Carvin vintage 33 - rebuild - what circuit?

[überfuzz]

Well... I'm not into metal distortion. The pcb is going out! Right about now I'm setting up a layout to copy the stock circuit. Mainly to get the stock channel 1. Now I've looked into the circuit a little bit closer and I wonder. Is it the 4 diodes that renders the harsh distortion in channel 2, cutting the top of the waves. Do you think channel 2 will be softer if the diodes are thrown out?

[hasserl]

Well... I'm not into metal distortion. The pcb is going out! Right about now I'm setting up a layout to copy the stock circuit. Mainly to get the stock channel 1. Now I've looked into the circuit a little bit closer and I wonder. Is it the 4 diodes that renders the harsh distortion in channel 2, cutting the top of the waves. Do you think channel 2 will be softer if the diodes are thrown out?

As I said in my first post I think, get rid of the clipping diodes and replace them with a grid to ground resistor, that will help tremendously.

[überfuzz]

Oh sorry! I forgot about that. Could you explain what a grid to ground resistor is? That is to a person with a somewhat limited English vocabulary. (I'm from Sweden. )

[hasserl]

Oh sorry! I forgot about that. Could you explain what a grid to ground resistor is? That is to a person with a somewhat limited English vocabulary. (I'm from Sweden. )

Sorry uberfuzz, those didoes are in between the grid of the V2A and ground. If you remove them there is no reference to ground for the grid, you'll need to provide one. I like 220k there personally, but have installed 470k as well. You can experiment with what value you like, the lower it is the more lossy it is (the voltage will decrease more); the higher it is the higher the less lossy (the voltage will not decrease as much). You could go as high as you want, but at values greater than 1M you might start to see some grid blocking distortion. I like 220k because the amp has plenty of gain and the lower value helps to tame some of the nasty distortion. You could try a 1M trim pot there and adjust it to taste.

[überfuzz]

Thank you very much! I'll use a 1M pot, must be the easiest way to get a feel for it.

[BackwardsBoB]

hasserl is famous in some circles for his Carvin Vintage Series mods, so you're talking to the right guy, but you can find some fun here too:

Carvin.com BBS :: View topic - Official Belair/Nomad Mod thread - Hasserl plus other folks.

[überfuzz]

So BoB, you're basically telling me that I'm sitting here muscling up against the Arnold of Carvin 33 mods. Oh dear!

The thread you posted gave me some good ideas to. My Carvin is starting the get it's shape, no wait, it's only in my head, so far.

[überfuzz]

A question that might be slightly off topic.

Usually I measure the effect in my tubes using a bias-probe or a 1 Ohm resistor in the negative to ground connection. Lets say I wanna check the effect over my tubes in an other fashion.

Is it possible to just measure the inner resistance of the OT, from main tap to each primary. And while heated measuring the voltage at each primary, seems to be pin 7 on the EL84 power section. Ohms law to get the current, and eventually current * voltage to get effect. This should give me an overview of what the effect is over each pair of tubes. Right..?

[hasserl]

Yes, that is correct, if by effect you mean "bias" or "dissipation". MEasure the resistance, measure the voltage, divide the voltage by the resistance to get the current, multiply that by the plate voltage to get power dissipated by the plates (this will be for a pair of tubes).

[überfuzz]

This is really embarrassing. I've actually studied engineering physic at the university. One could argue that the basics of physics should be clear to me.... But then again this is an other ballgame.

[überfuzz]

I'm going to leave the clean channel as it was, same circuit. Part from remaking it in a point to point wired fashion. Channel 2 has tree capacitors C10, C11 and C12, they're label so in the scheme.

C10
I'm not sure if it's called phase, but C10 is shifting it. Is that the only thing it does? Does it effect the signal/sound in any way?

C11 and C12
I've got an idea, but I don't feel like embarrassing myself further. If anyone could explain what they do?

(There's a link to the scheme in on of the earlier posts.)

Edit, has anyone seen a list of included components, or does anyone have one?

[hasserl]

I think C10 is there to isolate that channel when it is deselected. C2 does the job of blocking DC, and it is much smaller in value than C10. I think they sized C10 to be large enough to pass all frequencies that are passed along to it from C2.

C11 is a bright cap to allow higher frequencies a path around the volume control, very common in guitar amps.

C12 then dumps some of those same frequencies off to ground.

I'm not sure why Carvin tuned the amp the way they did. It seems more practical to me to just size C11 smaller and do away with C12.

[überfuzz]

All-right, I'm slowly grasping this. In my physics handbook it's stated that the the complex impedance is: Z = -j/(angular freq * C) and angular freq = 2*pi*f wish gives Z = -j/(2*pi*f*C) This fits nicely with your explanation.

This explains how a capacitor can be used to shield of low frequencys, further how a capacitor can be used to dump high frequencys off to ground. I guess next step for me would be reading up on frequencys rendered by guitars.

Thank you, again, for all help!

[überfuzz]

New sockets in place! I might just open a build thread to this...

[überfuzz]

I've gotten to the power section by now. There is one thing I I don't understand...

R44 and R47 seems to make the power tubes run unbalanced. What's the point of having 350 Ohm resistors connected to v5 and v8 and just leave v6 and v7 with full throttle..? Is there any reason not to remake the power section, to make the tubes run even? If I remake it, what voltage should I be looking for at pin 9, at the power tubes?