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Pilot 240 with too much bass-- circuit modification needed?

li303ve

Member
2016-01-26 11:03 pm
Hi all,

I have a Pilot 240 that has really overemphasized bass. It sounds like the loudness compensation is switched on, but this is not the case. Switching on the loudness just adds yet another level of bass bloat. I can sort of adjust to compensate with the bass control, but only if I adjust it near its extreme minimum where it becomes too sensitive to dial in the right tone easily.

All of the old resistors and capacitors have been replaced, so there shouldn't be any problem with parts that are out of spec. I keep checking to see if a cap or resistor somewhere was replaced with an incorrect value, but I haven't been able to find anything.

I'm starting to wonder if there is some kind of bass emphasis built into the circuit, or if the schematic has an error somewhere with a cap value or something.

I'm at a bit of a loss trying to get a more linear frequency response. Any guidance would be greatly appreciated. I have attached a schematic below
 

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I'm going to take a WAG and suggest the bass bloat maybe an artifact of the paraphase splitter. Any imbalance in the phase splitter will manifest as extra 2nd harmonic distortion which gives the impression of strong bass.
The "self balancing" paraphase splitter used is somewhat dependent on the choice of resistor value in that 12K/270K divider. That divider is assuming a gain of 1/(12K/(12K +270K)) = 23.5 for the inverting stage.
To test this theory/guess then make sure the splitter tube is good and then try different values for that 12K 5% which determines the level of the inverted output.

There is a reason why people stopped using that paraphase splitter and mostly went to using a long tail pair instead, although I note that some guitar amp guys are starting to use the paraphase splitter again. They probably like that bass bloat.

Just an idea which may prove wrong.

Cheers,
Ian
 

rayma

Member
2011-04-29 8:37 pm
I'm starting to wonder if there is some kind of bass emphasis built into the circuit, or if the schematic has an error somewhere with a cap value or something.

Is this bass problem on phono, line sources, or both?
The RIAA here leaves a lot to be desired. Also, you could delete the phono stage input coupling caps.
If it's on the line inputs as well, you could try bypassing the passive tone controls.
 
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li303ve

Member
2016-01-26 11:03 pm
I'm going to take a WAG and suggest the bass bloat maybe an artifact of the paraphase splitter. Any imbalance in the phase splitter will manifest as extra 2nd harmonic distortion which gives the impression of strong bass.
The "self balancing" paraphase splitter used is somewhat dependent on the choice of resistor value in that 12K/270K divider. That divider is assuming a gain of 1/(12K/(12K +270K)) = 23.5 for the inverting stage.
To test this theory/guess then make sure the splitter tube is good and then try different values for that 12K 5% which determines the level of the inverted output.

There is a reason why people stopped using that paraphase splitter and mostly went to using a long tail pair instead, although I note that some guitar amp guys are starting to use the paraphase splitter again. They probably like that bass bloat.

Just an idea which may prove wrong.

Cheers,
Ian

Thanks, I'm out of ideas so any theory or guess is appreciated.

Would you be able to suggest a range of resistor values that would make sense to try out in place of the 12k?
 

li303ve

Member
2016-01-26 11:03 pm
Is this bass problem on phono, line sources, or both?
The RIAA here leaves a lot to be desired. Also, you could delete the phono stage input coupling caps.
If it's on the line inputs as well, you could try bypassing the passive tone controls.

It's a problem with the line inputs. Not sure about the phono stage. I use an external phono preamp since I've never been crazy about how the Pilot phono sounds. Bypassing the tone controls is probably worth a shot.
 
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kevinkr

Administrator
Paid Member
Have you considered the possibility that this is entirely a speaker/amplifier interaction?

Before proceeding with any possibly ill-conceived changes measure the frequency response into a load resistor at 1W and perhaps full power. A scope and function generator at minimum are all that are needed. You may find that the bass tone control is not quite flat, but measure to know rather than guess.

I would suspect the amp has relatively high output impedance and is interacting in unexpected ways with your as yet unspecified speaker. Try the 4 ohm tap if the speaker is 8 ohms, this will reduce output power but significantly increase damping factor.
 
Hi all,

I have a Pilot 240 that has really overemphasized bass. It sounds like the loudness compensation is switched on, but this is not the case. Switching on the loudness just adds yet another level of bass bloat. I can sort of adjust to compensate with the bass control, but only if I adjust it near its extreme minimum where it becomes too sensitive to dial in the right tone easily.

All of the old resistors and capacitors have been replaced, so there shouldn't be any problem with parts that are out of spec. I keep checking to see if a cap or resistor somewhere was replaced with an incorrect value, but I haven't been able to find anything.

I'm starting to wonder if there is some kind of bass emphasis built into the circuit, or if the schematic has an error somewhere with a cap value or something.

I'm at a bit of a loss trying to get a more linear frequency response. Any guidance would be greatly appreciated. I have attached a schematic below

There are any number of possible causes for bass over emphasis:

1) Your speeks aren't playing nice with a hollow state amp. Solid state amps have way more damping, and your speeks were designed with that in mind. The lower damping may be letting your woofers run wild.

2) A rising frequency response introduced through the gNFB loop. Maybe when replacing all those resistors and capacitors, you did something to destabilize the NFB loop? This, also, could be a bad interaction between the amp and speeks.

3) You're losing your highs somewhere.

4) Flawed design. There are a couple of things I really don't like here. First, the paraphase is just about the worst phase splitter design. This falls into the "quick 'n' dirty" category of hollow state design. If you must use that, much better to either use dissimilar dual triodes with a high-u section for voltage gain, and a lower-u section to invert the inverted signal to get that 180deg phase split.

Doubling the bad is the use of 12AX7s here. 'AX7s have a high enough rp as it is. The plate load resistors (100K) are barely adequate here. An unbypassed cathode resistor (the driven side, obviously to further reduce the gain) just compounds the problem as it drives an already stratospheric rp into a LEO. You could be hearing the end result of badly distorting 'AX7s: excessive h2 that's manifesting itself as exaggerated bass.

(I have been seeing ESSSS-loads of complaints regarding 12AX7s and disappointing sonic performance. In all too many cases (and this is another one) I'm seeing plate loads that are excessively heavy. The plate resistance of the 12AX7 is into small signal pentode territory, but it's still a triode, and like all triodes, it likes the lightest plate loading you can manage. If you don't have the DC headroom, better to use active plate loading, or redesign for the 6SL7 with its much lower rp.)

The best thing to do would be to disconnect the phase splitter input from the rest of the circuit and o'scope the output while sweeping the frequency with a signal generator to see if there's a rising gain with low frequencies. Then work your way back to the input to see where your highs are going.
 

li303ve

Member
2016-01-26 11:03 pm
Thanks everyone, you've given me a lot to think about in terms of figuring out the problem.

I don't have a working scope available at the moment, so unfortunately that is limiting the extent to which I can troubleshoot. I have been able to figure a few things out though.

I ran another tube preamp that I know is working properly directly into the driver tubes on the Pilot. Bass still sounds bloated. I think everyone may be right in suspecting the PI.

I then ran a 1KHz test tone through one of the line inputs and measured AC voltage coming from from the driver and PI with a DMM. On one channel, I measured ~5.30V from the driver and ~5.70V from the PI. Similar level of imbalance on the other channel. I think I'm going to try replacing the 12k resistor in the voltage divider with a 9.1k resistor and a 5k trimpot in series and try to adjust for better balance.
 
Here is what else you can so since you said you hooked up another preamp at the 12ax7

It's a problem with the line inputs. Not sure about the phono stage. I use an external phono preamp since I've never been crazy about how the Pilot phono sounds. Bypassing the tone controls is probably worth a shot.

Notice they float the ground on the secondary so they can use it for feedback,
Try running the feedback from 8 ohm or 16ohm tap and see what happens.Also,when you hooked up the external preamp,did you disable the amp's existing preamp and then run a grid to ground resistor?