Crate V50 ?

[bill128]

I have a problem with my Crate v50 1-12. The crunch channel has low output even when the gain and level set to max. It is about as loud as the clean channel on 2. Also the tone controls have no affect. I tried changing the V1 and V2 tubes (12ax7). I am new to tube circuits and have downloaded pdfs for this amp. Any help would be greatly appreciated.
thanks,

Bill

[Jazz P Bass]

Crate V50

The tone controls work on Clean but not on Dirty?
Do you have the Crate V50 manual?
http://www.crateamps.com/pdf/manuals/V50-112_OM.pdf
On page 11 is the block diagram of the amplifier.
Clean Volume & Dirty Gain go through the same path. The tone circuit.
Dirty Level is after the tone controls.
Can you sort out which controls (Dirty Gain or Level) do work.

[bill128]

I do have the manual. The tone controls do not work on either channel. The presence switch has a minor affect on the tone. The Overdrive Level control does have an affect on the volume. When turned down the volume drops to zero. The Overdrive Gain control changes the signal but sounds like a tone control in that when it is on zero it has more bass and on 10 it is more treble but as far as volume, it has no affect on the signal.The Clean channel really honks volume wise. From this I am assuming the output section and pre-amp are working fine except when the the signal goes through the V1-b part of the first 12ax7. I don't understand how the amp can be so loud if the signal goes through the non functioning eq section.

Thanks,
Bill

[Jazz P Bass]

Crate V50

Please be aware that this amp has a SMPS.
Switch Mode Power Supply.
The line voltage cap can hold a charge for awhile after the unit is turned off.
Confine yourself to the preamp section.

Tone Control Inop.
There is an FET transistor that connects the tone stack to ground.
Q2. J112.
It may have failed, always on.
It is on page 2, over on the right.
Cw4,6 & 5 on your tone potentiometers.

[bill128]

Now What?

Thanks for the tip. I tried replacing Q2 and nothing changed. Next I tried shorting then opening the source/drain connections with no change in output. I would have thought that removing the JFET transistor would make this part of the circuit a simple passive tone control but they still had no affect on the sound or am I wrong in my assumption? I have checked all the pots with power off and the resistance is what is indicated on the schematic.
Is it possible that the two relays controlling the channel select functions may be faulty? Relay 1 and 2 should be turned on at the same time. I hear a relay click when the switch or footswitch is actuated. I know that at least part of the relays are switching because the Level pot has an affect on the volume when the gain channel is switched in. Could it be that the contact for relay 2c is non functioning and connecting R11 to ground all the time? And if this is the case would it render the tone control non-functioning?

[Jazz P Bass]

R 11

Good thought.
Why don't you get on R 11 (the relay side) & ground with your meter set on ohms.
Engage & disengage the relay & tell us what the meter reads.

[bill128]

Thanks. I checked it out and it has switches between 0 ohms and 1.45 Meg. So it looks like the relay contacts are functioning. Could C40 be open and blocking the signal? I wil borrow a scope tomorrow and try injecting a signal on the input and try tracing it.

[bill128]

Got a scope and began tracing a signal (mp3). It appears that V1-A is working as the signal is about .1Vp-p on the input and 1Vp-p on pin 1. The signal shows no ac loss on the scope at pin 1 of V1 through to pin 7 of V1. There is very little signal at all on pin 8. When I was looking I discovered that R75 on the schematic says 220K but the actual R75 is 100k. The voltage on top of R75 and R2 is 332vollts and on pin 1 of V1-A it is 250v but at pin 6 of V1-B is 330 volts. The resistor shows 100k out of circuit. Does this mean that the new tube I put in is bad? I have never worked on tubes before so I'm not sure of voltages at the terminals.

[pecorporation]

How is the solder on those tone and volume controls, reflowing them would ensure a good connection? I would reflow the input jack as well, just as a precaution.

[Jazz P Bass]

V1B

Can you read the Vdc on pin #8?
Compare it to pin #3.

[bill128]

I reflowed the connections but still have the problem. Pin 3 is 2.25vdc and pin 8 is 6.03 vdc.
Thanks,

[Jazz P Bass]

Crate V50

Can you please verify that V2b pin 7 (grid) is truly attached to V2b pin 6 (plate)?
I've never encountered this arrangement.

[Enzo]

OK, Jazz, I have looked, but I don;t see where he said pins 6 and 7 were connected. What am I missing? However there are many amps wher a triode stage is direct coupled to the following cathode follower stage, using the plate of the gain triode connected directly to the cathode follower grid.

Oh, I see, sorry. The schematic, not Bill. That is clearly a drawing error. The line from R16 to pin 6 should not be there.

If pin 6 has the same voltage on it that the top of R75 does, that means no current is flowing through R75. That is Ohm's Law. And that means the tube is not conducting or the circuit is not complete.

It takes about 10 seconds to try a different tube in the socket. So a new tube either works, or it doesn;t. If it works, then yes your other tube was bad. If it also doesn;t work, and in the same way, then the circuit has an issue. And finding 6v on the cathode of a non-conducting tube is problematical too.


Look, at this point, all we want to do is find out what it needs to make V1b stage work right. So we can steal a good tube from a later stage for now to find out. If this stage is dead, then later stages can still pick up its crosstalk signal and amplify that. The result will be a weak sound that tends towards the tinny. And you can turn the bass control up all you want, but if the bottom is not there, there si nothing to boost.

Fix the dead stage, and I just bet my lunch money your tone controls will wake up.

[Jazz P Bass]

Crate V50

Thanks for the clarification Enzo.

[bill128]

Tried another tube but it did not solve the problem. I tried removing R11 and R75 from the circuit in hopes that this would remove the affect of V1-B. I still did not have tone control. Is this correct logic? Also if I removed V2 completely and leaving R11 out jumpered pin8 on relay 2c to R19 would this eliminate the tone stack circuit from the problem and verify V1-B operation?