Hello-
I am building a two channel amp (Bass, Normal) with a pair of KT-88 into a very good output transformer at back end. I wish to voice one channel for bass. In doing research the Peavey Alpha Bass preamp attracted my attention. I am considering using just the first two triodes and tone stack in my build. I cannot figure out how to model the tone stack and wanted to ask any users of this amp to describe to me the action of the tone controls. I was particularly interested if, when "Mid" and "Low" pots are turned all the way CCW, will the volume level be very low similar to the early Fender TMB controls where you would lose all volume. In general, I wanted to get advice in regard to using this tone stack.
Also, in studying the preamp schematic, I was curious about why Peavey used additional coupling caps that most preamps do not have. Examples are C-33 (between input jacks and first stage grid) and C-34 (between preamp gain pot wiper and second stage grid). My guess is that Peavey designers wished the grid leak resistance value to be the constant value of the actual grid leak resistors (R-60 and R-59). Very curious to me and I am eager to learn the reason why this was done.
In doing my research for a bass preamp I relied on posts from a number of the different forums. I have no playing experience and am strictly a tube amp builder. So I am open to suggestions. The Orange AD-200 seemed to be very well regarded and the Baxandall -type tone stack with Mid pot was very interesting to me. In this particular build I am going into I have 2 of 9-pin (12AX7 type) sockets available for the Bass preamp plus front panel Treble, Mid, Bass pots.
Thanks all for any advice here. Attached is schematic for Alpha Bass preamp.
Byron W5FH
Peavey Alpha Bass Tone Stack Action
Moderators: pompeiisneaks, Colossal
Peavey Alpha Bass Tone Stack Action
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Re: Peavey Alpha Bass Tone Stack Action
Hi,
I am not familiar with the Peavey Amp and Tone-Stack, but the "stack" is only partially a stack, as the mid pot is not stacked under the other controls and bleeds off the signal for mid frequencies only. But the bass pot is still under the treble pot.
In the Bass community here in Germany I don't know anybody who plays the Peavey. Peavey is not considered as a goto brand for Bass.
The extra capacitors block any DC in case a tube fails and the grid gets DC for any reason. They are placed where a user might touch an input or a control which might have DC on it in a catastrophic failure. You don't see this very often. Any capacitor in the signal path adds some phase shift. Tube Bass amps in most cases sound best, the simpler the circuit is.
Preamp:
The orange is a very good sounding amp. Basically the Tone-Stack is a modified Fender Tone-Stack, optimized for Bass. Note: the coupling caps after the tubes are smaller as text books would suggest. In my build of a tube bass amp I experienced it too bass heavy with .1µF coupling caps. I used .022.
You say you have 2 systems available for the preamp. Do you mean for the preamp only or with PI and cathode followers?
What are you after? Ultra clean or with the option of distortion? For ultra clean 2x 12AX7 are plenty, depending on the parts around the tube you even get into distortion territory. My goal was ultra clean, so I used 47k on the anode and no bypass cap on the cathode. Otherwise it would distort too early. To boost amplification per stage use 100k or more + bypass cap. Cathode resistor needs to be calculated by load lines.
Note: both the AD200 and the peavey have the tone stack after the second triode (as has mine). Thus the second triode grid gets a signal that is not attenuated by the tone stack (-20dB or so) and it will distort earlier compared to a Fender Bass preamp which has the tone stack between the triodes.
Power-Amp:
KT88 tubes need a very small bias resistor to work properly and this loads the signal off the phase inverter. A 12AX7 will not deliver enough current to drive KT88s. It is strongly recommended to use cathode followers after the PI to drive the KT88s. otherwise you won't get the power you expect from a pair of KT88. If you use cathode followers, the tube for the PI does not matter anymore and you can stick to a 12AX7. The cathode followers should be 12AT7 or similar.
I am not familiar with the Peavey Amp and Tone-Stack, but the "stack" is only partially a stack, as the mid pot is not stacked under the other controls and bleeds off the signal for mid frequencies only. But the bass pot is still under the treble pot.
In the Bass community here in Germany I don't know anybody who plays the Peavey. Peavey is not considered as a goto brand for Bass.
The extra capacitors block any DC in case a tube fails and the grid gets DC for any reason. They are placed where a user might touch an input or a control which might have DC on it in a catastrophic failure. You don't see this very often. Any capacitor in the signal path adds some phase shift. Tube Bass amps in most cases sound best, the simpler the circuit is.
Preamp:
The orange is a very good sounding amp. Basically the Tone-Stack is a modified Fender Tone-Stack, optimized for Bass. Note: the coupling caps after the tubes are smaller as text books would suggest. In my build of a tube bass amp I experienced it too bass heavy with .1µF coupling caps. I used .022.
You say you have 2 systems available for the preamp. Do you mean for the preamp only or with PI and cathode followers?
What are you after? Ultra clean or with the option of distortion? For ultra clean 2x 12AX7 are plenty, depending on the parts around the tube you even get into distortion territory. My goal was ultra clean, so I used 47k on the anode and no bypass cap on the cathode. Otherwise it would distort too early. To boost amplification per stage use 100k or more + bypass cap. Cathode resistor needs to be calculated by load lines.
Note: both the AD200 and the peavey have the tone stack after the second triode (as has mine). Thus the second triode grid gets a signal that is not attenuated by the tone stack (-20dB or so) and it will distort earlier compared to a Fender Bass preamp which has the tone stack between the triodes.
Power-Amp:
KT88 tubes need a very small bias resistor to work properly and this loads the signal off the phase inverter. A 12AX7 will not deliver enough current to drive KT88s. It is strongly recommended to use cathode followers after the PI to drive the KT88s. otherwise you won't get the power you expect from a pair of KT88. If you use cathode followers, the tube for the PI does not matter anymore and you can stick to a 12AX7. The cathode followers should be 12AT7 or similar.