Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: newguitarsmell on May 24, 2019, 08:48:36 pm

Title: more problems with plexi build
Post by: newguitarsmell on May 24, 2019, 08:48:36 pm
Thanks all for the help. The bright pot and the master vol both have a buzz. I'm wondering if those resistors R10 and R11 and C5 need to be grounded? The pot is grounded but I don't have the R and C wired to the back of the pot...just to each other.. Hard to tell from the schematic. Thanks

https://el34world.com/Hoffman/files/Hoffman_Plexi_6V6-V2.pdf

I have good readings at A and B, (about what it says on the schematic)..then nothing (as in no current)  :BangHead:at C or D or E

Lights come one - tubes heat up and nothing... not even a buzz.
I used a MM OT - 18W - brown/red with a white center tap....

Here are some pics.

(https://i706.photobucket.com/albums/ww65/mmayer97801/Plexi%206V6%20build/IMG_1171.jpg) (http://s706.photobucket.com/user/mmayer97801/media/Plexi%206V6%20build/IMG_1171.jpg.html)

(https://i706.photobucket.com/albums/ww65/mmayer97801/Plexi%206V6%20build/IMG_1177.jpg) (http://s706.photobucket.com/user/mmayer97801/media/Plexi%206V6%20build/IMG_1177.jpg.html)
(https://i706.photobucket.com/albums/ww65/mmayer97801/Plexi%206V6%20build/IMG_1176.jpg) (http://s706.photobucket.com/user/mmayer97801/media/Plexi%206V6%20build/IMG_1176.jpg.html)
(https://i706.photobucket.com/albums/ww65/mmayer97801/Plexi%206V6%20build/IMG_1175.jpg) (http://s706.photobucket.com/user/mmayer97801/media/Plexi%206V6%20build/IMG_1175.jpg.html)
(https://i706.photobucket.com/albums/ww65/mmayer97801/Plexi%206V6%20build/IMG_1173.jpg) (http://s706.photobucket.com/user/mmayer97801/media/Plexi%206V6%20build/IMG_1173.jpg.html)
(https://i706.photobucket.com/albums/ww65/mmayer97801/Plexi%206V6%20build/IMG_1172.jpg) (http://s706.photobucket.com/user/mmayer97801/media/Plexi%206V6%20build/IMG_1172.jpg.html)
Title: Re: Problems with the Plexi 6V6 build Please help!!
Post by: SILVERGUN on May 24, 2019, 09:52:55 pm
http://el34world.com/Forum/index.php?topic=19361.0 (http://el34world.com/Forum/index.php?topic=19361.0)


http://el34world.com/Forum/index.php?topic=17701.0
Title: Re: Problems with the Plexi 6V6 build Please help!!
Post by: Colas LeGrippa on May 24, 2019, 09:54:25 pm
Did you check  r39 ?  If well soldered and value is aprox 20k you should have voltage in C
Title: Re: Problems with the Plexi 6V6 build Please help!!
Post by: newguitarsmell on May 24, 2019, 11:36:16 pm
Hi Thanks all - I did those things from the "do this first" list.
The problem turned out to be a dead cap - that Ajax .022. It literally came apart when I touched it and that was that.
I swapped it out and I'm getting good sound. The bright vol and the master vol have a buzz - I need to run that down...

Which capacitors have the biggest impact on tone in this kind of amp?

Also as the tubes go, which of the three preamp tubes most influences tone?
Title: Re: Problems with the Plexi 6V6 build FIXED
Post by: shooter on May 25, 2019, 07:27:04 am
as to which cap....

http://el34world.com/Forum/index.php?topic=19534.0

which tube & tone, me, I ALWAYS start with the speaker tube for tone, 4 or 5 different ones at least
 :icon_biggrin:
Title: Re: Problems with the Plexi 6V6 build FIXED added question
Post by: newguitarsmell on May 26, 2019, 05:08:03 pm
I have redone a lot of the wiring and I still have a bad hum that is in the bright volume and the master volume.
I am extremely frustrated since I have already redone one tube socket and the shielded wire hookups and... what am I missing? I get a great sound and a bunch of noise.
Title: Re: more problems with plexi build
Post by: dude on May 27, 2019, 11:12:18 am
Are you sure you did the shielded wiring to master and from input correctly? Might need it on the volumes too. Move wires around as you cause the hum, chop stick, does it change? Only ground the outer braid of the ground on the input and the the master vol gets a braided ground too. The other ends get tubed shrink (ground should not touch anything).


al   
Title: Re: more problems with plexi build
Post by: Tone Junkie on May 28, 2019, 11:51:29 pm
Oh do I remember my first build 50 watt plexi . If it wasn't for PRR,Tubnit, Slucky And a host of others I would have never finished . Seems I had a whole host of missing grounds, bad solder joints and a few other things. I think about 2011 brings back memories . Sorry for the high jack . Follow the gentleman who suggested the chopstick method, Definatly check how you put together your shielded wire, I spent months chasing ghosts on a splawn clone one wire from the shield was grounded at both ends. Bill
Title: Re: more problems with plexi build
Post by: newguitarsmell on May 30, 2019, 01:27:27 pm
YES!!
Title: Re: (even) more problems with plexi build
Post by: newguitarsmell on June 02, 2019, 01:22:02 am
I got the Plexi 6v6 up and running. The sound is promising, but there is still a hum. I swapped out the entire grounding-to-the-pot thing with something Slucky suggested and it helped a lot.. Adjusting the bias brought the hum down, too.

I did some readings on the DC voltages compared to what is supposed to be at A and B...(+) and found NO DIFFERENCE between A and B. Why? At first I thought maybe the choke wasn't working but I tested it and it came back with the following: 3.65H R=153.5Ohms.

No matter what I do I can't get the voltage to drop between A+ and B+.
The other funky reading was the VAC between the two 1N5408 diodes - is supposed to read 307 and it's reading 618? I don't get how there is a type of center tap between the two diodes that goes to ground. What am I missing?  :BangHead: :BangHead: :BangHead:

I looked through everything multiple times, and replaced some wires. I also traced through everything using the schematic - It is soooo close to sounding great - though right now I have it ripped all apart again.. HELP!
Title: Re: more problems with plexi build
Post by: sluckey on June 02, 2019, 02:59:38 am
Quote
No matter what I do I can't get the voltage to drop between A+ and B+.
That's normal. Look at the voltages on Hoffman's schematic.

Quote
The other funky reading was the VAC between the two 1N5408 diodes - is supposed to read 307 and it's reading 618?
That's normal. If you measure the voltage from one diode to chassis ground you will get half of 618.
Title: Re: more problems with plexi build
Post by: DummyLoad on June 02, 2019, 03:04:57 am
is the secondary of the OT wired correctly to the impedance selector switch? i see the orange wire tied to ground on the output jacks and the black wire tied to a switch pole. is the black wire the common? i believe that it is, however cannot confirm since the supplier of your OT is unknown. if you have the hoffman or classictone part, then swap the orange wire and the black wire.

orange is 16Ω tap
green is 8Ω tap
yellow is 4Ω tap
black is common/ground

deduced from the last picture of your opening post.

https://el34world.com/projects/images/DSCN1697.JPG (https://el34world.com/projects/images/DSCN1697.JPG)



--pete
Title: Re: more problems with plexi build
Post by: newguitarsmell on June 02, 2019, 12:51:17 pm
Pete - -

OT is Mercury MAR18-OT

PRI                       SEC

Red                       ORANGE - COM
White                    BL - 4 Ohm
Brown                   YEL - 8 Ohm
                            GR - 16 Ohm

That's why the Orange wire is where it is.....??
Title: Re: more problems with plexi build
Post by: newguitarsmell on June 02, 2019, 12:57:17 pm
In the lower RH corner of the turret board, there is a red wire attached to the last turret under the diode space.
The is not on the schematic - or on the layout. What is that?
This is from Doug's Plexi 6V6 build page 
Title: Re: more problems with plexi build
Post by: newguitarsmell on June 02, 2019, 01:04:47 pm
I've been through it and through it - my poor solder sucker is having a workout!
I redid all the pot and input wiring. I have not redone the speaker wiring - yet.

I am about ready to change out the heater wires... and redo all the tube sockets...
So close!

It's not looking as beautiful inside as it once did...
Title: Re: more problems with plexi build
Post by: sluckey on June 02, 2019, 01:11:50 pm
Quote from... https://el34world.com/schematics.htm

Quote
Mods and Projects 1
Note: When assembling the Hoffman Turret boards,
 use the documents under each board section.
Do not use my picture builds below to assemble a Hoffman board.
You may look at my picture builds as a rough reference only.
Things always change after I do a build.
That's why I do the test builds first.

Hoffman Plexi 6V6 Build
Title: Re: more problems with plexi build
Post by: newguitarsmell on June 02, 2019, 01:12:38 pm
 :l2: got it.
Title: Re: more problems with plexi build
Post by: sluckey on June 02, 2019, 01:47:16 pm
Your impedance selector and speaker jack are wired wrong. You have a green wire connected to the common lug of the switch. That common lug should connect to the tip of the speaker jack. Then the Black, yellow, and green OT wires should connect to the other three lugs on the switch.

The orange (common OT wire is properly connected to the ground lug on the speaker jack. But there is a second orange wire connected to the jack also. Where does this wire go? If it is supposed to be the NFB wire then it should not be connected to the speaker jack ground lug.

The yellow wire between the switch and the speaker jack is connected wrong on both ends. Just remove it and throw it away.

Now connect the common lug of the switch (mentioned above) to the tip of the speaker jack. The tip of the speaker jack is the lug that you have never soldered to.

I don't see a NFB wire unless the second orange wire I mentioned above is it.

I just completed a project using that exact same switch. My OT wires are white, yellow, and green. I used a gray wire between the switch common and the tip of the speaker jack. Here's a pic that may help you get yours wired correctly...

     http://sluckeyamps.com/phoenix/p12_big.jpg

When you get your impedance switch, speaker jack, and NFB wire connected properly, your hum issues may disappear as well.

Title: Re: more problems with plexi build
Post by: sluckey on June 05, 2019, 10:27:23 am
Any progress?
Title: Re: more problems with plexi build
Post by: newguitarsmell on June 06, 2019, 10:56:28 pm
HI Steve
I rewired the entire selector switch so it's right now. I haven't hooked it all back up again yet. I swapped out a bunch of wire and need to reconnect the choke. I rebuilt the shielded cables to the volumes... redid the entire grounding section.. we'll see. Wondering about NFB. I never used it on other amps - wondering if I need to hook it up for this one. I want to build your Plexi 6v6 does Doug make the turret board for it? Do you have a plexi with reverb?
Title: Re: more problems with plexi build
Post by: sluckey on June 06, 2019, 11:14:18 pm
NFB is important to the way an amp sounds. Doug will make a board for you if you provide the drawings. Ask him. I don't think  reverb goes with the Plexi sound.
Title: Re: more problems with plexi build
Post by: newguitarsmell on June 07, 2019, 12:41:27 am
I looked at your website and really liked that Deluxe Reverb you made. I've made two or three Princetons, a 5e3 and now the Plexi..I really like the sound of the 5e3 but had some issues with it.
I'm just a beginner. Most women aren't really into building amps - but I build guitars, too so I needed more of a challenge. When I was young, girls didn't make amps. Consequently, I make much better pies than amps.
Title: Re: more problems with plexi build
Post by: SILVERGUN on June 07, 2019, 09:28:04 am
Quote from: newguitarsmell on June 07, 2019, 12:41:27 am
I looked at your website and really liked that Deluxe Reverb you made. I've made two or three Princetons, a 5e3 and now the Plexi..I really like the sound of the 5e3 but had some issues with it.
I'm just a beginner. Most women aren't really into building amps - but I build guitars, too so I needed more of a challenge. When I was young, girls didn't make amps. Consequently, I make much better pies than amps.
Well, if you keep working on sluckey he might wind up trading you that Deluxe Reverb for a pie-of-the-month agreement. I don't think he gets emotionally attached to his amps, so you might have a shot.

I remembered seeing this video a while back where they show the workers in the Fender factory that do the handwiring of the higher end stuff. You will notice a large majority of women workers. At one point (3:35) the guy giving the tour says that "very precise hand work is predominantly done by women who have more patience and smaller hands".
So, you've got that going for you.

https://www.youtube.com/watch?v=54sNv1Ck6dc (https://www.youtube.com/watch?v=54sNv1Ck6dc)
Title: Re: more problems with plexi build
Post by: PRR on June 07, 2019, 10:58:47 am
> When I was young, girls didn't make amps.

I see what you are saying. When I grew up, boys didn't type, could not even take the class. So what do I do today??

But since before you were born, women workers in factories did nearly all the wiring in radios and related stuff like TV and guitar amps. I knew a gal who made TV tuners in the 1970s, and while she was taught funny-names for the parts, her pliers and solder skills were excellent.

If you are into academic literature: the book Capital Moves traces the history of RCA with women. They could be paid less, and could be dominated. Typically they got married and left in a few years, about the time they fully understood the exploitation, so didn't infect new workers with crazy ideas like fair quotas and pay. Nevertheless by the 1930s they were unionizing, and there were violent strikes. But "Capital Moves", labor is local. RCA moved to Bloomington and exploited those women until they struck. They moved to Memphis at a bad time. Then they discovered Mexico, or actually special "Trade Zones" just over the border. In a cash-poor desert, a young woman's paycheck might support a whole family, so she was under pressure at home to stay compliant to keep the job. Which was being threatened by lower-pay women in Asia, and RCA used that threat hard.

I'm not endorsing the book (https://www.amazon.com/Capital-Moves-Seventy-Year-Quest-Epilogue/dp/1565846591). It is long and dry and does not connect well. Only for the seriously curious.
Title: Re: more problems with plexi build
Post by: sluckey on June 07, 2019, 11:44:04 am
I worked with a lot of female techs in FAA. Probably 10 to 20% of the tech workforce are now female. I never knew one that considered electronics (especially building stuff) a hobby though. It was all about this is what the military trained me to do so I turned it into a career job. FAA was very hip to hiring veterans, especially minority vets.
Title: Re: more problems with plexi build
Post by: ALBATROS1234 on June 07, 2019, 12:08:01 pm
Yea the recent uprising where feminists state women weren't allowed to do this or that was true 100plus years ago maybe but strong women and women in the workforce including tech jobs have been around since the late 30s early 40s. The family structure thing was more important so less women chose to do those things and more of them focused on being the rock that held the family unit together therefore usually producing more well adjusted children who grew into productive citizens. They were the greatest generation and are why America reached the top but it seems to have peaked in the 1960s and has been going steadily downhill since unfortunately. And now we have millenials :BangHead: :cussing:
Title: Re: more problems with plexi build
Post by: shooter on June 07, 2019, 01:10:30 pm
Quote
could not even take the class.
we could , I was the only male in the class, ff 3yrs, boot camp commander ask for volunteer, I did, worked for the Admiral of the base while everyone else marched a lot  :laugh:
Title: Re: more problems with plexi build
Post by: vampwizzard on June 07, 2019, 02:40:58 pm
im 32.. the shift has been dramatically slow and long overdue in my early career. A lot of outreach and active 'Women in STEM' movement has helped. It's funny to me that there were so many women pioneers in early computing science and astrophysics/math and then the gigantic gap until about the last 10 years. IDK if thats related to the amount of skilled labor coming out of the military post WW2 or not.. just interesting.

FWIW i also wrestled in college and then coached for my alma maters first womens college wrestling team. Women kick ass too. Glad youre lighting the torch newguitarsmell
Title: Re: more problems with plexi build
Post by: newguitarsmell on July 01, 2019, 04:35:00 pm
Hi all - two comments - - yes A+ for STEM!  and - - those ladies who work for Fender do it for a job. I build amps out of a sick fascination. (ha ha) - if that was my job, I am certain I would feel like a human robot, but would probably spank the competition.

MY PLEXI BUILD: I attached the NFB and really not much changed. I still have hum and more irritating than that is the fact that the amp sounds over driven all of the time. I cannot get a clean tone. I am running one 6881, two 12ax7s and two Bendix 5992. I have 12ax7s I can try, but I generally love the clean feel of the 6881. Earlier in this post I linked a bunch of photos. I have since ripped the whole thing apart and rewired it all. I will post more updated, less pretty now, photos this afternoon. The main hum is in the second volume. There are three as seen in this pic here. The 470K and 500pf resistors in parallel are tied to a jumper which is attached to the back of the pot.
I couldn't get that to work so I just wired them all together and let them flow. Is that the problem?
Title: Re: more problems with plexi build
Post by: shooter on July 01, 2019, 04:48:28 pm
Quote
the amp sounds over driven all of the time. I cannot get a clean tone.

both channels?, one vol zero'd and just using 1 "side"
when I built mine I used an AU7 for "V1" to knock down gain.  The "clean" channel was TOO clean/bright/sparkly
the NOR channel was TOO dark/muddy
BUT when I dialed clean to ~ 4 and normal to ~ 2 I got a smile.  The "ratio" was almost always ~ 2:1
Then I got perma-grin when I matched up speakers in a 2X12  :icon_biggrin:
Title: Re: more problems with plexi build
Post by: Ritchie200 on July 01, 2019, 06:31:39 pm
Newguitarsmell

Hard see on the pics you posted but too dark.  Check value of R20. 

Backing up a bit...  I used to work at a fuse company that was established over 100 years ago.  Major components had always been manufactured but assembly was done by hand.  The majority of products were still hand assembled well into the 70's.  Women always did this work with some products requiring very intricate soldering, or handling of delicate element strips, some of them made out of silver less than a .001" thick.  God help you if you were a mechanic and the line went down.  This was piece-work and they had plenty of critical eyes looking over their shoulder while they did the repairs!!!  As sensors and other controls developed, much was automated but some products even today still require a steady hand.  Some of the women trained in PLC and CAM.  Some actually transitioned into engineering support roles.  Pretty cool as line experience was crucial in development of new products.

Sluckey I think I mentioned this before, but I aced my Controllers test twice and lost out to veterans.  Hard to be mad about that, but I sure wish they would have had room for one more!

Jim
Title: Re: more problems with plexi build
Post by: SILVERGUN on July 01, 2019, 06:48:13 pm
Quote from: newguitarsmell on July 01, 2019, 04:35:00 pm
- I still have hum
You're gonna have to identify this hum. Different folks have different definitions.
An exact frequency would REALLY help.

Quote from: newguitarsmell on July 01, 2019, 04:35:00 pm
- and more irritating than that is the fact that the amp sounds over driven all of the time. I cannot get a clean tone.
This could be the result of lower than expected voltages.
Please give us a list of all voltages on all tubes in a format that we can easily understand.

Quote from: newguitarsmell on July 01, 2019, 04:35:00 pm
-I am running one 6881, two 12ax7s and two Bendix 5992. I have 12ax7s I can try, but I generally love the clean feel of the 6881.
6881 ?  :dontknow:  https://tubedata.tubes.se/sheets/201/6/6881.pdf ?

Quote from: newguitarsmell on July 01, 2019, 04:35:00 pm
-Earlier in this post I linked a bunch of photos. I have since ripped the whole thing apart and rewired it all. I will post more updated, less pretty now, photos this afternoon.
Ok, but you're initial attempt was unpretty enough.  :icon_biggrin:
Your pics were good. Your issue could very well be somewhere in the messy wiring, so that's what you will want to fix going forward. (at least if you are going to want us to try to visually spot your issues).

Quote from: newguitarsmell on July 01, 2019, 04:35:00 pm
-The main hum is in the second volume. There are three as seen in this pic here.

Quote from: newguitarsmell on July 01, 2019, 04:35:00 pm
-The 470K and 500pf resistors in parallel are tied to a jumper which is attached to the back of the pot.
I couldn't get that to work so I just wired them all together and let them flow. Is that the problem?
Well, to be clear...the 470K and 500pf need to come from the pot wiper and connect to the 470K from the other pot wiper and then feed directly into the grid of V2-A....and yes, if you have got that bung'd up IT IS A problem.
No jumper to back of pot...I can see where that looks a little confusing in the layout but I would always refer back to the schematic for the correct wiring.

We all work better in schematic talk so whenever possible, reference the schematic.
If you aren't comfortable with the schematic, now is the time to trace out the circuit with a highlighter so you can truly attempt to understand all of the little pictures that make up the big picture.

Good luck...we have all been there 
Title: Re: more problems with plexi build
Post by: Ritchie200 on July 01, 2019, 06:53:41 pm
Sorry, more to the story.  Some used the tuition reimbursement program and got their EE degrees.  It takes some passion, beyond the assembly line, to go that route!  I was also told that back in the 60's and 70's an assembler at Marshall (the majority were also women) could build 4 - 50watt, 2 - 100watt, or 1 - 200watt in a shift.  I feel well spanked with every build I've done compared to that!

Jim
Title: Re: more problems with plexi build
Post by: newguitarsmell on July 01, 2019, 06:59:22 pm
Thanks for all the input.
I did trace the amp through the input but at this point what I am going to do is put it aside, and build another being super careful and neat with the wiring. If you thought the first pictures were messy, you wouldn't want to look at where I am at now.
ha ha :l2:
I will check voltages next.
m
Title: Re: more problems with plexi build
Post by: sluckey on July 01, 2019, 07:19:28 pm
Quote from: newguitarsmell on July 01, 2019, 04:35:00 pm
I generally love the clean feel of the 6881.
6881??? Has no use in any audio amplifier. Maybe you love the 6681?

Hoffman's 6V6Plexi is a proven circuit. Stick to the plans and you will be successful too.
Title: Re: more problems with plexi build
Post by: shooter on July 01, 2019, 07:28:28 pm
Quote
put it aside, and build another
:laugh:
after 18 of 'em I gave up and figured out how the wise here did "IT" and copied  :icon_biggrin:
study an Sluckey gut shot, a lot, then more, then sloooow down to a snails pace, walla, power up n jam
Title: Re: more problems with plexi build
Post by: newguitarsmell on July 02, 2019, 06:23:24 pm
Yeah... 6881. The mystery tube also known as the 5751. Just to remind everyone what a newbie I am. I ripped the thing apart in a rage last night - melted up all the cottage cheese solder and am putting it aside for awhile.
I was getting a reading of ZERO on #3 on either of the 6V6 tubes so I figured I needed a vacation from it.
I will return to it after I begin another - and like you say Shooter - SLOWLY and METHODICALLY putting it together.. bit by bit..
This time its the Proluxe version of the 5e3 from Robrob's site... we'll see how that goes. I am determined to get this right someday. Thanks everyone for the help. I will check back and start up the plexi again when I'm not mad about it anymore (a few weeks). I will not let that amp WIN! muoahahahahahah! :cussing: :cussing: :cussing:
Title: Re: more problems with plexi build
Post by: shooter on July 02, 2019, 07:12:10 pm
 :l2:
I'm the poster child for ADD, so I understand
I my profession I lernt the turtle ALWAYS wins, make a mistake, bad things happen to good ppl!

here's a before n after, us slow learners can learn  :laugh: