Crate V3112 - Clean Channel Noise Distortion

[TomCarlos]

Ok... stupid me.... or maybe this is how the amp is built... I had pulled the EL84s when taking the measurements on the 12AX7 tubes. When I put the EL84s back in, the voltages came close to the schematic.

But... Heater Voltages...

Tube 1
Pins 4 to 5 -> 11.9 volts
4 to 9 = -5.93v
5 to 9 = 5.97v

Tube 2
Pins 4 to 5 -> 12.5 volts
4 to 9 = -6.25v
5 to 9 = 6.32v

Tube 3
4 to 9 = 5.8vac
5 to 9 = 5.8vac

EL84's
I am seeing 5.8vac between Pins 4 and 5.

It looks like there is something odd with Tube 3. The Plate voltages on Pins 1 and 6 look ok. But the Cathodes tie together and instead of seeing 48v, I am measuring 32v. I pulled Tube 3 and measured R12 (1K) and R16 (22.1K). The resistors measure out ok. Not sure what else can be causing this low voltage.

Tom

[Audiotexan]

Ok... stupid me.... or maybe this is how the amp is built... I had pulled the EL84s when taking the measurements on the 12AX7 tubes. When I put the EL84s back in, the voltages came close to the schematic.

It's totally normal as voltages usually ride high when unloaded (or rather: missing the load the tubes put on).

But... Heater Voltages...

Tube 1
Pins 4 to 5 -> 11.9 volts
4 to 9 = -5.93v
5 to 9 = 5.97v

Tube 2
Pins 4 to 5 -> 12.5 volts
4 to 9 = -6.25v
5 to 9 = 6.32v

Tube 3
4 to 9 = 5.8vac
5 to 9 = 5.8vac

EL84's
I am seeing 5.8vac between Pins 4 and 5.

1st thing... did you make note of the "Test Point Conditions" as referenced in the upper left of the schem before measuring?

2nd thing... Are you sure you're at 120vac mains (as per TPC in #1)?
If mains were lower than 120vac, a drop to 5.8vac on V3 would be expected.

Kind of a unique way of doing things, but they've got stages/voltages split up in this design.

J1A (4-9) shows: -12.9v/F2NC
J1B (5-9) shows: GND/F2NC

J2A (4-9) shows: GND/F1NOT2
J2B (5-9) shows: +12.9v/F1NOT2

V1 and V2 are using one methodology, while V3 uses another (traditional ac heater)...

J3A (4-9) shows: F1/F2
J3B (5-9) shows: F1/F2

Now.. J24 shows F1/F2 as being 6.3Vac coming straight out of the mains tranny with a center tap to ground.
So V3 should be fine (Again, assuming mains was slightly lower than 120v).

Below that shows J9 as having F1/F2 as well...am guessing that that's just for reference, but I'm not spotting J9 elsewhere as yet. At any rate, I'd say that your voltages should be good given that you've got 12vdc on both v1 and v2, and 6vac on v3 as required (HOWEVER, you may want to double check your readings on V1 to make sure that you actually have a negative voltage from pin4-9. I'm just assuming you do have, if you actually DO have a positive voltage from 4-9 there, that could be worth looking into, as it's supposed to be a negative voltage.)

<Edit>Wait. Now I'm confused. Rereading once I posted to make sure I didn't lose train of thought....smacked me in the face with the realization that yeah, you had 12vdc, but NOT the way I was thinking.

Am I missing something, or are you missing a ground on V1 and V2, AND your rails for those two tubes are off?? <Edit>

<Edit 2> J9 is the 6.3vac power lamp indicator. </duh mode off lol>

[Audiotexan]

I heard some noise and distortion coming out of the clean channel. So I removed the EL84 power tubes and using the Line Out to drive a clean amp and good speaker. Yup, the distortion and noise are still there.

Putting aside the voltage 'question' above for a moment...

Another quick note to check if you have a scope, would be the TL072 (IC2a). This is the before the SEND (line out) that you were using to narrow things down. Check pin 3 and see if your signal is distorted at that point, if no, then try pin 1 of the same ic. If yes on pin1, then you've got your culprit!

[TomCarlos]

AudioT ... thanks for the reply and I will go through your suggestions in the next day or so. Will report back.

Tom

[g1]

It looks like there is something odd with Tube 3. The Plate voltages on Pins 1 and 6 look ok. But the Cathodes tie together and instead of seeing 48v, I am measuring 32v. I pulled Tube 3 and measured R12 (1K) and R16 (22.1K). The resistors measure out ok. Not sure what else can be causing this low voltage.

An error in the schematic would be the likely cause. According to the voltages they show for the plates and supply, the current through tube 3 would result in approx. 32VDC at the cathode, not 48V. So you can ignore that issue.

Edit: you mentioned you get distortion at the send jack. How does it sound when you plug your guitar straight in to the "return" jack, clean or distorted?

[TomCarlos]

Tone Meister - Yes, that is the wrong schematic. I appreciate the help but we already have the correct schematic. Please delete your attachments if you can.

G-One - You are correct. The 48V on the schematic must be wrong. I calculated and measured approx .75ma through J3A and J3B. We have 1.5ma flowing through R12. So what flowed in flows out of the tube. And 1.5ma is flowing through R12 and and R18, which gives us around 35vdc on the Cathodes. Mystery there solved. Thank you!

I did plug my guitar into the Line In Jack- it sounds clean through the amp. But I know that sooner or later I need to deal with R22. That cement resistor gets HOT- so the suggested replacement of aluminum resistors mounted to the case makes sense.

So I am still believing I have some type of issue in the preamp section.

My Mighty Mite tube tester showed all 12ax7s that were in the amp as testing good. However, the gain through J1A was only 1/3 of the expected value. As per the schematic, I measured 28mvRMS at pin 2 of J1A. The output at the plate was only 463mvRMS. I dropped in a new tube and then saw close to the 1.5vRMS as per the schematic.

I need to rig up my own tube tester circuit... I have enough parts around the house, including a Xformer- so that is on my To-Do list.

I will continue working my way through the preamp stages and compare to the Schematic.

AudioTexan- Thanks for the hint on IC2A. Teh signal on Pin 3 looks clean. However, the output on Pin 1 does have something odd looking on the top side of the sine wave- on the trailing edge. I'll look into this further and maybe try to post a picture.

But... next step... follow the signal patch through preamp and check those tubes!!!

More to come...

Tom

[g1]

My Mighty Mite tube tester showed all 12ax7s that were in the amp as testing good. However, the gain through J1A was only 1/3 of the expected value. As per the schematic, I measured 28mvRMS at pin 2 of J1A. The output at the plate was only 463mvRMS. I dropped in a new tube and then saw close to the 1.5vRMS as per the schematic.

When this happens, I usually then put the bad tube back in to double check. Just to make sure the issue is not a dirty socket, or bad solder connection if the socket is board mounted. Especially in this case as it is a preamp tube that checked good in your tester. Not saying that is the case here, but it's always a possibility.

Many good tubes have been discarded because the self-cleaning action of installing a new tube fixed the fault. Or the fault comes back later because it was actually a solder issue that was temporarily fixed by the action of swapping tubes.

[TomCarlos]

Ok... I think I am going to call this one "done."

As G-one suggested, yes I cleaned the leads on the tubes and the sockets. I always do that. I rotated the 12ax7s so that I could measure the ac plate voltage on Tube 1, pin 1. They ranged from 1.35vrms to 1.49vrms- close enough. I also cleaned the pots (Deoxit with F5 cleaner/lube) and all jacks.

I ran a signal to the Line in and monitored the output. I was getting 26.5 watts before visual clipping. The signal looked relatively clean and I let the amp bake for 15 minutes. Yes, R22 got VERY hot. But, the amp continued to put out max power without breakup.

I then ran a signal into the main input. Hey, this is a guitar amp, not high end stereo so the signal didn't look all too pretty at the output (thank you Enzo for reminding me of this). But again, I was able to crank the signal up to get my 26.5 watts at the output (before noticeable clipping). The amp baked once again and continued to produce that signal.

Playing a guitar through this amp doesn't seem to produce the greatest of all clean sounds- but maybe this is as good as this amp gets. I am waiting for my aluminum resistors to come is so that I can do the R22 mod as seen on other posts and the attached photo. I am hoping this will help lower temps on the chassis and run the output tubes a bit cooler.

Anyway..... other than the future replacement of R22, I did some scrubbing and cleaning and we'll call it good. I'm gonna return this to our young student.

Thanks everyone for schematics, replies, and weighing in with you help.

Much appreciated,

Tom