Go Back   Home > Forums > >
Home Forums Rules Articles diyAudio Store Blogs Gallery Wiki Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

Instruments and Amps Everything that makes music, Especially including instrument amps.

Modifying a 12ax7 HiFi valve preamp to be a guitar preamp
Modifying a 12ax7 HiFi valve preamp to be a guitar preamp
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 14th October 2019, 08:03 AM   #1
hoho is offline hoho
diyAudio Member
 
Join Date: Oct 2019
Default Modifying a 12ax7 HiFi valve preamp to be a guitar preamp

Hi,

I've got a HiFi stereo preamp with two 12ax7 valves. Attaching the schematics (the schematics uses 12au7 as the second valve, but I've put 12ax7 there as well, they seem to be compatible and 12ax7 gives more gain), it shows one channel.

I want to make a guitar preamplifier from it. I've been looking at some guitar preamp schemes on the internet and it seems like I mostly need to change some resistors (to the ones with the lower resistance), but as I'm not really experienced in the area, I am asking for advice.

The goal is to keep the preamp stereo and have two independent channels for two different instruments, and feed the result to a DAC (to digitally process it further).

Another thing I want is to supply each channel with two potentiometers: one for the gain (between the valves?), another one for the master volume (after the valves). To go from reasobably clean to reasonably crunchy being able to maintain the proper output level.

Could you recommend which resistors need to be changed? As for the gain and master volume I want to replace the two 470K resistors from the scheme, with 1M log pots — are these the correct targets for gain and volume?

Needless to say that as it is it works when I connect something like the headphones output, but when I try to connect the guitar, I cannot hear it.

I would appreciate any advice.
Attached Images
File Type: jpg diykit.jpg (22.9 KB, 158 views)
  Reply With Quote
Old 14th October 2019, 04:09 PM   #2
PRR is offline PRR  United States
diyAudio Member
 
PRR's Avatar
 
Join Date: Jun 2003
Location: Maine USA
R4 foils attempts to get good gain control between stages.

The low impedance of a ADC (not DAC) is a heavy load for a vacuum tube or a 1Meg(!) pot.
  Reply With Quote
Old 14th October 2019, 04:34 PM   #3
hoho is offline hoho
diyAudio Member
 
Join Date: Oct 2019
Quote:
Originally Posted by PRR View Post
R4 foils attempts to get good gain control between stages.

The low impedance of a ADC (not DAC) is a heavy load for a vacuum tube or a 1Meg(!) pot.
Oh, sorry, of course I was meaning the ADC. The way it is now, my ADC takes the output from the preamp when I connect the headphones output from my phone to the preamp's input just fine. I've set up an ADC-DAC pass through and when I put the preamp before the ADC — I get the pass through thing working ok.

That same pass through thing is silent with the guitar is connected to it.

Could you please advice? First thinking about your response is maybe like Klon Buffer (Guitar FX Layouts: Klon Buffer) in between the preamp's output and the ADC?

As I was saying before I'm not very experienced in the area. I might be asking pretty stupid questions because of it . It's just the guitar amp schemes seem pretty similar to what I have in that preamp. And I've seen 1M pots in the places of gain and volume...
  Reply With Quote
Old 14th October 2019, 04:36 PM   #4
hoho is offline hoho
diyAudio Member
 
Join Date: Oct 2019
Also, what would be your suggestion for R4?
  Reply With Quote
Old 14th October 2019, 04:55 PM   #5
DF96 is offline DF96  England
diyAudio Member
 
Join Date: May 2007
Guitar amp preamps are quite different from that circuit. It (unlike them) has been designed for low gain. You could use the same valves and some of the same components to make a guitar preamp, but that circuit is not the place to start from.
  Reply With Quote
Old 14th October 2019, 05:02 PM   #6
hoho is offline hoho
diyAudio Member
 
Join Date: Oct 2019
Quote:
Originally Posted by DF96 View Post
Guitar amp preamps are quite different from that circuit. It (unlike them) has been designed for low gain. You could use the same valves and some of the same components to make a guitar preamp, but that circuit is not the place to start from.
That's my main problem — lack of experience. I can reuse some basic working thing, but building it from scratch might be above my head. For that one I have the PCB with all the components properly labelled (I've got a quite reasonably cheap DIY kit from eBay).

When I look at something like https://www.audiotinker.com/support-...12ax7-reva.pdf, I can find similarity (of course to my limited experience extent). I'm totally fine with resoldering all resistors on that PCB, but I would really try to reuse the PCB if possible.

Last edited by hoho; 14th October 2019 at 05:08 PM.
  Reply With Quote
Old 14th October 2019, 06:50 PM   #7
JMFahey is offline JMFahey  Argentina
diyAudio Member
 
JMFahey's Avatar
 
Join Date: Mar 2009
Location: Buenos Aires - Argentina
Quote:
Originally Posted by hoho View Post
Hi,

I've got a HiFi stereo preamp with two 12ax7 valves. Attaching the schematics (the schematics uses 12au7 as the second valve, but I've put 12ax7 there as well, they seem to be compatible and 12ax7 gives more gain), it shows one channel.

I want to make a guitar preamplifier from it. I've been looking at some guitar preamp schemes on the internet and it seems like I mostly need to change some resistors (to the ones with the lower resistance), but as I'm not really experienced in the area, I am asking for advice.

The goal is to keep the preamp stereo and have two independent channels for two different instruments, and feed the result to a DAC (to digitally process it further).

Another thing I want is to supply each channel with two potentiometers: one for the gain (between the valves?), another one for the master volume (after the valves). To go from reasobably clean to reasonably crunchy being able to maintain the proper output level.

Could you recommend which resistors need to be changed? As for the gain and master volume I want to replace the two 470K resistors from the scheme, with 1M log pots — are these the correct targets for gain and volume?

Needless to say that as it is it works when I connect something like the headphones output, but when I try to connect the guitar, I cannot hear it.

I would appreciate any advice.
Shortest path to what you want:
* short R6 (replace it with a wire link)
* remove R4
* replace 12AU7 with 12AX7
* make its plate and cathode resistors same as on 12AX7 on the left
* Replace R7 with an Audio/Log 500 k pot.
Of course, if that is a Hi Fi Stereo Preamp, make 2 (mono) Guitar preamps, no use for Stereo there, so use 2 separate single pots, label them Ch1 and Ch2.
Those are Volume/Gain/Drive pots.
* Replace "623" with again 2 500k Audio pots, label them Master 1 and 2 respectively.

You will need to drill 4 pot holes in your chassis, plus needed In-Out jacks.
I suggest your front panel layout as:
Ch1 Input Jack, Volume/Gain, Master ; then Ch2: same thing.

Safety precaution: how much can the following stage (DAC or Preamp) take?

This tube preamp can easily put out 60 to 90V RMS; enough to NUKE anything after it (unless itīs a tube grid); so I suggest you add some 4V or so Zeners in parallel with output, in series but opposite ways, so peaks never reach 5V, which should be safe.
__________________
Design/make/service musical stuff in Buenos Aires, Argentina, since 1969.
  Reply With Quote
Old 14th October 2019, 07:32 PM   #8
hoho is offline hoho
diyAudio Member
 
Join Date: Oct 2019
Quote:
Originally Posted by JMFahey View Post
Shortest path to what you want:
* short R6 (replace it with a wire link)
* remove R4
* replace 12AU7 with 12AX7
* make its plate and cathode resistors same as on 12AX7 on the left
* Replace R7 with an Audio/Log 500 k pot.
Of course, if that is a Hi Fi Stereo Preamp, make 2 (mono) Guitar preamps, no use for Stereo there, so use 2 separate single pots, label them Ch1 and Ch2.
Those are Volume/Gain/Drive pots.
* Replace "623" with again 2 500k Audio pots, label them Master 1 and 2 respectively.

You will need to drill 4 pot holes in your chassis, plus needed In-Out jacks.
I suggest your front panel layout as:
Ch1 Input Jack, Volume/Gain, Master ; then Ch2: same thing.

Safety precaution: how much can the following stage (DAC or Preamp) take?

This tube preamp can easily put out 60 to 90V RMS; enough to NUKE anything after it (unless itīs a tube grid); so I suggest you add some 4V or so Zeners in parallel with output, in series but opposite ways, so peaks never reach 5V, which should be safe.
OMG! Thank you SO very much! I will research about what Zeners are and will try to rewire as you have suggested.

As for the ADC (I have written DAC by mistake, I'm so eager to get to the DAC stage that I've forgotten that I haven't even reached the ADC yet ), I have a cheap PCM1808 board (PCM1808 Audio Stereo ADC Single-Ended Decoder 24bit Amplifier Board Player Modul | eBay), if I'm reading the chip's spec correctly, it has 6.5V as absolute maximum, so 60 to 90V possible range definitely needs limiting.

Thank you once again!
  Reply With Quote
Old 14th October 2019, 07:55 PM   #9
hoho is offline hoho
diyAudio Member
 
Join Date: Oct 2019
Quote:
Originally Posted by JMFahey View Post
Shortest path to what you want:
* short R6 (replace it with a wire link)
* remove R4
* replace 12AU7 with 12AX7
* make its plate and cathode resistors same as on 12AX7 on the left
* Replace R7 with an Audio/Log 500 k pot.
Of course, if that is a Hi Fi Stereo Preamp, make 2 (mono) Guitar preamps, no use for Stereo there, so use 2 separate single pots, label them Ch1 and Ch2.
Those are Volume/Gain/Drive pots.
* Replace "623" with again 2 500k Audio pots, label them Master 1 and 2 respectively.

You will need to drill 4 pot holes in your chassis, plus needed In-Out jacks.
I suggest your front panel layout as:
Ch1 Input Jack, Volume/Gain, Master ; then Ch2: same thing.

Safety precaution: how much can the following stage (DAC or Preamp) take?

This tube preamp can easily put out 60 to 90V RMS; enough to NUKE anything after it (unless itīs a tube grid); so I suggest you add some 4V or so Zeners in parallel with output, in series but opposite ways, so peaks never reach 5V, which should be safe.
One additional question about the plate resistors. There is R1 there, should I short it too? Otherwise putting the same 100K resistor for the second valve won't really be the same.
  Reply With Quote
Old 14th October 2019, 08:00 PM   #10
hoho is offline hoho
diyAudio Member
 
Join Date: Oct 2019
And one more little question. Is it safe to try the modifications on the one channel only keeping the second one in the original state (in order to try the changes step by step and not all at once). I suppose it should be fine, but not really sure how the valve behaves when one part of it operates with much higher gain than the second one..
  Reply With Quote

Reply


Modifying a 12ax7 HiFi valve preamp to be a guitar preampHide 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

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
Valve preamp for bass guitar Philfr Instruments and Amps 13 7th January 2014 04:32 AM
Modifying Maplin preamp kit mfurseman Analog Line Level 32 17th June 2012 12:18 AM
Valve Preamp for voice / guitar purpose aguantesoda Instruments and Amps 4 19th August 2010 12:31 AM
Gain calculating in 12ax7 guitar preamp McBarry Instruments and Amps 4 19th November 2009 12:52 AM
Preliminary 12AX7 guitar preamp raidfibre Instruments and Amps 4 6th November 2007 10:24 PM


New To Site? Need Help?

All times are GMT. The time now is 09:08 AM.


Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2019 DragonByte Technologies Ltd.
Resources saved on this page: MySQL 14.29%
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2019 DragonByte Technologies Ltd.
Copyright ©1999-2019 diyAudio
Wiki