HRM needs some Lovin'

fred.violleau · Post by **fred.violleau** » Tue Mar 27, 2018 12:05 am

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fred.violleau · Post by **fred.violleau** » Tue Mar 27, 2018 12:07 am

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Post by **pompeiisneaks** » Tue Mar 27, 2018 1:01 am

Well basically make sure to put in some kind of sine wave maybe 400mV peak to peak so it's not too hot into the input, then probe with the scope probe just after the coupling caps from the output of the specific stage.

Step your way through each stage making sure it goes up more and more after. I also always test it at the input jack to make sure it's arriving, test it at the grid of the first stage, make sure it made it there, and then from there on, at each stage's either grid input or after the coupling cap. Most scopes can go up to about 300V no problems and if you have a 10x probe it can go higher. Many just get a 100x probe to be sure they're in the 'safe zone' for higher voltages if you probe B+ in the wrong way, which can fry your probe, input or entire scope, so be sure you're careful of that part.

You should see the output stage inverted from the previous as usual, unless it's a cathode follower. You should also see significant maginfication for each stage. Say you see 400mV into the first tube, the output s hould be like volts, 5 or maybe 10, can't remember. Then you should get more after each stage. Finally, the phase inverter should be many volts like 20 to 50 if I recall. You can then scope the output jack to see what you're getting there as well. ONe other thing of note, either have a dummy load in the output jack to save your ears from the annoying 'whine' of the sine wave, or put a speaker in but put some ear plugs in

I should really do a video of how to trace through the amp eh?

~Phil

fred.violleau · Post by **fred.violleau** » Tue Mar 27, 2018 1:04 am

@Phil thanks for the walkthrough, I will try that tomorrow! It feels like a Tuesday tips video

Thanks very much!

Humble and Dumble,

Fred.

fred.violleau · Post by **fred.violleau** » Fri Mar 30, 2018 7:10 pm

During One of my debug sessions, I noticed that one of my resistor on the HRM ground lift is 100k instead 100R and the other is 1 Meg. Would it be the cause for my signal to be so small?

Another thing : on the OD drive pot, I have 70ish AC volts coming in when the pot is completely closed, and around 50v when the pot is on ten. But on the out pin (the middle pin) I have less than 1v out. Is that pot failing?

Fred.

norburybrook · Post by **norburybrook** » Sat Mar 31, 2018 3:00 pm

fred.violleau wrote: ↑Fri Mar 30, 2018 7:10 pm During One of my debug sessions, I noticed that one of my resistor on the HRM ground lift is 100k instead 100R and the other is 1 Meg. Would it be the cause for my signal to be so small?

Another thing : on the OD drive pot, I have 70ish AC volts coming in when the pot is completely closed, and around 50v when the pot is on ten. But on the out pin (the middle pin) I have less than 1v out. Is that pot failing?

Fred.

I had asimilar mix up when I didn't measure before fitting. I had a rogue 100R in a bag of 100K from tube town. There's a huge difference between the two

My amp wasn't working well at all with the 100R so definitely change it to the correct one.It took a long time to trouble shoot that error!

M

fred.violleau · Post by **fred.violleau** » Sat Mar 31, 2018 11:03 pm

norburybrook wrote:
fred.violleau wrote: ↑Fri Mar 30, 2018 7:10 pm During One of my debug sessions, I noticed that one of my resistor on the HRM ground lift is 100k instead 100R and the other is 1 Meg. Would it be the cause for my signal to be so small?

Another thing : on the OD drive pot, I have 70ish AC volts coming in when the pot is completely closed, and around 50v when the pot is on ten. But on the out pin (the middle pin) I have less than 1v out. Is that pot failing?

Fred.

I had asimilar mix up when I didn't measure before fitting. I had a rogue 100R in a bag of 100K from tube town. There's a huge difference between the two My amp wasn't working well at all with the 100R so definitely change it to the correct one.It took a long time to trouble shoot that error!

M

Thanks for the input Marcus!
I spent some time trying to understand why my signal going into the overdrive was so weak. Turns out the shielded cable I was using is a shitty one! Almost all of the connections I did with it aren’t worth much. Besides RG174 is there any other recommendations for a good shielded cable?

Humble and Dumble,

Fred.

Post by **pompeiisneaks** » Sat Mar 31, 2018 11:39 pm

fred.violleau wrote: ↑Sat Mar 31, 2018 11:03 pm
norburybrook wrote:
fred.violleau wrote: ↑Fri Mar 30, 2018 7:10 pm During One of my debug sessions, I noticed that one of my resistor on the HRM ground lift is 100k instead 100R and the other is 1 Meg. Would it be the cause for my signal to be so small?

Another thing : on the OD drive pot, I have 70ish AC volts coming in when the pot is completely closed, and around 50v when the pot is on ten. But on the out pin (the middle pin) I have less than 1v out. Is that pot failing?

Fred.

I had asimilar mix up when I didn't measure before fitting. I had a rogue 100R in a bag of 100K from tube town. There's a huge difference between the two My amp wasn't working well at all with the 100R so definitely change it to the correct one.It took a long time to trouble shoot that error!

M
Thanks for the input Marcus!
I spent some time trying to understand why my signal going into the overdrive was so weak. Turns out the shielded cable I was using is a shitty one! Almost all of the connections I did with it aren’t worth much. Besides RG174 is there any other recommendations for a good shielded cable?

Humble and Dumble,

Fred.

dumble used RG59U and I've been using it on my builds as well. Interesting stuff. Thicker, harder to move around, but basically way better shielding I'm sure.

~Phil

fred.violleau · Post by **fred.violleau** » Sun Apr 01, 2018 12:21 am

pompeiisneaks wrote:
dumble used RG59U and I've been using it on my builds as well. Interesting stuff. Thicker, harder to move around, but basically way better shielding I'm sure.

~Phil

Thanks Phil!

Looks like a new order is on the way !

Humble and Dumble,

Fred.

norburybrook · Post by **norburybrook** » Sun Apr 01, 2018 10:48 am

yes that's just regular 75ohm coaxial cable. It is however very thick, but it's got great shielding and the center core is thick, solid copper.

It's cheap as well. I've just got some for my 2nd gen build.

M

fred.violleau · Post by **fred.violleau** » Tue Apr 03, 2018 7:52 pm

@Phil, @Marcus,

Where do you get your RG59 ?
Mouser only allows me to buy 100ft..
Thanks!

Fred.

Post by **pompeiisneaks** » Tue Apr 03, 2018 8:01 pm

fred.violleau wrote: ↑Tue Apr 03, 2018 7:52 pm @Phil, @Marcus,

Where do you get your RG59 ?
Mouser only allows me to buy 100ft..
Thanks!

Fred.

I bought mine at a local electronics shop called Vetco Electronics. They sell all kinds of wire by the foot. I may have to go get more, I'm down to the last foot of mine and have 2 more foot-ish length runs to finish on the build I'm doing now.

Here's the stuff: https://vetco.net/products/rg-59-u-coax ... on_id=2106

not sure if they'll ship it to france, but it's worth a try?

Edit: also I noted the picture there has a different shielding than what I bought, mine has braided copper. Not the film they show. Not sure if that's a different form of the same RG59/U or what.

~Phil

norburybrook · Post by **norburybrook** » Tue Apr 03, 2018 10:06 pm

Fred just put RG 59 Coaxial into your eBay search and you'll have plenty of options.

Tube town in Germany sell nicer sheilded cable by the meter.

RG59 is horrible to work with.

There are better modern alternatives IMHO.

M

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fred.violleau · Post by **fred.violleau** » Wed Apr 04, 2018 3:15 pm

@Marcus, I was on tube town, any cable you would recommend in place of the RG59?
It looks like the impedance and voltages supported by the cables varies :

Would that one be of use :
Signal wire 0,5 mm² shielded
Voltage Rating max: 50 / 75 V AC / DC
Capacity: ca. 260 pF / m
Temperature range: -10 bis +70 C°
Conductor resistance: 40 Ohm / km @ 20 C°
Test voltage: > 1 kHz

Thx

Fred.

norburybrook · Post by **norburybrook** » Wed Apr 04, 2018 3:23 pm

Fred,

I've used this:

https://www.tube-town.net/ttstore/Cable ... :1485.html

And I've also used the white coaxial which is 75ohm like the RG59 but it's very thin and the center conductor is very very thin so I don't like it.

The grey stuff is a different capacitance and impedance than the RG59 but it's a nicer cable. With such short runs I doubt the difference would be that noticeable.

I'm sure you can find a decent shielded cable your side of the pond. As long as it's shielded and of a decent quality I wouldn't get too het up on it.

RG59 is cheap as chips and you can get it anywhere as it used to be used for TV ariels etc and is still used in various audio/visual circles. It's just a bit ungainly to work with as it's so thick.

M

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