Looking for a little advice

[dude]

Hi all, hope everyone had a great Thanksgiving.

I posted a question a while back about lowering the gain on this amp, Crate VC2110.   I did some of the suggestions but decided to go back to stock and eliminate the third gain stage and make a cathode follower with that last stage. Attached is the schematic and the noted changes. After the changes the amp sounds very thin, the tone stack has little effect on the tone (basically doesn't effect tone at all). The gain knob (volume in schematic) and level knob has to be 3/4's up to get much of any playable sound, over all volume is much lower than before changes.

I didn't have to cut any traces to get the plate load -3rd stage to the cathode, I was able to jump a resister and use a  jumper. But I did change the two resisters before the grids from 475K to 270K, R9 and R10.  Could this be causing the thin response and lack of tone response?

Changes marked on schematic, PBC bd. Simple three gain stage, tried to get two stages and eliminate the third for a cathode follower. Any suggestions on what I did wrong.

Thanks as always,
al 

[tubenit]

I can NOT tell from your schematic "edits" what you did??  Sorry.

How do you have the cathode follower hooked up? 

Do you have caps or resistors inside the red space that I indicated on the CF schematic?  It sounds like you still have components there such as the 470k resistors being changed out? 

IF so,  that is probably your problem.  Look at Marshall, Bassman or Vox schematics. 

With respect, Tubenit

[dude]

Sorry about the dark print.

First question, (the red box), yes I have the two resistors and R9 and R10 in the box (both were 470K, I changed them to 270K) going to the grid of the cathode follower. Also other components coming after the second stage plate too, see schematic. If you look you'll see C6. .01uf, C29 .001uf then R9 and R10 that I changed.

On your second copy, you have the tone stack input going to the plate (without the plate resistor of course), isn't it suppose to be the cathode, above the 100K to Ground and not to the plate?

Basically what I did from the original schematic was:

Replace R11 with a jumper (high voltage direct to the plate)
Cut the connection of the tone stack from the plate and send it to the cathode above the replaced 100K to Gd. (R12)

I also changed the 33.2k (R13) going to the tone stack to 56K (Fender like).

Then I left "all the resistors the same as original schematic to grid from stage two", but I did change those two 475K's R,9 &10 to 270K right before the grid.

If you hit on the original schematic I posted it will get bigger and you can see.

I attached a Fender Bassman schematic, it's wired like your "red box" with nothing between the plate and the grid for the cathode follower but notice the tone stack is wired to the cathode...? I'm thinking take all the components out between the plate of the stage before the cathode follower, everything not even a .01uf cap...? Then run the input of the cathode follower to the cathode above the 100K to ground.

I want to take the board out, a little time consuming but makes for a cleaner job so hoping to nail it on the second try.   

Sorry about the poor image, but I appreciated your help.

Sincerely,
al

[tubenit]

On your second copy, you have the tone stack input going to the plate .... isn't it suppose to be the cathode

Yes, you are correct.  Simply an editing error done on my part from being in a hurry.

Should look like this ...............

I'm thinking take all the components out between the plate of the stage before the cathode follower, everything not even a .01uf cap...?

Connect it like in the schematic I am posting and hopefully that will resolve the issue. You don't want the caps and resistors between V1b & V2a

The 220p across the plate resistor of V1b is simply a smoothing cap and is OK to leave there IF you want to. IF not, remove it.


With respect, Tubenit

[dude]

Thanks tubenit,

I just wish I could change circuits around like this with out having to look up other schematics for reference to figure out what works but I guess that's ok. I'm finding over the years just solving things isn't the answer, it's the reasoning and knowledge behind the solution. I can read a schematic now and find all the components on a tag bd. or PCB which is nice, marked or not. I can make changes on a schematic then do them without looking at any layout. Something I couldn't do years back.

I'll post the results, I'm hoping for a better response from the tone stack and less gain.

thank you,
al     

[tubenit]

Yes, please let us know how it turns out.  Should be a reasonably easy fix.

With respect,  Tubenit

[HotBluePlates]

... eliminate the third gain stage and make a cathode follower ...

 the amp sounds very thin ... The gain knob (volume in schematic) and level knob has to be 3/4's up to get much of any playable sound, over all volume is much lower than before changes. ...

You made R12 (V2A's cathode resistor) 100kΩ to make it into a cathode follower. But you have no biasing resistor, and you don't have V2A fixed-bias by using direct-coupling from the previous stage. I'm thinking this is probably running V2A way into cutoff and requiring a huge signal to get anything through it.

So I agree with Tubenit. He's telling you to go the direct-coupled route, like a 5F6-A Bassman (see schematic fragment below).

That's probably easier than the other method, which is to cathode-bias the cathode follower and bootstrap it (see the other schematic below, with 1K between the tube and 56kΩ load resistor).

Both approaches bias the tube so it has a chance to work.

[dude]

I made the changes as tubenit suggested as posted in the changed schematic above.

The amp sounds slightly better but still need to have the gain and level all the way up to get any playable volume.  With gain at half and level at half, poor volume. Also the tone stack still is almost non-responsive no matter where I have levels.

I wired exactly as tubenit's changed schematic, plate from last gain stage direct to grid of cathode follower except I didn't made any changes from the cathode (pin 3) to the tone stack. But the only difference is I used a 56K (R13) instead of a 33K and I left in the 220p cap instead of the 500p (C9).

If this resistor and cap is causing the weak tone stack response I'd changed them but ...?

I was told by many here that fool'n with these amps to make them better is pot luck, seems to be true. Maybe I should just put it back the way it was and sell it.

Would that 56K feed to the tone stack cause my problem, I can't see why as the bassman uses that valve. The 220p vs 500p would just be brightness..?

al

   

[tubeswell]

Yeah, what Dr Gonzo said - try a 12AY7 in V1 and a 12AT7 in the phase inverter

[tubenit]

The amp sounds slightly better but still need to have the gain and level all the way up to get any playable volume.  With gain at half and level at half, poor volume

I wired exactly as tubenit's changed schematic, plate from last gain stage direct to grid of cathode follower

IF you still have too low of volume,  I'd say:

1)  Then some other part of the amp is broken or has become broken from working on the amp. And IF that is the case, then
     putting it back to "original" may still have low volume.

2)  It's not wired like in the schematic.  Have you quadruple checked your wiring????  
    
     You're absolutely 150% positive there is nothing in the path between  V1-8 pin and V2-2 pin??   You removed those other
     components and didn't just jumper over  them correct (which would still leave a path for signal to ground).   BLUE

     And V2-1 pin is directly connected to B+ correct?  And not connected to V1-6???  GREEN

Can you confirm again that you do have it wired correctly, please??   Because if you do then I don't think putting it back to original will change anything & that you will still have a problem.

with respect, Tubenit

[tubenit]

Keeping in mind that IF you actually did wire that correctly, that you probably have another problem with the amp that returning it to original may not fix?   So, ..................


IF I had a Crate amp,  I'd immediately gut it & use the PT & OT & chassis and cabinet to build something else that I wanted.

You could use an inexpensive tagboard (Doug sells them) and make a Plexi style amp.  You should be able to use the pots and the jacks.  You'll have to buy a few tube sockets and filter caps.

You can move the tube sockets to closer to the back panel. IF you don't have a hole punch,  I have traced a penny on to a steel chassis.  Then used a small 1/8" drill bit to drill holes inside the circle & then used a round drill grinder bit to smooth the hole. Worked very easily & only took a few minutes to do.

So, maybe for $125, you can make it into a really nice amp.

Check out this JCM900 that I converted into a Plexi 50 for a friend. It went from a very mediocre amp to a really GREAT sounding amp per his review/comments.    http://s28.photobucket.com/user/tubenit/media/CPlexi50chassisinteriordescription.jpg.html?sort=3&o=33

With respect, Tubenit

[tubenit]

Here is an example of a pretty simple build that sounds really good. You could even use paralleled terminal strips to build this.

IF you would like to try something like this and it is NOT beyond your amp building ability, I can draw you up a layout to go with it.

With respect, Tubenit

[sluckey]

The amp sounds slightly better but still need to have the gain and level all the way up to get any playable volume.  With gain at half and level at half, poor volume.

Put a jumper wire across R4 for more volume.

[tubenit]

Put a jumper wire across R4 for more volume

Here is an easier to read schematic:
http://www.el34world.com/charts/Schematics/files/crate/Crate%20VC210.pdf

I think Sluckey is on to something here that is worth noting.  You have a 221k resistor going into a 250k volume pot.

Now that Sluckey's drawn my attention to that, it occurs to me that eliminating the one gain stage may have reduced the volume that significantly that a 221k resistor going into a 250k volume pot could be part of the problem?

Try his suggestion of jumpering across R4 and see how much of a difference that will make?  Try that first before trying any of the other mods.

You could also add a 33k resistor from the ground terminal on the volume pot to ground (under the 250k volume pot) . That would also boost the volume.  Since you still have a master volume later in the system, you could use the MV to turn it off.

As a side note, Sluckey's help has resolved my amp trouble shooting problems more than anyone else's help on the forum. So please try his suggestion.

With respect, Tubenit

[dude]

Wow, holy S**t !

I didn't expect all this time and work on you're part Mr. Tubenit, THANK YOU, very much.

That's the amp, and thanks for a schematic I can see better.

Yeah, I was looking at the Bassman and it didn't have a resister between the first and second gain stage...?

Yes, there possibly could be something else wrong that has nothing to do with what I'm trying to do, the amp sits in a jam room and other players may have done something..? I never played it for a while before starting my mod. But I doubt that.

I like the idea about gutting the VC-2110, using the OT and PT for a plexi style little amp. I do a lot of recording of local bands, kind of for free so smaller amps are my choice. I have a few 18 watters, a super lite 18 watt, 5E3, Ampeg J12, a Alamo (all I built, or fixed) so maybe I'd like to do 6V6's but the original PT probably couldn't handle the current. I have a step drill so holes are no problem, I could use stand-offs too. I have all the parts on hand, too.

I'm really trying to learn more then what I know now, I have fixed many amps for friends but not on "my theory" but on my experience and this site. I just know what to do but don't don't why..., if you know what I mean. I'm now trying to learn "why" it works, not just the fix.

I'll do the last schematic changes you posted and double check what I did but I used a continuity test to check plate to grid and plate to the rail.

The only thing different is V2's plate, pin 1 goes direct to the B+ rail but there is a 10K down the rail from that connection - lowering the voltage to V1, B's plate resister. I'm sure that wouldn't have any effect....?  I just removed R11, plate R, and put in a jumper if you look at the schematic.   

I'll check and do all the changes and get back. You guys are great, with a little reading and your help I'm learning "why" and not just the fix.

Sincerely,
al

[dude]

 
 ]

I wired exactly as tubenit's changed schematic, plate from last gain stage direct to grid of cathode follower

IF you still have too low of volume,  I'd say:

  It's not wired like in the schematic.  Have you quadruple checked your wiring????  
    
     You're absolutely 150% positive there is nothing in the path between  V1-8 pin and V2-2 pin??   You removed those other
     components and didn't just jumper over  them correct (which would still leave a path for signal to ground).   BLUE

     And V2-1 pin is directly connected to B+ correct?  And not connected to V1-6???  GREEN

Can you confirm again that you do have it wired correctly, please??   Because if you do then I don't think putting it back to original will change anything & that you will still have a problem.

with respect, Tubenit

Thank you tubenit, this was the post were the problem was, human inexperience on my part. I moved the load resister to the tone stack to the cathode but only part of it, I forgot to cut the trace so I had the treble cap on the treble volume pot going to the plate still. No wonder I was complaining about no response from the tone stack.

When I jumped that 220K from the second stage to the grid, I got some tone stack response but the treble pot was wired backward and still weak....?  So thank you Sluckey for seeing that. Once I realized the treble was wired backward I knew I didn't make the changes correctly. That's were tubenit's tremendous determination to help me came in, and in a second with a magnifying glass (that schematic was hard for an old man to see) I saw that plate to the treble pot.

I didn't have to put the 33K off the ground leg of the gain (almost impossible on this PCB bd.) but I don't need it now. I did tweak the cathode caps as per tubenit.

Bottom line I learned a lot and it sounds pretty good for what it is now.

Thank you again Tubenit for the lessons and reasoning behind the changes.  A lot of things I wasn't clear on are coming to me, stuff I didn't understand now makes sense.

I learned one thing modding a PCB board is a pain in the butt, I hated working on them but cutting traces and jumping wires, using magnification and bright light to see. I'd rather work on a PTP, tag bd. any day.

So, again thank you all for your input.

I feel like I need to contribute to this site, maybe I can help a newbie, what goes around comes around.

PS.  Since the amps sounds pretty good right now I'd like to change out the stock speaker for a real nice old Fender alnico 10" speaker I had re-coned. It's 8 ohms and the VC has a 16 ohm OT, I have a Mercury Magnetics custom 5E3 OT that I can drop in for the Fender speaker I have. Do you think the MM OT would be to too small or muddy sounding?  I didn't really care for it in the 5E3 but that amp has a totally different circuit. I'm not into major over drive just rock and roll and blues.    

Sincerely,

Al Lang  

[HotBluePlates]

Thank you tubenit...

So thank you Sluckey ...

I think you're to be commended in accepting comments/critiques well!!

Many of us have figured out that when an amp build/mod doesn't work 100% properly, that "Hoffman's Law" is involved: "If it was wired correctly, it would be working now." Tubenit & Sluckey chimed in with some possibilities they thought of, but sometimes it's hard not to take info on how we might have messed up a build/mod personally.

So kudos for reading it in the spirit it was intended, and using that as a jumping-off point to find an issue Tubenit & Sluckey couldn't see themselves! Enjoy your amp!!!

[tubenit]

Congrats on getting it figured out!  BRAVO & hooray!  You did great catching the error (which would have been easy to make).

As an FYI,  HBP is my amp building mentor.  He was there at the start of my amp building walking me thru much of what I was needing to know & very patient with my lack of knowledge. . 

So, passing it on .................. is a cool thing for any of us who have benefited from guys like Sluckey and HBP. Glad to have you on the forum!

With respect, Tubenit

[Willabe]

I think you're to be commended in accepting comments/critiques well!!

So kudos for reading it in the spirit it was intended

Congrats on getting it figured out!  BRAVO & hooray!  You did great catching the error (which would have been easy to make).

Glad to have you on the forum!

What Tubenit and HBP said.


                Brad