bass preamp redux - diyAudio
Go Back   Home > Forums > Amplifiers > Tubes / Valves

Tubes / Valves All about our sweet vacuum tubes :) Threads about Musical Instrument Amps of all kinds should be in the Instruments & Amps forum

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 17th June 2009, 05:50 AM   #1
billr is offline billr  New Zealand
diyAudio Member
 
Join Date: Dec 2003
Location: new zealand
Default bass preamp redux

Apropos of something

Decisions loom on the bass preamp.

I've been modeling my preamp using LTC spice. One of the considerations that I've had was to try and determine the average input sensitivity that I require.

I've been doing some reading on this, and as far as I can see there seems to be little standardisation.

So i got the scope out and plugged in the bass geetar. I was surprised to see that with the volume all the way up, and tone set as loud as it could be, the with an average 'pluck' i could get over 400mv pk/pk and when i really pulled on the thing, nearly double that!

Bit more redeisgn fiddling called for, either that or is substitute 12au7s instead of ax7s.

I am also going down the path of dc heater supplies. probably a fad, but who cares, it's only for me.

DC regulators are a bit of a bugger at low voltages, so i was thinking that I will pair up two tubes so that I have heater voltage requirement of 25.2 volts or thereabouts.

My HT is going to be generated through two back to back 24 volt transformers. I'll hang a bridge rectifier, and regulator off of that.

This is more of a ramble than anything else. It also forces me to make sure that i do something.

i'll post some snaps later on.

all the best

bill
  Reply With Quote
Old 17th June 2009, 02:14 PM   #2
flysig is offline flysig  United States
diyAudio Member
 
Join Date: Apr 2009
Default Re: bass preamp redux

Quote:
Originally posted by billr
One of the considerations that I've had was to try and determine the average input sensitivity that I require.

I've been doing some reading on this, and as far as I can see there seems to be little standardisation.

So i got the scope out and plugged in the bass geetar. I was surprised to see that with the volume all the way up, and tone set as loud as it could be, the with an average 'pluck' i could get over 400mv pk/pk and when i really pulled on the thing, nearly double that!

Bit more redeisgn fiddling called for, either that or is substitute 12au7s instead of ax7s.

Bill, I found the same thing for 6 string guitars. There is no standardization of signal strength and a lot of variation! Electric guitars run anywhere from 100mV to 1V peak output into high impedances. 400mV to 800mV is a typical range for humbuckers. Acoustic guitars with preamps on board are in the ballpark of line level (1.23V rms) into 600 ohms. Mics? Those are all spec'd in dbU just to make it impossible to know a voltage without borrowing my son's scientific calculator and doing some math that I forgot 25 years ago!

Softer playing will result in smaller signals, obviously. Which makes it even more difficult to figure out what is "nominal". I built in lots of headroom and a master volume to cover all the bases.

Have you considered using a small signal pentode for the input?

FWIW, I dropped in a 12au7 into a Fender Super Champ XD guitar amp that had a 12AX7 in it from the factory. The aU7 tamed it a little bit, taking the edge off. It really sounds a lot better. But the gain didn't seem to change a whole lot, at least in a way that matters. It is quiet and clean, or it can be cranked up to painfully loud with full tube overdrive. I expected a big drop in volume but didn't experience that.

But subbing the 12aU7 for a 12aX7 in a standalone tube pre made a huge difference in gain. Some circuits seem a lot more sensitive to the swap. Is it the mu, or is it the bias point, or the load impedance? I haven't a clue.
  Reply With Quote
Old 18th June 2009, 12:13 AM   #3
billr is offline billr  New Zealand
diyAudio Member
 
Join Date: Dec 2003
Location: new zealand
Hi I'm going to put in a 'sensitivity' control between the first and second gain stages.

Also will reduce the Ht and play around with appropriate loadings to get the gain under control.

Not going down the pentode route, generally speaking, AFAIK, gain through a pentode is substantially higher.

I'll keep drilling and spicing to see what happens.

bill
  Reply With Quote
Old 18th June 2009, 12:24 AM   #4
ChrisA is offline ChrisA  United States
diyAudio Member
 
Join Date: Jan 2008
Default Re: bass preamp redux

Quote:
Originally posted by billr
[B]Apropos of something

Decisions loom on the bass preamp.

I've been modeling my preamp using LTC spice. One of the considerations that I've had was to try and determine the average input sensitivity that I require.
It depends so much on the player. But bass guitars don't have as much signal. Unless it is an active bass. I think this is why most amps have two inputs with one being 6 db down.

Also you might play more soft with a better amp. The good amp lets you just touch the string with the fingers and use the amp to make it loud With a cheap amp you play louder and the tone is not the same. So..... figure on a low minimum signal but then it could go really high. With a good enough amp maybe yu find yourself just barly striking the stings, Pulling them hard make the attack part of the note pitch higher. It's not just louder. Check it out, play while watching a chromatic tuner.

A hard question for me to figure out is how to set up the tone stack for bass. Maybe follow the trend and use an 8 band equalizer.

My dream bass amp I might build "some day" would have a tube input, then an effects loop that whent through an EQ, back to the preamp where we can mix the "wet" and dry" sounds and then I'd build one "chip amp", Gainclone type amp per speaker driver. So a 4x10 cab would have four cheap SS amps inside
  Reply With Quote
Old 18th June 2009, 12:44 AM   #5
billr is offline billr  New Zealand
diyAudio Member
 
Join Date: Dec 2003
Location: new zealand
Default Re: Re: bass preamp redux

Quote:
Originally posted by ChrisA


It depends so much on the player. But bass guitars don't have as much signal. Unless it is an active bass. I think this is why most amps have two inputs with one being 6 db down.

A hard question for me to figure out is how to set up the tone stack for bass. Maybe follow the trend and use an 8 band equalizer.


Hi, my guitar isn't active, hence my surprise when i put in on the scope. Its a Fender Squier Precision. [i'm just a beginner, which at my age is something of a miracle].

As for tone controls, go to www.duncanamps.com and you can download the tone stack calculator. I've taken the marshall and fender stacks and played with the values. You can see the results.

I plan to use one of these with an EQ afterwards, it will be switchable, ie. in or out.

Hope all goes well.

BIll
  Reply With Quote
Old 18th June 2009, 01:32 AM   #6
diyAudio Member
 
Join Date: Oct 2007
It seems to me that I read somewhere where the old Ampeg B-15 amp was designed so that it could handle something like a 5 volt input without distorting at the first gain stage! The reason was because a Gibson EB-O bass pickup could put out that much and it would distort just about any input it was plugged into due to it's super high output. This gave the Ampeg bass amp a clean solid tone with any bass.
  Reply With Quote
Old 18th June 2009, 01:53 AM   #7
ChrisA is offline ChrisA  United States
diyAudio Member
 
Join Date: Jan 2008
Default Re: Re: Re: bass preamp redux

Quote:

Hi, my guitar isn't active, hence my surprise when i put in on the scope. Its a Fender Squier Precision. [i'm just a beginner, which at my age is something of a miracle].

As for tone controls, go to www.duncanamps.com and you can download the tone stack calculator. I've taken the marshall and fender stacks and played with the values. You can see the results.
I'm an older beginner too. I know you can model a tone stack. But the problem is not calculation it is knowing if you should put the split at 50Hz or 100Hz. and how steep the rolloff should be. Which would sounds best? I read that Leo Fender didn't really calculate. He experimented and let musicians try out his prototypes and listened to comments from many musicians.

My plan is to build a stock Bassman tone stack and then "just try stuff". I might leave out a mid control. I'm designing a tone stack PCB. It will solder directly to the pots, or maybe attach to them with hot glue. And then coax cable with conectors on it will conect it THis why I can build several, even deferent topologies and swap them out.
  Reply With Quote
Old 18th June 2009, 06:31 AM   #8
billr is offline billr  New Zealand
diyAudio Member
 
Join Date: Dec 2003
Location: new zealand
Quote:
Originally posted by DaveMcLain
It seems to me that I read somewhere where the old Ampeg B-15 amp was designed so that it could handle something like a 5 volt input without distorting at the first gain stage! The reason was because a Gibson EB-O bass pickup could put out that much and it would distort just about any input it was plugged into due to it's super high output. This gave the Ampeg bass amp a clean solid tone with any bass.

Hi, i got a copy of the diagram for the B-15 and modelled the first two stages in LTC spice. I't distorts very badly with a 5v input!! heavily clipped.

thought you might find it of interest.

bill
  Reply With Quote
Old 18th June 2009, 07:21 AM   #9
diyAudio Member
 
Miniwatt's Avatar
 
Join Date: Nov 2004
Location: Den Haag
Default Re: Re: Re: bass preamp redux

Slightly o.t.

Quote:
Originally posted by billr

go to www.duncanamps.com and you can download the tone stack calculator.
Duncanamps isn't working. Any idea what's going on?

Cheers.
  Reply With Quote
Old 18th June 2009, 07:36 AM   #10
billr is offline billr  New Zealand
diyAudio Member
 
Join Date: Dec 2003
Location: new zealand
Default Re: Re: Re: Re: bass preamp redux

Quote:
Originally posted by Miniwatt
Slightly o.t.



Duncanamps isn't working. Any idea what's going on?

Cheers.

just tried it and it's fine. funnily enough i was downloading the pentode section of a ECL82 pspice model for a regulator i'm thinking of trying out.

just keep pushing F5!

all the best

bill
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Rotary Switch Redux ... udtman Parts 3 25th March 2008 12:15 AM
Maida Regulators Redux Eli Duttman Tubes / Valves 27 3rd June 2007 04:31 PM
Heater PS alternative / Redux FastEddy Tubes / Valves 7 4th May 2007 05:11 PM
PC Speakers Redux recca Multi-Way 0 11th March 2007 01:29 AM
Critical Q redux Dave Jones Multi-Way 18 20th February 2004 10:09 PM


New To Site? Need Help?

All times are GMT. The time now is 10:25 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2