Crate V5 mods... anyone?

[Tarin]

Deafelectro...
Know what, yesterday as i finished posting on this thread, i kinda wanted to get back at modding one of this since i never got the chance (sold mine at the first offer).
You're right, it does have a well built cab... saw one on the local CL for 80 bucks, mmmmmm what to do.

[jim p]

I posted this over at SEwatt and though I would post it here also for anyone thinking about adding reverb.

Well I am up to the point of ordering some boards made, just waiting on getting the Digital Sound Effector module form Belton somewhat. I have added enough holes to attach either the Sound Effector or the Digi reverb to the PCB. So figure I would go through a circuit description. The signal input to the board goes to W8 this connects to C5 in series with R4, R5 and R6 and the gate of Q1 a J211. The value of C5 a 1 nF and a total resistance of 1 Meg ohm will create a high pass filter to keep the bass out of the reverb (-3db approx 200 Hz). The connections W9, W10 and W11 are to replace R5 and R6 if a dwell (reverb input level) pot is desired. R4 can be used to set a limit on the maximum signal into the gate of Q1. Q1 the J211 jfet is used to make the signal input to the board a high impedance due to the fact the reverb module is a 10 k ohm input impedance device. W8 also connects to R3 that is connected to W6 and R2. W6 will be the connection to the control grid of the triode that was connected to the volume pot wiper. Moving back to Q1 (J211) its source is connected to R7 and C6 with C6 connected to R19 and C7. R19 and C7 is a low pass filter to limit the high frequency (approx -3db at 6.5 kHz) into the reverb module to prevent feedback the value of C7 can be increased if feedback occurs. The connection from R19 and C7 can be the input to the reverb module with R15 installed or the input to a common emitter amplifier with R10 installed. In the case of a VJ, Fender Champ, Crate V5 or any of the SE amps having the volume control followed by a triode connected to the output tube gain will be required before the input to the reverb module so R10 would be installed and R15 omitted.
The reason for adding gain to increase the signal into the reverb module is two fold, one the greater the input signal the better the signal to noise ratio this is the same for a digital reverb or a spring reverb. Second being a digital device if the signal is low less of the dynamic range of the system will be used and with low levels digital distortion may become noticeable. The gain required for a stock VJ is this, an EL84 will have an input voltage of approx 5 volts RMS at full power this is from the triode connected to its control grid with an approx gain of 50 so the signal at the control grid of the triode is 5 divided by 50 for 100millivolts RMS. Now with this board installed the signal to the control grid will be attached to a resistor divider R3 and R2 so twice the signal will be present at the volume pot wiper for 200 millivolts into the reverb module. The spec on the reverb module is 1.5 volts peak which is approx 1 volt RMS so 1 divided by 0.2 (200 millivolts) is 5 so a gain of 5 is required. Which leads to the common emitter transistor amplifier Q2 R8 and R12 are a voltage divider to bias the base approx 50 volts with a plate supply of 250 volts at W12 the input impedance to the base of the transistor is R8 in parallel with R12 in parallel with R13 times the transistor beta of approx 80 for approx 30k ohms. With R18 installed so the output of Q2 is connected to the input of the reverb module the AC impedance at the collector of Q2 is R9 50k in parallel with 10k for 8.3k ohms. The AC gain is 8.3k ohms divided by R13 so with 1.5k ohms for R13 it is 5.5. There is some signal loss at Q1 due to its source impedance of approx 160 ohms and R19 in series with the input impedance of 30k so overall the gain should be approx 5 from W8 to M4 (reverb module input). The output of the reverb module M2 through R16 is connected to W3 this point is to be connected to the reverb level pot. With the circuit as described the output of the reverb will be 5 times greater then the dry signal so you may want to half the input at Q1 by making R6 a 470k R5 a 470k and R4 1 ohms so it will just be 2.5 times greater. The reverb level pot connections are as above the top of the pot connected to W3 the wiper connected to W4 and the bottom connected to W5 (ground). To allow the use of a footswitch Q3 and R11 are on the board so when W13 is pulled to ground the mosfet is off and the wiper of the level pot is not shorted to ground with the switch open Q3 is turned on and the wiper is shorted to ground. When the reverb level is all the way up the mosfet will short the output of the reverb module to ground therefore you may want to use a 10k resistor for the value of R16.

If you were to add this module to an amplifier with master volume or a push pull amplifier where the signal level at the volume control is greater then 1 volt RMS R4, R5 and R6 would be used as a divider to limit the signal to 1 volt RMS. Then R15 would be installed so the signal directly from Q1 would be applied to the reverb module input. Then C11 a 100nf 100 volt cap would be installed and I would use a 20 to 25k ohm pot for the reverb level that will be connected to R17 with R18 omitted as well as R16 omitted. You will need to calculate the gain required for signal matching with 250 to 300 volts at W12 a gain of approx 22 is possible with an output of up to 180 volts peak to peak.
So unlike standard reverb tank connections with tube or solid state recovery circuits this circuit allows you to locate the reverb at a location in the amplifier with high peak to peak signal levels.

The power supply connections to the PCB are W1 and W2 with a 6.3 volt AC heater supply the two secondary connections go to W1 and W2. On an amplifier with AC heater supply D1-D4 are installed if a center tap is connected to ground it will need to be removed also if there are pseudo center tap connections through resistors in the amplifier they will need to be removed. W14 the AC ground should be connected where the center tap used to be connected. On an amplifier with DC heater supply D2 and D3 should be omitted to prevent ground loops due to diodes in parallel and W14 connected to the ground of the filter capacitor for the DC heater supply. If the amplifier has +15 volts you can connect the positive to W1 and ground to W2 then omit D2 and D4 replace D3 with a short and D1 with a 50 ohm 1 watt resistor.
With an AC supply D5, C3, D6, C4 create a positive voltage doubler. This is followed by R1 a series dropping resistor and D7 a 12 volt zener diode with filter cap C10. This provides Q1 the J211 with a drain supply of 12 volts because 5 volts would be insufficient to insure linear operation with device to device variation of jfets.

Regarding cost the parts are approx 8 bucks without pots the PCB 20 bucks each in quantity of three and the best price for the module is 15 bucks.

The schematic is here http://i1020.photobucket.com/albums/...matic31210.jpg

The layout is here
http://i1020.photobucket.com/albums/...rbpcb31210.jpg

Let me know if you find any errors, thanks.

[jim p]

I have three blank PC boards on order by what things are costing out to be if I had 10 or more people who wanted these the cost would be around 40 to 50 bucks. That would be all the parts on the PCB required and the reverb module you would need to do the assembly. It might be possible to include the level pot at this price also. (the knob?)

[deafelectromark]

I have three blank PC boards on order by what things are costing out to be if I had 10 or more people who wanted these the cost would be around 40 to 50 bucks. That would be all the parts on the PCB required and the reverb module you would need to do the assembly. It might be possible to include the level pot at this price also. (the knob?)

Reverb is better outboard as well as the effects. Amps are what they should be be. Amps- and nothing more- nothing less. KISS
M

[jim p]

Well I thought I would post what I have for right now. I have the PCB and have installed most of the parts for operation. I tested the FET buffer stage and common emitter stage using two 9 volt batteries for a power supply. The FET is OK with an input signal up to 1.5 volts RMS so no problem being the largest signal in should be limited to 1 volt RMS. Also with the VJ it will only be 200 millivolts at full volume out. Loading the common emitter stage with a 10k resistor I got a gain of 4.8 so that is OK also. The one down side of the cheapest PCB from ExpressPCB is all the unmasked copper makes it easy to have a solder bridge on the board (I had two from assembly). If I do the boards in a quantity I will see about solder mask and silkscreen to make life easier. I don’t yet have the Digi-reverb module but I do have the Sound-effector which is the same size so I used that to check dimensions and to take pictures for now. With the module mounted on the back using nuts for a gap the PCB and module are 1.5 inches high by 3.8 inches by 2.5 inches. If you mount the module off the edge of the PCB on the top the height is 1 inch with a width of 3.8 inches by 3.8 inches.

http://i1020.photobucket.com/albums/...dmodulepic.jpg

http://i1020.photobucket.com/albums/...dmodulepic.jpg

http://i1020.photobucket.com/albums/...verbPCBpic.jpg

[jim p]

Well I built up a board and first installed it in a Ver 1 VJ combo amp with AC filaments. Couple of problems with that being the first stage triode plate trace runs parallel with the control grid for the second stage so with 330k in series to the grid and 330k to ground you have signal with the volume pot turned all the way down due to coupling. The second problem was AC pickup that did not exist with the stock setup so instead of adding a DC heater supply and cutting up the pcb I put it back together to try a different amp.
Next I put it in a Crate V5 that has been modified by removing the op amp that was on the input and has the volume control between the triode stages. The AC heater supply for the power tube is tied to ground on one side in the amp so on the reverb board only one diode D4 is installed and a short is installed at D2. Then W1 is wired to ground and W2 is the AC input to the supply. I used coaxial cable to connect to the wiper of the volume pot to the input to the reverb board with the coaxial cable grounded only on the volume pot side also used coaxial cable on the output of the reverb board with the ground near the control grid. This uses the voltage divider of R2 and R3 on the reverb pcb to mix the signals. With this I was getting some AC pickup (ground loop or pickup of AC to DC supply on pcb) so I put the resistors for the signal mix down on the amplifier pcb with 330k from the volume pot wiper to the control grid and 330k from the control grid to the wiper of the reverb level pot. With this I get just a little pickup when the reverb level is at maximum which is surf guitar level. Adding a ground from the signal side of the reverb board to a ground near the triode control grid took a little more of the noise out.

I need to see what the signal levels are like on the pcb I split the input to the board with two 470k resistors for half the signal into the board. But found I lost a lot of gain at the common cathode amplifier output so the module input is less then 10k as the data sheet states.
I will have to look into getting some sound clips posted.

The reverb sounds fine at least equal to the digital reverb in the Bugera V5 I have. On the Bugera I did change the stock input to the reverb to a connection at the volume pot. I also changed values in the common cathode amplifier on its output due to clipping on the stock circuit.

I plan on adding this reverb to a 6AQ5 push pull amplifier that I just got a Hammond chassis for.

[jim p]

Looking at the input and output of the Digi-reverb I am getting numbers that are different then the data sheet. Part of the problem could be the approx 30 years old Phillips scope I am using, but instead of 1.5 volts peak it looks like the most you want at the input is 1.5 volts peak to peak. One thing I am sure of is the input impedance is closer to 4k then 10k ohms as the data sheet states. The reverb looks to peak with more gain at approx 2 kHz then level off. There is also some nasty looking high frequency odd harmonics in its output if you do overdrive it. So it looks like I may have just needed a jfet buffer on the input due to the low input impedance and you don’t need the common emitter gain stage so much. I might increase the frequency roll off into the reverb module by increasing the capacitance in the low pass filter I have after the jfet buffer. Wishing I had a digital scope to freeze the signal coming out of the module to get a better idea of what is going on. Also be nice to have one that has been calibrated in the past decade.

[jim p]

I have posted two sound clips of the Belton Digi-reverb mounted on the PCB I designed installed in a Crate V5 amplifier. It is my son playing with the amplifier connected to a cabinet with two Jensen Special Design 10 inch speakers. The reverb is set at the mid level in one clip and at maximum in the other.

[mars2010]

I have posted two sound clips of the Belton Digi-reverb mounted on the PCB I designed installed in a Crate V5 amplifier. It is my son playing with the amplifier connected to a cabinet with two Jensen Special Design 10 inch speakers. The reverb is set at the mid level in on clip and at maximum in the other.

after reading through all these pages of mods

these sound clips are great, very nice playing and tone...


it's incredible how much time and dedication jim p put into working on this amp
very interesting stuff...

[DeanEVO_Dude]

I just picked up my Crate V5 amp, changed the speaker (Eminence Patriot Ragin Cajun) and I kinda like the way it sounds... They should have added $50 to the price and equiped it with this speaker to begin with.

Anyway, my soldering iron will be aquainted with this amp either next weekend or the one after. The first thing that will change is the gain on the op amp, move it more 1.5:1 or so on the first stage and around 2:1 on second stage. Then to tackle the the tone control... But one mod at a time...

[DeanEVO_Dude]

Jim P,
I have a question for you, I want to add a volume control to my V5 and what I was going to do was replace R27 with a 10K audio taper pot, and Radio Shack has said 10k audio taper pot. R27 is a 1/2 watt resistor, and the 10K pot is also 1/2 watt... one leg of the pot tied to R16, the other leg tied to gnd. and the wiper tied to what used to be the junction of R16, R27 and pin 2 of the 12AX7/R7. Would this work OK? Are there better values for the pot and R16 that I could use for better control?

Thanks.

[deafelectromark]

Jim P,
I have a question for you, I want to add a volume control to my V5 and what I was going to do was replace R27 with a 10K audio taper pot, and Radio Shack has said 10k audio taper pot. R27 is a 1/2 watt resistor, and the 10K pot is also 1/2 watt... one leg of the pot tied to R16, the other leg tied to gnd. and the wiper tied to what used to be the junction of R16, R27 and pin 2 of the 12AX7/R7. Would this work OK? Are there better values for the pot and R16 that I could use for better control?

Thanks.

Hi,
The best place for a volume control is between the two tube sections. The wattage is irrelevant as minimal power is dissipated at these low currents. A 10K control is probably too low for a high impedance circuit, I would use at least a 250K control to not load down the preceding stage. Look back at the earlier posts and schematics and see that what you need to do is reduce the voltage going into the grid of the second half of the 12AX7 from the plate of the first half. The only 'trick' is to have it low enough to keep noise and stability under control (<1 meg or so). But if it is too low (<50K), you lose gain (and tone).
Hope this helps.
Mark

[deafelectromark]

Jim P,
I have a question for you, I want to add a volume control to my V5 and what I was going to do was replace R27 with a 10K audio taper pot, and Radio Shack has said 10k audio taper pot. R27 is a 1/2 watt resistor, and the 10K pot is also 1/2 watt... one leg of the pot tied to R16, the other leg tied to gnd. and the wiper tied to what used to be the junction of R16, R27 and pin 2 of the 12AX7/R7. Would this work OK? Are there better values for the pot and R16 that I could use for better control?

Thanks.

See post #16 for my mod for a starting point. Jim P takes it to the moon and back in 20 pages if you really want to get into it, but you can make this a great amp with NO added parts and very little work.
Mark

[DeanEVO_Dude]

Hi,
The best place for a volume control is between the two tube sections. The wattage is irrelevant as minimal power is dissipated at these low currents. A 10K control is probably too low for a high impedance circuit, I would use at least a 250K control to not load down the preceding stage. Look back at the earlier posts and schematics and see that what you need to do is reduce the voltage going into the grid of the second half of the 12AX7 from the plate of the first half. The only 'trick' is to have it low enough to keep noise and stability under control (<1 meg or so). But if it is too low (<50K), you lose gain (and tone).
Hope this helps.
Mark

Thats where R27 is, between the two sections of the 12AX7 and after the tone control (just like the Palo. 508). The only reason I chose the 10K pot value is because R27 is a 10K value. I just noticed that the Palomino uses a 10K linear taper...
If I use, say, 100K pot, should I change the value of R15? I know that the circuit is a voltage divider, but I ain't so good with theory, so...
My intent is to keep the "gain" control on the 2nd stage of the op amp and add a volume control to the circuit (put it where the power led is and move the led above the switch). So far, with the speaker change (Ragin Cajun) I like the sound of the amp, but it is just too loud (almost never move the "volume" past 9 o'clock). I want to be able to put the "gain" up to 1 or 2 o'clock and still be able to be tolerated by the others in the house... LOL
I am also going to go back and re-read alot of this thread...

Thanks.

[jim p]

It sounds like you are going to keep the exiting volume control as a gain control and then add a volume control after the first triode stage? As stated above in another post I would go with a 100k audio to a 250k audio pot for the volume control. Remove R15 and R27 connect one side of the pot to ground the wiper to what was the junction of R27 and R15 and the top of the pot to where R15 connected to R29. The reason the 508 has 10k there was to maintain levels for the line out that amplifier had.

If you want you can remove R7 also. If you need more bass change C24 to 50nf to 100nf but for overdrive you don't want a lot of bass going into the overdriven tube so stock value may be good for what you are doing.
You could also go all tube by moving the gain control to where R15 and R27 is and adding a master volume at R16 look around it has been covered in previous posts.

[DeanEVO_Dude]

It sounds like you are going to keep the exiting volume control as a gain control and then add a volume control after the first triode stage? As stated above in another post I would go with a 100k audio to a 250k audio pot for the volume control. Remove R15 and R27 connect one side of the pot to ground the wiper to what was the junction of R27 and R15 and the top of the pot to where R15 connected to R29. The reason the 508 has 10k there was to maintain levels for the line out that amplifier had.

If you want you can remove R7 also. You may want to change the value of C1 to 220nf the overdriven tube will sound better with less bass and reducing C1 will do that.

Yes, that is exactly my intent... To clarify, I want to put the wiper (onward towards R7 and pin 2 to gnd at minimum volume, right? So, counter-clockwise ties wiper to ground and clockwise (max volume) ties the input of R15 straight on to the tube (pin 2). Thanks.
What is the impact of removing R7? Will it change the tone (frequency respons)? or just the voltage going into the 2nd stage of the 12AX7 (level/volume)?

[jim p]

Yes how you are going to connect the pot is correct removing R7 will give you a bit more signal to the control grid it would matter more if you had a master volume at the R16 location for this mod not much difference.

[DeanEVO_Dude]

Yes how you are going to connect the pot is correct removing R7 will give you a bit more signal to the control grid it would matter more if you had a master volume at the R16 location for this mod not much difference.

Excellent! I have a 500K auido taper for "just such an occasion."

To all who have posted replies, thank you very much!

[jim p]

Warm or cold bias? Being you are going to over drive the tube you may want to change how the first triode stage is biased. If you look through the posts you should find a link to a web page that goes through tube bias and tone. In short using a high value cathode resistor from approx 5k to 7.5k is cold biased with the tube near cutoff leading to a crunchy rock tone. Warm biased is when you use a low value approx 750 ohms so the tube is near saturation for more of a blues tone. If you have enough tube stages being overdriven sometimes the setup is 1st stage warm 2nd stage cold or the reverse. Using the op amp you will only have one tube overdriven with the volume control between the tubes.

PS: Was thinking about the warm and cold bias points a bit more and on the down side if you do this to the first tube in the amplifier it may limit your clean headroom. With the op amp because you can get less than unity (1) gain it may not be a problem. But if a tube by itself was the first stage the signal from the guitar could be great enough to cause saturation or cutoff. So with tube only it may pay to play with the bias point if you have a master volume pot connected to the EL84 and just on the second triode.