Help with mod? Power amp and Input functionality

[asdf]

I own a Hot Rod Deluxe, a guitar, and a microphone. I want to plug the guitar into the input and the microphone into the Power Amp in, but that disables the preamp. Is there any way that I could modify the Power Amp In so that it works like a regular input that bypasses the preamp? I have my first real gig coming up but don't have a PA system. The Power Amp In seems to be linked to the Pre Out by using a stereo switch for a mono cable?? Could i just lift the wire to the unused stereo connection to get it to work? I cannot plug my mic into the second input because I need the distortion for my guitar so this looks like an only option? I do not have money for a PA at this time.

Here is a copy of the schematic.

http://www.blueguitar.org/new/schem/...rod_deluxe.pdf

[loudthud]

Not a very good solution but try connecting a 1 meg resistor from pin 2 of J3 to pin 6 of U1B. That should add the guitar signal back to the signal path when the microphone is inserted into the power amp in jack. The best solution is to get a small mixer, connect the output to the power amp in and run the preamp out to one of the channels. Then you would have a volume control on the microphone.

[asdf]

Mixer might have fixed it?

I plugged the mic into the channel one of my mixer and the Preamp out into channel two and mixed them to the Power Amp In. This seems to have fixed the problem but is there a possiblility that the mixer would be "robbing" my tone or compressing it? Heck I probably shouldn't be worrying too much, I am playing through a HRDx after all

Thanks for the suggestion. Do you think that your idea would sound or work better in any way? thanks for your time.

[Enzo]

Listen to the setup, does it seem to have less tone now? It doesn't matter if it is "robbing your tone or compressing it" if you can't hear the difference.

[asdf]

Amp Is Broken!!!!! PLEASE Help!!!!

There is NO sound on the Drive channel.
I think it is problably unrelated, but while I had the chassis open, I tried to rebias, but even with the pot maxed out, i could only get to around 20 mA. I replaced R77 with a 1 ohm resistor to make the biasing pot's range a little higher. I rebiased to 69 mA and went about my buisness. I goofed around with my new big muff for a while, hit the killswitch on my guitar, then walked out the room for some reason. I got caught up with my brother and we went outside for an hour. I came inside and walked into my room and was thinking "oh, i left my amp on." I went to go turn it off, and it was extremely hot. I think that if I left my hand on it for longer than a few seconds i would get some kind of burn. There was also a burning plastic kind of smell. Everything seemed normal except for a metal film resistor that was slightly discolored and left a small burn in the PCB. I replaced it even though it showed its normal resistance. There were also burns under a wirewound resistor that I added to simulate tube sag and pretty bad burns under the ceramic resistors R78 and 79. I tried to play it, and couldnt get any sound out of the drive channel, not even static, but everything works perfectly in clean mode. I spent about 8 hours today trying to find the problem with this amp by checking connections and touching up suspicious looking solder joints to no avail. I have read that the ceramic resistors might be the culprit, but each reads 470 ohms like they are supposed to. I retouched the solder joints as well. The only distortion pedal I have is that big muff, so I desperately need the drive channel. I am also broke and must have this amp up for practice tomorrow or by the very latest four days from now. No techs in my area can fix this before then, so im kinda desperate. Thanks for your time.

[Enzo]

I plugged the mic into the channel one of my mixer and the Preamp out into channel two and mixed them to the Power Amp In. This seems to have fixed the problem but is there a possiblility that the mixer would be "robbing" my tone or compressing it?

Sorry, I thought that meant you has sound.


You had not mentioned individual channel touble before that I recall.

When we refer to those cement resistors, the problem is not usually the resistor itself, but that it gets hot enough for its solder to fail. It can melt itself free or damage the pc traces around it.

First the power amp, with the low readings, my first thought is that one of the power tubes is not running. With power off, CLOSELY inspect the solder to the output tube socket pins. Resolder any with cracks.

Now power up and measure for B+ voltage on BOTH pins 4. And of course pins 3. And check for bias voltage on both pins 5. Bias is about -50vDC, and B+ is something like 400VDC.

If one tube is not running - and I mean electrically, the glowing heater tells us only that the heater works - then when you biased it so as to get 60ma, it might all be coming through one tube. And that tube would indeed get VERY hot.

Spec by the way is 60ma, not 69ma.

Did you change tubes? The durt channel is mostly the addition of V2b to the signal path. Swap places with a couple of the small tubes and V2. And check the drive and master controls themselves - a cracked control could do it.

[asdf]

No no no, let ME appologize. This happened earlier today I believe. I was singing and playing and all that jazz just fine with my mixer all hooked up before the drive channel died. Earlier today I checked TP10 which is supposed to be at 580mV but read out at 0 mV. Then I checked TP11 which is supposed to be 13.3V but I got 175.5V... I am pretty sure the trouble is on the second triode of V2 (V2B) because thats where the drive channel gets its juice. I think that the output section is fine because the clean channel works, but I will test those points tomorrow because I am so tired and I have slaved over that amp for hours today. And with the ceramic resistors, I touched up the solder joints so it should give me a temporary fix until I can get some cooler resistors in. Thanks for all the help so far.

[Enzo]

You are confused. The test points in the circle or oval are in AC volts and they represent the signal. They are approximate only and tatally depend on the input signal being as specified - 4mv of 1kHz into input #1. And even that in the case of these particular test points is only when the channels are set up to meet the conditions.

Only the square or rectangular test points are DC voltages.

I am sure there is not 175v of signal at that plate. 175v DC sounds about right for there.

Did you swap out V2? Just exchange it with V1. If the B side of the tube in V2 is truly bad, moving it to the V1 socket will mean that V1b was now bad, and that is the input stage. if it was bad, then in the V1 hole it would make all channels go out.

Please don't forget that the Drive control over by C1 could shut the channel down as easily as V2b could.

Let's use simple noise injection. Since you don't have any signal coming out of the dirt channel, we don't care at the moment about distortion - all we want it sound or no sound. Turn the amp on and set it to the dirt channel. usually when probing with your meter, we ground the black lead. This time don't. Set the black lead where it won't touch anything. Now if we touch points in the signal path with the red lead, it should cause some hum and noise. And that i9s all we need to test whether the circuit passes signal or not.

Turn the master up at least half way and probe its wiper - the middle terminal. Does hum/noise result in the speaker? How about when probing the top of that control? Probe TP10 - that grid should be pretty sensitive. Probe the other end of R20, after all K2B could have bad contacts on it, or for that matter the relay might not be energizing.

Wait, if V2 doesn't energize, we would just stick in the clean channel. But the contacts still could be bad.

Probe both ends of R45.

By injecting a signal - even if it is just noise - into each step of the circuit, we can find if the remainder of the circuit following is functioning.

Alternatively we could tace the signal through the circuit with a scope or an AC voltmeter. Apply a solid signal to the input, set the meter to AC volts, measure at places like TP10,11. If in doubt, turn the signal off and on and see if the meter follows. if you find signal on one end of a component but not the other, then that component is very likely what's wrong.

[asdf]

I goofed around with the tubes and put in a different set and even put in the originals for a bit.
For the points that you specified:
You can hear noise at the master volume pot and on both sides of R45, but nothing at TP10 or R20...
And with signal, the AC volts match up pretty nicely with TP10 and 11.(I think) I got about 250mV when moderately loud at TP10, and 22V at the same volume at TP11.
I kindof did this earlier but I had no idea what to look for or how to problem solve at all, but looking at the points I tested, do you think that it is a bad relay? If so how can I test it or get another one etc?
Oh and while I was doing the test at the TPs, I figured out that I can hear some guitar (low volume) if I crank the drive. The Master volume has no effect at changing the volume, it just scratches and makes a light tremolo kind of effect when swept. (i replaced it a while ago with a volume pot i out of a squier guitar i found, so the scratching isnt anything new...)

[asdf]

Weird resistor values

Resistors R19 and R20 both read 68k but R19 is supposed to be 220k and R20 is supposed to be 100k. Can resistors half blow? it sure looks like these two slipped a lot. Is that a problem?

[defaced]

If you're not desoldering one leg of the resistor, your reading is bogus because the rest of the circuit is affecting the resistance measurement.

[Enzo]

If you hear noise from the master, then K2A is OK - the noise had to pass through it. And you have 22v of signal at TP11? SO then we have only R25 and C10 between the TP11 and the master pot. Is R25 open, is C10 bad? Is there cracked solder on them or the traces that connect all this stuff. Follow the trace from TP11 through those parts and on to the master control. any unsoldered jumper wires or anything?

[asdf]

Everything connects. I dont know what to do. I guess Ill replace C10 and see if it does anything...

[asdf]

Swapping C10 didnt do me any good. Any other ideas?

[Enzo]

Start signal tracing. You have a strong signal at TP11 apparently, but not at the Master control. So trace the signal step by step from one to the next. That means detecting signal at each end of each part, and don't take the pc board traces as a given. There is alwaus a trace between point A and point B. The resistor does not solder directly to the cap, so check continuity from opart to part. Somewhere will be a point where there is signal on one side and not on the other - and THERE will be the problem. You could have an open somewhere, or you could have something shorted to ground. Most likely, anyway, I suppose there are weird things that could happen.