G-PULTEC problems, please help!

[gyraf]

looks almost right at a glance - but:

- You'll want second transformer 115V windings in series (giving 230V), not in parallel as shown (which will give 115V AC)
- You'll want to use both second transformer low-voltage windings - put those in parallel.

- you'll need to connect BOTH red and yellow to power-switch NO - otherwise you'll have one primary winding constantly powered

Jakob E.

 

[ding]

Jakob,

First off I want to thank you for what you do. This is very new to me so I appreciate your help. I forgot to mention that I am in the US. I imagine you are talking about the FS12-1000 and not the toroid when you refer to it needing to be in series. Is that right? I understand what you mean about the power, it was a bit late last night. :-[

 

[gyraf]

Yup, by "the second transformer" I mean the non-toroid.

It's clear from the drawing that you plan to run off 115V mains. You still need to step up to 230VAC at the second transformer.

Jakob E.

 

[ding]

Thanks so much Jakob. I think I got it. Is this right? The second transformers secondaries are 6.3v in parallel @ 2A.

EDIT: removed incorrect diagram to avoid it coming up in google searches, will post correct info up ahead. Reply #432.

 

[Harpo]

The second transformers secondaries are 6.3v in parallel @ 2A.

Connect both primary and secondary windings in series, IE 12.6V:230V
or both primary and secondary windings in parallel, IE 6.3V:115V.
Connected as drawn will give doubled stepup ratio (and blow up some parts that are not rated for this overvoltage).

As far as I have researched I need an ON-ON DPDT rotary switch. I am guessing 2 pole. Does anyone have any leads on where I can get one of these?

A common avail. Lorlin type 2x6 rotary switch, switches endstop washer limited to pos.2, will do the trick for cheap.
Please update your profile with your location data.

 

[ding]

Hello Harpo and thanks for your help. So I obviously misunderstood Jakob's quote below?

- You'll want second transformer 115V windings in series (giving 230V), not in parallel as shown (which will give 115V AC)
- You'll want to use both second transformer low-voltage windings - put those in parallel.

Jakob E.

So I will put the second transformer in series both on mains and secondary. It will give me a 12.6V @ 1A. I will then have to use a resistor to lower the voltage on that so I can power the on lamp. I have seen people pull the current from the tube PCB labeled 6.3v but I would rather not mess with that. Also thanks for the quick reply about the rotary switch. I was searching for something so specific not realizing I could use something right in front of me. I think I deleted that post about 5 minutes after I wrote it so you were paying attention.  Again thanks so much. I imagine this will be the beginning of an addiction.  ;D

 

[ding]

Just to double check. First transformer in parallel and second one in series like below. I used a 50 ohm resistor for the lamp.

EDIT: removed incorrect diagram to avoid it coming up in google searches, will post correct info up ahead. Reply #432.

 

[Harpo]

So I obviously misunderstood Jakob's quote below?

- You'll want second transformer 115V windings in series (giving 230V), not in parallel as shown (which will give 115V AC)
- You'll want to use both second transformer low-voltage windings - put those in parallel.
Jakob E.

Seems so. Up to your latest post, saying its a dual 6.3V secondaries, Jakob most likely was assuming, you use a dual 12VAC secondary windings for your 2nd. transformer as shown on pcb connections and schematic.

So I will put the second transformer in series both on mains and secondary. It will give me a 12.6V @ 1A.

...but this transformer is connected backwards. The 12.6V @ 1A is rated load current for the usual step down ratio. With this now backwards 12.6/230 or 1:18.25 step up ratio, avail.AC current goes down by the same ratio, IE 1A/18.25=54.8mA AC current. This AC voltage get fullwave rectified at cost of form factor 1.8, leaving 30mA avail.DC current for your HT rail.

I will then have to use a resistor to lower the voltage on that so I can power the on lamp.

If it is a lamp (from a previous post it might be a LED?), a 12V incandescent lamp can connect to the 1st.transformers secondary AC connections without series resistor. Lower voltage lamps or wanted lesser brightness will need a voltage dropping resistor.

I have seen people pull the current from the tube PCB labeled 6.3v

..probably connecting to a LED with needed current limiting resistor.

 

[Harpo]

I used a 50 ohm resistor for the lamp.

Maybe, but missing info.
Incandescent lamp @ ? V and ? W.
The 50R series resistor rated for ? W.

 

[ding]

this transformer is connected backwards. The 12.6V @ 1A is rated load current for the usual step down ratio. With this now backwards 12.6/230 or 1:18.25 step up ratio, avail.AV current goes down by the same ratio, IE 1A/18.25=54.8mA AC current. This AC voltage get fullwave rectified at cost of form factor 1.8, leaving 30mA avail.DC current for your HT rail.

Thanks. When I read that and look at this... http://gyraf.dk/gy_pd/pultec/gy_pd_sch.gif ...it makes sense. I'm taking this very slow and learning. BTW I was planning on using a fender 6.3v 150ma bulb but I imagine that a 12v bulb would be easier. I just did not want the light to be as bright as the eye of Sauron.  But reconsidering I think easier is better. I will look into a 12v bulb. Thanks.

 

[ding]

I have been having trouble finding the 12n and 18n WIMA or Orange Drop caps for the filter section. Right now I am using Sprague Goodman caps but they look weird in there with the ODs and WIMAs. So I got to wonder does anyone have a source for 12n and 18n ODs or WIMAs? Is there a recommended replacement brand for these? Does it even matter for this build as long as they are polyester or polypropylene?

Thanks

 

[audiophreak]

Hi,
      for the 12n I used WIMA 10n - 10%  and sorted out for highest value ( 11n I think , WIMA has tight tolerance )  , for the 18n I used a 15n and  pigy backed a 3n to the solder pads under the PCB.

 

[ding]

First of thanks to all that lent me a hand. I tried out my GPultec for the first time last night and it sounds very, very nice!!!! I have noticed that some of my diagrams of incorrect wiring were coming up in google searches for GPultec. In light of this I have removed them from my posts. I don't want people to be wiring their equipment wrong because of an image on google image search. I will update my diagram shortly but for now I just want to reference Patch's image which is the CORRECT way to do it. The only difference is that I wired my primaries on the toroid in parallel for the US.

Thanks all

BTW I got a 120v jewel light and just wired it up in series parallel with the lorlin on switch. It is bright as all hell but works like a charm.

 

[Harpo]

BTW I got a 120v jewel light and just wired it up in series with the lorlin on switch. It is bright as all hell but works like a charm.

..assuming in series might be a typo. If this lightbulb fails one day, this series connection will act like a 2nd mains switch that can only be switched to On again by exchanging the lightbulb. Hook it up in parallel to your mains transformer primary winding and a voltage dropping resistor in between (ask ohms law for resistance and wattage) will decrease the lightbulbs brightness to taste.

 

[ding]

Yes, you are right on that actually it is in parallel. I just had a brain fart while typing. I will try the resistor but I am just tickled that I have no trouble-shooting in my future so I had not thought that far ahead yet. Thank you, you and Jakob are a great help and an invaluable resource.

 

[APL]

Hi,
I am a Newbie . I have just received the excellent PCBs from PCB Grinder and want to get started, however I have been reading the various messages to see what the potential problems are. I can see human error and poor workmanship account for most problems (I’m capable of both of these) however there also seems to be some issues and improvements to the original design over last 10 years. I thought it would be better to ask questions before picking up the soldering iron.

I consider my electronics knowledge and skills are average, certainly less than many contributors here. I have made kits before but they were simply putting the supplied parts together following instructions, this project is more difficult. Hopefully more experienced people can tell me what is right or wrong.  Comments from experienced builders would be useful for me but also for others to produce a working Pultec with minimum problems.

Firstly I wanted to ask about Power supply. I should point out I am talking about a standard 1 channel Pultech as per Jacobs design.

1.Transformers
1.1      The original schematic shows 2 x 240:12V transformers. Jacob has said that this was because it was difficult to obtain a suitable single transformer with correct secondary voltages when he designed the G Pultec. The 2 transformers would be less expensive than Dan Audio type but take up more space.
Question 1  Are there any advantages/disadvantages having the original 2 transformers over the one (Don Audio type)? I am particularly thinking of noise issues which seem to common problem.

The original schematic shows 12V secondary’s, however Don Audio type are 9V
Question 2. If I decided to use 2 transformers could I use 240:9v 30 VA. or are 12V secondary’s essential to the 2 transformer design?

2. PSU board
Reading comments there seems to be a lot of recommendations to change and experiment with resistors and other components to get correct tube voltages. I really want to avoid this. I am ok soldering once, but unsoldering and resoldering will probably lead to problems, therefore I would like to achieve correct values first time if possible. So assuming I have chosen the Don Audio transformer:

2.1 Bridge Rectifier – I see in Reply #168 (June 2011) somebody recommends moving the 220v rectifier off the PSU board to the back of the case to reduce chance of AC noise.
Question 3 -  is this a good modification? If so what type and value rectifier should be used?

2.2 Resistor 3K3  should be replaced with 18K 10Watt resistor when using Don Audio Transformer (Reply #400 July 2013)
Question 4 – Why is such a large power value required compared to original 2 W?

2.3 Power regulator – If using Don Audio type transformer with  LM317? 
Question5  what difference (if any )does it make having input of 9v instead of the design voltage of 12v? Is the LM317 still the best option?

2.4 Resistors (heater circuit) – Linked to question 2.3 above, with 9v supply what values should the original 470R and 1K8 resistors be changed to?

2.5 trimmer (heater circuit) – again related to above, someone suggested replacing 1K8 resistor with 2.5k-5k trimmer to get 6.3 V on heater. Is this a good recommendation?

Any comments would be appreciated. I have similar questions about the rest of the build (sorry) but they can wait for another day.

Aidan

 

[ding]

I can't comment on all the technical stuff you wrote about because I don't really know and I am new to this like you but I can tell you about my experience building one. This is my first project besides a few guitar pedals. I used all wima and orange drop for the EQ section. I also used 2 transformers, the amveco and the triad. I followed the BOM exactly, no experimenting. The PCB and the transformers are as far apart as they can be. My box sounds excellent. Absolutely no noise and no trouble-shooting. It all just worked. I threw it up on waves q-clone and all the frequencies are pretty on and it sounds analog sweet. If you don't want to mess around I suggest you just follow the BOM exactly, take your time and you will get a nice sounding box. Good luck.

 

[Harpo]

1.1      The original schematic shows 2 x 240:12V transformers.

It doesn't. It shows two 220:12 VAC transformers.
Your 240:12 is a 20:1 step down ratio, the 220:12 is a 18.3:1 step down ratio.

Question 2. If I decided to use 2 transformers could I use 240:9v 30 VA. or are 12V secondary’s essential to the 2 transformer design?

Only if both transformers are 240:9. A 240:9 is a 26.7:1 step down ratio. When you connect this 2nd.transformer backwards to your 1st.transformers 9VAC secondary winding, you'll get 240VAC back on paper. If the 1st.transformer still is a 240:12 and you connect the 2nd. 240:9 transformer backwards expect about 12VAC*26.7=320VAC at your 2nd.transformer primary winding. This 320VAC gets fullwave rectified to about 320*SQRT(2)=452V raw DC voltage and your diodes and caps have to be rated for at least this voltage. As AC mains voltage isn't a constant, these numbers might vary by +/-10%. If you have 10% high mains, expect the HT raw DC voltage to increase to even more insane 498VDC when you want about 250VDC for your HT rail. The voltage dropping 3K3/2W from schematic will start to drop the HT voltage only after HT current is flowing. This will not be initially, as it takes a little time before the tube heaters are operating.

2.3 Power regulator – If using Don Audio type transformer with  LM317? 
Question5  what difference (if any )does it make having input of 9v instead of the design voltage of 12v?  Is the LM317 still the best option?

Less wasted heat. 9VAC*SQRT(2)=12.7V raw DC; 12VAC*SQRT(2)=17V raw DC. The differential to 6.3VDC times current demand is transfered to heat, so the 9VAC secondary requires lesser heatsinking. The LM317 is common avail., cheap and gets the job done.

2.4 Resistors (heater circuit) – Linked to question 2.3 above, with 9v supply what values should the original 470R and 1K8 resistors be changed to?

Nothing to change. The ratio of these resistors set the heater supply voltage to 1.25*(1+1800/470)+0.00005*1800=6.13VDC with perfect parts and typical parameters, but all parts come with tolerances, so they most likely will not be perfect.

2.5 trimmer (heater circuit) – again related to above, someone suggested replacing 1K8 resistor with 2.5k-5k trimmer to get 6.3 V on heater. Is this a good recommendation?

If it helps better sleep... just pretrim to 1K8 and dial in the final voltage to please your eye. The tube probably won't care.

 

[APL]

Thanks for replies regarding power supply questions, very helpful.

There has been a lot of discussion regarding grounding. I notice in photos some finished  Pultechs some have very little grounding wiring  while others look like a plate of green and yellow spaggetti. Is there a definititive method of grounding a single channel Pultec if so can you give me  reply number and date reference.

Thanks 

 

[gyraf]

General grounding:

- Power input gnd to chassis gnd.
- Wire XLR's pin1's together.
- Wire from xlrF(input)pin1 to input gnd on PCB.
- Also wire from this point xlrF(input)pin1 to chassis gnd.

Done

Jakob E.