5E3/AB763 Hybrid Questions...

[EETStudent]

I have completed a recent project where I converted a Bogen CHB-35 into an AB763 DR with a touch of 5E3. It is a single channel, AB763 normal channel preamp, LTPI, cathode biased 6V6 for about 18 watts, 5E3 OT and a JJ EZ81. The amp sounds really fantastic, but as always Im looking to expound upon that

1. Explain the effects of different resistance values in the negative feedback circuit. There currently is no negative feedback. Also, help me understand how a Presence control could be developed from negative feedback.

2. What are your thoughts on the phase inverter coupling cap? I've seen anywhere from 500pf to .01uF. I am using a .001 uf currently. Aside from more or less bass, how would larger or smaller values affect tone/distortion/output?

3. Explain the affects of the 'tail' resistor in a LTPI? I am using a 22k per AB763 cicuit, but Im seeing values as low as 6800.

4. Any experience with a triode/pentode switch in a 5E3 style output stage? Seems like an interesting thing to try

5. I can't find a lot of clear information on the internet regarding the EZ81 rectifier tube. Are these viable options for this application?

[tubeswell]

1. Explain the effects of different resistance values in the negative feedback circuit. There currently is no negative feedback. Also, help me understand how a Presence control could be developed from negative feedback.

Merlin's book has a chapter on GFB and where to put presence controls. Basically the NFB loop takes an AC voltage swing of the PT secondary and through a voltage divider puts the signal back into a (usually a pre-amp) stage, in such a way that the swing from the OT is in opposite phase to the swing at the point at which the NFB is connected earlier in the signal chain, in order to dampen the signal. Playing around with the NFB voltage divider is important to getting the 'right' amount of NFB without making the amp oscillate. This is dependent n the amount of swing you are getting from the OT in the first place. An OT with a higher Pr:Sec impedance ratio (i.e.; 'higher' being 1000:1 instead of 625:1) will pump out less voltage swing, and therefore need less in the way of NFB voltage divider strength for the 'same' amount of NFB. A presence control in a NFB loop, like in a 5F6A, is a RC tone control that selectively dumps some of the NFB frequency being fed back into the pre-amp, in order to accentuate higher frequencies or not as you turn the knob.

2. What are your thoughts on the phase inverter coupling cap? I've seen anywhere from 500pf to .01uF. I am using a .001 uf currently. Aside from more or less bass, how would larger or smaller values affect tone/distortion/output?

Size of this depends on the bandwidth associated with the PI tube you are using. In BF amps where they use a 12AT7, that has really high BW, therefore the coupling cap can be a lot smaller for the 'same' frequency. Bigger caps will tend to encourage more blocking distortion, so there is an 'optimum' size for each type of tube and where it is in the signal chain.

3. Explain the affects of the 'tail' resistor in a LTPI? I am using a 22k per AB763 cicuit, but Im seeing values as low as 6800.

The tail resistor is an attempt to provide a constant current source to the cathode resistor, so that when a signal is injected into the (inverting) stage of the LTP, and you get the expected cathode swing to feed the other (non-inverting) stage, so that most-all of the cathode swing takes place in the cathode resistor, and not in the tail resistor. However because the tail resistor is unbypassed, some swing inevitably happens in the tail resistor, albeit much less than in the cathode resistor, because of the relatively high value of the tail resistor. Now the tail resistor size also determines how much overall gain (and how well-balanced) the LTP has/is, because a larger tail resistor means that there is less plate-to-cathode voltage available (remember that the HT voltage is fixed, therefore it you increase the cathode voltage, you will decrease the plate-to-cathode voltage). This is why a typically a lowish tail resistor value, like 10k, means that you get more gain out of an LTP than a higher (say 47k) value, but also why the plate resistors in a LTP with a 10k tail need to be different (100k on the non-inverting side and 82k on the inverting side) in order to balance the LTP output signal strength, whereas an LTP with a higher 47k tail typically can get away with both plate resistors being the same, but at the expsense of lower overall gain.

4. Any experience with a triode/pentode switch in a 5E3 style output stage? Seems like an interesting thing to try

Nah. But you might want to put a high-value resistor across such a switch so that the screen isn't left instantaneously floating at the time you switch the switch

5. I can't find a lot of clear information on the internet regarding the EZ81 rectifier tube. Are these viable options for this application?

Did you look at Frank's tube datasheets page? Frank's Electron tube Pages

[JoeM]

I have completed a recent project where I converted a Bogen CHB-35 into an AB763 DR with a touch of 5E3. It is a single channel, AB763 normal channel preamp, LTPI, cathode biased 6V6 for about 18 watts, 5E3 OT and a JJ EZ81. The amp sounds really fantastic, but as always Im looking to expound upon that

1. Explain the effects of different resistance values in the negative feedback circuit. There currently is no negative feedback. Also, help me understand how a Presence control could be developed from negative feedback.

2. What are your thoughts on the phase inverter coupling cap? I've seen anywhere from 500pf to .01uF. I am using a .001 uf currently. Aside from more or less bass, how would larger or smaller values affect tone/distortion/output?

3. Explain the affects of the 'tail' resistor in a LTPI? I am using a 22k per AB763 cicuit, but Im seeing values as low as 6800.

4. Any experience with a triode/pentode switch in a 5E3 style output stage? Seems like an interesting thing to try

5. I can't find a lot of clear information on the internet regarding the EZ81 rectifier tube. Are these viable options for this application?

All of the questions you're asking can be explained technically, but you really should just get some parts and try the different component values etc. in the amp. Hard to explain what something sounds like, I think you have to hear it yourself.

[EETStudent]

Great information tubeswell. I think I am very satisfied with the current values in the amp. I think a .001 coupling cap sits perfect with my the 12AT7, and the 22k tail resistor is in a good spot. I just might leave it alone!

[HBamps]

I have completed a recent project where I converted a Bogen CHB-35 into an AB763 DR with a touch of 5E3. It is a single channel, AB763 normal channel preamp, LTPI, cathode biased 6V6 for about 18 watts, 5E3 OT and a JJ EZ81. The amp sounds really fantastic, but as always Im looking to expound upon that

1. Explain the effects of different resistance values in the negative feedback circuit. There currently is no negative feedback. Also, help me understand how a Presence control could be developed from negative feedback.

2. What are your thoughts on the phase inverter coupling cap? I've seen anywhere from 500pf to .01uF. I am using a .001 uf currently. Aside from more or less bass, how would larger or smaller values affect tone/distortion/output?

3. Explain the affects of the 'tail' resistor in a LTPI? I am using a 22k per AB763 cicuit, but Im seeing values as low as 6800.

4. Any experience with a triode/pentode switch in a 5E3 style output stage? Seems like an interesting thing to try

5. I can't find a lot of clear information on the internet regarding the EZ81 rectifier tube. Are these viable options for this application?

I have been working with a similar circuit and trying to tweak. Care to share the schem?