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shanx 27th March 2012 04:06 PM

West Mini IR rebuild
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?

Simon B 27th March 2012 09:17 PM

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.

shanx 27th March 2012 10:02 PM

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

shanx 27th March 2012 10:05 PM

2 Attachment(s)
Attachment 274148

Attachment 274149

hope that worked

shanx 27th March 2012 10:13 PM

Those were some shots as I had pulled it out of the cabinet. I think I will start with the master volume and try a tube swap first. But just curious if it would be worth changing some of the first stage resistors and maybe the cathode bypass cap as next step.

Simon B 28th March 2012 10:20 PM

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.

shanx 29th March 2012 01:56 PM

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.

shanx 31st March 2012 04:51 AM

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.:eguitar:

shanx 31st March 2012 05:10 AM

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.)

Simon B 31st March 2012 01:04 PM

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.

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