise.
Thanks again Mooly...if the that schematic were of such high gain that hum would be unavoidable, you would have thought there would be some mention of it. I did put a 1k source resistor on the circuit as is and still hums... The 1uf cap was put in there instead of the 150uf to raise HF only...The "source" will never be a guitar, but that preamp pedal.
The concept I am trying to convey is having all the volume and tone control be done before the preamplification stage in the tube amplifier... seems like a large part of the signal can be trampled the way tubed guitar amps all are (with volume and tone controls between fist and second gain stages)
I just dont understand how tubed guitar amps can have so many gain stages (upwards of four on some?) and somehow spare all the noise too.
The concept I am trying to convey is having all the volume and tone control be done before the preamplification stage in the tube amplifier... seems like a large part of the signal can be trampled the way tubed guitar amps all are (with volume and tone controls between fist and second gain stages)
I just dont understand how tubed guitar amps can have so many gain stages (upwards of four on some?) and somehow spare all the noise too.
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You have to remember that no amplifier, no matter how high the gain hums by itself. The hum gets into the chain through any combination of stray pickup, modulation from the supply rail or through incorrect grounding. The fact the gain is so how makes it so very prone to pickup though.
If you lift R1 and it still hums then try reducing R2 to say 10k and see if it changes anything. If not then its most likely rail or grounding issues.
If you lift R1 and it still hums then try reducing R2 to say 10k and see if it changes anything. If not then its most likely rail or grounding issues.
I am returning the ground from input to the signal ground on the Magnavox 138BB. It has the common ground to the chassis like most of the amps back then...
I will raise the source resistor from 220 ohm to 1.5k ohm to lower gain a bit, and remove the 1uf cap...
I will raise the source resistor from 220 ohm to 1.5k ohm to lower gain a bit, and remove the 1uf cap...
OK, so I reduced source to 1.5k and much better as far as hum/noise...lost a little bit of "snappyness"...may put that 1uf cap back in...
The cap brings the gain back to maximum as it appears as a 'short' to AC. If you must use it then try a resistor in series with it.