Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Skip Hagey on July 10, 2015, 04:36:20 pm

Title: microphonic PI
Post by: Skip Hagey on July 10, 2015, 04:36:20 pm: OK, so I've done my first power up on this AO-39 conversion. I don't get sound from the input yet, but when I tap the PI (V2) tube I get output in the speaker. Any Ideas?
Title: Re: microphonic PI
Post by: sluckey on July 10, 2015, 05:26:00 pm: Could be almost anything. Show us a schematic. Maybe some high rez pics too.
Title: Re: microphonic PI
Post by: Skip Hagey on July 11, 2015, 09:56:22 am: Steve, the schematic is yours...http://home.comcast.net/~seluckey/amps/hammond/hammond_ao39.pdf (http://home.comcast.net/~seluckey/amps/hammond/hammond_ao39.pdf) My wiring looks like a rats nest and is probably where the problem lies but here are a couple of pix:
Title: Re: microphonic PI
Post by: sluckey on July 11, 2015, 10:42:23 am: Measure voltages at the filter cap cans and for ALL tube pins and post the readings. Does V1 light up?

Divide and conquer. Turn the volume and tone to max. Now use a skinny screwdriver as a probe. Put the tip of the screwdriver on the wiper of the volume pot and touch the screwdriver blade with your finger (DON'T DO THIS TEST ON ANYTHING THAT HAS VOLTAGE ON IT!). Do you get a loud buzz in the speaker? If so, the problem is between the input jack and the volume pot. Very few components involved but plenty of opportunities to make a wiring error.

It's gonna be difficult to spot a wiring error in your pics, but you should be able to find it with your hands on the amp.

I suggest you move that speaker jack far away from V1. I put mine on the back panel. (This is not your current problem but will likely be an issue once the amp is running.) I also noticed you are using the old original filter caps. May want to change those.

You have some very unorthodox wiring in there so be careful. Lot of components supported only by a solder joint.
Title: Re: microphonic PI
Post by: Skip Hagey on July 11, 2015, 10:45:53 am: yep! Unorthodox is how I roll...lol

just got 600 volts DC from 1st filter cap. I'm guessing I have the secondary's and center tap mixed up...?
Title: Re: microphonic PI
Post by: Skip Hagey on July 11, 2015, 10:46:37 am: All tubes light but very dim....
Title: Re: microphonic PI
Post by: sluckey on July 11, 2015, 10:51:14 am: Quote
just got 600 volts DC from 1st filter cap.
I don't think that's possible. I believe your meter is lying.
Title: Re: microphonic PI
Post by: Skip Hagey on July 11, 2015, 11:19:41 am: you're right sir, my meter was measuring AC not DC. A=270, B=232 and that is a 4500 ohm resister 5W instead of a 2.2k cause that's all I've got, C=210 D=208
Title: Re: microphonic PI
Post by: Skip Hagey on July 11, 2015, 11:33:34 am: And 277VAC at pins 4 and 6 of the 5Y3
Title: Re: microphonic PI
Post by: Skip Hagey on July 12, 2015, 12:41:32 pm: Good afternoon, I'm wondering if my transformer is not up to par because all the filament and secondary HV values are somewhat 'starved". Since there are no voltage dropping resistors between the transformer and say pins 4 and6 and I'm getting a reading of about 280 each instead of the 315 on the schematic. Any help would be appreciated! I've checked all the connections and don't have any shorts, and I'm getting signal through to the speaker but very low volume. Thanks in advance...Skip
Title: Re: microphonic PI
Post by: sluckey on July 12, 2015, 01:25:15 pm: Quote from: sluckey on July 11, 2015, 10:42:23 am
Measure voltages at the filter cap cans and for ALL tube pins and post the readings.
Title: Re: microphonic PI
Post by: Skip Hagey on July 12, 2015, 01:49:16 pm: measure with or without tubes?
Title: Re: microphonic PI
Post by: sluckey on July 12, 2015, 01:56:25 pm: with
Title: Re: microphonic PI
Post by: Skip Hagey on July 16, 2015, 01:05:05 pm: Tube values, all pins, are posted; file 18493. Heaters on Rectifier are to high, I some how have the ctr tap and secondary's confused...but not sure how to fix it. I do have a very strong and pretty clean signal all the way to the spkr! Sounds pretty great, but I don't think the Rect. is right yet......
Title: Re: microphonic PI
Post by: John on July 16, 2015, 01:14:52 pm: Can you copy & paste the link to your voltages? On your rect. tube, many times the 2 yellow wires feed the filament, ~~and a yellow/stripe something is the CT.~~ Not always, but "usually".
Title: Re: microphonic PI
Post by: Skip Hagey on July 16, 2015, 01:18:58 pm: Doesn't copy and paste well but here they are. Two yellow wires(White really) to the htrs and the yellow stripe is grounded. voltage values at Cap cans is double what the schematic calls for also...

- Valve Pin 1 Pin 2 Pin 3 Pin 4 Pin 5 Pin 6 Pin 7 Pin 8 Pin 9
V1 12AX7 170 0 1.4 * * 170 0 1.4 *
V2 12AX7 257 111 80 * * 80 115 79 *
V3 6BQ5 * 0 8.5 * * * 259 * 247
V4 6BQ5 * 0 8.5 * * * 261 * 249
V5 5Y3 * 618 * 272 * 273 * 615 *
Title: Re: microphonic PI
Post by: sluckey on July 16, 2015, 01:31:34 pm: Quote
Heaters on Rectifier are to high
You're not measuring correctly. To measure filament voltage on a 5Y3 you must set your meter to measure AC and connect one probe to pin 2 and the other probe to pin 8. Should be about 5VAC.

I really doubt there's anything wrong with your rectifier or PT. If there was, you would not have the B+ voltages that you have.

Since you built this in the original chassis there was no reason to change any of the PT wires or the wires on the 5Y3 socket. You didn't change those did you?

PS... Here's the voltage chart...

http://el34world.com/charts/valve/ValveData.php?e=view&f=18493 (http://el34world.com/charts/valve/ValveData.php?e=view&f=18493)
Title: Re: microphonic PI
Post by: Skip Hagey on July 16, 2015, 01:34:39 pm: I gutted the original, but took note of where the tranny was wired and also measured the voltage before I took everything apart. Going to recheck my measurements....
Title: Re: microphonic PI
Post by: Skip Hagey on July 16, 2015, 01:37:18 pm: 4.1 volts AC across pins 2 and 8 on 5Y3
Title: Re: microphonic PI
Post by: Skip Hagey on July 16, 2015, 01:40:54 pm: Am I measuring the Cap values right? I'm measuring one lead on the cap and the other to ground...
Title: Re: microphonic PI
Post by: sluckey on July 16, 2015, 01:45:39 pm: Quote from: Skip Hagey on July 16, 2015, 01:40:54 pm
Am I measuring the Cap values right? I'm measuring one lead on the cap and the other to ground...
You have the probes connected correctly but I suspect your meter is not set to measure DC volts. If it is, then it's lying to you.
Title: Re: microphonic PI
Post by: Skip Hagey on July 16, 2015, 01:56:47 pm: Thank you, you are correct! Values seem to be low and these are the original cap cans. Does the low measurement mean the caps are "failing"? I have new ones but don't want to use them if I don't have to...
By Low I mean A=252v, B=240, C=227, D=226
Title: Re: microphonic PI
Post by: sluckey on July 16, 2015, 05:45:03 pm: Not necessarily. You measured 272VAC out of your PT. That could explain the lower B+ on the caps. Fresh caps might increase that. Using a SS rectifier would definitely take the B+ voltages up.

You also say you only have 4.1VAC on the rectifier filament. That's not good. Maybe your PT is actually putting out low voltages. Maybe your meter is faulty. How about plugging your meter probes into a wall outlet to measure your house AC. What do you get?

My PT# is AO-23279-1. What is your PT number?
Title: Re: microphonic PI
Post by: Skip Hagey on July 16, 2015, 05:50:49 pm: 115v AC out of the wall. and tranny is 23279. This thing is powerful and BRIGHT! It really sounds good...I could see adjusting res/cap values to roll a little more highs off, but I'm gonna get used to it first...
Title: Re: microphonic PI
Post by: sluckey on July 16, 2015, 06:02:12 pm: Quote
This thing is powerful and BRIGHT! It really sounds good
I wouldn't fret about the PT and cap voltages. Most people are more concerned with voltages being too high. Play on...
Title: Re: microphonic PI
Post by: Skip Hagey on July 16, 2015, 06:08:36 pm: Woohoo! It took a long time to get here! pretty happy to be here, thanks all! Till next time........