Breadboard Accessories - A Bench AC Supply & (may be ?) a multi node DC Supply ?

[kagliostro]

Before to start with my BreadBoard project I'm building my Bench AC Supply

based on some material I've collected for this purpose
(just yesterday I got the 3 x 230v/230v 250VA Isolation Transformers)

at the moment I'm missing the small transformer for the Bias Supply

and the Filament Transformer has all windings at 6.3v so I must dismount it and unroll one of the winding to have a 5v winding (if I'm able to do)

One thing I'm wondering is how to obtain the Bias voltage starting from low negative voltage till the -130v (required in amps like the Hiwatt 400), may be I'll use a 2 x 50v winding transformer (and also I'll never build an amp so big  )

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There is one other idea in my drawer this last times, a DC power supply for BreadBoard purpose

The idea will be to build a multi node supply that uses variable resistors as to be set very quickly at the right voltages for different projects

As to do this thing requires the ability to swap between different resistors values on the nodes and to use rotary switch requires a lot of different resistors, I'm wondering if is possible to use a power resistor paralleled with one other power resistor and an in series potentiometer as rehostat, if the potentiometer is of high resistance value I think it didn't requires to be a high Wattage potentiometer

But this is only something that appears to me a nice DC supply, but I've no idea if it can be done and also I'm confused about the values of the pot and resistors used in the nodes

What do you think ? Is this a Crazy Project ? Or do you think it is feasible in the way I've think to it or in one other way ?

Many Thanks for any consideration, council and help in fonding a solution to this thing

Kagliostro

[HotBluePlates]

There is one other idea in my drawer this last times, a DC power supply for BreadBoard purpose

The idea will be to build a multi node supply that uses variable resistors as to be set very quickly at the right voltages for different projects

... I'm wondering if is possible to use a power resistor paralleled with one other power resistor and an in series potentiometer as rehostat, if the potentiometer is of high resistance value I think it didn't requires to be a high Wattage potentiometer ...

Yes, that can be done.

But there is a problem: you have to know ahead of time roughly how much current may flow through those resistors (to figure appropriate power ratings) and roughly what range of resistances you need to have available for each network.

Can you answer those questions? If so, it's a pretty trivial math problem to come up with the resistance networks. If you can only answer one of the questions, then it should be how wide a range of resistances you need/want for each network.

[eleventeen]

I would not like to unwind a 6.3 transformer to make a 5 volt one. A 5 v rectifier-heater transformer sees your highest B+ with lots of ripple and just speaking for myself, I would *not* like to mess with it. I think I would prefer to place a resistor in series with the primary of a 6.3 trans to drop 20% of your line voltage (you have 230/240 volts where you are, yes?)

[kagliostro]

@ HotBluePlates

I think I must spent some time on looking to the PS schematic of a large number of amps as to identify which values are most commonly used as resistance values and Wattage values

I'm not sure if I'll go on looking to 100W amplifiers or may be 200W amplifiers

the Wattage value I think is more easy to be chosen as a resistor that is good (in Wattage ability) for a 200W amp may be easily used for a 10W amp

what I think will be more difficult is to establish the resistance value of the most often used resistors

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@ Eleventeen

I agree with you, I didn't like very much the idea to unroll one of the winding of the 5 secondary 6.3v transformer I've

I can't use the trick of the resistor on the primary winding with that transformer, because it has also 4 more 6.3v windings that I require to be at the right voltage

May be I unroll one other PT, I've some candidates on my stock, however no one was for filament use, may be I've some big 8v transformer, I must give a look for this thing

And, yes, here I've 230v line

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Thanks

K

[HotBluePlates]

@ HotBluePlates

I think I must spent some time on looking to the PS schematic of a large number of amps as to identify which values are most commonly used as resistance values and Wattage values

I'm not sure if I'll go on looking to 100W amplifiers or may be 200W amplifiers

the Wattage value I think is more easy to be chosen as a resistor that is good (in Wattage ability) for a 200W amp may be easily used for a 10W amp

But your supply shows node A and B separated by a choke, so these are likely going to be your output tube plate and screen nodes. So the rest separated by resistors will probably be phase inverter or preamp tubes, where the total output power for an amp won't matter.

Thinking more about it, since you already have a variac for a screen power supply, it would seem best to have one dedicated transformer for plate voltage, a 2nd dedicated transformer for the screen voltage and a third for the phase inverter/preamp.

I think DL and RicharD simply had 1 adjustable power supply voltage, and rigged a suitable supply for the circuit being tested. If you have more transformers or are willing to have separate supplies as your diagram seems to show, then you only need to figure out resistance range for preamp supply dropping resistors.

[kagliostro]

That seems one other way to achieve the result

I've two transformers for B+, and the idea was to use the first for amps requiring till ~300v B+ and using SS rectify

the second one was planned to be connected in series as to have a CT for tube rectify or to rectify via SS for ~600v B+

As I would like to try some tubes that requires a G2 voltage much lower than B+ the third transformer with his Variac is planned for that purpose

As you suggest I can use this lower output for a dedicated preamp section of a DC adjustable Power Supply

I've also some other small Variac and may be as you say, I collect one other smaller isolation transformer to be used only for the preamp section of the Bench Supply

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However the ideal will be to be able to obtain the whole voltages for the amp in test from B+, when established the right voltages it will be easy to simply look to the value setted on the nodes to duplicate it in the real PS with common resistors

To give a preset to the pots for the theoretical calculated value, then connect the tubes, measure the voltage under load and set it correctly, this will be fantastic

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As told, I like the idea and hope to be able to build this thing, but hope and succeed aren't the same thing

K

[jjasilli]

IMHO bench power supplies are useful for test/repair purposes, but not for amp design.  Voltage, compression & saturation, type of rectification, choke(s) or resistors, and sag (or not), are all "in the mix" for the resulting tone. 

[kagliostro]

I can agree with you that this kind of supply is better suited for repair

but I think that it may be also used as to stage the test of an idea

when the main line of the idea is developed

a proper PS must be used as to reach the fine tuning

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Today I had not very much time to dedicate to this thing, however searching in my stock

I've find one other good transformer that may be used

it is a 290VA and has a 0-220/380v primary winding - secondary are 0-220v / 0-24v / 0-12v(2A)-24v(2A)-48v(1A)

but at the moment no 5v transformers

K

[SILVERGUN]

Thinking more about it, since you already have a variac for a screen power supply, it would seem best to have one dedicated transformer for plate voltage, a 2nd dedicated transformer for the screen voltage and a third for the phase inverter/preamp.

I think DL and RicharD simply had 1 adjustable power supply voltage, and rigged a suitable supply for the circuit being tested. If you have more transformers or are willing to have separate supplies as your diagram seems to show, then you only need to figure out resistance range for preamp supply dropping resistors.

Great point HBP.....now you've got me thinking  .......just when I thought I was almost done with my bench PS

[xm52]

You might find the Lambda model 71 power supply interesting. Search for 'Lambda 71' on this site for manuals and schematics: http://www.ko4bb.com/manuals/index.php

[kagliostro]

The ideal solution of separated Variac and Transformer requires HV transformers to have the necessary B+ levels

or a doubled number of isolation transformers connected in series two by two without it it will not possible to have a high B+

with 230v we can have ~300v B+ if SS and less if we use Tube Rectify, so we need to use two 230v windings in series as to have 440v AC and ~600v DC (using SS rectify)

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The research go further on

K

p.s.: Thanks for the link Xm52, the purpose is to build something simpler than a stabilized supply, but those old schematic are always interesting and source of inspiration

[HotBluePlates]

You might find the Lambda model 71 power supply interesting.

If you buy anything like that, you'd better be able to pick it up without shipping. Better yet, have someone else pick it up for you.

Shipping costs will kill you. I was once given a huge tube-regulated power supply, somewhat like the Lambda 71 but very much more current capacity. Couple of 6.3v 10A filament transformers, big 20H 500mA chokes bigger than any 100w OT you ever saw, etc. It must weigh over 100lbs.

In my youthful ignorance, I dismantled much of it thinking I'd use the power transformers for the various supplies in an amp build. These days, I'm thinking about reassembling it to use for its intended purpose.

Sluckey may know a similar power supply: a big rackmount tube supply with maybe 6x 6AS7 pass tubes for an AF or FAA Interrogator.

[DummyLoad]

http://www.ebay.com/itm/TDK-Lambda-Genesys-GENH750W-Half-Rack-Programmable-DC-Power-Supply-600v-1-3a-/321122718614?pt=LH_DefaultDomain_0&hash=item4ac467d396

http://www.ebay.com/itm/TDK-Lambda-Genesys-GENH750W-Half-Rack-Programmable-DC-Power-Supply-600v-1-3a-/321122718614?pt=LH_DefaultDomain_0&hash=item4ac467d396

killer PS.

--pete

[xm52]

My thought was to use the lambda 71 as node A, then have modules for the other nodes. It would be an easy modification to have the unit produce more ripple if desired. It is nice because it supplies bias as well as heater voltages.

It is such a cool unit. Here is one currently available: http://www.ebay.com/itm/LAMBDA-71-0-500V-TUBE-AUDIO-POWER-AMPLIFIER-VARIABLE-REGULATED-300B-B-DC-SUPPLY-/190801289382?pt=Vintage_Electronics_R2&hash=item2c6ca4a0a6

[kagliostro]

WOW, I like this last unit

http://www.ebay.com/itm/LAMBDA-71-0-500V-TUBE-AUDIO-POWER-AMPLIFIER-VARIABLE-REGULATED-300B-B-DC-SUPPLY-/190801289382?pt=Vintage_Electronics_R2&hash=item2c6ca4a0a6

this will be a very good piece of iron also to be used only to be look at, something like you have a sculpture on your bench

for sure my interest for that thing will be its look much more than it's technical use (that for sure will be useful)

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with regard to similar units, Millet has also published on its website an interesting project

http://www.pmillett.com/HV_bench_supply.htm

but in practice it would be much easier to build something using something like in the attached files

only in adjustable version, like if you use a cascaded series of VVR

the use of cascaded VVR give you the ability to set perfectly the voltage of each node without any effort

and all this in a very small space, very far from the space to achieve the same thing using vacuum tubes, also if in a less romantic way

K

p.s.: All this interesting idea comes from Merlin B. whose books I recommend to have and read

[PRR]

> unroll one of the winding to have a 5v winding

Put a half-Ohm 10W resistor in series with that filament.

This gives 5.3V at 2A load or 4.8V at 3A load. This covers most rectifiers close-enough.

That's easier and a lot less risk than unwinding a transformer.

[kagliostro]

Thanks PRR

If you say that is doable I'll use your trick 

also to use a pair of switchable resistor to suit different current request will be very easy

--

Just a curiosity, I've never seen to be used with Tube Rectifiers a DC filament voltage instead of an AC filament voltage

in theory you can do ? (just an academic question)

Thanks

K

[sluckey]

Yes, you can do it. But you will need to float the dc filament supply. Not worth the effort IMO.

[kagliostro]

Thanks Steve

I was just curious to know that

Franco

[eleventeen]

I had one of those Lambdas very recently. It was a 75-80 lb beast, used 6 qty 6W6 tubes as pass elements which are fortunately very cheap, $3-4 each NOS easily. They run HOTTTT, use 1.2 heater amps each. It had some problems and I'm ashamed to say I was forced to chuck it, as I obtained it in weakish condition with busted meters and it was further damaged in shipping. Another possibility, also available used is a hp 712B. What's nice about these is that they handle the whole thing....filament, variable B+, and bias all in one shot. Much cheaper is a B&K model 16--......I am having a problem recalling the last two digits of the model number.

Fluke 407 <<Excellent choice (one or more for sale on ebay now) and KEPCO also make supplies. The Fluke 407 is almost knob-for-knob identical to the Lambda. Some of the Flukes often DO NOT have a separate bias supply, but usually sell cheaper than Lambdas and the bias part is easy to make yourself any number of ways because the current needed is so small.

The KEPCOs are excellent, and often use 6550 as pass elements, BUT, they seldom go over 350 volts which is a problem.
Secret: If you can find a nice KEPCO, you can steal the 6550's and substitute 6L6's (and the 6L6's can be kind of cruddy ones) at very slight sacrifice in current capacity. I got a sweet pair of matched 6550's out of a KEPCO and sold them for more than I paid for the supply.
Older one: KEPCO 245B.
BHK 1000
BE CAREFUL with the KEPCOs, many models only supply 10 ma which obviously won't cut it.

Let me just give one admonition about these supplies. The "BIG KNOB" in the middle of the panel is a variac, and that variac, more specifically, the "tooth" that contacts the windings around the variac has to be in VERY good condition or you will have arcing situations, and since that arcing drives the windings of other transformers, the arcing, if it occurs, RAPIDLY gets worse and worse and worse (arcing does that anyway) The condition of that tooth and the commutating rotor has to be maintained very, very clean. So, if you get one of these supplies surplus and it has 20+ years of grime on it, pay very close attention to the variac commutator.

[Heinz]

Maybe a low-tech solution would be sufficient...

I have built a pretty simple adjustable B+ power supply from 10 regular 230V/24V transformers. The secondaries are connected in series and I use a 12 position rotary switch to select an AC voltage between 0V and 250V from the chain of secondaries. When working with these voltages it doesn't really matter whether you can adjust the voltage continuously or in steps. By using this approach you can avoid the variac and use parts which are easily accessible and inexpensive. The transformers I used have 24VA but I used them mainly because I got them for free. In fact, you can use much smaller transformers (e.g. 12VA) and the current should still be ample. The whole assembly is compact, (relatively) lightweight and doesn't require maintenance.

The selected voltage then goes to a high voltage SS rectifier and nodes A, B and C which have "typical average" resistor values and 100µ/400V caps inside the chassis. If I want different values I use external resistors in parallel and additional caps. And if I want something completely different I take the voltage from node A and build the whole filter chain externally. With the rectification the voltage steps are roughly 35V and the DC voltage can be adjusted from 0V to around 350V. The DC outputs are the red connectors below the instruments.

The two instruments display the voltage at node A and the current. The current cannot be adjusted or limited. As a safety measure I have one 1A fuse on the primary side and another 1A fuse that protects the secondaries. The instruments were salvages from old broken bench top power supplies. Any type of instrument (digital of analog) can be used.

However, stacking the transformer secondaries has limits. You must be careful not to exceed the isolation voltage of the transformer windings. Most transformers have 1000V so you can go much higher than I did.

[eleventeen]

That's very clever! I'd want to mount the transformers on a piece of plastic, I think.

[Heinz]

They're actually mounted on a piece of wood inside the chassis. It was the easiest way to assemble them, has the benefit of being a decent isolator and it also dampens the mechanical hum from the transformers.

[kagliostro]

Hi Heinz

I remember your "Variac", some year ago I found the whole description

in a second time I was looking for that page but wasn't able to find it anymore

K

[jjasilli]

There are other vacuum tube, hi-power supplies like the vintage Heathkit PS-4 and similar (Eico, etc.): up to 400VDC & 150ma, plus 150VDC bias supply and 6.3VAC 4 amp filament supply.  I have one.  Not too expensive.  There's also a vintage Fluke 700 or 710???:  600VDC and lots of mA; but more expensive.  They make regular appearances on eBay.

Again, these are regulated power supplies.  Hence, to my mind, not well-suited to guitar power amps, which are famously unregulated.  Certainly OK for preamps.

Instead, for guitar power amp design my preference is 2 PT's: one for small bottle, the other for big bottle power tubes, fed by a variac.  Then, substitution boxes for power resistors, and for inductors.  Or just make a couple of likely choices available for A/B testing, by switching on the breadboard.