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Bias Adjustment & Test Points Modification
by spaceatl on 2010-02-18 19:42:47.7580

Ok cgtrox, I know that it really took entirely too long for me to get these pics, but I recently replaced my camera with a Nikon and my card reader sucks big one...

Anyways, this is a bias test point and trim pot modification. The BOM for the parts are listed below. Please note that this is an SV112 MK1 and it might differ slightly as it is a very early one I think. I wonder if they changed the bias resistor or the trims are 10K pots in later production runs...Mine will not go lower than 36 ma with 5K pots...That's a little too hot for the pot all the way down...And they are loose as a goose too...It took me about 1 hour to do the mod...On a pro production line this would add one a minute or two to the build time...The parts cost would be about $2 if parts were purchased in bulk...Don't see why these amps don't have this out of the gate...But it isn't too hard to add...

This is my actual Weber order (www.tdweber.com)

Total Order Amount: $ 4.95

2 items PNJ-P-Red ($ 0.45 each) subtotal = $ 0.90  (No Tax)
Pin Jack, Red

1 item PNJ-P-Blk subtotal = $ 0.45  (No Tax)
Pin Jack, Black

2 items wp253L-Slot-Lock-small body ($ 1.80 each) subtotal = $ 3.60  (No Tax)
25K Linear Pot, screwdriver slot, locking bushing, small body

Shipping was $8...A little steep, but I needed them quickly...Arrived in 3 days...

You have to remove the power amp pcb and remove the trim pots carefully...I used a 14 watt iron, coper braid and solder sucker and removed them...Those trims varied by 200 ohms (5k and 4.8k)...The lowest I could get the amp balanced was 37.5ma...4.8K was parked and the 5K was up a hair...measured 4.82K...Nice match on those stock tubes...

You have to drill 3 test point holes and 2 bias pot holes...There is a huge advantage here because the chassis is ground and the pots are sheilded...I swear my amp is quieter than it was before and I think the unshielded stock pots close proxemety to the board might be why...A theory at least...

Rear Panel

Weber SV Bias Mod Rear Panel.jpg

Inside The rear Panel

Weber SV Bias Mod Inside Rear Panel.jpg

Close Up of the PCB points

Weber SV Bias Mod PCB Points.jpg



Re: Bias Adjustment & Test Points Modification
by BigChas52 on 2010-02-18 20:05:09.0190

This is great stuff.  Thanks space.  I just ordered my parts from Weber.  I know this may seem like a dumb question.  What gauge wire are you using?  I haven't done any electronice projects in years.



Re: Bias Adjustment & Test Points Modification
by cgtrox on 2010-02-18 20:26:25.0540

Very cool, thanks again man! The only thing that scares me is the last time we talked on the phone you told me to be sure and keep one hand behind my back. I know it's a dangerous mod but I might just let the pros do it here at the local shop, they're cool. It's juuuust a little too invasive for me, I'll stick to modding guitars!!

cgtrox



Re: Bias Adjustment & Test Points Modification
by spaceatl on 2010-02-18 20:34:30.3720

Your welcome...I am really curious to know if you notice an improvement you can't quite put your finger on...but good...

I should have said that 10Ks would have been a slightly better choice but I did not know the stick trim values when I ordered...The 25Ks take it down to 24ma which is a nice safe setting for new tubes before adjustment...

I am pretty sure the wire I used is all 28AWG (maybe 30)...I will double check that...That's an overrated size for the voltage and current going through here as I think the tiny pcb hookup stuff would be fine...I recycled the wire from some 6 channel snake scraps...tinned copper...good shit...basically whatever fits in the trim holes nice is bigger than the traces are anyway...Larger wire is fine for the test points as the point lugs are big and so are the header pins...They don't need to be since this 30mv DC on a meter...real high resistance in meter leads...A solder gun is useful on the test point lugs and even the molex pins...



Re: Bias Adjustment & Test Points Modification
by spaceatl on 2010-02-18 20:43:28.4790

It's not that bad...Just turn the amp off without going to standby...The caps drain into the speaker in a couple of seconds...

The trim removal could be difficult if you have not removed parts before and no shame in leaving that to someone who knows how to do it properly...The chassis drills easy enough...I put a towel over the pcbs and vacumed it after just to be sure...5 holes and you are done...

I know if you can wire a guitar, you can do the test points completely, mount the pots with wires ready so your tech need only remove the stock trims and pop in the wires for you...It would save a lot of time on the bench...



Re: Bias Adjustment & Test Points Modification
by cgtrox on 2010-02-18 20:52:29.6120

Hmmm, what's a good inexpensive bias tool to buy? Now you got me thinking...

cgtrox



Re: Bias Adjustment & Test Points Modification
by Karl_Houseknecht on 2010-02-18 20:58:22.3320

cgtrox wrote:

Hmmm, what's a good inexpensive bias tool to buy?

Bias tool?   Any decent digital multimeter.  I got the one I use at Lowe's for like $25.  Works great.  And will work great with space's test point mod too.



Re: Bias Adjustment & Test Points Modification
by sofnwhat on 2010-02-19 04:18:13.6160

Dude, I really dig this mod and I think I need it. Although might to part of it and leave the rest for a tech. I tried biasing my Carvin TS100 and got a hell of a shock. My arm was numb for 15 minutes. I'd like to be able to do the biasing myself so this would be a nice easy and safe way to do so. Thanks man. It might be a while before I can get to it, but I'm gonna do it.



Re: Bias Adjustment & Test Points Modification
by Karl_Houseknecht on 2010-02-19 04:53:30.0230

That TS100 is an absolutely horrible amp to bias.  You have to disconnect transformer leads and connect meters inline....just terrible.  Not surprised you almost fried yourself.  But glad you didn't get seriously hurt.



Re: Bias Adjustment & Test Points Modification
by sofnwhat on 2010-02-19 08:58:26.5220

Thanks Karl. I don't have that amp anymore, I wish I did though. This happened about a year ago. I never biased an amp before and I had a feeling that this one was more involved than the norm. I'm more cautious now.



Re: Bias Adjustment & Test Points Modification
by cgtrox on 2010-02-19 09:35:27.6070

So that's how you got your Hulk personna, GAMMA RAY exposure from a biasing accident!! JK man, glad to hear you survived it, that's what scares me the most....

cgtrox



Re: Bias Adjustment & Test Points Modification
by cgtrox on 2010-02-19 09:37:54.0210

Wow, good to know, thanks Karl! I think I have one already, just gotta look for it. I guess use the "ma" setting? Milliamp?

cgtrox



Re: Bias Adjustment & Test Points Modification
by sofnwhat on 2010-02-19 09:41:26.0420

cgtrox wrote:

So that's how you got your Hulk personna, GAMMA RAY exposure from a biasing accident!! JK man, glad to hear you survived it, that's what scares me the most....

cgtrox

If only that were true!



Re: Bias Adjustment & Test Points Modification
by brianvalve12 on 2010-02-23 15:59:15.3330

Try www.eurotubes.com. They have a meter and a plug that goes right into the tube socket. You plug the tube into the socket plug and adjust from there. So easy a dummy like me can do it. Theres even video available on their site about how to do it. I think they charge no more then like 20 or 30 bucks for the meter and the socket.



Re: Bias Adjustment & Test Points Modification
by spaceatl on 2010-02-26 14:31:26.2830

Whether you use a meter on the SV test points or insert tube probe sockets, unless you move the trim pots you are still opening up the amp to adjust...probe sockets are a lot better than the test points that are on the amp since those tend to break the curcuit so you can get a real current reading...But if you are going that far, then you might as well do it right with a tone generator and O-scope...



Re: Bias Adjustment & Test Points Modification
by spaceatl on 2010-02-26 14:33:29.8960

It's not really ma...It's mv...So put your meter on 200mv and go to town...The test points are an approximation of bias current...reading current for real is real pain in the arse because you have to break the circuit and insert the meter...But this is close enough...If you really want to do it right you need an oscilloscope tagged on the speaker output. You send a 2K tone through the power amp in. When the bias is exactly right the sin wave will be smooth. When it is off, there will be a little notch near the x axis...The idea is to smooth off the notch. But the reality is that when you smooth the notch, this reading will usually be relatively balanced...

I bought a new meter not long ago from PartsExpress for $20...It's a really nice one...

Not going to posting much for a while...I got moved to another Dev group in the company I work for and I am having to ramp up on .net, C++ and a new product...I was nearly 10 years in with my last group and got pretty comfortable...Nice to have something new to do in the programming world...



Re: Bias Adjustment & Test Points Modification
by LoonyBin-Fizzbin on 2011-08-05 12:51:30.8510

P7170075.JPG

Hello everyone..Late to Post...

I finished the external Bias “Station” Mod... on my Spider-Valve MK1. And while I was at it (drilling holes) I decided to do the SV-Pre external Push Mode access points with pedal option....at the same time.

I also ordered the Parts from Weber Speakers (USA), and had them (the parts) shipped to Brasil, the order was 18 USD, so I didn´t pay import taxes (Duty).. Just the currency exchange rate.. which is low at this time....And it took only 2 weeks to arrive too...Got to love it... (when you are used to month’s!!!)

And I ordered new valves all-around. They have not arrived yet, Svetlana 6L6GC NSV-6L6GC matched pair, and Svetlana 12AX7 NSV-12AX7 with Matched Sections, from The Tube Store: http://thetubestore.com/nsvetlana.html

The Mod works Great...But I did/do have one issue..My V-4 25k Pot seems to be slightly defective..But I have learned to trim with it nonetheless. I´ll try to explain.. V3 Pot acts as expected, but the V-4 one... At it´s lowest setting. They both (V-3 & V-4) sit at +/- 1 mV difference from a reading of 11.5 mV. But once I turn V-4 Pot up only 1 mm (Clockwise) & stop, the mV´s rise without stopping gradually to the heavens..(I got caught of guard the first time and it went to 120 mV) I found I could control it by going back to the lowest setting, or going up about half way, then it seems to taper off to a “Normal' behavior... And Once “Locked” (using the lock Nut) it stays where I put it...I tested this over 3 days...and I have even tapped it on the shaft with a pair of pliers, to be sure it wasn´t a “fluke” (Digital Multi-Meters Batman)

As well before I did the Mod´s I wanted to have readings, so I found I was biased lowered than I thought at 31mV on both V-3 & V-4.. The last time I checked was about a year ago. I don´t use my Amp too often (or didn´t as much as I do now)

Then I metered the SV-pre to ground and as well to it´s pcb´s common, (could be isolated) reading from the DIP-Switches were so low in the mV range, that adding an external switching point & optional pedal Jack, seemed feasible, so why not? Even if I may never build a pedal. I wired a switchable TRS Female: SW-1 to Tip, SW-2 to Ring, and Common to Sleeve..

My overall rating of this mod is 5-Star...regardless of the V-4 pot...It makes taking knots (measurements) so easy.... The Difficulty for me was the actual drilling, because I used cheap hand tools, when in the past, I got used to having a full shop available to me, hand tools are not accurate as I´d like, nor do they have the torque that a Drill Press does, ect. Yes I did the Awl mark, then a 1mm pilot hole, then used the 6mm bit. But I was not filled with confidence...LOL even though it wasn´t a “hard” thing to do..LOL

Removing the old parts, (Trim Pots & dip Switch for the SV-Pre) was easy, the trick to this is to let the part/s cool off before you give it a second or 3rd go.. and if the solder isn´t liquifying, add a little fresh solder..and keep it cool... Take your time...

Due to my renown bad memory, I labeled everything using my printer, some photo-grade paper, and Spay on Glue. (Spray the back of the Paper on a cheap board after it´s trimmed & fitted, and not your Amp....LOL)



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Re: Bias Adjustment & Test Points Modification
by raed6 on 2011-11-21 21:21:03.1610

Help!! http://line6.com/support/message/330573#330573




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