5e3 Fixed Bias Balance Modification

[RundgrenRules]

My 5e3 build has always been pretty humm-y. I started tying to diagnose the cause of the hum, moving wires with a chopstick and whatnot, checking grounds, etc. Nothing really made the annoying 60hz hum go away until I popped in a pair of near perfectly matched new production 6V6 tubes to replace the (what I did not know at the time) were pretty out-of-match RCA 6v6s.

I popped the Bias Rite in and checked the idle current for the RCA tubes and it was 17mA off from one tube to the other. One is glowing blue and much hotter to the touch than the other. I pop in the new Tung Sols and they are within .01 mA idle current of each other and the hum is GREATLY diminshed.

I would really like to use the RCA tubes and add a circuit to bias, balance (and un-balance) the power tubes.

Would this one work just fine (from Rob Robinette's site):



I don't want to lose the characteristic of fixed bias 5e3 sound but I want to be able to use these (or any other unmatched) power tubes. It calls for a 50V AC tap, but my PT doesnt have a 50V tap. The amp is built from a Trinity Kit (see the layout at the bottom). So that means I would need to maybe add a high voltage tap like this one (again from Rob's website):



And COMBINE the adjustable fixed bias circuit with balance pot WITH the High voltage tap bias circuit....and now I am thoroughly confused. Could someone point me in the right direction or help me mash tehse circuits together? Thanks for reading!

 

[RundgrenRules]

I guess this would work? 220K resistor to drop the voltage coming off the AC on pin 6 (+-380v)?

 

[Paul G.]

Your 5E3 is not fixed bias, it is cathode biased. When an amp is referred to as "fixed bias", it doesn't mean there's no pot, it means that a fixed negative voltage is applied to the grid as opposed to cathode bias where the bias is more or less self-regulating. As more current drawn through the cathode resistor, it lowers the voltage (according to ohm's law), which then lowers the current drawn, until an equilibrium is established. Thus, it is not "fixed".

The easy way to balance the two tubes would be to separate the feeds to the cathodes. Instead of having one 200R resistor feeding both cathodes, you would have separate resistors feeding each cathode. Connect them both to the ground, along with their own cathode capacitors, then each one to its own cathode. I'd start with 470R. Then, measure your plate and screen voltages, then voltage drops across each cathode to calculate idle current. Adjust the value of the resistors until you get the current draw you need.

If you want to be fancy, you can put a linear pot (in addition to your cathode resistor) to balance different tube sets.

What you're proposing would require the cathodes to be tied directly to ground and adding a grid bias voltage -- it can be done but would completely change the character of the amp.

 

[Paul G.]

Oh, and the bad news is, one of your RCA tubes is weak. Tubes don't have to be perfectly matched, but they should be close. If one of your tubes is passing that much less current than the other, it's not a good tube. Forcing more current through it by limiting the other tube drastically probably won't give you the results you want. IMO. Just be cause they're RCA, doesn't make them automatically good. I've got a box of bad Black and Gray plate RCAs. Don't ask my why I keep them.

 

[RundgrenRules]

Thanks so much, Paul!

Does this look right? Start, here, measure, plug into the calculator and tweak the final resistor values?

Also, is this what you call a split cathode and does it have an effect on the character of the amp?

 

[RundgrenRules]

Well, damn thats two pairs then, same story....

 

[Ten Over]

If you want to be fancy, you can put a linear pot (in addition to your cathode resistor) to balance different tube sets.

Like this?

 

[dan40]

Also, is this what you call a split cathode and does it have an effect on the character of the amp?

Split cathode or shared cathode are terms normally used to describe the biasing of the first gain stage in the preamp. The 5e3 ties pins 3 and 8 together on the v1 tube so it's considered a shared cathode arrangement. Other amps like the Marshall Super Leads have separate resistors on the cathodes of the v1 tube and are considered split cathode. These two biasing schemes definitely have a big effect on the sound of the amp.

 

[Paul G.]

You're proposing major changes to your amp because you have a bad tube. I wouldn't bother.

 

[plumcrazyfx]

Look up humdinger. Might not work great with unmatched tubes.

Also, you're not using the Mojo Tone 756, are you?

 

[Ben B]

You're proposing major changes to your amp because you have a bad tube. I wouldn't bother.

This is the answer. A new set of RCA tubes (if that is what you want) is probably cheaper and much less invasive that modifying your amp.

 

[DCCable]

My 5e3 build has always been pretty humm-y. I started tying to diagnose the cause of the hum, moving wires with a chopstick and whatnot, checking grounds, etc. Nothing really made the annoying 60hz hum go away until I popped in a pair of near perfectly matched new production 6V6 tubes to replace the (what I did not know at the time) were pretty out-of-match RCA 6v6s.

I popped the Bias Rite in and checked the idle current for the RCA tubes and it was 17mA off from one tube to the other. One is glowing blue and much hotter to the touch than the other. I pop in the new Tung Sols and they are within .01 mA idle current of each other and the hum is GREATLY diminshed.

I would really like to use the RCA tubes and add a circuit to bias, balance (and un-balance) the power tubes.

Would this one work just fine (from Rob Robinette's site):

View attachment 999440

I don't want to lose the characteristic of fixed bias 5e3 sound but I want to be able to use these (or any other unmatched) power tubes. It calls for a 50V AC tap, but my PT doesnt have a 50V tap. The amp is built from a Trinity Kit (see the layout at the bottom). So that means I would need to maybe add a high voltage tap like this one (again from Rob's website):

View attachment 999446

And COMBINE the adjustable fixed bias circuit with balance pot WITH the High voltage tap bias circuit....and now I am thoroughly confused. Could someone point me in the right direction or help me mash tehse circuits together? Thanks for reading!

View attachment 999445

Your problem is that more than likely one of your output tubes has a heater to cathode short circuit causing the heater AC voltage to corrupt the signal from that tube. Changing the bias circuitry will not alleviate that problem.

 

[Paul G.]

Look up humdinger. Might not work great with unmatched tubes.

Also, you're not using the Mojo Tone 756, are you?

Humdinger matches the two legs of the heater circuit. This is not the problem here as a matched tube set is quiet.

 

[rschiller]

My 5e3 build has always been pretty humm-y. I started tying to diagnose the cause of the hum, moving wires with a chopstick and whatnot, checking grounds, etc. Nothing really made the annoying 60hz hum go away until I popped in a pair of near perfectly matched new production 6V6 tubes to replace the (what I did not know at the time) were pretty out-of-match RCA 6v6s.

I popped the Bias Rite in and checked the idle current for the RCA tubes and it was 17mA off from one tube to the other. One is glowing blue and much hotter to the touch than the other. I pop in the new Tung Sols and they are within .01 mA idle current of each other and the hum is GREATLY diminshed.

I would really like to use the RCA tubes and add a circuit to bias, balance (and un-balance) the power tubes.

Would this one work just fine (from Rob Robinette's site):

View attachment 999440

I don't want to lose the characteristic of fixed bias 5e3 sound but I want to be able to use these (or any other unmatched) power tubes. It calls for a 50V AC tap, but my PT doesnt have a 50V tap. The amp is built from a Trinity Kit (see the layout at the bottom). So that means I would need to maybe add a high voltage tap like this one (again from Rob's website):

View attachment 999446

And COMBINE the adjustable fixed bias circuit with balance pot WITH the High voltage tap bias circuit....and now I am thoroughly confused. Could someone point me in the right direction or help me mash tehse circuits together? Thanks for reading!

View attachment 999445

I would not go to fixed bias from cathode bias in a 5E3. The 5E3 power section inherent tone is based on the (1) Cathodyne drive with virtually no gain (2) Cathode "self adjusting" bias and (3) No negative feedback. A fixed bias will allow a bit more actual wattage - perhaps 10%-20% - but alter the tone and especially in the treble range. Also it may be necessary to add a negative feedback resistor to compensate.

No doubt AC humm can be caused by a bad tube, either preamp or power. Can also be layout, 6.3v filament properly grounded or if no center tap 100ohm resistors to ground, lead dress and using shielded cable to grids in the preamp (especially to the higher voltage II input) is a good idea. I have two original 1958 5E3s and one has a low level of AC humm and the other is dead quiet; having said that, the 68k input resistors on the board are likely better installed on the jacks as Fender did from 1960-1961 on.

My two cents worth.