Ordered a Schiit Aegir (single) for my Cornwalls....

[costerdock]

Been running my Cornwalls on VTA M125 Monoblocks for over 10 years - but having serious issues with the amps.  Working those out - I ordered the Schiit Aegir.

 

Seems it will be a good match - I'm going to run one in stereo mode - and they try it with XLR - mono mode - I'll use my working tube amp to complete it - to guesstamate what monos would be like.

 

If it works out - I might buy a second one - or simply return it.

 

Hoping it works out.

 

[captainbeefheart]

Is there a Stratocaster in that guitar case? I see Mr. Hendrix on the wall so that's my guess.

 

Anyway the VTA amps are very simple amplifiers, I don't see much going wrong with them except the modern metal can multi-section style caps are junk and they fail pretty often causing hum or worse blowing fuses. After 10 years of use I can see them going bad.

 

 

[costerdock]

4 minutes ago, captainbeefheart said:

Is there a Stratocaster in that guitar case? I see Mr. Hendrix on the wall so that's my guess.

 

Anyway the VTA amps are very simple amplifiers, I don't see much going wrong with them except the modern metal can multi-section style caps are junk and they fail pretty often causing hum or worse blowing fuses.

 

 

Not in the case - but in my office - a blue American standard.

 

Somewhat simple amps until you have a melt down.  I completely rebuilt it - worked for a week or so then failed.  I likely have to replace a tranny and quad cap.  Long story - full details here:  https://audiokarma.org/forums/index.php?threads/yellow-paper-mod-vta-m125s-did-it-help-my-situation.985784/

 

Thanks

[captainbeefheart]

Not going to lie but I don't have time to read through 15 pages.

 

The motorboating which is always a low frequency oscillation from a bad power supply capacitor is most likely the issue. Also these are ultra linear amplifiers and there is more voltage on the screens than the plates, modern tubes hate these amps and it's typical for a new tube to not like the high screen voltage with these types of amps. I doubt the transformer is bad but that's easy enough to test, typically when they fail they just pop fuses instantly. The motorboating from a bad power supply capacitor is an oscillation which can also cause the power tubes to go into a thermal runaway condition and red plate also drawing too much current and red plating the rectifier tube. Since no fuse blew I doubt you shorted a screen on a power tube or a transformer is shorted.

 

Did you replace the multi-section can capacitor during the 'rebuild'? I'm still sticking to my original hypothesis, a power supply cap went bad and caused low frequency oscillation.

[captainbeefheart]

It's very possible the mod you did with the silicon diodes which added a capacitance into the rectifier circuit caused a high amount of rectifier induced LCR ringing which will work the filter caps even more which caused them to fail faster.

[costerdock]

26 minutes ago, captainbeefheart said:

Not going to lie but I don't have time to read through 15 pages.

 

The motorboating which is always a low frequency oscillation from a bad power supply capacitor is most likely the issue. Also these are ultra linear amplifiers and there is more voltage on the screens than the plates, modern tubes hate these amps and it's typical for a new tube to not like the high screen voltage with these types of amps. I doubt the transformer is bad but that's easy enough to test, typically when they fail they just pop fuses instantly. The motorboating from a bad power supply capacitor is an oscillation which can also cause the power tubes to go into a thermal runaway condition and red plate also drawing too much current and red plating the rectifier tube. Since no fuse blew I doubt you shorted a screen on a power tube or a transformer is shorted.

 

Did you replace the multi-section can capacitor during the 'rebuild'? I'm still sticking to my original hypothesis, a power supply cap went bad and caused low frequency oscillation.

 

No I didn't - but I definitely plan to.  And thanks for the info - I plan to look into getting an ESR meter to begin testing all the caps and will open up the trannies and do a visual inspection first before asking what else I can do.  The issue that caused my rebuild was a bad sound followed by really foul burning smell where I had to unplug the amp to stop it.  I figured something must have not survived that - and is the reason I'm having issues now.

 

So - long story short my plan:

 

First test all caps (if you have a recommendation for a good esr meter - I'm all ears) and open trannies for visual look for burns etc - anything out of the ordinary.

 

Thanks for your interest - I'm open to all ideas - really appreciate it!

 

Cut and paste from AK:

 

Minimal road to recovery:
- ($$$$) Open Transformers and inspect (completely remove) and replace.
- ($)Replace CAN cap (assume it is bad being over 10 years old.)
- ($$) Build Bucking Transformer as my voltage - 122 is very high. Both amps will plug into.
- ($)Buy RUBLI delay board (http://rubli.net/HT_delay/)
- Only use my WS1/Thermistor SS rectifier - wow no money as I already spent it on them... doh! ($$)
- Adopt two fuses (assume 5 amp slo-blo perhaps) as outlined above in post 257
- Use my Sovtek 6550 tubes only (look into sending my KT88s to someone for testing - open to suggestions - Jim McShane much too busy)
- ($$) AutoBias boards

[costerdock]

29 minutes ago, captainbeefheart said:

It's very possible the mod you did with the silicon diodes which added a capacitance into the rectifier circuit caused a high amount of rectifier induced LCR ringing which will work the filter caps even more which caused them to fail faster.

Seemed like a well regarded mod - "yellow sheet" supported by folks I highly respect so I have to trust it is harmless and good to do.  Thanks.

[billybob]

Yes welcome back. You should get some feedback on the Schiit. It has been talked about abunch here.

While waiting, could do a search of the forum.

Cool...

[captainbeefheart]

47 minutes ago, costerdock said:

 

No I didn't - but I definitely plan to.  And thanks for the info - I plan to look into getting an ESR meter to begin testing all the caps and will open up the trannies and do a visual inspection first before asking what else I can do.  The issue that caused my rebuild was a bad sound followed by really foul burning smell where I had to unplug the amp to stop it.  I figured something must have not survived that - and is the reason I'm having issues now.

 

So - long story short my plan:

 

First test all caps (if you have a recommendation for a good esr meter - I'm all ears) and open trannies for visual look for burns etc - anything out of the ordinary.

 

Thanks for your interest - I'm open to all ideas - really appreciate it!

 

Cut and paste from AK:

 

Minimal road to recovery:
- ($$$$) Open Transformers and inspect (completely remove) and replace.
- ($)Replace CAN cap (assume it is bad being over 10 years old.)
- ($$) Build Bucking Transformer as my voltage - 122 is very high. Both amps will plug into.
- ($)Buy RUBLI delay board (http://rubli.net/HT_delay/)
- Only use my WS1/Thermistor SS rectifier - wow no money as I already spent it on them... doh! ($$)
- Adopt two fuses (assume 5 amp slo-blo perhaps) as outlined above in post 257
- Use my Sovtek 6550 tubes only (look into sending my KT88s to someone for testing - open to suggestions - Jim McShane much too busy)
- ($$) AutoBias boards

 

I wouldn't waste any time removing and opening transformers, it's just not worth it. Just remove the two red wires going to the diodes/rectifier socket and leave them floating. Don't power the amp up, measure resistance from each red wire to ground, one will be slightly higher than the other but somewhat close. e.g. 38/41 ohms. If you get anything close to a short, like near 0 ohms a winding has shorted. If you get a normal reading then power the unit up and measure AC voltage from each red wire to ground. Be very careful doing this, use only one hand to take measurements as there will be high potential. You should read somewhere around 430vac from each wire to ground. At this point the tubes filaments should all be lit as well and measure the DC voltage to pin 5 of each power tube pin to ground, they should all measure the same negative bias voltage, probably around -55vdc. If all this checks out you have zero issues with the power transformer, you should have answers in less than 20 minutes vs removing and opening them up which is useless, you won't see a short deep in the winding where they usually occur as that's where the most stress is on the magnet wire, they stretch and that's where they typically burn open first with excess current. My gut says you do not have a transformer issues since you didn't blow any fuses, the amp only red plated, most likely from a leaky power supply capacitor.

 

If you want to move onto testing the capacitors it gets a little more involved but you don't need an ESR meter. Since it's most likely the can capacitor just remove it from the amp. I recommend using a separate DC power supply, anything really, a walwart or computer/laptop power supply and place your multimeter to read DC current and place in series with the filter cap and DC supply, also add a 1k series resistor to limit current. I typically put the resistor in series with the + going to the capacitor and the meter in series on the negative side which for the can is the outside of the can. You'll see high current as the capacitor charges up but it should settle to a very low value, less than your meter can probably read which is less than 1mA. If after the voltage on the capacitor settles and doesn't seem to be increasing anymore take the DC current reading from the meter, if it's over 1mA it's junk throw it away. Typical leakage for a good electrolytic is less than 20uA, typically around 5uA, very little. 1mA is 1000uA. Test each section this way.

 

1 hour ago, costerdock said:

Seemed like a well regarded mod - "yellow sheet" supported by folks I highly respect so I have to trust it is harmless and good to do. 

 

There is nothing wrong with the mod, I suppose the amp is pushes current production rectifier tubes hard enough where a short can happen and the diodes will still rectify the AC if there is a short protecting the amplifier power supply from getting the raw high AC voltage. If you run a high quality vintage rectifier I wouldn't install the diodes. Any who, since the silicon diodes are doing the rectification they have different properties than a rectifier tube, mainly the high capacitance, 1N4007 is around 15pF, this causes a resonance every current pulse that can have a large overshoot and ringing on top of the current waveform, it's just more harmonics and junk the power supply caps need to deal with and filter out. You'll see many well designed amplifiers with solid state rectifiers have an RC conjunctive filter across the transformer winding to dampen this resonance. Some people get lucky using other diodes with less capacitance, they are typically called fast recovery diodes, although we do not need fast recovery for a 120Hz current pulse it will have lower capacitance changing the resonance frequency and amplitude to a more acceptable amount.

[OO1]

2 hours ago, costerdock said:

Been running my Cornwalls on VTA M125 Monoblocks for over 10 years - but having serious issues with the amps.  Working those out - I ordered the Schiit Aegir.

 

Seems it will be a good match - I'm going to run one in stereo mode - and they try it with XLR - mono mode - I'll use my working tube amp to complete it - to guesstamate what monos would be like.

 

If it works out - I might buy a second one - or simply return it.

 

Hoping it works out.

 

 

you can check with @SWL   he had a couple of these amps for sale

[costerdock]

1 hour ago, captainbeefheart said:

 

I wouldn't waste any time removing and opening transformers, it's just not worth it. Just remove the two red wires going to the diodes/rectifier socket and leave them floating. Don't power the amp up, measure resistance from each red wire to ground, one will be slightly higher than the other but somewhat close. e.g. 38/41 ohms. If you get anything close to a short, like near 0 ohms a winding has shorted. If you get a normal reading then power the unit up and measure AC voltage from each red wire to ground. Be very careful doing this, use only one hand to take measurements as there will be high potential. You should read somewhere around 430vac from each wire to ground. At this point the tubes filaments should all be lit as well and measure the DC voltage to pin 5 of each power tube pin to ground, they should all measure the same negative bias voltage, probably around -55vdc. If all this checks out you have zero issues with the power transformer, you should have answers in less than 20 minutes vs removing and opening them up which is useless, you won't see a short deep in the winding where they usually occur as that's where the most stress is on the magnet wire, they stretch and that's where they typically burn open first with excess current. My gut says you do not have a transformer issues since you didn't blow any fuses, the amp only red plated, most likely from a leaky power supply capacitor.

 

If you want to move onto testing the capacitors it gets a little more involved but you don't need an ESR meter. Since it's most likely the can capacitor just remove it from the amp. I recommend using a separate DC power supply, anything really, a walwart or computer/laptop power supply and place your multimeter to read DC current and place in series with the filter cap and DC supply, also add a 1k series resistor to limit current. I typically put the resistor in series with the + going to the capacitor and the meter in series on the negative side which for the can is the outside of the can. You'll see high current as the capacitor charges up but it should settle to a very low value, less than your meter can probably read which is less than 1mA. If after the voltage on the capacitor settles and doesn't seem to be increasing anymore take the DC current reading from the meter, if it's over 1mA it's junk throw it away. Typical leakage for a good electrolytic is less than 20uA, typically around 5uA, very little. 1mA is 1000uA. Test each section this way.

 

 

There is nothing wrong with the mod, I suppose the amp is pushes current production rectifier tubes hard enough where a short can happen and the diodes will still rectify the AC if there is a short protecting the amplifier power supply from getting the raw high AC voltage. If you run a high quality vintage rectifier I wouldn't install the diodes. Any who, since the silicon diodes are doing the rectification they have different properties than a rectifier tube, mainly the high capacitance, 1N4007 is around 15pF, this causes a resonance every current pulse that can have a large overshoot and ringing on top of the current waveform, it's just more harmonics and junk the power supply caps need to deal with and filter out. You'll see many well designed amplifiers with solid state rectifiers have an RC conjunctive filter across the transformer winding to dampen this resonance. Some people get lucky using other diodes with less capacitance, they are typically called fast recovery diodes, although we do not need fast recovery for a 120Hz current pulse it will have lower capacitance changing the resonance frequency and amplitude to a more acceptable amount.

Awesome - that is very helpful and similar to the manual for testing voltages w/o removing the red leads.   Thanks again!

 

Edit - only reason I want to look inside - especially the output transformer - is that the boom was substantial - I mean loud as hell.  I can't imagine there being no visible damage as a result.  The can cap - looks fine - no buldge - nothing - but sure inside it could be a different story.  I'd just like to see what suffered from that boom.   Anyhow - I really appreciate your comments - you clearly work on this stuff.  Thanks again!

[ODS123]

Like the pic!!  Reminds me of mine..

 

A music legend suspended above a legendary speaker

 

 

 

 

[costerdock]

2 hours ago, billybob said:

Yes welcome back. You should get some feedback on the Schiit. It has been talked about abunch here.

While waiting, could do a search of the forum.

Cool...

Thank you - yes - I've read some reviews here - seem favorable - some noise - which could be a concern - obviously would be a concern.  

 

I'll have to hear them myself and make a judgement - good to know there is a 15 day trial period.

 

Thanks again!

[costerdock]

1 minute ago, ODS123 said:

Like the pic!!  Reminds me of mine..

 

A music legend suspended above a legendary speaker

 

 

 

 

Absolutely beautiful - really nice - love the blue meters.

[ODS123]

19 minutes ago, costerdock said:

Absolutely beautiful - really nice - love the blue meters.

Thanks, and I like your rig and the guitars beneath JH.  ..Only wish I could play

[captainbeefheart]

24 minutes ago, costerdock said:

Edit - only reason I want to look inside - especially the output transformer - is that the boom was substantial - I mean loud as hell.  I can't imagine there being no visible damage as a result.  The can cap - looks fine - no buldge - nothing - but sure inside it could be a different story.  I'd just like to see what suffered from that boom. 

 

That's understandable but I don't think I have heard a transformer go boom, typically you can hear arcing or humming and it will get so hot they smoke and start to melt if the fuse doesn't blow, the fuse usually stops that much damage from happening because it's a high amount of current. What a fuse won't save is the DC leakage current in a capacitor, it might not be enough of a load to pop the fuse unless it's a dead short inside. The cap has a loss angle which dissipates heat from the internal leakage current, this causes the explosion after pressure builds it goes bang and you get a nasty smell. Now actually seeing the guts from the capacitor outside of the can depends on who made it, some make it like the original which yes it would show outside of the can. Other companies that just stuff the outer metal can with modern radials you probably won't see the aftermath of the internal radial cap exploding, it won't leak out of the large outer metal can, it's guts will just blow inside the can but you would still smell it I bet.

 

Of course it's not a huge endeavor to pull the transformers and pop the end bells off and have a look, just trying to save people some time from my experiences from these things.

[costerdock]

15 minutes ago, captainbeefheart said:

 

That's understandable but I don't think I have heard a transformer go boom, typically you can hear arcing or humming and it will get so hot they smoke and start to melt if the fuse doesn't blow, the fuse usually stops that much damage from happening because it's a high amount of current. What a fuse won't save is the DC leakage current in a capacitor, it might not be enough of a load to pop the fuse unless it's a dead short inside. The cap has a loss angle which dissipates heat from the internal leakage current, this causes the explosion after pressure builds it goes bang and you get a nasty smell. Now actually seeing the guts from the capacitor outside of the can depends on who made it, some make it like the original which yes it would show outside of the can. Other companies that just stuff the outer metal can with modern radials you probably won't see the aftermath of the internal radial cap exploding, it won't leak out of the large outer metal can, it's guts will just blow inside the can but you would still smell it I bet.

 

Of course it's not a huge endeavor to pull the transformers and pop the end bells off and have a look, just trying to save people some time from my experiences from these things.

 

Thanks again - really appreciated.

[costerdock]

16 minutes ago, ODS123 said:

Thanks, and I like your rig and the guitars beneath JH.  ..Only wish I could play

 

Thanks!   Me too - wish I could play - I'm working on it - weekly skype lessons with a pro - really helping me improve from years of self study where no one corrects your mistakes.

[captainbeefheart]

24 minutes ago, costerdock said:

 

Thanks!   Me too - wish I could play - I'm working on it - weekly skype lessons with a pro - really helping me improve from years of self study where no one corrects your mistakes.

 

That's great you have a pro helping you out in a skype platform!!  Definitely a lot better than just in a text form through a forum. I wonder if there online tech courses where it's a zoom platform with a teacher giving lessons on amplifier based electronics. Bruce Egnater (guitar amps) does an in person amp build class and they fill up pretty quickly so there's a decent interest in it. The zoom/skype thing could be anything from basic stuff to complete amp builds where the kits get sent out and you tune into a weekly with the class and teacher to go through the building/testing process while explaining and answering questions. 

[rjw1678]

I got my Schiit Aegir in November. I use it with a pair of 1996 LaScala's. It sounds amazing. I have attached a chart that PWK put together on amplifier power and sound level, the chart also includes Cornwalls.