Carvin vintage 33 - rebuild - what circuit?

[überfuzz]

I'm planning to rebuild a combo amplifier. It's a Carvin vintage 33.

There's noting broken or gnarly with it, but I would like to wire it in a point to point fashion. It plays fairly well, but I would like to get a more genuine vintage tune, a nice tweed tone is what I'm aiming for. (The amplifier is covered in tween cloth.)

I haven't opened it up as of yet and before I start working on it I like to get some advice. And yes, if anyone has the circuit scheme with voltages printed out on it. I'm planning to use the transformers, chassi, and most of the hardware. Should I go for the same circuit or is there a must-have-tweed-circuit I could fit with my hardware?

If it ain't broke... Fix it!

[überfuzz]

I haven't go any vox amplifier, so I'm looking at the ac30 circuit at the moment. Trying to figure out what to leave out and what I can't live without. Furthermore, how to fit the stuff in my chassi...

[Ronsonic]

Me, I'd leave out everything except one channel and probably not the top boost one. The non-top boost with or without a treble boost pedal is such a great classic rock sound for everyone from The Beatles, Deep Purple, Rory Gallagher, Tom Petty, Brian May, why mess.

Fitting it in the chassis is another thing. Based on most of the AC30 derived amps I've seen the approach is mostly dump parts and terminal strips in there and solder where they land.

[überfuzz]

I've seen ac30/reverb schemes on the e-net and I think I'll got for that.

The thing is, I don't know how to alter the scheme, properly. All schemes I've come across has got tremolo and I like to leave that out. Has anyone here a scheme for a ac30 with reverb and without tremolo. Or could someone help me to adjust a standard ac30 scheme?

[hasserl]

Well, you've got 8 nin pin socket holes to work with, and 10 holes for knobs on the front, plus a reverb tank. The transformers are nice trannies, but the power trans produces pretty high voltages, around 400vdc w/ ss rectification. So if you're going for a Voxish circuit the B+ is going to be a bit high.

HAve you considered just modifying the existing amp? If you're new to amp work that might be a better solution for you. The stock clean channel is actually pretty nice. The dirty channel can be fixed up by getting rid of the clipping diodes and maybe doing a bit of revoicing to suit your tastes. That will teach you a lot about how to work on amps, how to voice them, etc, without the danger of stripping apart a working amp and possibly ending up with nothing if the project gets too far over your head.

[loudthud]

I found this schematic: http://carvinmuseum.com/pdf/amps/VINTAGE33.pdf

There is quite a lot of stuff in there, it would be extremely difficult to cram that much circuitry in if the wiring was point to point or on an eyelet board. It will be a tight fit even with channel 2 removed. It's going to take a lot of planning and layout to make this work.

This amp has interesting and unique tone stacks. Kind of a James with a Midrange control added. Maybe just substituting a Vox tone stack will shift the tone towards the AC30. That wouldn't be too difficult to do modifying the circuit board. You wouldn't have to change the pots.

As for the output stage, hasserl is right. You will need some B+ reduction to get close to the near class A of the AC30 and that will knock the power down unless you change the output transformer. And it would run very hot, you might need a fan. Probably best to stick with what's there now but you could add a Cut control.

[überfuzz]

Thanx loudthud and hasserl for helping out.

The existing pcb board is in very bad condition so I'm going for a simple point to point wiring. The idea being, get some inches between tube sockets and the main board. The old pcb board is sitting right on top of the sockets, it's cracked from heat and fluid.

A fan is on the drawing board. I'm trying to figure out where to plug it. Is there a common solution to this. Right now I'm planning to plug it in on the 230VAC feed, to be fired when the amp is ON. Furthermore, how do I get the best cooling effect, where do I place it?

Even though I consider myself a newbie I'm quite confident. It's just the way I am, I rather do than don't. My first build was a Twin Reverb. The thing holding me back now is the circuit. As you probably understand I've only scratched the surface when it comes to fully understand whats happening in there.

Hasserl - you're right about the clean channel on this amp. It's actually on of the best I've ever heard/played. If I had the knowledge I'd definitely go for it, redrawing and rewiring the amp more like it's out of the box state.

[hasserl]

Well, there's no law that say you have to use all of the existing holes in the chassis, you can use only the ones you want/need, and use a faceplate to cover up any you don't use.

With that in mind, you have 4 preamp tube socket holes, if you want to retain reverb you can stick with the stock Carvin reverb circuit (uses only 1 tube), that leaves you 3 more for the preamp and phase inverter. You could do a JTM or plexi circuit in there, or even make it a single input and use the additional triode for a switchable boost stage. Though it's not like you'll really need additional gain with a JTM or plexi circuit driving EL84 power tubes, as that kind of front end will drive those little tubes really hard. Maybe still go with a channel switcher, share the input stage among the two channels, as the Vintage 33 does, maybe keep the stock Carvin clean channel circuit and pair that with a plexi channel.

[überfuzz]

Mhmm. You're leaving me with even more questions... Everything you write sounds reasonable.

Right now I'm sketching on a ac30-ish layout with reverb. Actually two, one with 3-pot tone control and one with a more ac30 styled 2-pot control.

If I post them here, would you have a look at them? Ehh... It'll take some days before they're ready to be shown. (I really don't have the knowledge to do it, so every step is a mile stone.)

[überfuzz]

I found an editor called expresssch and it speeded up the scrabbling.

There's a layout of the power-section, I've never tinkered with 9-pin sockets on the power tubes so I would appreciate thoughts on that. When I wire 8-pin sockets I always use blind lugs to connect resistors. Here I've just put them on the board.

The main board is inspired of or based on Hoffmans ac30 layout. I've made some alterations to it. With a ac30 reverb scheme by my side I tried to add reverb. Thoughts on this..?

[überfuzz]

I added a 1 ohm resistor and a pot in series at each power tube. Now I'll be able to adjust the current through each power tube. One question though, what voltage and current should I aim for. (I've never used EL84s) By the way is this a good way to set up the bias?

[überfuzz]

Right now I'm trying to fit the original circuit on a point to point board. There are actually a lot of space in the chassi, so it might be done...

One question though. There are caps without any value, instead their marked with N/U. What does this tell me..?

[überfuzz]

I just vent through the old pcb-board N/U seems to be Not Used or something. There's noting soldered where the value is N/U. The concept of redoing the original circuit seems more likely by the minute.

Could someone help me to adjust the overdrive channel. I like to ease it down a bit. Before I didn't really use the drive channel because it was so hard to find a nice breakup. It vent from clean to übermetal around 2 on the drive pot. (Carvin called it soak.)

[überfuzz]

Ok, maybe I should post the thing I came up with. Keep in mind, I'm not an amp tech!

(See the scheme posted by loudthud) Changing pot P5 from A500k to A1M or maybe an even bigger pot. In my mind, this should do the trick. However, this will only widen the area of nice drive. And the volume in channel two is somewhat low in the lower areas of the soak-pot. Is there a mod I can do to raise the signal level of channel two if I change the soak pot. The point being, I'll get nice a soak-pot without losing volume on channel two.


Sorry for this constant posting into this thread...

[hasserl]

I added a 1 ohm resistor and a pot in series at each power tube. Now I'll be able to adjust the current through each power tube. One question though, what voltage and current should I aim for. (I've never used EL84s) By the way is this a good way to set up the bias?

Boy, you lost me there. Where are you putting the pot? This amp is fixed bias, are you planning on keeping the fixed bias or converting to cathode bias? Voltage/current to aim for? EL84's are rated at 12 watts max plate dissipation. The voltage will be what it is based on the power supply and the load. The current is what you will adjust to keep the bias within safe limits. EL84's are also only rated for ~300 volts I believe, but they will take 400 if you keep the current down so as not to exceed max dissipation, set it for less than 70% at idle.

Changing pot P5 from A500k to A1M or maybe an even bigger pot. In my mind, this should do the trick. However, this will only widen the area of nice drive.

No, changing this pot to 1M will narrow the sweep of the pot where you find "nice drive".

While the stock lead channel circuit can be moddified to provide some nice tone, if you are going to the extent of stripping out the existing pcb and rewiring this with a tag or turret board, I wouldn't go with the stock circuit, I'd go with a nice Marshallish channel, something along the lines of a JTM45 or plexi. That circuit will drive the piss out of those EL84's you'll get plenty of distortion, plus some, and it's just a better sounding circuit IMO, and I've had experience with both.