Post got deleted - Using separate Amp/Preamp PT's.

[EL34]

Guys, something was screwed up in your original post
I could not figure out how to fix it


I copied all the text from the post and I will post it below
you can pick out your text below

[EL34]

I can email them if you wish

That's OK. You may want to get the custom Visio stencils on my website...


http://sluckeyamps.com/index.htm


I still see a couple wires missing on your wiring diagram. RE, your drawing... There should be a wire connecting the right side of R16 to that chassis ground lug between the 5U4 and the OT. Also, that red jumper you added... Look closely at your amp and you should find that those two points also connect to that chassis ground lug. Maybe not directly, but through another point that has a wire to that ground lug.


We're very close to having an accurate, working schematic of your amp. As soon as I have the voltage readings I'll post the Visio file that also includes a couple guitar preamps for you to think about. If the Visio file is too large to post here I'll send you a link so you can download it.
 ----------------------------


replace the bug



 -----------------------------

1. The resistor connected to V2-4 is a 100K.  Now designated as R6. It is orange/black/yellow/gold.  I read it as 300k by markings and 309k with the meter.

OK. I have corrected the schematic to show 300K also.

I am attaching some dimensioned drawings as well. Need to finish location of existing input, speaker connections, power cord etc. but everything else is there.

I can't see any attachments. In fact, I can't even see this reply through the normal viewing method. I had to select your name and then select show posts to view this reply. Please repost the dimensioned drawings and your latest wiring diagram.

As for the voltage readings... I am ashamed.  I messed up somehow. 

Connected speaker to speaker lugs.  Turned volume controls down to 0.  Plugged in the amp.  Everything lit fine.  Negative lead was grounded to chassis.  Touched positive lead to pin 5 (termination of yellow wire from PT)  as soon as I touched it, it instantly drew an arc and blew the 2A fuse.  So now I am dead in the water until I can pick up a fuse tomorrow.  Hoping I didn't seriously damage anything.   Will need to study up some more before I try again but I believe I followed instructions correctly...?

Pin 5 on the V5 socket is just being used as a convenient connection point. That yellow wire is the centertap for the 5V filament winding. It contains the B+ voltage for the amp. Should measure about 350 to 450VDC. I suspect you may have had your meter set to measure resistance rather than voltage. Probably didn't hurt the amp but may have damaged your meter. Which meter do you have? Brand and model number.

[EL34]

Sounds good. I will measure when I get home and create a scaled template of the chassis.

For the resistors I did a cut and paste from https://www.hobby-hour.com/electronics/resistorcalculator.php
If you input the value and ask it to display resistor it will show a color coded resistor,  rightclick on image and copy image into Visio.  You can size the object once you have it in Visio.

I am using the engineering/electrical engineering/fundamental items and fundamental items- visio 2013 stencils.  I tried to attach but getting an error message.  It will accept vss files
 but not vssx which is what they are.  I can email them if you wish.
------------




Sounds good. I will measure when I get home and create a scaled template of the chassis.


For the resistors I did a cut and paste from https://www.hobby-hour.com/electronics/resistorcalculator.php
If you input the value and ask it to display resistor it will show a color coded resistor,  rightclick on image and copy image into Visio.  You can size the object once you have it in Visio.


I am using the engineering/electrical engineering/fundamental items and fundamental items- visio 2013 stencils.  I tried to attach but getting an error message.  It will accept vss files
 but not vssx which is what they are.  I can email them if you wish.
 
 -----------------
 
 R11 should be 100K.
 
 ------------------------
 What are the chassis dimensions (excluding mounting lips)? L x W x D


EDIT... Just noticed you added reference numbers for the resistors and caps. I'll do that to the schematic as well.


Your resistor shapes look nice. Do you have a complete stencil set? I'd like to get the stencil file if you don't mind. You can directly attach vsd and vss files to your replies.
------------------------


OK, I see that and have highlighted in red.  Will make corrections tonight.  I think I spotted a couple other mistakes as well that I need to take a closer look at.


Thanks for the voltage reading tips.


I have added reference numbers for ease of discussion.


---------------


1. The resistor connected to V2-4 is a 100K.


2. The resistor between V2-1 and V3-5 should be 100K.


3. The resistor between V2-5 and V3-4 should be 240K.


4. The 50µF/6V cap connected to V1-8 should have the + on the top side that connects to V1-8.


5. Look at the bottom side of that 50µF/6V cap. Now look just to the right at the node where the 2K, 2K, and 6.2K join together. Those two nodes should have a jumper wire between them.


----------------

I have been taking resistor readings tonight.  I have a few that aren't decoding as standard values.   Color decode shows it as 48.4G Ohm?  Meter shows 520 in the 2000k range.  There are 2 of them.  Color banding is yellow, gray, yellow, gray, brown.  They are rather large connecting V2 pin 5 to V3 pin 5, and also V2 pin 2 to V4 pin 5.

Those are not resistors. They are capacitors. The value is .1µF. The color bands that tell the value are brown, black (not gray), and yellow. That reads 100,000pF or .1µF. Color code is the same as resistors.


Plug straight into the wall when taking voltage readings. DON'T USE A CURRENT LIMITING BULB!


All AC and DC voltages can be checked with the black meter lead connected to chassis...  ***EXCEPT*** for V5 pins 2 and 8. Those must be checked with one probe on pin 2 and the other probe on pin 8. Should be 5VAC. Pull V5 while making this check.


-------------------


OK.   Thank you.   I misunderstood what your drawing was.


I have been taking resistor readings tonight.  I have a few that aren't decoding as standard values.   Color decode shows it as 48.4G Ohm?  Meter shows 520 in the 2000k range.  There are 2 of them.  Color banding is yellow, gray, yellow, gray, brown.  They are rather large connecting V2 pin 5 to V3 pin 5, and also V2 pin 2 to V4 pin 5.


There are 3 wire wound "Tru-Ohm" resistors.  They are marked 10,000  225 and 1000.


Other than that, I think all values are correctly represented.


I will try to take some voltage readings tomorrow evening.


I have not done voltage reading on tube equipment before so hope I don't let the smoke out or curl my hair.


If I have the procedure correct it is as follows:


1. connect a speaker or dummy load to amp.
2. install incandescent safety bulb on bench power on and allow to come up to temp
3  clamp negative lead of multi-meter to chassis .
4. power on and allow to come up to temp
5. Verify meter is set to proper range
5. Using one hand and positive probe check and record all PT output voltages, check all filament voltages, check all plate voltages, check all cathode voltages.




If I have overlooked something or you have any tips, would love to hear them.


Here is my updated drawing.
------------------

[EL34]

Are 6SL7 cathode resistors 200r or 2K?

Good catch. They are both 2K.


Thanks for looking. Please continue and we'll get a good schematic yet.


-------------------




Are 6SL7 cathode resistors 200r or 2K? (Makes a real difference to the overall gain.) AGREE to know what it is now before getting ahead of yourself with changes.


My gut-shot reaction was to make V1 a 6J7/6SJ7 pentode. Changes input sensitivity from 150mV to 22mV, from HARD strum clean to e-z overdrive. But twin-triode with some EQ between is a well proven path, for sure.


Yes, pin 1 on metal octals goes to touch-safe ground, so an internal fault does not make the shell or base "live". (Also becomes a handy ground-point.) Many of these pinouts carried into Glass/Phenolic octals so pin 1 may do nothing in the tube you have, but leave the 1-Gnd strap in place for future re-bottlings.




----------------

There are a few things that don't make sense to me in the original amp wiring.  V1 pin 1 to V3 pin 1.  Pin 1 on V1 should be NC right?

Look at V3 pin 1. You'll notice it is connected to a chassis ground lug between V5 and the OT. IOW, V3-1 is a ground point. Most of the grounds in this amp are 'floating' and connected back to this ground lug. So, that means V1-1 is connected to ground. Pin 1 is internally connected to the metal shell of the tube. This is for shielding purposes. Pin 1 is not "NC" for a 6J5.

Your diagram shows the 6J5 left in V1 position.  Is this going to be changing.

Whoa! Don't get ahead. We're still working on getting an accurate schematic for the amp as it is. Once we have that it will be much easier to decide what to do with the amp. I suspect you will choose to replace the 6J5 circuit with a nice dual triode guitar preamp. I've got one ready to paste into the schematic for you to look at. Soon!


But first, let's finish the schematic. Then we need some voltage measurements. In particular, the secondary AC voltages on the PT and DC voltages on every filter cap. Tube pin voltages for all tubes will be nice too.


I can post the Visio file I'm working on if you want to work with that some. I've been drawing it by looking at your photos and your wiring diagram. I also have some pics from the net that I'm using. There are still a few components that I cannot verify the value. And I may have missed a wire or two. 


--------------------

Here's what I have so far. Still missing a few component values. I'm 98% confident the schematic is accurate. This would be even easier to install a twin triode guitar preamp than I originally thought. I'll add component values as you add to your drawing. BTW, you have some resistor values that can't be right.

I will verify my values.  Lighting was poor 


There are a few things that don't make sense to me in the original amp wiring.  V1 pin 1 to V3 pin 1.  Pin 1 on V1 should be NC right?   Strange.  If I am removing/re-wiring, I won't worry about it.


Your diagram shows the 6J5 left in V1 position.  Is this going to be changing.


I really appreciate you taking the time to help me out on this.


-------------------------------------------




Here's what I have so far. Still missing a few component values. I'm 98% confident the schematic is accurate. This would be even easier to install a twin triode guitar preamp than I originally thought. I'll add component values as you add to your drawing. BTW, you have some resistor values that can't be right.


----------------

[EL34]

--------------------




jjasilli, thanks for confirming my suspicions on pinout.  I guess I'm treading water tonite.


Sluckey,


I do a lot of work in Visio so it is more native to me than ExpressSCH.  I don't do much in the way of schematics and this is my first stab at a tube schematic so it has a bit of a hodgepodge that needs a lot of cleanup but it should be good enough to see the basic lay of the land.  Will try to touch it up tomorrow as time allows putting in cathodes, plates, etc..  I still need to get the values on all the resistors and one of the caps that is buried.




?'s   Besides it not looking real neat and clean an being a bit confusing, is the point to point wiring and keeping all the leads short better sonically than having everything on a turret board and looking all neat and tidy?  I read somewhere organ manufacturers were pretty anal about keeping leads short and eliminating wiring so as to limit noise.  Maybe a wives tale?  Audiophile lore?


Anyways, I'm falling asleep on the keyboard.  Here's what I came up with....


Edit, I cleaned it up a little. Still no values on resistors and caps... will be forthcoming.


-----------------------------------


My schematic drawing sucks.  So I learned to use ExpressSCH, which was hard enough for me.




Back to Pin 1 on the 6L6 has a wire connected and then has a resistor across to pin 5?  On a 6L6 pin 5 is the signal grid G1. Pin 1 has no connection in the tube, so it can be used for another purpose.  Seems like signal wire EDIT - Pin 5 1 - grid stop R - Pin 1 5.


----------------------------


Are you experienced?  With Visio? Specifically drawing schematics and electrical drawings? If not, you may want to just draw your schematics on paper. Visio adds a whole new layer of complexity to this task that can be very frustrating if not experienced.


---------------------------


Is it possible they just used this as a junction point?  For instance, Pin 1 on the 6L6 has a wire connected and then has a resistor across to pin 5?
Yes, it's possible.  Vintage tube devices often used true point-to-point connections - what I "affectionately" call the rat's nest approach.  Often, clever ad hoc connections were made to whatever was handy.  Tracing connections can be very confusing. 




That's one reason that I said you wouldn't be having these problems with a Weber (not Webster) amp.  The use of eyelet or turret boards vastly clears-up the wiring & layout. Even so, unused tube socket lugs might be used for ad hoc connections.  Jumping into vintage rat's nest wiring is another example of the deep end of the pool.




-----------------------


New plan based on today's feedback.  Scrap the dedicated preamp idea and keep it simple.


Save parts from the Eico 377 and turn it into a Champ.


Trace out the Webster tonite if hurricane prep allows time.


Thinking I will mod the Webster as neccesary to turn 6J5 into a pre-amp stage once I have a schematic I can share Per Sluckeys suggestion.


Question:


Looking at the tube pinouts and the wiring on the Webster, there are a lot of pins that have wires in place that show as NC on the tube pinout.  Is it possible they just used this as a junction point?  For instance, Pin 1 on the 6L6 has a wire connected and then has a resistor across to pin 5?


Pin 5 on the 5U4G has a wire yellow wire from PT connected to a red wire. red wire then ties with a bunch of other red wires on a standoff terminal, one of which goes to the OT. 






For now I will do the schematic in Visio as the ExpressSCH doesn't seem to be cooperating with me very well 


------------------------


I have just gutted an Eico Model 377 Signal Generator.


I had a 377; I think everybody did. Would have been useful as-is (once fixed).


However: mine burned a power transformer, when I wasn't looking. One month it worked, next month nada. I do not know if that was a freak even or if these things don't live forever. A risk you take when you "recycle" old-old parts of unknown history.


The 377 iron is far more than you need for a preamp; it would easily support a Champ. 6X5 is almost a 5Y3 but simpler heating. The 6F6 is the direct ancestor of a Champ's 6V6 and will do about the same work (different bias). Relative to 12AX7-preamped Champs, the 6SN7 is low-gain, the 6SJ7 is high gain, but that's more total gain than a Champ so something could be cobbled.


----

[EL34]

Is there a shallow end of the pool?
My solution was to bite-off chunks I could chew.  I quickly realized I started-off too deep.  So, I bought a used SF VibroChamp with no speaker in unknown condition - restored and hotrodded it with tried & true mods.  Then did the same with a SF Princeton.  While reading a handful of reference books by guitar amp guru's, and the posts on this Forum. 


-----------------


A good way to get your feet wet would be to attempt to draw a schematic of the Webster just as it is. The amp is very simple, neat wiring, and clean. You could post your schematic here and we could check it to see if it looks logical and suggest revisions if necessary. If this seems like an impossible task maybe you should consider selling the Webster and buying a kit to put together for your son.


Once you have a good schematic it will be easy to decide what to do with the amp. The style chassis almost begs to modify the amp to become a guitar amp. Here's what I'd do...


Just utilize the power supply and power amp just as they are. Rip out that 6J5 circuit and use the socket for a 6SL7 preamp. A fender AB763 preamp (like Rocky) or an Ampeg B15 preamp would be very easy to put in that Webster chassis using point to point wiring. My preference would be the Ampeg. Price would be under $50 even if you have to buy every component. Much less work than building a separate preamp chassis.


-----

You say Weber but I think you mean Webster.  If it were a Weber, you wouldn't have these problems.  By starting at the deep end of the pool, you're summoning the Spirits of Difficulty.

You are correct.  I meant webster but typed weber.  I went back and corrected the typo to avoid further confusion. 


Is there a shallow end of the pool?  At this point in time, a mud puddle to me is like the Atlantic, its all water to me


--------

By starting at the deep end of the pool

Ah, but you have sooo many parts to choose from for the next one


------


You say Weber but I think you mean Webster.  If it were a Weber, you wouldn't have these problems.  By starting at the deep end of the pool, you're summoning the Spirits of Difficulty. 


----------------------

[EL34]

I have a house with all the usual appliances and toys; I also have a chicken-shack with one light bulb.


Should I have the power company bring a line from the street just for the chicken shack? Or run a wire from the existing service in the house?


Usually it is a LOT cheaper to tap a small load from an existing supply than to bring in a new supply. For my chicken shack, stupidly cheaper. (About $4,000 to bring a new line, then $20/month meter fee, versus $200 for a line from the house and no ongoing fees. That's a lot of eggs!)


"Usually" a preamp load is much smaller than the power amp. Preamps run 2-20mA and power amps run 40-200mA. Nearly negligible. A many small bottle preamp may have to be checked for heater load.


OTOH, isolation is a factor. When my pump runs my house lights dim. If that would upset the chickens I might think about giving them their own supply. However there are regulated lamps (many new LEDs do), and you *could* put a regulator to a preamp.

So if the power company would provide you with a free hookup, waive the meter fee and just charge you for the extra power that the chicken coop used, would your chicken coop have its own power?




Since I am a newbie to tube amps and have not fully explored the gamut of how many lightbulbs I can run on an extension cord from the house.  I might need or want to run an incubator as well as a single lightbulb 


Those who saw my other post on the Webster amp are probably aware I am in a situation of inadequate gain and no tone stack on the present amp.  My choices are:


1. Leave the webster amp unmolested as is with all of its virtues and/or shortcomings and build a separate preamp/tone stack to front end it hoping for the best.


2. Modify the Webster's pre-amp stage. Possibilities would be to remove the 6J5 and replace it with a 6SN7 and use both sides of the 6SN7 for more gain as well as adding a tone stack between the new 6SN7 and the 6SL7.  I might be short on power to do this so options would be to delete the 5AU4 rectifier and replace with a solid state rectifier (which also opens up a base for the additional tube for tone stack)?


3. Modify the Webster amp pre-amp stage same as in option 2 but if short on power, change out the 6L6's for a matched set of 6V6 that will place less load on the PT.


4. Gut the Webster chassis and rebuild it to a known schema (like Sluckeys Rock-O-La amp or RCA/Jet 12 amp) that stays within the power budget of the Webster PT.


5. Just run an "extension cord" from the Webster PT to a couple new tubes on a separate chassis and hope it has the moxy to run them.




So, with those being the choices, let me deal the card up my sleeve.
(free power hookup for the chicken shack!)
I have just gutted an Eico Model 377 Signal Generator.  It was an octal based piece of equipment that gives me a free PT, filter choke, additional octal sockets, power switch, indicator light, and a really cool oscillator that I have been told makes a neat variable capacitor in a tone stack provided it will all power up.  I stole the tubes out of it long ago and the cord had been cut off, so I don't have an easy way of testing. 


 https://www.thegearpage.net/board/index.php?threads/eico-377-to-5c1-champ-conversion.1191665/


The PT gives me about 80ma of power to play with. Using this,  I'm figuring I can build a separate octal pre-amp section with its own dedicated, isolated power supply section.  It gives me room to add things like reverb, tremolo, the oscillator, FX pedal loop and whatever other ideas I might contrive without over taxing the main power amps PT.  It should also give me plenty of gain stages.


Once I get this all up and running and figure out what I like and don't like,  I would still have the option of doing a clean sheet build and putting all the working sections on one new, properly laid out chassis as a head unit. 


Is this crazy?


Sluckey, you definitely have my mental gears turning and I have many more ideas and questions.  I like the idea of versatility.  I think the preamp would likely have enough output to drive a set of headphones, handy for practice in a dorm room, or maybe a line out connection which would be good for recording via DI or connecting to a house PA etc.


I've got a small chassis/  PT from a Lafayette MPX receiver as well that would likely work for small preamp but it is setup for 12XX7 and I would prefer to stay all octal.


---------------------

[EL34]

I have a house with all the usual appliances and toys; I also have a chicken-shack with one light bulb.


Should I have the power company bring a line from the street just for the chicken shack? Or run a wire from the existing service in the house?


Usually it is a LOT cheaper to tap a small load from an existing supply than to bring in a new supply. For my chicken shack, stupidly cheaper. (About $4,000 to bring a new line, then $20/month meter fee, versus $200 for a line from the house and no ongoing fees. That's a lot of eggs!)


"Usually" a preamp load is much smaller than the power amp. Preamps run 2-20mA and power amps run 40-200mA. Nearly negligible. A many small bottle preamp may have to be checked for heater load.


OTOH, isolation is a factor. When my pump runs my house lights dim. If that would upset the chickens I might think about giving them their own supply. However there are regulated lamps (many new LEDs do), and you *could* put a regulator to a preamp.


---------------------------


I'll speak specifically about guitar amps...


Pros... Versatility. You'll have a totally self sufficient preamp that can be easily connected to any power amp. Only need one shielded cable if done properly.


Cons... Cost, size, weight, complexity. When you use a separate power transformer you will also need a separate fuse, power switch, indicator light, and filter circuit. These are a major cost of an amp. A separate tube preamp may only need one, two, or three tubes if no special effects. This requires a relatively small chassis. But when you add a separate power supply you will easily double the chassis size needed.


If you want a separate preamp that can easily be used with any power amp, then building a preamp with it's own independent power supply makes sense. But if the preamp, power amp, and speaker will all be housed in the same cabinet, it makes more sense (to me) to power the preamp from the power amp power supply. There have been several well known combo amps built with separate preamp that is powered from the power amp. I have one such project. Take a look. May get some ideas or raise more questions...


     http://sluckeyamps.com/vox/vox.htm
     
     ---------------------------
     
     
With 1 PT if the power amp sags, B+ voltage will also drop to the preamp.  Though this can be mitigated, if desired, with diode in the B+ rail after the PI node.




For hi-fi purists may claim that separate PT's and power supplies keeps signal from bleeding around.


-----------------------------------




What are the pro's and cons of using separate Power transformers for amp and pre-amp sections? 

[1blueheron]

Thanks for digging it out.  I will link to it in the new thread for those that are interested in backstory.  We had kinda gone down the rabbit hole and away from topic anyway....

[EL34]

Ok, good idea, just start a new thread
You can dig out any info above that is worth reposting