TJ 2A3 mesh plate bias

[Prairie Dog]

Hi everyone,  I'm having trouble with a pair of TJ 2A3 mesh plates I bought from diyhifisupply a couple of months ago, problem was right out of the box, though after several emails back and forth, they seem to think it's an issue with my monoblocks.  Here is a partial copy of one of my emails to them:

"I could not set the bias on the tube marked F254 to a value any better than -30V or so, with a plate voltage of 295V.  Recommended is -45 bias with plate voltage of 250V.  The tube marked F288 can be set to -44V, but this is at the maximum range of adjustment on my amp's bias setting.  This gives a plate voltage of about 268 volts."

This is actually the second pair of tubes from them, the first had an obvious defect in one of them, and it took forever to get them to replace the pair, I'm wondering if he is just brushing me off on this pair also?  He suggested there are ways to "adjust" the Paramounts to work better with their tubes, but has yet to offer any thing else.

FYI I've been using a pair of Shuguang 2A3Cs for 2 years and they set up beautifully: Vp 250V   bias -45V   Similar results with Russian variety.  So, they've had my money since March, and I'm still not able to use their product.

Any help is much appreciated.

As a sidebar, I built an Eros phonostage a couple months ago: huge improvement in all areas.

Thanks,
-Trevor

[Paul Joppa]

The amp you are using does not appear to be Paramount. So we'll need more information to understand your question.

[Prairie Dog]

Hi Paul, what info would you like?  I have Paramounts with soft start update.  Here are some more specific voltages:

with Shuguang 2A3Cs:

amp 1    A1 217.5             amp 2   A1 219.6
            A2  468                          A2 468
            A3  170                          A3 172
            A4  215.2                        A4 217.3

with TJ 2A3 mesh:

amp 1    A1 171                amp 2   A1 190
            A2 479                            A2 475
            A3 155                            A3 158
            A4 169                            A4 188


I'm using the driver tubes supplied with the soft start.

-Trevor

[Doc B.]

If one tube biases to 44V and the other only biases to 30V in the same circuit, it seems like there might be some sort of production variance involved. Have you tried swapping the TJs from amp to amp to see if the bias follows that one tube? It's PJs design and I will let him weight in on whether modifying the trimmer value is a good way to go to pull the amp around to working with that tube. My experience with modern tube manufacturers has been that they feel that if their tube does not behave like the original tube they named it after that the onus is upon the amp manufacturer to modify the amp to work with their tube. Our concern is that our customers get the result they want and thus we have spent a fair amount of time and energy modifying various circuits to work with these issues, like the servo circuit that was specifically designed to allow use of EH EF86s in the Eros (NOS EF86s don't require the servo), and the soft start in the Paramount which was designed to protect the EML 2A3 at startup. If the TJ 2A3s generally need this special requirement to function like the more standard 2A3 in our amps we will make some effort to accommodate them.

[Prairie Dog]

Hi Doc,
The lower bias follows the tube over to the other amp.  I think you're bang-on about manufacturers expecting us to mod our amps to suit their tubes -- Simon at diyhifi claims 25% variability to be acceptable, although he didn't give me a starting point to vary around, if that makes sense.  If one tube was 25% below nominal, and the other 25% above, that's a 50% difference!    It seems like a pretty big tube to tube variability for a so-called 'matched pair', at $239.00 a shot.  He seems very loathe to honour his "full warranty" , and rather blame the amp's design...

Thanks for the reply
-Trevor

[Doc B.]

claims 25% variability to be acceptable

That seems to be quoted fairly often as an industry standard. I suspect it is an artifact from the days of 10 and 20% tolerance resistors and -20+50% tolerance caps. With that kind of tolerance in the rest of the circuit it didn't matter as much if the tubes were a perfect match or not, since you would still probably have to make adjustments elsewhere in the circuit to match tubes up. Not that tubes are as easy to make as resistors these days, I'm sure it is much more difficult to get a consistent tolerance in some aspects of the construction. In the days when tubes ruled the high volume production meant that the really out of spec stuff could just go to the crusher. I can't imagine the demand for 2A3s being high enough to allow a manufacturer to replicate the legend of WE smashing all but 25% of their 300B production to maintain a quality standard*

There are tube vendors who charge a little extra for tubes that are matched to something like 5%. The 2A3s and 300Bs we get from New Sensor are usually very nicely matched. The 2A3 tubes we used to get from Valve Art we had to match ourselves, and every once in a while a couple out of a shipment would simply end up in the dumpster as unusable. Of course we payed a heckuva lot less than $239 for them.

* and perhaps it is an urban legend in the case of WE. I was told that when the 90s version of the WE300B was first prototyped one of the people who worked on the original 300B production said that the new one wouldn't sound the same because the equipment used pulled too hard a vacuum in the envelope!

[Prairie Dog]

After 6 months of frustration, shipping tubes back and forth between me and Hong Kong, and I don't know how many e-mails, the vendor admits there is something amiss with his 2A3 stock.  Here is a direct quote from his last email, received this morning:

"Hello Trevor,

The 2A3 we sent to you has low Transconductance. I tested our current stock of 2A3, the higher is 3.6mA/v, within +/-30% range but probably won't work on your amp. More stock will come but I guess TJ will not make it 100% up to the spec. Most amplifier will have a tolerance level. Can you ask Bottleneck for their tolerable Gm and mA/V figure? Then I will try to get one pair that will work on your amp.

regards,
Simon"

So far, he has all but refused to issue me a refund, and I have refused to mod my monoblocks to suit his products.  I'm running Paramounts with the soft start upgrade. I've used other Chinese and Russian 2A3 tubes with no troubles.   Any info / advice would be appreciated, as I'm just about ready to wash my hands of the 250.00 I spent on these TJ tubes.

-Trevor