CRATE BV amp Ch1 prob- "Crunchy" Noises.

[52 Bill]

V2 and V3 check is all I can physically do now- then Im totally stumped & need help.

I'm not that familiar with this amp and not having the preamp schematic makes it impossible for me to suggest specific things to check, so all I can suggest is to keep trying to isolate the noise source into smaller and smaller sections of the amp until you find it.

I also noticed that there are ICs in the audio circuits, so they can also suspect.

[Sea Chief]

Hi Bill- I have the schematic (PDF) kindly sent by Andrew to my hotmail. Alas I dont really understand too much of it, bar ID'ing the various components, ie C2 is 10uf etc (but I can see that on the board itself).

But Ive no idea how i could add the pdf schematic (attatched to a hotmail mssg) onto a reply here though.

IC means integrated circuit.. (means transistor does it?)

[Sea Chief]

Removing V2 I notice the top looks different to other 12ax7.. it actually looks a bit burnt/ this is the top coating bit which is usually silvery/ reflective.

Does that show symptoms of something?? a plate resistor on V2? (I am guessing entirely). Btw how do I check plate resistors are good, just test for 100k (or w'ever theyre meant to be from schematic) using an ohmeter across it, in situ, with amp off? They look physically ok/ no signs at all of ott heat to either V2's pin 1 and pin 6 resstors (which I think are the plate Rs like on V1-?).

[Jazz P Bass]

Crate BV50H Schematic.

Here is the schematic.
Thanks, Moz.

[Sea Chief]

Thanks for putting that up.

I see what I think are the V2 plate resistors as both being 221k 1/2w.

How do I test these again? can they be tested without draining the filter caps?

[Sea Chief]

Bill, (or anyone) I wonder if I can ask you on my Ch1 issue Im still trying to isolate the area to concetrate on..

The schematic [in reply #19] shows the Ch1 section, I think, that distinguishes it from the Ch2 section. This I think Im right in saying is the lower Optocoupler OC1 & the pots & components immediately to its left on the diagram alone.

-Schematic is best @ 300% min-

1) Am I correct in my judgement above? And am I right in thinking therefore that the two Optocouplers are OC1 (lwr one) soley for Ch1 duties, and OC2 (upper one) soley for Ch2? And what exacty do these things do- Ive never seen them in a fender amp for eg!
2) Do both Ch1 and Ch2 use both the preamp tubes in the amp, or is V1 used soley for Ch1, and V2 for Ch2?

If yes is answer to these 2 Qs, and I have this nasty 'dirty noise' soley on Ch1, then can I conclude therefore the cause -must therefore be- isolated to the lower Optocoupler & the [ch1] pots & components immediately to its left on the diagram??


(Ive ruled out all tubes/ V2 too you see by physical checks/ swaps.. and Im trying to boil down the possible causes of my solely ch1 bad noise to components therefore I guess.. and Ive got my beady eye on the OC1 thing now). The OC's look a bit like two beetles next to each other.

Any help I really appreciate. thanks Sea Chief.

[Enzo]

The schematic shows the preamp complete, it is all on one page, there is one basic path through the amp with the optos doing switching.


Optocouplers are just switches. They also call them LDRs = light dependent resistors. turn on the LED and the resistor goes low. Very much like a JFET.

V1a is the common input tube. pin 2 gets the signal from the jack. It comes out the plate at pin 1 and goes to a branching circuit. The upper branch is some tone shaping and a level control - GAIN - for the dirt channel, the lower branch is a common TMB tone stack with volume control for the clean. The signal from the two level controls then flow through optos to join as they enter V1b at pin 7. Turn on OC2 and the dirt channel comes through, turn on OC1 and the clean comes through. That is the only branching in the thing, the two control circuits. AFter that, there is only one path through the amp. Look at the schematic, signal in the grid and out the plate on each stage. What makes it clean or overdrive is the bypass caps switching in and out on the cathodes of V1b and V2a for gain. And after V2b, Q5,Q6 turn on and off to enable the tone stack and post volume over there. Oh, and Q3 controls the voltage divider level control of R15,16,17.

Noise in only the dirt channel or only clean channel may be related to that stage 1 branch, but could just as easily be an artifact of the other fain switching along the preamp.

[Sea Chief]

Hi Enzo thanks for that.

I realise the preamp is all on one page. I just need to locate the various sections of it, IE Ch1 section. Then I can narrow down the area in which the problem lies.

When you say V1a what do you mean by 'a' (pin1?), and v1b? and similarly V2b (b = pin2?).

A) Ok so if the octos are switches, why are there two? (surely one switch would mean either one way/ch1, or another way ch2).
B) Afaict an OC has 2 parts inside it, a resistor, and a diode: is the diode the switch part and is it possible that just resistor part of it could be shot?
C) Am I not correct in my isolating the ch1 section then, to the area I mentioned ie to the LHS of the lower OC1?
D) As before, are both preamp tubes used for both channels, or one for each?

thx SC

[Jazz P Bass]

Jumping right in here.
The 12A_7 tubes are two triodes in one glass envelope.
To distinguish between the two, pins 1,2 & 3 can be labeled V_A.
Pins 6,7 & 8 can be labeled V_B.
Plate, grid & cathode.
The heater pins (4,5 & 9) are common to both.
Now the circuit. (page 2)
The guitar signal enters V1A & is amplified.
The amplified signal leaves the plate (pin 1) & goes to both channels.
Ch 1 (clean) has tone controls & a volume control.
This goes to OC1.
The signal also goes through Ch2 gain control and onto OC2.
The channel switch function will select which OC to turn on, while the other is left off (normal state).
Either signal then enters V1B.
At this point JFets are employed to select different value components for each channel.

[Sea Chief]

Aha- ok got that JazzP.

Thats the kind of explanation I can cope with, just. Now the only thing with this now then, is that i cannot seem to distinguish any section therefore that -alone- can be attributed to ch1 (without the other ch2 beig used/ fed via or something) and therefore why in any way shape or form, I am finding this dastardly noise on one channel and not therefore also on the other.

Has anyone any ideas? anything at all? I dont understand why now then (even more confusingly now I roughly understand the signal paths so both ch and ch2 seem to be used at the same time) this noise is solely on ch1 and not also on ch2. Surely, just surely there must be/ there -has to be- something (there simply has to be) that distinguishes ch1 from ch2 in order for this noise NOT to be on both channels. Even if it is one component (indeed if it is one componnet only, then this would surly have to be the culprit).

If all of the preamp components, sections, tubes are used by both channels [as I think I understand the case to be now then] and Im getting noise only on one.. then that makes no sense whatsoever.

There has to be some sense I can follow, in order to try and at the least narrow down the problem to either an area (as I thought of doing b4 but now that seems to have no sense to it after JazzP's explanation) or further: to a few components, and further still: to a component. There has to be some logic I can follow/ apply here.

[g1]

As JazzP mentioned, there are 2 separate paths between V1A and V1B. Those are the different parts for the 2 channels.
Parts that are for clean only are:
C8,C9, C10
R10,R11
Low,High, and Volume pots
OC1

[Sea Chief]

As JazzP mentioned, there are 2 separate paths between V1A and V1B. Those are the different parts for the 2 channels.
Parts that are for clean only are:
C8,C9, C10
R10,R11
Low,High, and Volume pots
OC1

Right. So this is as I thought innitially, but it was dismissed. Im really confused now. I innitially asked if these very areas you now say (ie the lower OC1 + component area immediately to its LHS on the schematic) I can attribute to ch1? but was told 'no/ far more complicated than that/ ch1 is mixed in with ch2 etc etc'.. so I couldnt then distinguish even a ch1 area to concentrate on so I was stumped/ had nowhwere to think/ it didnt tbh make any sense.

All Im trying to do is narrow down where (if it is only heard on ch1- it is not heard on ch2) the noise can possibly be attirbuted to > then find the one, or more prob component(s).

--If anyone can help, Id be very very very grateful! Im almost begging for help now--

So if Im trying to do this narrowing-down, and you now (as I 1st thought/ asked) have concurred that these componenets ( C8,9,10,R10,R11,OC1, Low,Mid,High,Vol ) DO distinguish themselves as ch1 components... and I have noise in only ch1...

Can I rightly assume then that the problem --can only therefore be-- in one, or more than one of these components?? thx SC

[g1]

Right. So this is as I thought innitially, but it was dismissed. Im really confused now.

Who dismissed it? Enzo basically said the same:

It comes out the plate at pin 1 and goes to a branching circuit. The upper branch is some tone shaping and a level control - GAIN - for the dirt channel, the lower branch is a common TMB tone stack with volume control for the clean. The signal from the two level controls then flow through optos to join as they enter V1b at pin 7. Turn on OC2 and the dirt channel comes through, turn on OC1 and the clean comes through. That is the only branching in the thing, the two control circuits.

He did mention a few other parts that do some other switching later in the circuit, and that they could also be responsible, but that was not to dismiss the Ch.1 branch between V1A and V1B.

[Sea Chief]

Ok I need to try a different tangent here.

g-one (ar anyone). If you had an amp such as mine, and you found bad noise in ch1 -only- a crunchy rustling noise that remains whether vol is then turned up or not. What would you do? would you try and locate the ch1 area of components, or not?

[g1]

Yes, I would look at the branch around the Ch.1 tone controls. But what does the clean volume control do to the noise?
If you can get rid of the noise by turning down the clean volume, then the problem is BEFORE the clean volume pot.
If turning the clean vol. down does not affect the noise, then the problem is the vol. pot or something after it.

Edit: Sorry, I missed the part where you said the volume control has no affect on the noise. So that rules out anything before the pot.