Voltage divider resistor choice for DC elevated heaters

[Electrobumps]

I want to experiment with DC elevating an AB763 guitar amp heaters.

From what I've read there are a lot of people using values similar to 200k / 33K for around 60v.

I will make R2 a variable resistor as I want to hear the effects of different elevations some people suggest as low as 20v and others suggest around 60v. With the 20v being better for hum.

less load on the TX is good, would it make more sense to make R1 1meg as this will produce less current draw, unless I'm missing something?

Vdc = 430
R1 = 1 meg
R2 = Variable 200K

R2 set @ 180K = 65.5v
R2 set @ 49K = 20v

 

[Philbo King]

Elevated heater, as in adding a DC bias to the heater relative to the cathode? I question the benefit, but I don't know everything...

Since neglible current would pass through the resistance (if done right) the resistor value is likely not critical as long as the bias source is well bypassed to ground with caps, for the frequencies of interest.

 

[metalockpick]

Elevated heater, as in adding a DC bias to the heater relative to the cathode? I question the benefit, but I don't know everything...

Since neglible current would pass through the resistance (if done right) the resistor value is likely not critical as long as the bias source is well bypassed to ground with caps, for the frequencies of interest.

You could use the bias tap + a diode + cap for +60V.
You get benefits where the cathode voltage is closer to the filament voltages from raised filament voltages. In the '763, the pi cathode (a 12AT7), still doesn't have that high a voltage, and the vibrato circuit doesn't affect the audio this way. Tubes like the 12AX7 EH don't like a big difference in cathode voltage and filament voltage, however your amp uses a 12AT7 for the PI (unless you've modded it), so I don't know that it would matter.

If you're looking to cancel hum, you might try using a 12 gauge solid connector between the negative of the first brute force filter cap and the PT HV center tap.

 

[EthanSpaulding]

I hear that voltages as low as 12V are effective. I always use between 50V and 80V to drop the cathode to filament voltage difference that metalockpick mentions. 50V is definitely quieter than ground referenced. I wish I could elevate the heaters in my dual rectifiers, but heaters are on the same supply as the relay switching.

I did a bunch of experiments with a turret constructed 5150 block (very high gain). I ended up with SMPS 12.6V all preamp tubes. First I tried using pin9 as the center tap to reference, but found that the artificial center tap with resistors was quieter. Then, I experimented with where the elevated heater voltage came from. The noise signature was different depending on where I grabbed the reference from. In the end, I connected it close to the main filter caps and that was mainly because I had left room there. Later on I connected a capacitor from the first triode's pin9 (center tap of heaters to ground and it didn't lower the noise but, it did change the noise signature so it was less noticeable (less top end noise).

I know thats a long story, but I know people here know all about the finer details of optimization and heater elevation and balancing is a place where one can make great gains in noise reduction. So, if anyone has ideas, I would test them out on builds and report my findings. Looking forward to hearing about your findings Electrobumps!

 

[Electrobumps]

I know thats a long story, but I know people here know all about the finer details of optimization and heater elevation and balancing is a place where one can make great gains in noise reduction. So, if anyone has ideas, I would test them out on builds and report my findings. Looking forward to hearing about your findings Electrobumps!

The amp is pretty quiet, I've recorded far louder amps. It's a Mojotone twin reverb, built stock and trying out mods. Changing to coax cable has been a great addition and also just chopsticking and moving lead dress has been very beneficial.

I love the twin for recording and reamping, it's just the fade-ins and outs against the noise floor can be a battle. UAD C-suite is so impressive to deal with amp noise, but always want to get the best source and see how quiet I can get this amp. I'll try some SPL readings and see if I can get some meaningful results. I don't have the resistors in stock and with current projects before the holidays I think this will probably be a New Year's project.

 

[CJ]

A bias balance pot on the power tubes can sometimes help,

 

[ruffrecords]

Some tubes have specification for the maximum heater to cathode resistance. I suspect they all do but you rarely see it on data sheets. Where you do see it the typical value is around 20K (EF86 for example). For this reason I tend to use 22K for the lower arm. I usually set the elevation voltage at 25% of HT because my output stages are usually an SRPP where the top cathode site at 50% of HT. Setting elevation to 25% means the top cathode is 25% above the heaters and the bottom cathode is 25% below. This means my top arm is 66K. This means the total dissipation in the resistor chain is about 1W. For this reason i use 2W resistors and make the top 66K from two 33K resistors in series.

Don't forget to decouple the bottom arm to 0V with a suitably rated 100uF capacitor or you will get hum.

Cheers

Ian

 

[rock soderstrom]

Some tubes have specification for the maximum heater to cathode resistance. I suspect they all do but you rarely see it on data sheets. Where you do see it the typical value is around 20K (EF86 for example). For this reason I tend to use 22K for the lower arm. I usually set the elevation voltage at 25% of HT because my output stages are usually an SRPP where the top cathode site at 50% of HT. Setting elevation to 25% means the top cathode is 25% above the heaters and the bottom cathode is 25% below. This means my top arm is 66K. This means the total dissipation in the resistor chain is about 1W. For this reason i use 2W resistors and make the top 66K from two 33K resistors in series.

Don't forget to decouple the bottom arm to 0V with a suitably rated 100uF capacitor or you will get hum.

Cheers

Ian

I have to admit that I was not so aware of the meaning of Rfk. I used to use higher resistors so as not to excessively load B+.

However, a look at the data sheets for the EF12 shows that it can only handle a maximum of 20k as Rfk.

That's what I did in my current (not yet finished) project.

Very good information Ian, thanks!


Voltage divider + cap and the humdinger pot.

 

[Tubetec]

Can we see the circuit?

 

[rock soderstrom]

Can we see the circuit?

The final schematic has not yet been drawn. In essence it is a Gates Sa70/ Coil CA70 with classic CRCRCRC power supply for B+ and 6.3V AC for the heater. The tubes are 2x EF12k wired as triodes. Original would be 6j7/6C5.

NFB is adjustable, just as the capacitors in the NFB are switchable. Inspired by CoilAudio.

 

[ruffrecords]

What is the purpose of the pot R5?

Cheers

Ian

 

[rock soderstrom]

What is the purpose of the pot R5?

It is the gain pot. I understand your confusion, as it is in the NFB loop, which is not exactly considered optimal. It works...

In my circuit, R5 has another Rg connected in parallel. NFB can also be completely deactivated in my circuit.

 

[CJ]

Cool. Maybe a choke for R7?

Mesa has rectified DC on first preamp tube only for the last amp i worked on, not hard to build a pwr supply to handle 12ax7 heater current, seems like that would make the most difference on V1

 

[rock soderstrom]

Cool. Maybe a choke for R7?

Yes, this could help to optimise the somewhat weak output of the EF12k. I will also play around with the corresponding Rk...chokes are unfortunately expensive.

Mesa has rectified DC on first preamp tube only for the last amp i worked on, not hard to build a pwr supply to handle 12ax7 heater current, seems like that would make the most difference

True, normally I use DC heater for mic pres, but this time I will do it with AC only, just to prove me again that this can be good enough for studio work.

 

[myersbw]

The amp is pretty quiet, I've recorded far louder amps. It's a Mojotone twin reverb, built stock and trying out mods. Changing to coax cable has been a great addition and also just chopsticking and moving lead dress has been very beneficial.

I love the twin for recording and reamping, it's just the fade-ins and outs against the noise floor can be a battle. UAD C-suite is so impressive to deal with amp noise, but always want to get the best source and see how quiet I can get this amp. I'll try some SPL readings and see if I can get some meaningful results. I don't have the resistors in stock and with current projects before the holidays I think this will probably be a New Year's project.

I peeked at Mojo's schematic for for the AB763 Twin. One thing they didn't do was to include the heater balance resistors (a pair of 100 ohm resistors that center the heater circuit around ground. ) You'll see them on their Deluxe schematic off hanging near the pilot lamp/transformer in the lower right.

Of the amps I've built, I really never needed a reason to elevate or use DC heaters. Wire dress and quality parts have always been enough. Small builds, high gain (i.e. SLO-land) presents some other challenges. :O Best of luck!

 

[trobbins]

A few pertinent points could be:

If the amp has simple heater grounding, for example if the heater mid-point was grounded through a heater winding CT, or externally with a humdinger, then for starters retrofit a humdinger pot and see how significant that pot can lower any observable hum. That addresses one of the main hum ingress paths - stray capacitance coupling to input stage grids.

Heater elevation addresses a different hum ingress path - that goes via the heater stray resistance to cathode. That path is very dependent on the tube being used - so imho it is worthwhile firstly swapping in/out different tubes to observe any noticeable change. The 'elevation' aims to use a difference in voltage between the heater and cathode to increase the stray resistance value, and hence suppress that hum leakage path. As the decoupling capacitor is the part passing the hum current, the capacitor should connect to the quiet 0V point of the input stage, rather than say a noisier chassis or power supply 0V point.

More detail on this in the linked article: https://dalmura.com.au/static/Hum article.pdf

 

[CJ]

You can drop the hum quite a bit on an old blackface by removing and cleaning the brass plate, you would not believe all the crap under that thing,

 

[rock soderstrom]

Maybe a choke for R7?

This idea gives me no peace. I would like to try it out, maybe even wind such a choke myself.

I suspect it's not quite as "trivial" as a mains choke.

CJ, have you mentioned this topic anywhere in your countless (brilliant) posts?

How do you make a high-quality anode choke? What do you need to consider and what are the pitfalls?

Who else besides Sowter and Lundahl actually offers anode chokes?

 

[abbey road d enfer]

There are two motivations for elevating heater voltage.
One, as Ian mentioned is making sure the cath-to-heater voltage doesn't exceed a limit. For example, a cathodyne PI (aka concertina) has its cathode standing at about 50-70V, so elevating the heater voltage to 30-50V makes sense.
The other reason is reducing hum and distortion. Due to the non-linear characteristics of the heater insulating material, the capacitance and conductance between cath and heater is variable, with a maximum when the voltage is low, so in order to reduce capacitive coupling between the heater AC and the cathode, a bias is beneficial.
I have never seen that mentioned in tube manufacturer's datasheets, but it is mentioned briefly in RDH and a few disseminate articles (e.g. heater-Cathode Insulation performannce by Klemperer)

 

[trobbins]

Due to the non-linear characteristics of the heater insulating material, the capacitance and conductance between cath and heater is variable, with a maximum when the voltage is low, so in order to reduce capacitive coupling between the heater AC and the cathode, a bias is beneficial.

Afaik, it is the parasitic resistance, not capacitance, between heater and cathode that can be varied by dc elevation - the capacitance remains constant.