Crate V18: Mods and Conversions

[edge11]

Originally Posted by steeve_a

Good question! Would that be in front of the tone stack or the first gain stage? I guess you mean the tone stack since the one on the real SLO is fed by the cathode from the FX loop return. I have seen similar circuits for FX loops and they are supposed to be very transparent.

In the 5F6A bassman and early Marshall and Vox top boost designs there is a DC coupled pair with cathode follower that feeds the tone stack. This is strictly my own opinion but I believe that it is the DC coupling (no coupling cap between the two stages) that is the important factor, and with that design you need to take the signal from the cathode.

However, adding a cathode follower before the normal input stage can have a big effect on the sound, thickening it up and giving it some oomph! Mesa Boogie among others has used that on a switch in some of their amps.

Hopefully someone will experiment on both of these lines and post their results on The Gear Page.

Thanks for bringing this up!

Steve Ahola

yes i meant infront of the tonestack, from what i can gather, it will tame the gain a bit, but make the tone controls a bit more responsive, right?

[rock_mumbles]

According to Merlin B (Valve Wizard) there's a couple of reasons for the cathode follower, it is low impedance so there's less tone-stack loss, the other benefit is the asymmetric clipping which adds to the preamp's creamy overdrive characteristics

[steeve_a]

Originally Posted by rock_mumbles

According to Merlin B (Valve Wizard) there's a couple of reasons for the cathode follower, it is low impedance so there's less tone-stack loss, the other benefit is the asymmetric clipping which adds to the preamp's creamy overdrive characteristics

So would inserting an FET cathode follower ahead of the tone stack add those qualities to something like the SLO modded V18? Adding a single stage would reverse the polarity of the signal going into the tone stack. Some people say that polarity can be important as the signal goes through an amp circuit so a second stage to invert the signal might make it even better.

I have never really thought much about a cathode follower except as part of a DC coupled pair; what is the gain through the gain follower stage? I seem to remember that it will increase the current but not the voltage of the audio signal...

Steve Ahola

P.S. In my tweed bassman modded Peavey C50 I replaced the two OD gain stages with a DC coupled pair/cathode follower. But that signal goes to another tube stage which feeds the tone stack from the anode. So how would that fit into the scheme of things...

BTW several people have done that mod and made some suggestions that I will try on my own amp. The design was not quite ready for prime time- some of the pots did not respond very smoothly- you either had too little or too much...

"Shake and shake / the catsup bottle
first none'll come / and then a lot'll."

... from Richard Armour who also gave us:

"Nothing attracts / the mustard from wieners
as much as the slacks / just back from the cleaners."

.

[edge11]

Originally Posted by steeve_a

So would inserting an FET cathode follower ahead of the tone stack add those qualities to something like the SLO modded V18? Adding a single stage would reverse the polarity of the signal going into the tone stack. Some people say that polarity can be important as the signal goes through an amp circuit so a second stage to invert the signal might make it even better.

I have never really thought much about a cathode follower except as part of a DC coupled pair; what is the gain through the gain follower stage? I seem to remember that it will increase the current but not the voltage of the audio signal...

Steve Ahola

P.S. In my tweed bassman modded Peavey C50 I replaced the two OD gain stages with a DC coupled pair/cathode follower. But that signal goes to another tube stage which feeds the tone stack from the anode. So how would that fit into the scheme of things...

BTW several people have done that mod and made some suggestions that I will try on my own amp. The design was not quite ready for prime time- some of the pots did not respond very smoothly- you either had too little or too much...

"Shake and shake / the catsup bottle
first none'll come / and then a lot'll."

... from Richard Armour who also gave us:

"Nothing attracts / the mustard from wieners
as much as the slacks / just back from the cleaners."

.

Sounds good steve.
Btw did any one do the dumble,trainwreck, ecstacy mods?
i seem to recall someone on ppwatt wanted to do the dumble mod.
I was thinking about buying another v33 to do the xtc mod, so how does it sound?

[scihibmxer@juno]

Originally Posted by scihibmxer@juno

...I will take it to practice on Tuesday see how it works with the v18.

Tried out the bridged T attenuator tonight at practice, it still got pretty warm even with the v18 but not bad. And it worked really well. I was able to run the master volume at 7 and crank the gain to around 3/4 and not be too loud. Also nice having the reverb back... Could still get the amp on sustain and build to feedback, but at a reasonable volume. Just added a little treble on the tone control to brighten the amp back up. Nice, cheap little option, versus a real $370 attenuator for a $150 amp. Rad Shack actually has 20 watt 8 ohm resistors, little spendy at $1.99 each, you get two 10 watt 10 ohm for the same price but 25% off if you buy 5 parts. I may try it out this weekend with the "matching" post OT line out to the PA system.

[TomDanMac]

Originally Posted by TomDanMac

And thank you! The immortal saying is that two heads are better than one...and yours made mine a bit better, I think/hope...because...

In your (?) corrected schematic for the V18, the board layout is clearly labeled for the V33 AND the V50 as well...use in the V33 implies more current/power capability only (2 more EL84s), but for the V50 and two 6L6 power tubes, my 480 volts from the extra transformer winding idea would almost certainly have to be correct. The board layout actually shows the necessary missing parts from the V18 schematic, in the lower-left corner of the board layout. The V18 would be missing D104, C107, and R111, that would allow the transformer's other winding, pins 4 and 17, to contribute the extra 160 volt capability. Other than a different transformer and/or switching power device (and maybe some other minor changes?), it is looking more and more like this SMPS can go to 50 watts.

And I am with you on the "old school" PS replacement plan as well (the SMPS struck me as strange in the first place)...it's just that IF this thing can do this with minimal manipulation it really opens some doors...hopefully without letting the smoke out of anything...

Thanks!

Well...it's been awhile, working more and otherwise busy...I have actually had a chance to A/B my V18 supply with that of a dead V33 that I was asked to troubleshoot...they were identical to my eye, even down to the same parts missing, and I found a shorted D105 in the anode supply section, no other damage apparent from the very basic troubleshooting I was able to do, so I robbed one of the UF2010 diodes from my own PS and used it in place of the shorted one, and ran the V33 for over an hour...I am using the formerly dead supply in my own V18 amp for burn-in purposes until I can source a part to finish the repair, and I gave it a good workout again yesterday...my gut sense is that these diodes should be overkill for this low wattage supply (for voltage and current at least; not sure about switching speed), in which case this bad one could be a random and rare thing, or maybe the QC wasn't so good on a batch or two...the secondary is usually the easy part on repairing a switching supply anyway; it's when the primary side gets hurt that it gets tough...but they can be repaired with a bit of luck...and of course having the schematic!

Having examined the PS board closely now, the missing parts in the lower left section appear to be for the higher voltage supply for the 6L6 tubes in the V50, and in the upper left they are for a -6.3 VDC supply. I still need to determine if a different switching device and/or transformer is required for 50 watt output from the power amp section...

And I think someone mentioned somewhere here that the V18 is a bad (?) adaptation of the V33, and I think that's somewhat true...the V33 amp I heard yesterday after I had the PS working again has a beautiful clean sound, even with the stock speakers (I'm not sure if the 16 ohm speakers are doped like the 8 ohm ones are?), and the gain channel is better than what I had with my V18 to start also...that was using the same guitar I used when I made the decision to buy it, an Austin Strat copy with SSH...

Fun stuff!

[TomDanMac]

Nice work!

Originally Posted by scihibmxer@juno

Tried out the bridged T attenuator tonight at practice, it still got pretty warm even with the v18 but not bad. And it worked really well. I was able to run the master volume at 7 and crank the gain to around 3/4 and not be too loud. Also nice having the reverb back... Could still get the amp on sustain and build to feedback, but at a reasonable volume. Just added a little treble on the tone control to brighten the amp back up. Nice, cheap little option, versus a real $370 attenuator for a $150 amp. Rad Shack actually has 20 watt 8 ohm resistors, little spendy at $1.99 each, you get two 10 watt 10 ohm for the same price but 25% off if you buy 5 parts. I may try it out this weekend with the "matching" post OT line out to the PA system.

[TomDanMac]

Originally Posted by TomDanMac

Hello, fellow modded V18 owners!!

Yes...I took mine home on Saturday. I didn't follow any particular plan, I did tweaks to address specific issues. I removed R6 and C10, shunted the mid-boost on the tone stack, did the Leger Marshall/Bassman trace cut, dropped the 120k PI resistor to about 88k with a 330k across it, and the 470R PI resistor was accessible in mine, so I replaced it with 1.1k. I put a 33k across the reverb tank output. All was done by tacking resistors in place and simply cutting components out when they weren't needed; I hope to take the board out later and do proper component placements when I have things the way I want them, but there are no problems so far. The sound really improved, even with the Crate speaker still in the amp. It is loud, and still has a lot of solid punch in the low end; I feel it on my legs through my jeans. My guitar is Franken-wired and I use the pickups in series a lot, so it has a strong signal like that, and with full bass and treble; with the V18 the parallel pickup settings are a little better, cleaner. I have problems with hands, eyes, and working with my head down, so it took a lot longer than average, I am sure, for the little bit that I did, but it was certainly worth the time and effort. It's a good start for sure.

A bit more to add...I think I added the 680 ohm resistor to the other end of the reverb tank, and went with a 2M resistor between V1a and V1b...I did reduce the V1b plate resistor, and then I put a 1uF shunt on the V1b cathode resistor, rolling off the bass a bit and getting back some of the gain that was lost...it likes the parallel single-coil aspect of my guitar best, as noted...I have read about a couple of zener diode modifications in the power section, one by Paul Ruby (on the power tube grids to get rid of some crossover distortion) and another by a guy who goes by Chuck H (provides fixed bias performance above a certain level of drive to the output tubes)...when I can get parts...I swapped out the speaker for the Bugera that my LX112 had when I bought it, and it has the right tone, especially having tried the Wizard...YOW!!...loud, and not the nicest sound, too much of a good thing the way the amp shapes the tone right now, the Bugera is much smoother, just not as loud...I tried a GT 12AU7 in V1, and it isn't what I want...a DW7 is probably too expensive, considering I have so many preamp tubes now, so I think a split load might be in order for an experiment, the komet/fast-gradual modification...most importantly, I have tried it at a local small venue for a blues jam, with and without a drummer, and the sound and feel make me happy!

Really glad for the help and inspiration, and thanks to all!!

[TomDanMac]

...being the master of the piggyback reply as always...

I have been using the calculators here...

http://pentodepress.com/home/amplifi...ode-capacitor/

This may have been mentioned by someone else here; I am not even sure...but I like seeing what is going on along with hearing, it's just the way I am, and this site has really helped me a lot...

[scihibmxer@juno]

Originally Posted by TomDanMac

Nice work!

The best part is it worked really well. So a short jumper cable from the speaker out on the amp to the post OT line out box, level about 50% on the level pot, fed that to a passive DI box and into the PA. Soundman said sounded great, and good signal level. Then plugged in the attenuator in the other parallel speaker out jack on Line out box, then speakers to from the attenuator. With this setup, basically the same overdrive tone I hear on stage is fed to the PA, not the normal preamp only feed. My guess the volume on stage is less than a 5 watt amp, with the attenuator, just enough for me, and not so much as to overpower the stage. I already had all the parts, but even buying everything new probably around $20 for parts for both units.

[djsadowsk]

hi, i've been reading this thread for a while and am intrigued by all the mods you guys are able to do to this amp. i have one of the 212's that i am planning to do the SLO mod to. one of my main problems with this amp is it's insane loudness and while i was reading i saw you guys talking about the bridged T attenuator. to my understanding they are like the volume control boxes used so u can crank the amp and get the full tube sound without blowing your eardrums? and if so i saw in your pic a pot and in all the schematics there was no pots. i am a total noob at this so i guess what im asking is if you can post the schematic for what you made with the altoids box, and preferably in noob language. thanks for the awesome thread you have going.

[scihibmxer@juno]

Originally Posted by scihibmxer@juno

This is not exactly what I was talking about earlier as far as an attenuator but just finished this for a line out on a "Suprolious" valve junior, haven't tried it on the v18 but it might just do the trick. This is a line out that takes the signal from the output transformer. I know that "Undercool" added one to the internal of his "Z18," not sure the exact values he used. But they are great because you get the EL84 overdrive tone that you miss if you use the stock preamp line out. I built mine with 4 10 watt resistors parallel 2 hooked up in series. It gets warm with 5 watts but should be able to handle the v18, I have made dummy loads for testing before the v18 before out of 4, they get hot but didn't burn out. So you can totally dime the amp, run signal out to another amp, mixer-PA, etc. and adjust the level on the box, as well as on the main power amp. I hooked mine up to the FX return on a SS amp. Worked great. If you put the speaker in for the dummy load it will be just as loud again...



I built it in a mint tin.

OK, there are two different projects listed here, the one with the pot is the post OT line out. It also has large resistors that totally cut the speaker out of the circuit. The other altoids box, without the pot, is the biridged T attenuator. The bridge T is a fixed gain reduction. You can get all the parts for the bridged T at Rad Shck. You need to make one, then adjust the values some to get the volume you want.

So I used the Post OT line out to send signal to the PA, the pot is to adjust the level of signal that is sent to the PA (throught a passive DI box). The bridge T attenuation I put speaker wires and male - female jacks so you just insert it between the amp and the speaker. I showed it with 2 R2s and 2 R1s because Rad Shark didn't have 10 watt 5 ohm or 10 watt 16 ohm.

[TomDanMac]

Originally Posted by TomDanMac

Having examined the PS board closely now, the missing parts in the lower left section appear to be for the higher voltage supply for the 6L6 tubes in the V50, and in the upper left they are for a -6.3 VDC supply. I still need to determine if a different switching device and/or transformer is required for 50 watt output from the power amp section...

Well...I was both wrong and write...or rong and right... :-)

The negative PS on that board is for -63 VDC, but my guess about the +400ish VDC for the 6L6 tubes was on the money...some discussion and a link to the V50 PS schematic here...

http://music-electronics-forum.com/t16500/

So at least one other UF2010 power diode has failed in the "wild"...I have some NTE577 diodes on order; maybe they'll do the trick in that circuit...with a small amount of hunting the UF2010 has proved to be elusive, and I don't have as good access to cross-reference information as I did...

I wonder if it's possible to recreate the Traynor YGM3 circuit with the PS beefed up to that voltage...hmmm?

[steeve_a]

Originally Posted by TomDanMac

So at least one other UF2010 power diode has failed in the "wild"...I have some NTE577 diodes on order; maybe they'll do the trick in that circuit...with a small amount of hunting the UF2010 has proved to be elusive, and I don't have as good access to cross-reference information as I did...

The Crate amps come with a 5 year warranty (which is usually voided by any mods)- I was just wondering how they handle that warranty when they run out of replacement parts? Will they give you a brand new amp which is comparable? Or what

Just wondering...

Steve Ahola

[scihibmxer@juno]

Originally Posted by TomDanMac

...The negative PS on that board is for -63 VDC, but my guess about the +400ish VDC for the 6L6 tubes was on the money...some discussion and a link to the V50 PS schematic here...

http://music-electronics-forum.com/t16500/

So that must be the fixed bias tap for the 6L6 tubes. I had one (v50), but sent it back, it was an early version and anemic, I think they had screwed up the power supply and running it at ~300 volts on the power tubes. My wife's cousin picked up the later model and it sounded like a true 6L6 amp, and voltages at rail in the preamp were over 300 volts. But it also had a charred resistor in the power supply (R103?), he got rid of his also...