I have a West Labs Mini IR (1973) guitar amp that is a work in progress. I re-acquired the amp after it made the rounds with a few other musicians. It was my first tube amp that I bought with my own $$ so a bit sentimental about it.
The amp is now working rather well . I have re-tubed it, put a 3 prong grounded power cord, re-coned speaker, cleaned all jacks and sockets etc..
I was wondering if anyone may have some ideas or experience with these amps (or the Grande, Avalon, Filmore models)
I am looking to lower the noise floor a bit, and a couple of other things.
Would replacing the 1st/2nd stage preamp tube from 12AX7 to type 7025 make a big difference in the noise floor? I don't have significant hum problems.
It does not have a master volume, just a volume after 1st stage + tone stack. When the preamp volume is all the way up, the signal to noise sounds acceptable ( but rattles the windows!) So I will probably put a master volume but not sure if right before the phase inverter or post PI...Any opinions?
I could probably change out some of the old carbon comp resistors in the preamp stages to metal films? Is it worth it?
The amp is now working rather well . I have re-tubed it, put a 3 prong grounded power cord, re-coned speaker, cleaned all jacks and sockets etc..
I was wondering if anyone may have some ideas or experience with these amps (or the Grande, Avalon, Filmore models)
I am looking to lower the noise floor a bit, and a couple of other things.
Would replacing the 1st/2nd stage preamp tube from 12AX7 to type 7025 make a big difference in the noise floor? I don't have significant hum problems.
It does not have a master volume, just a volume after 1st stage + tone stack. When the preamp volume is all the way up, the signal to noise sounds acceptable ( but rattles the windows!) So I will probably put a master volume but not sure if right before the phase inverter or post PI...Any opinions?
I could probably change out some of the old carbon comp resistors in the preamp stages to metal films? Is it worth it?
Hi Shanx,
Well I can't find a schematic for a mini IR on the West Labs schematics page:
West Laboratories Guitar Amplifiers - Schematics
Have you made any drawings or taken any pictures you could post? Tedious though it is, actually doing a layout drawing of your amp does really help you be on the ball when it comes to repairing or modifying it. If you do a layout diagram I'd be willing to help sort a schematic, assuming it isn't already up there.
Wrt replacing carbon composition resistors with metal film - you need to be careful, metal film resistors, whilst excellent in many respects usually don't like the sort of high voltages found in valve amps, 250v is often a limit for them, but you need to actually check out some resistor spec sheets (don't get too excited now!).
The problem is that above a certain voltage, the resistance effectively decreases - they become non-linear. Not what you want.
Get whatever info you can up, I look forward to seeing it.
Well I can't find a schematic for a mini IR on the West Labs schematics page:
West Laboratories Guitar Amplifiers - Schematics
Have you made any drawings or taken any pictures you could post? Tedious though it is, actually doing a layout drawing of your amp does really help you be on the ball when it comes to repairing or modifying it. If you do a layout diagram I'd be willing to help sort a schematic, assuming it isn't already up there.
Wrt replacing carbon composition resistors with metal film - you need to be careful, metal film resistors, whilst excellent in many respects usually don't like the sort of high voltages found in valve amps, 250v is often a limit for them, but you need to actually check out some resistor spec sheets (don't get too excited now!).
The problem is that above a certain voltage, the resistance effectively decreases - they become non-linear. Not what you want.
Get whatever info you can up, I look forward to seeing it.
Hi Simon, sorry I did not post the whole schem. It is a variant of the Avalon type which is on that westlab site link you gave. It has an additional spring reverb circuit almost identical to the fender twin reverb. I have made copious notes , but all hand sketched.
Right about the metal films, I am going to look at that carefully. I am going toward Vishay FP69 series rated to 500 V, 2W or other flame proof type., maybe only on the lower voltage preamp signal path?
I will post a picture
Right about the metal films, I am going to look at that carefully. I am going toward Vishay FP69 series rated to 500 V, 2W or other flame proof type., maybe only on the lower voltage preamp signal path?
I will post a picture
Hi Shanx,
Just been taking a look at the schematic. Interesting that it appears to use an ultra-linear output txfmr. Are you running it with 6CA7s, or EL34s ?
My main guitar amp is a very old home-brewed amp, EL34s connected ultra-linear. It was originally built as a hifi amp, judging by the labelling of the inputs. No master volume and I probably won't add one, at home I often run through my dummy load, which has a variably attenuated output, I should add a power meter though really.....
Myself, I like the sound of an output stage working. I had a quite nice Marshall 50W with m/v once - I found the pre-amp distortion on its own just sounded like a pedal really.
If your amp has the effects loop shown on the circuit of the Avalon;
West Laboratories Guitar Amplifiers - Schematics
ie sniff and break send and return, you could try inserting an unconnected jack plug into the return to break the path from the output of the first valve to the phase splitter - if your noise problem gets much better doing this, you know that it's the preamp you should work on first.
Incidentally, if you've got 'em, you can use that pair of sockets to see what a pre phase splitter master volume control would be like.
What sort of voltages do you see along the power supply chain of caps and resistors? Points A,B,C,D on the diagram.
Just been taking a look at the schematic. Interesting that it appears to use an ultra-linear output txfmr. Are you running it with 6CA7s, or EL34s ?
My main guitar amp is a very old home-brewed amp, EL34s connected ultra-linear. It was originally built as a hifi amp, judging by the labelling of the inputs. No master volume and I probably won't add one, at home I often run through my dummy load, which has a variably attenuated output, I should add a power meter though really.....
Myself, I like the sound of an output stage working. I had a quite nice Marshall 50W with m/v once - I found the pre-amp distortion on its own just sounded like a pedal really.
If your amp has the effects loop shown on the circuit of the Avalon;
West Laboratories Guitar Amplifiers - Schematics
ie sniff and break send and return, you could try inserting an unconnected jack plug into the return to break the path from the output of the first valve to the phase splitter - if your noise problem gets much better doing this, you know that it's the preamp you should work on first.
Incidentally, if you've got 'em, you can use that pair of sockets to see what a pre phase splitter master volume control would be like.
What sort of voltages do you see along the power supply chain of caps and resistors? Points A,B,C,D on the diagram.
Hi Simon, Yes I did similar test at the Phase splitter and it did help. I will take some time and prepare a proper schem to post. You are right that I should try with a ''hot plate'' dummy load too, I am sure to get a better signal to noise with the existing preamp volume control turned up..That is certainly one issue.
Will need to check back on my notes for supply voltage readings.
Will need to check back on my notes for supply voltage readings.
tube swap on preamp
Well I finally took the easy route, and picked up two Sovtek 12AX7WA (7025 is also stamped on them). I swapped them in and gave them a run, they are on the 1st/2nd stage, and also on the reverb circuit.
I had originally retubed the amp with Electro-Harmonix (12AX7EH) triodes. The Sovtek swap made a very noticable difference on the noise floor. The E-H tubes were also Russian made but not sure if they were older Sovteks or Svetlana ? I would have to say the gain seems a bit lower on the Sovtek tubes, needed to dial down the treble a bit too..maybe their capacitance is a bit lower, I don't know.
The amp has a tonne of headroom, I can dime it and just starting to get a warm breakup. Is it the ultra linear config on the EL34s that keep it so clean? SHould I run them in standard mode and put screen supply/resistors? Running Mesa Boogie EL34s and not sure who made them.
I am going to swap the 1M and 68K resistors right at the input with metal films. That should kill a bit more noise, although now it is really quite respectable.
Well I finally took the easy route, and picked up two Sovtek 12AX7WA (7025 is also stamped on them). I swapped them in and gave them a run, they are on the 1st/2nd stage, and also on the reverb circuit.
I had originally retubed the amp with Electro-Harmonix (12AX7EH) triodes. The Sovtek swap made a very noticable difference on the noise floor. The E-H tubes were also Russian made but not sure if they were older Sovteks or Svetlana ? I would have to say the gain seems a bit lower on the Sovtek tubes, needed to dial down the treble a bit too..maybe their capacitance is a bit lower, I don't know.
The amp has a tonne of headroom, I can dime it and just starting to get a warm breakup. Is it the ultra linear config on the EL34s that keep it so clean? SHould I run them in standard mode and put screen supply/resistors? Running Mesa Boogie EL34s and not sure who made them.
I am going to swap the 1M and 68K resistors right at the input with metal films. That should kill a bit more noise, although now it is really quite respectable.
Oh and another thing..
I have seen more interest in West Labs on the web lately. I emailed Dave West, the founder, when I got this amp up and running, he was very helpful.
This amp was built in 1973, and is still very servicable. People who are lucky enough to own one swear by them. I just hope that when he does the re-issues, he builds them like that! (but please build a lighter cabinet.)
I have seen more interest in West Labs on the web lately. I emailed Dave West, the founder, when I got this amp up and running, he was very helpful.
This amp was built in 1973, and is still very servicable. People who are lucky enough to own one swear by them. I just hope that when he does the re-issues, he builds them like that! (but please build a lighter cabinet.)
Sovteks seem generally good, I like JJ/Tesla a lot, and their prices are comparatively low too. 'Badged' valves always seem a bit dubious to me.
Re switching to standard pentode mode, with a screen supply/resistors:
On my amp, I like the sound of it in ultra-linear, "warm breakup" is what I get too. I can push it to the point where there's plenty of that, and I know that the output transformer, which is quite massive is fine with it. I've seen and heard it pushed to output valve meltdown by the previous owner, with no other damage.
If you're in communication with Dave West, maybe you could ask what he'd think about increasing the gain to give you more drive?
Alternatively, you could use a three pole three way switch as a combined B+ & mode switch, with standby in the middle, ultralinear and normal either side of it, but again, I'd speak to Dave about running in normal mode.
Re switching to standard pentode mode, with a screen supply/resistors:
On my amp, I like the sound of it in ultra-linear, "warm breakup" is what I get too. I can push it to the point where there's plenty of that, and I know that the output transformer, which is quite massive is fine with it. I've seen and heard it pushed to output valve meltdown by the previous owner, with no other damage.
If you're in communication with Dave West, maybe you could ask what he'd think about increasing the gain to give you more drive?
Alternatively, you could use a three pole three way switch as a combined B+ & mode switch, with standby in the middle, ultralinear and normal either side of it, but again, I'd speak to Dave about running in normal mode.
Hi Simon,
The Mesa Boogie EL34s I think are pretty decent. I paid a pretty penny for them, but also bought in because a fellow musician has a Mesa Boogie Stiletto running the same tubes. No problems and a stunning sound from them. He can run them as ''triode'' connected, and pentode. I could probably check and see who builds them.
What is a bit strange is some of the web reviews gave the 12AX7EH from Electo-Harmonix (Russian sourced) a better noise rating than the Sovtek. What I think may have happened is Sovtek did in fact upgrade that line including the -WA version, they are stamped both 12AX7-WA/7025 and the construction looks a bit different.
The Ultra/Lin sound is probably the most focused and accurate of all modes. It sounds really good with my archtop's humbuckers. You get a warm note with a bit of singing overtones but retain a tight controlled sound. With single coils, Mark Knopfler or Steve Cropper like tone..Guess that is why it is the preferred in Hi-Fi amps.
I like your idea of the 3 way switch. I am not too worried about the OT handling it. It has gone through a lot of worse abuse.
Dave West seems to be getting his company off the ground again, He has released a new mini head, with promises of some reissues soon. I wish him luck.
PS If you have checked out Trainwreck schems by Ken Fisher it is quite similar to the Avalon (and also early Marshalls).
The Mesa Boogie EL34s I think are pretty decent. I paid a pretty penny for them, but also bought in because a fellow musician has a Mesa Boogie Stiletto running the same tubes. No problems and a stunning sound from them. He can run them as ''triode'' connected, and pentode. I could probably check and see who builds them.
What is a bit strange is some of the web reviews gave the 12AX7EH from Electo-Harmonix (Russian sourced) a better noise rating than the Sovtek. What I think may have happened is Sovtek did in fact upgrade that line including the -WA version, they are stamped both 12AX7-WA/7025 and the construction looks a bit different.
The Ultra/Lin sound is probably the most focused and accurate of all modes. It sounds really good with my archtop's humbuckers. You get a warm note with a bit of singing overtones but retain a tight controlled sound. With single coils, Mark Knopfler or Steve Cropper like tone..Guess that is why it is the preferred in Hi-Fi amps.
I like your idea of the 3 way switch. I am not too worried about the OT handling it. It has gone through a lot of worse abuse.
Dave West seems to be getting his company off the ground again, He has released a new mini head, with promises of some reissues soon. I wish him luck.
PS If you have checked out Trainwreck schems by Ken Fisher it is quite similar to the Avalon (and also early Marshalls).
Swap of PI tube to 12AX7
Running the preamp tubes now with the 12AX7WA/7025 and they are quiet.
That allowed me to try a tube swap on the PI from 12AT7 to the now spare 12AX7.
I left the 12AT7 on the reverb drive circuit.
With the gain increase at the PI , I am able to get into a nice harmonic distortion level with lower levels of guitar volume. The sound is slightly ''edgy'' ( need to roll off the highs a bit). Not a ''growling'' amp sound but definitely showing its teeth!
I still think the master volume would be nice adjustment to balance between a bit of preamp distortion and PI / Power tube overdrive. I will probably modify the NFB with a presence control pot (and some filtering to roll the highs off)
Running the preamp tubes now with the 12AX7WA/7025 and they are quiet.
That allowed me to try a tube swap on the PI from 12AT7 to the now spare 12AX7.
I left the 12AT7 on the reverb drive circuit.
With the gain increase at the PI , I am able to get into a nice harmonic distortion level with lower levels of guitar volume. The sound is slightly ''edgy'' ( need to roll off the highs a bit). Not a ''growling'' amp sound but definitely showing its teeth!
I still think the master volume would be nice adjustment to balance between a bit of preamp distortion and PI / Power tube overdrive. I will probably modify the NFB with a presence control pot (and some filtering to roll the highs off)
The PI and Power tube section, I think from what I see, are equally important to the contribution of output overdrive.The NFB & adding filtering should (I hope) re-correct the voicing of the amp. To me there seems to be more high frequency emphasis, with the higher PI gain. Makes sense the 12 AX7 has different transconductance, and I think the capacitance is different too ( didn't check though). Obvious I want to keep the harmonics..just don't want to be on the edge of oscillation (it is really close right now).
Haven't used one I'm afraid, but it's a very interesting idea.
You've got a couple of variables to consider here:
Do you reduce B+ to:
Output stage only (adjusting bias of course),
Output stage and PI,
Output stage, PI and pre-amp.
Also, do you want to feed the reduced voltage via a mosfet, which means you've got a stabilised supply, or do you want to stick with RC filtering, keeping some ripple in?
Just in case you aren't aware of it, there's opinion and evidence that ripple, and the way it dirties the ouptut signal is an important part of good rock guitar sound, which a stabilised supply would remove.
Let us know how you get on with this.
You've got a couple of variables to consider here:
Do you reduce B+ to:
Output stage only (adjusting bias of course),
Output stage and PI,
Output stage, PI and pre-amp.
Also, do you want to feed the reduced voltage via a mosfet, which means you've got a stabilised supply, or do you want to stick with RC filtering, keeping some ripple in?
Just in case you aren't aware of it, there's opinion and evidence that ripple, and the way it dirties the ouptut signal is an important part of good rock guitar sound, which a stabilised supply would remove.
Let us know how you get on with this.
Hi Simon,
I think the VVR Kit from hall is a power mosfet type, controling the power tube supply/grid bias.
I would like to keep reasonable amount of headroom at V1 and V2..and the 12AX7 on the PI has a lot more gain now. A master volume pre-PI is probably going in, I like a small amount of preamp drive control, and I can always leave it at max if needed (!)
I am just finishing up an effects pedal build for a friend right now, and I intend to do a bit more fiddling with the amp shortly.
I think the VVR Kit from hall is a power mosfet type, controling the power tube supply/grid bias.
I would like to keep reasonable amount of headroom at V1 and V2..and the 12AX7 on the PI has a lot more gain now. A master volume pre-PI is probably going in, I like a small amount of preamp drive control, and I can always leave it at max if needed (!)
I am just finishing up an effects pedal build for a friend right now, and I intend to do a bit more fiddling with the amp shortly.
Master Volume installed
After comparing my notes and some fender and marshall master volumed amps, I decided to go for it. I have not yet changed any filtering, or NFB.
The mod was a big payoff, she sounds ''right'' now. The edge of oscillation and high frequency over-emphasis is gone, is running dead quiet at reasonable volumes. It has that bit of preamp warm tone, actually sounds like a tube amp should now..it was too cold/clean before. WIth the 12AX7 in the PI position it was almost too much gain and getting tinny. The MV seemed to calm it down and move the voicing back to normal.
I know some people may say they don't like the MV added on some amps. I wired it in and it only took total 1/2 hour to do and it worked for this amp. Not saying a solution for every amp, but it worked really well this time. If it hadn't worked out, I would have just taken another 15 minutes to pull it off circuit.
Using a 500K Lin sealed clarostat type, instead of the fender 1M. Because this ihas a reverb unit, I put the pot right before the PI. You get MV control with both reverb and dry. Top of pot to previous stage and the bottom doesn't go to ground: goes to the top of the 100 ohm resistor of the PI tail (where the NFB goes back to ) Fender did that on their '73 fender twin. But I did not add that filter capacitor that they put on the wiper. Wiper direct to the 0.01uF entry to PI as usual.
Simon, I am going to add the presence circuit, just need a little time now. Life gets in the way and I got loaded with a bunch of stuff at work...but I am hoping the electrics will be all in order for next shoowtime. It has not been publicly played in something like 20 years!!
I am going to email Dave West some of these minor mods I have done so far on this. Wish list is to put in some power tube control now.
After comparing my notes and some fender and marshall master volumed amps, I decided to go for it. I have not yet changed any filtering, or NFB.
The mod was a big payoff, she sounds ''right'' now. The edge of oscillation and high frequency over-emphasis is gone, is running dead quiet at reasonable volumes. It has that bit of preamp warm tone, actually sounds like a tube amp should now..it was too cold/clean before. WIth the 12AX7 in the PI position it was almost too much gain and getting tinny. The MV seemed to calm it down and move the voicing back to normal.
I know some people may say they don't like the MV added on some amps. I wired it in and it only took total 1/2 hour to do and it worked for this amp. Not saying a solution for every amp, but it worked really well this time. If it hadn't worked out, I would have just taken another 15 minutes to pull it off circuit.
Using a 500K Lin sealed clarostat type, instead of the fender 1M. Because this ihas a reverb unit, I put the pot right before the PI. You get MV control with both reverb and dry. Top of pot to previous stage and the bottom doesn't go to ground: goes to the top of the 100 ohm resistor of the PI tail (where the NFB goes back to ) Fender did that on their '73 fender twin. But I did not add that filter capacitor that they put on the wiper. Wiper direct to the 0.01uF entry to PI as usual.
Simon, I am going to add the presence circuit, just need a little time now. Life gets in the way and I got loaded with a bunch of stuff at work...but I am hoping the electrics will be all in order for next shoowtime. It has not been publicly played in something like 20 years!!
I am going to email Dave West some of these minor mods I have done so far on this. Wish list is to put in some power tube control now.
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