Tubes too hot for OT?

[Robert M. Martinelli]

Bob,

you really spare no effort to help me with my problem. Thank you for that.
I'll try both speakers on both amps and compare the results. After that I'll try to find out why the voltage drops down to around 390v.
Maybe I'll try a different OT (still have a big spare one for two 6L6 tubes and 50-60W rating). THAT should be LOUD

Matt

Hi again Matt,
Don' t worry, I really hope you' re going to have everything fixed, I really think all the magic about this forum is about people helping people, maybe one day I' ll need some help or ask for advice, I won' t feel ashamed doing so, and I already know I' ll find a lot of knowledgeable friends ready to help me out....
That' s the spirit, share what little knowledge I have and improve myself by learning from others - I' m still pretty curious, even if I' m not that young anymore...

Looking forward to hearing from you soon about the results....

Take care

Bob

[txstrat]

The PT is the same type in both amps and reads 320v at each tap measured to ground before the rectifier (both amps). - This type is stated as 300v 200ma. ( two taps = 0 --------- 300V )
With the power tubes pulled both amps are at 419v at the plates and 418 at the grids (dropping one volt over the grid resistor) - less than I expected.
When I plug the power tubes back in, the plates are only 382v in both amps respectively 380v at the grids. ???

I believe the JJ 6V6S are fine with these values and put out a decent power recognizable in the speaker.
BTW: the 6V6 amp sounded even louder with the 2x10'' speakers and had a hefty low end.

The 6L6s don't seem to play off their capabilities at these voltages.

What to do or change or measure? Any hint is highly appreciated.

[Robert M. Martinelli]

With the power tubes plugged both amps are at 419v at the plates and 418 at the grids (dropping one volt over the grid resistor) - less than I expected.
When I push the power tubes back in, the plates have only 382v in both amps respectively 380v at the grids. ???

I believe the JJ 6V6S are fine with these values and put out a decent power recognizable in the speaker.
BTW: the 6V6 amp sounded even louder with the 2x10'' speakers and had a hefty low end.

The 6L6s don't seem to play off their capabilities at these voltages.

What to do or change or measure? Any hint is highly appreciated.

Hi Matt,
With the values you' re getting I find these statements of yours very true,
( also remember the 6V6 amp probably sounded even louder because of the lower impedance, as already stated ).

That PT of yours seems to be a perfect fit for 6V6s, but I find both voltage and current are low for 6L6s, they need a +B voltage around 450 VDC, and their current demand can go up to 200 mAmps when properly driven, add the 90-100 mAmps you' ll have to supply in quiescent conditions ( bias ) for plate and screen grids currents and you' ll see the currently available 200 mAmps simply won' t be enough. So my advice is to change the PT to one with a 330V secondary, and 300 or more mAmps current - this way you' ll have around 465 VDC out of the rectifier with the tubes out and around 445-450 VDC with the tubes in, and the 90-100 mAmps of bias will leave 200-210 mAmps available for the current swing imposed by signal, which is exactly what 6L6s need to release their full power ( around 50-55 W ) when operated in AB1 class at 450 VDC.

As to biasing, you' ll probably find that values around 35 mAmps for the plate current and around 8-10 mAmps for the screen grids will be ok.

( if you use the "1 Ohm cathode resistor" method remember that measuring the voltage drop across it you' ll read the sum of plate and screen grid currents, so you' ll need to read the voltage drop across the screen grid resistor to state the screen grid current and subtract it from the cathode current to get plate current alone and calculate the actual plate idle dissipation power ).

Hope this helps

Best regards

Bob

[txstrat]

Hi Bob,

your statements make sense to me.
I just wonder why 200ma shouldn't be enough. When you look at this website http://www.radiodaze.com/hammond19.htm the 6L6 amp PTs are at around 200ma. The Vibrolux PT even under 200. How come?
I also have different amp (with different PT and OT but same channel hook up) running on 6L6 with the same voltage, that is so loud you couldn't stand it. Absolutely no comparison with the amps I was talking before. But that's what I expected with the new 6L6 amp - not that the amp has to be that loud ever, I just wondered why it's not.

I found out, that the 35w OT for the 6L6s is barely bigger than the 25w OT for the 6V6. The OT of the third amp is almost double as big (does more iron give you more headroom?)

Would it make sense, to try the different OT first?

BTW: my PTs have a bias tap of 50v at 60ma

Matt

[Robert M. Martinelli]

There' s a difference in how the current is calculated, if you look it as P-P then the total swing will be 200, because in a class AB1 Push-Pull the output tubes are on for more than half of the waveform ( but remember you supply bias current ALWAYS, so almost 100 of the 200 mAmps are not available to give you output power as they go "wasted" in biasing the tubes ).

With a 250 or even 300 mAmps PT there 'll be less voltage drop with high current demands ( with 200 mAmps available, 100 mAmps won' t be a problem ) and the amp' s dynamic range and headroom will be much better ( provided you don' t have a tube rectifier, which "sags" when the current demand is high, due to its high differential resistance - but if memory serves me well you have a SS rectifier so this is not your case; BTW, If you like the sound "sagging" gives, get a 200 mAmps PT and switch to a tube rectifier, a lot of people love that ).

As to the OT, the bigger ( within reasonable limits ) the better, headroom will be higher because you' ll have less chances of saturating the iron.

Hope I' ve managed to answer your question

Best regards

Bob

[txstrat]

Well, you never know.
Only thing that was not equal in both amps was the PI.
The 6L6 amp had the PI like in the 6G16 schematic, the 6V6 amp had the PI like a deluxe reverb II (Rivera era amp).
I changed the cathode resistor from 820 to 680, the coupling cap before the PI from .0005 to .1 and the feedback resistor from 10k to 100k. And there it was.
The amp is noticeably louder, even if its (like ENZO wrote) barely louder it is there and that's all that I wanted. Also the bass response is way better.
Thank you all for your help.

Matt

[txstrat]

Bob,

just for your curiosity
I run the amp at 55ma now and it rocks. And yes, I use the 1ohm resistor method and it reads 57ma. That should be the 55ma for the plates and 2 for the grids.
Thanks again.

Matt

[txstrat]

And if anybody is interrested. The amp looks like this: (see attachments)
The protection board for the tubes is not installed on the 2nd pic.