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Modding the Crate V33H

So I had been itching for a new amp. I already have a few but everyone knows that you can never have enough amplification.

I had heard clips of the Crate V30 and Palomino V32 and thought they sounded pretty good. I also read a lot of comments on various sites about how great they were. So then I saw this guy selling the Crate V33H on ebay. There were not many reviews of it available but I thought it would be close to the V30 and V32. I wasn't sure about buying a Crate amp but what the heck...

When I first tried the amp the clean channel was OK. Actually it was really nice. However, I already have amps with a good clean sound, I wanted some gain.

This is where the amp was a let down. The overdrive channel was very dark and muddy.  Now this is just my opinion. If you have one of these and you love the sound, that's great. It just wasn't my thing.  I messed around with the tone stack and the presence and boost buttons but I could not get it to sound the way I wanted.

I seriously considered sending it back but then, about ten minutes later, the thing was taken apart on my bench so I could try my hand at modding.

There is not a lot of info on the web about these amps. I couldn't really find anything. I called up Crate and the woman I spoke to was very nice and sent me a schematic. You might be able to find one on the web if you hunt around but Crate can be reached here.

The first thing I did was compare it to the Crate Palomino V32 schematic that is at Blue Guitar. They are similar in their general layout but they also have their differences.

Anyway, I spent a bunch of time looking at schematics for inspiration, especially those of other 4 EL84 amps such as the Peavey Classis 30, Palomino V32, AC30 and the Trainwreck Liverpool 30 (I can dream can't I?) and came up with these mods.

 


Standard Tube Amp Warning

Tube amps can hold dangerously high voltages even after they are turned off and unplugged. If you are not familiar with the safety precautions for working on tube amplifiers do not try modifying one. Changing the tone of an amp is not worth your life. See here and here for more information.

Also, by doing modifications you will void your warranty and run the risk of breaking the amp altogether. The author assumes no responsibility for the use of any information contained in this document.

 

The first thing I wanted to do was make the overdrive channel brighter and the easiest way to do this was to use a "bright cap".

So I put a 300pf cap on the gain pot and that did brighten it but later I found better ways to brighten it up and the bright cap came off.

With these mods I was going for a brighter, vintage sort of tone. I've take the amp apart I don't know how may times and tried out all sorts of caps and resistors. To my ears, I believe below is the best combination I've heard and far removed from the dark and muddy tones that came with the amp.

I suspect there are more than a few V33 owners who are not happy with the overdrive channel so I'm sharing what I came up with in case anyone else finds it useful. I didn't cut any traces so if you don't like something it can be changed back.

Here is everything I did...

Preamp

  • Change R77 to 470k from 2.2m
  • Change R4 to 2.2m from 470k
  • Add 56k resistor in series to R17, the slope resistor (33k+56k=89k)
  • Change R75 to 100k (1 watt)
  • Change C4 to .0047uF
  • Replace C90 with a 47pF mica cap
  • Change R78 to 1.5k
  • Bypass R78 with a 2.2uF cap

Phase Inverter

  • Added 330ohm resister in series with R28

Output

  • Change C61 from 100uF to 220uF (100v)
  • Change R94 from 82ohm to 75ohm (10 watt)

 

Some explanation....

  • Swap the resistors in R4 and R77. Changing R4 to 2.2m puts the gain channel more in line with the signal path of clean channel where there is also a 2.2m resistor. This really helps the overall sound and I think this, along with R77, is the key to getting this amp's OD sounding right. Changing R77 to 470k from 2.2m will actually lower the gain. Adding the bypass cap to R78 will bring back a lot of the gain. I think the second gain stage was getting slammed and that caused a lot of the mud. The R4/R77 swap also helps the amp's dynamics and you don't even need to buy any parts.
  • Changing the slope resistor (R17) takes the amp a bit more in the Vox direction and cuts the bass. For comparison the slope resistor in the Liverpool, AC30 and lots of Fender amps is 100k while the Peavey Classic 30 has a 68K slope resistor. The V33H has a 33k slope resistor. A lower value slope resistor sends more signal to the bass and mids where a higher value emphasizes the treble. The higher value also makes the tone controls more effective so you'll have a better chance of dialing in a good sound. In the V33H one end of the slope resistor is under one of the tube sockets so that's why I put another resistor in series to up the value.
  • Remove C2. C2 is a 50pF cap that works as a bright cap (high pass filter) across R6 which is the 2.2m resistor in the clean channel. After changing the slope resistor the clean channel was sounding a bit thin. Removing C2 restores some beef to the clean channel.
  • The previously mentioned resistor swap lowers the gain so I did some gain stage tweaks to add some more ummph. I ended up changing R78, the cathode resistor, to 1.5K and bypassing it with a 2.2uFcap. This boosts the highs and mids. This mod will give you a nice increase in gain. I had a 22uF for a while but I finally decided that there was too much bass. There are a lot of options here and it's really up to your personal preferences. And when I say bypass I mean the cap is on top of the resistor with one end soldered to each leg. Again, it's not especially pretty but when working on a PCB that was not designed to have that cap you gotta make due.
  • R75 is the plate resistor and 100K cuts down on the fizz in the overdrive as opposed to the stock 220K though the 220K will produce more gain. If you do the 1.5k cathode resistor and 2.2uF bypass cap you should do this one too. Otherwise the OD might get farty and/or might reintroduce some mud.
  • I changed C4 from .001 to .0047uF to thicken the tone a bit. I thought it was sounding thin with the .001uF in there after the other changes.
  • I had to change C90 to a mica cap of the same value. Maybe it's just me but there should be a law against ceramic caps.
  • I did a slight bias tweak at the phase inverter. I added a 330ohm resistor in series at R28 that I think helps the sound of the PI clipping. The amp came with a 470ohm resistor but the AC30, Palomino, Liverpool, etc all had either a 1k or 1.2k resistor here which is the classic Vox value. However I've found in other amps that a 1.2k here can produce some fizzy overdrive. If I had a Palomino I would probably try changing this. I sort of split the difference and ended up with 800ohms. That's about the value you see in 18 watters. If you want a bit more of a Marshall crunch you might just leave the 470ohm alone there but I think a slightly higher value adds better bass definition. This is another location where the resistor is under a tube socket so I ender up putting another resistor in series. One end is sort of accessible.
  • I made some changes to the bias at C61 and R94. The 220uF cathode bias bypass cap at C61 is like the value in the Palomino, AC30, Liverpool and probably others. I think this gives the amp a little fatter bottom end. These amps might go lower than 75ohm for the cathode resistor (R94) but I was worried about running the EL84s too hot. There's definitely no red plating now. Anyway, I think these changes really did improve the tone. You could do the preamp mods first and skip these if you're worried about tube life.

High Gain Mod:

I decided to try to get a little more gain out of the amp and came up with this mod. I also wanted to get some drive from the clean channel where it can be controlled through picking dynamics. Here is what I came up with…

  • Change R14 to 470k
  • Added a 2.2k resistor parallel to R15 (this works out to about 1.2k)
  • Change C10 to .0047uF
  • Remove C3
  • Change C2 to 500pf mica (the 500pF cap is actually soldered on top of R6)

It sounds really good. If you want more gain give this a try. The C2 and C3 changes are optional clean channel tweaks. The high gain mod is an addition to the other mods listed above. (I had a more elaborate mod posted here before but decided I liked this simpler one better. This is the mod in the clip.)

 

These mods probably also apply to the V33 combo but I don't have one so I can't be sure. The fact that the main PCB and the OT both have V33-212 written on them is probably a fair indication that they would work.

 


 

At this point the gain is not too dark anymore and the tone stack is actually useful on the overdrive channel. Stock, the clean channel and the overdrive channel were radically different. Now they actually sound like they belong in the same amp. I might still go back in and tweak but I'm going to try it like this for a while. With humbuckers I think it sounds best with the gain at about 2 or 3 o'clock and the master at least at 9 o'clock. With single coils I like sort of the opposite, turn the master up then start adding gain. This gets closer to a Vox type chime.

I've swapped out all the 12AX7s because, IMHO, the stock sovteks are terrible. I'm still experimenting with different tubes but found this that I think has some good advice. Probably the biggest change was from using a much lower gain 12AU7 as the phase inverter (the preamp tube farthest from the OT). This really changes the amp. First and most notably the amp is not as loud any more. The output seems like 33 watts now, not 100 and it gives the amp a more vintage vibe. I highly recommend trying a lower gain phase inverter tube.

I'm running it through a pair of Jensen Mod 12-50's in a Lopo Line 2x12 convertible cabinet now. The speakers still need to be broken in a bit but for the price these speakers are an awesome deal. I think they sound great.

So overall I'm happy with the amp now and think it was a good purchase. There's a lot of sonic goodness in the amp waiting to get out. Also, the V33H is a rewarding amp to mod because with a few key changes you can make a huge difference in the tone of the overdrive channel. So far I've probably had more fun modding it than playing it.

 

Regards,

DvNator

Clips:

Here's a clip to give you an idea of the difference in the overdrive channel. This is a strat with a mini JB humbucker in the bridge. It's just an SM57 a few inches from the Jensen Mod 12-50 and no pedals or anything, just a bit of the amp's reverb.

Clip1

Here's a clip with the high gain mod. It's an old Ibanez with an older Gibson T-bucker in the bridge and the Jensen Mod 12-50s. It's just the guitar into the amp and I added some reverb on the computer.

Clip2

 

Also, these tube sockets are not so great IMHO. Be extra carefull when swapping tubes.

Click here for a schematic.

Note:I'm going to be moving my website here soon.

 

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