Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: AnalogDok on September 19, 2021, 06:46:54 pm
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I decided to use the Princeton Reverb bias scheme on my AB763 Deluxe Reverb since I don't have a 50V tap of the Hammond A029 PT I used.
My Bias voltage was -22VDC with the 100K resistor on the Bias board. I changed it to a 50K and the Bias is at -54VDC now.
I have zero Bias current or output from my output tubes. I ordered 68K and 75K resistors to swap out to see if I can get it close to -35V.
I may have miswired the amp though. I started it up on a variac on a dim bulb then added tubes. I'm straight into the AC now and so far, here are my voltages that I checked. Layout and voltages are on the file. I haven't wired in the MV yet.
Anyone have any idea?
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Anyone have any idea?
Yes. Export your diy file to pdf and post the pdf file. Then everyone can see what you're talking about. There are only a few diy users here.
Oh, you meant ideas about the bias voltage. :icon_biggrin:
-54v is too much for 6V6s. You have the right idea. But, put the amp in standby mode and monitor the voltage on 6V6 pin 5. Turn the bias pot from end to end to see the minimum and maximum bias voltage. Post the results.
I would think that with -54V out of the bias board that you should be able to adjust the actual bias voltage on pin 5 from -27v to -54v.
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Thanks Sluckey,
I'll run a test tomorrow and post results.
PDF posted.
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Why does it say "1V" on the screen grid pin?
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That is what I measured...
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That is what I measured...
Really! You show 1V on pin 6 of V7 and V8. But if you follow that yellow wire back to the board, you will see 460V on that end of the wire. You may want to check that again or just correct the drawing. :icon_biggrin:
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> I have zero Bias current or output from my output tubes.
If the Screen grid is at zero the plate-cathode current of the power tube will be very-very low.
If you have 460V at one end of a wire and 1V at the other end, there is something not right.
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You guys are great.....I have the interconnect from Pin6-V8 to Pin6-V7 but forgot to connect it to the board.
No wonder there is no voltage...
Yeah, something was not right....
Thanks...
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I checked the voltages and wrote them down.
If I turn it up to 10 on the volume, I get a little output but very weak and barely audible.
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Im a novice but double check your speaker output jack wiring.
Vr
J.
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I checked the voltages and wrote them down.
And they are very messed up!
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Something ELSE is disconnected. Perhaps the link from V6 cathodes to speaker circuit.
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For 6V6s (or 6L6s):
Pin 1 = no connection
Pin 2 = heater filament
Pin 3 = Plate
Pin 4 = Screen
Pin 5 = Grid
Pin 6 = No connection
Pin 7 = heater filament
Pin 8 = cathode
The pin socket tabs for Pins 1 and 6 (having no internal connection to electrodes inside the tube) are often used to anchor 'other' lead(s) - i.e. for the screen resistor or grid resistor (when these resistors are tied directly to Pins 5 or 4 respectively). But make sure, if using these pin tabs, that the connection for each pin tab is only to one of the resistor leads (and not to the leads on both the screen and grid resistor at the same time to the same pin tab - because that would be bad).
However, sometimes Pin 1 can be tied to Pin 8, and so be tied to the cathode. (when this is done, it is to help 'eat up' AC from the adjoining heater filament pin tabs). If this is done, make sure the Pin 1 tab isn't tied to the cathode as well as to the screen (or grid) pin tab at the same time.
In a PR, which is fixed bias, the Pin 8 tab needs to go to ground (usually either directly, or via a 1R cathode-current measuring resistor in series between the Pin 8 tab and the chassis ground). This connection is needed for the output tube to conduct - even though this pin voltage should be sitting at about 0VDC
Pin 5 needs to be connected directly to the bias voltage source (as well as to the coupling cap(s) from the Phase inverter). If using a grid resistor, this can be in series between the bias supply and the socket pin tab - usually with the other grid resistor lead connected to the Pin 1 tab (as described above). This pin voltage should be sitting at about -20VDC to -27VDC (so that you get about 18 to 24mA of tube current - measured at the cathode current measuring resistor)
Pin 4 needs to go to the screen grid resistor, (usually with the other resistor lead going to the Pin 6 tab - as described above). This connection is needed to turn the tube current 'on' (If the other connections are all connected, but the screen is at 0V, it will suppress tube current to such an extent that the tube won't conduct). The voltage on this should be close to or new the same as, the B+ voltage - or the same as the HT supply voltage for the screen supply node i.e. 410ish VDC.
Pin 3 needs to go to one of the output transformer primary winding ends. (and the Pin 3 tab on the other socket goes to the opposite OT primary end). This connection is also needed for the tube to conduct and supply current to the tube (through the OT primary winding back to the primary centre tap, which is connected to the B+ (incl reservoir cap). The voltage on this should be within a volt or two of the B+ voltage (i.e. 410- 430Vish)
The tabs for Pins 2 and 7 go (one each) to each side of the 6.3VAC heater circuit - which needs to be connected to the heater winding on the Power transformer. These connections are needed to heat up the cathode to the required temperature to ensure easy release of electrons. If the heaters aren't connected, the tube won't go 'on'. you should measure 6.3ish VAC between Pins 2 and 7, if everything is working properly.
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> For 6V6s (or 6L6s):
Yes, thanks, good for later. Right now I think it is the all-high readings on twin-triode V6 that are "very messed up!"
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Tubeswell: The 6V6GT tubes are wired exactly like the original Fender schematic and the Robinette layout for a Deluxe Reverb.
(https://i.postimg.cc/pTDFJkn2/DR-6V6.jpg)
72Blazer: Even a novice has something to add and I was thinking along the same lines last night. I wired this up as per the color codes as specified but I used a 1960's Hammond A029 OT and the color code may be different. I'll open up my 5E3 or PR to check today as they have the same OT and are working fine.
PRR: I quadruple checked the 6V6 connections after it was discovered I left out the connection from Pin6 to B+.
Since I'm running a PT that was designed for 117VAC. it is running a little high on the voltages at 122.5VAC.
What confuses me is that the 6V6 tube chart shows a maximum of 315V for the plate, the Fender schematic shows 415V.
Sluckey: I see 438V instead of 415V on the plates and screens which is as expected given the higher B+ on the PT.
I see -40.2V on the grid instead of -35V, again due to PT voltage differences.
I was able to get my filaments for 5.0V and 6.3V to 4.75V and 6.06V. Well within the 10% margin.
Do you see sonething else?
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V6 voltages are very messed up. I suspect a problem in the cathode circuit. Measure resistance from V6 pins 8 and 3 to chassis. Should be about 22.5K. What does your meter say?
Post some hi-rez pics.
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Sluckey,
With power off, I get OL..Open circuit
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.
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With power off, I get OL..Open circuit
Well, find out why! Ain't much between V6-8 and ground. Should be an easy fix. If you can't figure it out, post some hi-rez pics.
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This is a great way to double check the wiring on an amp.
https://el34world.com/Forum/index.php?topic=17701.0
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The solution lies on the pink brick road.
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Thanks Sluckey,
I got sidetracked today. The new PT for the S6606 came in today so I installed that and got the Bias under control. I ended up at 362V @19.2mA.
All I had was a 1K and a 1.5K in 5W so that's where it is. I'd order some more resistors in the 1.2 -1.4 range but I think it's fine where it's at.
Then I corrected the 5V and 6.3V heaters on the 5E3 and the AA1164. Got them within 4% on the 5E3 and within 1% on the AA1164.
I'll look at the pink brick load later tonight or tomorrow.
Much appreciated.
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OK, I got curious and looked at the pic Sluckey.
I missed the connection between the roach and that 25uF cap. That's some genius level talent there to figure that out without any info but voltages. Sucker worked right away after I bridged it and it sounds real good. Thanks Sluckey.
Now I have to figure out why the Tremolo doesn't work.
Here are the pics as requested. Sorry for the delay.
(https://i.postimg.cc/bNHRhxLD/DR-Full-Board.jpg)
(https://i.postimg.cc/HsCtRZ01/DR-Left-Board.jpg)
(https://i.postimg.cc/P5XQrCF6/DR-Right-Board.jpg)
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Now I have to figure out why the Tremolo doesn't work.
You must have a footswitch plugged in to enable the tremolo on an AB763 amp. Or you could just put a ground jumper on the vibrato F/S jack.
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Footswitch is plugged in, can't see it in the pics.
I turned the intensity all the way up and checked the switch several times with different speed settings. Nothing.
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Footswitch is plugged in, can't see it in the pics.
I can clearly see the vibrato footswitch jack in two of your pics and there is no footswitch plugged in.
Are you sure the Intensity pot is wired correctly?
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Sorry Sluckey,
I had the footswitch plugged into the reverb send and return. I plugged it into the correct jacks and bam.
Everything is working. It is a bit noisy but I need to install the doghouse cover and then address my lead dress etc.
The footswitch plugs sure are microphonic when plugged in. I'll have to sort that too.
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The footswitch plugs sure are microphonic when plugged in. I'll have to sort that too.
The reverb requires a shielded wire but the tremolo does not. Most footswitches do not use a shielded cable. So, get a real Fender footswitch if you don't have one. Be sure the plugs are connected properly. They are not interchangeable. Here's a pic of the Fender cable. The shielded plug must connect to the Reverb FS jack.
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Thanks, Good advice and I took it to heart when I built my Princeton Reverb.
The cables were very microphonic....wait for it......when plugged into the Reverb tank in/out connections..LOL...I make some dumb mistakes...
I have the Fender vintage foot pedals for both the PR and DR. Someone along the way made the argument for the Fender pedal for low noise and the shielded cable so that's what I use.
I am waiting for the self tapping screws to mount the doghouse cover before I start with the noise abatement. With it off, I expect some noise and it'll be hard to trouble shoot.
This amp has a lot of tone, reverb and volume...the tremolo works perfect too.
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Installing the doghouse is a safety thing. Has nothing to do with noise abatement.
Speaking of safety, those flying resistors on the rectifier socket have 463VDC on them! Makes me nervous just looking at the pic. :huh:
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Sluckey,
Actually installing the doghouse cover did improve the noise level a little. I've had so many other projects that I haven't been back in to try and abate any noise. I'll get to it.
The flying burritos/resistors are on the 5V and 6.3V lines and stiff. It would take a lot more than 5G's to move them at all and I have them far enough away from anything. It would take 10 G's to knock them out of place. The leads are heat shrink covered so even if the resistor body made contact with anything, no short would occur. The 5V ones aren't going to move at all.
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... those flying resistors on the rectifier socket have 463VDC on them! ...
The flying burritos/resistors are on the 5V and 6.3V lines ...
The yellow "5v line" has 5 volts a.c. present.
It also has the 440-whatever d.c. volts present that is found at the first filter cap. That's because the 5v winding is connected to the rectifier heater, which has one side connected to the rectifier cathode, which is connected to the filter cap.
So low-voltage a.c. plus high-voltage d.c. is present.
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So what do you suggest?
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Interesting issue I discovered.
I have the footswitch plugged in and if the reverb is turned on, the tremolo won't start.
If I turn off the reverb, the tremolo works fine and I can turn on the reverb and it all works fine.
It's just getting the oscillation to start I have to turn off the reverb.
I also have a noticeable nagging buzz. It's not loud but follows the volume knob and gets louder when turned up.
I'll have to go through all the tube pin solder joints to make sure none are cold.
Any suggestions?
The buzz was there before I put the doghouse cover on and before I added the voltage dropping resistors on the 6.3V and 5V circuits.
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Why does one of the capacitors in the trem oscillator have black stuff on the one end? Is it heat shrink tubing?
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I have the footswitch plugged in and if the reverb is turned on, the tremolo won't start.
If I turn off the reverb, the tremolo works fine and I can turn on the reverb and it all works fine.
It's just getting the oscillation to start I have to turn off the reverb.
Most likely an issue with the footswitch. Are you sure you are using a real Fender F/S?
Try this... Disconnect both F/S jacks. The reverb should be on and the tremolo should be off. Now use a jumper wire to short the tip of the vibrato F/S jack to chassis. This should enable the tremolo. If so, examine the F/S wiring.
Why does one of the capacitors in the trem oscillator have black stuff on the one end? Is it heat shrink tubing?
The heat shrink tubing prevents that huge cap from shorting to the turret above it. There is negative bias voltage on that turret.