18 Watt TMB build, amp plays radio signals

[mailnride]

I built this amp several years ago using the Visio layout, and the Lo inputs has never worked correctly. Hi and Master Volume inputs work fine, but the Lo input consistently seems to go from a moderately loud signal, to what sounds like the amp being turned off, then coming back up again. I think it may be the way I've wired the Cliff Jacks, the switching/shorting input jacks in the parts list.

Here's a pic of the schematic with the area circled. In this image, I assume the side of the jack with the little hump on it (the right side) to be the side where the guitar cable goes in. What does SHT and HOT stand for in this? I also pick up radio signals quite clearly through my speaker when I use that low jack.




Here's a picture of the actual jack wiring. The left green lead goes to pin 7 of V1, and the green lead on the right goes to the ground buss.

[Daviedawg]

As often happens it is hard to see exactly what the wiring is doing in a photo. I suggest that you look in the Classic 18 watt section of Downloads at Richies Improved Tremolo layout which shows the view of the connections from the back in which it is clear how to connect the two inputs. My approach would be to study that and draw a bigger version using that. Strip the wiring at the inputs and remake all the connections following the drawing you make.

Inputs are a common source of doubt. Good luck.

Dd

Edit. Just to be clear, on the Richies layout the blue coloured connections are grounded. It also shows screened cable. You do not need that as part of the troubleshooting exercise.

[JMPGuitars]

Post photos of your whole build. One thing your layout drawing doesn't show is your grounding scheme. You need as short as possible ground wires, especially regarding the input jacks and preamp section (which should have an isolated ground point from the rest of the amp).

If you're running long grounding wires, or running your input ground to somewhere far away (like a star ground), you're potentially creating an antenna to pick up radio signals.

[mailnride]

Post photos of your whole build. One thing your layout drawing doesn't show is your grounding scheme. You need as short as possible ground wires, especially regarding the input jacks and preamp section (which should have an isolated ground point from the rest of the amp).

If you're running long grounding wires, or running your input ground to somewhere far away (like a star ground), you're potentially creating an antenna to pick up radio signals.

Thanks, will do, I'm sure that's contributing. I can likely shorten ground leads. Let me ask you a question, i have a ground bus wire running parallel to the main board...is the goal shortest insulated lead wire to this buss, or to the chassis?

[mailnride]

As often happens it is hard to see exactly what the wiring is doing in a photo. I suggest that you look in the Classic 18 watt section of Downloads at Richies Improved Tremolo layout which shows the view of the connections from the back in which it is clear how to connect the two inputs. My approach would be to study that and draw a bigger version using that. Strip the wiring at the inputs and remake all the connections following the drawing you make.

Inputs are a common source of doubt. Good luck.

Dd

Edit. Just to be clear, on the Richies layout the blue coloured connections are grounded. It also shows screened cable. You do not need that as part of the troubleshooting exercise.

Yes, thank you. I will look at that. I notice in the updated Richie TMB that those input jacks are wired very differently. They have two separate leads coming off of V1 Pin 2, with one 68k resistor in line for each lead. I'm going to try rewiring it in parallel per the updated TMB schematic and see if that gets me a better result, along with the suggestion below to check the grounding.

[JMPGuitars]

Thanks, will do, I'm sure that's contributing. I can likely shorten ground leads. Let me ask you a question, i have a ground bus wire running parallel to the main board...is the goal shortest insulated lead wire to this buss, or to the chassis?

The easiest way to respond to that is to show you the way I do it. Here's my drawing of the Lite IIb(ish): files/JMPGuitars_18_Watt_Lite_2b_Layout.pdf

The PA and the preamp have separate busses. The PA goes to the star ground, while the preamp goes to the input jack, which then goes to ground near the input jack.

Thanks,
Josh

[mailnride]

Thanks! OK, re-doing the Hi and Lo inputs to the preamp tube solved that problem perfectly. I followed the wiring from the updated Richie's diagram. Adding the grid filter resistors at the tube socket & using shielded wire seemed to do the trick.

Question: for the input leads, what's the best way to route them through the chassis? 1) up and over everything else (when viewing the amp from the bottom) or 2) along the interior of the chassis, like I did with the heater wires? My shielded input wires are grounded at the jack end.

Also, I've been getting a high-pitched squeal if I turn up the Master Volume pot past 7. Happens whether there is a guitar plugged in or not. Any advice for that?

Chris