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Post by andychampionsound on Mar 10, 2021 10:42:57 GMT 12
hey all, it's been a long and painful wait for my Audio Research power amp to be re-valved, but once I do get it back, with a fresh compliment of approved valves, I'll want to stay on top of the valves. I can't say for sure it was the cause, but one tube arc'ed and blew a couple of resistors, and I'd been running it for a year or two without ever checking the valves, so I intend to improve my behaviour!
So I'm looking at al the options for multimeters from the likes of jaycar, and wondering if anyone has a preference or recommendation on a good option? TIA.
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Post by Graham on Mar 10, 2021 11:04:53 GMT 12
Either the Digitech QM1529 for $32.90 or the QM1321 for $46.90 from Jaycar will provide all the features and accuracy you will ever need.
I have the latter and use it regularly.
Cheers Graham
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Post by Owen Y on Mar 10, 2021 12:08:44 GMT 12
ARC amps typically have over 400vdc plate voltage. Although the biasing of output tubes usually involves measuring a much lower voltage across say a small resistor, you'd want a DVM with at least 600vdv rating. Especially if you have to probe inside the amp underside, as I expect in the VS55 (is this still your ARC amp?). So, of Graham's suggestions, I'd go for the 600vdc rated one, the Digitech. Check feedback maybe, on this DMM though. It's also supposed to be Cat III rated at this voltage. However its accuracy spec is not as good. Edit - I see that the QM1321 is CAT II safety rated to 1000v & has a lifetime wty, so that sounds pretty good too. It uses one of those pesky expensive 9v batts though, as opposed to a couple of AAAs, although it should auto-switch off.
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Post by Owen Y on Mar 10, 2021 12:27:08 GMT 12
You may want to check out Dave's EEV Blog video on our Digital Multimeter Buying Guide for Beginners thread. Dave doesn't mince his words but don't get hung up too much on all the specs, just note what he says about safety at around 30:00.
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Post by RdM on Mar 10, 2021 18:40:19 GMT 12
You may want to check out Dave's EEV Blog video on our Digital Multimeter Buying Guide for Beginners thread. Dave doesn't mince his words but don't get hung up too much on all the specs, just note what he says about safety at around 30:00.
Owen, some sort of typo:- your ink just links back to this thread, at present!
Just browsing 'other listings' from unrelated this afternoon, and happened to see this - if you just want a cheap one that also does 600V (0.8% accuracy) there's this ($16.99 & post):
I'm just noting it, not recommending it! Might be others worth looking at in that category, I haven't looked.
Having been away for some days, I need to catch up!
~ Ross
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Post by Owen Y on Mar 10, 2021 19:42:26 GMT 12
Hi RdM - sorry chaps, link now fixed.
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Member
Post by andychampionsound on Mar 16, 2021 9:21:02 GMT 12
thanks for the suggestions team, much appreciated. Jaycar is just down the road from work, and I'm hopefully picking up the VS55 with fresh compliment of tubes this afternoon, so looks like this should be easy enough!
As an aside, Getting the tubes supplied from Audio Research comes with some comfort and promises of great things, but my god it's expensive!
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Post by Owen Y on Mar 16, 2021 10:28:17 GMT 12
andychampionsound - One little trick that I used to use is, before taking out the old power tubes, is to first figure out which way the trim pots rotate to turn down the bias for ea tube. Then, turn down the bias a bit before swapping in new tubes. That way, when you fire it up with 4 new 6550s, you don't have to immediately fret about over-current & cooking new tubes. (The new tubes will vary a bit, plate current-wise, compared with old tubes & with different brands.) I see now that the VS55 has bias trimpots located conveniently on the top plate & the bias voltage test points on the rear panel. That's much safer than having to get inside, like on their older amps. Let us know how you get on 
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Member
Post by andychampionsound on Mar 16, 2021 14:17:32 GMT 12
Thanks Owen, unfortunately I just got the amp back with a compliment of new tubes from AR via Soundline, and the amp is already biased for these, so no old tubes to experiment on. I cannot overstate how absolutely deliriously happy I am to have my amp back, but if ever there was a crowd to get where i'm coming from, I figure you all know how it feels! And so begins a record playing binge! andychampionsound - One little trick that I used to use is, before taking out the old power tubes, is to first figure out which way the trim pots rotate to turn down the bias for ea tube. Then, turn down the bias a bit before swapping in new tubes. That way, when you fire it up with 4 new 6550s, you don't have to immediately fret about over-current & cooking new tubes. (The new tubes will vary a bit, plate current-wise, compared with old tubes & with different brands.) I see now that the VS55 has bias trimpots located conveniently on the top plate & the bias voltage test points on the rear panel. That's much safer than having to get inside, like on their older amps. Let us know how you get on
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Post by RdM on Mar 16, 2021 18:35:14 GMT 12
I was curious, still working out bias adjustments for the Wurlitzer amp, and saw www.arcdb.ws/model/VS55which in the right hand column shows: "Change of output tubes (2007)
VS55 had their tubes changed from 6550EH (Electro Harmonix) to 6550C (Svetlana) at the production line. The output tube bias was changed to 55mV, providing better sonics and improved reliability."I also downloaded the owners manual and service manual (really just a schematic). The earlier tubes/valves were set for 65mV (65mA, measured across a 1 ohm cathode resistor). Owen will I think have meant if you had had the chance with old tubes, bias to a lower value for a new starting pre-adjustment point, to be safe. Hopefully and presumably Soundline will have set all these new tubes/valves up nicely for you, so no need to worry, but you can at least check now every few months (?) how they are doing with your own multimeter, and feel more assured, and slightly tweak if found necessary. Well, that might not be so easy, since multimeters have typically thin probes, and the bias test banana jacks (their words) are much larger... I would buy a pair of banana plugs (Jaycar?) that had grub screw securing for the cable, insert the probes into those, screw down securely. I assume you have the owners manual, but in case not: audioresearch.com/ar-package/vs55-manual-pdf/audioresearch.com/ar-package/vs55_biasadjust-pdf/And for the schematic, circuit diagram, pass a Captcha test, and download from elektrotanya.com/audio-research_vs55_amplifier_sm.pdf/download.htmlImpressed! I still want to build my own valve amplifier(s), so it's all education and inspiration. Cheers! ;=})
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