Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: dpm309 on October 23, 2025, 05:37:09 pm
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My 5E3 build (1st amp I built several years ago) was experiencing crackling on the volume pots. Took it apart and cleaned the pots and is working. While I had it on my bench, I checked the voltages and bias. A few years ago I performed the Simplified Dumble 5E3 Tweedle Mods on it. The amp sounds great with this mod. I replaced the 5Y3 rectifier with a GZ34 along with other mods. My plate voltage on the power tubes is 372VDC which gives me a bias of 56ma. This seems high for a 6V6 but the amps sounds great and have not had any issues with the power tubes. Is 56ma too high? Will this affect the life of the power tubes?
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What is the cathode voltage? Assuming 250r bias resistor?
Is 56mA per tube, or both combined?
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To provide a valid response, more measurements are required.
Anode voltage
Screen grid voltage
Cathode voltage
Cathode resistor value
Screen grid current
And an accurate schematic, as it's not a standard 5E3 anymore.
Does reverting to the 5Y3 cause the amp to sound worse?
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I have attached the schematic for the Dumble 5E3 mods. The only difference is that I am using a choke instead of the 4.7K 2Watt resistor. I am getting 374 VDC on pin 3, 379VDC on pin 4 and 25VDC on pin 8. B+: 382VDC, B+1 : 380VDC and B+2: 300VDC. Haven't tried swapping rectifier back to the 5Y3. I'll try swapping the rectifier after I can locate one.
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you could just make the 250 cathode R a 470, 500, 530, whatever you have close to double the 250. then re-measure VDC at pin 3 n pin 8
do the ball park math ((plate VDC-cathode VDC) / Cathode R) / 2 (tubes) will yield total tube current including a few mA for G2
anything in the 35-45mA range is good for starts
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That's not a schematic. Also, by replacing the 4.7k dropping resistor with a choke you've stripped the circuit of any screen protection. If you're going to keep the choke, please add 1k screen resistors to each socket. This will also buy a bit of compression to offset the increased voltage.
25V at pin 8 with a 250Ω resistor and approx 350V plate to cathode comes out to about 125% idle dissipation. On a 1957 5E9-A Tremolux I recently restored, I had a similar situation. Increasing the cathode resistor to 300Ω and using a 5Y3 brought the idle dissipation down to about 105% and the amp was very happy with that.
Also: move your standby switch to after the reservoir node. This layout illustrates a hot switching arrangement that will lead to premature rectifier tube failure.
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Here is the schematic. With the 5Y3 rectifier, I am getting a bias reading of 47ma. Plate voltage dropped to 328VDC from 371VDC. Will add the screen resistors. I have never had any problem with the standby switch connected as per the layout. This is the method I have used on all of my builds. The amp seemed a little "beefier" using the GZ34. Besides adding the screen resistors, I will keep it as is. Thanks for everyone's comments and suggestion.
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I have attached the schematic for the Dumble 5E3 mods. The only difference is that I am using a choke instead of the 4.7K 2Watt resistor. I am getting 374 VDC on pin 3, 379VDC on pin 4 ...
That will make the screen grid supply node much stiffer and lower impedance, greatly increasing the potential to cause screen grid overheating; screen grid current limiting resistors would be a really good idea, eg 470R 1W.
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My 5E3 build (1st amp I built several years ago) was experiencing crackling on the volume pots. Took it apart and cleaned the pots and is working. While I had it on my bench, I checked the voltages and bias. A few years ago I performed the Simplified Dumble 5E3 Tweedle Mods on it. The amp sounds great with this mod. I replaced the 5Y3 rectifier with a GZ34 along with other mods. My plate voltage on the power tubes is 372VDC which gives me a bias of 56ma. This seems high for a 6V6 but the amps sounds great and have not had any issues with the power tubes. Is 56ma too high?
Yes
Will this affect the life of the power tubes?
Yes
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I am using a choke instead of the 4.7K 2Watt resistor. I am getting 374 VDC on pin 3, 379VDC on pin 4 and 25VDC on pin 8.
Shared bias resistor current = 25V/250R = 100mA, which equates to 50mA per 6V6. Assuming 2mA screen current per 6V6 (probably an underestimate in this case), that's 48mA plate current. 374V x 0.048A = ~18W.
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Here is the schematic. With the 5Y3 rectifier, I am getting a bias reading of 47ma.
Is that 1/2 the derived cathode current?
It would be good to provide the actual measurement readings, show your working, then we've got chance to check things properly.
Plate voltage dropped to 328VDC from 371VDC. Will add the screen resistors.
How about the other readings, as per previously. I have never had any problem with the standby switch connected as per the layout. This is the method I have used on all of my builds.
I suppose it depends how often you flip from standby to operate modes, and variables such as reservoir cap value and rectifier anode supply resistance.
But whatever, hot switching a valve rectifier will reduce its operating life and increase the chance of catastrophic failure.
https://www.valvewizard.co.uk/standby.html
The amp seemed a little "beefier" using the GZ34.
Yes, it will be, more voltage and same load impedance = more power, and higher screen grid voltage means more anode current means higher gm (stage gain).
The problem being that the 6V6 were being abused.
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The problem being that the 6V6 were being abused
most guitarists would find that a positive feature......"DUDE, this amp sounds amazing, it just keeps blowing up PA tubes....Can you fix that??"
:icon_biggrin: