Crate V5 mods... anyone?

[jim p]

The attached schematic is to make the first stage of the amplifier a JFET gain stage for overdrive. It has a minimum gain of two and a maximum gain of 10 to 20 depending on the gain of the FET installed. So if you have or are modifying the amplifier for preamp overdrive by using a master volume pot between the second triode and the output tube this will give you more gain to maybe get closer to the sound you want. I would also add a high level of brightness to the amplifier to go with this. Let me know if you need ideas on how to do that and I will post them. Also if more gain is needed the positive supply for this circuit could be increased by using a zener diode and series dropping resistor allowing for maximum gains of 30 to 60 and giving more head room for the JFET.

These changes are for an amplifier that has had the op amp removed from the amplifier.

[timothymegg]

Hey all you guys who know stuff about these things that I don't!

What tubes have you guys tried with good results for replacements to get a better sound?

I'm looking at Ruby Select tubes and JJ tubes right now so, if anyone has a recommendation with their experience on what they have done to change the sound then I am all ears.

I have already modded the amp and, the last stage seems to upgrade the tubes. First step was changing the speaker. Second, I bypassed the op amp. Third, I increased treble response in the tone circuit. Now the final step of getting some quality tubes in there.

Peace

[Rampant]

Hey all you guys who know stuff about these things that I don't!

What tubes have you guys tried with good results for replacements to get a better sound?

I'm looking at Ruby Select tubes and JJ tubes right now so, if anyone has a recommendation with their experience on what they have done to change the sound then I am all ears.

I have already modded the amp and, the last stage seems to upgrade the tubes. First step was changing the speaker. Second, I bypassed the op amp. Third, I increased treble response in the tone circuit. Now the final step of getting some quality tubes in there.

Peace

Hi matey

The valves were the first things I changed when I got mine!

I'm running a Mesa Boogie EL84 tube and a Mesa Boogie 12AT7 at the moment. I noticed an immediate improvement in tonal quality when I changed the EL84... not the night and day difference that changing the speaker makes, but a definite improvement nonetheless. Note that I'm using an alternative 12AT7 and not the standard 12AX7 - which gives a slightly lower output, but my practice room is a small box-room and has a massive mirrored wardrobe on 1 side - so I don't need the oomph. Even then I only normally play on volume 1 to 2. That's un-modified, of course, apart from the Jensen speaker...

HTH

Cheerz

Rampant

[jim p]

I don’t have an opinion on which is the best tube or tonal qualities of one vs. another. But one thing to look for is it really a different tube. In a lot of cases such as Grove tube or Mesa the tube is a Sovetek it may have been tested to meet there standards but the construction is the same. Electro-Harmonics and JJ tubes are at least a different construction then a tested and relabeled Sovetek so that is something to look for. If you search on this site on the subject of tubes you will find a survey on favorites.

[tunghaichuan***]

I don’t have an opinion on which is the best tube or tonal qualities of one vs. another. But one thing to look for is it really a different tube. In a lot of cases such as Grove tube or Mesa the tube is a Sovetek it may have been tested to meet there standards but the construction is the same. Electro-Harmonics and JJ tubes are at least a different construction then a tested and relabeled Sovetek so that is something to look for. If you search on this site on the subject of tubes you will find a survey on favorites.

Have to agree with Jim here. Tubes are only made in a few places these days. There are two versions of the Sovtek EL84, a Slovak version made by JJ, a Chinese version and maybe another made by EH, although it could be a tested/rebranded Sovtek. EL84s are cheap enough to buy one of each and let your own ears be your guide.

tung

[stoneattic]

jim_p,

I finally got around to starting the tone, master volume and VVR mods this evening. I started by swapping C1 with a 220nf cap and then did the master volume. At that point I put it back together to test it. The tone mod really helped and made the tone pot much more useful.

The result of the master volume is kind of strange. 0 to about 4 acts as expected, but from 4 to 6 the volume doesn't increase, but actually drops slightly. From 6 to 9 the volume increases but after 9 the volume drops quickly to nothing.

i believe I have everything wired correctly per your schematic and the pot is a 1meg, audio tapper, R31 is pulled and R16 was replaced with a 1meg.

Any thoughts?

thanks!

[jim p]

The solution would be to add a resistor in series with the control grid of the EL84 or a minimum value of resistance in series with the master volume pot. The idea of the master volume pot is to use the existing volume pot as a gain pot to overdrive the second triode for preamp overdrive then use the master volume pot to limit the signal to the output tube. So you would normally not turn the master volume pot up past ½ to ¾. If you want to allow for the master volume pot to be turned to maximum you will need to add a resistor in series with the pot to the coupling cap C7 say 50k to 100k ohms to set a limit. The way it is now if you turn the pot to maximum on the positive going signal to the control grid of the EL84 if the amplititude is high enough you will get grid current from the EL84 that will look like a low impedance to the 12AX7 driving it and it will not be able to supply that through R6. But as I said you should limit the master volume to ¾ max and use the gain pot to increase the volume if that is not enough.

As you change the volume the load on the 12AX7 is changing at the high volume settings this reduces the gain of the 12AX7 and the loss of signal when you set the master volume pot at a high level.

So basically you need to supply enough signal to the master volume pot by where you set the gain pot so it has something to work with. Every pot in the amplifier tone, gain and master volume are killing signal when you turn them down and what the master volume pot has to work with is dependent on what is before it that is the tone and gain pot settings.

To give you a better idea of what I am talking about check out this article.
http://www.tgpwebzine.com/?page_id=808

[jim p]

Some people have stated that the Crate V5 is a little boxy sound wise. In an early post I showed a beam blocker to reduce this and cut down on the ice pick tone from the speaker. Since then I have run across this article http://www.tgpwebzine.com/?page_id=424 and this looks like it may make for a better solution. They talk about reflections of the sound back to the speaker with a beam blocker which is something I thought might be a problem when I tried it. What is neat is this creates the effect in a way of a dual cone speaker by blocking the outer part of the cone from emitting sound at higher frequencies. The higher frequency signal from a 10 inch speaker are not as pure a tone as the lower frequency sounds. By physics from what I have found in other articles the frequency range of a speaker is relative to the cone diameter divided into the speed of sound and a factor of ten greater in frequency from that point. So for a 10 inch speaker this is 13584 inches/sec divided by 10 inches for 1358 Hz to 138 Hz so all those frequencies above 1358 Hz are less and less pure in tone. If you look at the response curve for 10 inch guitar speaker you can see some of this in the peaking of the curve at higher frequencies.

Anyway food for thought and maybe something you may want to try out.

[SciHi]

Any thoughts?

I posted this earlier: http://www.thefret.net/showpost.php?...&postcount=261
Maybe a litttle more conventional SE master volume and it worked great. I know Jim has had the discussion earlier about the grid resistor before or after the pot (or grid load), but after the pot is a little more conventional. Does it make any difference? I don't really know but the 250K pot with 6.8k grid resistor, post pot, sure added a lot to my amp. I did a half dozen other mods that didn't last as long as it took to put them on, but this master volume on my v5 is a real keeper.

[stoneattic]

jim_p, I tried the 100k resistor in series with the pot as you show on the schematic but have essentially the same issue. The control seems a little better but it still drops to nothing above 9 or so and there's still the dead spot/drop though the middle of the sweep.

SciHi, on your schematic there are a few other mods. Is the only thing relevant to the MV mod the pot and resistor shown in blue?

[jim p]

Looking at the markup I posted and doing some calculation if you still have C3 installed remove it. As you turn the pot up the capacitor could be working against you.
Also it may pay to check the pot with an ohmmeter sometimes they go bad where either the wiper looses contact to the resistive element or they wrap around and connect to the lower side which is ground in how this pot is wired.

The more I think about this the more I think your problem may be the pot is bad. I do think the 220p cap can work against you so it may be best to remove it. But as far as grid current that should not happen until full volume out of the amplifier so I think I was wrong on my earlier post. It is probably a good idea to add a series resistor with the pot but I thought I would make the mod as simple as possible. As far as how I marked up the first schematic it is the same as the master volume mod for the Valve Jr over at the Sewatt web site with exception of pot value which is 250k on the Vj mod. But higher value pot will give you greater gain from the 12AX7 driving the EL84 so that is why I suggested 500k to 1 Meg. As far as a grid resistor the pot is a resistor in series with the grid and connected to ground so you could add one but no real need.

[SciHi]

SciHi, on your schematic there are a few other mods. Is the only thing relevant to the MV mod the pot and resistor shown in blue?

That is correct. The other mods are for other reasons...

[stoneattic]

jim_p, I pulled C3 but the only difference it made is that now the volume change from 0-4 is minimal, and the volume change from 6-9 is more dramatic. It still drops to nothing from 9-10 and there's still the dip in volume from 4-6, but it's not quite as noticeable, which could just be because the overall volume is lower at 4 now.

[jim p]

jim_p, I pulled C3 but the only difference it made is that now the volume change from 0-4 is minimal, and the volume change from 6-9 is more dramatic. It still drops to nothing from 9-10 and there's still the dip in volume from 4-6, but it's not quite as noticeable, which could just be because the overall volume is lower at 4 now.

Have you tried checking the pot with an ohmmeter? The overall circuit is pretty basic and as far as others posted main difference is the value of the pot used. Just to check R16 the 220k resistor has been removed, yes? If not it should be. Another thing I noticed is that for some reason R6 in the schematic looks like it is 10k but it should be the stock value of 100k just in case you thought that was a change you needed to make. Have no idea how the screen shot made that come out that way. So if you ohmmeter the wiper of the pot to ground with power off of course should be 100k at the mid point to 1 Meg ohm at full clockwise position and should be the opposite if you test the C7 100k ohm resistor side 1 meg at full counter clockwise to zero ohms at full clockwise. Hope this helps.

[stoneattic]

Have you tried checking the pot with an ohmmeter? The overall circuit is pretty basic and as far as others posted main difference is the value of the pot used. Just to check R16 the 220k resistor has been removed, yes? If not it should be. Another thing I noticed is that for some reason R6 in the schematic looks like it is 10k but it should be the stock value of 100k just in case you thought that was a change you needed to make. Have no idea how the screen shot made that come out that way. So if you ohmmeter the wiper of the pot to ground with power off of course should be 100k at the mid point to 1 Meg ohm at full clockwise position and should be the opposite if you test the C7 100k ohm resistor side 1 meg at full counter clockwise to zero ohms at full clockwise. Hope this helps.

And the big d'oh, take his soldering iron away, award goes to me.
I had the screen grid and ground connections reversed on the pot. :

Thanks again for all your help and patience. I'm thrilled with this little living room practice amp right now. For want I plan on using this for it's perfect. I still have the VVR to add to it but honestly if I had done the MV first I may not have bothered ordering the VVR but since it's sitting here I'm going to (try to) install it.

[jim p]

Glad you have the master volume all sorted out and it is working. Figured there should be no problem but have not done the mod myself and I have been fooled by electronics before. Up side tubes take more abuse then transistors and FETs so they are more forgiving when you make a mistake.

If you have not done the variable voltage regulator mod yet just have a question on supplied hardware. I think you got a mica washer with that to isolate the tab of the T0220 FET from the chassis but did they supply thermal compound, a shoulder washer, metal screw and nut. Over at Sewatt someone was asking about chassis mounting and they were talking about a nylon screw and nut. Nylon is not good because it expands with heat and from thermal cycling and can loosen so you loose thermal contact also FET could move and short. Thermal compound will make for better thermal contact of the FET to the heat sink so it helps to use it for best thermal connection. Anyway over at Sewatt he posted that he got the shoulder washer from someplace so you could look over there to find where if you need one.

[stoneattic]

Glad you have the master volume all sorted out and it is working. Figured there should be no problem but have not done the mod myself and I have been fooled before by electronics before. Up side tubes take more abuse then transistors and FETs so they are more forgiving when you make a mistake.

If you have not done the variable voltage regulator mod yet just have a question on supplied hardware. I think you got a mica washer with that to isolate the tab of the T0220 FET from the chassis but did they supply thermal compound, a shoulder washer, metal screw and nut. Over at Sewatt someone was asking about chassis mounting and they were talking about a nylon screw and nut. Nylon is not good because it expands with heat and from thermal cycling and can loosen so you loose thermal contact also FET could move and short. Thermal compound will make for better thermal contact of the FET to the heat sink so it helps to use it for best thermal connection. Anyway over at Sewatt he posted that he got the shoulder washer from someplace so you could look over there to find where if you need one.

You are correct about the mica washer, but there was no compound or fastener hardware. I had planned on using compound though. But I'm really glad you mentioned using a shoulder washer. I would have just used a standard screw. When I saw the mica washer I realized that it needed to be electrically isolated but (mistakenly I guess) assumed that the hole was isolated. I have to dig though my random electronics hardware to see if I have an appropriate shoulder washer.

Due to the layout/location of where I plan to mount the VVR it doesn't look like I can mount the FET directly to the chassis. (I put the MV in place of the power indicator light and plan to mount the VVR in place of the on/off switch. I think relocating the on/off switch and light would look cleaner.) I had plan on mounting the VVR to a pretty large (relatively speaking) aluminum heat sink I have laying around which would be mounted to the chassis. Do you see any problem with this? The VVR is good for 50W so I don't think it's going to generate too much heat with this little 5W amp. Plus the heat sink is large in relation to the heat sink. I can take a pic of the sink I plan to use with the FET if you think it would help.

[jim p]

As far as the heat sink you will require as a guess figure you have 360 volts on one side of the FET and you dial the source of the FET to say 120 volts so you would have 240 volts across the FET. Figure the current that the tube is operating at will drop to say 25mA so the FET will have 240volts at 25mA for approx 6 watts. The maximum temperature the FET can operate at is 150 degrees C figure the amp is running hot at 40 C so can allow for a 110 degree C rise for the FET so you need a heat sink that can do 110C/6 watts or 18C/ watt or lower. So if you know the spec for the heat sink you are going to use or can find it or one that looks like it that should give you a good idea if what you have will work (look at heat sinks at Mouser electronics or Digi-Key). As I said the actual power on the FET is a guess. Looks like a TO 220 FET can do maybe 2 watts at 20c ambient temperature without a heat sink power greater then that will require a heat sink. Also air flow helps and you will have this in a box so you want to derate things a bit for that. I also left out the junction to case loss for the FET but looks like it should be just around 5 C so?

Probably best to get everything working run the amp for awhile and see how hot the heat sink gets before you button it up.

[jim p]

I was just surfing around and found Musicians Friend has the Crate V5 at 99 bucks with 10 bucks for shipping. I thought they stopped making these amps?
http://guitars.musiciansfriend.com/p...Amp?sku=487050

I am starting to think this posting needs an index to help anyone who is checking it out for the first time. Or even looking for something they thought they saw.