Crate V5 mods... anyone?

[jim p]

The screen grid resistor is attached to the wrong location on the PCB it should be on the other side of R25 (junction R25 R26 and C16). This will lower the screen grid voltage closer to (right at) its maximum rating of 300 volts. Best if you have the pcb out is to also replace R25 with a 7.5 to 10 k ohm resistor (approx 4ma across R25).

[jim p]

Another way to modify the amp would be to use the tone pot for a volume pot and the volume pot for a gain pot. This involves no cuts just jumpers and removing components with one being replaced (C24)
1) Remove PCB from chassis
2) Remove R13 C26 C24( to replace with 47nf for more bass or keep as is if you wish) C6 C5 C4 and R27
3) Replace C24 with a 47nf
4) Jumper junction of C6 C5 and CW2 (tone pot) to junction of R27 and R15
That’s it now the volume pot will control the gain of 2nd stage op amp for overdrive and the tone pot will control the volume.

[Big K]

Outstanding information!!!! But it makes my head hurt and tempts me to switch from coffee to vodka this morning with all the electrical theory.. I thought being able to solder effectively enough to swap out switches, pots and pickups made me feel like a genius when it all works but you amp modder guys are on a whole nother level : : : :
Thanks for all the shared information!

[tunghaichuan***]

Just as a heads up, I got the new flyer from Guitar Center with the V5 priced at $79. My flyer came with a 10% off coupon, so the V5 is a pretty good deal from GC right now. :

tung

[jim p]

What to do with the stock V5 speaker? Now I hope you aren’t going to sell it to someone so why not use it as a testing load for the amplifier. I put one of those cheep ¼ inch cables on it packed some fiberglass around it and put it in the box the amp came in. So now instead of just looking at the output with a resistor as a load I have a speaker instead. There is quite a difference in the waveforms on the scope between the resistor and the speaker. With a square wave you can see overshoot with the speaker you do not see with a resistor. Also you won’t drive everyone crazy (cats, dogs and neighbors)

[jim p]

Has anyone made the supply a choke input yet, just want to know exact voltage they are getting at C15+. I may want to make the same mod on one of my amps. This would also free up Junction of C16 R26 and R25 as the decoupling for the second triode. Also if the voltage is low enough can leave the screen grid connection stock.
Found with the volume control after the tone stack at minimum you still get signal at the output of the amp. I think this is due to not having separate decoupling on both the first and second triode. So being able to move R6 to the other side of R26 would cure this as long as it is not used for the screen grid.

[goonrick]

Has anyone made the supply a choke input yet, just want to know exact voltage they are getting at C15+. I may want to make the same mod on one of my amps. This would also free up Junction of C16 R26 and R25 as the decoupling for the second triode. Also if the voltage is low enough can leave the screen grid connection stock.
Found with the volume control after the tone stack at minimum you still get signal at the output of the amp. I think this is due to not having separate decoupling on both the first and second triode. So being able to move R6 to the other side of R26 would cure this as long as it is not used for the screen grid.

Haven't done it yet, as I'm waiting on some components to ship. I'll be sure to post what voltages I get when I do though.

[goonrick]

Jim,

I did the choke input mod and am getting 296V at C15 plus. Is that about what we were shooting for?

[jim p]

Around 295 volts sounds right it should have been close to peak with the capacitor input (240 rms x 1.4 = 335 volts). With the choke that would drop to 335 volts X 0.89 approx 295 volts.
Do you have both tube operating? Should check to see if choke is running hot at all. I ran a simulation and the power in the choke is higher then the stock Pi filter but should just run warm to touch?
Were you looking to lower the voltage below what you now have?

[timothymegg]

I just got this little guy. The speaker sounds pretty bad but, I hooked it up to a 412 cab and it sounds much better. I would like to do some of the mods posted on here but, I can't decide between deafguy and jp? Jp is more defined but, the deafguy mod seems easier and more class A without an opamp. Any comments? Also, anyone have any experience with adding a triode switch and how to do it? One last thing, does this amp need to be biased?
Peace,
Timothy

[jim p]

Haven’t made this mod myself but have simulated it with vacuum tube models and a jfet model. If you go without the op amp the tone control is just a treble cut no boost. The op amp was the treble boost (pre emphasis). This circuit has C501 a 1nf across a 330k ohm resistor that gives you a boost when the tone pot is at full treble. You can change the operating point by changing the values of both C501 and C502 keep the ratio at 1 to1.5 ( 1nf and 1.5nf) or maybe 1to1 (1nf and 1nf) I would stay in the range of 1nf lowest value used 3-4nf highest value Lower value equal to higher frequency operating point , higher value moves it towards the bass end.

[jim p]

The op amp used in this amplifier is in almost every stomp and effects box made. It is cheep and the specifications are fine, this stuff is AM radio frequency response realm 5 kHz max (modern op-amps go into the gigahertz range). The input impedance is higher then the 12AX7 so it will not rob the high frequencies from you pickups if you are going straight in. Once you put a box between the amp and the guitar you are not all tube. With the circuit as is or with the changes I have posted you also have a cap in series to help cut out the 60Hz that the guitar is picking up( also you can reduce bass if you want). The op amp also provides the treble boost for the tone control to work as both treble cut and boost. I have kept the gain of the op amp to a minimum so with overdrive the first stage tube will be clipping not the op amp.

But still some will prefer to have no solid state components in there signal path so I have posted a tone stack to use with the op amp removed.
Remember the people that first designed guitar amplifiers in some cases were TV and radio repairmen so it is not an exact science and nothing to get hung up about. But what they did created the sound we have come to love. Wonder how things would be if the transistor existed before the tube?

PS If you look at the schematic for the Fender Hot Rod Deluxe this op amp is used in four locations Line out, Line in, reverb drive (poor choice for that) and reverb recovery. Line out and line in are in the signal path.

[rock_mumbles]

I would like to do some of the mods posted on here but, I can't decide between deafguy and jp? Jp is more defined but, the deafguy mod seems easier and more class A without an opamp. Any comments? Also, anyone have any experience with adding a triode switch and how to do it? One last thing, does this amp need to be biased?
Peace,
Timothy

Hello Timothy,
The bottom line the amp will sound like a "real" tube amp when converted, stock they sound like ... (you know). Take your time and study the V5 layout and the mods and pick one. The "all tube" mod will be simpler and you will end up with a simple two tube guitar amp, if you do jim p's mod you will have more stages (and in effect more sound/tone manipulation) in the preamp.

I've just started to do an "all tube" mod to my V5 with a simple tone stack and volume control where the stock tone stack is placed. I want to do it without cutting any traces, by removing components and soldering new connections onto existing component pads (it could be converted later to something different if I wanted to change it). When I get done I intend to post a color coded "mod" layout that shows the parts to remove and how to connect the new components.

EDIT: I changed the above wording, because I think people could take my intention in the original statement in a wrong way, Sorry guys I should have been more careful with what I posted.

As far as your question regarding biasing, what I think you are asking is if the amp needs to be have the bias adjusted when the output tube is changed, the answer is no. The V5 is a cathode biased SE (single ended) amp (as are most small two tube amps) and as such does not have an adjustable bias, so you just pull one tube out and plug in another. That does not mean that the bias is correct out of the box, one of the "correction" mods for Epiphone Valve Juniors is to replace the EL84 cathode resistor with a larger value so the tube is not pushed quite so hard.

[goonrick]

I ended up doing an amalgam between the two mod types and am pretty happy with the results. I bypassed the op-amp stage and made a typical two tube amp. I also followed jp's recommendations on tweaking the output section and power supply and the amp is running stably and sounding damn good.

I would say the all tube mod gives you less to work with in the way of preamp tones, but it does the screaming tube amp thing very well when driven with an overdrive pedal. I was shooting for a small amp that would do an acceptable job of copping a big amp tone and I got it. The one downside, and this may be inherent to all amps of this size, is that there's not a lot of clean headroom and so you're not really going to get inspiring clean tones from the amp. That's okay with me because I've got other amps that do the clean thing very, very well (Lab Series L5, DI's, etc.).

I set out from the beginning to have an amp that would conjure some class A tube mojo in a small and relatively low volume package. I'd say I got it. I will also really enjoy capturing the overdriven tones on recordings. It sounds nothing short of excellent for what it is.

BTW, I ended up switching a bunch of tubes (12AX7s, 12AT7, etc.) and settled on the 12DW7. It's an AX front triode and an AU second triode. You can get a currently manufactured DW at eurotubes and I highly recommend it. It brought out the best of my lil' killer.

[jim p]

Was asking if the choke was getting warm when amp is converted to a choke input filter. Also what is the amplifier noise like? The amp I have with stock Pi filter is so quiet you don't know it is on.
Regarding headroom the amp I modified is fine did change 12AX7 to an Electro-Harmonix from the stock Sovtek. You can see by looking at the EH that it is not just a relabeled Sovetek.
What speakers have you played the amp through?

[goonrick]

Was asking if the choke was getting warm when amp is converted to a choke input filter. Also what is the amplifier noise like? The amp I have with stock Pi filter is so quiet you don't know it is on.
Regarding headroom the amp I modified is fine did change 12AX7 to an Electro-Harmonix from the stock Sovtek. You can see by looking at the EH that it is not just a relabeled Sovetek.
What speakers have you played the amp through?

Haven't checked on the choke. The amp has a low level 60 Hz hum (no buzz though) that it had since the moment I first plugged it in. It really is so low by comparison of the guitar signal that I'm not worried about it. From looking at the schematic, it appears that they used AC voltage on the power tube filament, and I'm betting this is the source of the noise. No hum reducing resistors to ground or anything. I looked the PC board over and it is too much trouble to change, so I'm not going to worry about it.

I'll pull the amp chassis from the cabinet and operate it at full volume for a while and check the choke temperature and report back.

BTW, I've plugged it into a 1 X 12" cabinet loaded with a Jensen Mod 12-70 and it sounds fantastic. I swapped the internal crate crap 8-30 for a Laney HH Invader speaker and a huge improvement there as well. It gets a nice, boxy tone which I expect from the type of enclosure it is. I simply love it.

[goonrick]

OK. After about 10 straight minutes of high gain playing, the choke is cool to the touch. I started out with the multimeter and temp probe ready, but since it's not even warm to the touch I skipped actually taking a reading. I'd say the choke input mod is stable.

[jim p]

OK. After about 10 straight minutes of high gain playing, the choke is cool to the touch. I started out with the multimeter and temp probe ready, but since it's not even warm to the touch I skipped actually taking a reading. I'd say the choke input mod is stable.

When I ran a circuit simulation it gave me 5 watts on the choke but I have been fooled by circuit simulations before. The ripple was higher in the choke input simulation also so the amp noise maybe higher from that. The amp I changed to a 6V6GT has no noticeable noise the other one may have a bad filter cap so it is hard to tell. For head room you could try some feedback on the triode stages say 27 to 40 pf (300 volt) from plate to grid. Not that I am totally pushing op amps but the one stage was inverting so you could go less then unity (X 1) if that was in the amp. Also you could do a resistor capacitor feedback on the input triode and really drop gain but would need the op amp as an input buffer for that.

[jim p]

For the speaker on the two V5 amps I have modified I have used Weber speakers in both the price is great and I think Weber designs speakers with tubes in mind. There is a lot more interaction between amp and speaker with a tube amp especially a single ended output type like this one. If you think of the speaker as walking a dog this type of amp is like having a six year old walk an Irish setter the dog will call the shots in some cases. While a push pull tube amp will move up to being a 12 year old walking the dog and a solid state amp is giving the job to a linebacker. A tube amp has high output impedance so the parasitic components of the speaker inductance and capacitance (also the transformers inductance and capacitance) have a greater effect on the output sound. You can see this if you have an oscilloscope and signal generator looking at the output with a speaker for the load vs. a resistor. The Ted Weber web site has a Q and A section that has a lot of good information there are also sound clips. One more thing they have is an amp load that is a speaker motor the microMass this would be a better way of overdriving the amp vs. a resistor and it has a line out and tone control.
If you have yet to buy am amp MF has Weber speakers so you could get both at the same time and save shipping.