Boost input gain on Carvin X100b

[glebert]

I'm in the process of fixing a late 80's Carvin X100b (gray rat fur covering, block letters). I have had a basically identical amp in the past and one of the things that I didn't like about it was the inability to get a full sound with single coil pickups. I have seen others make similar comments about this amp. I was thinking about trying to mod the amp a little to try to get a modest boost in gain on the input (or close to the input).

I have looked at tutorials and what not for drawing load lines and they all sound really easy, then you get to the actual circuit and nothing makes sense. Most tutorials seem to assume that there is a tube with a plate resistor that is connected directly to an ideal 200V (or whatever) voltage supply, but my circuit has a plate resistor and then there are a bunch of other resistors that drop the voltage down from 450V. You can't include those because there are other tubes drawing current from different nodes and dropping the voltage, yet it doesn't seem like you can ignore them either. Similarly the load impedance gets pretty tricky when you have relays and switches and tone stacks.

If anyone has any thoughts on a mod for this, or any advice on doing the load lines I would appreciate it.

Greg

[Helmholtz]

I don't see where you could use/need load line construction here. And the loadline won't tell you how the stage sounds. It only makes sense if you're designing new and untypical circuits.
It's much more important to understand how circuit values influence gain and frequency response.

Maybe a good pedal (Tubescreamer, RAT, OCD, Zendrive ..) solves your problem. That's what I use with an essentially clean BF Fender amp (even though I also have Marshalls).
As options for modifying the preamp are limited wihout adding a gain stage/tube, the right pedal is more likely to give satisfactory results.

Have you tried different speakers?

[Chuck H]

It's important that you verify the schematic matches your amp. There were a few different "series" of this model name.

This should only affect the high gain channel. This is a lot more gain. Maybe enough to make the amp unstable, but you can always chase that problem if it comes up. Changes in purple.

[glebert]

I don't see where you could use/need load line construction here. And the loadline won't tell you how the stage sounds. It only makes sense if you're designing new and untypical circuits.
It's much more important to understand how circuit values influence gain and frequency response.

Maybe a good pedal (Tubescreamer, RAT, OCD, Zendrive ..) solves your problem. That's what I use with an essentially clean BF Fender amp (even though I also have Marshalls).
As options for modifying the preamp are limited wihout adding a gain stage/tube, the right pedal is more likely to give satisfactory results.

Have you tried different speakers?

I mentioned the load line because 1.) I am interested in expanding my understanding of the theoretical operation and 2.) I have seen so many responses \saying "well, first you have to draw a load line..." Regarding your comment about boost pedal, speakers, etc, my point of comparison is a AB763 clone (Plush P1000S) using the same cabinet. Carvin is much weaker sounding on clean channel, as I said especially noticeable with single coils. Lead channel is better but can't compare that to a BF style amp. Regarding the need for more gain stages, are you saying that the gain in both V1 stages is already maxed out, even though they seem like they would have a very different operating point and load than say a typical Fender circuit?

It's important that you verify the schematic matches your amp. There were a few different "series" of this model name.

Yes, this matches my board (30-X100-4B). I always thought Carvin was stupid in making so many small changes to the models over the years but keeping the same model name, especially since they were almost entirely mail order. You see a guy playing a Carvin (insert model) amp and you want one and order it up and find out it is considerably different than the one he had, even though the name is the same.

[Chuck H]

Boosting gain on the normal and high gain channels can be done independently. There are good options for modifications to either or both. Adding another tube stage really isn't necessary.

[Helmholtz]

Carvin is much weaker sounding on clean channel

That wasn't clear from your OP.

Regarding the need for more gain stages, are you saying that the gain in both V1 stages is already maxed out, even though they seem like they would have a very different operating point and load than say a typical Fender circuit?

Gain in V1B certainly isn't maxed out.

Lead channel is better but can't compare that to a BF style amp.

The mod proposed by Chuck H might help. Especially reducing the value of R6 will increase plate current and gain as well as do away with cold biasing.
(Of course a BF amp doesn't have a lead channel.)

I have seen so many responses \saying "well, first you have to draw a load line..."

I don't recall having seen that kind of statement (relating to preamp gain) here since I joined the MEF (~3years).
Chuck surely didn't need load line construction to work out his proposal.

With single tube class A (e.g.preamp) stages load line construction helps to define a suitable operating point and plate load within tube limits. It also allows for rough calculation of gain. Not much more. And there are better/easier ways to determine gain.

[Chuck H]

As far as bumping the normal channel, a full bypass cap across the input stage cathode would go a long way. As for it not sounding "full", I'd attribute that to the severe T-filter following the input stage that has about 16dB of cut at a mid point of 400Hz. This is in addition to any tone stack or graphic EQ filtering that may cut more mids. FWIW the classic Fender designs filter mids about 12dB at 500Hz and only filter this way once. So that would sound a lot more "full".

[glebert]

I probably should have done some googling before I posted the question. Here are a couple threads about modding this amp. Some very similar to Chuck H's suggestions.

BFG mod
https://www.rig-talk.com/forum/threa...-mods.123615/\

Dan Torres mod
https://www.thegearpage.net/board/in...-mods.1730567/

Unfortunately these mods can be confusing considering the different circuit revisions that were previously referred to, but the concepts seem sound.

Thanks for advice! Happy for more opinions!

[Chuck H]

Those mods are for the most part addressing the same areas of the circuit. And for good reason. One thing I notice is that both the BFG and Torrez mods use a 4.7k to replace the 10k cathode resistor on the second gain stage of the high gain channel. I will admit that I almost did this too because it preserves more of the asymmetry of the original circuit (for better or worse). I decided to steer away from it though because this asymmetry can sound like there's a little fixed wah pedal tone going on. Also, the boost for the lead channel kicks in a full cathode bypass. I've read many opinions here that significant bypass of a cold stage doesn't sound very good. Since "nasally" is a repeated adjective for this amp I figure a more symmetrical clipped wave form should sound better and more full. YMMV and you can try the second stage cathode resistor as both a 1.5k and a 4.7k and decide for yourself. But while on that note...

This may be where any discussion of load lines came from. The 4.7k cathode resistor, and much more so the 10k, bias the second stage cold. But this isn't an extreme load line issue for small signal voltage amplification. In this case it really has to do with wave form symmetry during clipping conditions. Nothing that would be of concern to the operating condition for the tube.

[Enzo]

In defense of Carvin, Chevrolet has made the Corvette since 1953. It changes every year, but is always still called Corvette.

[glebert]

I never mail ordered a Corvette. If most people were sitting in a store and trying out a Carvin amp and they loved it and bought it who cares if it was different than the one from six months ago or even the one next to it on the floor. Since they were mostly mail order I think Carvin mostly sold on reviews, endorsements, and faith. I actually have had a few Carvin products that I really liked (I've been playing my Carvin LB40 bass pretty much exclusively since '95).

[Chuck H]

I ordered a Carvin DC400 based on it's features and artist endorsement. Absolutely fabulous guitar in acoustics and playability, but...

I ordered it with the 24 fret neck (because why order a smooth heel, neck through guitar if you're not doing 24 frets, right?) and they somehow located the bridge pickup about half an inch further from the bridge than typical!?! So the bridge tone was all wrong. I took it to a well known luthier at the time (because the instrument was so remarkable otherwise) and had the bridge pickup moved to the correct position and to hide the hole I had a third humbucker installed just ahead of it. Sort of a pseudo LP Custom thing, but with a coil split set up to select the coils nearest Strat type positions. So now I had a guitar that was toneful acoustically, unmatched in playability and with a "S" ton of pickup selections.

I used that guitar for about seven of the ten years that I gigged. I ultimately sold it for cheap to a friend that always coveted it when I was moving in a new direction musically.

Also, for the entire time that I gigged my band used a pair of large Carvin P.A. speakers and small Carvin stage monitors. All of which performed exceptionally well for many hard years. I ultimately sold the Carvin P.A. speakers to the same dude. He bought them because I was offering them for cheap. He used them as his home stereo speakers for years. Big ol Crown power amp too. Lot's of neighbor hate

So that's my personal experience with Carvin products. A good run overall.

[J M Fahey]

Mods suggested above will slightly increase gain at V1a and V2a stages, because they are already *almost* maxxed out.
The stage which is working with a boot over its head is V2b where R63 is dropping ton of DC voltage but not gain since itīs bypassed to ground through C41 and on the other side effective *audio* load for V2b is ludicrous 22k R64
WTF? What were they thinking?

I suggest you simply swap them, making R63 22k and R64 180k

DC voltage at V2b will stay exactly the same, but gain will increase dramatically to put it mildly.

You might have to increase C41 value if instability, specially motorboating appears, p,lease measure and post actual DC voltage at R63/R64 junction Ļ*after* the Mod, to know the actual cap rating needed there.
My question because higher value caps (say .22 or .47uF) may be too bulky for available space but on the other side 400V rating might be relaxed.
Or perhaps you could fit an electrolytic there, say 1uF but letīs check voltage present.

[Chuck H]

DC voltage at V2b will stay exactly the same, but gain will increase dramatically to put it mildly.

Absolutely! But...

Notice that this is the stage immediately prior to the high gain channel master volume.

I'm not sure this is where the increase in gain should be. I figure the gain "structure" at stages is likely set up to work with following processing stages like effects loops, EQ, etc. as well as balance with the output of the "normal" channel for intuitive adjustment of the high gain channel master volume. The OP mentions "the inability to get a full sound with single coils" and asked about an input boost. I interpreted that to mean more clipping is desirable PRIOR to the master volume. And then let the output there feed the function stages of the amp. Boosting after the clipping stages and before the master would make the master volume very touchy even at low settings, but wouldn't increase clipping. Well, unless the added drive to the SS processing stages is overwhelmed by the increased signal out of V2b. I don't think that's ideal.?. I think increasing gain at V2b would only serve to require very low master volume settings with no increase in preamp clipping.

Mods suggested above will slightly increase gain at V1a and V2a stages, because they are already *almost* maxxed out

The modifications to the preamp would increase gain into V2b (the last clipping stage) by about 8dB. This actually increases sensitivity and clipping, the guitarists version of "gain", very effectively.

[glebert]

As far as bumping the normal channel, a full bypass cap across the input stage cathode would go a long way. As for it not sounding "full", I'd attribute that to the severe T-filter following the input stage that has about 16dB of cut at a mid point of 400Hz. This is in addition to any tone stack or graphic EQ filtering that may cut more mids. FWIW the classic Fender designs filter mids about 12dB at 500Hz and only filter this way once. So that would sound a lot more "full".

Thanks for replies. I am wondering about the AC loading in general on V1b. In addition to the T-filter I'm wondering about the effect of that 220k R62 and if it is going to load down the circuit and reduce the gain as well. As I said, I am trying to use load line concepts to understand the circuit behavior, mostly using Rob Robinette's tutorioal. I tried plugging the AC load into LT spice and it starts at 55k ohm and drops as frequency goes up. If I run a Hot Rod Deluxe first gain stage the AC load starts at 84k doesn't drop as fast. At 1kHz the HRDlx AC load is 2x the Carvin.

Speaking of AC load, I don't get what is going on with the V1a load at all. I need to check the board and see what is there for the "C3 6.8k Resistor" (???) thing.