SVT Bias and balance setting

[pbman1953]

On my SVT setting the bias has no problem to set to .072 like the instructions state
With the balance ,the pot is not doing much to set to .01. The whole swing of the pot is .000


Plus it has a hum so it may be time for caps

[tubeswell]

Open it up. Check the screen resistors . One of them may be blown open

[pbman1953]

Thanks, I just bought this off a friend. It was a very good deal but I suppose it needs work. Since I'm not that familiar , I not sure if the hum that I hear is normal or that it needs caps. I thinking they need them because he told me that they were done around 2000.


The balance and hum pots are very noisy and need a cleaning.



I've watching some SVT repair videos and it seems that there's quite a few things to look for. One thing are the yellow box caps , if it has them. There's a history of 2 resistor's that go in the pre-amp.


On thing is that I tested the power tube heat and each one was around 90c. Not sure if that's common test but at least you know if a tube is running out of control.


The amp is still together. I was able to get my hand in there just to tap the pre-amp tubes . There's a a noisy JJ ECC 832, for channel 2 so I had a 12ax7 to  swap that out. That one makes channel 2 very scary. I may go with a lower gain.  The 6C4 was very noisy so I'd have to find one. A super light tap makes that go.   

[tubeswell]

Nope. First thing to check in those is the screen grid resistors. If one is open, it will throw out the output transformer current, resulting in hum. When all tubes are conducting, it’s easy to get the bias comparator working properly. So check those Rg2s with your R-meter. Which means taking the boards out. Sorry, it’s the only way to do it on a SVT-CL

[Latole]

If the tubes are very badly balanced / matched with too much difference they can generate hum.
Just measure the cathode current of each tube and you will know.

The filter caps; You have to measure the AC component on each of them. More than 0.3 volts, it is better to replace them. Otherwise they are not the cause of the hum.
Or if they show signs of damage; bubbles at the ends for example.

Wich SVT ?

[pbman1953]

1975

[Latole]

This one ;

removed incorrect schematic... sluckey

[pbman1953]

This one ;

Thanks, which one is the pre-amp page?

[Latole]

Look here ; https://el34world.com/charts/Schematics/files/Ampeg/Ampeg_Schematics.htm

I think it is this one;

[pbman1953]

Look here ; https://el34world.com/charts/Schematics/files/Ampeg/Ampeg_Schematics.htm

I think it is this one;

Thanks!




Can you suggest where to go get the cap kit? I know that Fliptops carries it but I'm wondering where else I could go?




https://www.fliptops.net/catalog/p-100084/cap-kit-for-ampeg-svt-w6550-v.1

[pbman1953]

Open it up. Check the screen resistors . One of them may be blown open

So you're saying all the 47k's off pin 5 to check?

[sluckey]

So you're saying all the 47k's off pin 5 to check?

No. Screen grid is pin 3. Screen resistor is 22Ω.

[pbman1953]

yes yes, sorry had grid on my mind too much, thanks.


From what I see on the schematic there's only 1- can cap, but inside there are 2. Plus they are the same value 70/40/40. Looks like the only one available is a CE. Are there any ones better out there? I don't see JJ or F&T making ones that size.

[pbman1953]

Also, If I decide to update the grid resistors there's no wattage requirement unless I go 1/2 watt. 


What rating would you all suggest?


Any suggestions for a 6C4 tube?


Thanks

[sluckey]

Better verify you are looking at the correct schematic. What kind of power tubes are in your amp?

[pbman1953]

Better verify you are looking at the correct schematic. What kind of power tubes are in your amp?

Valve Art- 6550. the tubes have a collar style lock at the base , not a spring and top plate system.

[sluckey]

Valve Art- 6550. the tubes have a collar style lock at the base , not a spring and top plate system.

Well then, the schematic posted by Latole is not the one you need. Look through Hoffman's schematic and find the correct schematic.

[pbman1953]

Valve Art- 6550. the tubes have a collar style lock at the base , not a spring and top plate system.

Well then, the schematic posted by Latole is not the one you need. Look through Hoffman's schematic and find the correct schematic.

I think it's this one. It has the 7-/40/40 can cap shown

removed incorrect schematic... sluckey

[sluckey]

Nope. Wrong power tubes.

[pbman1953]

Nope. Wrong power tubes.

you're right, maybe the earlier version.I'll check around

[Latole]

Look here ; https://el34world.com/charts/Schematics/files/Ampeg/Ampeg_Schematics.htm

I think it is this one;

Thanks!




Can you suggest where to go get the cap kit? I know that Fliptops carries it but I'm wondering where else I could go?




https://www.fliptops.net/catalog/p-100084/cap-kit-for-ampeg-svt-w6550-v.1

I never buy a capacitor kit, it's too rare and there are too few good ones.

You will have to buy them by the unit, choosing according to the MFD and voltage values and taking into account the physical dimensions.

You will have to take the values on the parts actually in your amp, not trust the schematic.

Where is your location; North America or Europe to give you the right references

[pbman1953]

Boston


I found most if not all from Antique supply in AZ. Very easy site . The carry the CE multi cap, F&T & Sprague


The only odd one was the 30uf/600v. An odd item , but I found a 30/500v.


Any suggestions on the 6c4 tube?

[Latole]

I know very well Antiques

You are looking to buy a 6C4 tube ?

[pbman1953]

I found this one that has 6550's.


I suppose the the cap section outlined with dashes are the can caps?

removed incorrect schematic... sluckey

[pbman1953]

I know very well Antiques

You are looking to buy a 6C4 tube ?

Yes, it's too sensitive and noisy even touched

[sluckey]

In reply #2 you mention 6V4. In reply #13 you mention 6C4. Which is it? They're totally different tubes.

[pbman1953]

In reply #2 you mention 6V4. In reply #13 you mention 6C4. Which is it? They're totally different tubes.

Sorry for making people crazy-


6C4

[sluckey]

I found this one that has 6550's.

I suppose the the cap section outlined with dashes are the can caps?

That's much closer. However, the two cap cans are different values. You mentioned earlier that the two cans in your amp are the same?

[pbman1953]

I found this one that has 6550's.

I suppose the the cap section outlined with dashes are the can caps?

That's much closer. However, the two cap cans are different values. You mentioned earlier that the two cans in your amp are the same?

Well I'm basing my caps on the Fliptops parts list. I'll go by what Latole said and open it and make a real parts list.

[pbman1953]

From moving the 6c4 in and out of the socket it may be making a better connection and it's not that noisy. One thing, it's sensitive to tapping like the most v1's are. Besides that it's quiet. Is this normal for that position and tube?




Couple more questions-




In my search for a 30uf / 600v, the most I can find is a 30/500v. Will that be ok or should I try for a higher value at 500v? or what combo would be safe?




For those who've done an SVT are there any tell tale things to look for and change with out testing?

[Latole]

 "  One thing, it's sensitive to tapping like the most v1's are. Besides that it's quiet. Is this normal for that position and tube? "

Yes, all V1, any tube, are noisy when tapping

Filter caps  ; two 50 mfd 300 volts ( or more ) in series = 25 mfd 600 volts

[pbman1953]

The 6C4 is v4, would that matter? I'm saying by being tapped it reacts like a v1 would


Thanks for the cap info!


Also, for one of the can caps, I found a 80/40/40 for the 70/40/40 stock size. Should I go with the 80uf or stay with the stock value?

[Latole]

The 6C4 is v4, would that matter? I'm saying by being tapped it reacts like a v1 would


Thanks for the cap info!


Also, for one of the can caps, I found a 80/40/40 for the 70/40/40 stock size. Should I go with the 80uf or stay with the stock value?

6C4 ; sorry for my mistake.  I don't know .

Read here ; "....that they're prone to microphonics in general and their use in the SVT doesn't help that. "

https://www.talkbass.com/threads/ampeg-svt-6c4wa-tube.1315801/

[pbman1953]

Maybe I should get one


Thanks

[pbman1953]

"  One thing, it's sensitive to tapping like the most v1's are. Besides that it's quiet. Is this normal for that position and tube? "

Yes, all V1, any tube, are noisy when tapping

Filter caps  ; two 50 mfd 300 volts ( or more ) in series = 25 mfd 600 volts

I found  2 MoD caps at 70uf/350.  Is that over kill? 35/700v

[Latole]

"  One thing, it's sensitive to tapping like the most v1's are. Besides that it's quiet. Is this normal for that position and tube? "

Yes, all V1, any tube, are noisy when tapping

Filter caps  ; two 50 mfd 300 volts ( or more ) in series = 25 mfd 600 volts

I found  2 MoD caps at 70uf/350.  Is that over kill? 35/700v

Good choice these MOD

[pbman1953]

First report-


This amp has never had a cap job. kind of sad to see and I hope my friend didn't pay for one on the service calls he had done.


Pre-amp-


Should the yellow box caps be replaced?


Change the yellow box caps and silver 40/450v caps?


Power amp-


This amp has 2- 100/40@450v volt caps. Not the 70/40/40 that i expected


100@ 450v
90 @200V
30@600V




Should I cut out the .047 white death cap?


Tube sockets-


The 47K's measure from 38-40k. Even the larger one on the end tube socket




22 ohm screen resistors- 


5 measure- avg of 22.9


1- won't measure


Change them all or only the bad one?


Should the yellow box caps be replaced?


Are here any other recommended parts to check?


Thanks










 

[pbman1953]

more pics

[pbman1953]

More tests-


5 ohm @ 5 w- power resistors-r's-27,30,32,38,41,44


5 measure- around 6


1- won't measure- should I replace only the bad one or all?






For r's- 35 & 36- 1 ohm @ 5w- They're at 2 ohm and not 1 ohm.- replace?


Thanks

[pbman1953]

power amp

[Latole]

More tests-


5 ohm @ 5 w- power resistors-r's-27,30,32,38,41,44


5 measure- around 6


1- won't measure- should I replace only the bad one or all?






For r's- 35 & 36- 1 ohm @ 5w- They're at 2 ohm and not 1 ohm.- replace?


Thanks

These are all goods, no need new one

[pbman1953]

Thanks but here's a few questions that may have slipped by-

1- Should I cut out the .047 white death cap?



2- 22 ohm screen resistors- 5 measure- avg of 22.91


1 won't measure.- replace all or only the bad one


possible tube subs-


Original value - 90uf@200v - I have an JJ- 80uf @500- use the 80 or get the 90?







The 47K resistors off pin 5  measure from 38-40k. Even the larger one on the end tube socket-   I bought new ones anyway

Thanks

[Latole]

I think you are brave and hardworking to start repairing this amp with so little knowledge of electronics

[pbman1953]

Thanks, I have an extensive background in car audio. So I've soldered a lot and have designed and installed some amazing systems. Many with 5.1 processors from Jim Fosgate the father of more current Dolby pro-logic.

Plus, with the help of the forum and some other techs, I'm gone through my Traynor YBA-3, Sunn 2000s & converted a Fender '75 Super twin into a bass amp. My first help on that was a tech from NJ Adam who used to work with Dennis Kager, from Central Music. Adam is an SVT expert. Dennis was the service manger at Ampeg/Magnavox before opening Central Jersey Music.

[sluckey]

1- Should I cut out the .047 white death cap?

No need since you have a properly wired three prong power cord. It's not a death cap. It's simply a line filter connected between neutral and ground.

2- 22 ohm screen resistors- 5 measure- avg of 22.91

1 won't measure.- replace all or only the bad one

I would replace only the one that won't measure.

[pbman1953]

Original value - 90uf@200v - I have an JJ- 80uf @500- use the 80 or get the 90?


I have a xcion 100@450v. Ok to use that or get a better one?

[Latole]

Original value - 90uf@200v - I have an JJ- 80uf @500- use the 80 or get the 90?


I have a xcion 100@450v. Ok to use that or get a better one?

Both will work.  JJ or Xicon

[pbman1953]

a question on security-


I sent a picture of the clamps that hole 2 caps. My question is if  can use a mounting tie -


https://www.homedepot.com/p/Commercial-Electric-8-in-UV-Mounting-Cable-Tie-Black-10-Pack-GTM-200STB-10/203531941

[Latole]

Yes.

I also use steel strapping;

https://www.homedepot.com/p/Oatey-3-4-in-x-10-ft-28-Gauge-Galvanized-Pipe-Hanger-Strap-33526/301505504

[pbman1953]

Yes.

thanks!,


In the pre-amp there's a 40uf@450V electrolytic  want to change. I have another JJ- 80uf@450, will that work or should i stay with the 40?