HT Transformers for Vacuum Tube Preamps

[ruffrecords]

I think I did not explain myself clearly. I am not concerned about the ability of the switcher IC to handle the current. What I am concerned about, if you generate HT from the heater supply, is HT switcher noise getting into the heaters and thence into the audio path.

Cheers

Ian

 

[thor.zmt]

I think I did not explain myself clearly. I am not concerned about the ability of the switcher IC to handle the current. What I am concerned about, if you generate HT from the heater supply, is HT switcher noise getting into the heaters and thence into the audio path.

Ian, I got the message just fine.

I think I did not explain myself clearly.

Modern Switcher IC's work at very high frequencies. This means the required components are small and have very low parasitics. It is quite trivial to substantially contain any noise on a small PCB.

The current flowing into the storage inductor is a sawtooth wave. Even the turnoff is somewhat gradual. Placing a few SMD 0805 format ceramic capacitors and then using a (rather small) LC PI filter with similar format parts, will kill any noise on the incoming line (hence my comment on using LISN & 5GHz spectrum analyser). This is the advantage of switching at 1.2MHz. Another LC filter on the output. All with small components. Noise contained.

Generally a well designed 4-layer board with a solid and well coupled ground and power planes is good practice, the PCB itself actually acts as distributed bypassing capacitor for very high frequencies. It will also help thermally.

I would worry a lot more about the switching noise from the off-mains step-down isolated heater supplies. They switch at much lower frequencies causing all parts needed to be physically much larger and expensive, so there is tendency by power supply makers to be economic. Of course, adding an LC filter after the SMPS will reduce noise a lot.

FWIW The EL84 PP Stereo Amp SMPS I referred to used CLC filter for Heaters (also used for digital circuitry), CLCLC for the HT and CRC for a +/-15V winding. No noise. On the mains side, proper filtering, also no noise to worry about.

Thor

 

[ruffrecords]

@Thor, looks to me like you just became the resident SMPS expert at groupDIY.

I try to keep abreast of advances in PSU design which is why I now use them for heater supplies but designing them is still very much a specialised art. Certainly switching frequencies above 1MHz helps in many ways. So, the bottom line is could you design a groupDIY 12V to 250V @50mA HT supply PCB we could all make and use?

Cheers

Ian

 

[soapfoot]

I can’t wait until someone with far more design chops than me designs a good SMPS for vacuum tube guitar amps.

I know the traditionalists will hate it, but I’d love at least one amp with a SMPS for maximum portability

 

[dmp]

At 50 mA I think this would be for preamp duties, not guitar power amps.

I know the traditionalists will hate it,

That was my first instinct thinking about it, but it's good to keep an open mind. Planning to order a cheap one and compare to a passive RCRCRC

I don't want to open a can of worms but I like to feel something is very reliable. And a properly spec'd power transformer into a RC filter I think could last decades with no problems. I look at a complex smd SMPS and wonder if it will be a failure prone as all the disposable electronic junk that's being sold today.
You have more parts to fail, you're less sure how close each part is operating to it's power/thermal limit, and with smd you have more compact size to dissipate heat AND no mechanical connection at joints - which with thermal expansion cycles may lead to joint failures.
Any thoughts?
Or the GDIY version could be thru hole, but then the larger size may be prohibative.

 

[soapfoot]

I don't want to open a can of worms but I like to feel something is very reliable. And a properly spec'd power transformer into a RC filter I think could last decades with no problems. I look at a complex smd SMPS and wonder if it will be a failure prone as all the disposable electronic junk that's being sold today.
You have more parts to fail, you're less sure how close each part is operating to it's power/thermal limit, and with smd you have more compact size to dissipate heat AND no mechanical connection at joints - which with thermal expansion cycles may lead to joint failures.
Any thoughts?
Or the GDIY version could be thru hole, but then the larger size may be prohibative.

My sense is that all things are tradeoffs. I won't be retrofitting my vintage guitar amps for SMPS anytime soon; the way they behave when overtaxed is part or the sound of the amp (and they've proven to be reliable over more than half a century).

The amps I use for recording/session work, good sound is the priority, reliability second, portability a distant third.

But to have an additional amp with portability as the top priority--made with an SMPS to be as lightweight as possible--would be appealing to me. I'd love to have a two-knob, single channel 2x EL84 1x12 amp that weighed under 15lbs or so. Something light enough that for a small hit it could feel less like "loading in" and more like "showing up."

With a neodymium speaker, SMPS, and cab made of lightweight material (pine or even an exotic like paulownia) my sense is that it might be possible.

 

[dmp]

My sense is that all things are tradeoffs.

Agreed
I've gone to head/cabs to make them easier to move (and they work better). Lot's of small amp heads have come out that are just what you're describing. A DIY would be cool

made with an SMPS to be as lightweight as possible

Physical size too. It would be interesting to compare the size & weight of a smps to a toroid RCRC or LCRC

the way they behave when overtaxed is part or the sound of the amp

Put a RC after the smps and you might get the sag / harmonics in the power rail.

 

[beatnik]

Are you using SMPS for heaters also in a self contained rack chassis ?

In that case the SMPS module needs special attention in terms of shielding ? does it radiate noise into audio path as a conventional transformer ?

 

[ruffrecords]

Are you using SMPS for heaters also in a self contained rack chassis ?

Yes I have in some cases

In that case the SMPS module needs special attention in terms of shielding ? does it radiate noise into audio path as a conventional transformer ?

I always use enclosed SMPS and I have found them to be significantly quieter than their linear equivalents.

Cheers

Ian

 

[Tubetec]

Guitar amps are often overdriven as part of the tone , I wonder how would an SMPS HT effect the sound .

In general push pull amps have good rejection of PSU noise , small single ended amps dont have the same PSRR and its harder to stop magnetic hum from the mains transformer getting coupled to the output .

Ive had a few friends ask about a battery powered tube amp for street busking or small acoustic sessions ,
Battery heater supply with an SMPS HT running off it might be interesting to try .

Combo amps tend to suffer from odd resonsances and microphonic effects more than heads , its easy to isolate the head from any vibrations from the cab , a layer of dense sponge material for the amp to sit on works very well.
Marshall have vibration absorbing feet , they make a difference when you have a 100W cranked into a couple of 4x12's .
Like the rest of you I dont intentionally make those kinds of noise levels any more ,

 

[Matt Syson]

SMPS have high frequency 'noise' and often mains supply 'frequency ripple superimposed on HF so can couple into your amp from heater to cathode, particularly if the cathodes are not tightly coupled to 'ground' at HF. Of course all switchers are not of the same 'quality' of HF and as the manufacturers data of all commercial supplies state, it is up to the end user to check whether EMC compatibility is up to scratch. HF coupled from heater to cathode will get amplified although whether that results in a problem for your individual 'design' becomes your responsibility. As another thought, overdriven valve amps with a suitably 'soggy' HT show a fair amount of modulation at 100/120Hz which 'add' to your tone. This can be clearly seen on a 'scope' when feeding a sinewave and push level to clipping so presumably occurs for sustained notes. Whether 'artists' notice the difference between 100 Hz and 120Hz modulation may be interesting to ascertain.

 

[soapfoot]

It's almost a certainty that a SMPS supply in a guitar amp wouldn't sound like the barely-adequate linear tube-rectified supplies found in classic amps.

But I somehow doubt that means it couldn't sound good. It might just be a different kind of "good."

 

[thor.zmt]

Guitar amps are often overdriven as part of the tone , I wonder how would an SMPS HT effect the sound .

That depends how it is designed. In principle it not a problem to simulate the sag of a tube rectified supply, for example. Being fully electronic you can pretty much do anything you decide to.

Ive had a few friends ask about a battery powered tube amp for street busking or small acoustic sessions ,
Battery heater supply with an SMPS HT running off it might be interesting to try .

Very possible. I had previously discussion what sort of products would "novel" for musicians and worth making into a product. Nobody ever mentioned a battery/portable tube guitar amp.

Thor

 

[thor.zmt]

But to have an additional amp with portability as the top priority--made with an SMPS to be as lightweight as possible--would be appealing to me. I'd love to have a two-knob, single channel 2x EL84 1x12 amp that weighed under 15lbs or so. Something light enough that for a small hit it could feel less like "loading in" and more like "showing up."

With a neodymium speaker, SMPS, and cab made of lightweight material (pine or even an exotic like paulownia) my sense is that it might be possible.

The possible solution for this kind of product would be "ZOTL" Principle from Dave Berning. It does away with any output transformer. Look it up.

Lightweight cab?

How about foam board layered between thinish layers of plywood?

Or maybe 3D printed honeycomb panels made from nylon sandwiched between thinnish layers of ply?

Thor

 

[thor.zmt]

SMPS have high frequency 'noise' and often mains supply 'frequency ripple superimposed on HF so can couple into your amp from heater to cathode, particularly if the cathodes are not tightly coupled to 'ground' at HF.

That would be, to be precise, BADLY DESIGNED "SMPS have high frequency 'noise' and often mains supply 'frequency ripple superimposed on HF".

Direct Main frequency (not harmonics) never directly pass through the SMPS. They literally cannot. They can however pass "around" the SMPS by a number of mechanism.

Here an example design of an AMPS for a Tube Amp with a low voltage "standby" switch for turn on/off (e.g. remote control) and +/-12V Auxiliary supply.



Here the actual implementation in a EL84 Tube Amp (HiFi):

As another thought, overdriven valve amps with a suitably 'soggy' HT show a fair amount of modulation at 100/120Hz which 'add' to your tone. This can be clearly seen on a 'scope' when feeding a sinewave and push level to clipping so presumably occurs for sustained notes. Whether 'artists' notice the difference between 100 Hz and 120Hz modulation may be interesting to ascertain.

This would be easily simulated in a suitable SMPS, it it is needed for the correct "tone". Dynamically reduce LF loop gain in the SMPS with current draw, so the ripple from the mains input capacitors modulates the output.

Thor

 

[thor.zmt]

I can’t wait until someone with far more design chops than me designs a good SMPS for vacuum tube guitar amps.

I know the traditionalists will hate it, but I’d love at least one amp with a SMPS for maximum portability

I can make a few suggestions.

First, NEVER EVER place heater winding and HT winding on the same switcher. I have done it one time and I will never do it again, if budget etc. allows..

This way needs to way too much circuitry to avoid the HT go through the roof when heaters are cold. I eventually got it to work reliable, but it was a slog.

As in Guitar Amplifiers often "less than perfect" power supplies contribute to overdriven "tone" this need to be simulated.

Thor

 

[abbey road d enfer]

The possible solution for this kind of product would be "ZOTL" Principle from Dave Berning. It does away with any output transformer.

There are actually two output xfmrs in them, much smaller I agree.
And with 8 power MOSFETS, is it still a tube amp?
When it comes to this level of complexity, why not not go full solid-state?
Although my fave amp is a Fender 63 Vibroverb, I also like my Quilter Mach 2 12HD combo. 200W, lighter and smaller than a 15W Fender Blues Jr.

 

[Brian Roth]

Something about an ancient Vox toooob amp sounds amazing for some songs.

Bri

 

[ruffrecords]

Something about an ancient Vox toooob amp sounds amazing for some songs.

Bri

Your namesake Sir Brian May swears by them.

Cheers

Ian

 

[abbey road d enfer]

Something about an ancient Vox toooob amp sounds amazing for some songs.

Indeed. However, I'm not convinced sag is a big part of its sonic signature.