Output Valve bias meter? how to wire in correctly?

[Whaam68]

Hi All
I've built this little valve amp for my office desk top using junk box parts on an old power supply chassis. The old chassis had quite a few more valve base holes than required and to tidy it up I've filled a couple using some mini bias meters (0-100 ma range). Could anyone give me a steer on how to correctly wire these meters in to the attached circuit to measure the output valve bias current? Pretty sure I need to measure the voltage drop across a 1 ohm resistor but not 100% sure of how to go about it. I'm not expecting accuracy from these cheapo ebay meters but more of a general guide and largely as a cosmetic exercise.
Mike

[Radio Wrangler]

It's considered bad practice to put current metering in the high voltage side of power amplifiers (RF and AF ones)

So it's usual to meter the cathode current.

But you don't have a separate cathode, it's inextricably linked with the heater and the heaters of the two bottles are combined. You'd need a separate, floating, heater supply for each output bottle.

Alternatively, you could put the meter in the cold (negative) end of the power supply, but that would also meter the current of the driver, and of both stereo channels all at once.

You could put the meter in the HT feed to the output transformer, but beware of voltage ratings... things like zero setting screwdriver slots can become live. Some transmitters have meters on HT in fitted perspex housings to keep safety clearances from people's fingers.

Having meters is trendy, having directly heated triodes is trendy, but together they create a lot of added complexity one way or another, and the attractive simplicity gets it in the neck.

David

[ms660]

Fixed grid bias for the output valve so there's no bias current.

With a 100mA meter you can measure the anode current of the output valve by connecting it in series with the HT supply feed, the anode current of the first valve will have little effect on the reading as it's likely to be less than 1mA.

Lawrence.

[Whaam68]

Thanks guys. I actually scratch built this a few years back from the grotty power supply chassis, a small box of random eBay mixed valves that happened to include a single 71A and the old school balloon rectifier. I already had a couple of small output tx and I was looking for a simple circuit to use all these bits to see what I could knock up and this one came along. It’s been an ornament for too long sat on a shelf but as a desktop amp driving an old pair of castle bookshelf speakers sounds very good. I will wire up the meters as suggested. The power supply had a couple of surplus valves and umbilicals so lots of holes to fill hence the meters. Even if not hooked in they look better than the empty holes.

I like it!

[usradcoll1]

Originally Posted by Whaam68

Thanks guys. I actually scratch built this a few years back from the grotty power supply chassis, a small box of random eBay mixed valves that happened to include a single 71A and the old school balloon rectifier. I already had a couple of small output tx and I was looking for a simple circuit to use all these bits to see what I could knock up and this one came along. It’s been an ornament for too long sat on a shelf but as a desktop amp driving an old pair of castle bookshelf speakers sounds very good. I will wire up the meters as suggested. The power supply had a couple of surplus valves and umbilicals so lots of holes to fill hence the meters. Even if not hooked in they look better than the empty holes.

I'm really surprised on your choice of valves. The 71A would be the furthest from my mind, maybe a 45 or a penthode, such as a 41.
Anyway, it's a very attractive job and proves what you can do with parts on hand.
Dave, USradcoll1

[Whaam68]

Thanks....I already had the 5Y3 and a single 71A and the edcor xse outputs. The blue paint was just rattle can from a local hardware store that was closing down and 6F5’s were very cheap, 45’s are rare over here but since I built this 71A’s now seen to be attracting a premium. All our rooms are very small and in near field with all kinds of music and 90db speakers it sounds very good, you would never know it is only 750mw. Really it was a curiosity exercise. Done again I would probably try 6v6gt.

[Whaam68]

This is probably OT for this section but if anyone is interested I just uploaded a quick clip of the amp playing on YouTube.. recorded on my phone just search for 71A stereo amplifier you should find it.
Mike

[usradcoll1]

Originally Posted by Whaam68

Thanks....I already had the 5Y3 and a single 71A and the edcor xse outputs. The blue paint was just rattle can from a local hardware store that was closing down and 6F5’s were very cheap, 45’s are rare over here but since I built this 71A’s now seen to be attracting a premium. All our rooms are very small and in near field with all kinds of music and 90db speakers it sounds very good, you would never know it is only 750mw. Really it was a curiosity exercise. Done again I would probably try 6v6gt.

Reading this thread, it made me hit the valve characteristics manual. Not having too much exposure to the 71A. I only have one radio that uses that valve, that being a 1928 Kolster, one of the early AC sets.
Before I looked up the schematic, I thought, how did he furnish the Filament voltage for the 71A. Then I remembered, it's an AC filament!
Dave, USradcoll1, supposed to know better.

[Whaam68]

Hi per the RCA data sheet the 71A was originally designed to work with DC filaments from a storage battery or AC. In the amp I built the filaments are DC. The full schematic is in a very old thread on diy audio.

[usradcoll1]

Originally Posted by Whaam68

Hi per the RCA data sheet the 71A was originally designed to work with DC filaments from a storage battery or AC. In the amp I built the filaments are DC. The full schematic is in a very old thread on diy audio.

If you get a chance, please include a schematic, hand drawn is fine.
I'm interested in the circuit design. It seems a little novel!
Dave, US radcoll1

[ValvoStef]

Originally Posted by Whaam68

Thanks guys. I actually scratch built this a few years back from the grotty power supply chassis, a small box of random eBay mixed valves that happened to include a single 71A and the old school balloon rectifier. I already had a couple of small output tx and I was looking for a simple circuit to use all these bits to see what I could knock up and this one came along. It’s been an ornament for too long sat on a shelf but as a desktop amp driving an old pair of castle bookshelf speakers sounds very good. I will wire up the meters as suggested. The power supply had a couple of surplus valves and umbilicals so lots of holes to fill hence the meters. Even if not hooked in they look better than the empty holes.

So cool.

[Diabolical Artificer]

About the only way you can measure Ik only is to use a current tfmr, a shunted voltmeter ( fixed R and preset) with a current scale on the voltmeter Power the amp up with the 71A biased to cutoff, measure the filament current only as a voltage off the current tfmr, null this out with a fixed R and preset then figure out the I/V characteristics of the cathode EG x volts = x current transferring the result onto a readable scale.

You could insert an AC ammeter into the filament/cathode but then you still have to null out that portion of current needed to get the filament cooking.

Having re read what I've just written I may be talking a load of old tripe and over thinking things.

Metering does complicate nice simple tooob amps but if you grew up with Joe 90, Thunderbirds, Quatermass et al, ones amplifier doesn't look complete without several dials, hundreds of knobs and flashing lights.

Andy.

I suppose you could put a hall effect sensor somewhere on the output transformer and measure the current with it. It would end up being much more complicated than the amplifier. If it was me I would simply put in the HT feed ignoring David's "bad practice" comment as the meters look all plastic and glass on top.

[Skywave]

Originally Posted by Radio Wrangler

It's considered bad practice to put current metering in the high voltage side of power amplifiers (RF and AF ones)
David

That I have heard before - but have never understood the logic.
Can you explain, please?

Thanks,
Al.

As David (almost) said, meters are not real "class II" (double insulated) devices and could lead to a shock risk. Very unlikely in this case.

[Whaam68]

Hi All
Thanks for the input tbh I wasn’t expecting much interest!

Usradcoll1 I’ve attached the power supply schematic. The rest is in the firs post. From memory I made a couple of mods to suit what I had on the guidance of Philip at Bluebell audio and to further separate the power supply feeding each channel.

Andy yes I’m old enough to remember Joe 90 and quatermass but the John Mills 80’s version...not the black and white original!

I have read up on how to measure the anode current in fixed bias manually which seems to be measuring the dcr of the output tx primary then the voltage drop from b+ to anode using I=V/R....what I’m still not 100% on is how to meter it in circuit (simply).

So to recap just double checking all I have to do ....accepting the sensible advice re the meters (they are all plastic and I’d only ever use an insulated screwdriver to adjust) is to wire a meter in series with the anode of each of the 71A output valve? So not measuring the voltage drop across a 1r sensing resistor? That almost seems too easy!

Out of interest a pic of the original chassis is attached.

Mike

[Whaam68]

As you can see I won’t be winning any prizes for layout....but amazingly there is no audible hum. Doubling up elements of the power supply really gobbled up the space. I did start to do something with the meters but clearly wasn’t confident enough to hook them up. It’s been a few years since and the amps been sat on top of a cupboard gathering dust so I forget.

[Radio Wrangler]

Originally Posted by Skywave

That I have heard before - but have never understood the logic.
Can you explain, please?

Thanks,
Al.

Some meters have metal fixings for the set-zero screwdriver slot adjuster and there isn't much insulation creepage distance (sometimes none) With a meter spring connecting the coil, adjusters can run live. The insulation grade of most meters is not marked, so general good practice is to think carefully and check that you haven't created a risk. By all means, go ahead if you're sure you're OK.

There had been a few accidents with amateur transmitters with 2-3kV HT which stirred concerns.

It was just a general heads-up to be careful with meters floating at HT.

David

[usradcoll1]

Originally Posted by Whaam68

Hi All
Thanks for the input tbh I wasn’t expecting much interest!

Usradcoll1 I’ve attached the power supply schematic. The rest is in the firs post. From memory I made a couple of mods to suit what I had on the guidance of Philip at Bluebell audio and to further separate the power supply feeding each channel.

Andy yes I’m old enough to remember Joe 90 and quatermass but the John Mills 80’s version...not the black and white original!

I have read up on how to measure the anode current in fixed bias manually which seems to be measuring the dcr of the output tx primary then the voltage drop from b+ to anode using I=V/R....what I’m still not 100% on is how to meter it in circuit (simply).

So to recap just double checking all I have to do ....accepting the sensible advice re the meters (they are all plastic and I’d only ever use an insulated screwdriver to adjust) is to wire a meter in series with the anode of each of the 71A output valve? So not measuring the voltage drop across a 1r sensing resistor? That almost seems too easy!

Out of interest a pic of the original chassis is attached.

Mike

Thanks for furnishing the schematic of both the power supply and the amplifier! It's a slightly different approach to old technology.
Keep up the good work.
Dave, USradcoll1