Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Doctor_A on May 24, 2011, 09:40:51 am
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Hi,
I just finished the basic work on my new amp, but it seems like it doesn't have the power it should with two 5881's in the output section.
Here are the pictures:
Although the wiring looks funky, it is wired according to the Hoffman layout and does sound great, just not loud enough, it has enough negative bias voltage to attain almost 40milliamps across the one ohm resistors, I have it set at 37 mA, it's got a Fender type OT tranny, a 40W like in the Hot Rod Deluxe. The sound is kind of bass heavy with all standard component values in the normal channel, but the bright channel sounds ok to my ears.
How can I get this amp to open up too full power?
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Post your tube voltages...
http://www.el34world.com/charts/valve/ValveData.php (http://www.el34world.com/charts/valve/ValveData.php)
...and provide a link to the chart.
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Hi,
I put my voltage chart into the data pages
The link is here: http://www.el34world.com/charts/valve/ValveData.php?e=view&f=11137 (http://www.el34world.com/charts/valve/ValveData.php?e=view&f=11137)
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Voltage readings for V3 are waaayyy off. Check again. If still way off, look for reason.
Voltage readings on V4 and V5 pin 8 should be only about 40mV. Recheck those also.
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From the picture it looks like the black wire from V3 pin8 is going in to wrong resistor.
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Good eye!
I believe that ties the grid and cathode together.
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Thank you VMS, and slucky,
I noticed the V3 pin 8 issue before your last two replies, I missed that on my re-check a few days ago, right now it's on the wrong end of the 1 meg resistor one lug over, it needs to go onto the tail end of a 470 ohm res.
I rechecked the 5881 voltage of pins 8 and 1 (tied together) it is about 37 millivolts, I think the ground of my voltmeter was falling off during checking.
I'll go over the amp better tomorrow and do a new voltage chart, and power output check, I bet the thing will be good after that if there is no damage to the tubes, these things are amazing that the amp actually works with a mis-wired tube socket.
Jim
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Well,
I made some changes and the amp works pretty good now at full power, around 42-45 watts as best I can figure with a plate voltage of 466 squared, seeing the primary impedance of 4,250 Ohms from the 40 watt Classic tone tranny.
The sound is clean and quiet up to about 3 fifths of the way, and past that the voltage at the speaker leads barely reaches 15 volts AC. But that is painfully loud, I don't play heavy metal music, I play blues, so this amp is king for that.
There is one tiny problem with the presence control giving a high pitched howl like a hetrodyne past 3/4's of the way, on the forum there was suggested to someone to swap the wires of the O.T. at pin 3 of the power tubes, should I try that?
One more question: The max ratings for this tube seem to be lower than it is operating at, the plate rating is only 400 volts for a 5881, so how long will these tubes last running at 466 volts at 37 mA? All the other voltages seem ok now, but...
This is my first build, and I know I didn't do the chassis layout in the most ergonomically correct way, but it works, and I will eventually get another chassis box and change it around, but for now are there any other suggestions?
Thanks again,
Doc
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There is one tiny problem with the presence control giving a high pitched howl like a hetrodyne past 3/4's of the way, on the forum there was suggested to someone to swap the wires of the O.T. at pin 3 of the power tubes, should I try that?
Yes.
but for now are there any other suggestions?
Using your gut shot pic as a reference... Notice how all the wires from the tube sockets cross over the board to the bottom side of the board? That's because the Hoffman board was designed for the tubes to be located on the bottom side of the board (in your pic). Doing so will tidy up the wiring considerably.
BTW, congrats on a successful first build. I hope you're hooked! :wink:
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If the OT primary wires are connected with the wrong phase the amp should oscillate like crazy at any setting of the presence control (or any other control for that matter). So, I respectfully suggest that the oscillation when the presence control is up is due to something else...