Crate V5 mods... anyone?

[Tele-Dave]

Jim, please excuse all of my questions, I'm quite new at this. Want to do the mods, but I want to understand everything that is being done so I will learn something, instead of just replacing things like a robot.
Thnx again,-----Dave

[jim p]

If you keep the volume pot as a gain pot you can leave C27 installed, it will just limit the maximum frequency the op amp will amplify at full gain to 13 kHz. Also may prevent possible oscillation of the op amp.

[Tele-Dave]

If you keep the volume pot as a gain pot you can leave C27 installed, it will just limit the maximum frequency the op amp will amplify at full gain to 13 kHz. Also may prevent possible oscillation of the op amp.

Thnx, Thats just what I was wondering.

So, if I just leave the existing pot in place, leave C27 in place, And not put the 10k in series with the pot, I can get max gain with good breakup because I'm not limiting gain on second stage of op amp. Without the 10k in series would probably be too much and go into clipping, but all I have to do is not crank gain up so much so as to control it, correct. I just want to limit amount of changes if necessary

What would the effect be if I just leave C26 and R28 in place also? Too much bass injection, Treble limiting into second stage of op amp?

Thnx again

[jim p]

Let me explain how the gain of an op amp works for an inverting amplifier. The 2nd stage op amp TL074-B is an inverting amplifier with the gain equal to the feedback resistor (CW1 the 250k pot) divided by the input resistor R14. So with the pot set at 250k the gain is equal to 250k divided by 27k equal to -9.3 if you figure that a strummed cord is 1 V rms (2.8 V p to p) the output will be 26 V p to p just within the supply rails of the op amp ( +15/-15). The minimum gain if you do not put a resistor in series with the 250k pot is 0/27k ohms or zero that is why I recommended to put say 10k ohms (27k would give you unity -1) in series with the pot so you wont have to guess where to set the gain pot if you don’t want to use it. Where are going to put the volume pot on the chassis with this mod?

Regarding the value of R17 I retested the amplifier after adding the reverb looking at the cathode current vs voltage across the EL84 found the cathode current to high giving 14 watts on the plate of the tube so I backed off that value and wound up with a slight tweak to the stock value. It looks that the stock value is not so bad. What I do have now is a 150 pf cap in parallel with R7 (I might try to lower this value to 100-50pf) the reverb required this to prevent feedback and this may have cured the oscillation problem I saw before.

[Tele-Dave]

Jim, I'm going to set up the volume pot just like you have shown it on the schematic. Will mount pot on other side of power switch towards edge of panel. Am going to use shielded wire since it's a long run.

O.K., understand about the op-amp now. (Just learned something-Thnx) Will place a 10k in series with pot.

Will leave value of R17 alone for now.

Am very interested in the Pentode/Triode switch setup

Going to order parts from Mouser tonight for the mods.

Went out today and bought another V5 at Guitar Center to use as a "Control" so I can see how much of a difference this makes. GC had them on sale for $85, plus I had a 10% off coupon, so I got it for $77. Very good price!!

Thnx-----Dave

[jim p]

Looking at the front panel I would consider putting the volume pot where the power indicator lamp is. Then put the power indicator above the power switch. You might be able to get a Knob from Crate tell them one of yours is broken or buy it from them. I don't think shielded cable is necessary for the volume pot you can just twist the ground wire around the other two wires this should be sufficient.
I bought another amp from MF told them Best Buy had them for 85 with in store pick up so they matched the price and shipped for free I saved the sales tax.
You can see the pentode/triode switch in the last pictures I posted on the back panel and inside the cabinet in the picture that shows the reverb vector board.

[Tele-Dave]

Looking at the front panel I would consider putting the volume pot where the power indicator lamp is. Then put the power indicator above the power switch. You might be able to get a Knob from Crate tell them one of yours is broken or buy it from them. I don't think shielded cable is necessary for the volume pot you can just twist the ground wire around the other two wires this should be sufficient.
I bought another amp from MF told them Best Buy had them for 85 with in store pick up so they matched the price and shipped for free I saved the sales tax.
You can see the pentode/triode switch in the last pictures I posted on the back panel and inside the cabinet in the picture that shows the reverb vector board.

Great suggstion on pot and light locations. Wasn't thinking about chassis in cabinet. See there is no room for pot with chassis bolted into cab. Glad you caught me on this. Would of drilled holes and then knob would of interfered with cab.--------Thnx

Will call Loud Tech today and see about knobbbbbbs.

Cant distinguish hookup locations for Pent/Tri switch on pic. Cant see wiring too good. Looks like you lifted leg on R18 and wired DPST switch between R18 and Pin 7 on power tube Can you do a simple drawing/diagram showing install points for hookup and switch wiring for Pent/Tri?

Also what is in R23. My board has nothing in it there. Is that a different value used for Pent/Tri setup???? Dont know too much about Pent/Tri Differences. Will google for info so I will understand it better than only that it changes power output of tube.

Jim, you don't know how much I appreciate all your help here. Thnx much!!


----------------Dave

[Tele-Dave]

Just got off of phone with Loud Tech (Crate)

Pots are $1.50 (Volume--- Part designator CW-1)
Knobs are $1.50

Surprisingly they had them in stock!

Ordered 2 of each so will have extra incase I want to mod other V5 I have.

[jim p]

For the pentode/triode switch I lifted R18 where it connects to the HV supply (J4) and hooked this side of the resistor to the wiper of the switch. Connected one contact of the switch to HV supply (plated hole resistor was connected to) this is the pentode connection. The other contact to the plate of the EL84 pin7 for the triode connection. I still want to check operating current and voltages of the tube in triode mode to see that maximum ratings are not exceeded, forgot to do that the last time I had amp opened up. Along with the drop in output power to approx 1/3 that of pentode mode the brightness of the amp also drops. You would need to lower the impedance of the stage driving the EL84 to over come the Miller capacitance to not have this drop in brightness I am going to go as is for now but in future may try a JFET or op amp between the 12AX7 and the EL84. A pentode has little to no Miller effect cannot get a good number on what the capacitance from plate to control grid is on an EL84 as a triode.

With the miller effect in mind you may want to limit the high frequency range of the first stage tube that you will be over driving with the op amp so I suggest putting a 20 to 33pf cap (300 volts minimum rating) from the plate (pin 6) to the grid (pin 7) of the 12AX7 to roll the high frequency off.

That is as great price for the knobs and pots from Crate did they charge shipping?

I called them for the schematic for the VC508 and the V5 is a variation on the 508 but as I said before done the wrong way and with bad values for a 10 inch speaker.

Oh I forgot that other resistor that you see installed, the board has a location for a resistor in parallel with the 1 H choke I installed a 1k ohm resistor there so a transit would not cause a slight oscillation also if the choke opened it is a little safer.

[Tele-Dave]

When in triode mode, how dark does it get? It isnt too bad, is it? Darker than amp was before tone mods? It is liveable, isn't it?
Going to do Pent/Tri mod anyway. When you come up with a fix, let me know.

Have to dig thru parts box for cap for Plate/Grid. Will try it if I can find one.
Poly cap? Correct?


Was better price for pots and knobs than I thought it would be. Shipping $6

Forgot to ask for schem for 508. Will call them back in few minutes.

As per R23, circuit looks like 1/2 watt or larger would be o.k., Correct?

Question, I want to put a standby switch in also. Would it be that all I would have to do is to install switch to break power at J1 and J2? Correct?


Thanx Jim----------------Dave

[jim p]

Because of the 1 H choke in the power supply you cannot just put a switch in the HV power supply and expect it to last. In a mechanical analogy a choke is like a flywheel it wants to maintain a constant rate it resists change at start up (building magnetic field) and if interrupted (clasping magnetic field). A choke is what is used to generate a spark on a spark plug in a gas engine. If you put the switch at pin 2 J6 (C15 side) when you open the switch the voltage on the choke will go positive to the point of arcing across the contacts in the switch and at some point the switch will fail. If you put the switch on the other side of L1 (J1 pin1) when you open the switch the voltage on the inductor will go negative but this side will work if you put a reverse biased rectifier diode to ground that can handle a minimum of 150mA and reverse voltage of 400 volts (1N4007 should work). Better is to put diode with a low value resistor in series (50 ohms 1 w) in parallel with the choke (cathode on pin 1 side) it will be forward biased when the choke goes negative with the switch opening. Also consider a bleeder resistor on the C15 side for safety.
Me I don’t believe in standby switches so much I don’t think cathode striping happens at these voltages and currents plus the amp is quiet.
If R23 is the one in parallel with the choke on the PCB 1 to 2 k 1 watt is fine.
For the plate to grid cap minimum 300 volt rating mica or poly is fine.

[Tele-Dave]

[QUOTE=jim p]If you put the switch at pin 2 J6 (C15 side) ------If you put the switch on the other side of L1 (J1 pin1) when you open the switch the voltage on the inductor will go negative
QUOTE]

Was talking about putting switch in leads from power trans to where it connects to J1 and J2 on edge of PCB. Just inserting a DPDT to break legs going to PCB and that way I will also have power to supply Indicator light when in standby. Just going to drop out 240V going to Plates
Shouldn't that be just like turning off main power switch as far as choke is concerned?
Was going to interupt both legs of power prior to going to PCB. Connectors are marked J1 and J2 on PCB, and on schematic it is prior to diode bridge and C14/R24 filter and with R24 at ground, should prevent problems that you describe with choke. Is that right or am I missing something????

[jim p]

Putting the switch on the AC side should be ok and avoids dealing with the choke.

[Tele-Dave]

Putting the switch on the AC side should be ok and avoids dealing with the choke.

That was my concern, and why I thought of puting it there. On some other amp schematics I was checking for switch placement, most of them placed switch on transformer secondary leads prior to any other electronics and if there was a choke farther downstream, and i noticed on some schematics that did not have chokes that they put the switch anywhere downstream prior to feeding plates.
Know about collapsing field in coils, that is why didnt want to break either side of coil, and thought that with R24 biased to ground, would bleed off pulse when field collapses. Should be just like turning off main power switch as far as coil is concerned, or that is what I thought.

Thnx Jim, just wanted to verify. Don't know too much about electronics, but was trained in automotive mechanics when there was Points, Plugs and condensers, (and coils) And understand about a collapsing field in a coil of wire producing high voltage surge.

Called Loud Tech again today 3 times, but message says "Our personel are in Training presently, please call back later." I will keep trying to get thru, would like to have copy of schem just so I can compare to V5.

[jim p]

Cathode bias with triode mode needs a higher value of cathode bias resistor vs. the pentode mode. On the amplifier I have modified have found that the stock value of 330 ohms is fine for either modes this results in a cathode current of 38mA in either pentode or triode mode. The result is a maximum plate dissipation of 11.3 watts in the triode mode 10.3 watts in pentode mode.
Driving the EL84 with a signal generator have calculated the capacitance from plate to control grid to be approx 2.2pf in triode mode with the Miller effect this becomes approx 30pf this in parallel with the 220pf has a slight effect on the high frequency roll off of the EL84 (20KHz in pentode mode 19KHz in triode mode) Some of the major changes between the two modes is output power 5 watts pentode mode and 1.5 watts in triode mode. Also plate impedance 38k ohms approx for the pentode mode 2k ohms in the triode mode the result the amplifier is more dampened in the triode mode closer to a solid state amplifier sound then in the pentode mode.

[Tomko]

Ok I now have a tube amp with a new/better speaker. This thing sounds great distorted but I can't get any clean headroom out of it. Can anyone help me? The op-amp circuit is gone. The tone circuit is gone replaced with a Marshall 18 watt tone circuit. The input resistor was changed from 1.5K to 47K. The resistor into the input of the power section was changed from 4.7K to 47K. I'm using a 12AY7 instead of a 12AX7. Still no clean headroom. I think all the distortion is power amp distortion. Can anyone give me some voltages to look for at the pins of the EL84? The bias is stock on the EL84 and I changed it to a Tesla tube. Any ideas?

[letter109]

My first post here folks. . .just wnted to say thanks to Tarin for the hi-res schematic of the V5. I tried calling the Loud Technology toll-free number but they're still out on training or something. . .been that way for several weeks now, eh? Anyway, I got the schem from this group and is greatly appreciated. I have three of those little V5s sitting around here, and I actually like 'em OK out of the box. But I also agree with deafelectro that this may be one of the next big things in little amps, and a great beginning place for a heck of a mod. So, if I have success with anything (or even failure) I'll try to let you all know how it came out. Thanks for a great group. I'm glad to be a member of it. -M

[jim p]

The most important voltage on the EL84 would be the cathode voltage to calculate the cathode current. With the 330 ohm resistor this should be approx 14 volts max equal to 42mA. Ballpark you would then have 38mA plate current and 4mA screen grid current. Not sure the 12AY7 may be problem free by the data sheets it is calling for a higher plate current (3mA) and a lower value grid resistor 100k ohms (may have high grid leakage current vs. 12AX7). I also noted on one data sheet a higher heater current so check to see that you have 12.6 volts across the heater. What are the cathode, grid and plate voltages for both locations of this triode? Have you tried the Sovetek tubes that were stock in the amplifier?

[jim p]

Looking further into the data sheets for the 12AX7 and the 12AY7 based on the plate impedance and the plate current vs. plate voltage you need to change the cathode bias resistors when using a 12AY7. The 12AY7 requires 4 to 3 volts at the cathode relative to ground so a resistor value of 2500 to 3300 ohms should be used for both R3 and R8. With R3 and R8 changed you should have approx 3.5 volts at the cathode and with no signal a DC voltage of approx 170 volts at the plate of each triode (pin 6 and pin 1) hope this helps.