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Caution on using 6GH8, 7687, 6U8 tubes to replace 7199 or 6AN8 in phase splitter circuits

B

Bob@Blue5

New Member
The pin out for the 6GH8, 7687, 6U8 tubes has a design weakness when used in a phase splitter circuit that can cause high frequency stability issues with some amplifiers originally designed for the 7199 or 6AN8 tube. The 6GH8, 7687, 6U8 tubes all feature the pentode grid (pin 2) right beside the output pin (pin 1) of the plate of the triode.
In almost all designs using the 7199 tube the input signal is amplified and inverted by the pentode and this signal is then applied to the triode's grid. The signal applied to the triode's grid is inverted onto the triode's plate. So now the pentodes input signal has been greatly amplified (often by 40DB or more) and inverted twice onto the triodes plate pin to be exactly in phase with the pentodes input grid pin. The design issue on 6GH8, 7687, 6U8 tubes is the input pin to the pentode grid (pin 2) is right beside the output pin (pin 1) of the plate of the triode. As the two pins are right beside each other in the tube and tube socket there is significant capacitive coupling between the two pins resulting in positive feedback.
The positive feedback effect of the capacitance between the two pins is greatly increased by the high gain in the pentode stage and by the fact many amplifiers designs leave the pentode input grid at high impedance from ground.
A spice simulation suggested that in some circuits as little as 0.06PF of stray capacitive coupling between the two pins results in as much as 30DB peaking of response at a few hundred KHZ. The true effect will of course be strongly determined by each individual unit being modified and is difficult to predict in advance.
Depending on many many amplifier design factors this high frequency positive feedback can be of little importance or in some amplifiers lead the amplifier into strong oscillation at some frequency well above the audio range. These stability issues can be hard to see unless the builder has a well equipped lab and often will result in a amplifier that sounds a bit "hard" or now lacks some "mid-range clarity" on complex music due the amplifier breaking into instability on some program material. Square wave testing "before" and "after" modification can be useful in checking for this issue.
I personally avoid 7199 or 6AN8 replacements where possible with a pinout that features the pentode grid and the triode plate being side by side pins.
 
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This may account for the unsatisfactory subtitution of these for 7199 via adapters in some amps and not others.
 
The ST-70 has no problems, but some amps used positive feedback around the pentode stage (Dyna SCA-35, Bogen DB-212, for instance) and these are more likely to have problems with the substitution. Plenty of amps were designed to use the 6U8, 6GH8. 7687 as gain stage and phase splitter after all.
 
If you are getting (parasitic) oscillation at hundreds of kHz the easiest solution is usually a well-placed low value capacitance to ground, probably at one of the grids. Without knowing the circuit design and frequency of the oscillation it would be impossible to say what value the capacitance should be. Or, you could just buy the correct tube and be done with it.
 
HH Scott used the 6GH8/6U8 in many amplifiers after they stopped using the 7199. They never seemed to have a problem. For that matter, I have rewired a few amplifiers from 7199 to 6GH8/6U8 simply because the tubes were either missing or bad.

"Your mileage may vary".
 
A well designed amp will have some bandwidth limitations built in to keep them stable as well. Most just have no gain past 100kc or so, and plenty roll off even sooner depending on the transformer. I own a few that I've cobbled up from junk that have to be rolling off by 30khz.
 
gadget73 said:
A well designed amp will have some bandwidth limitations built in to keep them stable as well. Most just have no gain past 100kc or so, and plenty roll off even sooner depending on the transformer. I own a few that I've cobbled up from junk that have to be rolling off by 30khz.
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I used to employ a "sniffer" probe to check preamp and driver stages for oscillation. Instability is often intermittent and doesn't show up in ways you would expect at the output. Unfortunately, I no longer have access to the analyzers since leaving my day job. :(
 
I'm limited to a scope and square wave testing into funky loads. Sometimes I've done the scope in xy mode with one at the input and one at the output to watch for phase shifts. PA and some instrument amp iron that wasn't used full range is pretty limited and prone phase shift issues.

Yeah the real answer is better transformers but i have an inescapable tendency to polish turds.
 
Tom Bavis said:
The ST-70 has no problems, but some amps used positive feedback around the pentode stage (Dyna SCA-35, Bogen DB-212, for instance) and these are more likely to have problems with the substitution. Plenty of amps were designed to use the 6U8, 6GH8. 7687 as gain stage and phase splitter after all.
Click to expand...
I had that stability issue in my DB212 when I rewired the socket and substituted a 6U8; I then converted to LTP and added a driver…plus 6U8/6GH8 require some component value adjustment, curves are different.
 
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I have a couple things that use a 6U8, and one set that run a 6AN8. Haven't felt any special need to sub the 6AN8 for anything else. They are reasonably available for not dumb money. I've picked up several spare sets for cheap. I don't think I've got more than 10 bucks into any of the pairs, and plenty were a dollar per tube, or 2 for $5 type things. I'm not picky about them, so long as I have 2 that are the same its good enough. I've had multiple of those tubes in these amps and can't tell any difference between them. Only reason I've even changed them is I had a set get noisy.
 
Am I reading this correctly in that the 6an8 is a direct replacement for the 7199? Not needing the adapter a 6u8 needs?
 
Johnnysan 1 said:
If you are getting (parasitic) oscillation at hundreds of kHz the easiest solution is usually a well-placed low value capacitance to ground, probably at one of the grids. Without knowing the circuit design and frequency of the oscillation it would be impossible to say what value the capacitance should be. Or, you could just buy the correct tube and be done with it.
Click to expand...
A low pass filter by itself is a bad band-aid... to properly address instability, the phase and frequency response need to be tailored. A step network will help with this, then adding an appropriate value cap across the feedback resistor will usual make the amp behave and be extremely stable.
 
WizardWG4 said:
I believe this is why Dave Gillespie recommends adding 10K grid stoppers in his PC-10A boards for the Dynaco SCA-35.
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This isn't likely to be a remedy for local oscillation at the 6U8 (or similar replacement).
WizardWG4 said:
My question though is why a 10K just for the inverted tube grid and not the non inverted?
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Only Dave knows for sure, but my guess would be that he arrived at this through empirical testing. I would only question why such a large value of resistor is required, and whether this actually points to a different root cause.

Jack
 
A lot of Japanese amplifiers use a 6AN8 and many US amplifiers use 7199 or 6GH8/6U8 in the AF amplifier-phase inverter stage. I have a Kenwood 1100u receiver that is just absolutely super. It was a real Bear to restore, many, many capacitors had to be replaced. But I had very little issues with the resistors. It uses the 6AN8 tube.
 
Bob@Blue5 said:
The pin out for the 6GH8, 7687, 6U8 tubes has a design weakness when used in a phase splitter circuit that can cause high frequency stability issues with some amplifiers originally designed for the 7199 or 6AN8 tube. The 6GH8, 7687, 6U8 tubes all feature the pentode grid (pin 2) right beside the output pin (pin 1) of the plate of the triode.
In almost all designs using the 7199 tube the input signal is amplified and inverted by the pentode and this signal is then applied to the triode's grid. The signal applied to the triode's grid is inverted onto the triode's plate. So now the pentodes input signal has been greatly amplified (often by 40DB or more) and inverted twice onto the triodes plate pin to be exactly in phase with the pentodes input grid pin. The design issue on 6GH8, 7687, 6U8 tubes is the input pin to the pentode grid (pin 2) is right beside the output pin (pin 1) of the plate of the triode. As the two pins are right beside each other in the tube and tube socket there is significant capacitive coupling between the two pins resulting in positive feedback.
The positive feedback effect of the capacitance between the two pins is greatly increased by the high gain in the pentode stage and by the fact many amplifiers designs leave the pentode input grid at high impedance from ground.
A spice simulation suggested that in some circuits as little as 0.06PF of stray capacitive coupling between the two pins results in as much as 30DB peaking of response at a few hundred KHZ. The true effect will of course be strongly determined by each individual unit being modified and is difficult to predict in advance.
Depending on many many amplifier design factors this high frequency positive feedback can be of little importance or in some amplifiers lead the amplifier into strong oscillation at some frequency well above the audio range. These stability issues can be hard to see unless the builder has a well equipped lab and often will result in a amplifier that sounds a bit "hard" or now lacks some "mid-range clarity" on complex music due the amplifier breaking into instability on some program material. Square wave testing "before" and "after" modification can be useful in checking for this issue.
I personally avoid 7199 or 6AN8 replacements where possible with a pinout that features the pentode grid and the triode plate being side by side pins.
Click to expand...
Yes, it seems to be unnoticed that the 7199 and later 6GH8 in the Dynaco ST70I and ST70II have a low pass RC on the triode grid, marked "not used in all assembly s" . Really, this is significant. This low pass is the audible range, amps with and without it must sound different.
I have circuit for 6GH8 used to substitute 7199 and used in variety of Sherwood amps and in Dynaco ST70 without RC low pass and work well. When put in a Harman A300 redesign, terrible oscillation with both 6GH8 and 6U8A. This very high amplitude oscillation is measured at 19.5 kHz and explains why RC low pass is placed so low frequency.

I use TINA for all design work, but only basic tube macros which do not show osculation. Evidently you have included the parasitic capacitance's which must make a big difference. I hope you can send your TINA simulation. I am very curious.

At this point I am thinking the VARIABLE in oscillation or not, MIGHT be the tube sockets and/or wiring ??? It would be interesting to try different sockets and experiment with removing as much of the solder lugs as possible. Also the different socket insulation, plastic vs. ceramic will have different parasitics . Or maybe the wiring is absolutely critical as well ? It be interesting to solve this long unresolved Mystery.

Dana
 
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