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Hi All,
First post and I've a number of questions, based on assumptions, re grid leak bias in a 1965 Alamo Titan (1x5Y3GT, 2x12AX7, 2x6v6 into 12") inc tremolo.
I heard one of these Titans a little while back and while they were designed to satisfy the cheaper end of the market, it had a fantastic (to my ears), vintage overdriven sound to it - so when one came up for sale on eBay recently (at a high price for its inherent design quality and age) I bought it! It's being shipped over from the US to the UK and I hope to be giving it a trial in few weeks after a thorough inspection.
I've also been reading an excellent book by Merlin Bencow called "Designing Tube Preamps for Guitar and Bass" and decided this might be a good opportunity to test what little of it might have registered in my grey matter and use it to better 'understand' the Titan. I also have an ancient degree in Electronic Engineering - so distantly familiar with principles of electronic circuitry but, at the time of my education, valves (as we call them in the UK) were definitely not part of the curriculum, being considered as antiques to chortle at like SMASH robots.
I've include an image of the first stage schematic of the Titan - it appears to be grid leak biased.
For connections beyond the segment shown - both the anodes of the 12AX7 are connected to +HT via separate 470K resistors. There is feedback connection from the speaker coil into the cathode of the 'lower' 12AX7 triode via a 100K resistor that also appears biased with a 2.2K resistor. The output from the lower triode is directly (DC) coupled to the input of the tremolo stage.
What struck me was the low value of the grid leak resistor on the first stage at only 220K. From my reading I would expect this to be in the range of 1M to 10M for grid leak bias and my questions are based around this design element and it's possible effects.
1. With such a low value is the grid essentially biased at 0v? - i.e. biased at the cathode voltage (no cathode resistor to raise the cathode potential, no very high value grid leak resistor to prevent accumulated grid charge bleeding to ground).
2. With such a low bias, is the anode current current continually running nearly at max (no negative voltage to turn it off) permitted by the 470K anode resistor in series with the inherent anode resistance? Is this considered a 'hot' bias?
3. The grid curves I have for a 12AX7 only show between 0 and -4v grid voltage. Positive beyond 0v appears outside typical operational range but with an AC signal at least half of it would appear to reside in this range if the bias is 0v. Does this configuration/bias therefore imply inherent and unavoidable distortion (for a guitar pickup level signal) and of what type / form / sound would it take?
4. Would the grid ever become more positive than 0v in this configuration, even when a signal is applied? - due to the grid, attempting to go positive under positve cycle signal influence, acting like a anode, collecting some of the cathode current and so counteracting the positive part of the signal - keeping it at near 0v regardless of positive input signal. i.e. compressing the positive part of the input signal.
5. Is distortion from this type of circuit configuration normally a desirable type that contributes to the amps voice and hence included by design?
6. Does the amplifier input resistance approximate 250K (47K+220K||Rg) - (assuming Rg ~1M) and is the input impedance similar (to AC signal)?.
7. What effect, if any, does Miller Capacitance have in this circuit and can it be estimated if so?
8. I understand noise generation in most amps is typically associated to Johnson (thermal) noise on input resistances in the first stage however there is no grid-stopper resistor in this circuit. Where are the main sources of noise generation?
9. Without a grid-stopper is this amp then highly prone to blocking distortion?
10. What influence will the impedance of the guitar pickup, and to some extent, the cable have to play - is this circuit design particularly sensitive to this aspect?
Many questions and thanks in advance for insights provided on any element.
First post and I've a number of questions, based on assumptions, re grid leak bias in a 1965 Alamo Titan (1x5Y3GT, 2x12AX7, 2x6v6 into 12") inc tremolo.
I heard one of these Titans a little while back and while they were designed to satisfy the cheaper end of the market, it had a fantastic (to my ears), vintage overdriven sound to it - so when one came up for sale on eBay recently (at a high price for its inherent design quality and age) I bought it! It's being shipped over from the US to the UK and I hope to be giving it a trial in few weeks after a thorough inspection.
I've also been reading an excellent book by Merlin Bencow called "Designing Tube Preamps for Guitar and Bass" and decided this might be a good opportunity to test what little of it might have registered in my grey matter and use it to better 'understand' the Titan. I also have an ancient degree in Electronic Engineering - so distantly familiar with principles of electronic circuitry but, at the time of my education, valves (as we call them in the UK) were definitely not part of the curriculum, being considered as antiques to chortle at like SMASH robots.
I've include an image of the first stage schematic of the Titan - it appears to be grid leak biased.
For connections beyond the segment shown - both the anodes of the 12AX7 are connected to +HT via separate 470K resistors. There is feedback connection from the speaker coil into the cathode of the 'lower' 12AX7 triode via a 100K resistor that also appears biased with a 2.2K resistor. The output from the lower triode is directly (DC) coupled to the input of the tremolo stage.
What struck me was the low value of the grid leak resistor on the first stage at only 220K. From my reading I would expect this to be in the range of 1M to 10M for grid leak bias and my questions are based around this design element and it's possible effects.
1. With such a low value is the grid essentially biased at 0v? - i.e. biased at the cathode voltage (no cathode resistor to raise the cathode potential, no very high value grid leak resistor to prevent accumulated grid charge bleeding to ground).
2. With such a low bias, is the anode current current continually running nearly at max (no negative voltage to turn it off) permitted by the 470K anode resistor in series with the inherent anode resistance? Is this considered a 'hot' bias?
3. The grid curves I have for a 12AX7 only show between 0 and -4v grid voltage. Positive beyond 0v appears outside typical operational range but with an AC signal at least half of it would appear to reside in this range if the bias is 0v. Does this configuration/bias therefore imply inherent and unavoidable distortion (for a guitar pickup level signal) and of what type / form / sound would it take?
4. Would the grid ever become more positive than 0v in this configuration, even when a signal is applied? - due to the grid, attempting to go positive under positve cycle signal influence, acting like a anode, collecting some of the cathode current and so counteracting the positive part of the signal - keeping it at near 0v regardless of positive input signal. i.e. compressing the positive part of the input signal.
5. Is distortion from this type of circuit configuration normally a desirable type that contributes to the amps voice and hence included by design?
6. Does the amplifier input resistance approximate 250K (47K+220K||Rg) - (assuming Rg ~1M) and is the input impedance similar (to AC signal)?.
7. What effect, if any, does Miller Capacitance have in this circuit and can it be estimated if so?
8. I understand noise generation in most amps is typically associated to Johnson (thermal) noise on input resistances in the first stage however there is no grid-stopper resistor in this circuit. Where are the main sources of noise generation?
9. Without a grid-stopper is this amp then highly prone to blocking distortion?
10. What influence will the impedance of the guitar pickup, and to some extent, the cable have to play - is this circuit design particularly sensitive to this aspect?
Many questions and thanks in advance for insights provided on any element.
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