Sunn 200s multicap can, etc. replaced but showstoping problem persists

[Tbass1]

Replaced the can with one from Dynaco rated at 550-600v and 30/20/20/20 mf and the two associated resistors with 33k-1w & 15k-2w metal oxide; and replaced the 2 bias 150v-50mf electrolytics. Also installed a 3 prong power cord and disconnected the polarity switch and wall power input on back.
Originally, this amps problem was boiled down to: blew fuse when powered up on standby & no tubes in place. Power transformer was suspected and pulled ( though this PT never smoked, stunk, or overheated ) , but couldn't find a problem with it so it went back in. The cap replacement/etc needed done anyway, but now the amp still has the same problem!?!? Blows fuse within 2 seconds of switching power on while on standby with no tubes installed. Being without my Sunn has become very frustrating since I'm forced to gig with an old solidstate carvin pb900. Since the same basic problem persists, and the power transformer is the main part in the circuit before the standby switch is closed, I think I have to buy the PT. Does anyone have any other ideas... possibilities I could check? Thanks

[Jazz P Bass]

You stated that you found no problem with the power transformer.
Does this mean you powered it up with the secondary disconnected & it did not blow a fuse?

[Tom Phillips]

Power transformer was suspected and pulled ( though this PT never smoked, stunk, or overheated ) , but couldn't find a problem with it so it went back in.

You didn’t explain how you verified that the PT was OK. The problem could be the PT or it could be a short in the amp’s wiring or part of the circuitry that is connected while the amp is on standby with no tubes installed.

... Since the same basic problem persists, and the power transformer is the main part in the circuit before the standby switch is closed, I think I have to buy the PT. Does anyone have any other ideas... possibilities I could check?

Let’s do some further troubleshooting. I assume that you already know how to safely work around high voltage. If not then get professional help.
If it were on my bench I’d disconnect all the power transformer secondary leads and leave the primary connected. There is no need to remove the transformer from the chassis. While you are doing that give the wiring a good inspection looking for shorts, burnet wires, bits of solder or loose wiring etc.

Now with the secondary leads arranged so they can’t touch anything, power the amp on. If the fuse doesn’t blow then the transformer is probably OK. We are now going to add one set of secondary connections at a time and check operation at each step.

Check continuity between each side of the heater circuit and the chassis. With the PT heater wiring disconnected both sides should be open circuit. If not then there is a short hidden in the wiring or a problem with a tube socket. If all is OK then connect the heater leads and power up again. If the fuse blows, check the heater circuit wiring again.

If all is OK. Turn power off and connect the bias secondary lead and the HV center tape. Power up and check for fuse blowing. Yes/No?

If there is still no fuse blowing then power down and connect are the HV wires. Since the rectifier tube is out the only fault paths are in the wiring or the rectifier tube socket.

Let us know what you discover.

Tom

[Tom Phillips]

I forgot to mention the 5V rectifier heater supply. Need to add that step. You get the idea. Connect one circuit at a time until the problem area is found.

[Rhodesplyr]

Replaced the can with one from Dynaco rated at 550-600v and 30/20/20/20 mf

That's interesting since Dynaco--as the company existed in the 1960s and 1970s--has been out of business for a long time.

A web search showed that someone's marketing a 550-600V can made in Germany. I'd wager it has 300V caps inside in series.

You can get a new PT for ~$105, not too painful--and the new ones do tend to run cooler.

[Tbass1]

Thanks All! I'll post results after further testing as outlined by Tom.

[Tbass1]

dynakitparts.com was my source for the multicap can, not Dynaco.

[troye]

Thanks All! I'll post results after further testing as outlined by Tom.

I have the exact same problem with my 200S. Switched to a 3 prong and replaced with a dynakit multi cap. Please let me know what you figure out. I've heard possible rectifier tube issue but haven't tried to replace yet.

[Tom Phillips]

I have the exact same problem with my 200S. Switched to a 3 prong and replaced with a dynakit multi cap. Please let me know what you figure out. I've heard possible rectifier tube issue but haven't tried to replace yet.

Tell us more about what troubleshooting you have done so far.
Does the amp blow fuses with all the tubes removed?
Do you have test equipment?

[kupervaser]

I have exact the same issue. My 200S blows fuses after a few seconds without any tubes in it, even the rectifier tube is out. Did you manage to find a problem?

[Jazz P Bass]

If the amp blows fuses with all of the tubes out, that pretty much narrows it down to the power transformer.
To verify this you should disconnect the secondary wiring & power up the transformer all by itself.

[kupervaser]

I am measuring some of the voltages on the PT. Without any tubes, so GZ34 is removed as well I measure around 740VAC between pin 4 and 6 of the rectifier tube. Is this ok?

[Tom Phillips]

I am measuring some of the voltages on the PT. Without any tubes, so GZ34 is removed as well...

First some clarification… In post #10 you reported that the fuse blew “Without any tubes, so GZ34 is removed as well.” Are you now saying the fuse doesn’t blow with all the tubes out or are you just making a quick measurement and then tuning the power off before the fuse blows?

…I measure around 740VAC between pin 4 and 6 of the rectifier tube. Is this ok?

That is OK.

[kupervaser]

Ok let me clarify some things.

First of all, when the GZ34 is inserted the amp blew fuses immediately. When I remove the rectifier tube the amp turns on, but if I flip the on/off switch a few times the fuse blows. I am sure it is not the rectifier, the tube works in other amplifiers fine. So I started by unsoldering some of the secondary windings of the PT. SO far, all grounds have come off, the 6,3V heater wires and the wire from pin 6 of the rectifier. Basically I still have the same problem, amp turns on but after flipping the switch on and off a few times the fuse blows. I first thought it was the switch itself, and I opened it and cleaned it properly, but didn't help. BY the way it sais 3A fuse in my amplifier but I read some people use 4A. Should I try?

[km6xz]

Turning the power on and off repeatedly is often a cause of fuse blowing is there is a thermistor in series with the primary to limit in-rush current. As the thermistor heats, its resistance drops, allowing higher steady state currents to be pulled. Cycling a power switch on a in-rush current limited unit is a good way to waste a lot of perfectly good fuses. I think you are following a red-herring and should focus on the real problem of a short on the B+ after the GZ-34.
It was a mistake to change all those caps when still trying to diagnosis the problem. It is likely that you introduced more unknown variables. AFTER the amp is working, you can restore it if you wish but changing parts in a random fashion usually makes things less predictable when trying to track down a problem.