Crate VC 20 too much gain

[dude]

I've been trying to lower the gain on this VC 20 practice amp a nice tone but way too much gain.

I've lowered the plate resistors to 100K, took out the cathode bypass cap on V1, V2 it's low at 2.2uf.

I'm now thinking about lowering the grid leak resistors. What's that 221K w/the bright cap off the plate in V1?

Any more suggestions to lower the gain?

I'm using a 5751 too but just way too much gain with humbuckers.

al

[HotBluePlates]

This amp looks to be designed from the ground up for gain. 3 gain stages then tonestack then output section won't be Soldano territory, but close.

You could turn the 3rd triode into a cathode follower. Then, you don't get that triode's current gain contribution.

I'm assuming this is a p.c. board amp. In that case, remove R11 (100k plate load) and replace with a small jumper wire. Remove C7. Remove R12 (1k cathode resistor) and replace with a 100k resistor (maybe the former R11). The last step may be the hardest, possibly a deal-breaker. Where the schematic says "TP5", you need to have go to the cathode, not the plate. Said another way, it needs to go to the side of R12 that's connected to the tube cathode.

How do you do that? It depends on exactly how the connection is made to the tone circuit components. If there is a trace that runs from the plate-side of R11 to the tone circuit (probably the case), you'll have to cut the trace to open this connection.

If you have to do this, I'd suggest taking a razor-blade and lightly scraping away some of the solder mask on this trace, then making 2 light cuts and pulling away a section of the trace. Think ahead and leave yourself some leftover trace on either side of this cut... you may decide to undo this change later and you'll need to bridge this opening.

On the side of the cut towards the tone circuit, you'll need to strip the end of a wire, lay it flat against the portion of trace that has the mask removed and solder it in place. Be careful about moving this wire until the solder is absolutely cool; it's easy to make a cold solder joint with this trick. Also, don't heat the trace and wire too long, or the trace may lift off the board. This is not a hard task for an experienced solderer, but it can be tricky for a newcomer.

Somewhere, I thought I had seen a picture to show what I'm talking about, but I can't find it right now. Perhaps someone has an example of soldering a wire to a trace to create a flying lead or jumper.

The other end of this wire needs to connect to the tube cathode. Depending on where the tone circuit is in relation to the tube and R12, you might do this a few ways. You want to keep this wire as short as practical. Maybe that means running it to the tube cathode pin itself, or perhaps to the cathode-side of R12, or even soldered to the trace at the cathode side of R12, using the same procedure of soldering it to the trace as you did for the tone circuit end.

[dude]

I'm assuming this is a p.c. board amp. In that case, remove R11 (100k plate load) and replace with a small jumper wire. Remove C7. Remove R12 (1k cathode resistor) and replace with a 100k resistor (maybe the former R11). The last step may be the hardest, possibly a deal-breaker. Where the schematic says "TP5", you need to have go to the cathode, not the plate. Said another way, it needs to go to the side of R12 that's connected to the tube cathode.

Yes PCB bd. I have modded bds before, this board is thin and has very small traces so a lot of care is needed but no problem. I can break the trace from the plate and use a jumper to the same hole of the 100K I just changed direct to the cathode.

So this eliminates the 3rd gain stage, what does a cathode follower actually do?

I assume I can redo the changes I made on the first and second gain stages, keeping the stock cathode bypass cap and resistors but I will keep that second plate resistor at 100K from 221K (R7).

What is the 470pf w/221K (C3, R4) off the first stage to the gain pot do? I haven't seem this before?

I appreciate you sharing your knowledge.

thanks, al 

[VMS]

Hi,

the first and easiest thing that I would try is changing the R10 resistor from 475k to maybe 47k.

[HotBluePlates]

So this eliminates the 3rd gain stage, what does a cathode follower actually do?

Well, you could remove it entirely. What we're doing instead is to take a stage that would multiply its input signal by perhaps 60 times, and instead multiplying it by around 0.95. So gain reduction first. A cathode follower also has a lower output impedance than a stage where the output is at the plate. So that drives the tone circuit easily.

A tweed bassman or early Marshall are 2 gain stages, a cathode follower and the tone circuit. Despite being thought of as distortion amps, both are clean until you whip the output tubes. So we're taking your amp with 3 gain stages, tone circuit and output section, and trimming down the gain.

the first and easiest thing that I would try is changing the R10 resistor from 475k to maybe 47k.

True, that would keep signal level low going into the 3rd stage. My concern is that doing this while keeping un-utilized gain might lead to noise. I figure reduce gain by pulling some out of the circuit. However, you've also raised a great point that the interstage voltage dividers may need to be adjusted after the 3rd stage becomes a cathode follower. They are like fixed volume controls, and could have too much loss after the extra gain is not there to offset the loss.

[bigdaddy]

Have you thought about removing V1/B completely from the circuit? Jump the volume pot into the V2/A grid.

That would cut down on the gain and you could mess with other values to dial it in.

In all honesty a lot of these types amps are what they are in my opinion. Every time I tried to make these type of amps better I ended up selling them or just ripping everything out and building something known to be good. I think the time spent trying to make this into another amp can be used making a new board with an amp that is either Brit or American. I think it would sound a hell of a lot better by just copying a simple Fender type amp with a long tailed PI or since it's an EL-84 one of the many 18 watt versions out there for EL-84's. I have heard some great amps. One amp I really liked that was one with just a EF86 driving the PI. I'm not really into EL-84s but this amp was amazing.

I did own one of the older ones and it sounded horrible. The cab was too small for a 10 and choked it off. Nothing I did made it sound good. I also had a few Blues Juniors and the other small EL-84 fenders and they sounded awful to me too. I also tried a PV 1-12 30 watt EL-84 amp and nothing I did made it sound good. If I knew then what I know now I would have just ripped everything out and made one of those EF86 18 watters. It's a very simple amp and I think there are pre-made boards for it. They take pedals well and sound very good. There are also many transformers for them now.

I really think by the time you're done you would have put a lot of time and effort into it and come up short. I have read other posts on forums with the same issues. Certain amps are what they are and unless you start from scratch you're chasing  some thing that's unobtainable from these amps...... A really good sound.

[VMS]

Since you have already changed R7 to 100k, you could also try a marshallesque approach by removing C5 and changing R6 to 10k.

[PRR]

Lift one end of C5 +and+ C7. That cuts gain by 4.

Tack 100K across R10.

Tack 100K across the ends of the Volume pot.

[bigdaddy]

Like I said those amps are what they are. Once you cut the gain maybe you cut the tone. It was designed around a parameter of a small low wattage higher gain amp and that's what it is. I think anything other than a complete redesign is chasing unobtainium. But that's how you learn, that's how I learned.

I hope these mods work......the thing some times with high gain amps is the speaker too.

If you use a lower efficiency speaker some of that gain is reduced in the end game. What speaker is in there? I find all too often when someone has been tinkering with amps like this they completely overlooked the speaker equation. It makes a huge difference. All this work could be for naught if you had the right speaker in there. Like sticking an original Jensen C10Q or Reissue C10R/P10R might make this amp just perfect. Or one of the Eminence lower wattage 10s like the Legend 1058.

[dude]

I hear you Big Daddy, I haven't tried any changes suggested here yet but plan to do one change at a time starting with the easiest, if any one or more do the trick great, if not I'll try the eliminating the third stage and change it to a cathode follower per Hot Blues Plate suggested but I'd have to cut a trace.

That amp has a 10" ceramic Crate speaker and it's sucks, the cabinet is very small and the speaker is boxed in. The secondary on the OT is one 16 ohm. I have been using two old Jensen type 12" (10/20 watt alnico) speakers made by Zenith in series at 14 ohms. I got them out of an old organ. These 12's did help a lot but the little beast has way too much gain. Turning the gain almost off gives the best tone but at volume sacrifice.

All I did was lift the cathode bypass cap on first stage and change the second stage plate R to 100K. I think I'll put that first stage bypass cap back, .68uf and remove the other two one at a time. If that doesn't do the trick I'll change that 470k grid leak, R10 to 100K as suggested.

I'm not sure what the 100K across the volume does, the volume off the first stage is marked gain on the amp and I'm assuming that's where the 100K goes but I'll try it.

Last I'll take out the last gain stage.

Still no luck, at least I'll learn like big daddy says

thanks all,

al

I thought about taking the PCB bd out and going with an 18 watt Lite llb but I already have a few different versions and wanted a little different tone.

   

[bigdaddy]

I'm thinking the design was to have a lot of gain because of such small size and that chokes off the speaker. I had an older one and it was terrible, the speaker sounded like it was being boxed in and choked off. Add gain from the amp to change that must have been the idea at the factory instead of making a bigger box. It was designed to be what it is.....a tiny PP EL-84 amp like a blues junior or something, it's about marketing with those companies not tone.

God luck and let us know. You could also put a new baffle in with an 8" speaker. A Weber like an F8150 and that will take the power yet maybe take down the gain because the speaker is in the right size box, that changes how everything works. Maybe thinking smaller will get you there. By using a bigger box with 2-12's you're adding to the problem so go in the opposite direction.

[dude]

Update on the little VC 20 Crate:

Lifted the 2nd and 3rd stage by-pass caps and lower the grid R10 to 100K from 470K. This lowered the gain a lot, actually the gain pot needs to be up at least half way to get any sound. I might put the second bypass cap back, 2.2uf, hear that and maybe up the grid R R10 to 200K to get some gain back.

Amp was overly bright so I lower the 470pf bright cap to 200pf, then 100pf this was the ticket, those el84s are bright sounding tubes.

I'm pretty happy at least the amp is usable to me now, thanks for all the help.

al