Tubes too hot for OT?

[txstrat]

Hi folks,

I wonder if I could blow the OT if I bias the output tubes too hot (I mean slightly hotter than the OTs value).

One of my amps (1x12'' Jensen C12Q) has a standard Blackface channel with a PI of two 1M resistors, cathode resistor 680 ohms and 22k over 4.7k to ground. Running two JJ 6V6S at 423V with a 25W OT (it says 25W on a sticker although I bought it as Fender replacement OT for a Deluxe Reverb - should be 22W, anyway).
This one is biased at 24ma and (according to webervst site the tubes should be around 10W output each = 20W altogether).

The second amp (2x10'' Jensen C10Q) has a standard Vibroverb channel (6G16) with a PI of two 1M resistors, cathode resistor of 820 ohms and 6.8k over 4.7k to ground. Running two 6L6GC power tubes (TAD) at 423V with a vibroverb replacement OT (35W). These are biased at 48ma which should give them an output of 20W each.

I'm telling you all this, cause the second amp seems to be not as loud as the other even though it has double the value in watts and bigger type output tubes. I'm aware that double watts is not double loudness, but shouldn't the amp with the 6L6 tubes be noticeably louder than the other?
I tried to compensate this with a bias set too hot for the OT. 40W to 35W. Will I kill the OT with this value eventually? What can be wrong?

BTW, the second amp seems to have not as round a bass response than the other - output tubes? I'm thinking about changing to Tung Sol 6L6GC STR tubes.

Matt

[voxrules!]

Hi folks,

I wonder if I could blow the OT if I bias the output tubes too hot (I mean slightly hotter than the OTs value).

One of my amps (1x12'' Jensen C12Q) has a standard Blackface channel with a PI of two 1M resistors, cathode resistor 680 ohms and 22k over 4.7k to ground. Running two JJ 6V6S at 423V with a 25W OT (it says 25W on a sticker although I bought it as Fender replacement OT for a Deluxe Reverb - should be 22W, anyway).
This one is biased at 24ma and (according to webervst site the tubes should be around 10W output each = 20W altogether).

The second amp (2x10'' Jensen C10Q) has a standard Vibroverb channel (6G16) with a PI of two 1M resistors, cathode resistor of 820 ohms and 6.8k over 4.7k to ground. Running two 6L6GC power tubes (TAD) at 423V with a vibroverb replacement OT (35W). These are biased at 48ma which should give them an output of 20W each.

I'm telling you all this, cause the second amp seems to be not as loud as the other even though it has double the value in watts and bigger type output tubes. I'm aware that double watts is not double loudness, but shouldn't the amp with the 6L6 tubes be noticeably louder than the other?
I tried to compensate this with a bias set too hot for the OT. 40W to 35W. Will I kill the OT with this value eventually? What can be wrong?

BTW, the second amp seems to have not as round a bass response than the other - output tubes? I'm thinking about changing to Tung Sol 6L6GC STR tubes.

Matt

Hi Matt,
half power actually means 0,707 ( 70,7% ) of the loudness, smaller differences are barely audible IMHO ( -3db or 0,707 is considered the slightest audible sound intensity difference )

Talking about power, it seems to me you' re confusing the quiescent plate power dissipation and the amp's output power, and they' re different - the first is bias-related, it' s constant, being a DC current, and it' s used to eliminate cross-over distortion in class AB1 designs. If you have a +B voltage of, say, 400 VDC and a bias ( =quiescent ) current of 25 mAmps per tube, then each of your output tubes is dissipating 10 W of power on its plate even when no power is fed to the speaker ( e.g. when the amp is on and you' re not playing ).

Output power is the product of the voltage/current sinusoidal ( thus alternating, not DC ) swing measured on a certain load ( speaker impedance ) under operating conditions ( e.g. when normally playing ). Output power can be then expressed as RMS ( root mean square, or continuous ) or p-p ( peak to peak, or "musical" power ), the first value is the "real" one and the only one which should be taken seriously.

There are some excellent threads about this subject to be found in this forum, so I won' t bother you any longer repeating things someone else explained better than I ever could ( due to my rather poor English :o ) so I encourage you to read them carefully.

Hope this helps

Best regards

Bob

[txstrat]

Bob,

thanks for your quick reply.
Seems like I didn't find the right words to describe what I mean.

Isn't the plate power dissipation of the tubes what the OT has to deal with?
If the OT is supposed to handle 35W, doesn't that mean 35W of the plate power dissipation? I thought so.

Actually I don't care if the amp has 35W or 40W output power or plate power dissipation, since it is already loud enough. But compared to the 6V6 amp I was just wondering why the 6V6 amp is subjectively louder than the 6L6 amp although the 6L6 tubes are supposedly higher in output.

I described the whole thing just to make sure that I measured both amps under the same conditions.

Hence I was asking myself (and the forum) what can I do to make the 6L6 amp louder.
Up to now I biased the output tubes at 48ma what should end up in 40W plate power dissipation - would this OR NOT harm the OT?. Any other suggestions?

Matt

[voxrules!]

Bob,

thanks for your quick reply.
Seems like I didn't find the right words to describe what I mean.

Isn't the plate power dissipation of the tubes what the OT has to deal with?
If the OT is supposed to handle 35W, doesn't that mean 35W of the plate power dissipation? I thought so.

Actually I don't care if the amp has 35W or 40W output power or plate power dissipation, since it is already loud enough. But compared to the 6V6 amp I was just wondering why the 6V6 amp is subjectively louder than the 6L6 amp although the 6L6 tubes are supposedly higher in output.

I described the whole thing just to make sure that I measured both amps under the same conditions.

Hence I was asking myself (and the forum) what can I do to make the 6L6 amp louder.
Up to now I biased the output tubes at 48ma what should end up in 40W plate power dissipation - would this OR NOT harm the OT?. Any other suggestions?

Matt

Hi again Matt,
The OT has to deal with both the bias, or quiescent DC current AND the AC signal current, they both flow in the primary winding at the same time, but, again, raising the bias only raises the power the plates have to dissipate, the OT is dissipating only a little power, because the power dissipation inside the OT in quiescent conditions is calculated simply dividing the DC voltage drop across the OT by the quiescent current, so if, say, 5 DCV drop across the OT , even with a quiescent current of 50 mAmps the OT primary is only dissipating 5VDC*0,05Amps =0,25 W. So, too much idle ( or quiescent ) current won' t be enough to harm the OT, but can make tubes' life a mess....

To state why the amps are different in loudness you need to measure operating conditions on both, and with the values at hand take a look to the threads I mentioned about how to calculate output power on PP output stages.

Hope this clarifies the matter

Best regards

Bob

[Enzo]

The idle current sets the dissipation IN THE TUBES. If your tubes are idling at 20 watts, the TUBES are dissipating the 20 watts, not the transformer. All the transformer does is carry the current from the power supply to the tubes. There is nothing to handle.

The power the transformer does handle is the audio power to the speaker. Idle dissipation in the tubes is NOT output power. Biasing the tubes for a total 20 watts idle dissipation is NOT 20 watts output.

Power tubes are valves, just as the rest of the world calls them. They don't make power on their own. They control the flow of power from the power supply through the circuit. Plugging 100 watt tubes into a 20 watt amp doesn't make it a 100 watt amp. The heftier tubes are capable of handling that power, but it must be there in the system they are plugged into in the first place.

The difference between 25 watts and 35 wats is barely audible.

Loudness is about a lot more than mere power. Unless the two preamps are identical, comparing the amps loudness alone is pointless.

As to the 2x10 amp, are the speakers in phase?

[voxrules!]

The idle current sets the dissipation IN THE TUBES. If your tubes are idling at 20 watts, the TUBES are dissipating the 20 watts, not the transformer. All the transformer does is carry the current from the power supply to the tubes. There is nothing to handle.

The power the transformer does handle is the audio power to the speaker. Idle dissipation in the tubes is NOT output power. Biasing the tubes for a total 20 watts idle dissipation is NOT 20 watts output.

The difference between 25 watts and 35 wats is barely audible.

Loudness is about a lot more than mere power. Unless the two preamps are identical, comparing the amps loudness alone is pointless.

As to the 2x10 amp, are the speakers in phase?

Thanks Enzo,
as usual you managed to make some of the points I was rising clearer ( I guess I still have to work on my English...and on tube amps, too :o ). I thought to have explained things in a pretty neat manner, but your straight-to-the-point answer is way clearer.
If I'm not asking too much, it would be kind of you to analyze
my answer and see what' s wrong and how can I improve the way I write my posts....

And, BTW, checking speakers' phase is a good idea, this could be the reason for the "missing" low end....

Regards

Bob

[txstrat]

It gets a little clearer now. Thanks for your patience.

The speakers are 8 ohms each in parallel to make it fit to the OT which has a secondary of 4 ohms. (both <+> and both <-> hooked together and leading to the OT output.)

I have two circuits hooked up identically (they differ only in the tone stack = Vibroverb 6G16 vs. AB763) the first amp is running 6V6 with a 25W OT the second one is running 6L6GC with a 35W OT.

I built the first amp and liked the sound very much. I just wanted it to be a little louder and with two 10'' speakers instead of one 12''. Thats why I built another one with the same circuit only with 6L6 this time.

The 6L6 amp is audibly less loud.

One thing I found out recently is that I have only 390 volts after the diode rectifier in the second amp. I measured the PT winding and it reads 305 volts that should be around 430V after the rectifier, shouldn't it?

Think I'm gonna measure everything in the circuit again compare the readings in both amps and tinker around a little to find the reason.

Any suggestions, where to look first?

BTW: should I have started a different thread for the missing low end?

[voxrules!]

Hi Txstrat,
You could have a combination of more factors rendering the second amp less audible; as you stated your +B voltage should be around 430 VDC out of the rectifier, so 390 VDC seems a little too low, I would also take a look at the OT ratio to check for an impedance mismatch. Is the OT ratio the same as the first amp with 6V6s or is this second OT optimized for 6L6s?

The OT ratio ( squared ) is equal to the ratio between Zin and Zout, e.g. the if the first amp' s has an output impedance of 8 ohm, and the power tubes are 6V6s in AB1 PP ( imped.around 9KOhm ) , then your first amp' s OT should have a ratio of (Sqrroot(9000/8))=around 33. This ratio is of course fixed, and if you change the tubes your output impedance will change accordingly - e.g. with 6L6s in AB1 PP ( imped. around 6Kohm ) using the same OT would give you an output impedance of (6000/1089)=circa 5,5 Ohm.

To state if your second amp' s OT matches correctly 6L6s with a 4 Ohm load you should verify its ratio to be (Sqrroot(6000/4))=around 39.
OTH, once you know the OT ratio, you can find, simply reversing the formula, which load matches best....

Summing up :

Ratio= Sqrroot(Zin/Zout)

Zout=Zin/(Ratio^2)

Zin=Zout*(Ratio^2)

Even the best built OT has losses in both the iron and the windings, and has a yield factor around 90%, this means that even if properly matched, only 90% of the output power goes to the speakers, this power is further reduced by impedance mismatches, and this, together with the low +B voltage could be the reason for the amp' s being "less audible".

Hope this helps

Best regards

Bob

[txstrat]

I bought both OTs as replacement parts. The first one was stated as a replacement for a Deluxe Reverb (6V6) OT with 8 ohms secondary. The second one was a Hammond replacement OT for a Vibroverb, Vibrolux etc. (6L6) with 4 ohms secondary. I would say up to now there is no reason to believe in mismatch.

I would rather look somewhere in the power supply to find the problem.

[Steve Conner]

You can't go making loudness comparisons like this unless you try both amps through the same speaker. Different speakers have huge effects on the tone and perceived loudness.

For instance, the 12" speaker in the 6V6 amp could have a midrange hump that makes it really roar out, while the 10s are flat, or have their hump further up, where the guitar has less energy. That would be my hunch, anyway.

Also, if your 6L6s are only getting 390V, they'll not be putting out much more power than 6V6s would.

[voxrules!]

I bought both OTs as replacement parts. The first one was stated as a replacement for a Deluxe Reverb (6V6) OT with 8 ohms secondary. The second one was a Hammond replacement OT for a Vibroverb, Vibrolux etc. (6L6) with 4 ohms secondary. I would say up to now there is no reason to believe in mismatch.

I would rather look somewhere in the power supply to find the problem.

Hi again,
If both OTs are correct, then I' m with you, the only thing left to find is the reason for that "off-spec" low +B voltage....I would suggest to check the AC / DC voltages with and without the tubes in to check for voltage drops, and also check the line voltage and the primary taps, looks like you' re off about 9-10 % on the secondary, just like having 110 VAC on a 120 VAC primary tap or 120VAC on a 130 VAC primary tap. Take a reading at the heaters' secondary, if you measure around 5,8 VAC with the tubes out then the whole PT is supplied lower than it expects to be, and this shifts all the secondary voltages on the low side.

As to the loudness, remember it' s a matter of perception, as the human ear has a non-linear response.

Hope this helps

Best regards

Bob

[txstrat]

You can't go making loudness comparisons like this unless you try both amps through the same speaker.

Right, I'm gonna swap the speakers first and listen to the result. Thats the easiest thing to do. Would there be a problem with the mismatch (8 ohm speaker at 4 ohm OT and 4 ohm speakers at 8 ohm OT) if I try it not too long.

I mean I don't want to ruin the OTs due to that mismatch.

[Enzo]

Bob, your English is fine. I wish the locals spoke as well here. I have yet to hear you say something like "Them are cool tubes." Unfortunately I hear that construction all to often here.

I often restate or summarize other posts, for no other reason than to have something explained yet another way for people trying to understand. HAving trained many technicians over the years, I know that sometimes you need to explain something five different ways until you find a way that connects with the student. The more voices in the chorus, the more likely someone will hear the words.

SO don't take my restating something you say as criticism in any way.

[voxrules!]

Right, I'm gonna swap the speakers first and listen to the result. Thats the easiest thing to do. Would there be a problem with the mismatch (8 ohm speaker at 4 ohm OT and 4 ohm speakers at 8 ohm OT) if I try it not too long.

I mean I don't want to ruin the OTs due to that mismatch.

Hi again,
OTs are usually very sturdy, so the chances to ruin yours swapping the speakers for a short test are practically zero; the point is, connecting the 8 Ohm 12" speaker to a 4 Ohm tap would limit current on the OT' s secondary, and your comparison would not give you the results you expect, because you would operate at a lesser power.
On the other amp you' re connecting a 4 Ohm load to a 8 Ohm tap, and this would force more current to flow, this will alter output power as well, compromising the comparison for opposite reasons.

( The "ultimate" test would be having the same speaker on both amps, as per Steve' s suggestion )

Oh, BTW I still think you should try to find out the reason for your low voltage, after all a 10 % lower input voltage makes for a 21 % output power loss...

Regards

Bob

[txstrat]

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

[voxrules!]

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.

[voxrules!]

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

[voxrules!]

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.