Blown PT?

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geoff 1965
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Re: Blown PT?

Post by geoff 1965 »

i built my amp to the lite2b spec initially and it has always been very quiet with no hum or hiss,what type of resistors have you used on the input jacks?
if you add some more pics of your jack's,pots,grounding etc someone might spot something.
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Riffmonster
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Re: Blown PT?

Post by Riffmonster »

Hi Geoff,

here are two pictures. I already tried separating the ground point of the pre amp from the power amp but it did not change the problem. Also, the shielded looking wire is not a real shielded wire but a desperate attempt of wrapping a normal wire in aluminium. I should buy some shielded wire.
I still can not find the problem. It is maybe just poor leaddres. I should have taken more time to build it.

Cheers
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geoff 1965
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Re: Blown PT?

Post by geoff 1965 »

if you fit shielded wire to the input you could also use a small value grid stopper 10 to 15K which will help with interference but won't affect the signal.
you've used good quality components and you have a lot of room for future mods if needed.there as been a lot of posts recently regarding grounding so have a browse through them.
good luck
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JMPGuitars
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Re: Blown PT?

Post by JMPGuitars »

Ground schemes: viewtopic.php?f=4&t=25372

Solder technique: viewtopic.php?f=4&t=25396
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Riffmonster
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Re: Blown PT?

Post by Riffmonster »

Thanks for the replies guys!
I reworked most of my solder joints and used real shielded wire but still there is no difference. I will try the 10k gridstoppers now.
I noticed that when I turn the tone pot all the way to the right, the buzz gets much more. The flaw must be before the control then, right?

Cheers!
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Re: Blown PT?

Post by JMPGuitars »

Post some updated photos. Check your voltages and post them too.
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Riffmonster
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Re: Blown PT?

Post by Riffmonster »

I was about to measure the voltages. Plates on V1 are about 130V and Plates of the El84 about 352V. Screens of the EL84 around 330V. When I try to measure the plate voltages of the phase inverter, a loud pop appears once I touch the pin with my probe. I can not really measure the voltages there.
:?
The pictures are from yesterday without the shielded wire and without the grid stopper.
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geoff 1965
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Re: Blown PT?

Post by geoff 1965 »

if you make a dummy load for the speaker jack you can test all the voltages safely without damaging your output transformer.
001.JPG
this one is 100watt 8ohm wirewound.
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Riffmonster
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Re: Blown PT?

Post by Riffmonster »

That’s true, I forgot about using a dummy load. I actually have one at home and will post the last voltages after work.
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Re: Blown PT?

Post by Riffmonster »

So I have 226V at the plates and 36V at the grids and 69 at the cathodes of the phase inverter. The 36V seems a bit low, given 20% tolerance. Cathodes of the El84 is 10V
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geoff 1965
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Re: Blown PT?

Post by geoff 1965 »

regarding grounding,my lite2 is a cage layout with the valves at the front so is a different layout to yours but here's the difference with the grounding; i have the input jack and tone pots grounded seperately from the B+, phase inverter ground. also my el84 bias is grounded seperately,you might want to try this and see if it helps.this is the layout i worked from with some ammendments to grounding.
18W_LITE2B_LAYOUT_BrownNote_TRANS_FINAL_v1.1.jpg
what value bias resistor for the el84's have you used?
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Riffmonster
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Re: Blown PT?

Post by Riffmonster »

Thanks for the input! I will probably try out the different ground and see if it helps.
I have a 150R resistor with a 47uf cap.
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Daviedawg
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Re: Blown PT?

Post by Daviedawg »

Your voltages seem a bit awry. On the power valves you should have a target of 12v on the cathodes and a difference a little less than that between pins 7 and 9 plate and screen. Can you post a full list of your voltages? Or you can refer to the standard examples posted in the downloads for a comparison, bearing in mind that they are for a traditional 18 watt. Then go from there.

Dd
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Riffmonster
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Re: Blown PT?

Post by Riffmonster »

Hey Dd,

sorry for the late reply. Here are my voltages:

V1:

Pin 1: 149
Pin 6:149

V2
Pin 1: 234
Pin 6: 230
Pin 3: 78
Pin 8 :78
Pin 2: 38
Pin 7: 37

EL 84s

Pin 7: 340
Pin 9: 323
Pin 3: 10

To me they seem not too weird, but I might be wrong.
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Daviedawg
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Re: Blown PT?

Post by Daviedawg »

Seeing them all together I agree that there is no reason to think that they are causing any issue.
Just as an aside. For best tone I would adjust your power valve cathode resistor to get the cathode readings up from 10v to as close to 12v as possible. It really is a noticeable difference. Try a 125 ohm.

Dd
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