Ampeg SVT-3 Pro Tube Gain Question

[Jerrold Tiers]

I have read elsewhere here on the forums and heard from an electrical engineering buddy of mine that the retube should not have affected the MOSFET bias. Rather, it can "drift" over time.

Correct.... The tubes are not involved in the bias, although they DO directly drive the Mosfets thru a blocking capacitor setup..

Bias drift really shouldn't happen, but it is possible that the adjustment control may slightly shift over time due to vibration etc, or get dirty, whatever.

It's MUCH better not to try the "by ear" adjustment. If you go too far, in the wrong direction, you can cause an expensive problem.

 

[JimmyM]

Anything you can do to keep the bias trimpot in place so it doesn't drift?

 

[Jerrold Tiers]

Anything you can do to keep the bias trimpot in place so it doesn't drift?

Actually, I don't really expect it to. If the setting drifts, that is one possible reason, I don't think it is very likely.

Overall, the temperature of the unit affects it lots more than most anything else. Bring it in from a cold truck in a Canadian January, and it might be buzzy for a little while, although the setting IS temperature compensated.

 

[Mulemute]

Hello there,

Getting a svt3 within 3 weeks, I have spent hours to precise my choice of changing serial tubes by Mullard ones.
Choice is nearly done, but I still hesitate for the 12AU7. RCA or Mullard ? And second question, concerning the V1 12AX7,
choose a different tube from the 3 other ones ?
Did anybody already faced this question ?

Final answering; does someone have a map, or pic, of the tube schema, inside this Ampeg beast ?

Thank you very much for your help there !

 

[Mulemute]

I have found a photo of the inside, looking around here, sorry about that lads.

I can precise my question here : concerning the 12AX7 V1, does anybody here, noticed some tasty differences between
Mullards, Tung Sol, RCA (5751 precisely) ? one is punchier, or warmer, or giving more gain...

Thank you, for any helping comments ...

 

[deaf pea]

...3) MOSFET bias...Since I did it by ear, I would like to try and find out the procedure for doing this with a meter (i.e., the right way) so I know it's correct for my new setup. Can anyone help in that regard?...

You don't use a meter . . . you need an oscilloscope! And the proceedure is to adjust the trimpot for the least cross-over notch distorsion, which is easily visible with a 'scope . . .

 

[Jerrold Tiers]

You don't use a meter . . . you need an oscilloscope! And the proceedure is to adjust the trimpot for the least cross-over notch distorsion, which is easily visible with a 'scope . . .

Actually, you DO do it with a meter..........

Assuming the mosfets all have matching color dots for each set*, the adjustment is to have the average of the voltage across the 0.47 ohm "source" resistors be 20 to 25 millivolts....

Some may be a bit higher, some lower. if you do it by ear or 'scope, it may not be right when colder, as in outdoor gig, etc.... until it warms up.

You do the adjustment when the amp is either 'cold", i.e. just turned on, or when it is thoroughly warmed up. AND you do it with NO LOAD (no speaker) and no signal.

* there are two groups of mosfets, a set of IRFP240 and a set of IRFP9240. All the ones of either set should have matching color dots, signifying that they have been tested and fall into a fairly narrow grouping.

if the mosfets are NOT matched, things might get scary, because they may not properly share the load. Bias may be jumpy and unreliable, and teh amp may fail (will fail, most likely).

I do not know if the chinese manufacturer still does the color dots.........

 

[deaf pea]

Actually, you DO do it with a meter... if you do it by ear or 'scope, it may not be right when colder, as in outdoor gig, etc.... until it warms up...

Thanks for the clarification . . .

 

[fenderthrupeave]

I must say Ijust changed the tubes in my SVT3 with these and holly crap it sounds GREAT and my bands even likes it when I play loud now. It is now close to an overdriven SVT. All that for 50 bucks I cant complain at all.

So I am finally done futzing with my SVT-3 Pro...

I have to say, NOW I am impressed and very happy!

I did 3 things:

1) Complete retube. I went with eurotubes.com and ordered 4 standard JJ ECC83S tubes to replace the 12AX7's, and 1 JJ ECC82 for the 12AU7. There are options within these choices, but I went with the "standard" tubes. I dealt with Jay who was very helpful and thorough with explaining all the options. The improvement is nothing short of AMAZING! The amp literally came to life. I have more gain, but I didn't lose ability to get a nice clean tone (which was a concern). I also have much more overall volume (which I believe is due to the ECC82?). I no longer need to engage the EQ and bump the EQ level slider to get more volume. The tone is rich, warm, fat and punchy. If I want growl, I can get growl (although at the moment I still like my SansAmp Programmable Bass DI for the dirtier tones). The fan seemed to run a bit slower after the retube (albeit still noisy). It still ramped all the way up when the amp was first turned on, but it gradually slowed down to the point where I didn't hear the motor anymore, just the grinding sound I described before.

2) Replaced the fan. There was definitely something wrong with the original one. The (exact) replacement is by no means quiet, but it is a significant improvement to the degree where I'm no longer distracted by it when playing at low volume (the grinding noise is gone).

3) MOSFET bias. I read here in the forums that the 12AU7 is what feeds the MOSFETS, so I assume that replacing the 12AU7 (if not all of the tubes) had an effect on the amount of signal feeding the power amp section. I did notice some very low "solid state" sounding distortion/fuzz after the retube, which sounded exactly like what others have described on the forums. I carefully used a plastic potentiometer adjustment tool and nudged the MOSFET bias pot ever so slightly clockwise until the fuzz was gone. It literally took a hair's thickness of a turn. Since I did it by ear, I would like to try and find out the procedure for doing this with a meter (i.e., the right way) so I know it's correct for my new setup. Can anyone help in that regard?

So there you have it. Thanks to all the people here on TalkBass for your replies and helpful suggestions!

 

[Wakey]

...the adjustment is to have the average of the voltage across the 0.47 ohm "source" resistors be 20 to 25 millivolts....

I'm not a tech. I did follow these suggestions (re-tube with JJs and adjusted bias) as described in this thread. As a result of studying other threads I believe Jerrold Tiers meant to say that the bias should be adjusted so that the current to each Mosfet is "20 to 25 milliamps". As other threads point out, this means that the voltage measured across each .47 Ohm resistor should be a maximum of 11.75 millivolts, not 25 millivolts.

I hope I have understood this correctly and that it may help someone avoid blowing up their SVT3Pro. If I have got this wrong, sorry.

 

[fisi_badtoworse]

In my svt 3, the tube gain control doesn´t change the sound at all? I thinkI´ll change the tubes to try solving it...
How i can find the 470 ohm resistor and the potenciometer who limits his current? Have a specific name ind the PCB? some Squematic?

PS: excuse my poor english

 

[TONYTHOMAS1956]

Hey everyone, thanks for the replies! Guess I'm OK. Gonna test the tubes on my trusty old Mercury tester and replace any if necessary. Anything else I should be aware of with this amp? Read about the MOSFET bias adjustment, but I think I'm OK in that regard. What I'm wondering is however, how come people have that distortion problem on new units? What can cause it to pop up over time?

Anyway, regarding the fan, I did blow it out and it still sounds like it's "rubbing". However, if it's normally obnoxious, has anyone ever found a suitable replacement? It's a DC fan, so I'm wondering if an ultra-quiet PC fan with similar specs can be substituted. There are fans out there that can move just as much air but are much more quiet. It is definitely an issue in a small home studio.

Take the top off, put a peice of black tape across the top of the fan, put top back on. Odd fix but it cured my noise. It cushsioned the top against the fan housing.(rattle)

 

[Foz]

fisi:

Bleed the caps! Be careful.

http://www.talkbass.com/forum/showthread.php?t=437406

 

[RickenBoogie]

In my svt 3, the tube gain control doesn´t change the sound at all? I thinkI´ll change the tubes to try solving it...
How i can find the 470 ohm resistor and the potenciometer who limits his current? Have a specific name ind the PCB? some Squematic?

PS: excuse my poor english

Before you go digging in there, the 3Pro only gets a tubey grind when the tube voltage control is set at 100 volts, and the gain is nearly cranked. If you still have trouble after doing this, proceed with the operation. Good luck.

 

[Foz]

Put a hot 12ax7 in the v1 [front left corner] and she'll get grindier quicker.

Put low noise / low microphonic tubes in her and she's more "hi-fi" all the way around

I did the latter and liked it.

 

[Jerrold Tiers]

I have read elsewhere here on the forums and heard from an electrical engineering buddy of mine that the retube should not have affected the MOSFET bias. Rather, it can "drift" over time.

It's possible that I simply didn't notice it before and was paying more close attention after the retube. I also played the living out of it that night.

Correct , it has nothing to do with mosfet bias.

Also, the bias buzzing is worst cold, typically, and will typically improve the more you heat it up playing.

 

[FresnoKiwi]

I have a 2001 SVT3. All original valves when I got it. Chap I got it from complained of low volume and I suspected the valves. Opened it up and found a Grove tube in pot VI which is usually the first valve after the input. The AU7 was Chinese and the rest were Russian and all looked pretty cooked. I tried a a couple different valves/tubes and all were an improvement of the original commie crud. Finally I put in a NOS GE AU7 for the phase splitter that feeds the slave. AU's apparently have a lower gain and are a bit cleaner. The Rest are now NOS RCA and the power just poured on. So much bottom I had to turn down the bass on my JB and that has never happened. And, a big increase in overall volume. But the richness is amazing, it is just not the same amp.
At the very least get a NOS AU in the splitter and NOS RCS for V1 which is where the Groove tube is.

 

[vin*tone]

FYI the Groove Tube is likely also "commie crud" - as are some of the finest tubes available.

 

[Groover]

What is the function of the 12AU7 in this amp? Phase Inverter?

 

[dalstar925]

I have a 2001 SVT3. All original valves when I got it. Chap I got it from complained of low volume and I suspected the valves. Opened it up and found a Grove tube in pot VI which is usually the first valve after the input. The AU7 was Chinese and the rest were Russian and all looked pretty cooked. I tried a a couple different valves/tubes and all were an improvement of the original commie crud. Finally I put in a NOS GE AU7 for the phase splitter that feeds the slave. AU's apparently have a lower gain and are a bit cleaner. The Rest are now NOS RCA and the power just poured on. So much bottom I had to turn down the bass on my JB and that has never happened. And, a big increase in overall volume. But the richness is amazing, it is just not the same amp.
At the very least get a NOS AU in the splitter and NOS RCS for V1 which is where the Groove tube is.

where did you get them? online or local shop?

i feel like i have to crank mine to even play at home...and i know it's time for new tubes...its just so lifeless, i played some other guy's backline saturday w/ a SOLID STATE SVT-450 head and whatever ashdown's low end 4x10 is...and i swear it sounded as good as my SVT-3 and brand new aguilar db212 (or even my 410 HLF ampeg cab). :scowl:

i just need help!!