Sunn 300T Bias circuit question/issue

[hiegdk]

I've got a Sunn 300T bass amp that I've been slowly chipping away at. I'm having an issue with the bias circuit . It keeps blowing the fuse - (F3) on the schematic - even when I've barely brought it up to 50-60V mains on the variac. I noticed that the bias winding on the PT seems to be 250V according to the spec sheet from Mercury Magnetics. Is it a problem that the bias winding is right at the voltage rating of the fuse?

Any suggestions for troubleshooting steps?

Service manual is here, too big to attach: https://el34world.com/charts/Schematics/files/Sunn/Sunn_300t_service_manual.pdf

[Lectroid]

Please post a complete schematic.

[Willabe]

Please post a complete schematic.

He did, it's in the service manual link he posted.

[Willabe]

I noticed that the bias winding on the PT seems to be 250V according to the spec sheet from Mercury Magnetics. Is it a problem that the bias winding is right at the voltage rating of the fuse?

Service manual is here, too big to attach: https://el34world.com/charts/Schematics/files/Sunn/Sunn_300t_service_manual.pdf

So, did you change the original PT with a Merc. Mag. PT?

In the schematic from the manual it shows PT secondary for -bias wind as CP18/CP19 both gray wires. No PT secondary acv's shown on the schematic.

Have you measured those -bias gray wires for acv? Take all the tubes out and leave them out until you get the fuse to stop blowing on the -bias circuit and verify that the -bias is working correctly. So you don't blow up any power tubes.

Now with all the tubes out turn on the amp and see if you can get an acv reading from the gray -bias winding wires, the PT secondary gray wire pair.

What do you get?

[Willabe]

After what I posted above, this amp has what looks to me to be a pretty complex -bias balance and adjust circuit along with a PI that has a complex(?) driver after it.

And their all tied together.

Merlin, HBP, tubeswell, Dummyload will probably understand it, maybe shooter too. Maybe a few others here. I get some of it, but.......

They will still need to know the answers to what I asked above. 

[mresistor]

What's the voltage at TP50?   To me the balance adjustment is adjusting one trio conducion level to be near the same as the other trio. It's a lot like that big Ampeg SVT bass amp but without the monitor circuit that would shut it down if too far off.  And of course it would be good to know what AC volts are on the grey PT leads.


A 250v rated fuse should be able to hande 250VAC. That fuse is only rated at 250ma though so the current is exceeding that rating.  There should be little to no current in a bias circiut that is why the fuse is rated so low in amperage.  It to me idicates there might  be a problem with one of the power tubes internally.

[Willabe]

What's the voltage at TP50?   To me the balance adjustment is adjusting one sextet conducion level to be near the same as the other sextet. It's a lot like that big Ampeg SVT bass amp but without the monitor circuit that would shut it down if too far off.  And of course it would be good to know what AC volts are on the grey PT leads.


A 250v rated fuse should be able to hande 250VAC. That fuse is only rated at 250ma though so the current is exceeding that rating.  There should be little to no current in a bias circiut that is why the fuse is rated so low in amperage.  It to me idicates there might  be a problem with one of the power tubes internally.

 I asked about the -bias wind gray wires acv because he said the Merc. Mag. spec sheet said they're 250acv.

But they go to a FWB SS rectifier, that would be 1.4 x 250 = ~350dcv. So that can't be a 250acv wind.

You have typo, it's not a sextet per side, it's 6 power tubes total, 3 per side.

That bias balance is limited by the -bias pot. And there's a lot more all tied in with the bias adjust and bias balance. The schematic shows the raw -bias C- being injected at 2 places, R333 and at R311/R312 in the bias and balance circuit. 

As far as the -bias drawing current causing the bias wind fuse to blow, C177 before the FWB, C166 just after the FWB and C162 in the bias and balance circuit, any of those could be bad. That would draw enough to blow the bias wind fuse. 

[hiegdk]

Thanks for the replies and questions all! Some additional info I should have shared right off:

All of my work on this amp so far has been without any tubes in it - I don't even have a set of power tubes for it yet as I wanted to confirm that the basics were at least in reasonably good working order before I lay out the $$$ for them.

I've replaced the following parts so far:
C171, C172, C173, C174, C175
C159, C156, C162, C166, C176, C170
CR99, CR98
R312, R311, R333, R353

I replaced all the big electrolytics with Nichicon radials and F&T axials as a matter of course. The 2 diodes and 4 resistors I replaced because they looked pretty cooked and the board under them had started to discolor. I mounted them in the air a bit and increased their power ratings (see photo).

To measure the bias winding I disconnected the grey wires from the board and measured across them. With 120VAC coming out of the variac I'm getting 245VAC across the grey wires.

I haven't been able to get a measurement at TP50 yet because the fuse keeps blowing before I get the variac up to a nominal voltage.

[SEL49]

Replace these four diodes. 1N4007 will work fine.

[Willabe]

On the schematic TP 50 C-, if there's a wire there you could disconnect so it's just the -bias FWB, then you could see if the fuse still blows. If it doesn't then the problems after that TP 50 point.   

And the 3 caps that could be causing the -bias to blow the fuse are C177, C166, and C162.

Those should be changed. They could be bad and shorting voltage/current to ground.

That's why you want to see if you can see if you can separate/disconnect the rest of the bias and bias balance circuit after TP 50.

[Willabe]

Replace these four diodes. 1N4007 will work fine.

Might be, but we don't know if the bias and bias balance caps, C177, C166, C162 are bad.

[hiegdk]

No wire after TP50, it's all on the board, but I should be able to lift the negative end of C166 to isolate the bias supply. I'll try that.

All of those caps are brand new F&Ts with the exception of C177 which is the original ceramic. Seems unlikely that they're bad, but certainly possible. Is there a quick way to test them? I don't own an ESR meter yet.

[SEL49]

I should be able to lift the negative end of C166 to isolate the bias supply. I'll try that.

That will not isolate anything except C166.

The FWB diodes and C166 are the prime suspects. Replace the four FWB diodes, or at least check them. You'll have to disconnect one end to accurately check them, so that's why I suggest to just replace them and be done.

[AlNewman]

I'm not pretending I completely understand the circuit, but I highlighted everything I see that is connected, and could potentially cause an issue.
If replacing parts in the initial power supply you shared in your 1st post doesn't solve the problem, there are a significant amount of components that COULD cause the fuse to go.  And if it's blowing fuses there are a significant amount of components that COULD have been damaged.
Basically the whole circuit should be gone through and tested initially without power to the best of your ability.  It wouldn't surprise me if the issue is fairly easy to diagnose and repair, as it's well designed, and there are many safeties built in.

[hiegdk]

I think there's a discrepency in the service manual, unless I'm reading it wrong. On the schematic it shows TP50 on the negative side of C166 and on the board layout page it shows TP50 on the positive side of C166 (which ought to be GND). I'm I simply misunderstanding something there?

[SEL49]

The schematic is correct. The layout is wrong.

[hiegdk]

The schematic is correct. The layout is wrong.

Thanks. I took your suggestion and replaced the diodes in the bias rectifier. I've now also lifted the negative leg of C166 as that ought to isolate C- (if I'm reading the schematic correctly). In that configuration and with the replaced diodes I'm seeing the same sort of behavior. I have a slightly overspec'd fuse in F3 (500mA instead of 250mA) and it starts to glow at around 30-40V from the variac. I don't have a replacement for C177 handy.

[AlNewman]

Build a lightbulb limiter.
You can't just increase the fuse value.
There's already plenty 2W resistors in that circuit.  You'll start frying traces.

[SEL49]

You can completely remove C177 for testing. It's not absolutely necessary for the bias supply to operate.

[hiegdk]

Build a lightbulb limiter.
You can't just increase the fuse value.
There's already plenty 2W resistors in that circuit.  You'll start frying traces.

I have one, and I use it. I have not/will not run the circuit at mains voltage with that fuse in place. I increased the fuse value just so I could monitor the behavior while slowly bringing up the mains on the variac as the 250mA was blowing too quickly.

[hiegdk]

You can completely remove C177 for testing. It's not absolutely necessary for the bias supply to operate.

Thanks, I'll pull it and see what happens.

[hiegdk]

You can completely remove C177 for testing. It's not absolutely necessary for the bias supply to operate.

Thanks, I'll pull it and see what happens.

Same behavior.

[AlNewman]

I have one, and I use it. I have not/will not run the circuit at mains voltage with that fuse in place. I increased the fuse value just so I could monitor the behavior while slowly bringing up the mains on the variac as the 250mA was blowing too quickly.

It doesn't matter what voltage you are running.  What matters is the current flowing through the circuit.  A 'slight' increase from a 250mA fuse to a 500 mA doubles the current through your circuit, which was designed for 250mA.  That means even the parts that aren't broken yet are working twice as hard.

[SEL49]

Pull all four diodes. What happens?

[hiegdk]

Is there a safe-ish way to measure the current draw in that part of the circuit? Not sure on the spec's for the Fender/Sunn iron but I read in another thread that the Mercury replacement for this amp has the bias secondary rated for 1A.

[hiegdk]

Pull all four diodes. What happens?

Pulled all 4 diodes. Fuse does not glow, and current draw on my wall meter seems much more like what I'd expect.

[SEL49]

Leave the diodes out for now. Measure resistance from C166 negative lead to ground. What have you?

[hiegdk]

Leave the diodes out for now. Measure resistance from C166 negative lead to ground. What have you?

Negative lead of C166 is still floating. Negative pad of C166 to ground is reading around 21K - started around 35K and slowly dropped.

[AlNewman]

Negative lead of C166 is still floating. Negative pad of C166 to ground is reading around 21K - started around 35K and slowly dropped.

If the lead of the capacitor is floating, measure resistance across lead to lead of C166.

[AlNewman]

Is there a safe-ish way to measure the current draw in that part of the circuit? Not sure on the spec's for the Fender/Sunn iron but I read in another thread that the Mercury replacement for this amp has the bias secondary rated for 1A.

There should be 2 spade connections, CP26, and CP 27, and that would isolate the power tube side of the circuit from the rest.

If your filter capacitor tests ok, you could disconnect those two connectors and see if you are still flashing your LBL.  That would be after reconnecting all the working parts in your initial power supply.

[Willabe]

I'm not pretending I completely understand the circuit, but I highlighted everything I see that is connected, and could potentially cause an issue.

And that's a lot of parts there.

[hiegdk]

Negative lead of C166 is still floating. Negative pad of C166 to ground is reading around 21K - started around 35K and slowly dropped.

If the lead of the capacitor is floating, measure resistance across lead to lead of C166.

Started at about 1M and steadily rising - at about 5M after a minute or so.

[AlNewman]

That sounds about right.  It's probably a good idea to replace it eventually, but for now, it should work fine.

Was the other one a ceramic capacitor?  You can test that the same way, if it tests in the Mohms to open, it can be reinstalled.

Next, try disconnecting the spade connectors and see if you are still reading short...  With a 250mA fuse.

[hiegdk]

That sounds about right.  It's probably a good idea to replace it eventually, but for now, it should work fine.

Was the other one a ceramic capacitor?  You can test that the same way, if it tests in the Mohms to open, it can be reinstalled.

Next, try disconnecting the spade connectors and see if you are still reading short...  With a 250mA fuse.

The ceramic measures OL.

WRT removing the spade connectors I presume you mean with the diodes and caps reinstalled?

[AlNewman]

Yes.  Rebuild everything, remove the spade connectors C26 and C27, and check with your LBL.

[hiegdk]

Same behavior as before, fuse pops almost immediately as I bring the voltage up.

[Willabe]

Same behavior as before, fuse pops almost immediately as I bring the voltage up.

Your not telling us what you changed, what you did.

[hiegdk]

Same behavior as before, fuse pops almost immediately as I bring the voltage up.

Your not telling us what you changed, what you did.

I did what AlNewman suggested. Bias rectifier rebuilt and ceramic cap back in, C- reconnected, CP26 and CP27 disconnected.

[SEL49]

Are you absolutely sure the diodes are installed correctly? Show us a hi-rez pic so we can have a look-see. Especially interested in this part of the board...

[hiegdk]

Are you absolutely sure the diodes are installed correctly? Show us a hi-rez pic so we can have a look-see. Especially interested in this part of the board...

I just have them temped in at the moment, hence why they are dangling in mid air.

[AlNewman]

Sooo, how much work have you done to the amp, and when did it start blowing fuses?

[hiegdk]

I got it dirt cheap in a non functioning state with no power tubes and no indication of what, if anything, was wrong with it. I was told it needed some work but non of the local techs wanted to work on it (I don't blame them). I decided to take it on as a learning project. So far all I've done is replace the parts I indicated earlier in the thread - which I did before I tried applying power to it. It started blowing that fuse pretty much right away. At this point I'm concerned that there could be an issue with the board itself - it's not the greatest quality.

[AlNewman]

Well, if the spade connectors were disconnected, and your bias supply is wired properly, all I can suggest is to test all the components highlighted in my original post.  There's only a few connected to ground, which could create a short.  There's some diodes which could be an issue.  There's a 10k FP resistor that may be suspect. mainly because it's probably there as some sort of fuse.  Basically test all the components with the amp off and see if you can find anything out of spec.

I still don't understand why you're blowing fuses with a light bulb limiter, but that's neither here nor there. 

[tubeswell]

That bias circuit only gives a result with a 12AT7 plugged into V4. The return for the 12AT7's cathode resistors is sunk to a fairly high NEGATIVE voltage (C-), and the plates sit at a fairly high positive voltage (source from X). You have to have current between the source and the sink (hence you need the tube plugged in). The cathode voltage on each 12AT7 triode is your bias voltage.


As to why the fuse keeps blowing - that indicates a short-to-ground in the power supply somewhere. You say it blows at 50-60V. What do the other voltages on the power rail look like before it blows?

[hiegdk]

That bias circuit only gives a result with a 12AT7 plugged into V4. The return for the 12AT7's cathode resistors is sunk to a fairly high NEGATIVE voltage (C-), and the plates sit at a fairly high positive voltage (source from X). You have to have current between the source and the sink (hence you need the tube plugged in). The cathode voltage on each 12AT7 triode is your bias voltage.


As to why the fuse keeps blowing - that indicates a short-to-ground in the power supply somewhere. You say it blows at 50-60V. What do the other voltages on the power rail look like before it blows?

Brought it up to 118V mains with the blown fuse in the bias section so I could measure the other voltages in the PS. TP44 and TP45 appear to be in spec but I'm seeing 2.6VDC at TP47, TP48, and TP49. One one side of R323 I'm seeing 365V and the other side is 2.6V. Disconnected one side of R323 to measure it and it measures fine.

[AlNewman]

Brought it up to 118V mains with the blown fuse in the bias section so I could measure the other voltages in the PS. TP44 and TP45 appear to be in spec but I'm seeing 2.6VDC at TP47, TP48, and TP49. One one side of R323 I'm seeing 365V and the other side is 2.6V. Disconnected one side of R323 to measure it and it measures fine.

Test diode CR92, and check resistance across C160.  You replaced all the electrolytics?

[Willabe]

I've got a Sunn 300T bass amp...... 

Service manual is here, too big to attach: https://el34world.com/charts/Schematics/files/Sunn/Sunn_300t_service_manual.pdf

[hiegdk]

Brought it up to 118V mains with the blown fuse in the bias section so I could measure the other voltages in the PS. TP44 and TP45 appear to be in spec but I'm seeing 2.6VDC at TP47, TP48, and TP49. One one side of R323 I'm seeing 365V and the other side is 2.6V. Disconnected one side of R323 to measure it and it measures fine.

Test diode CR92, and check resistance across C160.  You replaced all the electrolytics?

CR92 measures fine (with one leg raised). C160 measures 1.2M in circuit. I replaced all the high voltage electrolytics - there are a bunch of low small. ones in the low voltage sections of the amp that I haven't touched.

[Lectroid]

Please post a complete schematic.

He did, it's in the service manual link he posted.

I'm guilty as charged.  Reading too fast as usual... 

[AlNewman]

CR92 measures fine (with one leg raised). C160 measures 1.2M in circuit. I replaced all the high voltage electrolytics - there are a bunch of low small. ones in the low voltage sections of the amp that I haven't touched.

You're running 36.5 mA over a 10k resistor rated for 5W.  (R323)  That's over 13 watts across that resistor.  I would imagine it's getting pretty hot.  I wonder if there's maybe a bad solder joint or trace in that area?   

I just noticed C161 also should be tested for short.
If you're testing with no tubes in, i don't see on the schematic any other places where it could be dropping that much voltage, except C159. 
Maybe I'm missing other parts of the circuit fed by the Z node. 

Does R380 test ok?

As Tubeswell mentioned, the bias circuit is tied to the preamp nodes, so it's possible the 2 issues are tied together.

I'm kind of at a loss.  As I mentioned earlier, I would go through the whole amp, with the power off, and test each component to the best of your ability.  Test continuity across traces, look for cold solder joints and bad traces.  Look for parts that have overheated.  Make sure the pots are clean.

There ain't no glory in fixing amps, most of the time it's banging your head against the wall.  Until it's fixed at least.  Then you're a rock star.