A #102 Attempt - First amp

[Vertigo]

I just noticed something that I hadn't noticed before about the effects loop. There are a few variables, so I'll do my best to describe them.

Shortly after it was finished, I put a Strymon Mobius in the loop and I couldn't get it to sound good at all. This wasn't a huge surprise as I've read so much about how the d-lator makes an improvement

I've left the loop alone until today, and in the mean time, the Ayan treble peaker mod was put in, which made a big improvement to the OD section.

I pulled out an old RSP Intelliverb and have been playing with it in my different amps this past week so I plugged it into the ODS. To my surprise, if I crank the input and output levels, it sounded really good on the OD channel and OK on the clean channel, even when bypassed. The bigger surprise is when I pulled it out of the loop again, by comparison, the amp was really dark again. I think I've just grown used to the dark, choked sound but after playing through my Mesa again and then this little experiment with the Intelliverb, it's clear to me that there's a lot more that can be done to improve the sound.

While I was building, I used a combination of the "adapted layout" and "102 ford amp r5" because not all the connections were clear on either of them, so I used them both to double check. Other than the filter section and the way the power tube sockets are wired, the biggest difference between the two is on the FX loop. The "adapted layout" has a couple of 500pF caps and 220K resistor on the jacks, and rather than butcher what I think is the explanation, I'll just post the photos.

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So when nothing is plugged into the loop, they're in the cuircuit right? And when something is plugged in, the switches are open and they're taken out of the circuit entirely.

So why are they there? The obvious next step is to remove them from the circuit and hear the difference.

[Vertigo]

I'm leaving that post up, but after looking through the 102 FILES post, I see that aflynt posted that it's a treble reduction network.

[martin manning]

So when nothing is plugged into the loop, they're in the cuircuit right? And when something is plugged in, the switches are open and they're taken out of the circuit entirely.

I'm leaving that post up, but after looking through the 102 FILES post, I see that aflynt posted that it's a treble reduction network.

Correct, that network is active unless something is plugged into the loop. I don't know where that came from, but yes it will bleed off high frequencies. It doesn't look like a good idea to me, but aflynt must have had some reason to include it. There is another network on the loop jacks in #124, 250p parallel 220k. I think most people leave that out too.

[Vertigo]

So when nothing is plugged into the loop, they're in the cuircuit right? And when something is plugged in, the switches are open and they're taken out of the circuit entirely.

I'm leaving that post up, but after looking through the 102 FILES post, I see that aflynt posted that it's a treble reduction network.

Correct, that network is active unless something is plugged into the loop. I don't know where that came from, but yes it will bleed off high frequencies. It doesn't look like a good idea to me, but aflynt must have had some reason to include it. There is another network on the loop jacks in #124, 250p parallel 220k. I think most people leave that out too.

I just removed it and it made a massive difference in the high and upper midrange. I think that's what I've been trying to get rid of since I fired it up. I'll try taking the treble peaker out of the OD entrance since that killed a lot of the OD. I'm so relieved that this happened today as I was about to start going down the road of going to low plates.

There's a lot more base line hum and now a bit of what sounds like rattling when I hit chords when it's switched to the OD...so some new issues to track down

[pompeiisneaks]

Yeah we're around 265 today I just saw... it smells like fire outside... not fun. Take care.

~Phil

[Vertigo]

New Recording 2.m4a

I think this hum started after I removed the treble reduction on the FX loop. I can't help but wonder if it was filtering out the hum. All the tabs on those jacks have just been reflowed with no effect.

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I pulled the PI tube and the amp was dead silent.

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I've poked and pulled at every wire and connection inside it again and once got a scratch and pop by moving this bundle of wires in the yellow zip ties, but was not able to recreate it. I also chased each of the wires to their joints, poked at the joints with no effect.

I also tested resistance (I read somewhere else to give that a shot to look for ground loops) from every ground location to every other ground location. The maximum reading I saw was 1.2R and the lowest was 0.9R.

It increases when any of the pre-amp vol, master vol, OD vol (with OD on), presence, treble and when the FET is turned on, there's an additional SHHHH sound that increases when the pot is turned up.

Does this sound like a ground loop? Should the ground resistance be zero? Is it likely that the treble cut circuit was hiding it or more likely that it started when I removed that circuit?

[Vertigo]

I'm convinced that what I'm hearing is 60Hz as it was compared to a tone generator and a spectrum analyzer app I downloaded showed a higher volume spike in that range.

Another behavior I noticed today when attempting to diagnose it, when I turn up the preamp vol it gets louder until I get to about 8, at which time it gets a little quieter, then when I go past 9, it gets louder again.

I poked around in the signal path with my scope and got nothing at all. Thinking that I was doing it wrong, I probed the filaments of the relay power supply and got a predictable reading (my scope needs to be calibrated, but it showed a jagged wave over +- 3 divisions when set on 2V/Div)

When I probed the speaker output jack, I also got nothing unless I set the probe at its most sensitive setting with a 10X probe but it's all over the place so I'm guessing it's EMF. I get the same pattern when I touch the probe to anything metal in my office.



Any advice at all would be much appreciated.

[martin manning]

Yes, 60 Hz. Might be a poor ground. You can look for where it enters the signal path with your scope, but if it is affected by the preamp volume that suggests it is around there. I assume if you turn the master all the way down it's gone, and it's there in clean and OD modes? You can also pull tubes out to help isolate it. If it stops with V1 out, that reinforces the idea that its in the clean section.

[Vertigo]

Yes, 60 Hz. Might be a poor ground. You can look for where it enters the signal path with your scope, but if it is affected by the preamp volume that suggests it is around there. I assume if you turn the master all the way down it's gone, and it's there in clean and OD modes? You can also pull tubes out to help isolate it. If it stops with V1 out, that reinforces the idea that its in the clean section.

yes, if I turn the master all the way down it's gone. If I pull V1 out it's gone. I've swapped between three different tubes in V1 and it always hums. If I turn on the OD, it gets about twice as loud.

I'll keep poking with the scope, but I think I've touched everything attached to V1 and haven't seen anything

[ijedouglas]

Can you post closeup pics of your input jacks and wiring?

[Vertigo]

Can you post closeup pics of your input jacks and wiring?

Gladly. Thank you for taking a look. I've never noticed the dog hair until viewing these photos.

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[ijedouglas]

I might be missing it but I don't see the 1 meg input resistor on the NOR channel. I also can't quite make out the wiring on that jack either. I think I see a green wire to the tip lug, where does that connect to?

[Vertigo]

I might be missing it but I don't see the 1 meg input resistor on the NOR channel. I also can't quite make out the wiring on that jack either. I think I see a green wire to the tip lug, where does that connect to?

The 1M resistor is actually on the FET relay board between COM1 and NC2. The center of the coax to the grid stopper is also connected to COM 1, and the shield of that coax is connected to the other side of the 1M on NC2 and makes its way to the SW and S part of the jack and then to ground. The green wire on the Tip goes to NO2 on the FET relay board. None of this is on the layout that I made, so I must have been referencing something else. IIRC, I was using the pdf Martin made of all the eyelet boards.

As neat as I tried to be, it's a hot mess in that corner. I've tried to get a better photo

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[norburybrook]

Shot in the dark here:

Your speaker jacks; have you got an actual ground wire or are you grounding through the chassis?

I've had earth hum on anodised chassis because of this. I always wire an actual wire connection to ground from the speaker jacks. I Know these chassis are not anodised on the grounding places but might be worth checking.


M

[ijedouglas]

Have you tried disconnecting the input on V1a and injecting a signal to see if that eliminates the hum? I can't quite follow the wiring with the FET/NOR inputs and this would be an easy way to remove that from the equation.