Amp Tech Thread / Ask a tech Q

[Hemisaurus]

Yeah, your using the same term for two different things. Him flipping the signal ground to positive and positive to ground is a polarity switch, you should call it this as there is the possibility of a power component involved, ie. you can short out the output of a pedal, guitar or ground an amp the wrong way. The premier guitar article, and I agree, that the safe place to do this is at the speaker, not at the input.

Your inverting unity buffer is a phase shift, the signal is delayed, not just inverted. Inverting in a buffer is safe.

I will however reserve the right to laugh a lot if you paid big money for 'true bypass' pedals and then put that Radial buffer in your signal chain

[dunwichamps]

If you use isolation transformers in your ABY you can flip the signal by changing the leads of the secondary of the iso-tranny or you could do it by simply using a unity gain buffer or not using a unity gain buffer

[Hemisaurus]

Transformer, also phase reversal

Haven't seen a high impedance matcher anywhere convenient, got a link?

[dunwichamps]

Transformer, also phase reversal

Haven't seen a high impedance matcher anywhere convenient, got a link?

No i dont but BYOC has some op amp driven 1:1s in their design that work

[Hemisaurus]

Those little $3 RadioShack audio ones are 1K:8 if memory serves, two of them would work for isolating a line level, but not a guitar level just too low.

[dunwichamps]

you could drive it to line level then reduce it to guitar level, kind of stupid u know but would work

[Hemisaurus]

A couple of passive DI transformers back to back, Jensen spec theirs as 200K:1.5K be cheaper buying a couple of cheap passive DI's and tearing them apart probably.

Something tells me we're overthinking this, as it's not gonna happen

[dunwichamps]

yup. but thats engineering for you.

[Jake]

I will however reserve the right to laugh a lot if you paid big money for 'true bypass' pedals and then put that Radial buffer in your signal chain

I do use true bypass pedals as in, I truly bypass using pedals and plug straight into my amplifiers.

The pedals I do have are for shits and/or giggles.

[rayinreverse]

1st Gen Model T blowing a fuse in a gnarly fashion. Its blowing the 47 ohm resistor from pin 2 on power tube. Any ideas?

[dunwichamps]

pin 2 is heaters. That shouldnt be there I assume

Fuse blowing could be a short, or a blown cap or an arcing sockets

[Hemisaurus]

Why would there be a resistor on the heater? That would be weird.

According to the schemo, the 47ohm resistor is on pin 3, which is the plate, so I'd suspect a bad tube, sorry



Click for bigger.

[dunwichamps]

bad tube, and I did mean "shouldnt"

[Hemisaurus]

only time I've seen a resistor on the heaters is to drop the voltage on those old suicide series heater tube amps

owchy

[dunwichamps]

yea that is dumb....but oh well

[rayinreverse]

It's pin 3.
I didn't think a bad tube would do that kind of damage.

[dunwichamps]

if you say shorted the tube ud be dropping 500V across 47 ohm resistor eeecckkk...

[Hemisaurus]

5000W

[mortlock]

is it possible to fuck up a modular synth by patching in shit the wrong way..is there a wrong way??

[Hemisaurus]

All the control voltages on the synth will be at the same level, unless it's using modules from different manufacturers in the same frame, then there is a slight risk. The danger is in patching in external eqpt, that may be at a different ground reference, or have a different CV range.

[AgentofOblivion]

Hey fellas.  I'm a new guy.  Haze me if you must.  I have JCM 800 2205 with an FX loop.  I forget the year off the top of my head but I think it's around 1986.  Brief history:  a long time ago the volume would slowly fade out and it would just make a quiet fizzle when I played.  Took it to some guys and thought I just needed some new tubes.  I was just a young buck and I don't know if they screwed me or not, but they kept it for a long time and ended up saying the output transfer fried the caps and all sorts of shit and they ended up replacing all sorts of stuff (without permission).

It worked for a long time and still does.  However, I've noticed funny things happen when I use the FX loop.  Sometimes the volume will do the fade thing, but that's rare.  I noticed it tended to happen when I would change the state of the loop (meaning I would start using it or stop using it).  I ended up cleaning the jacks and it has seemed to work OK since.  However, I've heard the circuit they used for this loop is very poorly designed and leads to common problems.  Does anyone know what they are and if they might be responsible for my issues?  I pretty much have to use it for my Eventide Space.  One weird feature is that when using the loop your Master Volume control becomes much less effective but you can do crazy things with the volume using the pedal in the loop.  It almost seems like the volume control on the last pedal becomes the master volume of the amp.  What, if anything, should I do to make sure this amp is healthy and not about to make things worse?

[Hemisaurus]

Well bad jacks would explain your problem, there's nothing very complicated about the FX loop, as it's just tacked on the end of the reverb driver circuit. If you are using any kind of boost or buffered pedal in the effects loop, that's going to boost the signal going back into the amp and make your master volume seem less effiective.

Sounds like you cured your own problem, I wouldn't worry about it, unless you start having issues again. If you're looking for how to remove the effects loop, like I said, it's just the two jacks, you'd remove the jacks and bridge the connections between them permanently.

[AgentofOblivion]

But surely not all FX loops are created equal?  I've heard the first generation ones on the JCM 800s (which this is) are poorly thought out and were redesigned quite quickly.  Is this hogwash?  Thanks.

[Hemisaurus]

Well, I'm seeing two schematics for the 2205, one dated 1983 which includes a reverb, and one dated 1989, in both instances, the effects loop is the same, an interruption, post the reverb tank, and before the buffer tube stage. Same as it's done on the old Oranges, and so forth.

By the time you hit the JCM900's there's an op-amp buffering the send and return, but then the JCM900's are full of op-amps, and I suppose that's OK, if you like that kind of thing.

OK, I'm an arse, on the '83 schemo, the effects loops is drawn before the master volume which is post phase splitter, on the '89 it's drawn after. Yet that would mean they switched from a smart design to a stupid one. I suspect the '83 schemo is just drawn wrongly. Anyways, a bit of reading on the Marshall forum, and that's your problem area.

If you were brave, you could replace the master volume, with a 1M resistor, and move the Master Volume to the input of the tube stage after the effects loop, replacing that 1M resistor with it. That would probably help, if the thing is unlivable with. Or model it on the '83 schemo and use a dual 1M pot post phase splitter.

Depending on how much of the amp is on a PCB, this could be an easy mod, or a bitch. I never knew they made PCB'd jacks and pots on the JCM800 until Sunn showed me the innards of his. If you're lucky and the jacks are off the board, it should be a fairly easy mod.

[moose23]

IIRC there's some known problem with the switching jacks carrying too much current or voltage on the effects loop of some Marshalls, dunno if that's what's happening here though..