Thoughts on preamp tubes in a Nomad
- Thread starter Koreth
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When the supplier gives tube measurement data (say, transconductance Gm), for instance, from a Hickok 539C - what are NOS tubes supposed to measure, and how low do they go until their no longer good sounding ?
Are there differences in Gm between Mullard, Siemens, Amperex, RCA, Ratheon ? I've seen numbers ranging from 1500 to 2500.
When iplate is provided - the numbers seem all over the map - I've seen numbers from the low 2 mA to mid 4 mA....
AND - what is the effect of Gm and iplate on tone, tube life, etc.?
So much to learn, so little time and money.
Are there differences in Gm between Mullard, Siemens, Amperex, RCA, Ratheon ? I've seen numbers ranging from 1500 to 2500.
When iplate is provided - the numbers seem all over the map - I've seen numbers from the low 2 mA to mid 4 mA....
AND - what is the effect of Gm and iplate on tone, tube life, etc.?
So much to learn, so little time and money.
Looking at a spec sheet for the 12AX7, transconductance and plate current are dependent on plate and grid voltage, which probably varies from amp to amp and position to position in the amp depending on what role that section of the amp plays.
Specs on the sheet give a max plate voltage of 300V, and a maximum plate dissipation of 1W. The transconductance and plate current for 100 plate volts and -1 grid volts are 1250 microsimens/micromhos and 0.5 mA. At 250 plate and -2 grid, these are listed at 1600 and 1.2, respectively. While the latter is probably closer to what our amps run, the test numbers on eBay are dependent on what voltages and stuff the tester uses. So as long as tube has a transconductance or plate current near the listed specs, at not at some weak low number like 900/.02, the tube's probably okay. I would personally stay away from anything with a transconductance lower than 1000, but that's only because the RCA longplate I had that turned out to be a dud in my amp had one section at 925 or something thereabouts.
I don't know enough about these numbers to even begin to pretend how that will affect tone. So I'm just going to have to plug the tubes in and use my ears.
Here's the spec sheet I found and referenced.
Specs on the sheet give a max plate voltage of 300V, and a maximum plate dissipation of 1W. The transconductance and plate current for 100 plate volts and -1 grid volts are 1250 microsimens/micromhos and 0.5 mA. At 250 plate and -2 grid, these are listed at 1600 and 1.2, respectively. While the latter is probably closer to what our amps run, the test numbers on eBay are dependent on what voltages and stuff the tester uses. So as long as tube has a transconductance or plate current near the listed specs, at not at some weak low number like 900/.02, the tube's probably okay. I would personally stay away from anything with a transconductance lower than 1000, but that's only because the RCA longplate I had that turned out to be a dud in my amp had one section at 925 or something thereabouts.
I don't know enough about these numbers to even begin to pretend how that will affect tone. So I'm just going to have to plug the tubes in and use my ears.
Here's the spec sheet I found and referenced.
On eBay, I sometimes see auctions for tubes which the seller claims the tubes were tested in a pre-amp and sounded great, but haven't run the tubes on a tester at all and are thus sold as-is. Suppose I pick up a large lot of these as-is tubes at a steal (because they're as-is). Where should I start looking to find someone local who would have the proper testing equipment and be able help me pick the good tubes apart from the duds in the lot?
212Mavguy
Well-known member
Try looking for estate sales, pawn shops, and of course, there are buttloads of testers available on eBay... Stay away from cheap testers that rate on "emissions." A PM to Stokes will be a good decision, he knows a fair amount about testers. Do know that you can't go too far wrong with a Hickok for preamp tubes at least, power tubes need to be tested at real amp world voltages for accurate readings, like in a Maxi-Matcher. Good testers can be pretty spendy.
212Mavguy
Well-known member
The amp tech route is the best one, but you might be surprised by the number of techs that don't own one. It is an important part of my guitarism religion to seek out the most skilled tech in the area and develop quality rapport. And that definitely means giving him some business and referrals. The occasional gratuity in a non cash form is a great idea when work is done on your stuff. Find out what he likes.
There are a buttload of testers on eBay, can't go too far wrong with a Hickok for preamp tubes. Spendy but worth it. Look what it costs for a mesa these days...
There are a buttload of testers on eBay, can't go too far wrong with a Hickok for preamp tubes. Spendy but worth it. Look what it costs for a mesa these days...
I was browsing eBay, looking for some Sylvania shortplates so I had spares in case my current goes bad and I found this eBay auction. When I went down to look at the picture of the tubes, the first thing I noticed was that squashed octagon etching, which is indicative of an RCA, right? Taking a closer look, at the plates of the tubes shown and the plates of my own RCA longplate, the plate structure seems identical, save for my longplate is grey, those are black and they both have the bent square getter. So I'm pretty sure the tubes shown there are RCA blackplates, and not Sylvania tubes as described. Am I wrong? Does this sort of thing where the paint markings claim manufacture by one company, but etchings and plate structures indicate otherwise, happen often?
212Mavguy
Well-known member
Koreth,
these ARE Sylvania long blackplates. Notice how the rectangular halo is mounted by the corner and has no post that most getter halos are mounted on...With that said the Syl long BP tonally is a god tube, but as they get well used they can become microphonic. One of my all time fave blues tubes.
If you don't like the way they sound I'd be sad and surprised at the same time. The same tube will sound slightly different in different amps, though. Better to run a sweet tube like this in a head rather than shaking it to death in a combo. Or wrap it two-three layers thick in one inch heatshrink tubing and leave the metal preamp tube shield off it. 8)
these ARE Sylvania long blackplates. Notice how the rectangular halo is mounted by the corner and has no post that most getter halos are mounted on...With that said the Syl long BP tonally is a god tube, but as they get well used they can become microphonic. One of my all time fave blues tubes.
If you don't like the way they sound I'd be sad and surprised at the same time. The same tube will sound slightly different in different amps, though. Better to run a sweet tube like this in a head rather than shaking it to death in a combo. Or wrap it two-three layers thick in one inch heatshrink tubing and leave the metal preamp tube shield off it. 8)I figured I'd post some further thoughts and experiences with preamp tubes in my Nomad here then starting another thread.
I had a chance to pick up a quad of RCA 12AX7A shortplates and have spent the past couple days trying them out. As far as clarity goes, they're right up there with the GE 12AX7 shortplate I have. I think maybe the GE is slightly brighter, but overall the tubes sound very similar. So GE and RCA shortplates work well for V1, IMO. I'm swapping my GE longplates through V2, trying to hear if there's any noticable difference between them sonically (some have different ribbing or holes in the plates or different shaped getters). I found one that sounded beautiful, but then after a few chords it went microphonic and became noisy as hell. Damn.
Another GE I tried was noisy, having a constant and loud hum. Like the RCA longplate I tried before, there was quite a mismatch between sections 1700/1400, (the RCA was 925/1425). All of my quiet tubes have sections that are within 250 of each other. Can a strong mismatch between sections of a preamp tube be a reliable indicator that the tube will be noisy, or is it just coincidence?
I had a chance to pick up a quad of RCA 12AX7A shortplates and have spent the past couple days trying them out. As far as clarity goes, they're right up there with the GE 12AX7 shortplate I have. I think maybe the GE is slightly brighter, but overall the tubes sound very similar. So GE and RCA shortplates work well for V1, IMO. I'm swapping my GE longplates through V2, trying to hear if there's any noticable difference between them sonically (some have different ribbing or holes in the plates or different shaped getters). I found one that sounded beautiful, but then after a few chords it went microphonic and became noisy as hell. Damn.
Another GE I tried was noisy, having a constant and loud hum. Like the RCA longplate I tried before, there was quite a mismatch between sections 1700/1400, (the RCA was 925/1425). All of my quiet tubes have sections that are within 250 of each other. Can a strong mismatch between sections of a preamp tube be a reliable indicator that the tube will be noisy, or is it just coincidence?
212Mavguy
Well-known member
Hi Koreth,
No correlation between mismatched sections and noise.
No correlation between mismatched sections and noise.
I paid the premium for a handselected "balanced" NOS Jan Phillips 5751. It did not much other than give me 60 hz hum. Its in the trash.
I should be receiving my 1xRCA shortplate, 4x ratheons, and 4x slyvania's very soon. My ebay Xmas gift for the year.
I should be receiving my 1xRCA shortplate, 4x ratheons, and 4x slyvania's very soon. My ebay Xmas gift for the year.
Tommy_G said:
I am glad for the players that have saved a fortune with VOS used ebay tubes .
I think I save more by simply reading what works for others then buying N.O.S tubes
that are from an honest dealer with multiple tube testers . On power tubes you give them
the volts and your bias current and they can set you up with plug and play tubes .
I was tired of "strong matched pairs" only to find one redplate and there goes another.
Same with preamp tubes , they give you numbers , but what ones ?
I see strong at 1250/975 out of 1000 . Then another tester says 80/70 where 100 is new .
I no longer keep stashing tube after tube . I leave the amp on for two days after I have tested
the bias and played for a bit . New tubes will soften with time . If 4 VOS" used " pre-amp tubes cost 80 bucks
plus shipping that is cheaper than 4 of most new versions of preamp tubes ? NOS/VOS sound better .ImHo.
/best to you all
I forgot ,
While I was learning I did end up spending way too much .
I have found what I like and will soon be having a yard sale .
I will list the items that are worth selling , mostly new pairs and quads.
Some real N.O.S that were fine tubes but not of my liking.
This might save some of you money as I can describe the experiences of " this set " i.e.
/cheers
While I was learning I did end up spending way too much .
I have found what I like and will soon be having a yard sale .
I will list the items that are worth selling , mostly new pairs and quads.
Some real N.O.S that were fine tubes but not of my liking.
This might save some of you money as I can describe the experiences of " this set " i.e.
/cheers
