6550 Tubes for Ampeg SVT

[bobyoung53]

Are you really sure?
The pic I posted above was taken from a 12AX7 preamp version SVT at Ampeg website support.
My original schematic in paper print form reads (as well) exactly the same, its a Magnvox SVT made in ~1978

If 6146B tubes shall gain some more of power, the only way doing it was some improved conductance from plate to cathode at full throttle.
Are you really sure that it works this way with 6146B?

Yep. Although Wasnex did point out that it was 74 mV not 72 MV.
I have two late 70's Magnavox heads, they sound different than the 6146B head.

My 69 says 74mV on the chassis. Other than that, the procedure is the same as later amps.

 

[Wasnex]

Although Wasnex did point out that it was 74 mV not 72 MV.

Sorry, it's 75mV.

This is not my amp, but I confirmed mine says ".075V"

 

[ThisBass]

Yep. Although Wasnex did point out that it was 74 mV not 72 MV.
I have two late 70's Magnavox heads, they sound different than the 6146B head.

Doesn't matter if its 72mV or 74mV. Once the amp is "warmed up" and correctly biased, when playing a couple of tunes at rather highish loudness level, then the bias current drifts quite high anyway. Bias is (anyway) anything but something that was really consistent in real practice.
As long as it was calibrated in the range 70..80mV, its fine.

 

[ThisBass]

Also your SVT seems a bit extraordinary if it makes over 300W at 3%. Some do meet spec, but AFAIK many do not.

At least one of the reason might be the 235 VAC line voltage at the wall socket.
When I first time measured the output power of my SVT the line voltage was already raised (in the past the line voltage was 220V +/- 5%).

I have got a insulation transformer with adjustable output voltage. Thus it was theoretically possible to measure the SVT at 220 VAC. However its 300VA seem a little bit weak to power the SVT at full throttle.
At the other hand side, I'm quite happy with the amount of power the SVT does push out. If other SVT amps are weak, these are not mine...

 

[bobyoung53]

Doesn't matter if its 72mV or 74mV. Once the amp is "warmed up" and correctly biased, when playing a couple of tunes at rather highish loudness level, then the bias current drifts quite high anyway. Bias is (anyway) anything but something that was really consistent in real practice.
As long as it was calibrated in the range 70..80mV, its fine.

I know it.

 

[bobyoung53]

At least one of the reason might be the 235 VAC line voltage at the wall socket.
When I first time measured the output power of my SVT the line voltage was already raised (in the past the line voltage was 220V +/- 5%).

I have got a insulation transformer with adjustable output voltage. Thus it was theoretically possible to measure the SVT at 220 VAC. However its 300VA seem a little bit weak to power the SVT at full throttle.
At the other hand side, I'm quite happy with the amount of power the SVT does push out. If other SVT amps are weak, these are not mine...

U.S. line voltage is frequently at least 120 which is what's on the schematic, double that is 240 so 235 would be in the normal range. !970's SVT's can be wired for either.

Have you ever tried calculating the power output? I have to admit I have never heard of a 6550 SVT putting out anywhere near 350 watts unless it was driven way into clipping.

Here's a 6146B schematic that says 350 Watts, so I was probably wrong about them putting out 350, probably more like 320 W, as 330 for a 6550 is optimistic so 350 for a 6146B amp is probably optimistic too.

 

[Wasnex]

Doesn't matter if its 72mV or 74mV. Once the amp is "warmed up" and correctly biased, when playing a couple of tunes at rather highish loudness level, then the bias current drifts quite high anyway. Bias is (anyway) anything but something that was really consistent in real practice.
As long as it was calibrated in the range 70..80mV, its fine.

Before setting the bias, I let the tubes run for 15 minutes or so until they stabilize. I have never noticed the current drifting all that much from playing (on any of my amps). The current does drift a bit as the wall voltage fluctuates. But when I adjust the bias, I use a variac to set the line AC to 120V

SVTs are biased pretty cold, so if it drifts up to 80mV, I don't think it's a big deal. But if I checked and the bias was running 80mV, I would probably dial it back to spec.

 

[ThisBass]

Have you ever tried calculating the power output?

P = Vrms / Z

I have to admit I have never heard of a 6550 SVT putting out anywhere near 350 watts unless it was driven way into clipping.

Slight clipping at most, better little less.

 

[bobyoung53]

P = Vrms / Z


Slight clipping at most, better little less.

I'm sure you know the formulas

Before setting the bias, I let the tubes run for 15 minutes or so until they stabilize. I have never noticed the current drifting all that much from playing (on any of my amps). The current does drift a bit as the wall voltage fluctuates. But when I adjust the bias, I use a variac to set the line AC to 120V

SVTs are biased pretty cold, so if it drifts up to 80mV, I don't think it's a big deal. But if I checked and the bias was running 80mV, I would probably dial it back to spec.

Maybe he meant when he's actually playing? It does go all over the place then but that's normal.

 

[Wasnex]

At least one of the reason might be the 235 VAC line voltage at the wall socket.
When I first time measured the output power of my SVT the line voltage was already raised (in the past the line voltage was 220V +/- 5%).

I have got a insulation transformer with adjustable output voltage. Thus it was theoretically possible to measure the SVT at 220 VAC. However its 300VA seem a little bit weak to power the SVT at full throttle.
At the other hand side, I'm quite happy with the amount of power the SVT does push out. If other SVT amps are weak, these are not mine...

Yes I believe your AC system is a bit more stout than than our wimpy 120V system.

FYI from the Rev D schematic, originally dated 1974:

 

[ThisBass]

Before setting the bias, I let the tubes run for 15 minutes or so until they stabilize. I have never noticed the current drifting all that much from playing (on any of my amps). The current does drift a bit as the wall voltage fluctuates. But when I adjust the bias, I use a variac to set the line AC to 120V

SVTs are biased pretty cold, so if it drifts up to 80mV, I don't think it's a big deal. But if I checked and the bias was running 80mV, I would probably dial it back to spec.

Warmed up is not the same just as "real hot" when the tubes are pushed hard over some time.
Its not unusual for tubes to see a noticeable drift in characteristics with highish going temperature.
That's quite normal rather than something extraordinary.

 

[beans-on-toast]

Here's a 6146B schematic that says 350 Watts,

What they are saying is that the dummy load should be rated for at least 350W. This provides a bit of a safety margin above the amp’s rated 300W. They aren’t saying that when setting the bias the amp will put out 350W.

 

[ThisBass]

Maybe he meant when he's actually playing? It does go all over the place then but that's normal.

Yes, that's indeed what I meant.

 

[bobyoung53]

Before setting the bias, I let the tubes run for 15 minutes or so until they stabilize. I have never noticed the current drifting all that much from playing (on any of my amps). The current does drift a bit as the wall voltage fluctuates. But when I adjust the bias, I use a variac to set the line AC to 120V

SVTs are biased pretty cold, so if it drifts up to 80mV, I don't think it's a big deal. But if I checked and the bias was running 80mV, I would probably dial it back to spec.

I had a tech out here do something to mine, and when it got back I could feel the heat coming from the power tubes, checked the bias and they were both set to about 100 mV. It was noisy and hot, turned them both back to normal. I should have just done the work myself.

 

[bobyoung53]

What they are saying is that the dummy load should be rated for at least 350W. This provides a bit of a safety margin above the amp’s rated 300W. They aren’t saying that when setting the bias the amp will put out 350W.

I know it but they are supposed to be more powerful than the later versions. Same on the 6550 schematics, they say full power is 330, both optimistic to me.

 

[bobyoung53]

Warmed up is not the same just as "real hot" when the tubes are pushed hard over some time.
Its not unusual for tubes to see a noticeable drift in characteristics with highish going temperature.
That's quite normal rather than something extraordinary.

I wait about an hour or more usually.

 

[Wasnex]

Maybe he meant when he's actually playing? It does go all over the place then but that's normal.

Could be, but IMHO the TPs are not really measuring bias when you are playing.

To be a bit more specific, the bias on my amps returns to the proper level when I stop playing.

 

[ThisBass]

What they are saying is that the dummy load should be rated for at least 350W. This provides a bit of a safety margin above the amp’s rated 300W. They aren’t saying that when setting the bias the amp will put out 350W.

Bias can anyway calibrated with no load present to the output jack.
At least for the old SVT's where the output terminal is shortened when nothing is plugged in into the output phone jack.

 

[bobyoung53]

Could be, but IMHO the TPs are not really measuring bias when you are playing.

To be a bit more specific, the bias on my amps returns to the proper level when I stop playing.

Oh yes.

 

[bobyoung53]

Bias can anyway calibrated with no load present to the output jack.
At least for the old SVT's where the output terminal is shortened when nothing is plugged in into the output phone jack.

I wouldn't do it. I blew up a head one night, forgot to plug the cab in (two heads, shows I didn't need both ) course I was playing but I still wouldn't do it.