It is currently Mon Mar 16, 2020 10:53 am

All times are UTC - 7 hours



Post new topic Reply to topic  [ 73 posts ]  Go to page Previous  1, 2, 3, 4, 5  Next
Go to page Previous  1, 2, 3, 4, 5  Next
Author Message
Post subject: Re: Planning stage of building an 5F2A Princeton
Posted: Sun Nov 29, 2015 2:14 am
Offline
Rock Star
Rock Star
User avatar

Joined: Sat Jul 03, 2010 2:18 pm
Posts: 6544
" YouTube with a pretty good description of the functionality and purpose of the screen grid resistors at the input. "

Interesting. Will need to watch the YT --- as soon as I get off work. The vintage Fender SE amps don't need a separate screen resistor, as the tap off the PSU rail has a step-down resistor inline.

If you are talking about the grid-to-ground (G2G) resistor, that's cool.


Top
Profile
Fender Play Winter Sale 2020
Post subject: Re: Planning stage of building an 5F2A Princeton
Posted: Mon Nov 30, 2015 9:31 pm
Offline
Hobbyist
Hobbyist
User avatar

Joined: Thu Jun 20, 2013 8:55 pm
Posts: 78
Location: South Texas
Was hoping to figure this out on my own but I'm stuck and need some help.

When powered on the rect and OT heat up but not the 12AX7. No sound of any kind. I checked the voltage working from the preamp tube back to the OT and got this:

Preamp
pin 9 (Filiment) is 5.7v
Pin 4/5 (Filiment) is 0.5v

I ran it back to the 6v6 and all the way to the lamp and it's the same. Should I have 6v on both leads?
I'm using a Hammnond 291AX.
Image

I'm using a Hammnond 291AX.


Top
Profile
Post subject: Re: Planning stage of building an 5F2A Princeton
Posted: Mon Nov 30, 2015 11:15 pm
Offline
Rock Star
Rock Star
User avatar

Joined: Sat Jul 03, 2010 2:18 pm
Posts: 6544
You reading AC voltage, right? You have 6.3VAC off pins 4&5 to ground and 9 to ground --- tube out of socket.

The heater lines are running in parallel? Did you raise the line, by inserting two 100-ohm resistors to ground with each heater leg? Or is the PT's center tap off the 6.3VAc line grounded?


Top
Profile
Post subject: Re: Planning stage of building an 5F2A Princeton
Posted: Tue Dec 01, 2015 1:42 pm
Offline
Hobbyist
Hobbyist
User avatar

Joined: Thu Jun 20, 2013 8:55 pm
Posts: 78
Location: South Texas
BMW2002Ti wrote:
You reading AC voltage, right? You have 6.3VAC off pins 4&5 to ground and 9 to ground --- tube out of socket.

The heater lines are running in parallel? Did you raise the line, by inserting two 100-ohm resistors to ground with each heater leg? Or is the PT's center tap off the 6.3VAc line grounded?


They are twisted and in parallel.

CT from transformer is red with yellow stripe going to chassis ground.

Green legs from PT and green heater legs to tubes are soldered directly to each lug of the lamp - not the resistors. A 100 ohm resistor is soldered to each leg of the Fender style lamp and grounded to the chassis. Each resistor measured to ground shows 91 ohms.

Tubes Removed - AC volts:
One side of the lamp measures 0.62vac and the other at 6.2vac
Pin 4/5 to ground is 0.62v
Pin 9 to ground is 6.2v

Readings are the same at pins 2 and 7 of the 6v6 power tube socket. With tubes installed the voltage drops about .4v

The 6v6 and 5y3 tubes appear to glow normally but the 12ax7 stays cold.

The 291AX data sheet shows red and yellow are for the 5Y3 with the Red\yellow stripe as the 650 VCT but, as per my understanding of the layout and schematic, unless I also have a CT for the 6V6 and 12AX7 heaters (green 6.3v @ 2 amps) I needed to "make" one using the two resistors off the lamp as described above. If I had to have my doubts about anything in the board it would be the use of the CT.

I'm stumped but I hope this helps.


Top
Profile
Post subject: Re: Planning stage of building an 5F2A Princeton
Posted: Tue Dec 01, 2015 6:19 pm
Offline
Rock Star
Rock Star
User avatar

Joined: Sat Jul 03, 2010 2:18 pm
Posts: 6544
Yes, those VAC readings seem correct. Though, the low reading off 4&5 seems a bit odd. Is one leg of the heater line grounded to the chassis? Fender did this with their early single-ended amps, for simplicity and cost. You should only have the heater lines going to ground via the 100-ohm resistors on each leg. No where else should those line be touching ground.

Is the center tap off the 6.3VAC lines grounded or not? With 100-ohm lifting resistors --- the CT should NOT be grounded. This is usually a green-white or green-yellow line. The 100-ohm resistors act as an artificial CT point (0VAC).

Have you tried another 12AX7? Even, if the tube is new, could be a bad one. If the substitute 12AX7 doesn't work: Pull the 6V6GT. Plug in the 12AX7... does it now glow? If so, you have a drain somewhere in the green line. Check all solder points.

If the above doesn't help check your 12AX7's cathodes to ground. Sometimes, a tube's heater will light up, but you really can't see it. The grids are easier to see once they see B+. Check the cathode resistor and its bypass cap for continuity. Negative side of cap tied to ground. Positive to the cathode. Two of these pairs per 12AX7.

Try pulling the 5Y3GT. I know this tube gets AC voltage off another tap (red lines) --- but, sometimes the PT has issues. And the extra current draw off the rectifier lines causes the 6.3VAC (green) current to drop. If the 12AX7 now lights up... could be tranny or wiring issues.

Good luck! Keep us informed. :)


Top
Profile
Post subject: Re: Planning stage of building an 5F2A Princeton
Posted: Tue Dec 01, 2015 6:49 pm
Offline
Hobbyist
Hobbyist
User avatar

Joined: Thu Jun 20, 2013 8:55 pm
Posts: 78
Location: South Texas
I'll try 12AX with the 6v6 and 5y3 pulled.

I have no green/yellow CT on this OT.

I have a pair of reds, a red with yellow stripe that I sent to the chassis ground, a pair of yellow leads and a pair of green leads. Other than the 2 blacks on the primary side that's it. Here's a copy of the data sheet.

Image


Top
Profile
Post subject: Re: Planning stage of building an 5F2A Princeton
Posted: Tue Dec 01, 2015 6:58 pm
Offline
Rock Star
Rock Star
User avatar

Joined: Sat Jul 03, 2010 2:18 pm
Posts: 6544
Randy, ok. Correct tranny.

Measure the VAC across pins 4&5 and 9 of the 12AX7 socket. Should be around 12.6VAC. If it is... all connections are ok. Did you try another 12AX7?

Cathode-to-ground connections on the two halves of the 12AX7 ok? Pins 3 & 8 to ground. Each cathode tied to ground through a 1500-ohm, 1/2-watt resistor. One has a 25/50 bypass cap.

BTW... what VDC are you getting on each plate of the 12AX7? Pins 1 & 6. Should be in the 100's of VDC. I couldn't find the Princeton 5F2A schematic/layout with voltages... but the Champ 5F1 should be close:

http://www.electronicstudio.net/schemat ... layout.pdf


Top
Profile
Post subject: Re: Planning stage of building an 5F2A Princeton
Posted: Tue Dec 01, 2015 8:33 pm
Offline
Hobbyist
Hobbyist
User avatar

Joined: Thu Jun 20, 2013 8:55 pm
Posts: 78
Location: South Texas
I appreciate the help!

VAC from 4/5 to pin 9 is 6.16v

Cathode to ground off pins 3 and 8 is 0 and 0.

VDC off pins 1 and 6 is 260 and 260

I tested these at the pins and back to the board. Same thing on both ends and the meter shows solid continuity from the top of the turret lugs to the pins so I guess the next step is to pull the board and check the solder connections below the lugs.

About half way through the assembly I got to thinking I wasn't going to like turret boards, but I'm green at layouts and figured my point to point would be a mess.


Top
Profile
Post subject: Re: PlanCathode to grounning stage of building an 5F2A Princ
Posted: Tue Dec 01, 2015 11:38 pm
Offline
Rock Star
Rock Star
User avatar

Joined: Sat Jul 03, 2010 2:18 pm
Posts: 6544
"Cathode to ground off pins 3 and 8 is 0 and 0."

This is with the 12AX7 in the socket? You should be reading around 2 to 5 VDC across each 1500-ohm resistor. Try reading these value, again --- before yanking any boards. No voltage across the resistors = no conduction through the tube.

Anode voltages are a bit high. However, the PSU and dropping resistors seem to be ok. Maybe, there is no conduction from the anode --> to cathode ---> through to ground. The anode volts are high because of the lack of flow to ground. If you still get 0 VDC across the 1500-ohm resistors, check your jumper leads from the resistors to the grounding buss. And the continuity from the buss to the chassis.

You did try another 12AX7, correct?


Top
Profile
Post subject: Re: Planning stage of building an 5F2A Princeton
Posted: Wed Dec 02, 2015 10:00 am
Offline
Hobbyist
Hobbyist
User avatar

Joined: Thu Jun 20, 2013 8:55 pm
Posts: 78
Location: South Texas
I'll check it again but probably won't be back home until the weekend, which is really the only time I have to spend more than a few minutes working on it. I appreciate the help. It's very possible that I missed something simple even though I've checked the layout more times than I can count.


Top
Profile
Post subject: Re: Planning stage of building an 5F2A Princeton
Posted: Wed Dec 02, 2015 4:56 pm
Offline
Rock Star
Rock Star
User avatar

Joined: Sat Jul 03, 2010 2:18 pm
Posts: 6544
Check your continuity to ground. Esp ground points on circuit to chassis.

Good luck! :)


Top
Profile
Post subject: Re: Planning stage of building an 5F2A Princeton
Posted: Mon Dec 07, 2015 3:36 pm
Offline
Hobbyist
Hobbyist
User avatar

Joined: Thu Jun 20, 2013 8:55 pm
Posts: 78
Location: South Texas
Some progress but still very little heat in the 12ax7.

I now have voltage where it was absent before. A wire from the board to pin 8 of the rect. broke at the bottom of the turret lug so I moved all board leads to the top. Also checked grounds and they have continuity.

One thing I haven't done yet is connect the NFB and if that's it I'll kick myself with both feet. According to what I can tell it goes from the board to the speaker jack and nothing more. I've added a copy of the layout with voltages. My eyes aren't what they used to be. Got a bustin headache from looking at it all afternoon. Any help is appreciated. Thanks \ Randy

(All tubes in. Tested with spare 12ax's)
Image


Top
Profile
Post subject: Re: Planning stage of building an 5F2A Princeton
Posted: Mon Dec 07, 2015 6:06 pm
Offline
Rock Star
Rock Star
User avatar

Joined: Sat Jul 03, 2010 2:18 pm
Posts: 6544
With those voltages on the 12AX7 --- it should work. Which lines are going to the 12Ax7 heaters? EG: pin 2 of the 6V6GT to pins 4&5 (12AX7). And pin 7 to pin 9?

GNFB lead goes to (+) side of speaker jack.


Top
Profile
Post subject: Re: Planning stage of building an 5F2A Princeton
Posted: Mon Dec 07, 2015 6:56 pm
Offline
Hobbyist
Hobbyist
User avatar

Joined: Thu Jun 20, 2013 8:55 pm
Posts: 78
Location: South Texas
BMW2002Ti wrote:
With those voltages on the 12AX7 --- it should work. Which lines are going to the 12Ax7 heaters? EG: pin 2 of the 6V6GT to pins 4&5 (12AX7). And pin 7 to pin 9?

GNFB lead goes to (+) side of speaker jack.


I took care of the NFB earlier just to get it done. Pinned it to the + on the jack.

6L6 pin 7 is tied to pin 9 (12ax7)and pin 2 of the 6L6 is tied to 4/5 on the 12ax7, but I still have 0.5 vac off pin 2 on the 6L6. The 5y3 gets pretty hot and the 6L6 gets too hot to touch, as expected, but the 12AX gets only slightly warm and I still haven't heard a squeak out of the amp. Should the voltage be the same on both heater wires?


Top
Profile
Post subject: Re: Planning stage of building an 5F2A Princeton
Posted: Mon Dec 07, 2015 8:38 pm
Offline
Rock Star
Rock Star
User avatar

Joined: Sat Jul 03, 2010 2:18 pm
Posts: 6544
Try measuring the AC voltage across pin 2 and 7 of the 6V6GT (or 6L6GC). If it is 6.3VAC (or close), then the 12AX7 has to work. There is no grounding wire or any other connections to the heater line after the 12AX7?

Is the input jack hook to pin 2 of the 12AX7 (via the 33k-ohm resistor)?


Top
Profile
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 73 posts ]  Go to page Previous  1, 2, 3, 4, 5  Next
Go to page Previous  1, 2, 3, 4, 5  Next

All times are UTC - 7 hours

Fender Play Winter Sale 2020

Who is online

Users browsing this forum: No registered users and 2 guests


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum

Search for:
Jump to: