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Post subject: Re: Planning stage of building an 5F2A Princeton
Posted: Mon Dec 07, 2015 9:32 pm
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BMW2002Ti wrote:
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)?


Pin 2 on the 6V6 is 0.5vac.

I'm 99% sure of the resistor on pin 2 of the 12ax7 is a 33 but I'll check it again.

I really appreciate the help, BMW. Didn't know something so basic could be this much of a problem. I know I'm losing my trust with turret boards. That lead going to pin 8 on the 5Y3 went in from the bottom and had dropped down when I heated the turret to solder the cap, which is why I moved everything up top.


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Post subject: Re: Planning stage of building an 5F2A Princeton
Posted: Mon Dec 07, 2015 9:44 pm
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Randy, measure across pins 2 and 7 of the 6V6GT. Not to ground. That is, set DVM meter to read VAC. Put red probe onto pin 2 and black onto pin 7. Pull tube. You should get 6.3VAC (+/-0.3VAC).

Now, for the input... turn amp OFF. Set meter to resistance or DC ohms. Put red lead onto (+) portion of input jack. And black onto pin 2 of the 12AX7 (with tube out of socket). Should be reading 33k-ohms, (+/- 5% or +/- 1500 ohms).


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Post subject: Re: Planning stage of building an 5F2A Princeton
Posted: Tue Dec 08, 2015 8:04 pm
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BMW2002Ti wrote:
Randy, measure across pins 2 and 7 of the 6V6GT. Not to ground. That is, set DVM meter to read VAC. Put red probe onto pin 2 and black onto pin 7. Pull tube. You should get 6.3VAC (+/-0.3VAC).

Now, for the input... turn amp OFF. Set meter to resistance or DC ohms. Put red lead onto (+) portion of input jack. And black onto pin 2 of the 12AX7 (with tube out of socket). Should be reading 33k-ohms, (+/- 5% or +/- 1500 ohms).



I checked the input lead and have a 33Ω resistor soldered to pin 3 on the 12AX7.

The green heater wires... Now I'm tracking. I thought I was supposed to have 6.2v on each one - not total. I've got that across 2 and 7. What I was missing, and I'm not proud to admit this :oops: was the power from the 33uf to pin 8 on the 5Y3.

After correcting my mistake I powered the amp up and it showed its first signs if life but with a very high pitch screeching noise, probably around 2Khz. I thought possibly my input jack was wired wrong so I removed all wires and tried the amp with nothing at all on the input - the screeching at full throttle is still there and the volume control doesn't do anything except change the pitch just slightly.

I've looked at the layout many times and everything looks in order, but I missed the power wire coming off the 10K\3W resistor to the 5Y3 so I think I need to back away from it and look at the layout and schematic again after my brain has had a break. Hoffman's design may be functionally correct but it's not exactly like Leo's original, and I don't do this often and those subtle changes are not exactly rolling off the page.

I do appreciate the help! I wouldn't have gotten this far without it.


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Post subject: Re: Planning stage of building an 5F2A Princeton
Posted: Tue Dec 08, 2015 8:39 pm
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Try pulling the global negative feedback resistor off the speaker terminal. You may have something mis-wired in the GNFB circuit. This can cause the screeching instability noise you are hearing. If removing the GNFB resistor stops the screeching... reverse the connections onto the output transformer's primary lines. This setup is not as critical as in push-pull amps, but I have seen stability issues in some single-ended amps and OPT wiring.

Be sure all B+ are tied to the correct points. The B+ on the 6V6GT anode (pin 3) must be the higher voltage, as compared to the HV tie to the screen (pin 4). The voltage to the screen should be around 5-10 VDC less than the voltage applied to the anode.

Make sure that the grid stopper and grid-to-ground (G2G) resistors are properly placed onto the 6V6GT. Check the G2G resistor on the 12AX7. Must be grounded properly.

Check the wiring layout going to the sockets. Try not to overlap wires. If you need to overlap wires, keep them at 90-degree angles to each other.

After trying all these things and amp is still instable, you'll need to go step-by-step from input to output to find the culprit. Report back with results --- we'll go from there.

No worries, we'll get the amp working....

:D


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Post subject: Re: Planning stage of building an 5F2A Princeton
Posted: Wed Dec 09, 2015 9:40 pm
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With the NFB resistor pulled from the speaker tip on the jack the ultra high squeal is gone but there's no sound.

When you wrote "reverse the connection' were you referring to the common and positive leads from the OT to the speaker jack? I tried that for a second and got nothing.

The grid stopper and G2G appear correct on the 6V6 according to the Hoffman layout and how they are connected to the tube. Pins 5-6 are tied with a 1.5K resistor, and pins 1 and 8 go to the board with the 470K\3 watt resistor.

I'm not sure about the 12AX7. If pin 2 is the grid 2 then I have the input with the 33Ω resistor there. Pin 7 goes to the volume pot.

When it squeals it does it at full power regardless of the Vol control.

Volts to ground on pin 3 of the 6V6 is 340vdc and pin 4 is 317vdc.

The romance is wearing thin and I am experiencing the difference between a parts changer and a person who fully understands how something works. I see it all every day at work but it's no fun being on the other side of the fence.

I want to be sure about the G2G and grid stoppers so I'll do some research and dig in again tomorrow.


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Post subject: Re: Planning stage of building an 5F2A Princeton
Posted: Wed Dec 09, 2015 11:29 pm
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"With the NFB resistor pulled from the speaker tip on the jack the ultra high squeal is gone but there's no sound."

Double check all your connections in the output section. There is something wrong in the wiring between B+, the output tube, and the output transformer.

Removing the GNFB resistor should not stop sound from being heard from the speaker.

Can you take pictures of the current setup? This may help a lot.


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Post subject: Re: Planning stage of building an 5F2A Princeton
Posted: Thu Dec 10, 2015 7:19 pm
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Can't thank you enough Beemer. I'm going to look at all connections and the B+ again and see if I can find anything odd.

I thought about posting a pic days ago but the amp is looking rough after being on the operating table for several days. I've soldered, desoldered and resoldered parts many times during my testing and have a lot of dressing up to do once it is able to stand on it's own feet.

I'll post my results and possibly a pic later. I'm headed to the O.R. now.


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Post subject: Re: Planning stage of building an 5F2A Princeton
Posted: Thu Dec 10, 2015 11:13 pm
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Good luck!

Concentrate on the power & output stages. May want to follow the schematic and layout and check continuity of the lines. With amp off, power caps drained.

Then, after output seems ok, similarly check the input stage.

May also want to draw up old solder and re-solder as neat as possible. Lot of issues are solder related. Check for blobs touching other components and the chassis.

:mrgreen:


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Post subject: Re: Planning stage of building an 5F2A Princeton
Posted: Sat Dec 12, 2015 11:45 am
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SUCCESS!!!!!

Sort of...

It's working but the volume is low. I'd say almost half of what would be expected and there's not enough oomph to enter breakup.

So far there have been 4 main things wrong with the build.

1. I missed the power wire from pin 8 on the rect tube. Stared at it for days and didn't see it. 100% my mistake.

2. OT was wired according to the layout but I took your advice and swapped the primaries. No more shrill howling at full volume. I followed the layout and color codes I had printed out but somehow it was bassackwards. I supposed I should have learned to check the windings with a meter instead of trusting the colors.

Powered up again but this time it was deal silent - everywhere.

3. Checked resistance again on the input jack, which was correct at 33Ω a few days ago when you suggested I look at it again, but when I checked again this morning I had 33Ω for just a second and then it shorted. Pulled the coax and found a slight cut in the dielectric right at the braid where I had applied heat shrink. I couldn't see well enough to tell if a stray hair from the braid had gotten to the center conductor, but I had done a poor job when I made it in the wee hours one night a week ago so I replaced it. There's no chance of stray hairs getting to the center conductor now.

4. I'm fairly certain the volume problem is somewhere in the input side since the amp proved capable of screaming it's guts out when the OT was wired wrong.

Again, thank you Beemer!

Thanks to the help I've received at this site I've recapped my 65 Pro R and built my first amp.


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Post subject: Re: Planning stage of building an 5F2A Princeton
Posted: Sat Dec 12, 2015 4:20 pm
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Great news! Almost there. Pretty sure your volume issues are due to bad soldering, grounding, and/or partial shorts.

"OT was wired according to the layout but I took your advice and swapped the primaries. No more shrill howling at full volume. I followed the layout and color codes I had printed out but somehow it was bassackwards. I supposed I should have learned to check the windings with a meter instead of trusting the colors."

When you have the output tranny primaries wired backwards --- the global negative feedback becomes global positive. Almost always leading to a howling full volume screech.


:mrgreen:


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Post subject: Re: Planning stage of building an 5F2A Princeton
Posted: Sun Dec 13, 2015 7:06 pm
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99% there.

It's working and it's loud. It's clean to about 40% on the volume control and has a ton of crunch from 80% on up. The used Sylvania 12AX7 was badly microphonic so I replaced it with an old Raytheon and seems fine. Tapping on the chassis alone would cause it to howl and whine.

The volume problem was likely my eyes again. The schem called for a 100KΩ to P1 of the 12AX7 and I had a 100Ω in there.

One last thing to do as I begin my final tweaks is find the source of a 60Hz hum. It's not the worst I've heard but it's not right, and now that the amp is working almost perfectly there's no immediate need to start tearing things apart. I can take my time.

Your help has been much appreciated. Image


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Post subject: Re: Planning stage of building an 5F2A Princeton
Posted: Sun Dec 13, 2015 11:23 pm
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Good news. Lots of detective work, but you got there.

As for hum... there are two types:

1.) 60Hz is almost always due to heater line (two green lines and lamp). This is the only non-rectified line in most guitar amps. Hum is lower than open low E-string. Standard tuning.

2.) 120Hz, which is ground loops, power supply, shielding, etc. Post rectified hum. Hum is open A-string tone.


Here is chart of frequencies of standard guitar tuning:

1.) high (E) 329.63 Hz
2.) (B) 246.94 Hz
3.) (G) 196.00 Hz
4.) (D) 146.83 Hz
5.) (A) 110.00 Hz
6.) low (E) 82.41 Hz



Be sure only ONE end of the shielded input cable is grounded. I usually ground the input jack side. NOT the circuit board or 12AX7 tube side.


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Post subject: Re: Planning stage of building an 5F2A Princeton
Posted: Thu Dec 17, 2015 7:30 pm
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Randy, any luck with last bit of problems?

I sent you my email addy (pst)... in case you want to send more info.


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