SVT Bias and balance setting

[Latole]

Stay with the 40

[pbman1953]

Stay with the 40

Ok, let me know if these check out-


Pre-amp-
40UF@450- I found a F&T 47 @500v




Power amp-
30/600v- I will use your suggested MOD caps- 2 -70's in series to get 35uf@ 700v


100uf@450v & 90uf@200v- I have 2 F&T 50/50 snap in's that I'll parallel each one  to get 100 uf each- both @500v


Am I good to go with these?

[Latole]

I suggest you to study a little with, for example, the excellent books of Gerald Weber.

Because the questions you ask are the basis of amplifier repair.

These are great books, easy to read and understand. You will like them for sure.
I have them all and many more.

[pbman1953]

I suggest you to study a little with, for example, the excellent books of Gerald Weber.

Because the questions you ask are the basis of amplifier repair.

These are great books, easy to read and understand. You will like them for sure.
I have them all and many more.

Thanks for the book info!


For now, would you please respond about the cap list, only because I want to make the parts order so it will be here this weekend


Thanks!

[Latole]

Stay with the 40

Ok, let me know if these check out-


Pre-amp-
40UF@450- I found a F&T 47 @500v. 




Power amp-
30/600v- I will use your suggested MOD caps- 2 -70's in series to get 35uf@ 700v. 


100uf@450v & 90uf@200v- I have 2 F&T 50/50 snap in's that I'll parallel each one  to get 100 uf each- both @500v. 


Am I good to go with these?

Yes, good choices. 

[pbman1953]

Thanks so much!


I'll be back after the install!

[Latole]

Thanks so much!


I'll be back after the install!

Good luck my friend !

[pbman1953]

Thanks so much!


I'll be back after the install!

Good luck my friend !

Thanks!

[pbman1953]

As far as mounting the can caps-


The stock paper caps were held in by the insulator. I bought the metal plate , insulator and clamp. I was thinking of using the insulator that will be against the chassis and the metal plate.


Would you feel I'd need the clamp?

[Latole]

Show what caps you bought

[pbman1953]

Show what caps you bought

CE Can Cap- 100/40/450v

[sluckey]

The clamp is not used with those caps. You would normally use the metal plate since the cans of both caps are connected to ground. However, you may not need any mounting plates. Just use the plates that are already rivetted to the chassis if the twist lock cans will fit.

[pbman1953]

ok, thanks. Will do that.

[PRR]

removed

[pbman1953]

Are you saying not to use the CE and get the Fliptop cap.  Or could I cover the CE with a paper tube?

[pbman1953]

I'm playing safe. I bought the insulated caps and it's matching isolator disc.

[sluckey]

Are you saying not to use the CE and get the Fliptop cap.  Or could I cover the CE with a paper tube?

Look at the cans that are in the amp! Better yet, post a pic and we'll look for you. If your present cans have cardboard covers then you need cardboard covers (You can probably use the old covers) But if your cans are bare metal then you don't need cardboard. According to the "last" schematic you posted, both of the cans have the negative lead connected to ground. If this is the correct schematic then you don't need the cardboard.

One of your pics shows the cans mounted on a fiberglass mounting plate with a black wire connected to the tab for the can. That black wire should connect to ground somewhere.

Speaking of schematics... There have been several incorrect schematics posted in this thread. If you are sure the last schematic is correct (the cap cans match), I would like to go through the thread and remove all those incorrect schematics, with your approval of course. LMK

[sluckey]

I'm playing safe. I bought the insulated caps and it's matching isolator disc.

You only need insulated cans if the cans in your amp are insulated. ARE THEY INSULATED?

[pbman1953]

The stock caps are paper covered like a toilet paper tube. I haven't removed them as yet. I was waiting for the new parts. There is a fiberglas insulator visible. I can send pics later when I get home. Last night I checked all 4 twist tabs, on each cap. Each tab grounds out with a meter. Flip top only sells insulated caps for both cap models (70/40/40 & 100/40) for the old SVT.


At the beginning of the post  I sent the schematic with the 70/40/40 cap set. Mine has the 100/40 cap set.   




You can remove the schematics  from posts- 6,8 & 17


Posts 36 & 39 are what I have




Thanks


 
https://ampeg.com/support/files/Schematics/S%20Series/SVT%20(1972,%206550%20tube)/SVT%20SCHEMATIC.pdf

[pbman1953]

removed

You were right, THANKS!!


Fliptop said that that the amp required covered filters caps due to voltage on the outside. I'm somewhat surprised that Ampeg didn't design with discreet caps, like Fender did.

[sluckey]

Fliptop said that that amp required covered filters caps due to voltage on the outside. I'm somewhat surprised that Ampeg didn't design with discreet caps, like Fender did.

Nothing wrong or inferior with using multicap cans. Look at all those big Marshalls, Sunns, Magnatones, etc. Lots of multicap cans. Leo probably bought train car loads of axial caps for pennies each.   

Last night I checked all 4 twist tabs, on each cap. Each tab grounds out with a meter.

The metal cans of those two multicaps are connected to chassis ground. There is no voltage on the cans. The schematic says so, I've been shouting it, and you have proven it with your meter. Those particular caps don't ***NEED*** to be insulated. However, a lot of the cans used by Ampeg were stacked in series so the cans on the top of the stack will have B+ on them. Surely Ampeg decided to just stock/use all cardboard insulated caps rather than have some insulated and others not insulated. Avoids assembly confusion/errors and probably cheaper to stock only one kind. The same goes for the fiberglass mounting plates even though your amp could have used metal. Why risk some minimum wage assembler putting a metal plate in the wrong place?

[pbman1953]

Fliptop said that that amp required covered filters caps due to voltage on the outside. I'm somewhat surprised that Ampeg didn't design with discreet caps, like Fender did.

Nothing wrong or inferior with using multicap cans. Look at all those big Marshalls, Sunns, Magnatones, etc. Lots of multicap cans. Leo probably bought train car loads of axial caps for pennies each.   

Last night I checked all 4 twist tabs, on each cap. Each tab grounds out with a meter.

The metal cans of those two multicaps are connected to chassis ground. There is no voltage on the cans. The schematic says so, I've been shouting it, and you have proven it with your meter. Those particular caps don't ***NEED*** to be insulated. However, a lot of the cans used by Ampeg were stacked in series so the cans on the top of the stack will have B+ on them. Surely Ampeg decided to just stock/use all cardboard insulated caps rather than have some insulated and others not insulated. Avoids assembly confusion/errors and probably cheaper to stock only one kind. The same goes for the fiberglass mounting plates even though your amp could have used metal. Why risk some minimum wage assembler putting a metal plate in the wrong place?

"the cans used by Ampeg were stacked in series so the cans on the top of the stack will have B+ on them. "  That's what Fliptop said




Also, Fliptop said that they are out of the 100/40 so they are subbing the 70/40/40 and I will get 110 instead of 100. He said that it's been done many times over.


I'll have pictures to show soon


Thanks

[sluckey]

"the cans used by Ampeg were stacked in series so the cans on the top of the stack will have B+ on them. "  That's what Fliptop said

Well, some are. BUT NOT YOUR AMP! WHAT DID YOUR OHM METER TELL YOU???

[pbman1953]

Got it.


I was just writing you to what they said

[pbman1953]

Paper cap and insulator

[sluckey]

Why did you drill the rivets out? The fiberglass mounting plate should have stayed attached to the chassis. This is supposed to be easy, especially since the tabs are not soldered to the mounting plate.

The "correct" way to remove a twist-lock can is to remove any wires and solder from the twist-lock tabs. Then use your needle nose pliers to straighten the tabs. At this point the can will easily slip away from the mounting plate.

To properly install a twist-lock can simply insert the four tabs into the four slots of the mounting plate. Use your needle nose pliers to give each tab a quarter turn. This securely locks the can to the mounting plate.

FYI, those original fiberglass mounting plates are far superior to a phenolic mounting plates that you will probably get from F/T.

[pbman1953]

Why did you drill the rivets out? The fiberglass mounting plate should have stayed attached to the chassis. This is supposed to be easy, especially since the tabs are not soldered to the mounting plate.

The "correct" way to remove a twist-lock can is to remove any wires and solder from the twist-lock tabs. Then use your needle nose pliers to straighten the tabs. At this point the can will easily slip away from the mounting plate.

To properly install a twist-lock can simply insert the four tabs into the four slots of the mounting plate. Use your needle nose pliers to give each tab a quarter turn. This securely locks the can to the mounting plate.

FYI, those original fiberglass mounting plates are far superior to a phenolic mounting plates that you will probably get from F/T.

A tech told me to drill out the rivets. I'll remove the can from the plate and re-mount.

[pbman1953]

It's been a while since I did a cap job




I have a variac so what would be good steps to follow as far powering up?


Thanks

[Latole]

I use bulb limiter. I don't think we need to that.
  IMO, at the amps building plants, they fire the amps with no variac , no bulb limiter

[pbman1953]

Success, no smoke.


All kidding aside , I used the variac, anyway, and brought it up slow. The pilot light lit up at around 90 volts. When I switched the stand by I had no noise and very minimal hum.


I found a NOS  Mullard 7147/12dw7 for v1 and a NOS Sylvania 6c4 which is quiet.


The amp biased easily and the hum control took away some slight back ground hum. But, the balance control doesn't seem to do anything. All you hear is a chaffing sound when moved.  the meter moves while I move the dial but still settles on .000 or .001 anywhere in the travel.


I thought the bias/ balance section was similar to a Sunn 300t, but I may need a scope to really balance it

[tubeswell]

The amp biased easily and the hum control took away some slight back ground hum.


I thought the bias/ balance section was similar to a Sunn 300t, but I may need a scope to really balance it

With 3 pairs of tubes, you still need to do measure individual tube current and swap around the set to see which combination of which tubes on which side achieves best balance on both sides for the balancing control to work optimally.

[sluckey]

But, the balance control doesn't seem to do anything. All you hear is a chaffing sound when moved.  the meter moves while I move the dial but still settles on .000 or .001 anywhere in the travel.

You've already set the idle bias. Now you're gonna adjust the PI balance. The instructions are on the schematic. Did you read and understand? You will need a signal generator and a 4Ω 330 Watt dummy load. Do you have proper equipment?

You will connect a signal generator to the amp input. Apply a 200mV 40Hz signal. Adjust the amp's volume until you measure 25V_rms across the 4Ω dummy load connected to J4. The amp will be putting out 150 watts of power at this point! Now connect your dmm between test points K1 and K2. Adjust the PI balance for zero VDC. You're done.

CAUTION!!! BE ABSOLUTELY SURE THE DUMMY LOAD IS SECURELY CONNECTED TO THE SPEAKER JACK J4. A POOR CONNECTION WILL LIKELY CAUSE A MELTDOWN.

[PRR]

...Now you're gonna adjust the PI balance....

Disagree. Ampeg was knob-polishing. Their fussy process is correct, but IMHO it is NOT needed. Even precision High Fidelity amps don't all go through this busy-work (and the ones that do may just be supporting dealer service profit).

I had a Fisher which did have this AC Balance plan, I turned it night and day for a month with many meters, and if it was anywhere near close it was all the same.

Are we still working from the schematic in Reply # 68?

Then, with power OFF, trim VR3 for 4Kohms +/-1Kohms. Done. Have a beer.

Personally I'd rather have a 3.9k fixed resistor than an unreliable trim-pot.

[Latole]

The amp biased easily and the hum control took away some slight back ground hum.


I thought the bias/ balance section was similar to a Sunn 300t, but I may need a scope to really balance it

With 3 pairs of tubes, you still need to do measure individual tube current and swap around the set to see which combination of which tubes on which side achieves best balance on both sides for the balancing control to work optimally.

Right , with a 1 ohm resistor solder at each cathode or a bias probe

[pbman1953]

...Now you're gonna adjust the PI balance....

Disagree. Ampeg was knob-polishing. Their fussy process is correct, but IMHO it is NOT needed. Even precision High Fidelity amps don't all go through this busy-work (and the ones that do may just be supporting dealer service profit).

I had a Fisher which did have this AC Balance plan, I turned it night and day for a month with many meters, and if it was anywhere near close it was all the same.

Are we still working from the schematic in Reply # 68?

Then, with power OFF, trim VR3 for 4Kohms +/-1Kohms. Done. Have a beer.

Personally I'd rather have a 3.9k fixed resistor than an unreliable trim-pot.

Are we still working from the schematic in Reply # 68?-  yes




I'll use my Bias Rite to see here I am with this set.


Question- is the balance control similar to Fender bias  balance control?

[pbman1953]

The tubes are a set of Valve Art 6550A

1st measurement-


Average voltage is 690v


measurements are in watts



1-17
2-16
3-15
4-15
5-17
6-15


Seems to be very cold






2nd  after boosting the bias




1-25
2-24
3-18
4-25
5-27
6-25




Could the balance pot be bad? If it's suppose to work like a Fender, it does nothing to shift or take away from either side

[Latole]

Could the balance pot be bad? If it's suppose to work like a Fender, it does nothing to shift or take away from either side

Read all bias voltage at tubes's grid.
You'll see if pot work or not.

[pbman1953]

The cathode current avg is 35, less on #3

[sluckey]

Could the balance pot be bad? If it's suppose to work like a Fender, it does nothing to shift or take away from either side

Not likely. This PI balance is nothing like the Fender bias balance.

1. Did you read and understand the instructions on the schematic?

2. Did you read and understand the instructions I wrote in my previous reply?

3. Do you have the proper test equipment?

If you answer no to any of these 3 questions then you cannot properly adjust the PI balance. In that case, just follow PRR's advise...

Then, with power OFF, trim VR3 for 4Kohms +/-1Kohms. Done. Have a beer.

[pbman1953]

Could the balance pot be bad? If it's suppose to work like a Fender, it does nothing to shift or take away from either side

Not likely. This PI balance is nothing like the Fender bias balance.

1. Did you read and understand the instructions on the schematic?

2. Did you read and understand the instructions I wrote in my previous reply?

3. Do you have the proper test equipment?

If you answer no to any of these 3 questions then you cannot properly adjust the PI balance. In that case, just follow PRR's advise...

Then, with power OFF, trim VR3 for 4Kohms +/-1Kohms. Done. Have a beer.

Looks like I'm going in the direction of the beer. I don't have the test gear.






Do you feel the tubes are still good? They are relatively close.


Bias wise, for a 6550, this seems to be on the low side at an average of 24. I was thinking of bringing them up 27-27 watts

[sluckey]

Looks like I'm going in the direction of the beer. I don't have the test gear.

Good idea, but first set VR3 to 4K since you've been tweaking it.

Do you feel the tubes are still good? They are relatively close.

Absolutely. You did set the two bias pots for 72mV at test points K1 and K2? When you did this you balanced the idle bias for the two sides. Nothing more to do.

Bias wise, for a 6550, this seems to be on the low side at an average of 24. I was thinking of bringing them up 27-27 watts

Nothing wrong with running like that. Ampeg thought it was a safe operating point that still produces more power than any sane person needs. A Bass amp is supposed to have low distortion, which means running the tubes cool. You don't want to buy a six pack of those tubes every six months. I highly recommend setting the bias pots exactly as Ampeg suggested. Instructions are on the schematic so a year from now you can simply readjust using the same procedure. Easy peasy.

[pbman1953]

Looks like I'm going in the direction of the beer. I don't have the test gear.

Good idea, but first set VR3 to 4K since you've been tweaking it.


All set, the v3 is set to 4k

Do you feel the tubes are still good? They are relatively close.

Absolutely. You did set the two bias pots for 72mV at test points K1 and K2? When you did this you balanced the idle bias for the two sides. Nothing more to do.


The calibration of the back of the amp says to bias vr1/k1 with the common port to 72mv. Then you're supposed to balance with the probes in k1 & k2 to .01,using the vr2 bias pot. If I do that, then k2's adjustment goes to 83mv, if tested like the k1. Should I just go with individual testing and not balance between K1 & K2? Or use the Ampeg way?

[pbman1953]

Some follow-ups-


The paper filter caps measure a so-




Stock was 100/40 @ 450v


They both measured


165/64

[sluckey]

Then you're supposed to balance with the probes in k1 & k2 to .01, using the vr2 bias pot. If I do that, then k2's adjustment goes to 83mv, if tested like the k1.

THAT AIN'T WHAT THE PROCEDURE SAYS! Read it again and again and again until you understand what it says. BTW, both procedures say exactly the same thing.

You're supposed to adjust VR2 for ZERO volts between K1 and K2. Think about it before you answer... If K1 reads 72mV and you have zero volts between K1 and K2, what voltage must be on K2? If you answer this wrong I'll never speak to you again. 

I've attached a stripped down PI with an explanation of the circuit and what happens when you adjust VR3.

[pbman1953]

If you measure k2 and common it should be the same as k1. 72mv

[sluckey]

If you measure k2 and common it should be the same as k1. 72mv

OK, and if you measure voltage between K1 and K1, what will you have?

[pbman1953]

If you measure k2 and common it should be the same as k1. 72mv

OK, and if you measure voltage between K1 and K1, what will you have?

between K1 and K1   


How is that done? There's only 1 k1

[sluckey]

If you measure k2 and common it should be the same as k1. 72mv

OK, and if you measure voltage between K1 and K1, what will you have?

between K1 and K1   


How is that done? There's only 1 k1

Now I'm going crazy too! Let me try again...

OK, and if you measure voltage between K1 and K2, what will you have?

[pbman1953]

If you measure k2 and common it should be the same as k1. 72mv

OK, and if you measure voltage between K1 and K1, what will you have?

 On the lowest scale-


I rechecked k1 and common, then k2 and common- both are at 72mv each


k1 & k2 are now .8

[sluckey]

I give up. I'm gonna go play with my pet rock.