Crate V5 mods... anyone?

[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.