Help me stop my preamp from farting! - Page 2

SY · 24th October 2005, 01:32 PM

Yes, you need it; with no decoupling, the circuit will be prone to oscillate.

Johan Potgieter · 25th October 2005, 09:22 PM

From my side, I see that the original C6 would have given a -3 dB point at about 30 Hz, so one wonders if that had anything to do with a certain bass effect. All the time constants in the shown circuit could certainly have caused some audible effect (desired by the original designer?) I see no feedback, so there would not have been a peek, though feedback is possibly occuring through the ht line. One just wonders whatever could the purpose have been with such a low C6 - hardly decoupling!

I was unable to download the full circuit for some strange reason, so am not able to check what kind of total gain is available. I would thus also support the suggestion of increasing C6 to a few uF to start with. After that it begins to be difficult to come up with a diagnosis without seeing the exact nature of the disturbance.

kevinkr · 25th October 2005, 09:48 PM

I was wondering whether or not you are using an ernie ball musicman/stingray bass and whether or not it has fishman piezo undersaddle pickups? Some of the electronics in these things produce substantial subsonic cr*p which could be at the root of your problem. A small coupling capacitor right at the input of the amplifier might help.

Kevin

Buckeye Sound · 26th October 2005, 02:08 AM

No bass guitar, this is a regular guitar amp. The amp has two channels and this circuit is the overdrive preamp channel for leads and such. So lows are not critical other than shaping the overall tone, but of course that's what it's all about -- good tone. After thinking about it, I think I will try increasing the value of C6 as suggested, but I will also increase the plate and cathode resistor values. By doing this, the 12AX7 should work more efficiently, reducing the amount of power the circuit uses to get that cool overdrive sound. Hopefully keeping the power reserve for tight bass sound.

Any other thoughts are welcome!

Buckeye Sound · 26th October 2005, 02:21 AM

Also, Channel 1, the clean channel, I'm very happy with. In fact, this channel that I'm tinkering with, channel 2, sounds very good when I run it clean, no overdrive. So, I'm leaning toward the idea that I'm starving the preamp of power, channel 2. I notice in later versions of this amp, Music Man used higher value resistors for the plate and cathode, which would make the tube work more efficiently. I think they used something like 450k plate and 4.5K cathode, or something like that. Any opinions on this idea?

24th October 2005, 01:32 PM	#11
SY diyAudio Moderator Join Date: Oct 2002 Location: Austin, TX Blog Entries: 1	Yes, you need it; with no decoupling, the circuit will be prone to oscillate. __________________ “Listening to records is like ****ing a picture of Brigitte Bardot.” - Sergiu Celibidache

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25th October 2005, 09:22 PM	#12
Johan Potgieter diyAudio Member Join Date: May 2005 Location: Pretoria, South Africa	From my side, I see that the original C6 would have given a -3 dB point at about 30 Hz, so one wonders if that had anything to do with a certain bass effect. All the time constants in the shown circuit could certainly have caused some audible effect (desired by the original designer?) I see no feedback, so there would not have been a peek, though feedback is possibly occuring through the ht line. One just wonders whatever could the purpose have been with such a low C6 - hardly decoupling! I was unable to download the full circuit for some strange reason, so am not able to check what kind of total gain is available. I would thus also support the suggestion of increasing C6 to a few uF to start with. After that it begins to be difficult to come up with a diagnosis without seeing the exact nature of the disturbance.

25th October 2005, 09:48 PM	#13
kevinkr diyAudio Moderator Join Date: Sep 2004 Location: Boston, Massachusetts Blog Entries: 6	I was wondering whether or not you are using an ernie ball musicman/stingray bass and whether or not it has fishman piezo undersaddle pickups? Some of the electronics in these things produce substantial subsonic cr*p which could be at the root of your problem. A small coupling capacitor right at the input of the amplifier might help. Kevin

26th October 2005, 02:08 AM	#14
Buckeye Sound diyAudio Member Join Date: Jun 2005 Location: Here	No bass guitar, this is a regular guitar amp. The amp has two channels and this circuit is the overdrive preamp channel for leads and such. So lows are not critical other than shaping the overall tone, but of course that's what it's all about -- good tone. After thinking about it, I think I will try increasing the value of C6 as suggested, but I will also increase the plate and cathode resistor values. By doing this, the 12AX7 should work more efficiently, reducing the amount of power the circuit uses to get that cool overdrive sound. Hopefully keeping the power reserve for tight bass sound. Any other thoughts are welcome!

26th October 2005, 02:21 AM	#15
Buckeye Sound diyAudio Member Join Date: Jun 2005 Location: Here	Also, Channel 1, the clean channel, I'm very happy with. In fact, this channel that I'm tinkering with, channel 2, sounds very good when I run it clean, no overdrive. So, I'm leaning toward the idea that I'm starving the preamp of power, channel 2. I notice in later versions of this amp, Music Man used higher value resistors for the plate and cathode, which would make the tube work more efficiently. I think they used something like 450k plate and 4.5K cathode, or something like that. Any opinions on this idea?

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