Early SVT distorting...early?

[Knobby]

I have a somewhere between '71 and '74 SVT that, well, gives it up earlier than one would expect.

I'm running into a Berg NV610 with either a '68 Telecaster Bass, or '71 Jazz (mostly), and anything above 3 is crunchy and getting crunchier.

This was a fun thing last year (with the bands I was working with), but this summer there has been some country gigs on the calendar, with some outdoor stuff where I need to get up there on the dial. In short, I needs me cleans!

It's had a recent retube with WingC's, not 100% sure about the pre's. (But I know one went bad last year and was replaced.)

Channel 2 is not as loud and cleaner. If that's any help.

Gents, where would you start your search?

[beebassdude]

im no expert, but a pre amp tube could definitely be causing early breakup. id start with those first, test all of them or swap em out and see what you get.

[ampegfuzz]

What are your eq settings and have you pulled and pre tubes to see what they are. I know i have had some probs with some JJ 12dw7 tubes in my ampeg of the same era. How long have you had the amp.

Borrowed this from a thread i keep bookmarked:

Originally Posted by Hoss

The top row is the tubes for the preamp.
The whole bottom is the poweramp.
The pre is connected to the poweramp with the fat cable and the plastic plug (the fan is also connected to that)

Looking from the backside of the amp into the cabinet:

Topside metal box screwed to the top of the cabinet (=preamp) from left to right:
6C4=output stage (cathode follower); 12DW7=chan.1 tonecontrols; 12DW7=chan.1 midrange-eq; 12AX7=chan.2 preamp; 12DW7=chan.1 preamp

I find the 12DW7=chan.1 midrange-eq- position the most sensitive one for microphonics (highest gain pobably). Swapping tubes can help.

Bottombox with transformers and power tubes = poweramp:
front: 6x6550 power tubes
1st back: 2x12BH7 driver tubes for power tubes (one drives each triplett - can be adjusted with each bias pot)
2nd back: 1x12DW7 phase splitter (splits signal for either triplet - can be adjusted with the balance pot.)

[JimmyM]

with a precision, my svt starts to break up at around the 11:00 mark on channel 1 and i think 1:00 on channel 2. channel 2 is also cleaner and brighter and not quite as loud. in other words, i think your amp is working fine. ampegs just break up early on the dial.

[johnk_10]

mine stays clean up to about 10:00-11:00 (depending how hot my particular bass' output is) and starts distorting after that, but it's darn loud at that point.

[Knobby]

Originally Posted by JimmyM

with a precision, my svt starts to break up at around the 11:00 mark on channel 1 and i think 1:00 on channel 2. channel 2 is also cleaner and brighter and not quite as loud. in other words, i think your amp is working fine. ampegs just break up early on the dial.

Dang, yeah it's not yet 11:00 where it's breaking up with the Precision Bass. I might sqeeze some more volume with the Jazz Bass. Shoot.

AmpegFuzz, my EQ settings are all noon, and I've had it for a couple years now.

Yeah specifically 2 Saturdays from now I know for a fact I'm going to need a shade more headroom. I just sold my SVT-3 Pro too that might have got me there easier (ton of EQ possibilities). I have an Eden and a GK in the arsenal, but dang it, this type of gig is why I bought SVT!

It just made a weird noise tonight during rehearsal though. I may look at the pre's.

Thanks for the responses so far everyone!

[zachoff]

I split my signal and use both channels. I have channel 1 on about 10:30 and channel 2 at 10:00. All of the dials are at noon except the mid on channel 1 is +4. Low and high boosts are off and xover is at 800hz. Anyway, I'm far from getting undesireable breakup. I get a little but I like it & I can carry this amount of breakup until both channels are past noon. The preamp tubes would be good to check but maybe try splitting your signal & see what happens. Also maybe roll off the tone a bit and you'll get a warmer breakup.

[Seans]

Check V1 valve position in the pre amp, this is the valve nearest the input jack sockets, it should be a 12dw7, I think you'll find its a 12ax7 in there which most people stick in to get the SVT's to break up earlier.
If I'm right, have a look see if you have any other 12dw7's in there, if so make sure V1 has a 12dw7 and you should be ok for now, have a look and let us know.

[maxbass]

Mine is as old as yours, and I have the same NV610 cab.
Channel 1 starts to break at 11 o'clock with the P-Bass, unless I don't play quiet tunes.
It stays cleaner with the J-Bass, even with the volume a bit higher to compensate for the lower output of the J.

p.s. All tubes should be the original ones, and I have a quite heavy right hand

[MRuss]

I don't SVT's were meant for clean country. They are a loud hard rock amp.

[Groover]

Tube BIAS Voltage too low may be the cause of this.

[JimmyM]

Originally Posted by MRuss

I don't SVT's were meant for clean country. They are a loud hard rock amp.

you can play any kind of music on any amp. and you can play an svt quietly and it still sounds awesome. you must be a younger guy because only younger guys say that

[ampegfuzz]

I knew someone was going to razz him a little

[beans-on-toast]

Originally Posted by Groover

Tube BIAS Voltage too low may be the cause of this.

+1 Well worth checking the bias out because it is the easiest thing to try.

Next since you've already replaced the power tubes, check the phase inverter, driver and pre-amp tubes.

The distortion could be caused by a lower level problem. Your caps could be functioning below spec. If your amps has insufficient power supply capacitance then the power supply will sag and as a result, distortion will be present. Even if you don't have the telltale hum associated with a bad cap, their marginal performance could be responsible for the distortion. If the caps are old, it might be worth having them replaced.

[coreyfyfe]

Originally Posted by beans-on-toast

+1 Well worth checking the bias out because it is the easiest thing to try.

Next since you've already replaced the power tubes, check the phase inverter, driver and pre-amp tubes.

The distortion could be caused by a lower level problem. Your caps could be functioning below spec. If your amps has insufficient power supply capacitance then the power supply will sag and as a result, distortion will be present. Even if you don't have the telltale hum associated with a bad cap, their marginal performance could be responsible for the hum. If the caps are old, it might be worth having them replaced.

+1 to all of this. I have an SVT of about the same vintage as yours (was told 74, haven't tried actually dating it) and I started getting odd distortion. It wasn't constant like yours, but mostly around certain frequencies/notes. Brought it in for clean up and a cap job. Cost more than I was happy to pay but it's now in tip-top shape, the pre-amp is back to original spec (had been converted to 12ax7s) and it's sounding great.

[Groover]

Is there an easy way to check for caps health? (poor man's way) - and I am aware that that is where you can actually get voltage zapped and feel the pain

[rbonner]

If it hasn't had a cap job, it's time for one. If you wanted to go the cheap, you'd need to solder in the new caps one at a time. If you could put a tone generator on it and look around with an oscilliscope, find the offending circuit and replace the caps providing power and bypassing there only. But since this amp is already due, just get it done and call it a day.

How do I know I need to do a cap job? I have already done one of my SVP1600 power amps and it was only 5 years old. It distorted in the stereo and dual mono modes. Four new power supply caps fixed it.

The hardest part is finding the "Right Tech" a really good tech will make the amp LOOK BETTER inside than it did coming from the factory. A hack will make it look like it was repaired by a MASH unit under fire.

BOB

[Zentner]

I didn't know my SVT got clean.....

It's an earlier blueline (6146's) and it breaks up at like 9:30/10:00 with a ric.

I'd try dialing the volume back on your bass juuust a smidge, channel 2, with the EQ switches in. That will give you the least aggressive midrange.

[anderbass]

Originally Posted by Knobby

Dang, yeah it's not yet 11:00 where it's breaking up with the Precision Bass.

That's about were my 72 SVT (recently serviced/new caps/nos. GE 6550's) starts to break up too, although mine is WAY loud at that point and I rarely get it past 9:30 or so even with fairly loud bands. I run very similar tone settings as you, passive and active P-Basses and a NV610 cab too. My volume seems to double between 9 and 11 o'clock.

Maybe adding your Goliath Jr. would help for your outdoor gig. If nothing else, stacking your NV on top of it would be elevating your NV up were you could hear yourself better. I've found my SVT/NV610 projects out into the audience (at inside or outside gigs) much louder than I would have guessed without any help from the P/A needed.

[beans-on-toast]

Originally Posted by Groover

Is there an easy way to check for caps health? (poor man's way) - and I am aware that that is where you can actually get voltage zapped and feel the pain

When the caps are over ten years old, I find that it is more cost effective, in terms of diagnosis time, to just replace them. As Bob said, sometimes they need changing even sooner.

I have a capacitor tester that reads the capacitance and DC leakage of electrolytic caps. I paid $10 for it at a hamfest. That's a poor man's aproach if there ever was one .