Volume / Voltage Issues with Bassman 6g6B

[tubeswell]

Re the h-k voltage - I elevated the heaters in my amp to about 50V. (The humdinger wiper is going to the heater elevation point - which is at the output/knee of a 235k/22k voltage divider from B+ to ground that is decoupled with a 47uF 100V cap). The CF voltage on the bass channel in mine is 133V, so having the heater elevated is keeping h-k to 83V in this case.


I think you need to re-do the grounding in your amp. Have you read this article? http://www.valvewizard.co.uk/Grounding.pdf

[HommeMarrBuckley]

I was checking out old photos of the build and came across this one.  I think I corrected the error here but not sure if I did.  Can't check because amp is not here.  Never wished to have a mistake before but I am kinda hoping I didn't fix this and that the filter caps currently look this way under the cap can.  Wouldn't this be a contributor to the hum?  Can see the connection in second pic.


http://minus.com/i/57XrDpbxrnPq


http://minus.com/i/bnLSFr22IEmuM

[tubeswell]

The way you have the buss bar done certainly looks like it could produce extra hum. Read Merlin's article again.

[HommeMarrBuckley]

Tubeswell,


You wired your jacks differently.   Could you explain how you did yours?  I was following another build on Gretsch Talk that used coax and wired them similariy to yours.  It's different than the layout in that you don't have the sleeve and tip of hi and lo jacks together.


I followed lower left doubleinput
http://el34world.com/Hoffman/files/CommonHookups.pdf


I have the coax ground (near jacks) going to the bus.  Also, ground lug off Hi jack going to bus. 


It seems you have both ground lugs connected together by wire.

The way you have the buss bar done certainly looks like it could produce extra hum. Read Merlin's article again.

Yeah, I've never built an amp before.  It seems to work correct but the hum part.  Just kind of losing it with hunting and pecking. 

[tubeswell]

Tubeswell,... You wired your jacks differently.   Could you explain how you did yours?  ...
It seems you have both ground lugs connected together by wire.

Not wired different in reality. Just looks different. The switch lug of the lo-gain jack is wired to the input lug of the high gain jack (with a little black wire). The hi-gain jack input (in each case) has a 1M-to-ground (grounded at the jack's ground lug. There is a 3nF cap in parallel with each 1M (to snub RI). The 68k are each connected to the respective input lugs of the jacks. The bass channel's ground lug is also connected to the ground buss wire. (But the normal channel's ground lug isn't connected to the ground bus - no need, and it would just cause hum if it was)

The way you have the buss bar done certainly looks like it could produce extra hum. Read Merlin's article again.

Yeah, I've never built an amp before.  It seems to work correct but the hum part.  Just kind of losing it with hunting and pecking.

Read Merlin's article on grounding. For these amps it works better to have everything connected in 'galactic ground' fashion on the buss bar - even the output stage ground, and only have one ground connection to the buss bar (attached at the input of the most-sensitive channel - in this case the bass channel has more gain than the normal channel)

[tubeswell]

Here's a pic with words and confusing arrows of what I mean. (This is my 6G6B-type amp when I was at the stage of having the normal channel wired in, but hadn't then finished wiring the bass channel.)

[HommeMarrBuckley]

 help
Alright, I am hesitant about posting this but I am kinda at my wit's end.  I realize that I have some grounding issues and Tubeswell gave me great advice that I would like to try but I just don't know enough.
It involves the layout of my bus bar grounds and if hum ensues, elevating the heaters.
Could someone identify these on the schematic and layout:
output stage ground returns
bias ground return
screen filter node return
reservoir cap return
phase inverter tail, filter cap, and ground return

and then how to implement them in a logical way on the bus bar in terms of the layout?

I understand how to go about the jacks and the grounds for both channels and their filter caps on the bus but am lost after that.

My filter caps are exactly as shown on the drawing.

Then a voltage divider involving a 220k resistor, 22k resistor and 47uf cap given I have a CT for ground reference.  Would this be put on the standby switch?


Yeah, I know-asking for a lot.  I cringe writing this.

[tubeswell]

The reservoir cap(s) are the 2 parallel 20uF caps you have tied together with the red wire at the +ve end. The screen cap is the 20uF you have connected to the blue wire at the +ve end. The PI cap is the one you have connected to the brown wire at the +ve end. The two other caps are for the respective pre-amp channels.


If opting for a galactic ground circuit with a buss bar, all the ground returns for the output stage would connect to one point - i.e. the ground side of the (2 parallel) reservoir cap(s) and the screen cap would be connected together with the PT's High Tension secondary winding centre tap (or the ground side of a FW bridge if you are using a bridge rectifier). Also to be connected to this point would be the 6L6 cathodes and the ground return for the bias supply circuit and the ground return for the voltage divider that you are using for the reservoir cap bleeder/heater elevation network. Then you run a wire from this connection node to the floating (non-grounded) end of the buss bar. (i.e. Don't ground these separately - rather, ground them at the floating end of the buss bar)


The PI cap ground would connect to the ground return for the LTP inverter tail/presence pot. Then these would be connected to the buss bar (just after the connection point for the output stages).


Then the ground returns for each pre-amp channel would connect to the buss bar at the same location as the ground return for each respective channel's filter cap. The buss bar gets connected to the chassis at the bass channel's input jack sleeve terminal (which is the higher gain channel of the two pre-amp channels in this circuit). You don't need a separate connection to the buss bar for the normal channel's inputs (The 1M there can just ground at the input jacks for the normal channel)


If using a humdinger pot, the pot wiper can go either to the nexus/node where all the power amp grounds are going, or if you are elevating the heater circuit, then the ground return for the voltage divider for the heater elevation goes to that point instead (and the humdinger pot wiper would then go to the output of the voltage divider).

[HommeMarrBuckley]

Well, I took Brad's advice and shortened up a lot of the wires.  Still have the hum.
I listened to Tubeswell's advice (many, many thanks for his patience) and it made me look at something.
I looked at my PT from Classictone.
Here is the spec sheet:
http://www.classictone.net/40-18005.pdf


It says that the 2ndary filament tap is internally grounded.
Reads: "NOTE: GRN/YEL is internally grounded" on lower right side of spec sheet


I have this lead; my green yellow wire ground to a PT bolt.
Could this be a source for my 60 hz hum ie create ground loop?
Should I un-ground this lead, tape it up and tuck out of the way or will that do damage?

I doubt this is the answer because I wonder why they would have that wire avail in the first place. 
However, in all honesty, I hope it does serves a purpose in some way for someone else.....but in my case it has no use and just needs to be ignored and shunned.


Just thought I would post before re-working my bus and ground returns from the filter cap, different bias pot, bias posts, etc.


Lastly, in case the above (grn yell wire) doesn't amount to/mean anything, if I rework the ground returns from the filter cap board:
Should I keep the ground return wires seperate and each have their own channel back to the bus?  Like is it bad practice to have 2 or 3 ground wires share the same grommet hole.


Cheers and thanks,
Matt

[sluckey]

Reads: "NOTE: GRN/YEL is internally grounded" on lower right side of spec sheet

I have this lead; my green yellow wire ground to a PT bolt.
Could this be a source for my 60 hz hum ie create ground loop?
Should I un-ground this lead, tape it up and tuck out of the way or will that do damage?

That would have certainly caused a problem if you had tried to elevate the CT to some voltage rather than just connecting it to ground. But you are fine the way it is.

I just looked at you pics of the main board and the cap board. Your ground wires from the cap board are fine. However, there are some poor solder connections on the cap board. A couple connections just have the solder balled up on the eyelet. That's not good. Put some flux on all the eyelets and resolder until the solder flows smoothly and coats the brass eyelets. One eyelet doesn't even have any solder!

Most of the eyelets on the main also need fixing. Again, put some flux on each eyelet and resolder until the solder flows smoothly and coats the eyelet. You don't want any solder balls just sitting on the eyelets.

DON'T USE ACID FLUX! (like you would find on the plumbing aisle at Lowes) Use rosin flux that is meant for electronics.

[HommeMarrBuckley]

After occupying myself with other things and thinking over this, I decided to try and figure out a best method for Tubeswell's route in terms of better grounding with a bus.  I decided to bust up my cap doghouse because I thought it is best to keep things as short as possible.  However, before cutting any more wires and setting things to permanency, I thought it might be good to see if the placement of these caps is a good idea.  My bass, normal and PI cap are still on doghouse board. 



In the pic, the two to the right are the reservoir caps and the first on left is the screen cap.  I thought it best to keep these output grounds here.  I was going to tuck the bias pot up and have those grounds connect here as well along with the output cathodes.  I still have to drill for those bias points but I have them marked.  These will all head up to end of buss. 

Wasn't sure if it was ok to have all grounds connect together on one point or should they meet at one node first.  I was going to use the same circuit board and eyelets.  I am not going to screw in the bias pot so what do you think is they best way to hold it in place? (silicon-what type? or any glue?) I also saw a member on here use ties to hold the caps (soldered but wrapped around the middle)

Any input is appreciated, thanks.

Yeah, some soldering points weren't done too well and need redone.  still learning.

[tubeswell]

Separating the filter caps into local supply nodes with galactic grounding is the way to go. (Make sure the filter cap polarity is the right way around)

[HommeMarrBuckley]

This should be short.  In terms of repositioning and wiring these caps, my screen cap is now near my output tubes.  In looking at the schematic, can I just connect the + end of my screen cap to one of the pins (4 or 6) of the output tubes?  The rest of that connection is alright.  56 k  1 watt (other side doghouse) to choke black (circuit board) onto output tubes.
 
Thanks

[HommeMarrBuckley]

Bueller?  Anyone?


So is it ok to wire the + end of the screen cap to one of the screen grid pins on the output tubes?
http://www.gad.net/Blog/wp-content/uploads/2011/06/1963-Bassman-6G6B-Schematic.jpg


It looks ok but I don't know because I don't know much in terms of schematics, point to point etc.  I dunno because I don't think I have ever seen this on output tubes gut shots.   Just thinking its better for me than having to run a wire the whole way back to the doghouse where the 56k 1 watt resistor is.

[sluckey]

So is it ok to wire the + end of the screen cap to one of the screen grid pins on the output tubes?

No. The only thing that should be connected to pin 4 (screen grid) is one end of a 470Ω resistor.

[HommeMarrBuckley]

In terms of grounding my bus, I know folks go about things that they are more comfortable with but I was wondering in my limited know how if this seems correct in the route I am deciding to go.
If I ground my bus bar at the Bass HI jack, I should use a star washer here to help it bite to chassis.  All my grounds will be going this route except power ground and speaker jack.
But then by doing so, I should then probably use insulated washers for the rest of the jacks and pots, correct? Because I want them isolated from the chassis.
I know you use 2 insulated washers--(know this sounds dumb asking but I don't know) but do you put one on either side of jack or pot because of dual contact to chassis?
Thanks
 
 

[Willabe]

I've never heard of anybody using isolating washers on pots.

I guess your concerned about the ground buss running along the back of the pots making a ground loop?

Technically it is a ground loop, but it is a small loop. Many guys do use the back of the pots for the ground buss wire and have NO problem.

Yes on a star washer for good contact to the chassis but you always use a star washer there even when using isolating washers so the jack doesn't come loose.

Yes on using 2 isolating washers, 1 inside/1 outside. At least 1 of them needs to be a shoulder isolating washer, depending on the chassis/faceplate thickness. And you need to use the long style bushing Switchcraft jacks to be able to get the nut on.

Might I suggest looking at the all the different amp builds that our host Doug and 1 of the forums mods, Sluckey, have documented and posted here for great examples of quite grounding. And their layouts and lead dress are top shelf also! You can't go wrong copying from them. 

Do it like they do until you get a better handle on the principles.   

While it's a very good thing to learn and understand these principles and be able to apply them why re-invent the wheel?

Another thing is classic Fender amps are not high gain amps and don't really need as much attention to possible ground loop and lead dress problems.


                             Brad     

[Willabe]

You will find Doug's builds in here;

http://el34world.com/schematics.htm

And Sluckey's builds in here;

http://home.comcast.net/~seluckey/amps/index.htm

[Willabe]

In terms of grounding my bus,

HommeMarrBuckley, see this thread I started on grounding schemes for more info;

http://el34world.com/Forum/index.php?topic=18828.0

(As we have several members asking questions on grounding in the last week.)


                         Brad   

[HommeMarrBuckley]

Brad,
Thanks for the grounding info.


I went ahead and redid my cap layout but unsure if this is alright.
I left out all of the other connections on this drawing because I am fine with them, but just had a question in regards to my Phase Inverter cap and Screen cap.
On the left, is the traditional way it was done with the doghouse, I altered mine to whats on the right.
My question is, will this work?  Is it pretty much the same-connection wise?

I realize I could have just left the PI cap where it was originally but I wanted to move it closer to it's ground node and space the caps out.


Thanks
Matt

[Willabe]

Why do you have the normal preamp and bass preamp separated by the PI and screen cap B+ nodes?

(You can do it that way.)

The preamp cap B+ node is usually last in the B+ series chain, so it is the quietest B+ node, has the most B+ filtering.

The more signal the tube see's at it's input the less filtering it needs, the less signal a tube see's coming in the more B+ filtering it needs. As the B+ string goes through each cap (node) and choke/resistor then to the next cape/resistor each node is cleaner then the node before it. (But they do have large/larger R's feeding the preamp caps so that does increase the ripple filters effectiveness.) 

Edit: I just looked at the Fender schemo and it's correct the way you have the B+ string set up. (I don't recall seeing a Fender amps B+ string set up this way.)   

[HommeMarrBuckley]

Why do you have the normal preamp and bass preamp separated by the PI and screen cap B+ nodes?

(You can do it that way.)

Thanks Brad for verification.


In terms of seperating the preamps from the PI and screen cap.  In all honesty as to why, Im just trying to learn as I go.  When I started this, there weren't many clones posted out there.  Many had vintage ones that they recapped, serviced etc. but they all incorporated the brass plate.  The man that built a clone that inspired me to do this (Mosrite forum--great, gracious guy) used a brass plate too.  There's a guy that sells a clone with just the normal channel and added reverb.  He was cool but didn't seem to have the time to help a greenhorn which is how I ended here.  So, I have no idea of how he grounds his blonde.


I built the circuit and it seemed to have the tones.  When I started, I remember reading about bus bars and coax on jacks for eliminating noise and I decided I was going to do that.  I tried to follow other similar Fender builds with these elements.  It kind of derailed me though when I switched it on and I got a lot of hum.  Then tubeswell came along with his build in the post and made me reconsider my grounding.  At this point, I tried to find out and thought why not.  It isn't a high gain amp but still is pretty loud.  Again, super thankful that some skilled guys are patient and decided to break it up in stages and keeps grounds close to each.  Steve did mention looking over solder points but by that time I disconnected caps and took out the bar to rework, etc. 


I just hope this route gives me a hum free result.  I added bias points between the fuse and power switch.  The proximity worries me a little but we will see.  I just wish I had more time to do this because I would have been done a long time ago.  I feel like I owe it for all the help I have got like I am not holding up on my end but I will post what happens when it does.  Finished the grill cloth on the baffles only to find that I need to shave just the tiniest, frustrating amount of wood on the cabs to make it fit.  Gritting teeth when neat, cloth corners start to wear and fray alittle from fitting over and over.


On a funny side note, Ive been asked numerous times why I chose this as a first amp and not a Deluxe or Champ etc.  My top hero Jeff Buckley (seen a lot of acts live and never saw anyone come close or have the skillz he did--Im a more melody, mood guy than technical proficiency) he had a Tremoverb so I got one--you know how it goes--anyway--had a chance to talk to him before he died about what he was using--said a "Bassman--you know with the piggyback" so everything kind of fell into place with me doing this---only to check online around the start of this and see that he had the Bassman 135--Ha!  My memory could be off and I have yet to ask my friend that was there with me but I swore he said the blonde--could be just one of those for the studio deals--Who knows.  Now, how to construct a Black Beauty---nah that's alright.

[tubeswell]

So how does it sound now?

[Ed_Chambley]

Lots of guys have used this Bassman.  Tom Petty, George Harrison and John Lennen, Cliff Gallup.  You will also see this amp behind a lot of steel guitars.  Brian Setzer of course really loves his, but it is a little different circuit so may not compare.

I have had one for a while now and the honeymoon period has been over a long time.  When I can use it in a place that is large enough it is there along with the old blond 2, 12 with Altec 417.  I own a lot of amps and played many more.  IMO in a 50 watt it is my favorite just edging out my Super Reverb which I still carry and use as my effect amp for larger shows.

[Willabe]

IMO in a 50 watt it is my favorite just edging out my Super Reverb which I still carry and use as my effect amp for larger shows.

WOW, that's saying a lot, from you, knowing your attachment to BF SR's. (I love BF SR's too.)

[Ed_Chambley]

IMO in a 50 watt it is my favorite just edging out my Super Reverb which I still carry and use as my effect amp for larger shows.

WOW, that's saying a lot, from you, knowing your attachment to BF SR's. (I love BF SR's too.)

It is saying a lot.  I guess after playing a Super Reverb for over dare I say 40 years I have changed my main preference in amps once.  I will say I still use a Super Reverb a lot and my favorite Tweed is a 5F4 which is sort of strange.  I like to hear the guitar I guess.

I know you have strummed a Super a time or 2 yourself.  I am liking the simpler circuits these days seem like.

[Willabe]

and my favorite Tweed is a 5F4 which is sort of strange.  I like to hear the guitar I guess.

Have you ever disconnect the FB loop that's around the CF stage, 4M7 R, to hear what that sounds like?

[Willabe]

I know you have strummed a Super a time or 2 yourself.

Played it for years and never really had a problem with it. A lot of the older blues guys and the younger guys too, where I hung out at all either had a BF TR or a BF SR. Just very good sounding amps that held up to the work load.

Had a heater/CT 100R burn up once and the black plastic/steel strap broke but other than that, I can't remember anything else going out.   

[Ed_Chambley]

and my favorite Tweed is a 5F4 which is sort of strange.  I like to hear the guitar I guess.

Have you ever disconnect the FB loop that's around the CF stage, 4M7 R, to hear what that sounds like?

Never done that.  I have 2 of them.  One is a head and I have tried all sorts of things with it, but usually put it back.  My other one is on loan to a Friend right now.  It is the "V" front cabinet, but it is a clone.  He is gigging with a modern hipster group with a retro sound almost like Rock-a-billy.  He likes the look.  They call themselves the Cleftones.

What does the mod do, make it a little raunchy?

[HommeMarrBuckley]

"We might be done with the past, but the past aint through with us" - one of my fave quotes from Magnolia which kind of describes my situation.
I take some between posting and to help with recall---I built the 6g6B circuit.  Had some flubs along the way.  Got it working good with sound but had to deal with some 60 HZ hum.  Fellow member recommended altering to galactic grounding.



Was more nervous this time than last when I powered it up.  Let it warm up a bit and then flipped the standby.  Immediately it powered off/pilot light cut out.  I quickly flipped both switches off--but there wasn't any smoke, sparks, noise or anything alarming.  I was in the weeds and super down.  All this time into this and all for what?  Checked fuse and sure enough--blown.  With all this new wiring, I was trying to make sure nothing crossed.

[HommeMarrBuckley]

Bias Points

[HommeMarrBuckley]

Reserve/Screen Caps


Bias pot isn't attached to chassis.  Tight snug fit on 1st layer circuit board.

[HommeMarrBuckley]

Bus Bar
I didn't apply a node for the PI cap--just attached those three connections to bus.  Added .0033 caps to input jacks for RF

[HommeMarrBuckley]

Here is where I am hoping the culprit is and will only be a minor fix. 
The PI, Normal and Bass caps.
Notice the red wire.  I thought of it for a second when I screwed the doghouse (not shown) on but obviously not enough.  The doghouse edge put it tight to the chassis.

[HommeMarrBuckley]

Look close.  I guess this looks enough to be a short/arc, doesn't it?  The black.  That red wire connects underneath between 56k and 4.7k resisitor and goes to + end of screen cap.I will have to take aim here next week--and decidely this amp will either not have a doghouse or will with routed hole/pathway.
Will also help me look under the added reserve/screen cap board to make sure nothing is crossing there as well.
I am just hoping that this is the only thing that caused the issue.
Will have to wait until next week.

[Willabe]

Was more nervous this time than last when I powered it up.  Let it warm up a bit and then flipped the standby.  Immediately it powered off/pilot light cut out. 

If you don't have a light bulb limiter than you need to build 1;

http://home.comcast.net/~seluckey/amps/misc/Amp_Scrapbook.pdf

[SILVERGUN]

Notice the red wire.  I thought of it for a second when I screwed the doghouse (not shown) on but obviously not enough.  The doghouse edge put it tight to the chassis.

HMB, please take this as a constructive, well intended safety warning that you absolutely need to hear....

TAKE A TIME OUT AND READ ABOUT ELECTRICAL SAFETY

Our entire goal while building these death traps are to isolate the high voltage from the chassis by whatever means necessary.
In general, all electrical appliances are unsafe by nature even when wired by professionals and stamped with UL approved markings.

By "thinking about it for a second when you screwed the doughouse down" you have proven to have decent instincts.
Learn to listen to them and continue to question the result of your actions.

Be glad you are not recovering in the hospital right now.

Respectfully and with genuine concern,
SG

[HommeMarrBuckley]

First,
Here you go Brad.  Something I should have made a long time ago.  I dunno why I was so resistant to make one--probably because I don't have much time and thought it would take too long.  I found out quite the opposite.  Pretty simple and quick to build.  I used this site because it's explained simply and  Im borderline technical lunkhead.
 https://robrobinette.com/5e3_Modifications.htm#Light_Bulb_Current_Limiter
so mine looks exactly like his.


SG, I appreciate your concern and was not offended at all.  With what limited knowledge I have, I try to be careful as possible.  Laugh, I learned how much I take for granted when you goto the store or somewhere and plug in and flip the switch compared to doing that same process to an open amp that you made. 


Well, I removed the shorted wire and put in a new one and put in the new fuse and plugged into the limiter.  I used a 150 watt bulb and read what I should expect which is what basically happened.  The bulb's filaments got bright red upon power up but then cooled and dimmed.  I flipped the standby and nothing.  No change in the bulb and no loss of power or sound/noise.  I powered down and plugged the amp directly into the wall outlet and checked the input jacks for clearance with guitar cable.  Plugged in Normal channel.  Powered up--all ok---flipped the standby---NO HUM!   I increased the amp volume to about 3-4--still no hum.  Rolled guitar volume and success!  Out came the slight grainy clear sound.  Man, I was smiling like a little kid.


 I pushed the volume to the max and there was a slight bit of hum with no guitar but I figure that's not unusual considering it's fully dimed.  I checked the remainder of the inputs--all good.  All good except the bass channel.  Plugged in and it sounded max volume right away.  Amp volume knob was at zero. ??? Played with amp volume knob and didn't do much.  It was hella loud.  Took a second---sure enough.  Looked at volume pot and saw bit of solder residue on tab.  Left over from when I replaced the bus bar which means something was there before.  Checked layout--sure enough I forgot to add the wire there that's grounded.  Regardless, that's an easy fix.  I am just happy that I got the end result.


I plan on checking voltages again and bias but I didn't have much time before work.  I will try to add video showing that the hums gone but I am pretty much at the end of the road.  Not whole lot to do with the cab and head but that's where Im going to now.  Every story has three arcs (no pun intended) so that will be the last and will be short.   I will post when it's complete.


Still blows my mind because it's about one of the coolest things Ive ever done.  But I would have never got here without the help and generosity of others.  Thanks to all admins and members that have chimed in and helped me get here. 
And a very special thanks to you Pete.  Thank you for patience and input and helping me get over this last and challenging hurdle.  You're right, galactic grounding is the way to go.
Cheers!