jwyse's first build: 50W

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jwyse
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Re: jwyse's first build: 50W

Post by jwyse » Sun Dec 05, 2010 9:38 pm

syscokid wrote:I hope the "more-educated" chime in here, soon. Those Mercs are suppose to be putting out about 420 volts B+..... :what:

Are you sure that PT isn't marked "G3"?
It's definitely marked -G2 on the sticker, but I suppose it's possible that a -G3 or -G3S (515V/500V) was labeled incorrectly...? :what:

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bulatovic
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Re: jwyse's first build: 50W

Post by bulatovic » Mon Dec 06, 2010 8:01 am

Can you check AC voltages from your PT to the rectifier?

If your PT is 350-0-350, with the rectifier it should be 350*1.4 = 490VDC or thereabout without any load.

Edit*
Just seen from one of you pics that your HT voltage taps are 360-0-360V AC. When you multiply that with 1.4 you get 504V DC plus or minus a few volts. The math seems alright. So, if you want 420VDC you need taps of 300 to 330V AC in order to get the desired B+.
Look at George's manual and voltage chart. He has 335V AC on the rectifier which when rectified gives out 440VDC - the multiplier then is equal to 1.313, but it can vary a bit...

So my guess is that you got a PT with higher voltages. Nothing wrong with that, you'll just get a bit more headroom and punch :rock:
Metro'd Marshall 1979 Super Bass / '80 1960A G12H30 55Hz
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bulatovic
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Re: jwyse's first build: 50W

Post by bulatovic » Mon Dec 06, 2010 8:16 am

Another thing, on biasing your tubes.

You have 510V B+ voltage.
You want to run your EL34 powertubes on 70% of their maximum plate dissipation. that is 25W * 0.7 = 17.5W
The bias formula is
(25W*0.7)/B+ = required A (mV on pins 8 and 1 measured to the ground)
in your case it's
17.5w/510V = 0.0343A = 34mA - or 34mV between pin 8 and ground.

That should set up your powertubes properly with those voltages ;)
:rockon: :toast:
Metro'd Marshall 1979 Super Bass / '80 1960A G12H30 55Hz
1974x 18w Clone - Trinity/Ceriatone

2008 Gibson Les Paul 1958 VOS
2005 Fender Stratocaster

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Re: jwyse's first build: 50W

Post by jwyse » Mon Dec 06, 2010 1:44 pm

bulatovic wrote:Can you check AC voltages from your PT to the rectifier?

If your PT is 350-0-350, with the rectifier it should be 350*1.4 = 490VDC or thereabout without any load.

Edit*
Just seen from one of you pics that your HT voltage taps are 360-0-360V AC. When you multiply that with 1.4 you get 504V DC plus or minus a few volts. The math seems alright. So, if you want 420VDC you need taps of 300 to 330V AC in order to get the desired B+.
Look at George's manual and voltage chart. He has 335V AC on the rectifier which when rectified gives out 440VDC - the multiplier then is equal to 1.313, but it can vary a bit...

So my guess is that you got a PT with higher voltages. Nothing wrong with that, you'll just get a bit more headroom and punch :rock:
It took me a while to figure out where you were seeing 360-0-360. I was looking at my voltage chart and didn't see 360 ANYWHERE. :palm: So, I think you're right about the voltages being reasonable considering a 360V output from the PT. But all the documentation I have on the MM PT says that it should be 323-0-323 VAC, so I think I'm going to contact MM and ask them about it.

Am I endangering my tubes, caps, or any other components by running it at these voltages? And if all that IS ok/safe, what do I need to swap out to get my bias within the recalculated range of ~34 mV? I assume i change the resistor connected to the bias pot; how do I calculate the value to change it to?

Thanks again!!

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Re: jwyse's first build: 50W

Post by jwyse » Mon Dec 06, 2010 2:27 pm

Paul at Mercury confirmed that I somehow ended up with a 360-0-360 version of the P4550JT-G2, instead of the 323-0-323 version. So apparently there are at least 2 versions. This being my first build, I was too naive to catch the discrepancy from the documentation during the unboxing/build process.

Anyway, Paul was kind enough to get a replacement on its way to me, but in the meantime: is it 'safe' (for my components) to keep running as-is (band rehearsal, and practice at home) until the replacement arrives?

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Re: jwyse's first build: 50W

Post by bulatovic » Mon Dec 06, 2010 7:18 pm

Glad you figured it out!

The thing is, it's a matter of taste, higher vs lower voltages. As i said, if you're looking for more punchy, ballsy tone with more headroom, high voltage is the key!
On the other hand, i'm not sure how long would the electrolytic caps hold on for, as they're rated at 500VDC...
It's ok on 100w amps because you double up capacitors (first in parallel, then in series) to handle high voltage, but since this is a 50w version you only have a single cap for each part of the amp circuit that is rated at 500VDC... So i'd be careful...

As for the tubes, yeh, don't worry about them, because you don't want to go over 80% of the max plate dissipation, but would like to be in 60-70% area. That means that you are exceeding the dissipation if your bias voltage is exceeding 39mV, while you want to be between 29mV and 34mV where 29mV would be 60% plate dissipation, and that's perfectly safe for the tube.

But as i said, i wouldn't worry about the bias current/voltage and tubes, i'd worry about capacitors, especially Mains and Screen caps which is where you get more than 500v DC. (Mains is the first can cap that connects HT Fuse and STDBY switch - if you followed George's manual - screens cap is one part of the second can cap that connects to both powertubes - with yellow wire in the manual)
I'd power up the amp and measure voltages at those points just to check if they're ok, if you're really inpatient to play the amp till the PT arrives.
:toast:
Metro'd Marshall 1979 Super Bass / '80 1960A G12H30 55Hz
1974x 18w Clone - Trinity/Ceriatone

2008 Gibson Les Paul 1958 VOS
2005 Fender Stratocaster

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Re: jwyse's first build: 50W

Post by jwyse » Fri Dec 10, 2010 12:32 am

The replacement PT arrived from Mercury right on schedule, and replacing it wasn't quite as difficult as I had feared. Bias was still a bit off, but I replaced the bias resistor with a 3.3k, and that did the trick.

I didn't power up the amp since my last post, because I realized (and was reminded by the next reply) that most of my caps are only rated for 400V. Anyway, I think the amp sounds better now -- maybe it's just in my head, but it seems like it's breaking up a lot more/earlier since there's less headroom. ??

I used the amp in rehearsal tonight, and it sounds amazing. A little bit brittle on the highs, but I think that's partially because of the new tubes and caps, and partially because I was playing through a 4x12 loaded with G12T75s. Now I'm trying to burn it in a little faster by playing my iPod through it, on standby, with all controls dimed and the power tubes removed (according to George's post somewhere around here).

Next up is a 2x12 loaded with Scumbacks -- I've got an H75 on the way, and I'll order an M75 to go with it after Christmas. :D

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Re: jwyse's first build: 50W

Post by bulatovic » Fri Dec 10, 2010 7:03 am

Now, that's great news!
Glad to hear you got your amp fixed and working well!
I suppose voltages now are in the correct range?

Yes, lower B+ lets powertubes saturate more easily, hence earlier breakup and lower headroom which is essential for later 50w. As far as i know they had something about 370V B+ which makes those amps instant crunch machines, especially with humbuckers - if you dial your volume knob on those amps to 3 or even less, and still have clean sound, then something is wrong :)

As for harshness, i bet on those speakers which in my opinion are just nasty! You should see a huge improvement with your Scumbacks! And to be honest i wouldn't worry about burning the amp in with iPod, i'd rather do that with a guitar :shred: :mrgreen:

Which tubes are you running there?

Congrats for having such a great amp!
:rockon:
Metro'd Marshall 1979 Super Bass / '80 1960A G12H30 55Hz
1974x 18w Clone - Trinity/Ceriatone

2008 Gibson Les Paul 1958 VOS
2005 Fender Stratocaster

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Re: jwyse's first build: 50W

Post by neikeel » Fri Dec 10, 2010 9:01 am

jwyse wrote: burn it in a little faster by playing my iPod through it, on standby, with all controls dimed and the power tubes removed (according to George's post somewhere around here).
Ok I am puzzled? Normally expect burn in with amp fully turned on with OT in circuit and doing some work.

Please can you find the original post as I am having difficulty understanding :what:
Neil

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Re: jwyse's first build: 50W

Post by jwyse » Fri Dec 10, 2010 9:41 am

neikeel wrote:
jwyse wrote: burn it in a little faster by playing my iPod through it, on standby, with all controls dimed and the power tubes removed (according to George's post somewhere around here).
Ok I am puzzled? Normally expect burn in with amp fully turned on with OT in circuit and doing some work.

Please can you find the original post as I am having difficulty understanding :what:
neikeel, here's the post I was referring to: http://forum.metroamp.com/viewtopic.php?p=28477#p28477" onclick="window.open(this.href);return false;

VelvetGeorge wrote:The difference is subtle, but you should hear the notes become more defined and the harmonics get sweeter.

Sozos seem to have a more obvious change over the first 25 hours than other caps. In fact, I never really noticed it until the Sozos.

You can take your output tubes out if you're worried about wearing. Run a CD into the amp with the standby on and dime everything.


George

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Re: jwyse's first build: 50W

Post by neikeel » Fri Dec 10, 2010 10:48 am

Ah, ok!

That is just for your coupling caps. Other items that need to bed in are your OT and filter caps but of course they need to be making some noise :twisted:
Neil

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Re: jwyse's first build: 50W

Post by bulatovic » Fri Dec 10, 2010 11:56 am

Well, that needn't be loud - especially if you have a nice 100w 8ohm resistor, but wait when it comes to breaking-in the speakers! :rock:

:mrgreen:
Metro'd Marshall 1979 Super Bass / '80 1960A G12H30 55Hz
1974x 18w Clone - Trinity/Ceriatone

2008 Gibson Les Paul 1958 VOS
2005 Fender Stratocaster

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Re: jwyse's first build: 50W

Post by jwyse » Fri Dec 10, 2010 1:01 pm

bulatovic wrote:Now, that's great news!
Glad to hear you got your amp fixed and working well!
I suppose voltages now are in the correct range?

Yes, lower B+ lets powertubes saturate more easily, hence earlier breakup and lower headroom which is essential for later 50w. As far as i know they had something about 370V B+ which makes those amps instant crunch machines, especially with humbuckers - if you dial your volume knob on those amps to 3 or even less, and still have clean sound, then something is wrong :)

As for harshness, i bet on those speakers which in my opinion are just nasty! You should see a huge improvement with your Scumbacks! And to be honest i wouldn't worry about burning the amp in with iPod, i'd rather do that with a guitar :shred: :mrgreen:

Which tubes are you running there?

Congrats for having such a great amp!
:rockon:
Yep, voltages are all within the expected range now. :D

I'm not a fan of the G12T75s at all. My 2x12 currently has a 25W Greenback in it (along with another speaker) so I was worried about pushing it too hard at reharsal -- so I took the 4x12. Really can't wait to get the Scumbacks, especially after all the ridiculous praise for those speakers around here.

I'm running the Mullard reissue tubes that were an option back when I bought the kit from the Metro store.

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Re: jwyse's first build: 50W

Post by jonamojo » Sat Dec 11, 2010 5:09 pm

Interesting you chose mm trannys, I did myself also I found it took quite a bit of playing for all the components to settle in. After that it was pure bliss. play the amp on ten as much as you can and the OT will adjust and not sound overly bright anyways good luck. :rock:

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Re: jwyse's first build: 50W

Post by neikeel » Sat Dec 11, 2010 5:24 pm

jonamojo wrote:Interesting you chose mm trannys, I did myself also I found it took quite a bit of playing for all the components to settle in. After that it was pure bliss. play the amp on ten as much as you can and the OT will adjust and not sound overly bright anyways good luck. :rock:
Isn't that just fried auditory 'hair' cells :rock:
Neil

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