How To Match Power Tubes
Example: Here is the procedure for the ARC VT50 when retubing the amp. https://www.arcdb.ws/model/VT50 Trim pots for biasing the 6922 input and driver tubes are found behind the left and right channel side panel cover plates of the amp. FWIW, nothing is mentioned in the VT50 manual about biasing the 6922 input and drivers tubes when installing new 6922 tubes. VT50 Wiring schematic diagram. https://www.arcdb.ws/Database/VT50/ARC_VT50_schematic_and_parts_list.pdf
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What model ARC amp do you have? The VT series therein, VT50, VT100, VT100 MK 2 series requires the 6922 signal tubes must also be biased. I believe, looking at the schematic diagram for the ARC CA50, the 6922 tubes also require biasing. Especially the 6922 driver tube. The 6922 input and driver tube must be closely, tightly, matched. Especially each section of the driver tube. If each section of the input and driver tube are not tightly matched you will not be able to bias the tubes to meet the required ARC specs. https://www.arcdb.ws/Database/CA50/ARC_CA50_schematic_and_parts_list.pdf
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Hello to everybody.
I’m from Italy, the name of my Company is dBevolution. I would like to write something about my experience with Maximatcher and the Sales Manager Mr. Peter Toms. I ordered and paid one Maximatcher and one Maxipreamp on 8 November 2021 (order No. 606). Today 28 May 2022 (after more than 6 months) I didn’t receive the instruments, Italian customs told me that the attached documentation from the seller was incomplete and the parcel was sent back to the seller MORE than 3 months ago, but Mr. Peter didn’t refund me. I sent him many email, but usually he reply to me after some days (probably it’s not his interest to resolve the problem), and he never sent me a refund and I never received the instruments. Lately he stopped to reply to me (he don’t reply anymore). If the situation will be not resolved, I will take a legal action.
So…pay attention before to buy from him |
I have to disagree with the methods discribed to match tubes. You can not match tubes using a tube tester along " (AVO 160, Hickok 539b & TV DU/7) ". You do need a device to host the tube that's being tested for matching. That device could be an amp or a tube tester. You then need a way of measuring bias current. You can buy a bias probe and meter on ebay, or you can make a bias probe cable and use a multi meter or you can modify your amp to behave like a bias probe by adding resistors which alow the measurements to be made. So lets say you measure your tubes....and you make this simple....lets say you use a tube tester to host the tubes and in adition you use a bias probe. So you run 4 tubes thru the process and you find you have IP (bias current readings from probe) of 24.1, 23.9, 22.9, 23.1 and you find your tube tester says the transconductance values are GM 15000, 14900, 14000, 13900. what does this mean. well it means you have two matched sets. The pair that comprise of IP of 24.1, 23.9 with GM's of 1500, 14900 and the pair that comprise IP 22.9, 23.1 and GM's of 14000, 13900 are the two matched sets that must be used together . In the example I gave, we are testing 6550s' which have an expected transconductance value when new on my tester of 11000 which would make all tubes in this example score 126% or better. |
By the by, does anyone know someone who repairs Hickok equipment? There is an open circuit somewhere and it won't measure pentodes properly. There's probably a thousand solder joints in this thing, and 1/2 mile of wire. I would have take up methaphetamines to even consider trying to figure it out. The schematic looks like one of those optical illusion drawings. |
For matching power tubes, short of a curve tracer, a Maximatcher would be the best and easiest to use. It measures plate current and transconductance at two different plate voltages and has several bias voltages available. At 425V plate, it provides a more "real world" test than most tube testers for 6550 type tubes. |
Dennis, I got into this a while ago when I owned an ARC VT130se. This particular amp required 4 matched 6550's since the bias adjust controlled all 4 of them. There were 8 total. After contacting a couple of tube persons and waisting time and money on testers, (AVO 160, Hickok 539b & TV DU/7) I came to the conclusion that you're better off finding a trusted source to do this for you with power tubes. I had good luck with Jim Mcshane and Andy Bowman at Vintage Tube. I would have to agree that the maxi matcher would be your best bet if you were going to try this yourself. Note that I still use my testers for testing tube condition and signal tube comparisons. As I understand it, an accurate way to see if the tubes are matched is to test their plate current while they are idling on your amp. Tube testors don't have the equivelant power that your ARC amp probably has. I have been told this by a number of authorities on this. You would have to collect a number of tubes to find a matched set. I've been told that the plate current at the Amp's equivelant current level is what you match to. Regarding matching, note that ARC uses a push pull method for its power tube - that's why they should be matched. The tester that I like is the TV7 D/U. There's a lot of them out there and there's a well respected person, Dan Nelson that calibrates them. If you need further sharing of my experiences with this, drop me an email. I don't consider myself an authority on tubes but I have played around this stuff quite a bit. Steve |
it seems that your tube amp does not have bias adjust screws on the outside of the chassis? The type can be adjusted clock-wise/anti-clockwise using a screw-driver. Is this a correct assumption on my part? If you do have such bias adjust screws, you don't really need matched tubes. If you don't have these bias adjust screws, I've heard from people who have vastly more tube experience than me that you STILL do NOT need to have matched tubes! Tubes mismatched as much as 20% work just fine & the sound is not compromised. So, is it possible that the technician simply scared you for no reason at all? Someone correct me if I'm wrong. Thanks! |
