HRM needs some Lovin'

[fred.violleau]

Is your bias circuit still like this? http://ampgarage.com/forum/viewtopic.ph ... 65#p380765

Yes it did note change.
I just extended my B+ rail.
I will resume the troubleshooting tomorrow as I have friends for supper coming within minutes.
Again any advice is welcomed and much much appreciated!

Thanks.

[ChopSauce]

... looks like I used a cable gage a little too big, that made the wiring a bit difficult and cluttered the available space as well...

Are you sure? Your chassis does not seem to be as wide as many others.

Good luck with the sequel, anyway. I wish I could be more helpfull

[10thTx]

I am thinking you have an effects loop given you listed d-lator voltages?

IF so, plug into the return jack of the effects loop. Do you get sound that way from the speaker? If so, your signal problem is prior to that.

Let's say that doesn't produce sound. Then ......................

IF you have another amp around with an effects loop, you could try sending the preamp signal of your HRM into the return FX jack of that amp. IF you get sound then you know the signal problem in after the return FX on your HRM.

Hope that makes sense?

Once you figure out whether it's prior to the FX return ......... OR after the FX return, then you can use carefully use insulated alligator clipped wires to "jump" over & bypass a triode gain stage to isolate where the problem is.

with respect, 10thtx

[martin manning]

Hold on, I think get the power stage bias sorted out first and then move on to other issues.

Scanning the posted pin voltages:

The Dmbleator supply voltages look too low (about half of what they would be in an outboard D'lator), and the send stage should have ~30V on the cathode (maybe 15 in your case given the low supply voltage). What does the D'lator power supply branch look like?

The PI cathode voltages should be exactly the same since they are tied together; you're off by 0.5V.

The 60mV on one power tube (pin 1?)... Do you have 1 ohm current sense resistors on the cathodes, and are pins 1 and 8 tied together? If so, that says one tube is drawing 60mA (which is too much!) and the other is not drawing any current.

[fred.violleau]

Hold on, I think get the power stage bias sorted out first and then move on to other issues.

Scanning the posted pin voltages:

The Dmbleator supply voltages look too low (about half of what they would be in an outboard D'lator), and the send stage should have ~30V on the cathode (maybe 15 in your case given the low supply voltage). What does the D'lator power supply branch look like?

I have a 100k dropping resistor that feeds V3a with 189v cascading into another 100k dropping resistor that feed V3b with 138v

The PI cathode voltages should be exactly the same since they are tied together; you're off by 0.5V.

They are now both 60.3v

The 60mV on one power tube (pin 1?)... Do you have 1 ohm current sense resistors on the cathodes, and are pins 1 and 8 tied together? If so, that says one tube is drawing 60mA (which is too much!) and the other is not drawing any current.

I have now 10mV on pin 1

I also changed the boas resistor for a 27k which allows a range from -45 down to -60

I have now -49.8 and -49.7 on the power tubes.



Humble and Dumble,

Fred.

[martin manning]

OK, progress!

Which node are you taking the D'lator power supply from? What is the voltage there? I'd take it from the screen node, and use ~47k or so to get ~350V at the first filter. Then the 100k going to the next filter should get you to ~250V there.

10mV on both power tube pin 1's? Are the power tubes installed? Do you have 1 ohm current sense resistors on the power tube cathodes?

[fred.violleau]

Thanks for the help Martin, I really appreciate it.

here is a little picture to clarify things about the B+ rail:

Power stage with readings.png

As to answer your previous questions :

10mV is only for tube 1 pin 1,
0 on the second tube pin 1

Are the power tubes installed? yes they are both, nothing else plugged between the wall and the amp

Do you have 1 ohm current sense resistors on the power tube cathodes?
I have a 1ohm resistor wired with pins 1&8 for each tube that goes to ground
here is the pic of the resistor :

Current sense resistors.png

I will change the first of the 2 100k resistor on B+3 with a 47k and report my readings

Also here are the new readings I took this afternoon :

V1a : 184v. - 0 - 1.88v.
V1b : 194v. - 0 - 1.84v.
V2a : 201v. - 0 - 1.76v.
V2b : 221v. - 0 - 1.61v.
V3a : 197v. - 10.53v. - 1.87v.
V3b : 97v. - 0 - 0.71v.
V4a : 324v. - 20.5v.- 61.1v.
V4b : 318v. - 21v. - 61.1v

[martin manning]

V1 and V2 look good, as does V4. Note you can't measure the grid voltage on the PI directly since your meter upsets the bias point. Take that measurement from the bottom of the bias resistor, which is the 820R (or something like that) connected to the cathodes.

V3a is the "send" stage, yes? Looks like something wrong there. Check the wiring and component values carefully. The recover stage (V3b?) seems ok, except for the low plate voltage. That will come up when you reduce the dropping resistor on that branch to 47k.

Have you tried to bring the power tube current up with the bias pot yet? With only 10mV/0mV on the cathodes it appears they are in cut-off.

[fred.violleau]

Hey Martin, you really are a blessing, thanks for your time, patience and expertise.

Here are updates on the readings.
I cannot understand why the readings jump up after a few minutes of running. amp warming up ? false contact somewhere ?
I have decided to let the amp run for a while and see how much of a difference my readings will be.

So far, I changed the third dropping resistor to 47k 3W
This brings new voltages :

PI B+: 474v.
V2 B+: 335v
V3a B+: 245v
V3b B+: 177v.

V1a: 182v - 0 - 1.83v
V1b: 189v - 0 - 1.89

V2a: 196 - 0 - 1.73v
V2b : 215 - 0 - 1.58v

V3a : 249 - 13.07 - 2.14
V3b : 122 0 - 0.89v

V4a : 322v. - 20.3 - 61.5v
V4b : 316v. - 20.8 - 61.5v

I will review the wiring on the dlator and get the readings from the resistor you mentionned for the PI.

Thanks.

[pompeiisneaks]

Something else I think you mentioned. You probed one of the anodes and heard noise at the speaker, that usually means from that point on, you've got a good signal path, if there's a point where you get no noise look somewhere around that point. (But it could be that there's output but super soft too I guess?)

~Phil

[fred.violleau]

While reviewing the wiring of the dlator, I noticed that the secondary output jack ground was touching the side of the tip and was grounding all the output signal!

Bended the culprit a little, plugged in and Houston we now have sound!



I have sound ... and a good hum. Looks like all the tone knobs and switches are functional on the clean channel. When I tried the switch to OD, I lose the signal and keep the hum..

More to investigate, and more to come during the week.



Humble and Dumble,

Fred.

[fred.violleau]

Something else I think you mentioned. You probed one of the anodes and heard noise at the speaker, that usually means from that point on, you've got a good signal path, if there's a point where you get no noise look somewhere around that point. (But it could be that there's output but super soft too I guess?)

~Phil

Hey Phil, thanks for jumpin in!
I do have a weak signal output: I could play a little but with the amp almost at full volume and hearing less than bedroom volume...

I will keep looking and see if there are weaker spots than others!

Thanks for the tip!


Humble and Dumble,

Fred.

[fred.violleau]

@10thtx thanks for your debug tips!

I am thinking you have an effects loop given you listed d-lator voltages?

I do

IF so, plug into the return jack of the effects loop. Do you get sound that way from the speaker? If so, your signal problem is prior to that.

Let's say that doesn't produce sound. Then ......................

IF you have another amp around with an effects loop, you could try sending the preamp signal of your HRM into the return FX jack of that amp. IF you get sound then you know the signal problem in after the return FX on your HRM.

Hope that makes sense?

Totally!

Once you figure out whether it's prior to the FX return ......... OR after the FX return, then you can use carefully use insulated alligator clipped wires to "jump" over & bypass a triode gain stage to isolate where the problem is.

with respect, 10thtx

Meaning that I would take the signal from the plate of one stage and bring it into the input of the next closest working stage, bypassing whatever isin between right?

I have tried to isolate if the problem was before or after the FX loop, but that was prior my finding that the output amplified signal was sent to ground.

I will try again tonight and see if my weak signal comes from the preamp stage or the power stage. My guess for now is the power stage : when I tried to probe on the power tubes , the little crack I was hearing before had turned into monster cracks.

It looks like the signal is amplified, but Martin was suggesting the readings on the power tubes were too small and would have them in cutoff mode.

BTW, how could I create a cheap load to avoid waking up everyone in the house when probing/debuging the amp?

Thanks again for your help and support.


Humble and Dumble,

Fred.

[norburybrook]

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



Marcus

[pompeiisneaks]

Another place I've gotten hum was having my output transformer leads too close to the pots, make sure you run them directly away from any of the early signal path.

~Phil