70s Ampeg SVT re-valve and callibration

[Jaseandwave]

Hi everyone,

I have been having various issues with my early 70s Ampeg SVT (pops crackles etc) so decided to change all the valves.

When I came to do the job there were all 12AX7s instead of 12DW7s.

I've put all the correct new valves in their correct places but now there is a fair bit of hum so I want to correctly callibrate the amp.

The directions are to put a short across pins 2 and 3 of connector P3, and a 4ohm dummy load across pins 1 and 4.

I've studied the schematic for the amp and taken the power amp out of the chassis but I can't find P3.

Anybody help me identify P3 and also any other tips would be gratefully accepted.

I'm well aware that I need to be very careful whilst carying out this operation!

Jase.

 

[beans-on-toast]

They are referring to the connector at the end of your speaker cable. The very early models had a 4 pin XLR type connector. Later 70's models have a 1/4" plug on the end of the speaker cable. Sometimes the amps are modified and the speaker cable is removed and a 1/4" jack is installed on the chassis.

If you do not have the 4-pin connector, you do not have to short pins 2 and 3, just connect the 300W, 4 ohm dummy load.

For the crackling, should also clean your socket pins and re-tension them (pinch them tighter) if necessary. Some people simply plug in the 12AX7's in place of the 12DW7's, other times the amps are modified. You should check that your amp was not modified to run on 12AX7 tubes. They would have changed some resistor values to lower the gain of one half of the 12AX7.

Otherwise, the noise could be caused by a bad solder joint or component.

 

[coreyfyfe]

Some people simply plug in the 12AX7's in place of the 12DW7's, other times the amps are modified. You should check that your amp was not modified to run on 12AX7 tubes. They would have changed some resistor values to lower the gain of one half of the 12AX7.

+1 to this. When I got my SVT it had been modded to 12AX7s. I had it put back to spec which was relatively quick and easy. I think it's a difference of three or four resistors. I believe JohnK provided me with the service bulletin which had the values, I'll see if I can dig it up.

 

[Seans]

There is a thread on here that goes in depth in to the balance control sequence, I think Groover was the Op. But for now I would just go through the bias and keep the balance control where it is. I moved mine just a little and now it sets up an annoying crackle, and yes I have deoxed it and even taken it apart to clean it.

 

[Jaseandwave]

+1 to this. When I got my SVT it had been modded to 12AX7s. I had it put back to spec which was relatively quick and easy. I think it's a difference of three or four resistors. I believe JohnK provided me with the service bulletin which had the values, I'll see if I can dig it up.

Thanks, that would be really useful.

 

[Bassmec]

Well Go On! Give it a proper birthday! its about time to check the electrolytic's in the old girl, I would change them all before the hum gets too bad or you have cracking on dynamics.
Them's the only bits apart from some tubes, and some potentiometers where, "nice and Vintage", "well tried and tested" don't really help much.

 

[coreyfyfe]

From this thread:
http://www.talkbass.com/forum/showthread.php?t=664585

here it is:

if the previous tech used that method, all you have to do is check resistors #R25, #R7 and #R6. if they are not the stock values and have been changed, change them back to:
Stock SVT:
R25= 1.5K
R6=47K
R7=100K

I'd also definitely agree that it may be time for a cap job. When I got mine it had a few weird hums and crackles. I ended up replacing most of the tubes, recapping, and having the preamp set back to stock. Sounds great now.

 

[Jaseandwave]

Cheers,

That's great. I had some of the old paper caps replaced earlier on in the year when the problems began so hopefully I can get it all hunky dory now.

Quick (possibly stupid) question... Should I do the calibration with the preamp connected or not?

 

[coreyfyfe]

The easiest thing would be if you still have some 12ax7s laying around see if you get less noise with them instead of the 12dw7s. Then you could adjust everything else for the least noise with the less noisy set of preamp tubes.

Checking the resistor values will be your best bet, but can be a huge pain in the back side. If you pull out the preamp section and take off the cover over the tubes all the resistors are numbered. Find the correct ones and check the value (unless you know them offhand you'll have to look up which resistor values are coded by which color patterns).

Like I said, taking the preamp out is a pain unless you have a lot of bench space, patience, and proper tools. You have to disconnect the pre from the lower chasis (the connector in the back near the fan) then undo the screws in the top of the headcase to drop it down and then slide it out. From there, just take the plate around the preamp tubes off and you can look around. I'd caution you against touching anything unless you really know what you're doing in there, but you would at least be able to know for sure which preamp tubes you should be using.


Edit - I just reread your post, I thought you said preamp corrected (as in 12ax7 vs 12dw7) haha, so I guess my post is kind of irrelevant to your actual question - should you leave the pre connected. To which I would say you probably should.

 

[Seans]

Cheers,

That's great. I had some of the old paper caps replaced earlier on in the year when the problems began so hopefully I can get it all hunky dory now.

Quick (possibly stupid) question... Should I do the calibration with the preamp connected or not?

Are you setting the bias on the amp or have you done that and you're going to do the balance, which should be done after you have set the bias.

 

[Jaseandwave]

Are you setting the bias on the amp or have you done that and you're going to do the balance, which should be done after you have set the bias.

I'm going to bias first, then balance after.

 

[Jaseandwave]

OK,

I've set up bias as per instructions, put all the correct valves in all the correct places but now I've got a loud hum which I can't tune out with the hum pot. Also three of the 6550 power tubes are clearly running too hot - they glow blue and are very hot to the touch compared to the other three, I've swapped them around (they're a matched sextet of svetlana winged c's) and it's always the same positions, ie the three on the right as viewed from the rear (V4, V5, V6).

If I reduce the bias on VR1 to about 0.2V the hum goes away and the tubes glow nice and orange, but the sound from the amp is weak and distorted.

Any ideas?

P.S. thanks to everyone for their help during these traumatic times!

 

[Seans]

OK,

I've set up bias as per instructions, put all the correct valves in all the correct places but now I've got a loud hum which I can't tune out with the hum pot. Also three of the 6550 power tubes are clearly running too hot - they glow blue and are very hot to the touch compared to the other three, I've swapped them around (they're a matched sextet of svetlana winged c's) and it's always the same positions, ie the three on the right as viewed from the rear (V4, V5, V6).

If I reduce the bias on VR1 to about 0.2V the hum goes away and the tubes glow nice and orange, but the sound from the amp is weak and distorted.

Any ideas?

P.S. thanks to everyone for their help during these traumatic times!

Are you setting bias at x10, it's possible, i've done it, it's .072 v.

What reading do you have across K1 and K2, it should be zero.
Or,

Do you have any original paint on the balance pot to show where it sat originally, if not turn it fully left, then turn it clockwise until its slot is vertical, this should be roughly half or less distance of travel. Set bias again.

 

[Jaseandwave]

Are you setting bias at x10, it's possible, i've done it, it's .072 v.

What reading do you have across K1 and K2, it should be zero.
Or,

Do you have any original paint on the balance pot to show where it sat originally, if not turn it fully left, then turn it clockwise until its slot is vertical, this should be roughly half or less distance of travel. Set bias again.

Thanks,

I know what you mean about the x10 thing. I'm sure i've got it set at .072V now. First time I set it the sound was very weak, distorted and quiet and after some head scratching I'm convinced I had it set to .0072V.

Either way it's either weak and quiet or running hot and humming.

I've got the balance set roughly in the middle - but it doesn't seem to make a fat lot of difference where I set it, the same 3 power tubes are still running hot.

I've got a gig tomorrow and it's looking like i'm gonna have to use the Peavey combo!

 

[beans-on-toast]

Sounds like you are trying to read AC volts.

Check that you are setting your meter to DC volts when you read 0.072V, or 72 on the DC millivolt scale, K1 to ground, and K2 to ground.

Turn on the amp, volume controls should be set off, fully counter clock wise, the amp should be in playing mode, not on standby, let it sit like that for at least 20 minutes before taking your readings. If you take a reading, it should be more or less the same if you take it again in five minutes, otherwise the amp is not warmed up enough.

 

[Jaseandwave]

Sounds like you are trying to read AC volts.

Check that you are setting your meter to DC volts when you read 0.072V, or 72 on the DC millivolt scale, K1 to ground, and K2 to ground.

Turn on the amp, volume controls should be set off, fully counter clock wise, the amp should be in playing mode, not on standby, let it sit like that for at least 20 minutes before taking your readings. If you take a reading, it should be more or less the same if you take it again in five minutes, otherwise the amp is not warmed up enough.

Thanks,

I definitely have meter on DC scale but I didn't let it warm up for anywhere near 20 mins. I'll give it anoter whirl.

 

[Jaseandwave]

I've looked at the resistors and it seems they're not stock, but they're not all as the procedure posted above.

R25 is 4K7 (as per mod quoted above by coreyfyfe)
R6 - cant make out all the colours but there are orange and green together so 35K 350K or 53K or 530K - certainly not 220K or 47K though
R7 - cant make out all the colours but there are green and black together so I reckon 50K or 500K - certainly not 100K or 220K

Does this make any sense to anyone?

 

[johnk_10]

if you take some very clear photos of the resistors it might be easier to help you.

BTW, tubes are generally not supposed to glow orange (very brightly at least), and a little blue is usually normal (it's just the gas in them).

it sounds to me like you aren't very familiar with working on these (they're definitelynot for beginner techs), but if you post pics, and we sart at the beginning, we may be able tohelp you.

BTW, there's a hum balance control that has nothing to do with the bias. it balances the heater circuit. the other balance control is to balance the phase inverter section.

 

[Seans]

Just another stick in the fire, check that the 12BH7's are ok, they are drivers for the 6550's and each one does 3 of the 6550's.
Have them tested on a valve tester, now these are notoriously hard to test, by reducing the neg grid volts a little they should test, but at least you can tell if they're matched within acceptable limits.

OR. Have a look at the 6550's and see if you can see little grey patches/blotches, they will be near the core of the valve and will stain the shrouds, if you have this then these valves are shot and have been into meltdown/damaged by incorrect bias. They will test ok, but once they have been up there that's it.

 

[Jaseandwave]

Here are pics of resistors as discussed.

I reckon the only dodgy one is R6 (the brown one) certainly been changed.

The other 2 in question (R7 and R25) are original I think.

Should I change R6.

By the way. After a bit of time and effort I've got the thing running pretty nicely now.