[lbj]

The good news is, the fix is simple, it's a bad center tube.  Lloyd's going to send a replacement.  Btw, Lloyd is great to work with and I can't recommend him highly enough!
 
Bad news is I can't enjoy the amp until I get a new tube.  I have a 12AT7 on hand, but I'm a little nervous using it in the amp and I'm sure sonically it'll be subpar anyway.

 

[replytoken]

Quote:

The good news is, the fix is simple, it's a bad center tube.  Lloyd's going to send a replacement.  Btw, Lloyd is great to work with and I can't recommend him highly enough!
 
Bad news is I can't enjoy the amp until I get a new tube.  I have a 12AT7 on hand, but I'm a little nervous using it in the amp and I'm sure sonically it'll be subpar anyway.

If the tube is a 5751 or 12AX7, you could easily pick up a new one at a guitar store, or you could start finding a good supplier of NOS or used tubes.  When I purchased my unit several years ago, Lloyd was shipping new Sovtek 5751's, nothing to get excited about.
 
--Ken
 
 

 

[diego]

@digic25
 
The heater supply is designed for 12v and it´s one of the advantages of the Ear HD. Working at 12v allows the heaters to work with half the current which makes them quieter, in addition the amplifier produces less heat. I suppose it´s possible to add resistors to drop the voltage but they will get very hot and I don´t know if the transformer is rated for the additional current. Personally, I would not do it.
 
@lbj
 
I think 220v is the right high voltage for the output tubes on that amp. The input tube plate should be around 200v. You get a higher reading when you turn it on because the tubes have not heated and are drawing less current. So, what you measured is perfectly normal.
 
Regards,
Diego

 

[tme110]

Very cool.  You made quick work of that amp and thanks for all the pics.  Of course, now I want to buy another one just so I can build one too.
 

 

[lbj]

I tried the 12AT7 today and found out the stock tube wasn't defective after all.  The 12AT7 was also missing the right channel!  I took the amp back to the bench and discovered the true culprit:  one of the capacitors had partially separated from its double-sided tape attaching it to the top of the chassis and was possibly shorting one or both terminals against the bottom cover.  This is what Lloyd had to say about the theory:
 
Well that makes sense. If one of the terminals of C8 shorted to the chassis, you would just get one half of the heater of the 12AX7 energized. If you look at the schematic, you will see that the center tap of the heater is connected to ground so that the heater supply is not floating with respect to the plate voltage. With one of the terminals of C8 shorted to the chassis, one half of the 12AX7 heater would also be shorted. I don't think both terminals of C8 shorted or you would have blown a fuse.
 
So the amp is back up and sounding beautiful, after judicious use of some hot glue.  Mystery solved, thank goodness.  Luckily Lloyd hadn't unnecessarily shipped out the replacement tube.

 

[lbj]

Thanks Diego, I figured there couldn't be anything drastically wrong if the amp sounded so good.
 
Ugh, you guys are making me empty my wallet.  I purchased a GE 5-star and Raytheon windmill getter 5751 to start my tube rolling adventures.  And I'm seriously grappling with whether to snap up a Sylvania 3 mica BP/GP and/or a Tung Sol.  Thanks go to Twinster, MacedonianHero, and eucariote for the suggestions.

 

[Twinster]

I think you got a good selection to start with. The Sylvania are very expensive but very good (based on what I read I do not own one yet). I also used a RCA Black plate Command Serie and liked it a lot.

 

[tme110]

a
Well, I got a sylvania windmill getter but I'm sending it back. It worked but it just didn't seem right and I don't think it was a taste thing - so I'll try to get another.  I also got a GE 5-star - it sounded pretty good but I couldn't tell enough of a difference from stock.  buying tubes still confuses me.  I'll see one tube listed for $20 but see a pair with the exact same discription for over $200.  I don't see any place with all the details listed here, ie no 'red lable' or even basic stuff like grey or black plate.  And what is a windmill getter?
 
 
 
 
 
 
 
 

 

[tme110]

Also, does the MAD care if 2 headphones are connected?  I've had hd600s and 650 connected without detecting any issues but it's more likely that I'd have grado's and hd650's connected.

 

[lbj]

As I understand it, the amp doesn't care if two headphones are connected, just total power for each will be reduced.  Also, unless they are identical cans, levels will be different.
 
I have a Raytheon windmill getter 5751 that I greatly prefer over the stock 12AX7.  To my ears there's very little difference between the Raytheon and GE 5-star, though I haven't done a very comprehensive comparison.
 
I have a question, anyone try a K601 with the Ear+ HD.?

 

[tme110]

Sylvania TMGP?     triple mica grey plate???
 

 

[lbj]

Quote:

Originally Posted by tme110 /img/forum/go_quote.gif
 
And what is a windmill getter?
 

A windmill getter is the metal structure near the top of the tube that looks like a propeller.
 


Quote:

Sylvania TMGP?     triple mica grey plate???
 

Yes that's right.
 

 

[lbj]

Should also add that even if there's no description of the tube, you can visually determine black/gray plate and # of micas if there's a picture.  The color refers to the vertical plates visible in the tube, micas are the horizontal plates (sometimes I see tubes with 3 micas but 2 are sandwiched together at the top of the tube, not sure if those qualify as triple mica, I think so?).  Sorry if this is stuff you already know.

 

[tme110]

didn't know you could 'see' the difference in grey/black plate - thanks for all the info!

 

 

[lbj]

You're welcome!