NAD/Project SVT Blueline

[greengoblin]

Perhaps your dyslexia is acting up: Shematic shows R46 is 47R I am not aware of any mod that involves cutting the feedback loop. I would think this would be a big no, no with this amp.

LOL, that is quite possible. I have been combing over schemes to see what is going on at the point on this board. From what I can find, it looks at though the two points should not join. What made it look odd to me is that someone cut into the board at this point so my first thought was someone did this for a reason. Similar cut style to the mods around R50 per Ampeg recommended mod. And they must have gone too far as they had to fix the connection between R9 and R8. I included a pic. Guessing that someone joined the points with solder and this was someones way to fix it...



So far it is coming along nicely, but thankfully I have a Hakko FR-301 as I removed a ton of unneeded solder gobs. Fixed a few bad traces as well.

 

[Wasnex]

LOL, that is quite possible. I have been combing over schemes to see what is going on at the point on this board. From what I can find, it looks at though the two points should not join. What made it look odd to me is that someone cut into the board at this point so my first thought was someone did this for a reason. Similar cut style to the mods around R50 per Ampeg recommended mod. And they must have gone too far as they had to fix the connection between R9 and R8. I included a pic. Guessing that someone joined the points with solder and this was someones way to fix it...

View attachment 3504310

So far it is coming along nicely, but thankfully I have a Hakko FR-301 as I removed a ton of unneeded solder gobs. Fixed a few bad traces as well.

I believe the solder bridge on the trace on the far right connects R9 and R8. The mounting holes for R4 and R46 are just to the left of this and it does look like the trace between them has been cut. It also looks like they may have also cut the trace between R4 and C7 as well; so possily three cuts total (although hard to say for sure). I suspect this might be a bodged attempt to install node H of the power supply. I.E. they were cutting on the wrong side of the board.

I have revised this for clarity...Mod to add node H.

Here's a board with the cuts to add node H and you can also see the area you are concerned about in the upper right corner.

 

[Wasnex]

Here's another board modded for Node H, where the area you are concerned about is shown more clearly. I don't see any cut's in the upper right corner.

 

[greengoblin]

Looks like they used a dremel with a cutoffwheel to cut between R4 and R46 and in doing so they accidentally cut R8 to R9. Or could be, like you said, they cut the wrong part of the board. I cleared the solder bridge and redid it with an internal piece of wire for strength. R46 to C7 is just a green line from the board coloring, but I will double check continuity before proceeding. All of my board repairs are complete as there was one missing trace from R27 and also had one lifting trace. Waiting on a few resistors and a couple other small parts before I go further.. Cant wait to get this thing finished, but want to make sure everything is right....

 

[Wasnex]

Looks like they used a dremel with a cutoffwheel to cut between R4 and R46 and in doing so they accidentally cut R8 to R9. Or could be, like you said, they cut the wrong part of the board. I cleared the solder bridge and redid it with an internal piece of wire for strength. R46 to C7 is just a green line from the board coloring, but I will double check continuity before proceeding. All of my board repairs are complete as there was one missing trace from R27 and also had one lifting trace. Waiting on a few resistors and a couple other small parts before I go further.. Cant wait to get this thing finished, but want to make sure everything is right....

I look forward to seeing a photo of the completed repairs.

 

[greengoblin]

Here are a couple fixes so far, magnified for ease of viewing.

First is the redone solder bridge. I extended the length, added an internal wire for strength and then super glued it for additional strength.



Second was a trace that was completely gone. Used a copper tape to replace the trace, then lay a thing solder on top. After that I added a piece of wire to help with strength and conductivity. Again, finished it off with superglue to help strengthen the bond to the board.



There is one more trace that I repaired (no pic yet), but think I will end up redoing it as I don't like the looks of it. Same process as above, but the solder/copper is a bit wide. Guess it adds better conductivity though. It is the trace from R27 to the tube socket wire.

And as you can see, I adopted the process of the red marker to show what has been re-soldered or re-flowed.

They look much better in a non-magnified view and will add a full board shot once finished.

 

[JimmyM]

Such a gift to be able to do that stuff yourself!

 

[greengoblin]

Well, here is the full board finished on the solder side so you can see all of the fixes. Not thrilled with my R27 trace fix, but I am going to leave it.

 

[greengoblin]

So I guess I read to much, but research is supposed to be a good thing. Right? I have my power amp board complete and ready to drop in once i have finished cleaning the chassis more. Regarding the screen resistors I have seen many recommendations on what the values should be. I have seem some state numbers that seem out of the norm... The orig scheme shows 22ohm, but all state not to use them. Some state to bump up to 220ohm with the watts being slightly different, anywhere from 1/2w to 2watts.. But then I have seem some other posts recommend much higher values such as 1k, 1.5k and higher wattage. If memory serves me correctly, it is stated that the higher value helps prolong tube life and possibly make them more stable..??.. For a 6146B amp, what is the best value to go with?

And then that begs to ask on the plates. Early scheme lists 10ohm, slightly later in 70 they went to 3.6 ohm. The newer versions have a ferrite bead, but they also run 6550s. Hopefully not stupid questions as I am possibly rethinking some options while I have it out.

 

[Wasnex]

So I guess I read to much, but research is supposed to be a good thing. Right? I have my power amp board complete and ready to drop in once i have finished cleaning the chassis more. Regarding the screen resistors I have seen many recommendations on what the values should be. I have seem some state numbers that seem out of the norm... The orig scheme shows 22ohm, but all state not to use them. Some state to bump up to 220ohm with the watts being slightly different, anywhere from 1/2w to 2watts.. But then I have seem some other posts recommend much higher values such as 1k, 1.5k and higher wattage. If memory serves me correctly, it is stated that the higher value helps prolong tube life and possibly make them more stable..??.. For a 6146B amp, what is the best value to go with?

And then that begs to ask on the plates. Early scheme lists 10ohm, slightly later in 70 they went to 3.6 ohm. The newer versions have a ferrite bead, but they also run 6550s. Hopefully not stupid questions as I am possibly rethinking some options while I have it out.

Since you have wired in node H you have taken care of the biggest threat to the tubes.

The 6146B has a lower screen dissipation than 6550. Per the datasheet, 6146B screens are only rated for 3W VS 6W for 6550. As long as the screen voltage is kept relatively low I don't think the tube wants to draw a lot of screen current. Of course when you push the tube to overload, you never know what will happen.

My 69 came to me with 15 ohm plate resistors. I put 10 ohm plate resistors back in and also a fresh set of 22 ohm screen resistors. If I started having problems I would bump the screens up to 220 ohm 1/2W. I have probably read some of the same articles you have. I think have bought various sets of resistors up to 1.5K.

FYI Ampeg released a Technical Information Bulletin that applied to basically all models of 6550 equipped SVTs. See attached for the Technical Information Bulletin and 220 ohm 1/2W resistor recommendation.

 

[greengoblin]

Since you have wired in node H you have taken care of the biggest threat to the tubes.

The 6146B has a lower screen dissipation than 6550. Per the datasheet, 6146B screens are only rated for 3W VS 6W for 6550. As long as the screen voltage is kept relatively low I don't think the tube wants to draw a lot of screen current. Of course when you push the tube to overload, you never know what will happen.

My 69 came to me with 15 ohm plate resistors. I put 10 ohm plate resistors back in and also a fresh set of 22 ohm screen resistors. If I started having problems I would bump the screens up to 220 ohm 1/2W. I have probably read some of the same articles you have. I think have bought various sets of resistors up to 1.5K.

FYI Ampeg released a Technical Information Bulletin that applied to basically all models of 6550 equipped SVTs. See attached for the Technical Information Bulletin and 220 ohm 1/2W resistor recommendation.

I performed those on my SVT 2 Pro and SVT II non pro. The non pro I replaced the resistors with the proper ferrite beads as well so I have extras. Right now it sits with 10 ohm and 1k and I may just stick with that for now. Wasnt sure if I should drop them to 220ohms units.

 

[rickdog]

My understanding (based on advice here from some of the tube gurus) is that the 10 ohm plate resistors are intended to act as fuses and protect the output transformer from a shorted tube. Newer SVTs with the LED bias system include a protection circuit that shuts off B+ if a tube draws excessive current, so the resistors are no longer necessary.

So in your Blueline, you probably want those resistors. They are supposed to be a very specific type. Perhaps someone like @beans-on-toast, @BassmanPaul, or @Wasnex (or others who don't come to mind quite as quickly) can provide specifics? Or even correct me if I've garbled this advice?

 

[BassmanPaul]

I’ll leave advice on the SVT to folk who are more familiar with the units.

 

[Wasnex]

I performed those on my SVT 2 Pro and SVT II non pro. The non pro I replaced the resistors with the proper ferrite beads as well so I have extras. Right now it sits with 10 ohm and 1k and I may just stick with that for now. Wasnt sure if I should drop them to 220ohms units.

Since you have the amp on the bench. It might be interesting to do some experiments with different value screen resistors to see how much it impacts tone, feel, and power.

I reread some of the posts about 6550 equipped SVTs yesterday and there are a lot of suggestions to run 1.5K screen resistors with claims the amps never sounded better, and there was no noticeable different in amp volume. I actually find it a bit hard to accept that the larger resistors had no impact on output power and tone, but maybe the impact is so small that it is is not really relevant from a playing standpoint.

There are no guarantees a 6146 will respond the same to such a large screen resistor. I do think the larger resistors will protect the tubes better, but they may also limit the power too much; it's basically a trade off. If the amp will not be pushed hard, perhaps it's not worth worrying about.

Although I have the original 22 ohm screen resistors on my 69, the screen supply (node E) has a voltage dropping network to compensate for the aftermarket PT with 350V screen taps. The voltage dropping network is comprised of a 16K series resistor, a 120K shunt, and an extra capacitor.

Node E sits at 219V at idle, but if I feed the amp a test tone, power is limited to about 120W and node E drops to about 165V. When I play the amp normally, it sounds great and produces only slightly less clean volume than my Skunk, but it sounds a bit more compressed.

I suspect the compression may be due to the sag induced by the voltage dropping network. I would think drastically increasing the value of the screen resistors would also make an amp a bit more compressed. Depending on your tone goals this may or may not be desirable.

So maybe run some experiments and then make a value determination based on the results.

 

[Wasnex]

I performed those on my SVT 2 Pro and SVT II non pro. The non pro I replaced the resistors with the proper ferrite beads as well so I have extras. Right now it sits with 10 ohm and 1k and I may just stick with that for now. Wasnt sure if I should drop them to 220ohms units.

Did you do any AB testing when you installed the larger screen resistors. Any idea how much power the amps make with the new resistors?

 

[Wasnex]

My understanding (based on advice here from some of the tube gurus) is that the 10 ohm plate resistors are intended to act as fuses and protect the output transformer from a shorted tube. Newer SVTs with the LED bias system include a protection circuit that shuts off B+ if a tube draws excessive current, so the resistors are no longer necessary.

So in your Blueline, you probably want those resistors. They are supposed to be a very specific type. Perhaps someone like @beans-on-toast, @BassmanPaul, or @Wasnex (or others who don't come to mind quite as quickly) can provide specifics? Or even correct me if I've garbled this advice?

There is probably some truth to this, but what you usually see is open screen resistors. When the screen resistors open up, typically the tubes stop drawing current.

 

[greengoblin]

My understanding (based on advice here from some of the tube gurus) is that the 10 ohm plate resistors are intended to act as fuses and protect the output transformer from a shorted tube. Newer SVTs with the LED bias system include a protection circuit that shuts off B+ if a tube draws excessive current, so the resistors are no longer necessary.

So in your Blueline, you probably want those resistors. They are supposed to be a very specific type. Perhaps someone like @beans-on-toast, @BassmanPaul, or @Wasnex (or others who don't come to mind quite as quickly) can provide specifics? Or even correct me if I've garbled this advice?

There is probably some truth to this, but what you usually see is open screen resistors. When the screen resistors open up, typically the tubes stop drawing current.

I am running Vishay fusible 10ohm 5w 5% resistors since i had them. Believe they are AC-5's for power supplies.

Did you do any AB testing when you installed the larger screen resistors. Any idea how much power the amps make with the new resistors?

Unfortunately, no. All of the heads I picked up over the last year or so have been non-working projects. Did a few tests on them and then just began the rebuilds.

 

[DrMole]

Despite the fact that I have mostly posted on humor and gig-type threads, this thread and technique posts are by far the most interesting and useful threads for me.

Looking forward to the finished product but am truly loving the journey and informative posts by all.

More, please!

 

[greengoblin]

FYI... Took some readings of the Power Transformer laying on the bench. All voltages AC and read while fed 120v from my variac:

All measured AC:

Red to Red 523.2
Blue to Blue/Yellow 36.25
Blue to Orange 173.1
Blue to Green/Yellow 310.4
Blue to Green 347.2
Blue/Yellow to Blue/Green 274

Wasnex added:

Here are pairs:
523.2V B+
347.2V Screen Supply
274.15 Bias Supply


Transformer # 893007 682 (maybe 082) 034

 

[Wasnex]

FYI... Took some readings of the Power Transformer laying on the bench. All voltages AC and read while fed 120v from my variac:

All measured AC:

Red to Red 523.2
Blue to Blue/Yellow 36.25
Blue to Orange 173.1
Blue to Green/Yellow 310.4
Blue to Green 347.2
Blue/Yellow to Blue/Green 274

Wasnex added:

Here are pairs:
523.2V B+
347.2V Screen Supply
274.15 Bias Supply


Transformer # 893007 682 (maybe 082) 034

Thanks brother...I have been looking for this info for quite a few years, so I can have a proper PT wound for my 69.

Here is some more info on SVT transfomers that supposedly came from an SVT old service manual:

Transformer Specs

Early version with 6146s:
PT 8930007:
600v @ 0.75A
200v @ 0.07A
150v @ 0.025A

Heater transformer 8930008:
6.3v @ 10A

OT 8950004:
Primary 1850 ohms
Secondaries 2, 4 ohms

Later version for 6550s:
PT 8930072:
600v @ 0.75A
330v @ 0.07A
150v @ 0.025A

Heater Tranny 8930073:
6.3v @ 12.05A
​

Notice these specs provide the resultant DC voltages under load. Per @agedhorse you need both loaded and unloaded voltages to determine the impedance of the transformer, or voltages under two different load conditions.

The impedance is what determines where the voltages will settle when the amp is actually operating. So if you get the impedance wrong, the transformer will not perform properly.

Here's a post I found explaining the way Ampeg publishes transformer load specs: [Invalid or Expired Link Removed]