Modifications for Crate Vintage Club 20

[oldmacman]

I finished tweaking this little amp last week and it sounds really good. I figured I'd share what I did in case someone else has one of these and wants some ideas.

These are cool little amps with good (made in Chicago) transformers, an Eminence speaker, and a stout cabinet. I picked mine up for $100 on craigslist — it was originally listed for $160, but the seller offered it to me for $100 after I played it and found a couple of minor issues.

The designer, Obeid Khan (who I wouldn’t be surprised to see hanging out here), seems to like old Vox amps, and this is designed in that vein: lots of chimey highs, EL84 power tubes cathode biased hot, and no global NFB. However, the tone stack is Fender with some changed values and an added mid boost, and the phase inverter is cathodyne, very similar to a Princeton Reverb. In addition, there are three cascaded gain stages before the tone stack, probably to tick off the design requirement that the amp be capable of high gain at low volume. Stock, it’s good sounding, and it has a cool and unique vibe. My goal with this was to get the amp closer to a blackface Princeton, with usable cleans and not as much gain.

Stuff you can do:

— Simple tweaks. The preamp gain stages are all biased hot (100K/1K); you can alternate hot-cold (e.g. 100K/1K followed by 100K/1K8 or 100K/2K2) to get a more balanced (less overtly warm) sound. The .001u capacitors bypassing two of the plate resistors can be lowered in value to extend the highs a bit, and the 470p and .001u capacitors bypassing the resistor dividers in the first couple of gain stages can also be lowered in value if you like an amp that’s less trebly/Voxey. You can lower the 100K resistors in the cathodyne PI to 56K a la Princeton Reverb; you really don’t need the extra swing with EL84s. You can also change R31 to 1K5 for a more centered bias for the gain stage in the cathodyne PI. Add some aluminum foil tape across the back panel to improve the shielding. Add a 68K grid stopper resistor on the first 12AX7 to guard against picking up RF (replacing the jumper wire that’s there in the stock amp).

— Revoice the tonestack. I replaced the 220p cap with a 270p cap and the 33k slope resistor with a 150k resistor, which brings the response more in line with a Fender or a Dumble.

— Increase the values of the cathode bias capacitors in the preamp. I went up to 22u throughout. One of the common complaints about this amp is that it’s too boxy sounding, and some people attribute that to the small cabinet for the 12” speaker, but that’s not really the case. The .68u bypass capacitor on the first preamp stage takes out a lot of bass and leads to that sound.

— Add a grid stopper resistor to the cathodyne phase inverter. Per Valve Wizard, this guards against bad-sounding frequency doubling distortion, and since the stage works at unity gain (no Miller capacitance), the frequency response of the stage won’t change to any appreciable degree. Use a 470K resistor.

— Add some negative feedback. Look at the schematic for a Princeton Reverb and modify the common-cathode gain stage before the cathodyne PI to accept feedback through a 47 ohm resistor below the bypassed cathode bias resistor. Then run the feedback resistor from J9 (output transformer secondary) to the 47 ohm resistor. The stock Princeton Reverb feedback resistor is 2K7 with an 8 ohm speaker; multiply that by sqrt(2) to get the value for our 16 ohm speaker to find that a 3K9 resistor will provide the same amount of feedback.

— Reconfigure the gain stages. With negative feedback added, the drive to the PI isn’t enough to drive the power tubes all the way. We can rearrange the gain stages of the amp to have a stage after the tonestack, which will fix this problem at the expense of being able to have high gain (3 cascaded gain stages) at low volume. To do this, we remove C29 and R9, cut two traces (as shown on the schematic), and add three jumper wires (also as shown on the schematic). After this, the topology of the amp looks like this:

Gain stage (V1A) -> Gain control -> Gain stage (V2A) -> Tonestack -> Level control -> Gain stage (V2B) -> reverb mix -> PI (gain stage and splitter)

The gain and level controls continue to work as expected.

[Stevem]

A common reoccurring problem with all tube Crate amps are the solder connections failing on the tube sockets with enough heat up and cool down sessions, the only way to fix it right is to suck out all the factory solder and reflow the tube pins with a leaded solder! There circuit boards are so dam thin that the expansion and contraction rate is huge!

[johnnyreece]

A common reoccurring problem with all tube Crate amps are the solder connections failing on the tube sockets with enough heat up and cool down sessions, the only way to fix it right is to suck out all the factory solder and reflow the tube pins with a leaded solder! There circuit boards are so dam thin that the expansion and contraction rate is huge!

Yep. My cousin had one that I redid socket connections on a couple times. He got tired of messing with it, so I gutted it. It's now a Rocket.

[oldmacman]

A common reoccurring problem with all tube Crate amps are the solder connections failing on the tube sockets with enough heat up and cool down sessions, the only way to fix it right is to suck out all the factory solder and reflow the tube pins with a leaded solder! There circuit boards are so dam thin that the expansion and contraction rate is huge!

At least some of the heat issues must be due to how hot they run the EL84s -- they're biased at ~95% of dissipation at idle, and the cabinet isn't very well ventilated. The control panel gets really hot after the amp has been on for a while.

[johnnyreece]

Not to mention the high plate voltages.

[Jags]

I just joined this forum as I'm trying to improve the tone on my Crate VC20 and found this great post. I'm really not needing to do all the mods that oldmacman presented here but I am trying to get more bass out of this little amp. Compared to my Fender Princeton Reverb this little Crate has no bottom end at all.

Question: If the only change I do is replace C1 to 22u will it improve the bass? Do I also need to change C5 and C7 to 22u? What volt rating should the 22u be?

I'm a real novice at all of this. Usually just mess with the electronics in my guitars. This is my first attempt at doing mods in an amp.

Any help will be greatly appreciated. Thanks!

[martin manning]

Welcome! I'd start with increasing the value of the cathode cap on the first stage as you propose, and then the others to see if you need it. The nice thing about trying a big increase in a cap value is that you can just parallel the new larger one across the old one, or across R3 in this case. You can solder it to the pads on the bottom of the board, watching the polarity of course. This is a low voltage area, so a 25V cap is fine, but don't let it touch any other solder joints. Adding the 68k resistor on the input grid is something else to try, since that will take out a bit of the extreme high end.

[roberto]

If you do that, you could like the second stage without bypass and snubber.

[Leo_Gnardo]

Question: If the only change I do is replace C1 to 22u will it improve the bass? Do I also need to change C5 and C7 to 22u? What volt rating should the 22u be?!

Some mods I make to all Vintage Club Crates (unless the owner prefers nails-on-chalkboard highs) is to remove brightening cap across the volume control, or "peaking" cap as on the VC20, that's C3. With the hi end tamed a bit, it brings out the mids and lows with little further effort.

I'd be careful not to overdo C1 value. Try just a couple uF here, 2 or 3, maybe 4.7 or 5 uF should be enough depending on your guitar, speaker & ear.

Also to dial back those aching baking EL84's I'd go 160 or 180 ohm cathode resistor R22, also remember you can select replacement EL84s with low Pc (emission) ratings from Antique & Ruby & I'm sure a couple more suppliers. JJ has been making good tough EL84s, and EH's EL84M are pretty good too. Get some fresh output tubes in your gem, bias dialed down to sane levels, & I'll bet you like it a lot more.

[Jags]

Thanks everyone for your help and input! What I have already done to tame the high gain is to replace V1 with a NOS RCA 12AU7 long black plate. I also replaced V2 & V5 with NOS black plate GE 12AT7's. This has helped a lot with the high gain harshness the amp originally had. V3 & V4 EL-84's have been replaced with NOS GE tubes I had laying around that my dad brought home from NASA when he was working on the Apollo project back in the 60's. I also replaced the speaker with a MOD 10-50.

Right now the amp sounds pretty good. Definitely more mid rangy compared to my Fender Princeton Reverb but I like the difference. Only problem is the bottom end. It has no balls!

I just bought a few 22u caps off ebay and they should be here in a few days. I'm here in Hawaii and there are no local sources for electronic parts. I'll try the 22u in C1 and see what it does. I also like Leo's suggestion of changing the R22 cathode resistor to keep the EL84's from cooking. Luckily I do have big box of resistors that my Dad collected so at least I have resistors available to experiment with.

Thanks to everyone once again and please keep the suggestions coming!

[Jags]

OK, My caps came in and I went ahead and did what Martin suggested and soldered the 22u cap across the R3 resistor. That did make a small change in bringing out a bit of the bottom end. I went ahead and did the same with the other 2 stages by soldering the 22u cap across R6 and R12. That helped a bit more. It was pretty easy to solder them across the resistors and I didn't have to disassemble the entire circuit board to do it. I then went ahead and cut the C3 "bright" cap as Leo suggested. That helped to tame the edginess.

The amp definitely sounds a lot better now. As oldmacman said the 22u caps remove the "boxy" sound the amp had. It almost had a transistor radio sound to it before. Now it is a lot more sonically rounded and balanced sounding. Closer to the Fender Princeton Reverb but still with it's own more midrangy character to it. A very useful tone for rock and blues as compared to the Princeton that is great for jazz.

I'm sure the different non spec tubes and the change in speaker are contributing to the overall tonal change also. It's pretty much given me what I was looking for. It transformed it from a Screaming Banshee into a nicely behaved Tone Monster with a nice distinct character.

I would still like to change out R22 as Leo suggested but I don't have a 5volt 160 or 180 ohm resistor. Will that change lower the output of the amp significantly? Right now I get a good loud clean bedroom volume with the gain at 5 and the level at 10. I get a nice dirty overdrive at the same loud bedroom volume with the gain at 10 and the volume at 5. I don't really want to make this amp any less loud so I worry what that R22 resistor change would do to the overall output.

One other thing I may try is to put a 12AY7 into the V1 position as that should give me a little more boost compared to the 12AU7 I have in there now. I just can't seem to find a nice GE Black Plate 6072/12AY7 5 star at a decent price out there to try.

Any other comments or suggestion would be appreciated and I again thank you all for your help.

[pstephan]

Sorry to resurrect an old thread, but I want to do the same mods. Can you update how this turned out for you and what you finally did and how it sounded?

[akuster777]

Thanks, oldmacman, for the mod suggestions!

I picked up a used Crate Vintage Club 20 (without reverb) a year ago from Craigslist. It badly needed new power tubes from the get go, so I put in a new set of JJs.

This is really the perfect amp to mod the preamp section and listen to how each mod changes the resulting overdrive or distortion sound. It uses two 12AX7 tubes configured for three gain stages and a cathodyne phase inverter. And it’s not difficult to disconnect the components from the underside of the board, but be careful disconnecting the caps with slightly fish-hooked wires under the board so you don’t pull up the traces from the board. Even if you do, if you’re careful not to tear them, you’ll still be able to connect to them.

I played around a bit with a few configurations, and I found something that offered options for medium-gain overdrive for blues or rock, higher gain without fizz, and also some very nice cleans.

I really learned about how each component works together looking at this amp in conjunction with my copy of Merlin Blencowe's book Designing Tube Preamps for Guitar and Bass. The preamp in the Crate VC20 is almost exactly the same schematic as in his Medium-Gain Preamp Design on p. 256.

Here's how I modded my amp:

PREAMP VOICING CHANGES:

JM1 change input jumper to 68k resistor

preamp plate resistors:
R2 leave, or upgrade to 100k dale (V1A plate resistor)
R7 change from 221k to 100k dale (V1B plate resistor)
R11 leave, or upgrade to 100k dale (V2A plate resistor)

preamp cathodes:
R3 change from 1K to 1K5 (V1A cathode bias resistor)
C1 leave, or upgrade from .68uf tant to .68uf 50v electrolytic (V1A cathode bypass cap)
R6 leave 2K2 (V1B cathode bias resistor, or add OPTIONAL SWITCH 1, below)
C5 remove 2.2uf (V1B cathode bypass cap)
R12 change from 1K to 820R (V2A cathode bias resistor)
C7 remove 2.2uf (or add OPTIONAL SWITCH 2, below)

voltage divider between V1B and V2A:
(if you decide to use a 10K bias resistor in R6 for V1B cold clipper, then leave these at 475K to match Marshall 2204)
R9 change from 475K to 680K
R10 change from 475K to 300K

phase inverter:
R16 upgrade 1M grid leak to correctly wire to C12 and new 470K grid stopper (see photo)
R15 change 100k anode load to 56k
R18 change 100k cathode load to 56k

power section:
R22 change 120R 5W to 150R 5W (cathode bias resistor)

BETTER QUALITY COUPLING CAPS

C2 upgrade 10nf (coupling cap between V1A and V1B)
C6 upgrade 10nf (coupling cap between V1B and V2A)
C12 upgrade 10nf (coupling cap between V2A and V2B PI, in conjunction with 470k grid stopper and R16 1M grid leak—see photo)
C13 upgrade 22nf (coupling cap between V2B PI and V3)
C14 upgrade 22nf (coupling cap between V2B and V4)

BETTER QUALITY SILVER MICA CAPS for keeping treble less fizzy

C3 upgrade 470pf (bright cap)
C9 change 220pf to 470pf (treble tone stack cap)
C28 change 1nf to 100pf (V1A plate bypass cap)
C29 upgrade 470pf (treble peaker, or remove this cap if sound is too bright for you)

REPLACE OLD FILTER CAPS

C4 10uf 450v
C8 10uf 450v
C16 47uf 450v
C19 100uf 450v

OPTIONAL SWITCH 1 (V1B cathode bias resistor and bypass cap):

R6 change 2K2 resistor to three-way on-off-on switch with three resistor choices: 2K7, 10K, and 1K8 with a 22uf 50v bypass cap

Inspired by Rob Robinette (https://robrobinette.com/Voicing_an_Amp ... d_Clipper_), this three-way switch gives three options:
1) 2K2 stock cool bias (which offers the best balance both clean and overdriven)
2) 10K cold clipper (like Marshall 2204, a more contained distortion, but quieter, good with an OD pedal)
3) 1K5 center bias with bypass cap on (sweet cleans, but farts out with gain over 5)

OPTIONAL SWITCH 2 (V2A cathode bypass cap):

C7 change 2.2uf cap to two-way on-on switch with 22uf 50v cap

This two-way switch gives two options:
1) unbypassed, tighter distortion
2) bypass cap on (acts like a boost, sweeter cleans, and also nice for high gain solos but splattier with chords)

RANGE OF TONES:

1) Quite a wide range of nice tones from clean to overdrive over the whole range of the gain knob, with optional switch 1 set to 2K2 and switch 2 set to unbypassed
2) Very good clean tones with gain set at 5 or less, optional switch 1 set to 1K5 with bypass cap on, optional switch 2 set either way for more articulate or wetter cleans
3) Some fun higher gain lead tones with gain knob on 9 or 10, optional switch 1 set to 2K2, and optional switch 2 set to bypass cap on

The Crate VC20 has been great fun to mod and to play.

Best wishes to all,

Andrew

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[samrock55]

I just bought one of these for 149.00 in excellent shape I think I will Dumble tweak it. Move the tonestack eq and add the switches? Anyone have any suggestions on how easy? I wonder how little it would take to tweek it?