Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: phsyconoodler on November 15, 2010, 12:39:38 pm
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Well when you have a window of time,you like to play.
I have a Fender Custom Vibrolux amp that I use at Jams that I modded to Vibroverb specs.(huge improvement).
I had a that unused normal channel sitting there so I modded it to a Marshall preamp layout.I changed the plate resistors on V1 to 220k,the slope resistor to 56k,the resistor from the bass pot to 10k from 6.8k and changed the coupling caps from .1uf to /.047uf.
I stopped there because I didn't have the rest of the parts to complete my Fenshall mod.
The results?It has considerably more gain,is quite nice sounding at about 4 and above,drives the crap out of the speakers and generally is a keeper.I'm going to complete it with .022uf coupling caps and the 220k to ground off the second coupling cap and may add a raw control in the low input jack.
Now I will use an A/B box instead of a pedal in front of that amp.I'll test it on thursday at the jam and see how if fares in a live situation.
Keep in mind it's still a PC board amp and I haven't hand-wired it yet.If this mod works as good as I think it will I'm going to do a hand-wired board with the plexi normal channel on it.
I like!
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What would happen if you lifted the tone stack from its current location and moved it to the other side of V2? That would also give a more Marshally sound, yes?
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What would happen if you lifted the tone stack from its current location and moved it to the other side of V2? That would also give a more Marshally sound, yes?
You would still have the TS loading from the high-Z output after the triode instead of the low-Z from a CF stage retaining more of the original signal which is part of what gives the Marshall it's flavor & gain.
But, I think it might simulate well enough as phsyco says and he's happy with it? When I moved the TS once in a bit of a re-build w/out a CF stage at the time the TS in the middle of two triodes gave a gain structure more w/ a blues-ie feel and when put after the 2nd triode it gave a more rock-ish gain & feel to the amp.
I think if a new board were to be made then phsyco could even add a CF stage > TS and do it "Marshall correct" if he wanted?
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When I build the new board it will have an extra tube and the Marshall plexi circuit all the way.The reverb channel will be left stock.
As it is it is loud but gets a really nice growl.An extra cascade gain stage would be nice indeed.
I swapped out the Alnico speakers for a pair of Celestion Vintage 10's but it made the amp pretty dark,so I swapped out the bypass caps for .68uf ones.That perked it up nicely and gave it the sparkle that was present with the alnicos and the stock bypass caps.It really takes pedals much better now.
My suggestion to Fender guys who want to hot-rod their boring old Fender amps is to use a nice set of Celestion speakers and drop those bypass caps down.The flubby bass is all but gone,the sparkle is still there and you can rock out all you want with pedals.
Win-win for this guy.
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New V-10's get a bad rap for some reason but I like them too. Much better than the darker less efficient greenback 10 anyway. Another good ceramic w/ a touch more brightness & sensitivity is the Weber Blue Pup 10.
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Being a total noob, I have to ask a couple more questions about this mod, or I'd have to dig out the Marshall Schematic and do a comparison.
Which coupling caps are you talking about changing to .047, and you mention taking a 220k to ground? Where in the circuit is this (before or after the coupling cap)?
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This sounds like a Gerald Weber mod (he takes credit for it anyway). I built a board similar to this and am waiting for some cathode bias resistors so I can switch from fixed to cathode bias.
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It's actually a 2.2meg to ground at the junction of the coupling cap and the grid wire from the volume pot.