Help w Volcano hot EL34 on Music Man 210HD 130

[bluesfreak]

normally 400Va is the max for a 6V6 but JJ 6V6 are more robust and can handle up to 480V

[LBrewski]

After further (static) inspecting and measuring resistors and whatever else in the areas of concern, it all looks ok - which I more or less expected given that it works fine until it heats up.

So I put together a Mouser order for resistors and signal caps to replace those that are the most likely suspects, based on the advice given. When those come in my plan is to replace parts, re-tension the socket contacts and then power up and adjust the bias setting cooler.

Have I missed anything?

[Albert Kreuzer]

Sounds like a plan

Don't forget to resolder the tube socket connections and make sure none of the contacts are broken inside the socket.

[LBrewski]

Update: After replacing grid resistors on sockets & coupling caps on the driver board, re-tensioning socket pins and reducing bias from .5v to .32v...

I played it for almost 2 hrs with no problems, the tubes had a perfect glow and the thing sounds great.

Although I need more time to be certain, for now I am calling this amp repaired!

I'm not sure what exactly fixed it, I'm leaning toward the cooler bias.

Thanks to everyone's advice here, this is a great forum.

I added some pics of the parts replacement, before and after. Note that the coupling caps leading to the output tubes had been previously replaced with a large disc type that I'm not familiar with.

Music Man 210-HD pictures by ElBrewski - Photobucket

[Albert Kreuzer]

Congrats!



PS: Those .1/1kV disc caps were probably ok, but ODs sure are a better choice.

[LBrewski]

So after a few playing sessions over the past week+ the amp still runs fine but this animal has a dual power switch labeled Hi, off and Lo. I've been testing in the Hi (130W) mode so that it runs as hot as it can. But I thought I better try the Lo (65W) mode as well and it turns out it has a moderately distorted sound - almost like a low quality overdrive pedal. I don't know for sure if this mode had a problem before I started working on the amp, but I really don't believe so.

I am guessing that I have brought the bias down too low for the low power mode to run properly - I had moved it from .5V to .32V - and what I'm hearing could be cross-over distortion. Does this sound likely?

I have no experience with hearing what that would sound like. I am tempted to plug in my (recently acquired) sine wave generator and looking across the speaker contacts with my o'scope for a look see.

From the schema it appears that the Hi/Lo switch uses more primary windings on the PST when in the Lo mode, thereby lowering the secondary voltages.



I was thinking that raising the bias up by some amount and trying it, maybe up to .38V, then higher; but I don't want run this thing too hot or I could be back to square one. Any thoughts?