are my tubes matched or not ?

[FatGaz]

I have a traynor yrm-1 amp and I've put a pot in to adjust the shared fixed bias to the pair of 6ac7/el34 tubes.

the old tubes were sounding tired so I bought a new 'matched pair' of EH 6ca7 but i can get them biased properly with the shared bias voltage. I checked the bias is the same on each socket.

with a plate voltage of 430V for 70% idle current i should be at around 40-41mA per tube.

BUT, measuring across 10ohm cathode resistors, I get
0.363V and 0.198V
swap the tubes in the sockets and then i get
0.200V and 0.398.2V (may have bumped the bais pot when swaping)

Second thing I try is to increase the bias voltage (less negative) and I get
0.400V and 0.528V with a bias of - 38.5V
dropping down again I get
0.194V and 0.374V with a bias of - 43.5V

SO Are my tubes badly matched ???

Should I install separate bias pots for each tube ???

The shop I got the tube from don't believe my measurements. Further when I gave the tubes back to be tested they claimed that they were matched quoting currents of 59 and 57 mA on their tube tester at 430V.

What's going on ? Is my amp bad ? is the shop lying to me ???

Help !

Gareth.

Ps i double checked the resistance of the cathode resistors both 10 ohms.

[Amp Kat]

Most tube testers don't check matched parameters. It takes a special set-up or at least a bias rite and I highly doubt a Distribtion store has one. You're tubes are not matched and they don't want to except the fact.

[FatGaz]

Most tube testers don't check matched parameters. It takes a special set-up or at least a bias rite and I highly doubt a Distribtion store has one. You're tubes are not matched and they don't want to except the fact.

So, do you think I should return them ?

G.

[Enzo]

Power off and discharge the amp.

FInd the output transformer center tap connection - it will be the + connection at your main rectifiers, where it meets the first filter cap.

Now measure resistance to each power tube pin 3 from that point. You are measuring the resistance of the output transformer winding. if they are about the same, then your tubes are not so well matched. If the readings differ, then that is part of why the tubes have different cuurent draw. The winding resistance is effectively a series resistor.

[Robert M. Martinelli]

Hi,

What kind of resistors did you put in series between cathode and GND to measure the voltage drop ?

If you want to get accurate measurements, then you have to use precision resistors (1% or better).

I was also wondering why you're using 10 Ohms resistors, you'd better use 1 Ohm ones, so the voltage drop across each resistor (in mV) will be equal to the cathode current flowing (in mA), making for a simpler procedure.

About the cathode current reading you're getting, you should also remember that, in pentodes, the cathode (=total) current is the sum of plate and screen grid current, so, to correctly state the real amount of the sole plate quiescent current, you have to measure the screen grid current (by measuring the voltage drop across each screen grid resistor, and dividing it by the screen resistor's value) and subtract it from the cathode (=total) quiescent current flowing.

BTW, Enzo's got a good point (as usual), the two halves of the primary winding aren't built to be symmetrical "DC-wise", but only to oppose the same impedance to the output tubes, so their DC resistance usually differs somewhat, making for a different DC quiescent (bias) current, should this be the main reason for the different current reading, if you're willing to bias the output tubes at exactly the same point, you'll need to install two separate bias pots.

Hope this helps

Best regards

Bob

[FatGaz]

Thanks for the advice.

I thought that by swaping the tubes between the sockets and still measuring a large difference in idle current ( the difference following the tubes and not the sockets), that issues with gross asymmetry in the amp circuit are thus unlikely ?

I'm vaugely aware that the PI is not quite symmetrical, wasn't aware that the OT is often different too. I'll measure every thing again to see if there is anything odd, including the PT. I did check voltages before and they match those quoted on the schematic.

Question I have now is, do I put two bias circuits + pots in the amp to set the idle currents around the same point ?
In other words, are these tube so badly matched that I shouldn't even bother trying ? (and have an unplesant argument trying to get a refund)

thanks all !

Gareth.

PS, 10 ohms was what was to hand at the time, I did measure their resistance to double check. (0.3V =30mA isnt hard maths !) I could try to get 1 ohms in case this might be throwing the circuit, but 0.3V -vs 430V still seems insignificant.

[pdf64]

It's unusual to be matching at the tube's max dissipation 25 watts.
Re the cathode resistors, be aware that when the amp is cranked, the tubes will be passing >200mA, so a 10ohm 0.5watt will be getting too close to it's dissipation rating for a long life with good stability / accuracy (and will cause a slight amount of local negative feedback, unless bypassed). Whereas a 1ohm 0.5 watt will be fine.
A bias pot for each push/pull side is a good thing.

[MWJB]

Gareth wrote "I'm vaugely aware that the PI is not quite symmetrical, wasn't aware that the OT is often different too. " Why isn't the PI symmetrical, have you changed something in it? The typical BF Fender style LTPI (which is what the YRM1 has) is usually pretty symmetrical, to the point that you have to start changing component values to unbalance it...even with a non-balanced PI tube installed.

If your plate current is much more than +/- 5mA adrift then you might start noticing a drop in fidelity...not everyone dislikes this. Mismatched plate current will have more sonic effect that any imbalance in the PI circuit.

Peter wrote: "HotRox or Watford Valves will accept them back for re-testing if you've got concerns." Watford certainly will, but not my experience with Hot Rox, they claimed that it was not possible to match tubes in an amp !?!

[Steve Conner]

Hi Gaz,

Just a thought: You measured the bias voltage and it was the same at both grid pins, right? But once the tubes were installed and running, did the grid pins still both show the same voltage? Or is one of the tubes maybe leaking grid current and pulling its bias voltage out of spec? Does the bias voltage continue to drift as the tubes warm up?

If so, maybe your bias feed resistors have drifted up. Or maybe you should change them to 100k, if they were 220k. 220k was always marginal for EL34s.

The Hot Rox thing is pure bullsh!t on their part. They match tubes for you so that they'll be matched in your amp! That's the whole point of it. I think what they mean is that if they don't match in your amp, there's something wrong with the amp that needs fixing. But in that case, the problem would stay with the socket, and yours followed the tube. Grid current is the only explanation I can think of for that.

Maybe you should get another pair of matched tubes from the "other" supplier and try again :-) If they work, you have another stick to beat your existing supplier with: "These other tubes work fine"

[Enzo]

Yes, if the difference follows the tube, then my scenario in the transformer doesn;t apply. But be aware of it for future reference.

[Amp Kat]

Well I've done this so many times as some of you other guys have also when you see the large mismatch to only realize that the box marked match really isn't matched afterall. Retailers are strickly going by what their distributors tell them and send them and Distributors are not always the most reliable source as even some tubes marked matched and tested are bad out of box. Pulling tubes out of similar lots is just flat laziness and often the culprit at the end-users expense unfortunately but I do like Steven's recommendation of being sure and getting a good known matched pair from a reliable source like Eurotubes or maybe Dougs and make absolutely sure that you have the current sense resistors acting and measuring correctly.

[pdf64]

Sorry, I edited out my comments re Watford Valves and HotRox, as I hadn't read the original post properly (which says the tubes have been bought from, and re-tested, by a shop).
Given the low quality and fragility of most modern tubes, and careful handling of the post office and couriers, it can't be easy selling tested tubes. I can sympathise with them wanting to screen out spurious claims, but it get's irksome when your own claim gets rejected.

[Steve Conner]

Oh PS, if you're totally stuck, you could always send the offending tubes up here for a second opinion, I'll test them out in the lab and send them back. It would be worth knowing if there's a UK tube supplier peddling crap.

But I'd suggest trying another matched pair from another supplier first.

[FatGaz]

Sorry, I edited out my comments re Watford Valves and HotRox, as I hadn't read the original post properly (which says the tubes have been bought from, and re-tested, by a shop).
Given the low quality and fragility of most modern tubes, and careful handling of the post office and couriers, it can't be easy selling tested tubes. I can sympathise with them wanting to screen out spurious claims, but it get's irksome when your own claim gets rejected.

Peter wrote: "HotRox or Watford Valves will accept them back for re-testing if you've got concerns." Watford certainly will, but not my experience with Hot Rox, they claimed that it was not possible to match tubes in an amp !?!

err, what ever your comments were, they may well be relevent to my experence! : these tubes are mailorder from HR. First time I've used them - had used W before. Statements HR made on the phone were not helpful, and numbers quoted to me were " '57' and '60' on our tube tester", yet the tubes arrive back with 57.7 and 57 newly written on them. The olny reason I ended up with the tubes being sent back was beause he flat refused to replace them. We didn't get to the refund discussion.

At least I won't go bankrupt over just a pair of el34's.

I'll swap the 10 ohms for 1 ohms and I'll swap the 220Ks out too for 100Ks. At the same time I'll re-jig the bias to give independent controls.

Thanks for all of the advice.

cheers for the testing offer steve, I think i will get another pair from W. I'll make local inquires to see if I can get the tubes tested/measured somewhere else first though. these tubes are in danger of getting frequent flyer miles.

Gareth.

[Wakculloch]

Keep us all informed on how you get on. It's interesting to know how the British suppliers stack up.