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p2p marshallized VJ question


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I just had a go at building a p2p valve junior using tagstrips and mostly recycled parts from my previous mods, stock PT and stock 5.2K OT... and it actually works, would you believe! There's something up with the volume though, it sounds sweet until the volume goes past 1/3 then it gets increasingly distorted as if it cant power the speaker properly. Not sure why that is but off the top of my head thought maybe it was because I was thrifty (ie cheap!) and resused the old (stock value) filter caps. Could beefing up C6 to 47uf or 100uf possibly fix this? Any ideas gratefully received, thanks!

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I'm kinda confused because they are mostly overdrive sounds.. a third of the way up and getting into distortion sounds normal.

at any rate C6 up to 100uF just improves filtering and helps it to sag a bit less when you hit a big chord.. far as I know.

I suppose that might have some small effect on the break up point, but I'm guessing.

mine sounds sweet AS it gets increasingly distorted!

TWANG

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Thanks for the reply, sorry I should have been more clear, the distortion I described was not the marshall 'crunch' we all know and love but more like the spluttering distortion of an amp unable to power the speaker sufficiently.

In any case I put a 100uf in C6, also changed the centre tap resistors for the heaters were dinky little 0.5w which was all i had, swapped those for 1W resistors, not sure if that last one made a difference but it seems all good now, lots of rockin crunchy goodness even on 10=p~

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Looks like I spoke too soon, my problems with the spluttery distortion when cranked is still there but only at higher volumes. After reading Stargeezers post I tried another VJ OT in case the problem was there but there was no difference, however it got me thinking that perhaps the stock OT isn't up to the challenge of the marshallized circuit and recommended Hammond OT upgrade is in fact essential to the marshall mod? If anyone can confirm this that'd much appreciated, thanks!

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Looks like I spoke too soon' date=' my problems with the spluttery distortion when cranked is still there but only at higher volumes. After reading Stargeezers post I tried another VJ OT in case the problem was there but there was no difference, however it got me thinking that perhaps the stock OT isn't up to the challenge of the marshallized circuit and recommended Hammond OT upgrade is in fact essential to the marshall mod? If anyone can confirm this that'd much appreciated, thanks! [/quote']

 

Sounds to me like what you are experiencing is blocking distortion?

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Yeah I left out R6 as per the marshall mod, and tried different tubes but was still pretty much the same. I was wondering about the OT as my first mod ever was a marshallized mod on a store bought VJ, which worked without fault, the specs are identical to the one i built except the first one had a hammond OT. I don't know much about blocking distortion, is that to do with the value of R2? Perhaps a 33K would be better there instead of the 10K there now..

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Tried putting a 100k resistor in R6 (which was previously jumped) and changing R7 back to 100k, with no change, if anything it seemed worse so I returned it to how it was. However I tried raising R15 from 10k to 18k which seemed to help a lot, at the cost of some high end. Now when you play a power chord on high volume (8+) its sweet for about 6 seconds but just splutters out suddenly, which does sound like blocking distortion, i need to read more on that.

At a pinch maybe swapping the 500k pot for a 250k would help but i thought if the circuits identical to my modded factory VJ which works perfectly both should work pretty much the same. Oh well back to the soldering iron, thanks guys!

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Thanks, Gil, I tried that and it was a little better but it still doing the same thing at high volumes.

I'll have to do a bit more research to troubleshoot whats wrong, or at least know what questions to ask. Just wondering though, are there reports of blocking distortion occurring after others have done the marshall mod? I don't recall reading of it , and my first VJ mod had no trouble at all so I thought an exact copy of the modded circuit using a VJ PT and OT should work the same. I thought this was a fair enough assumption but could stand to be corrected, thanks again!

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Thanks' date=' Gil, I tried that and it was a little better but it still doing the same thing at high volumes.

I'll have to do a bit more research to troubleshoot whats wrong, or at least know what questions to ask. Just wondering though, are there reports of blocking distortion occurring after others have done the marshall mod? I don't recall reading of it , and my first VJ mod had no trouble at all so I thought an exact copy of the modded circuit using a VJ PT and OT should work the same. I thought this was a fair enough assumption but could stand to be corrected, thanks again![/quote']

 

I have a really simple idea for you to try....cut JP-1 on a PCB...or lift c-3 from the circuit....same thing and see what you think. If you stil have that farty thing going on your problem is later in the signal chain IMHO. There could be quite few causes actually.....tryin' to get to the bottom of it!

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If you go with the turretboards.com layout for the Marshally mods, the easy way to stop blocking distortion is use a lower gain tube. JJ's 12ax7 has considerably more gain than a Sovtek 12ax7WA, the modern Chinese, or most NOS 12ax7's. Those may be "normal" enough in terms of gain to prevent that blocking distortion from occurring. But assuming you've already tried one of those, or something even lower in gain; then the R6/R7 tweak would be the next logical step. After all, you're going for the gusto-gain if you do the Marshall mods anyway, so a lower gain tube really seems kind of counterproductive. On the other hand, a 5751 does sound so sweet when overdriven.

 

Gil...

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My money is still on a bad VJr OT(sounds EXACTLY like the problems I had)....if everything is OK in the pre, and he's changed the power tube...

Sizz, you could try the little P-T-31 8W OT from AES... CHEAP(13 bucks ) and they sound really good ...

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