My first tube amp...

[alliclaytor]

Left switch is standby, right switch is full power. Previous owner gave instructions, and it works just like my Fender tube amp.

[Erik Mandaville]

Welborne Labs is out of stock on that transformer, anyway. Don't sit around worrying about your power trannie, though. It may be fine for years to come, and if something happens, than it can always be fixed.

So, when you first power up the amp, you should be operating it so only the filaments glow. Does doing just that cause the thud?

[alliclaytor]

No worries on the transformer. The thud only occours when I turn on the second switch to finally produce sound. When I turn on the first switch, the tubes begin to glow, but the second is where the magic happens.

[Erik Mandaville]

Okay -- thanks for explaining that. Does your Fender amp exhibit anything similar? The amps of guitar players I've played with sometimes had a small 'bump' like that, but nothing that really rattled woofers.

How long is the standby on before you hit the other switch to play music (on the Dynaco)?

[Donkeyshins]

"The "pop" i refer to is a low thud, two actually."

A 'thud' is really mutch different than a snap or pop sound, and so is not the power switch but in-rush current thud. Do you happen to know which of the two switches is standby (tubes will glow, but no high voltage will be present) and which is the high voltage switch?

Your power transformer is actually being relieved of one of it's main current draws -- the filament supply that would normally be used for the vacuum tube rectifier. In fact, it may run a little cooler because of that. There can still be noises and stress associated with SS diodes working in conjunction with hefty filter capacitance -- often higher-pitched or sort of oscillation that can be filtered to ground on the AC side of the power transformer secondary (before rectification).

Do you know which switch is used for standby? It's important to know which is which and how to use each. The tubes may be getting slammed with high voltage before they even have a chance to warm. There are some robust replacement power transformers available for Dyna ST-70 if you even need one. Welborne Labs is one source. The photo is helpful, but just a little two distant to make out how the standby is wired -- whether it lifts the center tap of the power transformer or is in-line with B+ voltages (those are the high voltages of your amplifier).

Additional comments:

• Invest in a good Current Inrush Limiter like a CL90 (http://www.diytube.com/cl.pdf) - that will prevent the B+ slam
• Try Triode Electronics (http://store.triodestore.com/mq060.html) if you need a new PT - cheaper than Welborne
• You could also replace the PSU diodes with HEXFREDs - might reduce noise
• Check out http://www.diytube.com - lots of good information on Dynaco gear

Welcome to the wonderful (and maddening) world of thermionics!

-D

[jacksonbart]

I am glad you like your amp, but wow I could not take the soldering, thumps, pops, power supplies that will fail, shocks after the dang thing is unplugged. I just love music.

[johnyholiday]

could be those big filter caps charging up,one reason why speaker relay-delays are used in modern SS amps....nice el34 amp....looks like it could have "trippple sluggg of the shock of stock" .... johny likes the jangle of EL84's and the tone of EL34's,sometimes the punch of a KT77,,johny is also a 70% bias guy http://www.aikenamps.com/Why70percent.html

[Marvel]

Try Triode Electronics (http://store.triodestore.com/mq060.html) if you need a new PT - cheaper than Welborne

I have one of the triode electronics PTs in my ST-7-. That thing is huge!

Bruce

[Erik Mandaville]

An in-rush current limiter may help with this, and it may not. I used a couple in series in the 6SN7 preamp I made several years ago, and in that case there was an improvement, but I still have to be conscious of turn-on sequence -- preamp first then amps -- to keep that little bump out of the speakers. It's an idea to try here, though. Those things were a source of some interesting discussion around here a few years ago.

I haven't ordered from Welborne Labs in a long time. I never, ever had a problem with them in the past, and can only speak about my own experience. Triode Electronics would be a great place to check for a new PT if one is needed. It seems the one in question is working okay.

As usual: Caution when working with any electronic component, even if it's unplugged. If there isn't a bleeder circuit in the modified power supply in this amp, the storage charge in those big filter caps can shock hard even days later.

[alliclaytor]

Thanks for all the info. I have a niles speaker switcher that I think might be a good solution...turn the switch on after I have the amp turned on. When the thump was the worst, I probably let the amp warm up (first switch on) for 3 min. After HarryO suggested I waited 5+ minutes, I tried that and the "thump" was significantly less and really not even a concern for my speakers. It is now similar to the sounds I get out of my fender amp when I first turn it on. So long as I am not doing anything to hurt the amp now, I feel ok. Now I need to find some time to sit down and get intimate with my amp...lol.