MAKING PROGRESS ON A FRUSTRATING BUILD

[TIMBO]

Hi guys, The title says it all,  but we have made some great progress.

I found a DC follower that worked well ( in circle)but it worked a little too well, I couldn't dial it down enough to get a clean sound and when turned to max it got ugly, but I was getting close and I felt that the tube was CLIPPING nicely at a certain level.

I looked in my schem files and found Sluckey's Plexi50-JCM800 and liked the idea of the JCM part,not sure that this was going to do the trick and about a hour latter the change was GREAT, I was able to dial it back and still have plenty volume and quite clean and rolling back the volume on the guitar was even better. Turn things up and got some nice breakup and CLIPPING,so i'm in the ballpark.

I progressively removed the bracketed caps as it was way tooooooo brite even with the neck pickup on the LP and ended up removing then all, this also removed some of the fizz and buzz I was getting.

I also removed the 100p cap across pin 6&8 of V1 cause this was reacting with the treble pot making it scratchy and rough.

By removing these cap didn't make much difference to the overall sound other than making it a little less trebly but a lot quieter.

I would like to remove a bit more of the briteness,  but I don't know what more I can do 

My next want is to add a SOMETHING    I'm getting close to what I am aiming for, I getting the breakup but it is lacking depth and a bit thin maybe cause it too trebly 

I want to make it FATTER and LUSHER, I get it close using the neck pickup on the LP but it could do with more.
I need an OD that will ENHANCE the sound and not over distort it, so I am thinking that what ever I do would be best added between V1 & V3.

I have a spare tube socket and three pot holes

My thoughts are :-
 To do a one tube OD similar to T's D'Mars ODS & FX but with out the TONE control - Maybe this will give it a bit more depth but will it give it too much distortion 

To do DC follower as per the "Double C" amp without the tone control - but will this add too much GAIN or could it be biased a bit cold to only add a little gain and would the trim and level do a similar job

Or to add an OD of sorts that could parallel V1a just to FATTEN the signal 

I'm close. Thanks

[tubenit]

Timbo,

I want to make it FATTER and LUSHER

I would try a 1.8k/2.2uf  or 1.8k/5uf on V1b cathode.  The 2.7k/.68uf is very trebly.

With your second tone stack you have a .001 in series with a 220p cap.  That is giving you 180p treble cap which is going to be VERY thin and possibly ice picky?

http://www.kusashi.com/series-capacitors.php?a=220&b=1000&c=0&d=0&e=0&f=0&g=0&h=0&i=0&j=0&k=0&l=0&stage=results

On my TBM and my D'Mars ................. I use the mid boost on almost all the time.  So, I am using .0012 or .0013 most of the time for my treble cap.

IF you will use a spdt  to switch between your .001  vs.  .001 & 220p in series ..................  I think you will find a MUCH fuller tone.

With respect, Tubenit

[tubenit]

Just another idea to ponder ..................  I really do not have any agenda for you.

The active FX loop does had some warmth, IMO.

With respect, Tubenit

[SILVERGUN]

Hey TIMBO,,,,,glad to see you're still having fun...

One thing that stands out to me is the .047 bypass cap on V3b, which is helping to amplify the treble coming from the tone stack treble cap that T pointed out.....kinda treble+treble
If you can deal without it,,,getting rid of that cap will also help lower your overall preamp noise

The other thing that caught my eye is the 22K value of the PI tail resistor......that seems quite high,,,and could be part of the cause of an anemic tone
Most schematics I have seen have values of 1K or below in that spot (a lot have 470R),,,,and if you lower that value you will definitely hear a huge difference acrtoss the board......as-is, you're really not getting much PI push   ( as an example,,,I recently dropped mine to 230ohm,,,,and WHAM---lightning) )
Try clipping in a paralleled 10K to get a feel for where that will take you

Also, paralleling V1 is not a crazy idea, and when I did it, it was a keeper,,,for me
You can use a lower plate and cathode resistor value, and wind up with a lower noise floor
Just don't expect a huge gain increase or OD boost,,,,,,,just kinda pushed the front end a little harder for me and added a more powerful feel 

I agree with T's comment about living in the same city,,,, from the other thread you posted.....if we all lived on the same street,,,this could be as easy as borrowing a cup of sugar from one another 

Maybe we should start an amp builder's retirement community? 

[TIMBO]

Thanks for those ideas guys, they look great.

SG, those resistor values were wrong,cut and pasted and didn't check the values.

I am going try the Paralleled V1 and see what it does as it is an easy rewire.

T, i'll give try a few things with that TS also.

I've drawn paralleled V1 on sheet 2 so it is somewhere to start,i found two schems with a paralleled V1 (texas raptor and plexi od draft ) that will give me some plate/cathode values to play with.Thanks

[sluckey]

The Marshall 1974 18 watt normal channel runs parallel triodes in the preamp. using 100K and 820Ω.

You have a lot of gain stages in that amp. I would be tempted to eliminate the V3B gain stage completely, feeding the MV wiper directly to the PI input cap (like a Marshall). Then use V3B as a direct coupled cathode follower to drive the tone stack (again, like a Marshall). Using less gain stages will lower the noise hiss.

[SILVERGUN]

One other thing I noticed while experimenting with the paralled V1 approach...
I noticed that the value of the shared bypass cap in that position was pretty critical to the overall tone of the guitar
Try some higher value in that pos. to fatten up the sound,,,,,something like a 4.7 uf will boost more of the freq. range of the low end
And if you like the boosted range, you can increase the gain in that stage and get more of the winning combination

Rivera did 33K input grid stopper, 47K plate, 470R cath, 47uf cap (they must like the #47)
Dr.Z did 68K plate, 1.2K, 22uf

I wound up with 22K input, 68K plate, 680R cath, 25uf.......I recently tried to go back up to a 1K cath. resistor, but immediately just wound up turning around and going back to 680R.......for me, that was where the magic happened 

[SILVERGUN]

TIMBO,
Check out this page on Merlin's site that features this chart that graphs the effect of gain (in db) produced by different values of bypass caps throughout the freq. spectrum (in HZ),,,,and then the chart that shows the corresponding freq. of each note on the guitar (notice that the entire range of the guitar is contained between 80hz-1.2K approx.----not including harmonics)
http://www.valvewizard.co.uk/OtherStuff.html

I mention this because it is something I just learned and found very helpful in choosing bypass cap values.
Notice that with a 1uf bypass cap, your not getting the full gain boost until around 1K, where 2.2uf or 4.7uf cover more of the low frequencies (maybe the fullness your looking for)
If you're looking to fatten it up, that might be valuable info.....

You might already know this stuff, and I might sound like an ass for repeating it,,,,,,but I'll take that chance in hopes that it might possibly help you find what you're looking for.

With all due respect my friend,  

[TIMBO]

Hi guy's, Progress is going great, I've tweaked the absolute SH*T out of it and now I'm to where I think I've got it sounding its best.

I can dial it down to pretty clean and there is still volume in the power tubes to make it loud.

Turn the gain up to half and dial in some nice bluesy breakup 

Turn the gain to 3/4 and get some ZZ  

The endless RESEARCH and Math that has gone into this build was meticulous, What a load of SH*T , I winged IT  , I guess if you change enough bit you are bound to hit the mark eventually 

So if there is some things you see that don't look right, please tell me cause it was all "trial and error"

There is still more to tweak.Thanks   

[tubenit]

If it sounds great to you, then you "got it right" and nothing needs changing!

Glad you're persistence has paid off for you.  Sounds like a good amp.

I am noting some changes that I would make if it were mine.  It might not do anything to improve your amp?  These changes are simply based on my personal preferences and what I've experienced over the yrs of tweaking.

I would not like a 820p across that first gain stage plate resistor.  I've tried that before and felt like it cut out quite a bit of transparency to the tone. The exception of that is when using a 5879 pentode.

With 6V6's, I'd probably change the preamp coupling caps from .02 to .01.

I'd move the 470k going into the first volume pot to the wiper side so it can function like a grid resistor and maybe help with any oscillation issues?

I personally don't like the 47p across the LTPI plates.  I have found even a small cap like that kills the tone that I am looking for.  I have found that using the "enhance cap" across the LTPI entrance plate resistor to NOT kill any tone for me and in fact has helped with a number of builds to increase "blooming".

I'd probably change that one LTPI .01 cap to .02 or .047.

Just some thoughts to consider.   Hope you're having lots of fun with the build.

I always enjoy your threads and the build reports!  Thanks for sharing them and sharing your innovative approaches.

With respect, Tubenit

[SILVERGUN]

I winged IT  , I guess if you change enough bit you are bound to hit the mark eventually 

There is still more to tweak.   

Great news TIMBO,,,,there is ALWAYS more to tweak 
That's a big problem for me now.....I'm not sure I'm gonna know when it's time to button it up move on 

If it sounds great to you, then you "got it right" and nothing needs changing!

Glad you're persistence has paid off for you.  Sounds like a good amp.

I always enjoy your threads and the build reports!  Thanks for sharing them and sharing your innovative approaches.

+1 

Please forgive me if I step out of line......this stuff gets me excited
And I like trying to help guys who help me.....

Tweak on Sir 

[TIMBO]

OK guys, Tubenit thanks for those changes I think they have made a small amount of difference so i'll keep them in   I still find that the 820p on V1 plates keeps it quiet without any major tone loss.
Stage 1 complete (maybe just some minor tweaking)This has turned out to be a fantastic OD and will be kept as is for the OD channel
Stage 2 is under way. V2a is a spare triode, now I'm thinking a CLEAN channel is needed (sheet 2) I started with the values shown and so far sounds great but turning up the clean volume it gets a little overdriven but that is OK as expected.
What I was wanting was too be able to dial in a clean sound at a similar volume level to that of the OD when flicking the switch (footswitch)at the moment the levels are quite different as you would expect. Thanks    

[DummyLoad]

bias @ -42V? that's a fairly frigid bias for 6V6 @ 370V... :-)

--pete

[TIMBO]

Hey DL, I'm A little rusty when it comes to FIXED biased amps, I was just had it at a setting as a starter, Now that you have pointed it out to me I looked into it a bit further.

Yes it a bit chilly, Checked with webers bias calculator and with 370v on the plates,it should be about 19-20mAs, I had it at 14mAs

If i'm right it should be this:-

12/370x0.7=.0227 (22.7mA) So set at approx. 20mA is around -36 bias 

[DummyLoad]

yessir. that looks good! :-) with 370V you could probably safely pull up to -34V.

 

--pete

[TIMBO]

Hi guys, Still making progress but in what direction I really   I don't think it's a plexi ANYMORE  
The new schem says it all but with a few minor problems

I like the sound of the OD, I can dial it in pretty clean and up to a heavy distortion (mid dial is where I like it the best, nice and BLUSEY) The OD volume pot is not in yet, my thoughts to put it in was to get similar volume level between the two channels if needed.

The clean channel is clean to a point but will get into some nice breakup when the clean vol is turned up which is good

Now that I have the relay installed, its easier to compare the two, clean channel is clean to about half on the dial and goes into light distortion at 8-10 and the OD is fairly clean to about 2 and is equal to the clean channel at about 5 (clean channel 8-10) and can get heavy distortion up to 10 which I think is great

I don't think that I have much of a problem with this circuit but I think I could have achieved a similar sound with one less tube.  :think1

I would like to have the clean to stay cleaner up to about 8 on the dial as I think it goes into too much distortion too quickly but not loose volume
OR add the OD volume to keep the OD at a similar level as the clean but doing this may loose overall volume, if this makes any sense  

With the latest changes the fizz at volume seems to have reduced itself so I been able to remove some of the caps on the plate resistors and on that subject I removed the 3w metal oxide and used 1w carbon film which there was on real changed and I ended up using the .75w metal film ( I was not using them cause I thought they might be under rated for plate resistors but they stay very cool.)so don't know if these helped with the fizz

I do have a small problem with the relay and I don't think that its is switching noise of the relay itself but because it is not MBB so I getting clean break of the actual sw. that makes a loud noise when changing from clean to OD where as when switching from OD to clean is not too bad. any thoughts. Thanks

[sluckey]

I do have a small problem with the relay and I don't think that its is switching noise of the relay itself but because it is not MBB so I getting clean break of the actual sw. that makes a loud noise when changing from clean to OD where as when switching from OD to clean is not too bad. any thoughts.

Put a de-spiking diode across the relay coil. Cathode connects to positive voltage (your regulator chip). 1N4148 is good, but a 1N4007 works too. This will take care of the big CEMF spike created by the relay coil when it is de-energized. This diode is standard for any dc relay used in sensitive equipment.

If the pop is actually caused by the contacts you may be able to eliminate it by putting some big resistors across the contacts.  

PS... Does your 12 V LED have a built in current limiting resistor?

[tubenit]

I would like to have the clean to stay cleaner up to about 8 on the dial

These 3 ideas would help with that.

Relay section B would allow a paralled resistor for the OD increasing gain. And when not engaged have a 100k/2.7k for cleaner tone on the clean channel. 

The other relay would put two caps in series and create a lower value coupling cap for the clean.

You could have two relays that switched simultaneously.  

Just some options to consider?  
 

Tubenit

[TIMBO]

Hi guy's, This one just about pushed me over the EDGE             

I had it pretty close to where I wanted it so I took it to a mates place to give it a bit of a thrash, all was great and sounding real good and played it for about 10 minutes when it lost power and the sound was distorted and breaking up 

Got it home and checked all voltages and nothing that stood out

It now has an intermittent crackling sound and when strumming it just breaks up and distorts

The only thing that stands out is when turning up the presence it crackles and squeals which it did not do before 

I chop stuck it and nothing presented itself. Thanks 

[Tone Junkie]

Timbo my friend, Ive had similer problems before myself.  Both times something differant, 1st was a bad tube, second was a bad solder joint on a ground wire. both showed up when i tried to play it hard for a while.
 I must have replaced half the  board before I found that one, chopsticking didnt help either. Usually with me Im finding its always a ground wire somewhere, either i miss it or dont solder it properly I build late at night often, but thats just me. Good luck
Bill

[TIMBO]

Hi Tone, I found part of the problem. I put a 120p cap across the 82k plate resistor of the PI and it looked intact but once removed the cutting out stop, now I left with a loud HUM  Thanks

[tubenit]

Timbo,

Man, I feel for you! The first tube amp I ever worked on had the most complicated problem to find. I worked on it almost every wkend for maybe 7 wks before I found the problem.  Never had anything so difficult to diagnose since. It ultimately proved to be a totally bizarre anomalie that I've never seen since.

PLEASE report back when you find the resolution to the issue.

I'm rooting for you!  With respect, Tubenit

[TIMBO]

Hey T, I'm not really bummed by this as I always thought it a long shot to work, I still see it as a learning thing.

I'll keep probing and see what I come up with.

A friend is looking for a JCM800 and I got the silver jubilee sitting in the cupboard that I will gut for parts

I'm starting to think that I need to build a basic great amp and then add to it, to achieve MY ultimate amp.
So building this JCM it could be a great building base

Doug 50w plexi layout is proven to work so i'll start with it ( have not tried this type layout )

Thanks again 

[TIMBO]

Hi guys, this one is not quite dead and buried yet. The 120p cap (silver mica) was the problem, must have SH*t itself and caused the crackling and cutting out. I also tracked down the hum, it turned out to be an old ax7 the I stuck in for testing purposes

It still has a small amount of hum but I will put that down as it being HI gain.Thanks

[SILVERGUN]

Nice work Timbo...way to stick with it and keep us updated the whole way....

It's nice to have the luxury of learning from someone else's pain  .....

The JCM 800 builds are very well documented and a guaranteed winner,,,
I look forward to the next set of posts, where you detail how everything went perfect and you have no issues to deal with 

[TIMBO]

Hi guys, Back on track again and things are sounding good. Tweaked the james TS to reduce some of the bass and push up the treble a bit as the clean was sounding a bit dull......

But by doing this it makes the OD TOO trebly, so by adding a one knob TS to the OD I have more control over the gain as well the tone

I not sure the better place for it as most of the OD circuits have it after the gain stages as drawn in the insert but I have not got room for a level pot

Both of these positions work well so I can't decide which is the better.

If there is another way to insert a tone (one knob) that can also act as a LEVEL control, that would be neat

Sluckey, I put a 1N4148 across the coil this has reduced it a bit, I also tried a large resistor (2M2 and 10M) across the relay switch but this did nothing

I'm thinking of adding a PPIMV as I am unable to turn the MV down enough to play at bedroom level and at 1 on the dial it compresses the sound in an ugly way and 2 is toooo loud, if I was to add the PPIMV would the PI be subject to too much GAIN as if appears that the MV controls some of the gain. Thanks

[tubenit]

If there is another way to insert a tone (one knob) that can also act as a LEVEL control, that would be neat

Interesting idea!  With the level turned down .................... what do you want to happen to the tone?  More treble or less treble?

IF you want differing levels of treble to mids increased or lowered ........... you could try this approach with a dual gang pot.

You can reverse the positions of the caps on the tone section to either increase treble or add mids.

With respect, Tubenit