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 Post subject: “Free” (nearly!) B+ boost for audio amplifiers.
PostPosted: Jul Tue 23, 2019 1:32 am 
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Location: Thornhill, Ontario, Canada
BACKGROUND:
One of my audio interests is designing and building high-performance audio amplifiers in the 3 to 60 wpc range, using “parts to hand”, i.e. no expensive purchases of tubes or “iron”. (1)

The ideal PT is not always to hand, i.e. I can generally find the current rating I want, but not always the rectifier P-P voltage. “Too high” is quite easy... use choke input and/or add series WW resistors. “Too low” and you have to get creative... and there is a way!

THE FIX:
Add a full-wave rectified AC source in the PT centre tap to ground line. (2)
To do this, determine the B+ increase you want; typically it will be in the range 25 to 100 VDC.
Find a small PT (the “boost transformer”) with a secondary RMS AC voltage about equal to the B+ DC boost you want. It need not be high-power as it only has to supply the amplifier B+ current at a low voltage.
If you juast happen to need an additional 25 VDC on the B+, a useful “boost” source is a 25 VAC bell transformer, often found as scrap.
Find a full-wave rectifier: an easy way... part-out an old PC power supply.
The current rating must be above the peak current delivered by the rectifier (3), Salvaged PC rectifiers are fine, being over 4 amps rated.
Connect the “AC in” terminals of the S/S FW rectifier to the secondary of the boost transformer.
Connect the primary of the boost transformer tgo a switched mains AC source.
Lift the centre tap wire of the PT HV secondary and connect it to the positive terminal of the S/S FW rectifier.
There is no need to put a filter capacitor on the output of this FW rectifier as its sole function is to add ½ cycles of voltage alternately (120 per second) to the PT centre tap, effectively adding to its original plate-to-ground secondary voltage.
See pic attached (used on a home-brewed Williamson a while back.)
Of course, you need the space on or under the chassis to put the boost transformer!

TO TEST:
Bring up the unit slowly on a Variac. Note the AC voltage on the tube rectifier plates, this should be the original PT voltage plus the “boost transformer” voltage. Note the various DC voltages, in particular the resultant B+ voltage to the O/P tubes (at OPT primary centre tap) and the unfiltered boost voltage at the positive terminal of the added FW rectifier (an RMS-reading meter will give this.)
Of course, confirm that the B+ voltages across the B+ filter are within limits. If necessary adjust series resistance in the filter to establish correct B+ voltage to the earlier stages (typically the voltage amplifier and phase splitter.) Generally, a higher B+ voltage at the phase splitter is a “good thing” as it allows the tube to stay in a more linear region. If the voltage amplifier is directly coupled to the phase splitter, ensure the grid1 voltage is about a third of the local B+, but this is a design issue beyond the scope of this note.

Finally, if you have the equipment (signal generator, 'scope, etc) do a full amplifier test to establish target performance has been obtained.

Interestingly, you can do a before and after comparison by disconnecting “boost transformer” primary from its 120 VAC input and shorting this primary winding. This makes the “boost transformer” secondary become a “near short” to ground, i.e. PT centre tap being effectively grounded through the S/S FW rectifier, except for 2 x 0.7 volts forward drop in the S/S rectifier diodes plus the PT leakage reactance. Of course, I need hardly say, do NOT short the S/S FW rectifier output, NOR open the PT center tap!

Important! Note 4, below, applies to all this!

For the record, I have done this modification to three audio amplifiers with complete success.
Cheers,
Roger

PS.
Special “Q&D” kluge!
If you have an amplifier (or AC radio) that you believe would benefit from just a small increase in B+ and it has one side to the 6.3 VAC heater voltage grounded, you can get a “free” 8 VDC B+ boost this way:

Connect a single, ½ wave diode rectifier from the 6.3 VAC heater line to a terminal, call it “point A” so that this point is positive wrt ground (diode cathode to “A”).
Connect a “large” (see below) electrolytic capacitor from “A” to ground (right polarity!)
Reconnect the PT centre tap from ground to “A”.
Now B+ will be about 8 volts higher.
What happens?
The heater voltage is ½ wave rectified to nominally 1.414 times the RMS voltage, less a 0.7 volt diode drop... about 8.2 VDC.
On the positive ½ cycle the cap charges up and raises the PT CT voltage by an amount a bit less than 8.2 volts. On the negative ½ cycle, the capacitor delivers all the B+ current to the amplifier, its voltage dropping a bit depending on the capacitance value and the average B+ current. It then gets re-charged on the next positive ½ cycle.

My results:
I did this on a low-powered P-P amplifier in a circa 1960 AM/FM receiver. It had an anemic 200 VDC B+, a poor design that delivered barely 4 watts at clipping. I used two paralleled 1N4002's as I was concerned about ripple current peaks. The cap is 1,700 uF, 30 volt, as it was to hand... Note: it needs to be large enough to supply all the B+ current during the negative ½ cycle without dropping “too much” voltage... the transient voltage drop here changes the balance of the PT secondary and places a 60 Hz component on the B+ filter. My final B+ boost was about 7.7 VDC. Max output power went up just a fraction... about 1/3 of a watt! Not really worth the effort, but “free” as I have a large stash of near useless 1N4002's (too low a PIV voltage for most tube-based uses, the default is the 1N4007) and the cap was surplus (reformed, tested very good.) My real cost? Just my time experimenting, aka fun!
YMMV...

Note 1:
All my decent amplifiers are checked on a PicoSope digital ocilloscope... this measures power output, S/N ratio and noise floor, TH and IM distortion (all harmonics to high order) and frequency response. Bottom line: there's no place to hide if you get something wrong!

Note 2:
We all know that mains voltage is higher these days, e.g. 125 VAC, compared to the 110 or 115 VAC prevailing when vintage amplifiers were sold, or any given vintage PT was designed. Before doing the above, ensure the chosen PT gives the right heater voltage when plugged into your mains supply. All my vintage tube amplifiers run off bucking transformers so that the heaters to close to 6.3 VAC. B+ voltage adjustment (up or down, by any means) comes after that.

Note 3
Rectifier current.
If the amplifier uses S/S B+ rectification (rare for vintage amplifiers, but the Heathkit W-7M uses them - with a surge limiter!), the switch-on inrush will typically be high (charging the B+ filter.) If it uses the 5U4 or similar directly heated tube, the inrush is spread out a bit, the only concern is now the ripple current at each half cycle into the reservoir capacitor. Make sure the cap value does not exceed the limit for the rectifier tube (check the RCA tube manual.)

Note 4:
Don't so any of the above unless you are competent in tube amplifier design and refurbishing, with a full understanding of how to deal with the lethal voltages in a tube amplifier chassis. These things are not AA5 radios!


Attachments:
B plus boost circuit.jpg
B plus boost circuit.jpg [ 146.86 KiB | Viewed 850 times ]

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 Post subject: Re: “Free” (nearly!) B+ boost for audio amplifiers.
PostPosted: Jul Tue 23, 2019 1:05 pm 
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How much increase from just replacing the rect tube with solid state?

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 Post subject: Re: “Free” (nearly!) B+ boost for audio amplifiers.
PostPosted: Jul Tue 23, 2019 8:35 pm 
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Simple boost for HV. Add the 5 volt rectifier winding in series with the 300 volt secondary winding. Use the outer two secondary winding connections as AC output. Then use full wave solid state rectification.
Personally, extra effort, with little overall gain. Wow factor is also low.

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 Post subject: Re: “Free” (nearly!) B+ boost for audio amplifiers.
PostPosted: Jul Wed 24, 2019 4:38 pm 
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Location: Metzger Oregon
Thanks for posting this, I had thought about something similar a time or two, but never did follow through. Would a voltage doubler off of an unused filament winding be an option as well?

I hope this isn't too far off topic, but I am wondering what you use for output transformers. I have thought of doing the same, but the limiting factor has always been the output iron - it's hard to justify the time invested into building an amp, only to use output transformers robbed from an old radio that presumably have limited performance.


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 Post subject: Re: “Free” (nearly!) B+ boost for audio amplifiers.
PostPosted: Jul Thu 25, 2019 2:52 am 
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classicelectronicsguy wrote:
... Would a voltage doubler off of an unused filament winding be an option as well?
This sounds like a good idea, as long as you get a very low impedance positive point to connect the HV secondary centre tap to. I've not done the circuit analysis... it may need a large cap to bridge any 1/2 wave issues.

classicelectronicsguy wrote:
... I am wondering what you use for output transformers. I have thought of doing the same, but the limiting factor has always been the output iron - it's hard to justify the time invested into building an amp, only to use output transformers robbed from an old radio that presumably have limited performance.
Agree, good OPT's are expensive to buy. In a recent AM/FM chassis refurbish from cheap SE stereo to better P-P mono (posted on ARF), I used a Hammond 125E OPT I had to hand. The problem is that this OPT is worth more than the scrap value of the whole chassis! When I dispose of it, likely for parts, I'll take out the 125E. If anyone wants it left in, it'll be $50 more, i.e. it won't sell!
Cheers,
Roger

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 Post subject: Re: “Free” (nearly!) B+ boost for audio amplifiers.
PostPosted: Jul Thu 25, 2019 3:45 am 
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What am I missing?
Can't the higher B+ be achieved by employing a voltage doubler or tripler in the filter area?

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 Post subject: Re: “Free” (nearly!) B+ boost for audio amplifiers.
PostPosted: Jul Thu 25, 2019 6:35 am 
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Joined: Sep Thu 23, 2010 6:37 am
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Location: Powell River BC Canada
The B+ boost circuit with the 60 volts kind of
makes me want to probe the current waveform of the input filter
for snivets caused by rapid conduction turn on of the bridge diodes.

Maybe hang an AM transistor radio close to rectifier tube when its
working.

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 Post subject: Re: “Free” (nearly!) B+ boost for audio amplifiers.
PostPosted: Jul Thu 25, 2019 5:01 pm 
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Pbpix wrote:
What am I missing?
Can't the higher B+ be achieved by employing a voltage doubler or tripler in the filter area?
We are not trying to get 2 or 3 times the B+, just a few percent increase, e.g adding 60 VDC to a 300 VDC B+, or so.
Cheers,
Roger

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 Post subject: Re: “Free” (nearly!) B+ boost for audio amplifiers.
PostPosted: Jul Thu 25, 2019 6:14 pm 
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You should gaze carefully at your bridge boost circuit to see if it has enough uniqueness.

:wink:

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 Post subject: Re: “Free” (nearly!) B+ boost for audio amplifiers.
PostPosted: Aug Sat 24, 2019 10:23 pm 
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radiotechnician wrote:
The B+ boost circuit with the 60 volts kind of
makes me want to probe the current waveform of the input filter
for snivets caused by rapid conduction turn on of the bridge diodes.

Maybe hang an AM transistor radio close to rectifier tube when its
working.



Shouldn't be an issue if fast recovery diodes are used.

Engineer, that is quite interesting and is something I've never thought about.


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 Post subject: Re: “Free” (nearly!) B+ boost for audio amplifiers.
PostPosted: Sep Sun 01, 2019 10:43 pm 
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What I don't get is why did you draw the schematic like that?
Shouldn't the electrons flow from left to right?
:lol:

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 Post subject: Re: “Free” (nearly!) B+ boost for audio amplifiers.
PostPosted: Sep Mon 02, 2019 12:12 am 
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Electrons don't flow, they drift, and if we waited for them to get anywhere,
it would be longer than a selenium argument.


And, Ill gladly argue charge velocity if you are buying the drinks. 8)

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