Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Richs1979 on February 09, 2025, 12:41:56 am
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Hello again, and good Sunday morning to you all.
This has been some learning curve so far. Since this is my very 1st build, I had a lot to learn.I have gained quite a bit of knowledge in the last month or so since starting this project. I have actually started to understand and read schematics. It's too bad that the circuit flow doesn't follow a physical layout more closely. Anyway, attached are some pics of what I've done so far. I do have some questions though.
First, I stopped at soldering the first 2 filter caps, so I could ask if they are correct. I followed Mr. Hoffman's diag and schematic and placed both those caps on the a A-B+ node. Is this correct?
My 2nd question concerns the Mojo752 - 125p5d power trans that I will be installing tomorrow. I'll be running both 5v yellow wires to a GZ34 rectifier with diodes. Do I need to run the red/yellow center tap to ground? I've read conflicting statements on this one. I think that it should be tucked away along with the 6.3v grn/yllw wire. Help is much appreciated.
Thank you.
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Hi Richs
I looked at the Mojo build documents and Sch, and I would have to agree with past comments of Sluckeys ie Rubbish!! I understand your confusion :laugh:
So simply, Yes. you could fit the HT CT (Red/Yel) with the Heater CT (Green/Yel) wire to a good Earth Lug near Power Tx BUT don't use the Tx mounting screws. Or you could use 100R ristors to ground as shown in Mojo Sch BUT not shown in their layout.
Please, next time include the Tx wiring details to help our colleagues. And please note the second attachment is from Mojo :w2:
Regards
Mirek
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Nice build
From Fender Deluxe Reverb;
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Thanks for your help. Since I'm using two 100 ohm resistors with the heater wires I'm pretty sure the green and yellow should not be hooked to ground. The red/yellow center tap should. My first question was if my filter caps are correctly installed. Thank you for the compliment btw. This started as a bare 20.250 x 8 inch chassis and I just designed it to taste.
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Since your heater winding is center tapped, you could just ground the green/yellow and NOT bother with the 100ohm resistors to ground.
It's fine either way, just don't do both. If I have a center tap, I'll use it. Saves space . . .
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Good evening. I finally finished the Hoffman AB763 1 channel amplifier kit with 2 6l6 output tubes and a GZ34 rectifier. Since this is my 1st time building anything electronic from scratch, I'm a little nervous to fire it up. I went over the circuit dozens of times and it seems everything checks out with Mr. Hoffman's diagram. The only changes I made were placing the bass pot first, and adding a future NFB switch. Also a future Master pot and an extra (V6) preamp socket for future modification. Everything else is a copy of the Mr. Hoffman's design. Is there a way to check my work without powering up? And how do I go about making sure the cathode bias is correct once I do fire it up? I do have a Variac which I got just for this project. Since all my other amps have solid state rectifiers, I don't have a spare rectifier tube. I have one on order which should be here in a few days. Anyway, I'm a few months shy of the big seven-O and I'm hoping i can show this thing off to some of the younger session guys I play with. Help is much appreciated. Thanks.
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The first fire up
I'll use bulb lamp current limiter, not variac. 2 tests
1- No tubes
2 - Only preamp tubes. NO power tubes
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" And how do I go about making sure the cathode bias is correct once I do fire it up "
-if the amp starts up correctly with the bulb lamp current limiter. Only preamp tubes. NO power tubes
1- Pug amp in the wall outlet ; power on , stanby in play .
2- Read negative bias volts at pins 5 on each 6L6 socket
Turn bias pot to reach than maximum negative voltage ; You must have more than -30 volts dc.
3- Il you can't reach -30 volts ; issue in the bias circuit ; DON'T put power tubes in the amp
4- If you have -30 and more ( it is better) leave the bias pot at its maximum.
5 - To adjust at the right bias, you must have a bias probe or 1 ohms resistor from each 6V6's cathode to ground.
Read here ;https://www.mozartproject.org/how-to-bias-your-amps-tubes-for-the-best-sound/
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Is the photo in reply#5 the finished amp.
It looks like a couple connections are missing. Rectifier pin8.
I would ground the PT center tap red/yellow to the A node caps negatives.
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Hello -
Posted is a closeup of the PT wiring. I copied this build from one of Doug Hoffman's post's. The PT wiring is identical to his build. My main question was how to make sure my wiring is good. On paper it's all correct. Nothing was missed. I have a Variac and I was wondering if I can do a lo voltage circuit test before installing any tubes. When I ordered this kit there were parts Doug no longer carried but I followed his recommendations to build a 6l6 based single channel ab763.
Thanks again.
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On paper it's all correct. Nothing was missed.
Maybe it looks good on paper but you are missing a very important wire inside the chassis. You MUST connect a wire from pin 8 of the rectifier socket to point A on the board! Hoffman puts a standby switch in line with this wire. Without this wire you will have no high voltage anywhere in the amp (except the rectifier socket).
I noticed in the first pic you posted you had a red wire correctly connecting the standby switch to point A on the board, but this wire disappeared in later pics. What happened?
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You're right. Stupid beginner mistake. I took out the standby switch out since after reading dozens of post's saying it's only necessary with ss rectifiers. Thank you for pointing out that blunder.
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Using a lightbulb limiter is correct procedure for first startup as outlined in reply #6.
Can't see the A/C wiring to check if it's correct.
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It's tucked under the lip. Blk to fuse to switch to transformer. White to tranny white(neutral), green to ground.
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Light bulb limiter to test for shorts not variac for start up.
https://el34world.com/Forum/index.php?topic=32424.0 (https://el34world.com/Forum/index.php?topic=32424.0)
How to build a light bulb limiter;
https://www.sluckeyamps.com/misc/Amp_Scrapbook.pdf
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I have an inline multi ac power meter. I thought I could use the Variac, the display meter, then the LBL. I need to make the LBL first. Can I rig a table top lamp instead of buying a switch box panel? Also, I bought that variac to use with my old Fender's since the voltages in my house get up to around 123, 4.
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Put the variac aside, use just the light bulb limiter. You don't need the ac power meter for 1st time start up.
The bulb limits the current and that will protect the amp from damage if there's a short. For 2 x 6L6GC use 100w bulb.
Read this;
https://el34world.com/Forum/index.php?topic=32424.0 (https://el34world.com/Forum/index.php?topic=32424.0)
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I need to make the LBL first. Can I rig a table top lamp instead of buying a switch box panel?
The principle is there; Picture .
Use or build the way you want but respect this wiring and use an incandescent lamp, not a led.
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Will do. I know there's a box of 60s. Would that do if I dont have a 100? Thanks for your help.
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60's are fine.
Also, you can use halogen bulbs instead of the more common "incandescent" bulbs.
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Thanks again. I'll be back after start-up. I'll use a 1 amp slow blow for the 1st test. Then I'll put the 2.5 back in if it didn't smoke. It's like my first small block build breaking in a new cam. That 1st fire-up is nuts. Fingers crossed. Thanks again.
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I'll use a 1 amp slow blow for the 1st test. Then I'll put the 2.5 back in if it didn't smoke.
No, that's what the light bulb limiter is for.
Use the correct size fuse.
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Ok.... Will do.... Got the lamp done. Bulbs were LEDs. Need to get the right ones L8r. Thanks.... I'll be back. :icon_biggrin:
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Hello again.
Made the light bulb limiter and had buy bulbs from Amazon. Bulb works... Tested it with another amp to get an idea how it works. Bright for a second then dim. Anyway, powered up the AB763 and the bulb did not turn on. The pilot light is working and there was no smoke, hum or anything unusual. I'm thinking that my B+ wire to the rectifier tube is wrong. Shouldn't it be on pin 7 and not 8. Because I added the diodes everything shifted one over. Here's a pic. Help is much appreciated as always. Thanks
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NO! your rectifier is wired correctly. Do you have any tubes installed? If you looked over the documentation of this Hoffman circuit you would know that. https://el34world.com/Hoffman/files/Hoffman_AB763_1.pdf (https://el34world.com/Hoffman/files/Hoffman_AB763_1.pdf)
the exception being you added safety diodes.
https://dynacotubeaudio.forumotion.com/t1006-tube-rectifier-yellow-sheet-diode-mod
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I'm thinking that my B+ wire to the rectifier tube is wrong. Shouldn't it be on pin 7 and not 8. Because I added the diodes everything shifted one over.
No, everything is not shifted over 1. You only shift the red B+ acv wires over 1 pin to mount the diodes.
You still keep the rectifier yellow 5acv heater wires on pins 2 and 8, and take the B+ dcv from pin 8.
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No tubes..
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I got the 0 volt ted/yellow center tap going to ground. Can that be wrong?
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Are you using a 100w incandescent bulb? Not LED.
Read this on 1st time start up;
https://el34world.com/Forum/index.php?topic=32424.msg359486#msg359486 (https://el34world.com/Forum/index.php?topic=32424.msg359486#msg359486)
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Incandescent Edison bulbs from Amazon. Tried it with my 15 water and it worked liked they said it would
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I got the 0 volt ted/yellow center tap going to ground. Can that be wrong?
Yes, B+ CT goes to ground.
Did you have the power tubes in when you tested with the LBL? Or just the just the rectifier tube?
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PLUG THE TUBES IN!
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No tubes. Should I have put the rectifier in? And why did I not get a complete circuit even without the tubes?
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Read this on 1st time start up;
https://el34world.com/Forum/index.php?topic=32424.msg359486#msg359486 (https://el34world.com/Forum/index.php?topic=32424.msg359486#msg359486)
You have to set the -bias to max before you put the power tubes in.
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Pilot light ON proves you have a completed primary circuit.
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So max out that 10k tremelo pot. I read the startup instructions over and over. And it still tells me that the limiter bulb should glow from the get go. I tried it on another tube amp and it works. It did not even flicker when I turned on the new build. The pilot light did go on. But that's it. No complete circuit back to the the limiter bulb. Am I misunderstanding something?
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So max out that 10k tremelo pot.
No, not the trem pot. The little white -bias pot on the eyelet/turret board.
You measure for -dcv at the power tube pin 5 of both power tube sockets.
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Correct, since it's wired to the intensity pot I just assumed it was for that. Since this is cathode biased I thought the value was fixed by a resistor. Please forgive my ignorance. Never did anything this deep before.
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I read the startup instructions over and over. And it still tells me that the limiter bulb should glow from the get go. I tried it on another tube amp and it works. It did not even flicker when I turned on the new build. The pilot light did go on. But that's it. No complete circuit back to the the limiter bulb. Am I misunderstanding something?
When you tried it with a different amp, that amp had all it's tubes in it. The tubes will draw current, that causes the bulb to light up.
The new build, you have no tubes in it, so no current draw, bulb will not light up.
I don't see how you could have read the LBL start up instructions "over and over" and are not understanding what to do. It's very clear.
"Yes, just plug the amp into the LBL, no variac, no tubes, no rectifier tube, if amp has 1.
This tells you that all the PT secondaries, and whats wired up to them, on/off switch, fuse, pilot light, heaters, 5acv rectifier tube socket, and -bias supply, if amp has 1, have no short. If SS rectifier, this also tells you power supply, B+ filter cap string should be good, no shorts.
If the amp passes that test, then add the rectifier tube, if it's got one. This tells you power supply, B+ filter cap string should be good, no shorts.
If the amp passes that test, then if adjustable fixed -bias, verify -dcv at power tube socket. DCV present, set for max -dcv.
Now with the speaker plugged in, or dummy load, put ALL the tubes in.
Passes that test, your should be good. Unplug the amp from LBL, and now if amp has adjustable -bias, set the -bias."
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So max out that 10k tremelo pot. I read the startup instructions over and over. And it still tells me that the limiter bulb should glow from the get go. I tried it on another tube amp and it works. It did not even flicker when I turned on the new build. The pilot light did go on. But that's it. No complete circuit back to the the limiter bulb. Am I misunderstanding something?
I bet the tubes were plugged in on the other amp. The fact that the pilot light is on means you have a complete circuit on the limiter bulb. But, that 1 watt pilot light does not draw very much current. That's why you don't see the limiter bulb lit up. PLUG IN ALL THE TUBES!
On another note... It looks as though you have the black wire from the PT connected to the end terminal of the fuse holder. For safety sake, connect the power cord black to the end terminal and the PT black to the side terminal.
One more note... This is not a cathode biased amp. It is a fixed bias amp. And the 10K trim pot on the board sets the fixed bias grid voltage.
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Correct, since it's wired to the intensity pot I just assumed it was for that.
Forget the trem intensity pot.
The little white pot on the power tube end of the eyelet/turret board is the -bias pot. That's the pot to set the -bias with.
Since this is cathode biased I thought the value was fixed by a resistor.
This amp is not a cathode (K) bias amp. It is an adjustable fixed bias amp. That's why you have that little pot on the board.
Take your meter. Set it for DC voltage, with alligator clips on your meter leads clip the black lead to ground and the red lead to pin 5 on 1 power tube socket. Turn on the amp, no LBL, measure dcv at full counter clockwise (CCW), then measure at full clock wise (CW). Post those readings here.
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Did measurement at pin 5.
Counter clockwise = -63.0
Clockwise = -25
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Ok, that's good.
Now set the -bias full CCW, -63. Now put ALL the tubes in, including rectifier tube.
Plug amp into the LBL, turn on the amp. Be ready to turn it off if the bulb stays bright after turn on and does not dim back down. Just like when you tested it on the other amp.
If it dims back down you should be good. Take the amp off the LBL and set the -bias.
Set the bias for fixed AB with this; https://robrobinette.com/Tube_Bias_Calculator.htm
I like to set the bias for 60% dissipation.
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I appreciate you putting up with my inexperience. I just wish I knew how to do this stuff 50 years ago :laugh: I'll be turning it on in after the dishes are done. I'll look at the link you sent me. Thank you. :thumbsup:
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One more quick question.
Do I need to put a load on the output now?
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Do I need to put a load on the output now?
Yes, always a load when you turn on an amp.
It's in the instructions I posted for you.
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Did you use 1 ohm resistors on the power tube cathodes (K)?
They go between the tubes K, pin 8 and ground. Then you can measure the tube current safely at low voltage.
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Yes..... 1 and 8 to 1 ohm
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Wow..... I just looked at this thread. Could write a book. Thank you all for your help. Set bias to 26.4 light bulb stayed a nice dim color after initial semi bright turn on. Pots work. Only Volume por is scratchy in a few spots. A bunch of rotations back o'and forth should fix that. Will plug in my Tele and see what happens. Didn't check the tube color though. Will do that now. Thanks again. I'll post the final results L8r in the evening. :icon_biggrin:
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If the LBL went dim with all tubes installed then turn the amp off and take the LBL off the amp and put it aside. You will now have the bias set to max negative voltage. Then turn the amp on and then set the bias. You cannot set bias when on the Light bulb Limiter LBL.
Reread Willabes post 41 again.. and the posts after that and make sure you know what you are doing with setting the bias to 60% of max dissipation. If not ask some questions. We don't want to burn up some expensive tubes.
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You will now have the bias set to max negative voltage.
You set the bias pot to max -bias before you put the power tubes in and test with the LBL.
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Have hum coming off the C node cap. When I chopstick it it goes away. When just touching it it goes away
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Set bias to 26.4 light bulb stayed a nice dim color after initial semi bright turn on.
26.4 what? DC volts, mA's?
Pots work. Only Volume por is scratchy in a few spots. A bunch of rotations back o'and forth should fix that. Will plug in my Tele and see what happens. Didn't check the tube color though. Will do that now.
You don't test those things with the amp still plugged into the LBL. Just get the amp to pass the smoke test, LBL test. After amp passed the LBL test, plug straight into the wall, no LBL, then set the bias.
After bias is set, then you can plug a guitar in and test all the controls.
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Have hum coming off the Code cap.
What is a "code cap"?
And why are you playing around with that?
Stay focused.
1st - LBL test.
2nd - set bias on power tubes.
The you can fool with the rest.
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26.4 volts
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26.4 volts
No.
You set it for MAX -bias before you put the power tubes in. Not the least -bias.
Now set the -bias full CCW, -63. Now put ALL the tubes in, including rectifier tube.
And you mean (negative) -26.4dcv?
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Setting the bias on power tubes is like setting the idol on a carburetor.
The more negative the bias the less current can flow through the tube. So -50dcv is more negative than -25dcv.
We set the bias for max -bias to protect the power tubes when we do the LBL smoke test.
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I redid the LBL test with the bias pot at -63. I watched the light carefully and it did dim. But not as much as the little 15 water did. I'm reading all this in millivolts. I'm stopping here so not to burn something up. The calculator came up with 26.4 being on the standard side. I disconnected the LBL and I turned the pot clockwise till I got
to this reading using millivolts again. I have a pic of the tubes.
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Yes -26.4 vdc
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Ok. I'm exhausted. Redid the startup thing. With the bias pot completely ccw I put the tubes back in. Turned it back on with the LBL and it did dim steadily after turning it on. The calculator gave me a number of -26.4 after Inputting a 6L6GC. I will stop here with the LBL disconnected waiting for some guidance. There was a hum sounding like a bad ground and when i tapped on the 22 500 filter cap connected to the boards C-node it lessened a bit. Tubes had a red glow but not too bright. They seemed off colored compared to my 69 super bassman which had more of blue 4han red. Anyway appreciate everyone's help. Thanks
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Your 69 Super Bassman has a +410V HT and -37V nominal Bias Supplies!!
Assuming your Mojo TX "Mojo752" with Secondary rated at 355-0-355 VAC will probably give you approx 450V DC.
Your Bias "safe starting point" should be no less than -50V DC!! Even if you aren't using the LBL.
You haven't told us you HT Voltage either with or without 6L6GC fitted :w2: But we know you have 1R0 fitted to pins1+8 on O/P tubes.
At 450V HT and available -63V bias supply you should be able to dial in 40mV DC across you 1R0. ((Tube bias at approx 60%)
For whatever reason you are trying to dial in ?-26.4V, drop it and go back to Post 41 as Willabe kindly outlined :worthy1:
-26.4V on Grids of 6L6 with HT> 400V will lead you to redplating and destruction!
...and good O/P tubes are getting expensive :laugh:
Regards
Mirek
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Again, my inexperience comes to play. I typed in what I thought was general 6l6gc 18watt. Looked now and I was supposed type in 30 or 40 watts. Which I did and came up with -50 vdc as safe. Still have a slight hum, but my tubes look better. 22 500 cap on board node c seems the culprit. Seems microphonic when I tap on it. The bias is set though. Here's the best pic I can send you with this phone. Thanks again for responding.
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The hum is only slightly worse than the sf super. Since the tubes looked normal I plugged in my strat set the volume to 3, mid scoop and a little top end and geeze..., I was blown away. With a small blackheart enclosed cab I blew the roof off the house. My wife loves me now. Anyway, other than something with that cap, and now I found that my number 1 input with the 1 meg resistor doesn't work, number 2 with the 2 68k caps works fine. Overall my 1st build works. As far as voltages go, please tell how to go about measuring them. I know how to keep one hand in my pocket, just need to know where and what measurements. Thanks again
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Ok Richs, now that your wife loves you, can you please let us in on a secret.
What is ie can you please measure your +HT on idle, your current in both 1R0's on O/P tubes and just for fun, final Bias voltage on O/P grids (or grid resistors). With your two friends ie reliable Multimeter and 1R0 resistors, you can learn a lot and achieve good confidence in self builds.
?What calculator were you dialing numbers into?? Definitely NOT as useful as guidelines from mresistor and Willabe :l2:
Regards
Mirek
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I had the meter on the whole time I was playing and the bias stayed at -50. As for measuring ht, please Instruct. Is it just the voltage off the 2 red transformer wires or the current. Would hate to burn another meter. I feel stupid asking this. But I'm just a guitar player with a soldering iron.
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One more quick question.
Do I need to put a load on the output now?
Wow !!! :BangHead:
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Erase
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One more quick question.
Do I need to put a load on the output now?
Wow !!! :BangHead:
Latole, you need to be a little more respectful to members. That reply was over the line.
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I had the meter on the whole time I was playing and the bias stayed at -50.
Always put what the voltage is, AC or DC.
And if the -bias supply is new, it should/will be stable. What you want to measure after you set the -bias is across the K bias R. The current through that R will increase as you play through the amp. You just really want to know the current through the tubes at idol. (Unless you have some kind of problem.)
As for measuring ht, please Instruct. Is it just the voltage off the 2 red transformer wires or the current.
No. You have to measure the power 6L6GC plate dcv. It's pin 5.
Use you gator clips on your meters leads. Red lead goes to pin 5, the black lead goes to ground, set meter for dcv. Make sure the red leads clip is NOT touching anything but that pin 5. You don't want to short pin 5 to anything else!
Post the readings here of both tubes.
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In reply #63, the screen shot you posted of the bias calculator, you put the wrong dc plate voltage in.
That B+ power supply must be putting out more than 350dcv. So you biased the wrong.
You have to measure the 6L6GC plate dcv 1st, and put that into the calculator. Then calculate the -bias. After you set the new bias, you have to re-measure the 6L6 tubes plate dcv again, if different put the new dcv into the bias calculator and calculate it again.
The more current the power tubes draw, they will drop the B+ dcv. The less current the power tubes draw, the less they will drop the dcv. The current draw is a load on the B+ power supply.
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So take reading off pin 3 on both tubes, correct? What should those voltages be? I know from reading it's never what the transformer rating is.
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Yes, you measure both tubes, pin 3, they will be close in their readings, within a few dcv's.
Probably will be ~ 450dcv or a little more.
Then put that number, average the 2, into the bias calculator.
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Got it. I appreciate you sticking thru this with me. I noticed a comment from one of the other guys. Not cool. At least I got this running without smoking anything. Thank you. I'll post the numbers and redo the bias again.
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I noticed a comment from one of the other guys. Not cool.
I called him on it, just not necessary.
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Checked voltages on the power tubes.
V6 was 426
V7 was 428
Do I average these before calculating bias, or do I use the higher reading?
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There 2 volts different, doesn't matter.
Go with 427dcv.
I get; 427dcv, 6L6GC @ 30w, Class AB fixed bias, 60% dissipation, 42.2mA. You can try 70% dissipation. Tubes will run 10% hotter.
After you set the bias, go back, measure the plate dcv again. If 5 to 10 dcv higher or more or 5 to 10 volts lower or more, re-calculate the bias again. Should be good then.
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Now v6=439, v7=438
So re-do bias with 438, correct?
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Now v6=439, v7=438
So re-do bias with 438, correct?
Yes, I would. Easy to do, doesn't take much time.
DCV went up, so the tubes were pulling a little more current before.
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Set the bias to -45 gets more hum if I go lower.
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Set the bias to -45 gets more hum if I go lower.
Do you mean -45mA, measured across the K 1 ohm sensor R?
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Yes ..... Pin 5
The hum seems to dissipate when I turn up the volume slightly. I know the noise floor goes up and maybe veils the lo frequency hum, but it seems it does lessen when the volume gets turned even slightly
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Set the bias to -45 gets more hum if I go lower.
Do you mean -45mA, measured across the K 1 ohm sensor R?
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Yes -45mA
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I'm not sure how your setting the bias.
You put your meter probe's across the 1 ohm K R with the probe gator clips. Set the meter to dcv.
1mv = 0.001
And then 0.001dcv converts to 1mA.
So lets say, 0.042dcv = 42mA.
I get tangled up in numbers, but I think that's right. :laugh:
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I'm using pin 5 on the 6l6 and black to ground
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Pins 1 and 8 go to a 1 ohm. Then to ground
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With my probe across the 1 ohm on v7 I get 22.0mv
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V6 I get the same 22.0 mv at that 1 ohm resistor
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I'm using pin 5 on the 6l6 and black to ground.
No.
Pin 5 is the tubes grid, that's where you measure for -dc bias voltage. Bias dcv is not the tubes current. When setting the bias you want to know how much current is flowing through the tube.
The tube calculator is telling you how much current you run through the tube. You check that across the 1 ohm bias current R.
Pins 1 and 8 go to a 1 ohm. That ;looks like ground
It's not ground, but it is grounded. As current is pulled through that 1 ohm R, a voltage will develop across the R. The more the current, the larger the dc voltage at idol. We use that dcv to set the bias.
To set the bias with the bias calculator number;
Put your meter leads across the 1 ohm K R.
Set the meter for dcv.
If the calculator says, lets say 40mA, then turn the bias pot until you see 0.040dcv.
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V6 I get the same 22.0 mv at that 1 ohm resistor
What does the bias calculator say to set for mA's for 60% dissipation? Set it for that #.
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If I set it that low the hum gets annoyingly loud. If I set it at 70 percent it is acceptable. Is this safe or do I live with the hum and set it at 60 percent. I noticed also if I put my hand on the reverb input jack, the hum is lessened. I am trying to keep this on the cool side to stretch the life of the tubes. I wish I were more knowledgeable. I appreciate your patience with me. I wish I could buy you a drink. Thank you.
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:laugh: I don't drink anymore.
We'll get to the hum. Lets just get the bias set correctly.
What's the 6L6GC plate dcv, pin 5, now?
Edit; Pin 3.
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-43.5
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I'll be back
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-43.5
I'm sorry, I meant pin 5, B+ high dcv.
Edit: Sorry, I'm tired. :laugh: Pin 3! The plate.
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So I did the following.
Measured pin 3 on both V6 and V7
V6 = 418v
V7 = 417v
At 1 ohm resistor
V6 = 31.0mv
V7 = 31.0mv
Pin 5
V6 = -49
67 = -49
This actually sounded the best with a single coil going through it. Beautiful full sound crystal clear with no artifacts.
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Ok, good.
So the bias calc says; 418dcv, 30w tube, 60%, 43.1mA. So your getting close.
So set it at 43.1mA.
Then re-measure the plate dcv, pin 3, then put that dcv in the bias calculator. Re-set the bias. Then measure the plate dcv 1 more time, pin 3, if dcv is close, your fine, if it moved up/down more than ~5dcv, run it again.
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Will do. Fingers crossed.
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Your B+ is low. Should be more like at least 330dcv to 350dcv, fully loaded.
You don't have the amp still plugged into the LBL do you? You don't need that any more. It will throw off ALL your measurements.
You sure you have a GZ34/5AR4?
We need the acv on pins 4 and 6 of the rectifier tube and the dcv on pin 8 and pin 2.
Measure each of those 1 at a time, red probe to the pin, black probe to ground.
Then measure the B+ dcv at each filter cap node and post those #'s.
Post the #'s.
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I'm going out to get a sandwich, be back in a while.
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I'm back, did you take those measurements?
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Here are the numbers
Rectifier pins 8 and 2 = 5.4 vac
Rectifier pins 4 and 6 both @164.5vdc
Nodes.....
A = 439 vdc
B = 433 vdc
C = 425 vdc
D = 409 vdc
I'm going to have to quit for now. But I'll be at it in the morning. Appreciate all the time you spent on this with me.
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You don't have the amp still plugged into the LBL do you? You don't need that any more. It will throw off ALL your measurements.
You sure you have a GZ34/5AR4?
We need the acv on pins 4 and 6 of the rectifier tube and the dcv on pin 8 and pin 2.
Measure each of those 1 at a time, red probe to the pin, black probe to ground.
You did them backwards.
Need ACV on pin 4, ACV pin 6. (See below.)
I forgot; You have protection diodes on the rectifier tube socket. So What I'd like to see is the ACV from the PT's red wires.
DCV on pin 2, DCV pin 8.
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Measured pin 3 on both V6 and V7
V6 = 418v
V7 = 417v
At 1 ohm resistor
V6 = 31.0mv
V7 = 31.0mv
Pin 5
V6 = -49
67 = -49
Nodes.....
A = 439 vdc
B = 433 vdc
C = 425 vdc
D = 409 vdc
Somethings wrong here, 20+ dcv difference between Node A and pins 3 on V6/V7.
It should be the higher dcv, that would seem correct for that PT.
You have to get these measurements right, so you can bias the amp correctly.
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Will take those measurements again later today. In the midst of cleaning up around here with the ice storm we got hit with. What should the voltage be for that PT.? It says 355 vdc on the sheet.
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What should the voltage be for that PT.? It says 355 vdc on the sheet.
Don't know. Spec sheet number is unloaded, it will be less fully loaded, with all the tubes in.
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What should the voltage be for that PT.? It says 355 vdc on the sheet.
Don't know. Spec sheet number is unloaded, it will be less fully loaded, with all the tubes in.
Answer #1
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I had to stop the cleanup, now it's pouring rain. Anyway I checked these points again.
Rectifier = both yellow are 5.4 ac
Pin 4 = 160 dcv max
Pin 6 = 160 dcv max
Could those 2 diodes be messing something up. The voltages dropped a few times down to 142 vdc before stabilizing at 160. Stupid question but shouldn't this voltage be around 175 dcv with thos ht secondaries rated for 355 v?
Filter caps nodes
A = 426 (2 in parallel 22uf 500v)
B = 418 22uf 500v
C = 408 22 uf 500v
D = 385 22 uf 500v
These are F&T caps that Mr. Hoffman put in this kit
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Pin 4 = 160 dcv max
Pin 6 = 160 dcv max
Could those 2 diodes be messing something up.
Yes they are. The AC voltage Willabe wants you to check is actually on pins 5 and 7 (the pins where the PT red wires are connected).
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Ok will do. Be back in a little while. Trying to clean up here b4 the deep freeze 2night.
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No read on 5 and 7. Meter just jumps around. Should I just pull the diodes out and rewire to pins 4 and 6?
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No read on 5 and 7. Meter just jumps around. Should I just pull the diodes out and rewire to pins 4 and 6?
No, leave the diodes.
Set your meter for AC voltage.
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We need DC voltage on pin 2, DC voltage pin 8.
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A = 439 vdc
B = 433 vdc
C = 425 vdc
D = 409 vdc
A = 426 (2 in parallel 22uf 500v)
B = 418 22uf 500v
C = 408 22 uf 500v
D = 385 22 uf 500v
These reading shouldn't be that different.
from now on, measure your AC voltage at the right wall before you take any more power supply voltage readings and post that 3 too.
Is your meters battery old?
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I was ready to check the wall outlet when I read this post. I did remove the diodes and rewired to pins 4 and 6. I get a max of -19 on both and then it just jumps around. The dc reading on pin 8 is 428 vdc. I will check my battery and outlets right now. Thank you again.
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Do you know if your wall ac voltage goes up and down a lot?
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I get a max of -19 on both and then it just jumps around.
Is that - as in negative?
19 what?
AC? DC?
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Yes That was -19 on both 4 and 6 after I pulled the diodes out. Battery reads 9.4 volts but wall outlet reads 118
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Just wanted to add that we've had brown outs lately due to icing. And who knows with the new smart reader they put in our bill went thru the roof. Nother subject sorry. Yes it does fluctuate. I had a read last month of 123 volts in one if my basement outlets.
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Post a pic of your meter.
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I measured new outlets in basement and they're all 120. Here's my old meter.
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You either have a faulty meter or operator error. But really, there's no reason to get hung up on measuring the AC voltage on pins 4 and 6 of the rectifier tube because the rectifier is putting out the correct DC B+ voltage. You're way past all that. Just set the bias and call it done.
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You either have a faulty meter or operator error.
He's given 3 different B+ dcv, 22dcv difference.
With 355-0-355 @ 300mA, with a GZ34, the readings he's giving look a little low.
I was trying to find out what was going on. :dontknow:
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Yes That was -19 ...
-19 what?
You need to post AC or DC on any voltages you post. (Not that it matters with that reading it's wrong any way.)
On your meter, looking at the pic you posted, going clock wise from the off setting, 1st is DC voltage, 2nd is AC voltage. DC is the straight line, AC is the wavy line.
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So, measure the plate DCV of the power tubes, pin 3, then put that number into the bias calculator.
Use the 6L6GC 30w tube setting.
Set the bias at the 1 ohm bias R in dcmV, then re-measure the plate dcv, pin 3, then plug that number into the bias calculator and re-calculate the bias. Then re-set the bias, should be good then.
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Doides back in measured 414 dcv at pins 3 on 6l6gc. Set bias to 36.7 mA at 1ohm fed from pin 1and 8 power tube
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So, how does it sound? (https://el34world.com/Forum/Smileys/default/smiley-music014.gif)
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Sounds strong and clean. But the hum is still there. When you turn up the volume the hum goes away. There's no ground buzz from the guitar and when I let go there's no noise.its the hum when the volume is turned all the way down. Can I post a sound file or video on this forum so you can hear it.
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Yes, if you want.
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There's no ground buzz from the guitar and when I let go there's no noise.
So, it humms only when your touching the guitar with the amps volume all the way down/off?
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It hums when the volume is completely off. No guitar hooked up. Just the amp on with volume completely down. When raising the volume even slightly, the hum dissipates. I'll be back in a while. Gotta start dinner.,,, Thanks ...
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I turned on the amplifier again and I noticed that if I put my hand above a certain part of the amp, the hum subsides. You don't have to touch it, just put a part of your anatomy over the top of that section. I'm wondering if it just needs some type of a faraday shield. Most of the Fenders I've seen have some aluminum glued or stapled to the cab where the amp sits. I have a piece of thin stock I'll try placing over the top to see if that works.
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Can you take new pin5 power tube measurements and post them.
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Pin 5 is -41.3 volts
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I was right about the shielding. Put some foil over cardboard and the hum is acceptable now. Sounds like my 69 SF. :l2: Thank you to everyone especially Willabe who stuck through this newbies 1st build. Thank you again. Time to build a cab.
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The finished AB763 build in it's new cabinet.