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substitute 7199 for ECF82/6U8A

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jkojic

jkojic

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Joined 2017
#1
hi-substitute 7199 for ECF82/6U8A- with converter- i know very little about electronics

forums talk about using other models with some adjustment- i dont want to do that

is the above exactly the same as 7199- sellers say it is
 
#2
The 7199 and 6U8 do not have the same pin out connections.
But that is not all, they do not have the same characteristic specifications.
The 7199 triode section u = 17; the 6U8 triode section u = 40; and the pentode sections specs are somewhat different too.

Just changing the wiring to the tube socket will probably not be enough to get the circuit to work as good as it did as when the working original tube was used.

Using a 6U8A to substitute for the 7199 is best left to someone who understands the circuit that the tube will be used in, and how to change the values around the circuit to make sure it works as well as the 7199.

Perhaps if you post the schematic of the circuit you are using the tube in, someone reading this post will go through the math to see if there are appropriate modified part values that will work.

I used to use 7199 tubes in my Dynaco Stereo 70.
But I now use that amp as a platform for me to try different topologies and tubes;
single ended stereo, push pull mono block; DHTs, triode-wired pentode/beam power tubes, etc.
 
#6
jkojic said:
dynaco sca-35
Click to expand...

Is your old SCA-35 driver tube bad, and must be replaced?
Sometimes just cleaning the tube pins (or the socket) fixes noise or other
intermittent problems. Be sure to switch the bad tube between channels,
before deciding that the tube is the problem. Also check solder joints on the board.

If you must sub the tube: 6U8 to 7199 Adapter
| TubeDepot.com
 
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#9
jkojic said:
hi-substitute 7199 for ECF82/6U8A- with converter- i know very little about electronics

forums talk about using other models with some adjustment- i dont want to do that

is the above exactly the same as 7199- sellers say it is
Click to expand...
The tube combined with the adapter will behave close to the 7199. The fact that the triode have different amplification is of little consequence as it is used
as a concertina, a function that acts like a cathode follower.
The penthode section is close enough .
Plug it in and enjoy the music !
 
#12
I should have seen this coming . . .
One of the very few companies that ever used the 7199 was Dynaco.
And Yes, I knew the 6U8 has been used successfully as a substitute in those Dyna amps, with nothing more than rewiring the connections.

But read on:

The ST35 may have done the same thing as the ST70, and that could be a problem.
The filament supplies were allowed to float, the only reference to ground was ceramic capacitors from the center taps of the 2 filament windings (probably only 1 filament winding on the ST35). But the floating allowed any leakage current from the EL34 heaters and the 7199 heaters to their cathodes to dictate the DC level of the heaters.

After many hours of operation, the 7199 triode section developed significant leakage from the heater to the cathode. This leakage resistance was in parallel with the bottom resistor in the triode concertina circuit. That caused unbalanced signals of the top concertina resistor, versus the bottom concertina resistor that was in parallel with the leakage resistor.
As a result, 2nd harmonic distortion was caused, which the global negative feedback "fixed" somewhat but that created 2 x 2 = 4th harmonic and 2 x 3 = 6th harmonic distortion (reference Norman Crowhurst).

One solution was to elevate the filaments slightly. An example can be found in:
"Sound Practices" issue 10, article: "Save those 7199s".
I believe a CD of the Sound Practices issues may still be available on the web.
 
#14
jkojic said:
no they are 5 months old - i also have one spare nos set
Click to expand...

If you really have 4 new 7199s, there's nothing to worry about.
They will last for many years and work well. Just don't idle the amp
for long periods of time, unused.

At some point you should convert the output stage to matched pairs
instead of a matched quad, by splitting the 100R bias resistor
(and shunt capacitor) into a 200R (and shunt capacitor) for each pair.
 
#15
6A3sUMMER said:
I should have seen this coming . . .
One of the very few companies that ever used the 7199 was Dynaco.
And Yes, I knew the 6U8 has been used successfully as a substitute in those Dyna amps, with nothing more than rewiring the connections.

But read on:

The ST35 may have done the same thing as the ST70, and that could be a problem.
The filament supplies were allowed to float, the only reference to ground was ceramic capacitors from the center taps of the 2 filament windings (probably only 1 filament winding on the ST35). But the floating allowed any leakage current from the EL34 heaters and the 7199 heaters to their cathodes to dictate the DC level of the heaters.

After many hours of operation, the 7199 triode section developed significant leakage from the heater to the cathode. This leakage resistance was in parallel with the bottom resistor in the triode concertina circuit. That caused unbalanced signals of the top concertina resistor, versus the bottom concertina resistor that was in parallel with the leakage resistor.
As a result, 2nd harmonic distortion was caused, which the global negative feedback "fixed" somewhat but that created 2 x 2 = 4th harmonic and 2 x 3 = 6th harmonic distortion (reference Norman Crowhurst).

One solution was to elevate the filaments slightly. An example can be found in:
"Sound Practices" issue 10, article: "Save those 7199s".
I believe a CD of the Sound Practices issues may still be available on the web.
Click to expand...
A note :
Dynaco ST35 don't use 7199 and don't use EL34. It does however use
12dw7 aka ecc832 and EL84 in a concertina arrengement. The filaments
are tied to the EL84 cathodes at app 14V, which must be considered a good compromize.
 
#16
petertub,

Post # 5 says the actual amplifier in question is an ST35.
If the user has 7199 tubes in it, that may be why the 7199 tubes do not work.

I mentioned the ST70 in case the ST35 filament arrangement was similar, but thanks for the filament info on the ST35.

Are you sure no ST35 uses 7199 tubes?
If it is supposed to be 12DW7/ECC832, then the real problem seems to be trying to use 7199 pentode/triode in a dual triode socket.

Perhaps the amp in question is 35 Watts times 2, and is actually an ST70, not an ST35???
 
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#18
rayma, jkojic, petertub,

petertub says the ST35 uses 12DW7/ECC832 tubes.

jkojic says the amp is an ST35, and he says he wants to replace the 7199 tubes (from it).

Were there 2 versions of ST35???

Is this an SCA-35, ST35, or ST70???

There is a pretty serious difference between a 7199/6U8A versus a 12DW7/ECC832.
 
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