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Old 15th December 2007, 08:21 PM   #1
horace is offline horace  United Kingdom
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Default Retubing my amp (EL84, ECC81, ECC83) - Tube recommendations?

Hi

I have a Tube Technology Unisis Signature amp, which I plan to retube and would be grateful for some advice on choosing which brands/types of valves to use.

The amp's output stage uses 4 EL84 per channel + 1 Ecc81 per channel.
Inside the amp are 4 Ecc83, two of which belong to the built in mm phono stage.

Another Unisis user has recommended EI EL84 - these are about 5 each, so definitely worth a try. Could I do better by spending more and if so how much and on what?

Instinct tells me to focus my attention on the ECC83s in the phono stage, as they will need to be of good quality (I intend to add an mc step up transformer soon). ECC83s are plentiful, which is great for the experts, but can be a little daunting for beginners like me.

Basically, I've budgeted around 150 (ish) for all the tubes (8x EL84, 2x ECC81 and 4x ECC83). I could spend more if need be (I want to do the job properly) but have no experience of tube rolling, so would be grateful for some pointers before launching into the unknown.

Many thanks

Martin
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Old 15th December 2007, 10:40 PM   #2
EC8010 is offline EC8010  United Kingdom
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So you're thinking of spending 150 on valves hoping that your amplifier will sound better because some brands of valve might be better than others. OK, so let's start by thinking about valve specifications...

To be called an ECC83 a valve has to (ought to) meet ECC83 specifications. It may have other unique properties (perhaps unique to that manufacturer) but they're not specified, and therefore not guaranteed.

Does the circuit rely on unspecified properties? Not such a daft question as you might think; circuits have been developed (perhaps inadvertently) that required properties not specified in valve data sheets. This sort of behaviour resulted in selections of general purpose devices acquiring special part numbers. Tektronix certainly did this. It doesn't necessarily mean it's any better for your application, though.

Linearity (distortion): Rarely specified on valve data sheets, but quite important. CNC machines potentially allow modern valves to be made with better linearity, but whether they really are (and what their quality control is like) is another matter.

So, you see, it's a bit of a lottery.

More pertinently, the ECC83 is not the ideal phono stage valve. For that application, you want lots of gm (find out about that), which the ECC83 doesn't have, so it doesn't really matter about choosing a nice lemon...

Enough of the negative, go and find out a little about how valve electronics works. I won't say it's cheaper (because it isn't) but it will be more satisfying in the long run.
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Old 15th December 2007, 11:24 PM   #3
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EC8010 is right in what he says, the only trouble is that it doesn't match my own experience - many tube brands do sound different to me. The trouble with suggesting recommendations is that I don't have experience with how your amp sounds, and further, what your own sonic preferences might be. But perhaps I can quickly talk generally.

EL84's I've had little experience with and premium NOS US or UK types tend to attract high prices. A few of current manufacture apparently dont sound bad at all, but I'll leave this for someone who's had recient experience.

for the ECC81/12AT7 driver, you're just going to have a roll a few different types to see what you prefer, I'm afraid. the AT7 family seems to have a 'dry' sound, which I don't really like. But I've heard some nice sounding gear with them, and they do driver-duty largely fine it seems. Sonics-wise, the best for clarity and insightfullness tend to be the Siemens 3-mica varieties. (Though in the wrong system this tend to be too much of a good thing). And at the other end, best for lushness is the Sylvania 3-mica versions. Both appear as pairs on eBay about once or twice a month. In between these; the RCA black-plates are overlooked a little, but are nice sounding. And Valvo's are good too with a bit more of a 'modern' sound than the RCAs. NOS Mullards sound good, but with a slightly emphasized midrange and lack of 'sparkle' to the treble (in fact, they often sound 'rolled-off' - good perhaps if the rest of your system was strident). Ive had no experience with current production 12AT7/ECC81's

For the 12AX7, in current production, a few good ones exist; but I'd suggest the Sovtex 12AX7LPS for good sound and because it tends to be quiet (important for phono). If you want NOS, I'd suggest searching for a couple of E83CC's of Holland, German or UK manufacture because in my exp they too seem to be quieter than average ECC83's and generally very good-sounding (for a 12AX7) too.

I hope that helps a little.
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Old 16th December 2007, 12:36 AM   #4
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For the EL84, I've been pleased with the Russian 6pi14pi tubes. I've tried the standard variety, as well as the -K and the -EB versions. To me, they all sound good and seem to hold up well. I've run them UL with 390 volts between plate and cathode, and never had a meltdown.

You can usually find them on eBay at very reasonable prices, even including the shipping charges. The last set I bought was about $4 (USD) per tube, shipping included. At those prices, what's not to like?

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Old 16th December 2007, 02:51 AM   #5
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Horace,

Use the Sovtek 12AX7LPS in the phono section, as the variant is both clean and quiet.

Use the Saratov, Russia, made "reissue" Mullard long plate ECC83 in the remaining 12AX7 positions, for tonal reasons.

NOS Mullard made CV4024s are still affordable. So, that's the 12AT7 variety I advise you buy.

Use the JJ EL84, as it is the best sounding current production variety. The Russian 6p14p-ev previously mentioned is a true 7189 equivalent that's TOUGH. The Russian tube has decent sonics, but the JJ is a bit more refined.

YMMV applies in spades to tube recommendations.
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Old 16th December 2007, 02:53 AM   #6
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I agree with Ty. The Russian 6pi14pi valves have been great for me. I've also used the -EB version and been very pleased. These are built even heavier than many NOS EL84/6BQ5s and sound quite good. And at their price they are certainly worth a try to see what you think. As Ty said, no meltdowns, no trouble whatsoever.
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Old 16th December 2007, 03:58 AM   #7
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Eli, FWIW, the JJ ECC81 in grounded cathode have half the distortion and a more favorable spectrum than the Mullards.

EC8010's comments are most pertinent; more will be generally be achieved by optimizing circuits than by swapping tubes or caps or whatever.
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Old 16th December 2007, 11:03 AM   #8
horace is offline horace  United Kingdom
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My thanks to you all for the advice.

The 6pi14pi are indeed very cheap (you can barely buy decent light bulbs for that kind of money - aren't I glad I didn't choose a 300B for my first serious valve amp!), so I'll try those first. Once I've lived with them for a while, I'll try some JJ EL84 (more expensive, but still cheap).
Likewise, Sovtek 12AX7LPS will hardly break the bank, so two recommendations here is enough for me.
The "reissue" Mullard long plate ECC83 mentioned by Eli hasn't come up in my ebay searches yet, so I'll broaden my search a little.

If I've learned anything at all from browsing the forum, it is that there seem to be few absolute truths in valve audio (that's true of most things of course, but seems especially true here) - one man's equivalent valve type is another man's 'no it isn't'. Can be quite confusing to the beginner, but I'm picking things up slowly as I go along. I will definitely start reading on the subject - I'd love to build my own valve amp one day - it's a long way off, but I have taught myself how to make a reasonably neat solder joint, so I have taken the first faltering step. I will shortly be building an mc step up device and am looking into sourcing a decent pair of transformers.

I'm grateful for the benefit of your experience. Fortunately, my amp makes valve biasing easy (plug a bias meter into a socket on the side and follow the instructions - I'm sure even I can manage that), so the process of replacing the tubes shouldn't be too challenging.

Thanks again

Martin
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Old 16th December 2007, 12:45 PM   #9
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Quote:
I will shortly be building an mc step up device and am looking into sourcing a decent pair of transformers.

Martin,

You can get what you need close to home. Sowter models 9570 and 9580 get the job done with all, except the lowest O/P, LOMCs.
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Old 16th December 2007, 01:22 PM   #10
horace is offline horace  United Kingdom
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Thanks Eli - I hadn't found those. Do you think it would be worth paying the extra for the OCC wire option?

The other transformers I'm considering are these (at the bottom of the linked page):

http://wduk.worldomain.net/acatalog/AmpKits.html

They are 150 a pair and apparently the same as those used in this Audio Note kit:

http://www.hificollective.co.uk/kits...te_stepup.html

The second link also has a downloadable build guide, which seems very straightforward. What do you think?

Regards

Martin
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