Hello,
This is less of a question about a specific amp then about a design topology. I've noticed in the Redbear MKX-50 and Marshal JCM800 amps, the second gain stage has the plate of one side directly connected to the grid of the next side, with no coupling cap in between. Is this a common topology used to achieve overdrive? Also, wouldn't this make the tube wear out extremely quickly?
That may explain why the user manual says to change out preamp tubes once a year.
But, it does not explain why the Redbear MKX-50 schematic says that pin 1 (plate) is 263V, and pin 7 (second stage grid of same tube) is 0 V.
I must be misreading, or missing something.
View attachment redbear-cubcombo-mkx50-amplifier-schematic.pdf
This is less of a question about a specific amp then about a design topology. I've noticed in the Redbear MKX-50 and Marshal JCM800 amps, the second gain stage has the plate of one side directly connected to the grid of the next side, with no coupling cap in between. Is this a common topology used to achieve overdrive? Also, wouldn't this make the tube wear out extremely quickly?
That may explain why the user manual says to change out preamp tubes once a year.
But, it does not explain why the Redbear MKX-50 schematic says that pin 1 (plate) is 263V, and pin 7 (second stage grid of same tube) is 0 V.
I must be misreading, or missing something.
View attachment redbear-cubcombo-mkx50-amplifier-schematic.pdf
V2B is a cathode follower, not an amplifying stage. The important issue is not grid voltage (with respect to the ground), but grid-cathode voltage.
The voltage table is wrong. V2B grid voltage must be the same as V2A anode voltage.
The voltage table is wrong. V2B grid voltage must be the same as V2A anode voltage.
This is right out of the Fender 5F6-A Bassman. The cathode follower is used to drive the tone stack. See how it passes by V3A's plate to the 33k resistor? This is the slope resistor which sets up how the tone stack operates. Some amps have this as a 56K like the Bassman, some at 100K. It just depends on how you want your tone stack to operate. Of course the value of the caps matter too. The cathode follower cannot add gain and usually operates at a slight loss but serves as an impedance matching circuit to drive the tone stack more easily.
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Interesting. I've worked on a few cathode followers, the most memorable being the Ampex 601, but this had a coupling cap in it.
If I might "drift" the topic of this thread slightly, the amp in question is (you guessed it) the Redbear MKX-50. The issue is "lack of bass response".
My plan is to check all voltages, check the PSU for its performance and AC ripple, and go from there. Anyone got any "AHA" ideas that are common pitfalls for bass response in these amps?
Otherwise, I hope to nail it down once I get this on the bench.
I'm not familiar with this specific amp but you could try increasing the value of the .022uF coupling caps to .047,.068, even to .1uF. Watch out if you do it in the stage between the phase inverter and power tubes. It could possibly lead to blocking distortion if that value gets too high. Try the preamp stages first. You could temporarily parallel caps to the existing ones to see what value you like best. You could also bypass the 470pF cap after V1B to reduce some treble. Less treble=more bass? Not sure how much that one will help. Make sure to observe voltage ratings of the caps.
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The Redbear is full of highpass filters in the preamp stage. Mainly the 470pF's parallel to the 470k resistors and gain pot. The one just after the 'low' input attenuates the frequencies below 338Hz by -3.3dB. The one after V1B attenuates with -6dB below 720Hz(!). So yes, bass response will suffer greatly. And were not even talking about the partially bypased gain stages! The 22nF after V1B is fine. It's -3dB point is around 7Hz. Remove the 470pF caps and the amp will probably sound a lot warmer. Or remove them one by one to listen what the effect is per cap.
EDIT: concerning the 'buy new tubes every year' part. It's for commercial purposes only.
EDIT: concerning the 'buy new tubes every year' part. It's for commercial purposes only.
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Hi funk1980, how did you establish the resistance value to use in the formula for calculating the -3dB freq. for the .022uF cap after the V1B stage? Also, would he gain enough bass to use say a 10-25uF cap on both V1 stages in place of the .68uF cathode bypass cap to make a difference? Is fully bypassing a gain stage as effective as changing a coupling cap for tone shaping, or as you suggested, removing the 470pF caps which essentially does the same?
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Hi Guys
This may be an entirely uphill battle if your speaker cabinet is too small to produce decent bass output, or if you are using a tiny driver. Some info about the rest of your system would be helpful.
The Redbears are just copies of standard Marshalls with a minor difference here and there - nothing that would effect tone. The use of 5881s should put it in the "bassier than an 800" category already inasmuch as the harsh high-end of the EL-34 is not there.
skewing the impression of tonal balance.
Sometimes the need for more bass is really the need for less treble. That's where the posts above are headed: get rid of the bright cap on the 'gain' pot, then try lifting or increasing the value of the 680nF on the first stage., then maybe remove a 470pF from one or more of the interstage attenuators. All of these parts were added to the plexi (and retained in the 800) to make their overdriven sound tighter and less flatulent than their original Bassman values.
Note that the cathode follower does not effect distortion as it has no gain. It does however kill tone and make the EQ far less effective. The simple movement of one connection allows tying the EQ to the plate of the preceding gain stage and suddenly the EQ will be lively and really do something. TUT6 explains why this is so and why this stage is even there.
Have fun
This may be an entirely uphill battle if your speaker cabinet is too small to produce decent bass output, or if you are using a tiny driver. Some info about the rest of your system would be helpful.
The Redbears are just copies of standard Marshalls with a minor difference here and there - nothing that would effect tone. The use of 5881s should put it in the "bassier than an 800" category already inasmuch as the harsh high-end of the EL-34 is not there.
skewing the impression of tonal balance.
Sometimes the need for more bass is really the need for less treble. That's where the posts above are headed: get rid of the bright cap on the 'gain' pot, then try lifting or increasing the value of the 680nF on the first stage., then maybe remove a 470pF from one or more of the interstage attenuators. All of these parts were added to the plexi (and retained in the 800) to make their overdriven sound tighter and less flatulent than their original Bassman values.
Note that the cathode follower does not effect distortion as it has no gain. It does however kill tone and make the EQ far less effective. The simple movement of one connection allows tying the EQ to the plate of the preceding gain stage and suddenly the EQ will be lively and really do something. TUT6 explains why this is so and why this stage is even there.
Have fun
The resistance acting with the 22nF cap are basically the two 470k resistors in series. The 470pF cap can be ignored since its reactance at the low frequency point is huge (48MegOhm @ 7Hz).
If using a bigger cathode bypass caps gives the desired tone can easily be tested, since using the 'out' setting will have the same effect on frequency as using a large bypass cap. The gain will of course be higher with a bypass cap in place. Using a small(ish) bypass cap is a very effective way for tone shaping! And many amps use it as the well known 'presence control' by making the effect variable with a pot between the bypass cap and ground.
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