The Stromberg Carlson ASR-120 console amplifier used them. I think that's where mine came from (I parted it out).
Jack
6U8 and 6GH8 as 7199 sub data
- Thread starter gadget73
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Jack
Yes, the very nice sounding Scott LK-150 tube amp uses 7199's, the LK-72 (w/7591) & LK-48 (w/7189) integrated amps, and for a hot moment the 222C ('61) also both used 7199's; with 7189 power tubes with 420V plate voltage, and 345V on the screen grids.
The wonderful "Vacuum Tube Valley" periodical, issue #20 (2003) stated: "Modern issue 7199's can be problematic; good 7199's are the key for proper phase inverter balance, and low distortion. It's best to use NOS 7199's from Sylvania or RCA as they seem to sound best and last longer". 7199's are prone to develop filament to (triode) cathode leakage. On top of finding a good NOS matched pair, (very hard now a days), they're also prone to failing, out of spec. sooner than later, making you have to replace it even more often than you would like.
I highly recommend anyone who loves audio tubes to read the "Vacuum Tube Valley" magazine issues. These past issues can be found here; its a treasure trove:
VACUUM TUBE VALLEY: 90's and 00's tube technology
Vacuum Tube magazine from the turn of the 21st Century
www.worldradiohistory.com
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... Also 299C and 299D...
33RPMThorens
New Member
I think the difference would be really minor at best.
There is also a not insignificant amount of feedback to correct most ills so unless the tubes are really screwed up you probably wouldn't ever hear it.
Just for reference, this is the SCA-35 circuit
Feedback and servo mechanism is R33. It biases the screen, but it also couples some AC signal in via C18. Discussion about this and why this specific circuit can be moody about it's tubes here
FWIW, that France Amperex 6GH8A in my test data looks to be the same one Dave shows in that post.
Wondering if I could do 2 resistors in series with a cap to ground at the mid point. That should bring over the DC without the AC signal. If they are high enough in value it shouldn't present too much of an extra load for the cathode.
Just for reference, this is the SCA-35 circuit
Feedback and servo mechanism is R33. It biases the screen, but it also couples some AC signal in via C18. Discussion about this and why this specific circuit can be moody about it's tubes here
Hidden Treasure: Bringing Out The Very Best of Dynaco's SCA-35
This thread presents a significant work for this amplifier, examining all of its virtues and warts, providing explanation of all the various issues, and presents real modifications for resolve. It will be presented in a number of sections as the thread develops, which will allow for questions or...
audiokarma.org
FWIW, that France Amperex 6GH8A in my test data looks to be the same one Dave shows in that post.
Wondering if I could do 2 resistors in series with a cap to ground at the mid point. That should bring over the DC without the AC signal. If they are high enough in value it shouldn't present too much of an extra load for the cathode.
The R33 C18 combination does 2 things : stabilizes the idle voltage of the pentod and : inject positive feedback to increase gain.
Positive feedback isn't good for distortion. And despite the application of GNFB, I believe there is less in this design than would typically be present in a straight pentode amplifier. It might indeed be true that not everyone can hear a difference, but I would want the most linear tube available in this position. That's my story and I'm stickin' to it.
Jack
And what about using 7687 tubes as a replacement for 7199? Supposedly, it was Sylvania's attempt of a 7199 for audio that did not take off?, And phase inverter replacement for the 6U8.
Although, I think finding a good pair of them is even harder to find than a 7199.
Although, I think finding a good pair of them is even harder to find than a 7199.
Typically, integrated amps used a gain stage to make up for the losses of the tone controls. Positive feedback gave this amp enough gain to omit that stage. Although positive feedback would increase distortion in this stage, it's likely still less than that generated in the output stage. And negative feedback reduces distortion from both sources.
Much harder, I've been looking on Ebay for the better part of the year and haven't seen a single one come up. I did work on a 299a 2 years ago for a guy who found someone in Canada selling them. One was bad and we sent it back, thankfully he was able to get a replacement from the guy, he's since sold out though.
And then there was this thread, which I haven't read in a while but might be worth looking at: https://audiokarma.org/forums/index...9-or-6an8-in-phase-splitter-circuits.1047124/
On another note, great thread, I'm working on an ST70 again and before I saw this thread I ordered a sleeve of 5 NOS 7199's. I've never owned trued 7199's for my ST-70. I'm hoping these 7199's are good, since they are all matched originals in a sleeve, perhaps they aren't picked over junk.
I did get the best distortion performance with 6GH8A's so far though, I've tried those, 6U8's, 6BL8's and 6F1P
CVElectronic
Member
Some Hammond Organ (H-100 series) internal amplifiers used 7199s.
I think the problem is that no tube manufacturer is ever likely to produce a small pentode/triode tube ever again.
I think the problem is that no tube manufacturer is ever likely to produce a small pentode/triode tube ever again.
I'm kinda curious why the ST70 circuit is so picky about which specific tube you use.
I spent quite a bit of time over the weekend working on one. It took quite a bit of swapping around to find tubes that had acceptable THD performance.
For example, the JAN Sylvania 6u8a's that Tubedepot ships with the 7199 to 6u8a adapter, both tested perfect on my RoeTest. However in circuit (1khz, 1v signal to the amp), one had great THD+N (0.4% or so) and the other 1.5% under the same test conditions.
I ended up having to sort thru about 20 tubes before I ended up with two pairs of 6GH8A's that had acceptable distortion performance.
I had one 6gh8a in particular that tested perfect on the RoeTest but measured 6% THD!! under the same conditions as the others that were good.
Out of the sleeve of 7199's I bought, 3 of them had acceptable performance in the ST70.
I've seen the same thing with 6u8a, 6bl8 and 6f1p's in the ST70. Seems like its specific to the tube itself, and not necessarily the manufacturer/batch
I spent quite a bit of time over the weekend working on one. It took quite a bit of swapping around to find tubes that had acceptable THD performance.
For example, the JAN Sylvania 6u8a's that Tubedepot ships with the 7199 to 6u8a adapter, both tested perfect on my RoeTest. However in circuit (1khz, 1v signal to the amp), one had great THD+N (0.4% or so) and the other 1.5% under the same test conditions.
I ended up having to sort thru about 20 tubes before I ended up with two pairs of 6GH8A's that had acceptable distortion performance.
I had one 6gh8a in particular that tested perfect on the RoeTest but measured 6% THD!! under the same conditions as the others that were good.
Out of the sleeve of 7199's I bought, 3 of them had acceptable performance in the ST70.
I've seen the same thing with 6u8a, 6bl8 and 6f1p's in the ST70. Seems like its specific to the tube itself, and not necessarily the manufacturer/batch
The 7199 was used only to save cost, and good performing examples were more readily available back in the day. I also believe the tube manufacturer graded the tubes before stamping the Dynaco name on them. That was a common practice at that time. In any event, the best thing that can be done for the ST-70 is to simply replace the board with one that can accept low distortion audio-grade tubes. I know people who have searched for years in an attempt improve the sound quality, eventually settling on tubes like the EF86, 6SN7, 6CG6, 12BH7, etc. Considering how prices for these amps have skyrocketed over the last few years, someone could probably do well by offering a point-to-point replacement for that board. It's not much additional cost to fabricate an aluminum plate with terminal strips and standard tube sockets.
Jack
Sounds strange. What is your test conditions ? Is it a real unmunged dynaco st-70 ?
Yes, I have two ST-70's here now, mine and another persons that I just restored. Both are stock (other than the 80/40/30/20 can cap, yellow sheet diode mod, and silicon rectifier in the bias circuit). Both have had critical resistors replaced on the driver board (the latest one, all with metal films). Mine has IC polypropylene caps but I left the Black Cat PKM mylar caps in the 2nd st-70 (they test perfect and they don't have any leakage)
The distortion follows the tube between the two ST70s. This seems to track with what El Paso tube amps found during some of his testing many years back in this vid:
In the notes of the video he says:
"Update: 3-8-2013I went through a dozen 6U8's until I found two that performed very well and the good ones perform as good substitutes for the 7199 also and have the same pin-out as the 6GH8. Both the 6GH8 and 6U8 require the adapter socket to be used in place of a 7199."
I wonder how many people are actually checking these things with a distortion analyzer these days. As much as I love my test equipment sometimes it's a bit of a curse!
Test conditions are 1 volt input signal at 1 kHz, into an 8 ohm load. I have a levear vp-7725d and rohde and Schwarz upl analyzer.
I have 3 pairs of good tubes for the ST-70 in my spare stock now. Two pairs of good 6GH8A's, one pair of 7199's. I should be able to get a few more once I decide to spend more time on it, since I did find several 6F1Ps and 6U8A's that had acceptable performance.
Strange as ST-70 is not known as a poor performer.
It’s not, but it seems picky about driver tubes. I wonder how many people out there are rocking high distortion ST-70's and don't even know it.