VTVM doubt - Page 1
Forum section: electronics / testgear

author: SAUL BRITTO
author_rank: Frequent Contributor
timestamp: March 08, 2016, 01:56:36 pm
content: Hi all. Somebody said me that two 1N40xx can replace a 6AL5 in a VTVM has anybody tried this with a good result?Saul.

author: PA0PBZ
author_rank: Super Contributor
timestamp: March 08, 2016, 03:41:38 pm
content: It depends I think. If it is used as a mains rectifier go for it, but if it is some special secret function it could be that you have to make some adjustments to get the correct indications on the meter. It will not blow up or destroy your house.

author: SAUL BRITTO
author_rank: Frequent Contributor
timestamp: March 08, 2016, 03:49:52 pm
content: Quote from: PA0PBZ on March 08, 2016, 03:41:38 pmThe 6AL5 is the detector diode in a VTVM.I think that just works well with fast smal signal diodes.

author: phoenixcomm
author_rank: Newbie
timestamp: March 08, 2016, 04:14:24 pm
content: The 6AL5 is just a twin diode. No biggie but why bother?There cheap. And there is nobut if it is some special secret functionabout them, it is just a twin diode.If you do replace them make sure to check your output voltage of the new rectifier (smaller voltage drop) as is will be a little higher, so you might need a small resistor to adjust the voltage.C. Harrisonthis is fromhttp://www.soldano.com/amp-help/tube-vs-solid-state-rectifiers/Please pay attention to the underlined bits..A tube rectifier has internal resistance. The more current that travels through a tube rectifier, the more the voltage drops. When the voltage drops, the power of the amplifier also drops. The tube rectifier has the drawback of not being able to provide a consistent voltage when it’s under load. The other drawback is that the tubes themselves run hot, and can be relatively short-lived. Unfortunately, modern day sources for rectifier tubes are not very reliable and, even in their prime, these tubes were usually the weak link in most amplifiers.An amp with a tube rectifier tends to sound much spongier in the bottom end. Low-frequency notes take more current through the power tubes to reproduce. This increased current causes a voltage drop in the rectifier tube and the amp loses power. So, when more power is actually needed, the amp gives less. Because of this, a tube rectifier amp will sound spongy and more distorted at high volumes. This, probably more than anything, is what gives a vintage amp its sound and color.A solid state rectifier has no internal resistance whatsoever. It has a very consistent fixed voltage drop that occurs both at zero or full current (approximately .7 volts).When an amplifier needs power at low frequencies, there will be no limit to the current that travels through the rectifier. This results in an amp with more headroom that is punchier, more articulate and able to deliver the goods in the bottom end.

author: FlyingHacker
author_rank: Frequent Contributor
timestamp: March 08, 2016, 04:25:39 pm
content: In looking at your quotes there, phoenixcomm, isn't this one of the advantages of a VTVM? This behavior helps protect the meter from stupid overloads, or even (predictable but) unforeseen dumps of inductive overloads.I think replacing the mains rectifier with something solid state could work OK. But, as was mentioned, why bother? Most of my VTVMs have their original tubes from the 50s and 60s.

author: Ian.M
author_rank: Super Contributor
timestamp: March 08, 2016, 04:28:22 pm
content: Quote from: SAUL BRITTO on March 08, 2016, 03:49:52 pmEven with an appropriately chosen diode and a series resistor to match, its likely to upset the calibration on the lower part of each range due to differences in the non-linearity of the If vs Vf characteristic between a thermionic and a silicon diode.However as a6AL5is only rated for 9mA continuous, 54mA repetitive pulse, and was specified for use as a FM detector (typically at an IF of 10.7MHz), you can be certain that a 1N40xx 1A power diode which isn't even satisfactory much above 15KHz is a very poor substitute.Quote from: phoenixcomm on March 08, 2016, 04:14:24 pmThat is an over-simplification to the point of being grossly inaccurate. *ALL* silicon rectifiers have a finite, measurable slope resistance, which although it can be fairly small, and varies with device construction, forward current and temperature is certainly not ZERO! Vf also varies with current and is *NOT* a fixed 0.7V.(0.7V Vf is approximation that is little better than a rule of thumb for back-of-the-envelope circuit analysis.)In fact, that link reads as Audiophool bullsh-t.

author: FlyingHacker
author_rank: Frequent Contributor
timestamp: March 08, 2016, 04:30:55 pm
content: Quote from: Ian.M on March 08, 2016, 04:28:22 pmWould the frequency response be important in the VTVM? For higher frequencies you be using an RF probe, no? Not disagreeing; just trying to expand my knowledge here.

author: Ian.M
author_rank: Super Contributor
timestamp: March 08, 2016, 04:39:55 pm
content: It depends. I'd need to see the original specs of the VTVM to be certain. However, when the frequency response of the possible replacement is 2 to 3 orders of magnitude worse, one expects trouble!I suspect that a pair of low leakage Schottky signal diodes would be a better choice, although some circuit mods may be needed as well.

author: PA0PBZ
author_rank: Super Contributor
timestamp: March 08, 2016, 04:50:33 pm
content: Quote from: phoenixcomm on March 08, 2016, 04:14:24 pmI was going to react to this, but I see that you already did it yourself and several others after you.

author: SAUL BRITTO
author_rank: Frequent Contributor
timestamp: March 08, 2016, 04:59:09 pm
content: Quote from: Ian.M on March 08, 2016, 04:39:55 pmI agree with you Schottky would be the better choice. In truth this about 6AL5 and SL diodes are just curiosity or a good topic to discuss.As I do have one VTVM a kind of Frankenstein here I will try this ideia and return the result soon.

author: SAUL BRITTO
author_rank: Frequent Contributor
timestamp: March 08, 2016, 05:05:09 pm
content: I don't know if all of you agree with me but, VTVM has some problems in low voltages,naturally a limitation of your detectors,is this that bother me.

author: Ian.M
author_rank: Super Contributor
timestamp: March 08, 2016, 05:35:54 pm
content: Sure, it has limitations at a small percentage of FSD on the range selected, but a commercially produced VTVM should have a scale that is calibrated to compensate for the published forward characteristic of the detector diode used, so the only remaining issues should be device to device variation and ageing.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 09, 2016, 04:51:23 pm
content: Substitution with 1N4148 would work well enough, but it wouldn't be best.6AL5 works down to -0.6V or so. (Yes, negative.The heated cathode acts as a heat pump, much as light causes a photodiode to generate a little power.) For currents under 1mA or so, it has significantlybetterperformance than silicon or schottky diodes.So with this substitution, you'll have something of a hole on the lowest AC voltage range.My VTVM has a pretty reasonable 1.5V scale; you'd have an offset of about -1V (including a deadband for <1V).There's also the matter of breakdown voltage; 6AL5 is rated for 330V, and probably arcs over at well more than that. Whether the full AC input is applied to the diode, will depend on the meter design though.Tim

author: SAUL BRITTO
author_rank: Frequent Contributor
timestamp: March 09, 2016, 06:21:41 pm
content: Quote from: T3sl4co1l on March 09, 2016, 04:51:23 pmCoherent Tim.Very good explanation thank you.

author: phoenixcomm
author_rank: Newbie
timestamp: March 09, 2016, 08:25:35 pm
content: Hold on please!You VTVM's Power Supply ie. (Transformer, Tube/Rectifier) not including the filter ie. ( Choke[if any], Capacitors) have little to do with the basic specifications of your VTVM.QuoteNO!Stupid overloads (voltage/current) canDESTROYyour meter The power supply has nothing to do with it!!Please seehttps://www.element14.com/community/videos/15419/l/german-safety-test] https://www.element14.com/community/videos/15419/l/german-safety-testThe sensitivity is from your meter's detector circuit ie. (Wheatstone bridge null detector) and how closely and with which precision you matched the resistors on the range switch.Cris H.

author: phoenixcomm
author_rank: Newbie
timestamp: March 09, 2016, 08:59:33 pm
content: Hold on please!You VTVM's Power Supply ie. (Transformer, Tube/Rectifier) not including the filter ie. ( Choke[if any], Capacitors) have little to do with the basic specifications of your VTVM.QuoteNO!Stupid overloads (voltage/current) canDESTROYyour meter The power supply has nothing to do with it!!Please seehttps://www.element14.com/community/videos/15419/l/german-safety-test]https://www.element14.com/community/videos/15419/l/german-safety-test] https://www.element14.com/community/videos/15419/l/german-safety-testThe sensitivity is from your meter's detector circuit ie. (Wheatstone bridge null detector) and how closely and with which precision you matched the resistors on the range switch.Cris H.BTW1. A 1N4001 will not work. but 1N4005/6/7 will.http://www.diodes.com/_files/datasheets/ds28002.pdf] http://www.diodes.com/_files/datasheets/ds28002.pdf Please look at the Reverse Voltage RMS figures. If it were me I would use a 1N4007 and get over it. Remember that tubes run on the High Voltage that is the what the secondary of the transformer is rated at.2.QuoteOkI see Ian that you have no Idea about tubes used in stereo gear!Tubes have a warmer sound and yes bigger base response.The tube will have better harmonics..I hate the 60hz in an amp and alway trying to kill it.Or just read for yourself:http://kenrockwell.com/audio/why-tubes-sound-better.htm.

author: FlyingHacker
author_rank: Frequent Contributor
timestamp: March 09, 2016, 10:38:09 pm
content: Quote from: phoenixcomm on March 09, 2016, 08:25:35 pmOf course the power supply has nothing to do with it. I was not referring to the power supply. I meant the input circuit can help protect the meter movement (at least vs. a VOM). While most VTVMs were kits, and some have questionable assembly I would still put most of them up against a cheap DMM any day.

author: CustomEngineerer
author_rank: Frequent Contributor
timestamp: March 10, 2016, 03:31:48 am
content: Quote from: phoenixcomm on March 09, 2016, 08:59:33 pmFrom link:These are subtle effects most audible to musicians and very dedicated music lovers; casual listeners (people who "listen" with their eyes open while doing something else) usually won't notice, but sometimes the difference is so obvious that people's wives will comment that "wow, that sounds much better" when people use tubes at home.Well, that certainly proves it.

author: vk6zgo
author_rank: Super Contributor
timestamp: March 10, 2016, 03:56:05 am
content: A whole lot of people have gone off at a tangent about tube rectifiers.A 6AL5 is a twinsignal diode,It is used to provide an ac range on a VTVM.It is not a power supply rectifier--some VTVMs were made that operated from batteries.1N4005,7s,and the like are power rectifiers.As such,they are not suitable to replace a 6AL5.

author: med6753
author_rank: Super Contributor
timestamp: March 10, 2016, 04:25:10 am
content: Quote from: vk6zgo on March 10, 2016, 03:56:05 amThe other common application for the 6AL5 is ratio detector in FM tuners.

author: vk6zgo
author_rank: Super Contributor
timestamp: March 10, 2016, 05:53:24 am
content: Quote from: med6753 on March 10, 2016, 04:25:10 amYep,& sometimes for noise clippers in HF radios,or in TV keyed clampers,or any of a myriad uses,before solid state took over.

author: med6753
author_rank: Super Contributor
timestamp: March 10, 2016, 07:39:24 am
content: There were commercial solid state replacements for the 6AL5 as well as several other vacuum tube types. But good luck finding one today.Here's an example:http://www.philipstorr.id.au/radio/technical/The_Fetron.pdf

author: SAUL BRITTO
author_rank: Frequent Contributor
timestamp: March 10, 2016, 07:54:17 am
content: Quote from: med6753 on March 10, 2016, 07:39:24 amI didn't know this device.