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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets. |
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19th Feb 2021, 12:40 pm | #1 |
Hexode
Join Date: Feb 2007
Location: Luton, Bedfordshire, UK.
Posts: 456
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Output Valve bias meter? how to wire in correctly?
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 |
19th Feb 2021, 1:34 pm | #2 |
Moderator
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Location: Fife, Scotland, UK.
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Re: Output Valve bias meter? how to wire in correctly?
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
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19th Feb 2021, 1:38 pm | #3 |
Dekatron
Join Date: Apr 2011
Location: Cornwall, UK.
Posts: 13,454
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Re: Output Valve bias meter? how to wire in correctly?
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. |
19th Feb 2021, 3:32 pm | #4 |
Hexode
Join Date: Feb 2007
Location: Luton, Bedfordshire, UK.
Posts: 456
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Re: Output Valve bias meter? how to wire in correctly?
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.
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19th Feb 2021, 4:08 pm | #5 |
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Re: Output Valve bias meter? how to wire in correctly?
I like it!
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20th Feb 2021, 2:34 am | #6 | |
Heptode
Join Date: Mar 2017
Location: Cedar Grove, Wisconsin, USA.
Posts: 823
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Re: Output Valve bias meter? how to wire in correctly?
Quote:
Anyway, it's a very attractive job and proves what you can do with parts on hand. Dave, USradcoll1 |
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20th Feb 2021, 8:24 am | #7 |
Hexode
Join Date: Feb 2007
Location: Luton, Bedfordshire, UK.
Posts: 456
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Re: Output Valve bias meter? how to wire in correctly?
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.
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20th Feb 2021, 11:29 am | #8 |
Hexode
Join Date: Feb 2007
Location: Luton, Bedfordshire, UK.
Posts: 456
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Re: Output Valve bias meter? how to wire in correctly?
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 |
20th Feb 2021, 6:09 pm | #9 | |
Heptode
Join Date: Mar 2017
Location: Cedar Grove, Wisconsin, USA.
Posts: 823
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Re: Output Valve bias meter? how to wire in correctly?
Quote:
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. |
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20th Feb 2021, 6:55 pm | #10 |
Hexode
Join Date: Feb 2007
Location: Luton, Bedfordshire, UK.
Posts: 456
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Re: Output Valve bias meter? how to wire in correctly?
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.
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21st Feb 2021, 12:30 am | #11 | |
Heptode
Join Date: Mar 2017
Location: Cedar Grove, Wisconsin, USA.
Posts: 823
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Re: Output Valve bias meter? how to wire in correctly?
Quote:
I'm interested in the circuit design. It seems a little novel! Dave, US radcoll1 |
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21st Feb 2021, 3:07 am | #12 | |
Hexode
Join Date: Mar 2014
Location: Fareham, Hampshire, UK.
Posts: 482
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Re: Output Valve bias meter? how to wire in correctly?
Quote:
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21st Feb 2021, 9:39 am | #13 |
Dekatron
Join Date: Jul 2009
Location: Sleaford, Lincs. UK.
Posts: 7,657
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Re: Output Valve bias meter? how to wire in correctly?
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.
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21st Feb 2021, 1:50 pm | #14 |
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Re: Output Valve bias meter? how to wire in correctly?
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.
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21st Feb 2021, 2:47 pm | #15 |
Rest in Peace
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Re: Output Valve bias meter? how to wire in correctly?
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21st Feb 2021, 3:29 pm | #16 |
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Re: Output Valve bias meter? how to wire in correctly?
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.
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21st Feb 2021, 4:11 pm | #17 |
Hexode
Join Date: Feb 2007
Location: Luton, Bedfordshire, UK.
Posts: 456
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Re: Output Valve bias meter? how to wire in correctly?
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 |
21st Feb 2021, 4:26 pm | #18 |
Hexode
Join Date: Feb 2007
Location: Luton, Bedfordshire, UK.
Posts: 456
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Re: Output Valve bias meter? how to wire in correctly?
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.
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21st Feb 2021, 4:40 pm | #19 | |
Moderator
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Location: Fife, Scotland, UK.
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Re: Output Valve bias meter? how to wire in correctly?
Quote:
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
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Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done |
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21st Feb 2021, 7:44 pm | #20 | |
Heptode
Join Date: Mar 2017
Location: Cedar Grove, Wisconsin, USA.
Posts: 823
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Re: Output Valve bias meter? how to wire in correctly?
Quote:
Keep up the good work. Dave, USradcoll1 |
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