HRM needs some Lovin'

[fred.violleau]

you have Hum? do your heaters have a center tap, if not have you got a pair of 100r registers to ground at some point from the heater wiring?

Chek the tightness of your speaker output jacks if they're grounging through the chassis, or put a groung wire to a ground point close by.

That's about as far as my knowlewge can help in this Fred I'm afraid. Dont be dissheartened though as this kind of trouble shooting is where I really learned about amp building, it can be frustrating but in the end for me it was really the part of the process where I really learned about things and made progress. If all my builds had worked first time I'd still be painting(building) by numbers [emoji1]



Marcus

Hey Marcus, thanks for your help!

I did not put 100R resistor on the heaters. I was re-reading Merlin blencowe’s chapter on hum cancelling on he advised the exact same thing or even better: a “humdinger” (500ohms pot wired on both heaters with wiper to ground.

My jacks output (as well as all the knobs and jacks plugs all share a ground bus on the back pannel. Forging on your experience I though I would try to leave no room for that possibility.

I retried the amp tonight and with humbuckers I have to push it a lot to get the hum (all knobs on 10 basically...). So I might tackle that issue but not as priority no1 [emoji6]

Mid boost works only on one side, signal is off on the other side.

Will try to get the power amp stage working 100% and then move back up the chain.

Is it normal to get huge cracks on the speaker when probing the power tubes?
Seems like I would have the amp running at 100% even if the volume knob are shut off!

I should have more time tomorrow to debug some more.


Humble and Dumble,

Fred.

[dorrisant]

Probing the power tube input pins will make a pop at the speakers... That is normal. Look at your schematic and you will see that probing there is downstream from any volume control, therefore you have no way of attenuating the pop when the amp is connected directly to the speaker. Hot plates, dumm loads, etc... if you want to reduce it or not hear it at all. If you expect it to pop, then it is no surprise when it does.

[fred.violleau]

Probing the power tube input pins will make a pop at the speakers... That is normal. Look at your schematic and you will see that probing there is downstream from any volume control, therefore you have no way of attenuating the pop when the amp is connected directly to the speaker. Hot plates, dumm loads, etc... if you want to reduce it or not hear it at all. If you expect it to pop, then it is no surprise when it does.

@dorissant thanks for your input!
Indeed, there is always a first time

[fred.violleau]

So I took my voltages today :

B+ 1 (OT) 498v
B+ 2 (Power Tubes) 493v
B+ 3 (PI) 470v
B+ 4 (OD) 332v
B+ 5 (Cln) 325v
B+ 6 (Send D-lator) 244v
B+ 7 (return D-lator) 174v

Pins 1 2 3 4 5 6 7 8 9
V1 177 0 1.83 - - 185 0 1.79
V2 192 0 1.71 - - 211 0 1.56
V3 245 2.14 13.04 - - 116 0 0.89
V4 317 20 60 - - 307 20.5 60

P1 18mv 0 485 480 -45 480
P2 1mv 0 490 486 -45 487

Tried to switch Power tubes even rolled tubes, no change. So I am assumig I made a mistake in the 1/2 power switch
Reviewing the wiring as we speak..

[fred.violleau]

Found some time yesterday evening to troubleshoot the amp.

Found two weak ground connections which I repaired. One was on the first power filtering node and second was on the FX loop board. Seems like these had a very positive impact on the hum level. I can still hear some but the amp is running at 100%, all knobs on ten. I am assuming a small noise at that level is decent and acceptable.

Amazing what a bamboo stick can do ! I found a microphonic preamp tube on the PI which I replaced and change the OD tube which was apparently blown.

So now OD channel is receiving signal. But it is weaker than the clean channel. So I think there might be a problem with my HRM ground lift. Seems also the level and drive knobs are not having any effect on the OD Output volume. I will have to investigate on this side now that I can hear something.

Still having trouble with the power section. One tube is almost drawing no current (2mv) while the other is drawing around 30 mv. Rolled tubes, same diagnostic.

Nothing wrong with the half power switch which I checked.

There is one area I need to review: the 1r resistors to sense current. Maybe one cable is not grounding properly. But could that be the origin of the problem, or should I check more on the OT side?

Thanks for your input!


Humble and Dumble,

Fred.

[norburybrook]

Fred.

depending on which layout you used it is possible that adding the 1R to ground will short out the power valves if you have the say 5k1 resistors wired across the valve to pin one rather than them going straight to the P.I. which the original dumble amps do.

Just a thought.


Marcus

p.s. just re read this..100r?? they are for the faux center tap on the heaters you need 1R for the bias points. Are we talking about the same thing?

[fred.violleau]

Fred.

depending on which layout you used it is possible that adding the 1R to ground will short out the power valves if you have the say 5k1 resistors wired across the valve to pin one rather than them going straight to the P.I. which the original dumble amps do.

Just a thought.


Marcus

p.s. just re read this..100r?? they are for the faux center tap on the heaters you need 1R for the bias points. Are we talking about the same thing?

Hey Marcus,

Sorry for the mistake, I was writing on the train and out of my (bad) memory. It is indeed 1r resistors not 100r.

I also have 1k 5w resistor across pin 4 and 6 on each tube and have a 3k3 1w resistor on pin 5 of each tube connecting to PI.

I will try to bypass the 1r resistors on the weekend and check the end result on the tubes.

Thanks for your time and wisdom.


Humble and Dumble,

Fred.

[norburybrook]

if the 3k3 are going to the PI then no problem, the problem is if you follow some of the layouts where the 3k3 is connected to pin 1-8 if you then add the 1R resistor like that it will ground !!

I made that rookie mistake hence my thinking about it when I read you post. Apologies if that's rather obvious.


Marcus

[fred.violleau]

@Marcus,

Apparently the problem on the power tubes was on the half power switch. After removing it, all voltages started to align better although a bit hot.
B+ rail:

B+1 : 474v (OT)
B+2 : 470v (power tubes)
B+3 : 450v (PI)
B+4 : 325v (OD)
B+5 : 316v (Clean)
B+6 : 269v (send D-lator)
B+7 : 263v (return D-lator)

Power tubes voltages:
P1 35.6mv 0 470 467 -42 469
P2 35.8mv 0 470 467 -42 468

It looks like I had wired the half switch shutting down one tube while halfing the other... I might do something about it once the amp is working.

I cant seem to find the right dropping resistor for the D-lator, but I will eventually!

Next is debug the voltages on the D-lator.

Side question : when the amp is on but on standby and I measure the voltages at the end of the rectifier bridge I get around 140v AC that disappears when the amp is running. Is it normal ? I though I would only get DC at the end of the rectifier bridge. Are my diodes still good or are they blown? Can I test the diodes while on the circuit?

Thanks for your help!


Humble and Dumble,

Fred.

[67plexi]

Fred. What I see is you need to drop your voltages B+2 to B+3 by 150 volts so your PI is in the 300 volt range.

[fred.violleau]

@67plexi, thanks for the suggestion.
I am getting a little worried on my B+ rail as I have soldered/unsoldered quite a few times the resistors. I hope I did not damage my F&T caps, nor the resistors, and my leads are getting shorter each time.

So before I solder I would rather submit and validate my configuration :
voltages dropping resistor
B+ 2 (Power Tubes) 470v 22k 3w
B+ 3 (PI) ~350v ~2.2k 2w
B+ 4 (OD) ~325v ~2.2k 2w
B+ 5 (Cln) ~319v ~2.2k 2w
B+ 6 (return D-lator) ~310v ~10k 2w
B+ 7 (send D-lator) ~265v

Any thoughts ? Does it makes sense ?

Any issue with my B+ for the D-lator being fed after the clean and OD B+ ?

Thanks for your input.

Fred.

[fred.violleau]

I have tweaked my B+ rail, still needs a little adjustments but I am getting there.

I have the clean channel functioning and for now the plate voltages are on the low side (184v and 187v for V1).

When I switch on the OD channel, signal drops to almost inaudible... i can hear the overdriven sound, Level and drive knobs are still effective.. but the signal is not entering the FX loop (send pot has no effects when it does work on the clean channel).



Humble and Dumble,

Fred.

[pompeiisneaks]

I have tweaked my B+ rail, still needs a little adjustments but I am getting there.

I have the clean channel functioning and for now the plate voltages are on the low side (184v and 187v for V1).

When I switch on the OD channel, signal drops to almost inaudible... i can hear the overdriven sound, Level and drive knobs are still effective.. but the signal is not entering the FX loop (send pot has no effects when it does work on the clean channel).



Humble and Dumble,

Fred.

Do you have a scope and sig gen or phone app? You can see where in the process it's disappearing to narrow it down to a specific triode or past a triode etc.

sounds like something is dumping most of the signal to ground instead of letting it pass.

~Phil

[martin manning]

...and ~185V on V1 plates is fine.

[fred.violleau]

I noticed something weird :
When switched to the drive channel, I have some ~20v AC coming on pin1 of the drive pot, but nothing on pin2 no matter where the wiper is..
Is it normal ? looks to me like a huge voltage drop...
Can I bypass the pot and try to jump pin1 on to pin2 and see what happens ?
Since I reused a lot of pots, I might have a failing one, although I tested all of them before installing these on the chassis to make sure I had the proper values...

I have tweaked my B+ rail, still needs a little adjustments but I am getting there.

I have the clean channel functioning and for now the plate voltages are on the low side (184v and 187v for V1).

When I switch on the OD channel, signal drops to almost inaudible... i can hear the overdriven sound, Level and drive knobs are still effective.. but the signal is not entering the FX loop (send pot has no effects when it does work on the clean channel).



Humble and Dumble,

Fred.

Do you have a scope and sig gen or phone app? You can see where in the process it's disappearing to narrow it down to a specific triode or past a triode etc.

sounds like something is dumping most of the signal to ground instead of letting it pass.

~Phil

Hey Phil,

I do have a scope.
I watched my first tutorial this morning, trying to figure out the does and don'ts.
I have used this in my childhood and I have to remember how this thing works.
Any good tuto to recomend is welcomed.
Thanks again for your help !

...and ~185V on V1 plates is fine.

Thanks @Martin for the reinsurance, much appreciated!

Fred.