First amp build

[AlNewman]

The B+ is high because the transformer was designed for an organ.  Organs have a massive preamp section which creates significant draw on the B+.
I tried to explain this 2 pages ago.

[Mongrel714]

The layout indicates 9V caps in the Bias supply. Should be a minimum of 63V based on a calculated Bias range of -11VDC to -49VDC with a 25K Bias pot. Typo or problem?

Is the B+ high because there are no power tubes in or they are not conducting?

Hello, The Caps in the bias supply are 63v, typo, The tubes test good, 95 on a scale of 0 to 120, emmission test is good, heaters are good, no shorts, (I have a tube tester) and it is a 50k bias pot.

[Mongrel714]

The B+ is high because the transformer was designed for an organ.  Organs have a massive preamp section which creates significant draw on the B+.
I tried to explain this 2 pages ago.

Im sorry, I am new at this and taking in a lot of information, I am reading everything I can on this subject, and I am going to check out all the information all you gentleman have provided here! I really appreciate all the answers and comments, I am just learning this but I do feel im getting close here, thanks again everyone!

[Mongrel714]

B+ is too high. What does it measure when you install a rectifier tube?
It appears that the AO39 power amp used a 5U4GB and two 6BQ5. It worked for Hammond. It should work for you. Hammond used 10 watt dropping resistors and two 30uf capacitors in the B+ feed.
Sluckey has been down this road. See his conversion!!!
https://www.sluckeyamps.com/hammond/hammond_ao39.pdf
The cap can next to the power transformer looks very old. Does it test good?
more conversion problems solved at:
https://sluckeyamps.com/index.htm

My rectifier tube gave up the ghost, Have to order a new one, I will read through Sluckeys post and check out his site, the can cap does test good, probably would not hurt to replace it as it is old, on a slight budget here, but trying to pick up pieces weekly, It is the first of the week so I am not sun stroked out from my job, will cram all this info.

[astronomicum]

The B+ is high because the transformer was designed for an organ.  Organs have a massive preamp section which creates significant draw on the B+.
I tried to explain this 2 pages ago.

The OP keeps posting pin 3 is 0V. There is a 1R monitoring resistor. Is this a measuring error or is it really 0.000V? It should read 0.somethingV if the tubes are in. Is the meter on the right scale?

[Mongrel714]

The B+ is high because the transformer was designed for an organ.  Organs have a massive preamp section which creates significant draw on the B+.
I tried to explain this 2 pages ago.

The OP keeps posting pin 3 is 0V. There is a 1R monitoring resistor. Is this a measuring error or is it really 0.000V? It should read 0.somethingV if the tubes are in. Is the meter on the right scale?

The meter may not have been on the right scale now that you mention that, it was on the 600vdc scale.

[Mongrel714]

New Rec tube is on the way.

[Mongrel714]

Ive got S Luckeys schematics printed out along with the original Hammond schematics from my old organ, I will study all this and try to figure this out, I really appreciate the help guys!
Oh and re tested the EL84s, they passed all the tests and on the "Quality meter" registered 100, My tube tester is a B&K Model 600Dyna Quick, prior owner said resistors were replaced (which I checked and they are new) and calibrated.

[Mongrel714]

And I did ask Sluckey if he minded that I study his schematics!

[ac427v]

Check your original Hammond Organ schematic to see if it had a second Power Transformer used for the preamp.

If it had two power transformers, the 140ma specification listed for the power amp power transformer may allow you to get B+ similar to the organ and similar to Sluckey's conversion.

If the organ had only a single power transformer it will not work with your plan.

[Mongrel714]

Check your original Hammond Organ schematic to see if it had a second Power Transformer used for the preamp.

If it had two power transformers, the 140ma specification listed for the power amp power transformer may allow you to get B+ similar to the organ and similar to Sluckey's conversion.

If the organ had only a single power transformer it will not work with your plan.

They have an extra input Tranny and output tranny for organ voicing but no extra PT, the PT and Rec tube itself was in a separate enclosure from the main amp.
  I am re vising my plan at the moment, working towards using a 3 tube pre amp and a power section based on the organ power supply, this approach has been in the back of my head for awhile now, after looking at Sluckeys schematics it seems like the way to go, Ill have to replace some parts as I go, the 5U4GB rec tube should be here this weekend, and a new load of resistors, Ill keep going till I get it figured out with everyones help! Thank you!

[AlNewman]

Have you figured out yet what your tubes are dissipating, or the current draw of your amp?

[Mongrel714]

Have you figured out yet what your tubes are dissipating, or the current draw of your amp?

Haven't got that far yet, not had a safe voltage for the tubes to run for any amount of time, Myself, being a noob at this, I don't know if you can or should run the plates and screens at +425vdc, was afraid of frying the screen, no body's really said if they will stand up to this voltage for any amount of time.

[tubeswell]

Have you figured out yet what your tubes are dissipating, or the current draw of your amp?

Haven't got that far yet, not had a safe voltage for the tubes to run for any amount of time, Myself, being a noob at this, I don't know if you can or should run the plates and screens at +425vdc, was afraid of frying the screen, no body's really said if they will stand up to this voltage for any amount of time.

A JJ6V6S will handle those voltages.

[Mongrel714]

Have you figured out yet what your tubes are dissipating, or the current draw of your amp?

Haven't got that far yet, not had a safe voltage for the tubes to run for any amount of time, Myself, being a noob at this, I don't know if you can or should run the plates and screens at +425vdc, was afraid of frying the screen, no body's really said if they will stand up to this voltage for any amount of time.

Yep, If my new Rectifier tube and resister network don't do the trick, Ill up grade to the JJ6V6S


A JJ6V6S will handle those voltages.

[Mongrel714]

Hey Guys, My Rec tube will not be here until Monday, so I am going to re build the power supply, make a new board to mount the resistors on, I will get the schematic scanned in to photo shop and make it readable, if some one would check it out when I post it make sure I didn't screw something up, (I'm sure I will) once again, I really appreciate the help, all of you!

[Mongrel714]

Here is a quick sketch showing my power supply layout, the Resistors can be changed of course if they need to be changed, Can some one tell me if this looks correct??

[AlNewman]

As far as providing a power supply, your drawings make sense.

[Mongrel714]

As far as providing a power supply, your drawings make sense.

Thank you for that.....Now looking at the original power supply for the organ itself, you can see a 30 Ohm? resistor R93 (they do not verify on the schematics if its ohms or K ohms) after the rectifier tube and before the first capacitor, that is affecting the voltage before it hits the first capacitor, is it dropping any voltage? what is the purpose of that resistor?
As you all told me, not getting the voltage drop I would like with the new rec tube, still showing 430vdc coming off pin 8 and at the plates.

[ac427v]

The power supplies that you built don't work because your errors created excessive voltage.
The amp is not safe because dropping resistors do not have enough wattage rating to dissipate all the heat created by the voltage drop in the high amperage portion of the circuit.
Your standby switch is wrong and will burn parts.
You say you don't know what you are doing.
These are reasons you should stop being inventive. Build your first amp exactly like knowledgeable experts built theirs.
Sluckey has been down this road!!! He knows the theory. He has the experience.
Make your power supply exactly like Sluckey's Hammond AO-39 conversion to Marshall 18 Watt Lite. It works!
That means 250 ohm / 10 WATT dropping resistor before the first capacitor and center tap of the output transformer. Do not use a different value until you learn how to calculate how many watts of heat are created by a resistor in this location.
Use a 2.2K ohm / 5 WATT resistor for the next dropping resistor for the power tube screens.
8.2K ohm/ 2 WATT for the phase inverter.
1 K ohm / 2 WATT for the preamp tubes.
If it does not work, you did not follow the plan correctly.

[AlNewman]

As far as providing a power supply, your drawings make sense.

Thank you for that.....Now looking at the original power supply for the organ itself, you can see a 30 Ohm? resistor R93 (they do not verify on the schematics if its ohms or K ohms) after the rectifier tube and before the first capacitor, that is affecting the voltage before it hits the first capacitor, is it dropping any voltage? what is the purpose of that resistor?
As you all told me, not getting the voltage drop I would like with the new rec tube, still showing 430vdc coming off pin 8 and at the plates.

It looks like that 30R resistor drops about 4V @ 115ma.  The resistors in series from center tap to ground add up to 242R and drop about 28V @ 115ma. 

The EL84s draw around 70ma at idle.  The pre amp on that organ draws somewhere around 45ma. How much current does your pre amp draw?  What value and wattage resistor will you need to drop to your ideal voltage at your estimated current draw?

[AlNewman]

The power supplies that you built don't work because your errors created excessive voltage.
The amp is not safe because dropping resistors do not have enough wattage rating to dissipate all the heat created by the voltage drop in the high amperage portion of the circuit.
Your standby switch is wrong and will burn parts.
You say you don't know what you are doing.
These are reasons you should stop being inventive. Build your first amp exactly like knowledgeable experts built theirs.
Sluckey has been down this road!!! He knows the theory. He has the experience.
Make your power supply exactly like Sluckey's Hammond AO-39 conversion to Marshall 18 Watt Lite. It works!
That means 250 ohm / 10 WATT dropping resistor before the first capacitor and center tap of the output transformer. Do not use a different value until you learn how to calculate how many watts of heat are created by a resistor in this location.
Use a 2.2K ohm / 5 WATT resistor for the next dropping resistor for the power tube screens.
8.2K ohm/ 2 WATT for the phase inverter.
1 K ohm / 2 WATT for the preamp tubes.
If it does not work, you did not follow the plan correctly.

Sluckey's design uses a different transformer.  You're right though, the stand by should be after the reservoir cap.

[acheld]

With respect for your perseverance, you are in over your head on this build.   Re-purposing transformers and circuits require some experience beyond watching YT videos. 

Seriously, put this project aside.   Pack it up and plan to come back to it later in your career!   

A great first project is to build one of Hoffman's smaller amps.  You should be able to complete this project first, and build on your success from there.

I'm trying to give realistic advice here.  Your ability to push ahead is impressive.

[AlNewman]

I disagree.  I think the OP is going to build this amp one way or another.  Lots of experienced people have given them several options and advice, and they want to proceed in their own direction.
I think everyone here has built an amp that we aren't proud of.  We *could* open that amp back up and make it right, but we don't.  Sometimes the hard way is the way.

[Mongrel714]

Guys, This amp is not built, I am asking questions now to find the correct way to go, I don't want to aggravate everybody by asking these questions, but if I don't how will I learn?
 I think a lot of this confusion come from my lack of direction, and changing designs to much at the beginning of all this, I wish I would have listened and went straight for the 18 watt build, but the alure of that 50 watt Marshall had me hypnotized, I realize that was a mistake, another mistake I've committed is trying to work on this amp after working in the sun for 10 hour a day and being to burned out to even work safely on this project, that wont happen again.
 I started this because it is something I have always wanted to do, I've played Marshall all my life, now I want to build one,  this was meant to be a fun experience learning something new, but if I continually make people mad, well that's not fun.
I'm not going to stop trying to build this, If you all want me to move on and find help elsewhere I will, I really didn't start this to make people upset.

[Mongrel714]

My thinking on this is that I do have a different PT than Mr Lucky, so I was looking at the original schematics to see if I needed to use the values of those components since someone mentioned that it worked in that configuration, I don't have all the extra parts the organ amp had, so I'm trying to see what to use and what to not use.
  May have to use a mixture from both to make this right.
Thank you
Oh, I have 2 new Can caps headed my way

[Mongrel714]

And from what I've been reading, for pre amp current draw, I have to measure voltage drop over the plate resistor on the 12AX7s, before the resistor, B+voltage, then after the resistor off the plate pin, then divide the voltage drop by the resistor value, but this needs to be done on a amp that is running correctly, so estimated pre amp current draw with 3 12AX7 tubes would be 9.5 mA.

[acheld]

You're over thinking pre-amp current draw.   12AX7's draw roughly 3mA plate current, and about 300mA heater current (when in parallel config).  You don't need to measure this -- it's right there on the datasheet.   For most guitar amps (that would include Marshall style amps), most of us do not even consider pre-amp plate current.   For 4 tubes, it will be about 12 mA at our usual voltages, probably in the range of 5 - 10 % of your plate current budget

Personally, I don't even think about pre-amp plate current when designing my amps.   The real difference is in the selection of output tube(s).   

You must also consider heater current.  That will be a limiting factor with many transformers . . . 

I do have a secret weapon of sorts:  Hammond makes a lot of great transformers.  On their website, they have listed their guitar amp trannies by both Fender and Marshal amps. https://www.hammfg.com/electronics/transformers/classic/290?referer=1101  So, if you want to build a JCM 800 50 watt, you can look it up there.  All the work has been done, though I always draw up a power budget myself and compare it to what is being spec'd in a transformer.   They also have a similar table for output transformers.   

[Mongrel714]

You're over thinking pre-amp current draw.   12AX7's draw roughly 3mA plate current, and about 300mA heater current (when in parallel config).  You don't need to measure this -- it's right there on the datasheet.   For most guitar amps (that would include Marshall style amps), most of us do not even consider pre-amp plate current.   For 4 tubes, it will be about 12 mA at our usual voltages, probably in the range of 5 - 10 % of your plate current budget

Personally, I don't even think about pre-amp plate current when designing my amps.   The real difference is in the selection of output tube(s).   

You must also consider heater current.  That will be a limiting factor with many transformers . . . 

I do have a secret weapon of sorts:  Hammond makes a lot of great transformers.  On their website, they have listed their guitar amp trannies by both Fender and Marshal amps. https://www.hammfg.com/electronics/transformers/classic/290?referer=1101  So, if you want to build a JCM 800 50 watt, you can look it up there.  All the work has been done, though I always draw up a power budget myself and compare it to what is being spec'd in a transformer.   They also have a similar table for output transformers.

Thank you Ac, I'll check that out on my break, the pre amp current draw, Al ask me what it would be a few posts back, I believe he was just trying to kickstart my brain!

[Mongrel714]

That Hammond page is great, I've got it book marked for future reference!

[AlNewman]

There's voltages on your build schematic.  You can find out what current the pre amp and phase inverter are supposed to draw.

[Mongrel714]

There's voltages on your build schematic.  You can find out what current the pre amp and phase inverter are supposed to draw.

See, I just learned something!! 😁 Thanks!

[Mongrel714]

Hey guys, sorry no progress, The Wife's Birthday weekend!

[Mongrel714]

There's voltages on your build schematic.  You can find out what current the pre amp and phase inverter are supposed to draw.

Just wanted to verify that I'm going about this the correct way, looking at the Tube town schematic,
Find the plate resistor for pre amp tube, take the B+ voltage before the resistor, then subtract the voltage after the resistor, then divide by the value of the resistor itself, gives you the current.
First preamp tube
B+4:260v subtract voltage after 100k resistor:195v =65v drop
65v divided by 100,000ohms=0.00065mA
Do this on all pre amp and PI tubes then add together to find total current?
Or is it the 1.8v, 2.8v, 1v, 50v (PI) below the tubes?

[AlNewman]

Yes that's the correct way to calculate current.  You can use either plate voltage or cathode voltage, (PI gets a bit trickier using cathode voltage).  You can also look at the power rail and see what resistors feed what parts of the circuit and take one measurement for that section.

[astronomicum]

Product is in Amps so:
Answer is 0.00065A
Converted to mA product x 1000= 0.65 mA

[Mongrel714]

Product is in Amps so:
Answer is 0.00065A
Converted to mA product x 1000= 0.65 mA

Thank you! Noted and corrected on my worksheet!

[Mongrel714]

Yes that's the correct way to calculate current.  You can use either plate voltage or cathode voltage, (PI gets a bit trickier using cathode voltage).  You can also look at the power rail and see what resistors feed what parts of the circuit and take one measurement for that section.

Thank you Al, Give a man a fish, feed him for a day, teach a man to fish, feed him for a lifetime! will work more on this after work.