Tubes for Groove Tubes SuPRE

[reginator]

Hello all,

I bought a SuPRE around four years ago and I love it. Recently the left channel has been cutting out, and intermittently spiking when I rotate the gain knob. The left channel also has bad distortion when it's working.

I tracked the problem down to the GT-6922 tube. The 6922 in the left channel is very microphonic and the VU meter swings wildly when I tap it with a wooden poker. The 6922 in the right channel and both channels 6GH8 tubes rock solid when tapped. I also verified it's the 6922 by swapped it to the right channel and the problem followed.

Here's the real problem - I can't find GT-6922 or GT-6GH8 tubes anywhere. I did an unscientific comparison of the tubes in my SuPRE and photos of 6922 and 6GH8A tubes on the internet. What I found is the GT-6922 looks like the Electro-Harmonix 6922EH (not gold edition), and the GT-6GH8 looks like the Sylvania 6GH8A. But I don't know for sure?

Does anyone know for sure what Groove Tubes used for their GT-6922 and GT-6GH8 tubes?

 

[pucho812]

1.swap tubes from one channel to the other to confirm beyond doubt the issue is in fact the tubes.
2. If it is tubes, buy yourself some 6922 tubes and some 6gh8a tubes. I see 6gh8a’s and 6922’s all over the place. The 6gh8a are going to be nos most likely.

For the 6gh8a see about getting rca or ge
For the 6922 go with Jan for nos or eh for modern production.

3. If it’s not tubes, schematics are in the tech docs

 

[reginator]

Thanks for the tips, pucho812!

I mentioned in my post that I swapped the tubes and the problem followed to the other channel, so it's confirmed the 6922 is dying.

I do have two sleeves of GE 6GH8A's, but I bought those before I realized their internal makeup is different than the GT-6GH8. I haven't tried them because the 6GH8 isn't giving any trouble at the moment.

What I'm hoping to find out is what brand of tubes did Groove Tubes use for their 6922 and 6GH8(A). I'd like to keep using that brand/series of tube in my SuPRE. I guess I can buy a couple of sleeves of 6922EH and Sylvania 6GH8A to see if they're the same as GT.

 

[moamps]

According to the appearance of the mica insulator, I would conclude that it is probably a rebranded vaccum tube 6n23 or similar from a Russian manufacturer. And I would probably put some TFK or Siemens E88CC in there instead.

 

[reginator]

Thanks for the insight moamps. I checked both of your recommended tubes and they're not cheap, but luckily they're still affordable. If I get them, I'll need to get at least two tubes (one for each channel) so there won't be a tonal and performance difference between each side.

Cheers

 

[reginator]

Quick update, I asked Fender about the GT-6922 or GT-6GH8 and they sent this reply today:

"Unfortunately, it does look like this particular tube predates our acquisition of Groove Tubes in 2008 and has not be sold by us since.

As a result, we unfortunately don't have further information on this tube."

Just an FYI for anyone looking for exact replacements.

 

[Gavin Pursinger]

I thought 6922 was equivalent to 6dj8 and was a common tube in Tektronix oscilloscopes.
Did you try Antique Electronics
(tubesandmore) ? I used to have. a Rode NTK that I subbed an NOS 6922. It is a cathode follower in that mic I believe and there I did not notice a great improvement

 

[reginator]

I tried everywhere I could think of, and I let Mr Google search in other areas across the globe. There's no GT versions of these tubes for sale that I can find.

But you are right, there are 6922's, 6GH8's, and variants available today. Since the right channel of my SuPRE is in good condition, I'll take voltage and frequency measurements from the test points listed in the schematics to use as a baseline. I'll probably also take readings from the left channel for good measure. Then I should be able to plug in new tubes and compare the readings and sound... and adjust one of the channel calibration trimmers <if necessary> to tighten up the performance.

 

[Gavin Pursinger]

Does the 'GT' stand for groove tubes or is it a variant like
6L6 GC ?
I think I have a pair Amperex
6922. Since I have moved well past my Rode NTK. I'll check. I believe they are Nos in original
Boxes...it's been a while. I'm in Portland Oregon are you in USA . IF YOU NEED THEM I can priority mail them to you and you can mail me a ten spot for the postage. I'll let you know this eve when I get home from work.

 

[reginator]

Hey Gavin, yes GT stands for Groove Tubes.
I'm in Georgia but I do travel quite a lot. In fact I'm heading to Singapore tomorrow night and won't be back till May.
That's a generous offer for your Amperex. I "DO" need at least one 6922 for the bad channel, but I won't be able to install it until May.
We can continue with direct messages for convenience after you've had a chance to check your stock.

 

[pucho812]

Does the 'GT' stand for groove tubes or is it a variant like
6L6 GC ?
I think I have a pair Amperex
6922. Since I have moved well past my Rode NTK. I'll check. I believe they are Nos in original
Boxes...it's been a while. I'm in Portland Oregon are you in USA . IF YOU NEED THEM I can priority mail them to you and you can mail me a ten spot for the postage. I'll let you know this eve when I get home from work.

Yes correct.
Any groove tubes tube will be stamped with gt on it. Nothing special
About it other than it went through the gt test process and was screen groove tubes.

 

[Organic Vibes]

Hello Reginator,
I knew Aspen Pittman personally and worked on various projects with his staff from about 2004 ~ 2009. Steve Goods was in charge of the Supre project admin and IIRC, the Supre circuit was based around a stereo version of the Brick, which is an awesome DI box favored by the likes of Leland Sklar and others. Mitch Margolis was the lead engineer at G/T and a gifted mind. He also spear-headed the Viper and Glory Comp products. He has also worked on projects with Manley Labs and others over the years.

I own a hopped up Supre and have spent considerable hours refining the unit build to levels not available from any commercial unit. I also own G/T Vipres and Glory Comps. I can save you a lot of time by sharing some of the things I learned and did to optimize my Supre, for which I will NEVER sell of course. Probably the biggest weakness of the Supre units are the Chinese sourced magnetic resistors employed; (yes they will stick to a household magnet, telling us something very important) and "average" Chinese poly caps employed. This was done to keep the BOM cost at modest cost as Aspen was always trying to offer great value for the given price-point.

Upgrading all resistors to PRP 9372 non-magnetic audio resistors will really smooth out the tone. The Chinese blue colored magnetic resistors have a slightly grainy sound which is "OK", but not the finest possible. Unfortunately, stripping the boards down and executing all these upgrades takes skill and lots (tons) of time. Its worth it, however, if you have the time and resources. For poly caps, I prefer Vishay/Roederstein which are still made today in the Portugal Vishay factory and the current capacitors available are excellent subs for the modest cost. Yes, you can use Wima's too, but I think if you do an A/B you're going to prefer the Vishay's which are still made to the previous German Roederstein specs.

For tubes, the OEM G/T 6922 and 6GH8(A) tubes were sourced from Russia and perhaps China. They went through Aspen's stringent testing rig at the G/T factory and silk-screened accordingly. You should be able to "tube roll" these with NOS and current production tubes. Nothing unusual. What I learned is that if you are using the stock resistors and poly caps, then you will probably find that the choice of tubes is more forgiving. Once you upgrade the majority of components then you'll notice the sound quality with the improved components favors the higher end delicate tone tubes such as Siemens and TFK. So its really all about matching high quality components with high quality tubes. This is no different than building high end hi-fi tube amps etc. The more you experiment and listen the more you will realize that cross mixing premium fuel with lower cost auto parts makes for a mixed bag of performance. I think you get the analogy now. I wish it was different, but its not.

My favorite tube picks are TFK 6922 or 6DJ8 on the front end (Siemens also good) and vintage RCA 6GH8(A) on the output end. The RCA's will give you that thick fat 60's studio sound. A GE will sound a bit leaner and TKF did not make a 6GH8(A). The other kind of "cool guy" 6GH8(A) is the "Yugo" (Yugoslavian) tubes found on eBay (as of about 1 year ago), which utilize TFK (Telefunken) tooling. A quick look at these and you will see that they are very similar in construction to the stock G/T 6GH8(A) tubes but with higher quality and a more TFK like clean sound. A nice sub for the RCA's. Two different tones indeed.

The "ALPS like" carbon pots on the Supre are decent. No need to upgrade them. There are power supply upgrades you can do too; mainly with voltage regulators & rectification diodes and my favorites are the various Telefunken diodes, now produced as the same by Vishay. The stock Rubycon electrolytic caps are decent. There are electrolytic upgrades such as Rubycon Kaisei series too, but they are expensive. They will take your Supre up a few notches however in tone and there are few preamps that are built to these levels of quality. You can P/M me for more details. Lots of work.... but if you have the time (50 to 60 hours) you will own something unique. A few fix attached with mods in progress. I executed these mods in 2021 during COVID lockdowns.

Also for all you G/T studio gear owners, "THE PLACE" for G/T service is Paul Morte (Tech Services), 946 N. Main Street in Orange, CA. Aspen and Paul were colleagues and good friends, (as was I), and Paul is the authority for all G/T studio products service. He has all the calibration and tech docs to service Supre, Vipre & Glory Comps and others. Fender will not release these calibration docs, so I highly recommend contacting Paul and his team for service. No one knows these units like Paul & his team. No one; as no one was as closely tied to Aspen for service as Paul.

I hope this info is helpful for all. I'm not on this forum too much due to time constraints and too many projects on my plate. Cheers, Brad Pfeifer - Audio Engineer

 

[reginator]

Hey Brad,
Thank you for explaining the work you did to improve the quality of your Supre. I'm definitely intrigued and I have the resources to maintain and modify my gear. I don't have a lot of tube experience, but I do know electronics. I'd be interested to hear about all the work you did to modify your Surpre. The thing I noticed is there seems to be a lot of afterthought that went into the design. The way the PCBs are wired up and the way some of the components are soldered in a kludge brings me to that conclusion. But this could have been the way Groove Tubes chose to fine tune each channel. So yes, I believe some improvements would be nice, and would probably add to the longevity of the preamp.

It'll be three weeks before I'm able to work on my Supre since I'm out of town. We can PM to continue the conversation.

Cheers,
Reggie

 

[Organic Vibes]

Hello Reggie,

Nice to hear your comments and that you are "intrigued" with the potential of your Supre unit. The G/T studio gear circuits are pretty darn good, but the build execution is not as good as it "could" be; and that's what this is all about. To save costs, Aspen had these units built in China by Frank Hee who owned a tube hi-fi company for many years. I believe he is retired now, however.

Subsequently, there is much more potential in these products. (Vipre included); if you have the time and resources to invest in them. I am happy to learn that you know electronics and its OK if you are new to vacuum tubes. no worries. You'll catch on in due time. What we are trying to do is take the same approach exactly; as if we were building a hi-end hi-fi tube amp etc. (Examples, such as Audio Note Ongaku or Dynamo ST-70 for example, come to mind). I've been building tube amps since the early 1990's and love the tone and performance of tube gear, of course. I am an audio engineer /consultant these days and you can look my profile up on Linked in.... (Bradley Pfeifer - Audio Engineer)

Also before I forget, if you'd like to learn more about "The Man", Aspen Pittman and what he was all about (an incredible asset to our MI audio community), please watch Warren Huart's You Tube- Aspen Pittman: Interview & Studio Tour with Aspen Pittman (2016). I am so glad that Warren took the time to "film and capture" my good buddy Aspen in all his glory! He was larger than life; and as you will see when watching this timepiece; he was ahead of his time with so many concepts. The comments to this specific You Tube post prove it. You will then understand more about the features and the history behind the Vipre, Supre and Glory Comp products and how they were unique for their time and I believe are classic legends in their own right. I am honored to have been a small part of Aspen's career and involved in working with him over the years. We truly lost a legend in the Summer of 2019 at the time of his passing.

If you'd like to learn more about tube circuits, check out Uncle Doug's You Tube Channel as he has some of the best educational lessons on tube circuits & many (I included), are very grateful for him passing on his critical wisdom to our next generations. So hats off to Uncle Doug too, as he is another hero passing on his wisdom is such a humble manner.

Moving onward...Your best source for parts & components is The PartsConnexion in Canada as they are a wonderful "one stop source" for many different high quality components. They stock the USA (made in Iowa) PRP 9372 non-magnetic (made for audio) resistors at great prices as well as countless other products that are well known in the audiophile circles. There are many brands of resistors on the market today and some (few) even better than the PRP 9372, but IMHO, you really are going to find it tough to beat the 9372's in terms of cost/performance.

It appears that the guitar amp (boutique) builders have not discovered the PRP 9372 audio resistors' value and performance as I don't see many firms employing them. They are smoother sounding than the old carbon comp resistors of yesteryear (which have a more "grainy" sound by comparison), & are apparently still preferred in Fender amps etc., However, 9372's are high precision resistors & don't drift like carbon comp resistors (over time) and they sound really warm and smooth without all the grain. Many people don't know that both of these resistors are "non-magnetic", thus not able to attract EMF/RF into the circuits they are employed in. This is super important in all audio applications and I constantly try to educate my clients about this. Too many audio product brands don't pay attention to the lower noise floor that is achieved when using all non-magnetic components. IMHO, it really is a must for all audio products, if your goal is to build the highest quality. Brands like Nakamichi and T+A in Germany (Dennis Prager's fav brand of audio gear) have known this little secret for years. So its really all about a certain level of "build discipline" knowing that an all non-magnetic circuit will sound superior for audio applications. I am hoping the industry will someday wake up to this wisdom. This applies to capacitors as well and you have to keep an eye on the electrolytic caps these days as some employ OFC copper lead wires (non-magnetic of course), and others still employ steel (magnetic) lead wires which of course are ferrous and will stick to a magnet. This of course, is Not good for audio high resolution!

So try keeping a small household magnet at your tech bench and start learning about what's out there by simply "magnet testing" all the parts you put into your audio circuits. You will find it very intriguing again, I promise! Collectively, we as an audio community can help shape the future of our audio components if we become educated and start demanding non-magnetic components for all our audio applications. The component manufacturers will listen in time. I hope all who are reading this find this helpful and valuable.

Safe travels overseas Reggie, and I look forward to conversing with you when you get back to the States.

Warm Regards,
Brad