Hi,
I have built a tube amp for a friend who plays Lap Steel.
He supplied me with transformers and a circuit from an old Stromberg Carlson HiFi amp Model AP 437.
I recreated the circuit in a new chassis and added a new front end to the circuit to give the amp a tone stack.
From the input through to the Phase Inverter the circuit is a direct lift from a Fender Delux Reverb Amp Model 763, the Normal channel.
There is no Presence or Bright switch on the circuit I used although I've seen them on other diagrams for this amp.
Here is my issue,
The amp works fine, has guts and tons of gain but, the high end seems somewhat rolled off and the tone controls don't have a lot of effect.
I'd like to tweak this and get a better high end range as well as better response from the tone controls but I'm stuck on where to start with this. I'm hoping someone here will have some suggestions on how to tackle this.
Here is the current circuit I've used.
Thanks much,
Chris
I have built a tube amp for a friend who plays Lap Steel.
He supplied me with transformers and a circuit from an old Stromberg Carlson HiFi amp Model AP 437.
I recreated the circuit in a new chassis and added a new front end to the circuit to give the amp a tone stack.
From the input through to the Phase Inverter the circuit is a direct lift from a Fender Delux Reverb Amp Model 763, the Normal channel.
There is no Presence or Bright switch on the circuit I used although I've seen them on other diagrams for this amp.
Here is my issue,
The amp works fine, has guts and tons of gain but, the high end seems somewhat rolled off and the tone controls don't have a lot of effect.
I'd like to tweak this and get a better high end range as well as better response from the tone controls but I'm stuck on where to start with this. I'm hoping someone here will have some suggestions on how to tackle this.
Here is the current circuit I've used.
Thanks much,
Chris
You have probably just wired the tone stack wrong- it catches people out every time, no matter how many times you think you've checked it! Either that or some of the components simply have the wrong values.
Got it, I'll check everything again but I'm pretty sure I have it right, at least wired right. The stack does work but the range on each tone pot seems to be much less than it should be. The volume works fine. I've been wondering if the capacitance for the whole stack is wrong, too high, and it's rolling off the highs.
Thanks for the suggestions.
Thanks for the suggestions.
You have 2 potential problems that I can see:
1. The tone stack and volume control are within the global negative FB loop, which will cause the NFB to try to cancel their effect.
2. The input to the tone stack would be better taken from a low impedance source, IMHO. A grounded cathode 12AX7 does not have a low OP impedance.
The thing might work better if you can add a cathode follower front end and insert the tone stack and volume controls between the cf and the first 12AX7 gain stage.
1. The tone stack and volume control are within the global negative FB loop, which will cause the NFB to try to cancel their effect.
2. The input to the tone stack would be better taken from a low impedance source, IMHO. A grounded cathode 12AX7 does not have a low OP impedance.
The thing might work better if you can add a cathode follower front end and insert the tone stack and volume controls between the cf and the first 12AX7 gain stage.
Thank you Ray_Moth. I appreciate your observations. I'm hoping to avoid adding another tube so I'll try to address this using your two suggestions.
Ray, I don't think the global NFB is actually doing anything- the input cathode is bypassed with a 22uF cap! Seems to be a redundant bit of fiddling that the designer has done.
Just noticed, the slope resistor in the tone stack is not labelled. What value are you using? I'm guessing not less than 100k.
Just noticed, the slope resistor in the tone stack is not labelled. What value are you using? I'm guessing not less than 100k.
There are 2 x 100K anode resistors in parallel on the tube after the vol pot, might make the DC a bit high on the input pin 2 to the phase splitter? The 0.5uF cap on pin 7 is returned to B+, is this intended?
Ian.
Ian.
The input cap is sure working to cancel out the NFB, but the unknown element here is how much feedback is actually being supplied by the tertiary FB winding. Dedicated windings like that are usually of a higher level than the VC winding is, so there may just be enough NFB to limit the tone control range -- although that's a pretty high FB resistor. Either way, that needs to be reworked, and the values for the second gain stage are off from typical Fender values. The plate load is usually 100K, which means the cathode resistor should be 1500 ohms since it's not doing double duty for the vibrato channel in this case.
Dave
Dave
Thanks everyone. I tried to get back to you last evening but work got in the way..
Merlinb - What 22uF cap are you referring to? Is it the 25uF on the plate?
FYI the NFB does not have much effect at all and in fact, when disconnected, some of the high end comes back. Also the slope resistor is 100K
Ian44, there is only one 100K resistor, the one that runs from the anode to the B+. The other is an error on the diagram! The .5uF cap on pin 7 of the phase splitter does return to B+ on the original Stromberg Carlson schematic.
dcgillespie - What cap are you referring to as the input cap? The FB resistor on the original circuit is specified as 15K. As far as the Fender values go I took these from a "Delux Reverb Amp model AA 763" schematic. In that schematic the plate for V1b and the plate for the vibrato are connected to ground through a single 820 ohm resistor.
I appreciate the help guys, I really do.
If anyone is interested I can post the original Stromberg Carlson circuit.
Thanks again,
Chris
Merlinb - What 22uF cap are you referring to? Is it the 25uF on the plate?
FYI the NFB does not have much effect at all and in fact, when disconnected, some of the high end comes back. Also the slope resistor is 100K
Ian44, there is only one 100K resistor, the one that runs from the anode to the B+. The other is an error on the diagram! The .5uF cap on pin 7 of the phase splitter does return to B+ on the original Stromberg Carlson schematic.
dcgillespie - What cap are you referring to as the input cap? The FB resistor on the original circuit is specified as 15K. As far as the Fender values go I took these from a "Delux Reverb Amp model AA 763" schematic. In that schematic the plate for V1b and the plate for the vibrato are connected to ground through a single 820 ohm resistor.
I appreciate the help guys, I really do.
If anyone is interested I can post the original Stromberg Carlson circuit.
Thanks again,
Chris
I would eliminate that feedback, at least for now. Check the value of the "mid" resistor which is the 6800 in the tone stack. If it's 68k, for example, you would have the tone-control problem you describe, as well as tons of "gain" (in the guitar amp context.) I switch in/out an extra resistor in that spot to get 2 modes.
PI looks a little strange but I'm not well versed on them.
PI looks a little strange but I'm not well versed on them.
P Jazz -- Yeah, in my rush to post quickly and rush out the door, I meant to say "input stage cathode cap", but the words stage and cathode got left out! Sorry for the confusion. This is the same cap that merlinb is referring to as well. With the feedback attempting to be injected into the cathode of this stage, that cap is effectively shorting it out for little if any feedback at all. Feedback should not be injected into that stage. More on this later.
As for V1B, Fender ties the cathode of this stage to the same cathode in the vibrato channel, except that in your case, there is no vibrato channel to help pull current through that 820 ohm resistor. Since the current flow is cut in half, then the resistance needs to be doubled to maintain the same bias on the tube. Therefore, it should be raised to 1500 ohms.
The .5 uF in the phase inverter stage could be returned to B+ or ground, it makes only little difference. Since it's function is to ground the grid of the inverter's cathode driven section AC wise, either of these connections will work since the B+ rail it's connected to also represents AC ground as well -- or at least should! Since the previous stage is direct coupled into the inverter, connecting the .5 cap to the B+ rail helps to maintain inverter stage linearity during periods of significant B+ sag to the stage.
As for the value of the feedback resistor, if it is the original value, that lost all meaning when you changed the front end section and where the feedback is connected to now as opposed to the original design. The best thing you could probably do at this point is to remove the cathode bypass cap on V1B, and inject the FB at that point. This would be after the controls, so that they could work to maximum effect, and the feedback could be effective as well, unaffected by the control settings. To do so however, you would need to check the phase of the FB winding to make sure the FB remains negative, since it should now be reversed relative to the correct phase for insertion into V1A.
Of course, you could carry the Fender topology further and change the tail wiring of the inverter stage to be like Fender's, and inject the feedback there as well. To do that however would require a tube change at that point, as the 12AU7 has so little gain to offer for the configuration it's used in for the inverter function that any feedback injected there -- as is -- would hardly be effective at all.
Others may have other suggestions, but these changes would make for a great amp! Good luck!
Dave
As for V1B, Fender ties the cathode of this stage to the same cathode in the vibrato channel, except that in your case, there is no vibrato channel to help pull current through that 820 ohm resistor. Since the current flow is cut in half, then the resistance needs to be doubled to maintain the same bias on the tube. Therefore, it should be raised to 1500 ohms.
The .5 uF in the phase inverter stage could be returned to B+ or ground, it makes only little difference. Since it's function is to ground the grid of the inverter's cathode driven section AC wise, either of these connections will work since the B+ rail it's connected to also represents AC ground as well -- or at least should! Since the previous stage is direct coupled into the inverter, connecting the .5 cap to the B+ rail helps to maintain inverter stage linearity during periods of significant B+ sag to the stage.
As for the value of the feedback resistor, if it is the original value, that lost all meaning when you changed the front end section and where the feedback is connected to now as opposed to the original design. The best thing you could probably do at this point is to remove the cathode bypass cap on V1B, and inject the FB at that point. This would be after the controls, so that they could work to maximum effect, and the feedback could be effective as well, unaffected by the control settings. To do so however, you would need to check the phase of the FB winding to make sure the FB remains negative, since it should now be reversed relative to the correct phase for insertion into V1A.
Of course, you could carry the Fender topology further and change the tail wiring of the inverter stage to be like Fender's, and inject the feedback there as well. To do that however would require a tube change at that point, as the 12AU7 has so little gain to offer for the configuration it's used in for the inverter function that any feedback injected there -- as is -- would hardly be effective at all.
Others may have other suggestions, but these changes would make for a great amp! Good luck!
Dave
I once owned a Twin Reverb, actually a Super Six Reverb but same thing with 6 speakers, I sold it and never regretted it. I built a Fender preamp and hooked it up to a SS power amp and that was much better. But now 15 yrs later, now using tubes for hifi, I think a hard cold hifi 100W power amp with nice tube pre might be the go. Each to their own. Being a geetarist and into hifi, I learned that hifi is about clean sound, with all details revealed. So I think to myself, why not apply that to the guitar amp, then it will cut through the mix with minimum noise and max clarity.
Ian
Ian
P-Jazz -- Two other points I just noticed:
1. Your stand-by switch must be changed. As is, it only switches off the plate supply to the output tubes, but leaves the screens still activated. In this configuration, if the plate B+ is turned off, the screens will try to carry all of the quiescent current by themselves and be ruined quite quickly. The stand-by switch should be placed after the first PS filter cap, but switch the power to the plate and screens of the output tubes. The small tubes should be kept on all the time to prevent a strong pulse on the output tube grids when coming out of standby. The B+ to these tubes will rise significantly when in the standby mode, so all of the filter caps must be able to handle the increased voltage under that condition.
2. The bleeder for the screen dropping circuit is severely under-rated. As is, it is dissipating nearly 18 watts under quiescent conditions, but is only rated for 20 watts. That sucker just might be glowing in the dark! While it will do a fair job of stabilizing the screen voltage for the output tubes, it also wastes power. You might seriously consider using a VR tube (ala' Leslie style) or zener string to provide the necessary drop. For the purposes here, it will do at least as good a job, and eliminate a major source of heat and wasted energy in the amp.
Dave
1. Your stand-by switch must be changed. As is, it only switches off the plate supply to the output tubes, but leaves the screens still activated. In this configuration, if the plate B+ is turned off, the screens will try to carry all of the quiescent current by themselves and be ruined quite quickly. The stand-by switch should be placed after the first PS filter cap, but switch the power to the plate and screens of the output tubes. The small tubes should be kept on all the time to prevent a strong pulse on the output tube grids when coming out of standby. The B+ to these tubes will rise significantly when in the standby mode, so all of the filter caps must be able to handle the increased voltage under that condition.
2. The bleeder for the screen dropping circuit is severely under-rated. As is, it is dissipating nearly 18 watts under quiescent conditions, but is only rated for 20 watts. That sucker just might be glowing in the dark! While it will do a fair job of stabilizing the screen voltage for the output tubes, it also wastes power. You might seriously consider using a VR tube (ala' Leslie style) or zener string to provide the necessary drop. For the purposes here, it will do at least as good a job, and eliminate a major source of heat and wasted energy in the amp.
Dave
Blimey, you've spotted a whole raft of problems! That circuit is full of gaffs...
Guys,
Thanks again for your time and comments.
I've got it back on the bench and will be doing some rework over the next week.
Merlinb, The gaffs are mostly mine although I can't claim the underrated bleeder in the screen dropping circuit, That's vintage Stromberg Carlson.
While I'm a bit chagrined at having missed all this stuff, I'm grateful for the help. This is only the second Amp I've tried to build and I value the assistance you guys are providing.
Any time you're passing through Buffalo, NY with time to kill, let me know and I'll stand you to some beers and wings.
Chris
Thanks again for your time and comments.
I've got it back on the bench and will be doing some rework over the next week.
Merlinb, The gaffs are mostly mine although I can't claim the underrated bleeder in the screen dropping circuit, That's vintage Stromberg Carlson.
While I'm a bit chagrined at having missed all this stuff, I'm grateful for the help. This is only the second Amp I've tried to build and I value the assistance you guys are providing.
Any time you're passing through Buffalo, NY with time to kill, let me know and I'll stand you to some beers and wings.
Chris
Heh, a couple of years ago I stood on the bridge at Niagra and looked over at Buffalo, but thought twice about entering American soil 😉
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