erwin_ve wrote: ↑Sun Jan 12, 2020 8:51 am
Changing the mains fuse in 230-240v 50Hz territory isnt simply half of the USA 120v 60hZ.
The frequency from 60hZ to 50Hz has a bigger current draw. Because: Z= 2*pi*f*L
For a 50w in 230v-50hZ use a 2A sloblo.
For a 100w in 230v-50Hz use a 3A sloblo.
I know, living on the 50Hz side has gained me real life situations. Yet I see the advise take 1/2 of the rating and then being surprised the fuse blows without finding a cause.
I know, living on the 50Hz side has gained me real life situations. Yet I see the advise take 1/2 of the rating and then being surprised the fuse blows without finding a cause.
Tony,
Agreed! Without knowing the actual specs of a PT, some may work a lifetime with a 1/2 rating.
But I've seen more than once, the fuse, when powering up, is the culprit.
A pt also has a current flow without load. Afaik you need to add the current unloaded pt to(calculation based on the formula above, doing 60Hz vs 50Hz.) current draw at loaded pt.
The actual calculation I've done in the past gave me a headache but at least gave me a understanding why a 1/2 rating doesn't always work.
talbany wrote: ↑Sun Jan 12, 2020 11:42 amJust as a point of reference though.
Parts manual in the 65 Twin reissues specs 1/2 rating 2A for 240V and 4A @ 120V
Reissue also has 3 more preamp tubes?
But note that the export PT has a different part number. Perhaps that compensates for all but the voltage difference?
talbany wrote: ↑Sun Jan 12, 2020 11:42 amJust as a point of reference though.
Parts manual in the 65 Twin reissues specs 1/2 rating 2A for 240V and 4A @ 120V
Reissue also has 3 more preamp tubes?
But note that the export PT has a different part number. Perhaps that compensates for all but the voltage difference?
But I've seen more than once, the fuse, when powering up, is the culprit.
Erwin
Could be! Since inrush current varies transformer to transformer. Noting wrong with upping it .5 for a bit more headroom if it's blowing on startup... I am with that!
Tony
" The psychics on my bench is the same as Dumble'"
Thanks everyone for the discussions and suggestions. In the end I will go with 1.5A for my Bassman PT 50W and 2.5A with my Twin PT 100W. If it doesn't blow no need to go higher..
rootz wrote: ↑Sun Dec 29, 2019 12:16 am
I see mu metal sheets are available at eBay. https://www.ebay.nl/itm/151912090773 Not cheap, but $22,95 might be well spend. Anyone got experience with shielding with mu metal tape?
I bought the exact item above. It arrived on Saturday. It is not very long, maybe the size of an A4 paper. It is a bit hard but still malleable.
I tried laying it flat between the transformer and the pan, didn't make a difference. I tried folding in a U shape and place it aroung the PT and also couldn't hear much difference. I tried laying it like a reverse U on top of the Pan but also couldn't hear much difference. Unfortunately I could not wrap it around the Pan without "damaging" the sheet, as I think once I put it in that shape it will be very fizzy to make it flat again...
Should I try grounding the muMetal to the chassis?
Any suggestions..
[EDIT] seems like the older two rock amps had a similar issue..Any Mayer Sig builders in EU experienced the same? I am runing 240V and probably in the higher magnetic filed category. https://www.thegearpage.net/board/index ... g.1332229/
I spent some time yesterday playing around with caps across the reverb return pot (like the Mayer sig). I had in hand 220pF / 470pF / 500pF / .001uF / .003uF / .01uF. For the testing I had the pan just next to the PT. I noticed the higher the value the weaker the reverb, and seems to roll off the bottom end too. With the pan next to the PT, the .01uF made the hum almost unadible, but as soon as I mounted the chassis back in the head with the pan underneath the EMI was very audible again.
I wonder how Two Rock fixed this from the earlier models. I think the new Custom Reverb Signature have a somewhat small PT which gives a good enough clearence...
I have been thing of adding caps to ground on the reverb return, maybe from 220pF all the way to 3000pF, to see if there is any improvement. I see princeton add a cap across the send too..
Another fix might be a RC filter to roll off the hum frequency, something like a LNFB
Also, any tips on how to properly use the MuMetal?
Can you stick just the output end of the tank in the head box so that it is in the same location as it is when it is mounted in place, except rotated 90 degrees?
martin manning wrote: ↑Tue Jan 14, 2020 2:18 pm
Can you stick just the output end of the tank in the head box so that it is in the same location as it is when it is mounted in place, except rotated 90 degrees?
Hi Martin, do you mean when I test for the caps across the return pot or in general? If it is for the testing It is fairly hard to work inside the chassis on that configuration, hence why I placed the pan just next to the PT for the testing. If it is in geral, the pan 90 degress or placed vertical (where the grill cloth is) has no hum, but my head is fairly small and I do not have a lot of room to wiggle, hence why my grill is removed for now for all the testing.
My black head for reference (sorry for the blurry shot), and I added a shot of a brown two rock head. Doesn't seem like they have much more clearance than me?
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Martin, I was doing some reverb pan experiments. If I place it 90degrees to the PT with the output (red) side underneath the PT I have a lot of hum, if I place the input side (black) I have hum but less, If I place it vertical as of it was my grill cloths I have no hum at all.
With the pan grounded on the output (red) I have hum and a high pitch noise, and with it grounded on the input (black) I only have hum.
Marcus, thanks for sharing! I wonder what is going on in my amp? Did you do yours as per Wonderland layout exactly? Do you know how is your pan grounded?
EDIT: I just did the chopstick test all over and couldn’t find any issues. And an interesting fact is that this amp is fairly sensitive to the cel phone waves. Maybe a relationship to the sensitivity of the pan?
EDIT2: removed the .01uF across the return, definitely more audible the hum now even at much lower settings
norburybrook wrote: ↑Tue Jan 14, 2020 6:40 pm
Raphael,
I've got huge transformers in mine and it's as quiet as a mouse.
JM wonderland rear.JPG
JM transformers.JPG
M
Marcus,
The reason yours is quiet is because the stand-up power transformer has a different magnetic orientation then the horizontal or z-mounted transformer.
-jack
Jack,
Of course, it's staring you in the face when you look at it a happy accident of using vintage USA transformers
@Raphael, the amp is exatly the same as the layout except for the voltage divider , I think the 1m/68k off the top of my head , was changed to allow more gain.
Voltages are higher as it's a B+ of just over 500vDC.
Jack is correct in the reason it's so quiet though.
Thanks for the input Marcus and Jack...you live and learn...now have to find a way to mitigate this, still wonder how Two Rock did it as the transformers seem to be similar....but surely will go for stand up PT in the future, in amps with reverb..
I think I will try .01uF / .02uF /.03uF and .05uF across the reverb return and try to find a compromise between the tone and the hum. I know .01uF reduced it a fair bit...
What I'm suggesting above is to rotate the pan 90 degrees in-plane (keeping it flat) so that just the output end is inside the head box. Most of it will be sticking out the back. This would be the equivalent of rotating the PT 90 degrees in plane with respect to the output transducer. That Two-Rock head has a lay-down PT, doesn't it?
Last edited by martin manning on Wed Jan 15, 2020 11:07 am, edited 1 time in total.
with that!