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Post subject: Re: Help Bandmaster blowing fuses
Posted: Sat Aug 04, 2012 10:27 am
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aclempoppi wrote:
Can't say I'm confident with the situation. Switching that standby, slams the first filter cap pretty hard. But since the first reservoir is post standby, it kind of takes the brunt. That's good news for the choke and the rest of the guys following on the B+ supply. They get a slight break, while those caps charge up. To be honest, I use the standby function as little as possible. Prefer to turn volumes down, if I'm taking a 10 minute break. As far as using that function as a cool-down, I'm willing to debate the heater running on a hot tube vs ambient temperature cool-down. Art



Art. Most all amps, at least the ones I own, have stand-by.
I use stand-by religiously to warm up the amps before playing.

I know that when shutting off amps, there's no difference if standby or power-off
is used first (according to past discussions & readings). I still use stand-by first
to power-off first, but some do say it is of no consequence.

Anyway, is it your opinon not to use stand-by at all?
or to use stand-by in certain proceedures?
or to avoid using stand-by in certain ways to protect tubes or capacitors?

Thank you for sharing your strategy. Toppscore :)

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Post subject: Re: Help Bandmaster blowing fuses
Posted: Sat Aug 04, 2012 12:47 pm
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Retroverbial wrote:
BMW2002Ti wrote:
I've put snubber caps across switches in amp, to reduce pitting. And early failure. Another option. FWIW.


+1

Especially since contemporary Carlson switches are but a pale substitute for the US-made OEM versions of yore.

Arjay


How are snubber caps implemented on power/standby switches? What values do you use? My 5F6A clone uses Carling switches and has a 0.047 mFd cap connected to the standby switch where it connects to the output of the rectifier. What is the purpose of that cap (C7)?

https://taweber.powweb.com/store/5f6a_schem.jpg

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Post subject: Re: Help Bandmaster blowing fuses
Posted: Sat Aug 04, 2012 1:44 pm
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Bluesky, It's really part of an RC circuit across the switch to absorb transient spikes. Often a cap is good enough, as long as it's rated high enough voltage. I used 600VDC-1KV rated caps.


"Originally Posted by semitone: Tech, can you suggest a value and voltage rating?"

"Value isn't really critical...anything between say .01 and .05 should be just fine. Get the highest voltage rating you can find. I'd go for 600-630v rated 'cause that's a DC voltage rating, not AC."

If that isn't enough and in some standby switches which "see" near or over 600VDC --- an RC circuit maybe needed:

" With both AC and DC, contact arcing can be minimized with the addition of a "snubber" circuit (a capacitor and resistor wired in series) in parallel with the contact, like this:

Image

A sudden rise in voltage across the switch contact caused by the contact opening will be tempered by the capacitor's charging action (the capacitor opposing the increase in voltage by drawing current). The resistor limits the amount of current that the capacitor will discharge through the contact when it closes again. If the resistor were not there, the capacitor might actually make the arcing during contact closure worse than the arcing during contact opening without a capacitor!

While this addition to the circuit helps mitigate contact arcing, it is not without disadvantage: a prime consideration is the possibility of a failed (shorted) capacitor/resistor combination providing a path for electrons to flow through the circuit at all times, even when the contact is open and current is not desired. The risk of this failure, and the severity of the resulting consequences must be considered against the increased contact wear (and inevitable contact failure) without the snubber circuit.

The use of snubbers in DC switch circuits is nothing new: automobile manufacturers have been doing this for years on engine ignition systems, minimizing the arcing across the switch contact "points" in the distributor with a small capacitor called a condenser. As any mechanic can tell you, the service life of the distributor's "points" is directly related to how well the condenser is functioning.

With all this discussion concerning the reduction of switch contact arcing, one might be led to think that less current is always better for a mechanical switch. This, however, is not necessarily so. It has been found that a small amount of periodic arcing can actually be good for the switch contacts, because it keeps the contact faces free from small amounts of dirt and corrosion. If a mechanical switch contact is operated with too little current, the contacts will tend to accumulate excessive resistance and may fail prematurely!

This minimum amount of electric current necessary to keep a mechanical switch contact in good health is called the wetting current. "

Normally, a switch's wetting current rating is far below its maximum current rating, and well below its normal operating current load in a properly designed system. However, there are applications where a mechanical switch contact may be required to routinely handle currents below normal wetting current limits (for instance, if a mechanical selector switch needs to open or close a digital logic or analog electronic circuit where the current value is extremely small). In these applications, is it highly recommended that gold-plated switch contacts be specified. Gold is a "noble" metal and does not corrode as other metals will. Such contacts have extremely low wetting current requirements as a result. Normal silver or copper alloy contacts will not provide reliable operation if used in such low-current service! "


From Marshall Forum (where I got the last set of specific values):

http://www.marshallforum.com/marshall-a ... andby.html


" ...this thump is almost always caused by the transient of the switch changing state, complicated by poor grounding schemes in the amp.

shunt the standby switch with a snubber network made of a .01uF/4kV cap in series with a 47ohm/2W resistor. "


I believe this is from Valve Wizard site. Which is temporarily down, right now. :(


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Post subject: Re: Help Bandmaster blowing fuses
Posted: Sat Aug 04, 2012 5:07 pm
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Guess I wasn't clear enough! I think there is an underlying problem, yet to be found . The standby comment was just to speak to the repeated use of the switch not being benefical. Especially because it blew the fuse. A can of worms? That's the way it goes! Beemer, I share Merlin's view on this subject. Art

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Post subject: Re: Help Bandmaster blowing fuses
Posted: Sat Aug 04, 2012 6:34 pm
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BMW2002Ti wrote:
Bluesky, It's really part of an RC circuit across the switch to absorb transient spikes. Often a cap is good enough, as long as it's rated high enough voltage. I used 600VDC-1KV rated caps
.
.
.
.
" ...this thump is almost always caused by the transient of the switch changing state, complicated by poor grounding schemes in the amp.

shunt the standby switch with a snubber network made of a .01uF/4kV cap in series with a 47ohm/2W resistor. "


I believe this is from Valve Wizard site. Which is temporarily down, right now. :(


Interesting. I do notice that the standby on my 5F6A clone will thump periodically, but not that often. What I do notice more often is the momentary dimming of the power indicator when I throw the standby switch. I assume that the snubber would be of value in both conditions? My only other question as I noted earlier relates to what is the purpose of C7 in the Weber schematic?

https://taweber.powweb.com/store/5f6a_schem.jpg

It wouldn't interfere with the operation of the snubber would it?

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Post subject: Re: Help Bandmaster blowing fuses
Posted: Sat Aug 04, 2012 7:05 pm
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Sorry if the thread is straying, a bit. But, C7 looks like it functions very similar to the (in)famous "death cap." :shock:

But, in this case not to filter the "hot" line, but as a "buffer" function for the GZ34. To absorb flyback or some such event. I see this type of cap in many of the newer amps which use Russian and Chinese made rectifiers, some which are noted for flash-over.

FWIW--- a good mica cap (I have several rated to 4KV) works just as well as a axial NP --- to "snub" the switch contacts. For obvious reasons (leakage & failure), I would not recommend any PIO caps in this position.

Honestly, I don't use standby swtiches on amps below 50 watts output. An old tech told me around 600-700 VDC B+ is about when you start needing these devices. More for transmitter circuits and less for receiver. I just wished they made 600VDC rated switches still... Pipe dreams... :lol:


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Post subject: Re: Help Bandmaster blowing fuses
Posted: Sat Aug 04, 2012 7:16 pm
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" I think there is an underlying problem, yet to be found. "

Art, you are 100% correct. I've never seen a fuse pop due to a noisy standby switch. BUT that switch does do one thing. Cause an in-rush when activated. And for whatever reason --- this is popping the fuse.

I'd check the 6L6GC sockets, the wiring on the OPT (esp the one to ground), and ALL grounding points for good solders. Also, like Art mentioned before --- that choke just after the standby switch. Pull the choke and check each lead's resistance to the outer frame. You may have a shorted winding.

If the above looks ok... may need to take that OPT out and exam it.

Good luck! Keep us posted.


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Post subject: Re: Help Bandmaster blowing fuses
Posted: Sat Aug 04, 2012 7:47 pm
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BMW2002Ti wrote:
Sorry if the thread is straying, a bit. But, C7 looks like it functions very similar to the (in)famous "death cap." :shock:

But, in this case not to filter the "hot" line, but as a "buffer" function for the GZ34. To absorb flyback or some such event. I see this type of cap in many of the newer amps which use Russian and Chinese made rectifiers, some which are noted for flash-over.

FWIW--- a good mica cap (I have several rated to 4KV) works just as well as a axial NP --- to "snub" the switch contacts. For obvious reasons (leakage & failure), I would not recommend any PIO caps in this position.

Honestly, I don't use standby swtiches on amps below 50 watts output. An old tech told me around 600-700 VDC B+ is about when you start needing these devices. More for transmitter circuits and less for receiver. I just wished they made 600VDC rated switches still... Pipe dreams... :lol:


Yeah. Sorry if I took us off track. :oops:

I do want to note that C7 IS in the original 5F6A 1959 Bassman schematic, so it was not put there by Weber because of Russian or Chinese rectifiers.

http://ampwares.com/schematics/bassman_5f6a.pdf

Ok. Back to our regularly scheduled program. :D

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Post subject: Re: Help Bandmaster blowing fuses
Posted: Sat Aug 04, 2012 8:32 pm
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Bluesky, yeah on the way to work I thought: "Another brain fart." That cap is to reduce any AC hash riding on top of the DC (ala a bypass cap). Bypassing the main 'lytics and always in upstream from them.

Just couldn't get to the darn site. Too much work left behind for me. Ruining my posting and social life... :lol:


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Post subject: Re: Help Bandmaster blowing fuses
Posted: Sat Aug 04, 2012 9:01 pm
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BMW2002Ti wrote:
Bluesky, yeah on the way to work I thought: "Another brain fart." That cap is to reduce any AC hash riding on top of the DC (ala a bypass cap). Bypassing the main 'lytics and always in upstream from them.

Just couldn't get to the darn site. Too much work left behind for me. Ruining my posting and social life... :lol:


That's kind of what I was thinking it was for. Given the smallness of the cap value it would only be acting on any higher harmonics or other dirt that may be riding on the recitifed but unfiltered output from the rectifier tube. Just wanted to be sure as I wasn't clear on how a snubber would work on the standby switch.

NOW back to our regularly scheduled program. :lol:

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Post subject: Re: Help Bandmaster blowing fuses
Posted: Sat Aug 04, 2012 10:04 pm
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Shimmy, please tell us about the re-temper concept, concerning precious metals. I only worked four winters, as a staff QC member in the heat treat department, during the early '80s. Thanks in advance! Art

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Post subject: Re: Help Bandmaster blowing fuses
Posted: Sat Aug 04, 2012 10:41 pm
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OK... back to original post. :lol:

1.) Pull all tubes. Replace fuse with proper value one.

2.) Have speaker(s) connected. No input, zero volume.

3.) Turn amp on. Fuse pop? Check PT, choke, wiring (esp ground) in power supply. Wiring from PS to output section. Check the wiring around the main power supply rectifier bridge.

4.) Fuse ok? Put 6L6GC back in. Turn on amp. Fuse pop? Cehck for bad tubes, tracings on socket(s), dressing of wires on the socket.

These are the usual suspects and should be ruled-out before venturing further. If the amp pops fuses under all of these conditions...

5.) you may need to look at that PT. Easiest way to tell is to lift the secondary leads off the PT. Fuse pops? PT needs to be pulled and inspected.

Good luck! Keep us posted.


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Post subject: Re: Help Bandmaster blowing fuses
Posted: Sat Aug 04, 2012 11:29 pm
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Yeah Bro, I let the real deal go!!!! Lets get right back at the amp issues!!!! Art

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Post subject: Re: Help Bandmaster blowing fuses
Posted: Sun Aug 05, 2012 3:01 am
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aclempoppi wrote:
Yeah Bro, I let the real deal go!!!! Lets get right back at the amp issues!!!! Art


So what amp issues are we still trying to solve? MusicMans's last post stated:

"Well, I got the amp working!! Hooray! I replaced the .01's and the .02 caps and rechecked my voltages. It stayed on so I tried to switch the standby off and on which I did about 5 times in about 1 min. It blew the fuse in my iso tranny. I let it sit for about 10 min in the on position then I turned it back on. It held... I plugged my guitar in and played... WOW! Still a little worried about my voltage on the pin on the reverb tube. Should be 280 but I'm still getting 460. Maybe the sockets which I'm sure could be replaced. Gonna give it a few more test runs before I put it back into the cabinet. Now to find a new project. I would just like to say a BIG THANK YOU to ART, ARJAY and all who so graciously gave me info and the help to get to this point.. I'm sure I'll be needing more soon!"

MusicMan then stated:

"It was a 3 amp, I replaced it with a 2 amp which I had as replacements for my amp. Do you think I still have a problem in there? I thought it could have been switching the standby off and on. I didn't have my speaker load on at the time."

Which I assume is in reference to the fuse in the isolation transformer.

So it looks like there are two issues left before the sidetrack on the standby switch:

1. "Still a little worried about my voltage on the pin on the reverb tube. Should be 280 but I'm still getting 460." If I am reading this right, it seems to me that this issue is pretty isolated to the 1/2 7025 used for reverb recovery. If it were a power supply issue, it would also affect the other 1.5 7025s which are getting their plate voltage from point "D" and no such issues have been mentioned. I assume we are still using this schematic:
http://www.webphix.com/schematic%20heav ... _schem.pdf. So doesn't that point the finger at the 100K/1 watt plate resistor?

2. Then we have "It blew the fuse in my iso tranny." MusicMan stated "It was a 3 amp, I replaced it with a 2 amp which I had as replacements for my amp." So it is not clear to me whether it was a 3 amp fuse or a 2 amp fuse that blew in the iso transformer. But it IS clear that the 2 amp fuse in the primary side of the amp's power transformer DID NOT blow. So my question is, how does a problem in the DC side of the rectifier get reflected back through to the primary side of the power transformer and blow the 3 amp (or 2 amp?) isolation transformer fuse and NOT blow the 2 amp power amp fuse? If there were a power supply problem, I would be more concerned that the amp's primary side fuse DIDN'T blow more than I would be concerned that the iso transformer fuse did blow. Does anyone think that it might have just been bad luck or a coincidence that the iso tranny fuse blew?

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Post subject: Re: Help Bandmaster blowing fuses
Posted: Sun Aug 05, 2012 7:54 am
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Bluesky,

OK... Looked back and actually read the thread this time.

Very serpentine tale, for sure.. for sure. :D

MusicMan, prolly best for you to update us on the current status.

1.) Is the amp making good vibes?

2.) Are you still blowing fuses? Which ones? In amp or in isolation tranny or both. And what is the VA rating of that isolation tranny?

3.) Are the voltages still wacky on which specific reverb tube {driver: 12AT7 or recovery tube: 12AX7 (7025)}. Does the reverb work? How does it sound?

Thanks. Thread re-boot. :mrgreen:


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