New tubes for SVT 3 Pro

tom once dead · #1 12-13-2010, 04:56 AM

I'm thinking about getting a full new set of tubes for my 3 Pro. Im getting a bit of distortion before the clip light goes on, which didn't happen when I first got the amp.

So before I go out and buy some new tubes I thought I might find out what some other 3 pro users have re-tubed their amp with. I'm going for a really midrangy tone with a bit of bottom end.

spego · #2 12-13-2010, 06:10 AM

I just retubed mine less than a month ago. I used all JJ's in mine.....I think it was $54 shipped from Amazon for all the tubes. I went totally based off a friends experience with an SVP and he said the JJ's sounded great.....so I took his advise. It sounds like it never has....better than it did new. It growls....lots of bottom/low mids and just sounds louder to me (could be just me though). If I can help further just ask.

kmqu601 · #3 12-13-2010, 10:39 AM

I'm actually curious about this as well. Just bought new (used) svt 3 pro and was thinking about changing the tubes so that I have a point of reference for my own records.

Verminaard · #4 12-13-2010, 10:46 AM

I put a JJ set in mine a year ago and they seem to really jive with the 3 pro. Sounds way better than it used to with the original tubes. I'd say go for it!!

TheNightLife · #5 12-13-2010, 12:43 PM

ive put mesa sp12ax7's and there 12au7 into 2 svt3's and it sounded amazing. cleaned everything up and it had much more growl to it. it cost 100 dollars total to get all the tubes from guitar center.

tom once dead · #6 12-13-2010, 08:40 PM

So the JJ's seem to go really well with the 3 Pro then?
Will it be cheaper to buy the tubes as a set or get them all separately.

spego · #7 12-13-2010, 08:52 PM

The cheapest I found all the tubes (in one spot) shipped was Amazon through a seller called AmplifiedParts.

JJ ECC82 / 12AU7 Vacuum Tube
Sold by: AMPLIFIED PARTS
Condition: new
Quantity: 1
$9.20 each
Item subtotal: $9.20

JJ ECC83 / 12AX7 Vacuum Tube
Sold by: AMPLIFIED PARTS
Condition: new
Quantity: 4
$9.75 each
Item subtotal: $39.00

---------------------------------------------------------------------

Item Subtotal : $48.20
Shipping & Handling : $5.95
Total : $54.15
Paid by Visa: $54.15

tom once dead · #8 12-24-2010, 07:24 PM

So I just went to order the tubes off Amazon, but they wont ship them to Australia. Is there another US store that sells JJ tubes and will ship them over here?

Travis_Bass · #9 12-24-2010, 07:27 PM

Im glad im not the only one thinking of retubing. I bought my amp used and it has the original 5 yr old tubes. Whenever I boost on my bass, or use a wah effect, I get ugly clip distortion. Tubes might be at their max.

tom once dead · #10 12-24-2010, 08:01 PM

yeah, same thing happens with me. I play on a different (brand new) 3 Pro all the time and it doesnt start to distort for ages, unlike mine that distorts before the clip light comes on.

If you're in the states you can get them off Amazon spego posted all the details.

JimmyM · #11 12-24-2010, 08:07 PM

tom, your mosfets might need rebiasing. classic symptom in the svt 3 pro.

tom once dead · #12 12-24-2010, 08:22 PM

Is that a thing I'll need to see an amp tec about?

JimmyM · #13 12-24-2010, 10:43 PM

yep. it could easily burn your amp up if you bias it too hot. the mosfets tend to drift over time and just need tuned up every now and then. but it has to be done on a scope to really close tolerances.

audiomitch · #14 12-24-2010, 11:01 PM

Quote:

Originally Posted by JimmyM

yep. it could easily burn your amp up if you bias it too hot. the mosfets tend to drift over time and just need tuned up every now and then. but it has to be done on a scope to really close tolerances.

I agree it should be done by a tech if you're not familiar with working with electronics. But according to Jerrold Tiers all you need is an accurate multimeter. You measure the voltage across the mosfet transitors and adjust the bias pot to get it to spec.

I biased my mosfets using his procedure and didn't burn anything up. Mine were really low - spec is an average 25mv across all the transistors, mine were measuring an average of 3mv.

The original threads are all in this forum, but here's some excerpts (unfortunately out of context) I pasted in to a text document -

Jerrold Tiers

The adjustment MUST be done with NO LOAD. Any load will draw a slight current due to small DC offsets, and messes up the reading. Just about any SS biasing should be done no-load.

Then also, it is 25mv, not 10 mv, measured across individual source resistors. That is the average per "set" of N-channel or P-channel mosfets.

This is for a unit with graded and matched mosfets. There should be a color dot on each device, and all N type or all P type devices should have same color, although the N and P may be different.

Jerrold Tiers

The current can be higher than what you have. The expectation was literally that the average would be "at" the bias and some would be a bit high, others low. This is for either group......... measure across each resistor in the group, and average them. Range should not have any really high ones, which should be Ok for matching color dotted parts.

There is a tendency for the bias to even out at slightly higher currents.

The actual spec for matching is that the "Vgs(on)" must be within a 0.3V range for all parts in one grade (color). That is the voltage from gate to source when the drain current is 1 mA.

Obviously, that works "backwards" also, so that if the gate voltages are all the same, the drain currents will tend to match due to the source resistors. The issue isn't at low currents, it is the matching at high currents. With about a half ohm of source resistor, the mosfets must share within about 0.6 A, ensuring nobody hogs all the current.

The bias is simply to get as much current in the parts as possible, within reason and without getting the devices too hot at idle, so as to avoid crossover "notch" at all likely temperatures (the usual bias circuits never quite track precisely as they should). The SVT-3PRO has zero feedback at audio frequencies (it has DC feedback), so crossover distortion is particularly critical.

When I wrote the original matching spec proposal, it was for matching to 0.25V. But it turned out that the manufacturer was already matching to 0.3V, and could do that very cheaply, so we took them up on it.

JimmyM · #15 12-24-2010, 11:08 PM

really? did not know that. a meter can be had cheaper than a bench charge, that's for sure.

okcrum · #16 12-24-2010, 11:12 PM

Quote:

Originally Posted by JimmyM

really? did not know that. a meter can be had cheaper than a bench charge, that's for sure.

True, but you have to measure across the source resistors, not the transistors. Hopefully the guy can identify them.

audiomitch · #17 12-24-2010, 11:18 PM

Quote:

Originally Posted by okcrum

True, but you have to measure across the source resistors, not the transistors. Hopefully the guy can identify them.

Sorry, brain fart.

okcrum · #18 12-24-2010, 11:22 PM

Quote:

Originally Posted by audiomitch

Sorry, brain fart.

Ya had me worried for a sec there.

JimmyM · #19 12-24-2010, 11:24 PM

i wouldn't know what the hell to touch in there. i'm best kept away from that stuff.

tom once dead · #20 12-24-2010, 11:28 PM

I think I'll just take it to a tec. I'm not touching anything in there.

Still interested to know if anyone knows of some US sites that ship JJ tubes down under.

Edit: I just had a look at the tubes that are in there now.
The first one right at the input jack is a Groove Tubes, then the two other pre amp tubes are both Sovteks. But the interesting part is the driver tubes. The 12AX7 is another Sovtek but the 12AU7 is some no name brand with 12AU7 stamped on it and a Made in China sticker on it.

I think changing the tubes is going to help a lot.