Ampeg SVT-3 Pro Tube Questions

[randysmojo]

I do know that AX7s, AT7s, AU7s, and AY7s all are of the same basic design, just different gain, and can be changed out in some cases anyways in old tube amps like old Fenders, but I didn't know if a little more gain in that stage of a 3pro would help tighten up the response, or not make any change that would be helpful. Also, if in that stage if it might change how the Mosfets bias. Probably best to leave alone like you say.

 

[mazdah]

It should be safe to use AT7 in preamp, but I'd not recommend it for driving Mosfets, just like you wrote - best to leave the AU7 and AX7 alone there

But I'd start from throwing Sovteks 12AX7 WA away if you have them

Sovtek have 3 versions of 12AX7, their gain level is different, WA have the lowest gain.

 

[f.c.geil]

I definitely am getting rid of the SovTeks. Probably using Sylvania, RCA, and/or Telefunken instead.

 

[randysmojo]

Mine has all Electroharmonix. Who knows where they where manufactured. Probably low level Sovteks! I have some other tubes around here I'll dig up. Groove Tubes and NOS RCA's. Maybe some other stuff. from back in my lead guitar days.

 

[f.c.geil]

Well, I re-tubed it with a NOS RCA 12AX7 and a NOS RCA 12AU7 in the driver slots, and three Tung-Sol 12AX7s in the tone section. So, how does it sound?

LOUD!​

It also has all the tone, funk, cheese, stuff, Ampeg goodness, etc. that one could want. Yeah, the SovTeks were definitely choking this thing out! I went from diming the gain and almost never lighting the LED to having it at 9 o'clock with the LED coming on very often (basically flickering every note). I haven't rehearsed yet, but when practicing, the whole house rattles if I bring the Master up above 9 o'clock. This thing frigging RAWKS now.

Thanks so much to everyone for their help!

 

[randysmojo]

I dug up a RCA 12AX7 and 12AU7 Black plates from the mid 60's that I pulled out of an old Naval O-scope, so like they are going in my drive section ASAP! The amp is at our drummers house and I am down with Bronchitis so I might not get to it for a few days!! Now if I can dig up some awesome 12AX7's for the preamp....

 

[f.c.geil]

Let me tell you, you will not regret it! Make sure you're using the 17mm plates to get all you can (tone and volume) from it. Put those RCAs in the drive section, get RCAs, Telefunken, Mullard, Siemens, or (minimum) Tung-Sols for the tone section, and you're rocking!

 

[randysmojo]

Are these the 17mm you speak of? One is an AU7, the other an AX7. biggest plates I've ever seen in a preamp tube.

 

[f.c.geil]

Looks right. Measure the plate length as best you can. The short ones are 14mm, the long ones are 17mm. You want the 17mm.

 

[randysmojo]

I've got a gig this afternoon and I put the NOS RCA 17mm 12AU7 and 12AX7 in the drive stage, and another 17mm 12AX7 and Groove Tubes 12AX7's in the preamp. Made sure it worked late last night at low volumes, so we'll se what that does, but the replacement tubes light up a lot more than the cheap Electro Harmonix tubes that where in it when I got it!

 

[f.c.geil]

You'll love it, but let me know what you think afterward.

 

[f.c.geil]

On a side note, I decided to try the method recommended by the majority on this one, and installed five carrots. I must have done something wrong, as it just doesn't sound as good as it used to, and there's no volume to speak of. I don't get it, this is the USA, so isn't the majority always right?

But, since that didn't work, I went back to my previous RCA and Tung-Sol tube setup.

 

[audiomitch]

Sounds like your old tubes weren't functioning correctly, and replacing with any new tubes would have had a dramatic effect. I did some tube rolling with my old SVT-3 and noticed minimal change. I ended up with Tung-Sols and a Sylvania NOS 12AU7 in J12. I had a much more dramatic effect after biasing my Mosfets.

 

[f.c.geil]

Did you bias them yourself, or have it done? If you did it yourself, how is it done? I'm pretty sure mine needs it done, but the nearest Ampeg licensed repair facility is 120 miles away.

 

[B-string]

Did you bias them yourself, or have it done? If you did it yourself, how is it done? I'm pretty sure mine needs it done, but the nearest Ampeg licensed repair facility is 120 miles away.

PM Jerrold Tiers, I'm pretty sure he posted the procedure here before but the search function left me irritated. Of course I'm not searching for my own amp or one I need to repair

 

[audiomitch]

Did you bias them yourself, or have it done? If you did it yourself, how is it done? I'm pretty sure mine needs it done, but the nearest Ampeg licensed repair facility is 120 miles away.

I did them myself. You need a multi meter and a good understanding of the thread here that explains it. It's not too complicated, but it is a little nerve racking.

 

[f.c.geil]

I sent Jerrold a PM as suggested, and this is what he said:

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.

What you need to do is to set the bias with the adjustment control on the PWB ("pot") so the average voltage across the 0.47 ohm resistors is 25 millivolts. Average means that you might have some at 30mV, and some at 20 mV, for instance. or maybe some 15mV and some 35MV..... but the average is about 25mV or a tad less.

You should NOT have any that have near 0mV, or over 40mV. I would not like to see even 40mV on an individual 0.47 ohm. If your unit has decently matched parts, which it should, it will bias up nicely without really extreme readings.

You do this with no signal and NO SPEAKER.

Watch out, because the bias control on these is touchy, if you go too high you can fry things.

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.

JT

 

[randysmojo]

You'll love it, but let me know what you think afterward.

Holy Moly! First off, the replacement tubes glow way brighter than the cheapies. Also gain a dramatic amount of volume and clarity. I'll put it this way, I removed my ad for which i was trying to sell this amp to get something else. That good!!! So don't change the type of tube (12AX7, 12AU7..), but change them for better quality tubes!!

 

[f.c.geil]

Holy Moly! First off, the replacement tubes glow way brighter than the cheapies. Also gain a dramatic amount of volume and clarity. I'll put it this way, I removed my ad for which i was trying to sell this amp to get something else. That good!!! So don't change the type of tube (12AX7, 12AU7..), but change them for better quality tubes!!

That's what I wound up doing. I'm still playing with the driver tubes (Sylvania Gold, RCA, Telefunken, GE) to find the best pairing, but have settled on brand new Tung-Sol tone tubes. They really sound good there, maybe as good as RCAs. I won't be needing a different amp anytime soon!

 

[fuadramsey]

randysmojo,

How did your amp sound before? My amp sounds good and I was wondering if this would make it sound great. Mine is fairly new, I bought it new about 4 years ago.

Holy Moly! First off, the replacement tubes glow way brighter than the cheapies. Also gain a dramatic amount of volume and clarity. I'll put it this way, I removed my ad for which i was trying to sell this amp to get something else. That good!!! So don't change the type of tube (12AX7, 12AU7..), but change them for better quality tubes!!