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Post subject: '65 TRR Dummy Load and Bias Range
Posted: Tue Sep 24, 2013 7:10 pm
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Some information on measurements on my '65 TRR, using 2 power tubes instead of 4 etc: http://www.jensign.com/TwinReverb/


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Post subject: Re: '65 TRR Dummy Load and Bias Range
Posted: Wed Sep 25, 2013 4:44 am
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Hi MichelG,

Cool stuff! Thanks for taking the time to illustrate and share this, and I might add, a very nice job. :)

You should try two output tubes with a 4 ohm load and check the distortion and power out from the amp, and to the speakers. I was curious as to what problems you think this would cause. An impedance mismatch within a 2 to 1 ratio is not a problem for a tube amp, and the results might surprise you. :idea:

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Post subject: Re: '65 TRR Dummy Load and Bias Range
Posted: Wed Sep 25, 2013 6:21 am
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Thanks! Operating with a load impedance of half the design value (e.g. driving into 2 ohms with 4 tubes, or 4 ohms with only 2 tubes) is strictly speaking not a good idea. Sure you'll be driving the tubes harder for a given input level, with more tube distortion, but the tubes will be dissipating (under signal) more power than their nominal "Maximum Plate" power, typically ~ 25W per tube for a GT6L6B. This could cause early tube failure. Although I don't do regular tube amplifier design, a quick check of the RCA Tube Manual shows that push-pulls are designed with output load impedances so that the plate dissipation is slightly below the maximum safe value, so halving the load will definitely drive those power tubes into a danger zone with considerably more plate current. That being said, I know that some folks have told me they operate their Twins with 2 tubes into the 4 ohm paralleled speakers. I'll do some power measurements on a 5 ohm dummy load (briefly :-) to see how hard those tubes are working ... then briefly with the actual speakers. But since the Twin is so loud, it might be hard to hear a real difference at ear-shattering levels. I really need to get a DRR although I'm learning how to safely carry my TRR.


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Post subject: Re: '65 TRR Dummy Load and Bias Range
Posted: Wed Sep 25, 2013 7:27 am
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Pulling two output tubes will prolly raise the plate voltages of the two remaining tubes. You will need to rebias the amp to compensate. For the GT 6L6GB (5881WXT+), I would not go above 20-22 watts per tubes, at idle.

I believe as long as you have stable bias on the tubes & are in 60-70% max dissipation range --- you will not do harm to capable tubes, despite what load the secondary of the PT is seeing. Now, as for the PT itself --- you need heavier winding and plenty of iron for heat dissipation, when playing with load impedances.


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Post subject: Re: '65 TRR Dummy Load and Bias Range
Posted: Wed Sep 25, 2013 9:27 am
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BMW2002Ti wrote:
Pulling two output tubes will prolly raise the plate voltages of the two remaining tubes. You will need to rebias the amp to compensate. For the GT 6L6GB (5881WXT+), I would not go above 20-22 watts per tubes, at idle.

I believe as long as you have stable bias on the tubes & are in 60-70% max dissipation range --- you will not do harm to capable tubes, despite what load the secondary of the PT is seeing. Now, as for the PT itself --- you need heavier winding and plenty of iron for heat dissipation, when playing with load impedances.


Power tranny?

:?

Arjay

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Post subject: Re: '65 TRR Dummy Load and Bias Range
Posted: Wed Sep 25, 2013 9:42 am
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Here are some measured results:
4 Tubes: Vcc = 450VDC Vgrid=-46V Ibias = 35mA
2 Tubes Vcc = 456VDC Vgrid = -46V Ibias = 36mA

So very little difference. Shouldn't need to adjust idle bias conditions.
The *idle* plate dissipation is only part of the consideration. When you change the load impedance (by changing speaker impedance) you are changing the active signal plate dissipation dramatically. Here are some more measurements:

2 Tubes with 5 ohm load: Peak output voltage(clip) = 20Vpeak so 40Wrms output
2 Tubes with 10 ohm load: Peak output voltage(clip) = 30Vpeak so 45Wrms output

The power outputs are similar. However the 5 ohm load pushes 35% MORE current, which also reflects back to the primary current supplied by the tubes. The plate current swing will therefore push the tube dissipation considerably higher, no matter what you do the idle bias adjustment. So for example a (proper) 8ohm load which has an ACTIVE tube dissipation of 20W will be pushed up to 27W if you use a 4 ohm load. A bit too close for comfort.


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Post subject: Re: '65 TRR Dummy Load and Bias Range
Posted: Wed Sep 25, 2013 10:32 am
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shimmilou wrote:
Hi MichelG,
--
You should try two output tubes with a 4 ohm load and check the distortion and power out from the amp, and to the speakers.
..... and the results might surprise you. :idea:


OK after listening with this config. with my Fender Jag, I'll grant that this does have that extra growl/dirty factor even if you are pushing those 2 tubes into certain early death (if you rock really hard!).
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Post subject: Re: '65 TRR Dummy Load and Bias Range
Posted: Wed Sep 25, 2013 1:47 pm
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MichelG wrote:
Here are some measured results:
4 Tubes: Vcc = 450VDC Vgrid=-46V Ibias = 35mA
2 Tubes Vcc = 456VDC Vgrid = -46V Ibias = 36mA

...

2 Tubes with 5 ohm load: Peak output voltage(clip) = 20Vpeak so 40Wrms output
2 Tubes with 10 ohm load: Peak output voltage(clip) = 30Vpeak so 45Wrms output

....However the 5 ohm load pushes 35% MORE current...


:?

Where do you see 35% more current? The lower-ohm load shows very little current increase. The OT being an inductor will resist current changes, so the harder you push, the more it resists (to a point). If you check your tubes across range of output power, you should see a peak near the "matched" load, and less power out with a higher or lower load impedance (check your RCA tube manual curves). If we were talking DC, sure, the current would roughly double with a halved impedance, but tubes, trannies and speakers just cannot work that way due to the AC on them and the inductance of each. There is no extra power to be dissipated, it just doesn't develop to begin with. The fact that your have a largely purely resistive load will affect the results somewhat also. A speaker(s) alone will have more influence (inductance) to change the equation.

Sometimes the halved speaker impedance on a tube amp can get you that earlier distortion desired, as you've noted, but man, what a lot of work to get a TR to distort. :lol:

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Post subject: Re: '65 TRR Dummy Load and Bias Range
Posted: Wed Sep 25, 2013 1:57 pm
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Retroverbial wrote:
BMW2002Ti wrote:
...load the secondary of the PT is seeing...


Power tranny?

:?

Arjay


Getting old can be rough. :lol: I'm sure that he meant OT.

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Post subject: Re: '65 TRR Dummy Load and Bias Range
Posted: Wed Sep 25, 2013 2:04 pm
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Mike, oh man, I meant output tranny. If the plate voltages are in the ballpark (2 versus 4 tubes) --- prolly means that the OT (or OPT) has enough heft in the primary stage to compensate for the number of output tubes connected. As you stated: AC inertia.

MichelG, have you tried using a 12AU7 in the phase inverter position? The lower peak-to-peak swing drive may give you the same effect, without the resistance load.

Remember, that impedance is not constant throughout the frequency range. So the set resistance load maybe affecting the overall tone of the amp, as well as the output loudness. The spectrum of frequency response may become dominated by your resistors and not the characteristics of the speakers.


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Post subject: Re: '65 TRR Dummy Load and Bias Range
Posted: Wed Sep 25, 2013 2:21 pm
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I'll have to try an AU in the PI to see how it works. Interestingly, Carvin uses a lower Plate voltage on the PI to achieve less output power and early distortion in many of their amps. Although the distortion is from the PI and not the output tubes, I really like it.

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Post subject: Re: '65 TRR Dummy Load and Bias Range
Posted: Wed Sep 25, 2013 2:44 pm
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shimmilou wrote:
MichelG wrote:
Here are some measured results:
4 Tubes: Vcc = 450VDC Vgrid=-46V Ibias = 35mA
2 Tubes Vcc = 456VDC Vgrid = -46V Ibias = 36mA

...

2 Tubes with 5 ohm load: Peak output voltage(clip) = 20Vpeak so 40Wrms output
2 Tubes with 10 ohm load: Peak output voltage(clip) = 30Vpeak so 45Wrms output

....However the 5 ohm load pushes 35% MORE current...


:?

Where do you see 35% more current?


For the 5 ohm load, peak current is 20V/5 = 4 Apeak
For the 10 ohm load, peak current is 30V/10 = 3 Apeak
so the 5 ohm load definitely draws 33% more current ... no question.
This current draw is reflected back to the transformer primary so plates will swing with larger plate current. As a rough approximation for determining safe operating conditions you have to assume something simple like the plate load being resistive. Of course speakers are complex but assuming that the output load reflects back as a plate load is a good starting place. As in an example in that RCA manual with a typical design (assuming resistive load line for load-reflected output transformer), you figure out the max. plate dissipation under max. signal swing and if its too high for that tube, you have to increase the load impedance (or equivalently raise the output speaker impedance). Frequency response of the output load obviously complicates that.
Haven't tried using a different driver tube. Yeah the TR is hard to push but I play fairly clean ... just an interesting little adventure. Hopefully some of the results will be useful to somebody.


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Post subject: Re: '65 TRR Dummy Load and Bias Range
Posted: Wed Sep 25, 2013 9:17 pm
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Which math are you going with? Your earlier post, you listed 40W rms output at 20 V, now you've somehow doubled the wattage. :? No way you'll get 4 amps from the OT, especially at 20 volts. Use RMS, not peak. :wink: I think that your first numbers are much more reasonable, with the load halved, it's closer to 10 % to 12 % difference in output wattage, and the current difference would be even less, percentage wise. Also, as pointed out earlier, impedance changes with frequency, and 4 ohms or 8 ohms is nominal, and at a particular frequency. The amp rating is also derived with a purely resistive load at a specific frequency, so the rating will be somewhat different with a speaker and a guitar signal.

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Post subject: Re: '65 TRR Dummy Load and Bias Range
Posted: Thu Sep 26, 2013 5:54 am
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shimmilou wrote:
Which math are you going with? Your earlier post, you listed 40W rms output at 20 V, now you've somehow doubled the wattage. :? No way you'll get 4 amps from the OT, especially at 20 volts. --.


Not sure why you think I've "doubled the wattage" ... maybe I wasn't clear enough so:
For the case of 2 tubes driving a 5 ohm resistive load:

The measured PEAK voltage across this load is 20Vpeak (or 14.1Vrms).
This means the PEAK current through this load is 20/5 = 4Apeak (or 2.8Arms).
The power dissipation in this 5 ohm load is therefore:
Pdiss = Vrms*Irms = 14.1*2.8 = 39.5Wrms
or equivalently in terms of PEAK voltages and currents:
Pdiss = (Vpeak)^2/(2*Rl) = 40Wrms

Following the design procedures used in the RCA tube manual, it is easy to transform this measured output load power back to the plate circuit (using the known turns ratio of the TRR transformer of ~ 24) to get actual TUBE dissipation for this maximum signal case. For the 5 ohm load case it is ~ 26W/tube and for the 10 ohm load case (close to recommended 8 ohm) it is ~ 12W/tube so as i said earlier you are operating in dangerous area if you play real hard and use 2 TRR tubes into a 4 ohm load. I'm going to add details of how these numbers are obtained along with scope traces (showing those Vpeak values) to the article later today. Of course transformers are not ideal and speaker loads are not purely resistive .. complicates things ... but the general trend will definitely be that using a 4 ohm load in the TRR with 2 tubes will push your plate dissipation way up there.


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Post subject: Re: '65 TRR Dummy Load and Bias Range
Posted: Thu Sep 26, 2013 6:06 am
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You seem like a smart guy, so I'm sure that you will eventually figure it out. :wink:

This is what you posted first:

[quote="MichelG"]...
4 Tubes: Vcc = 450VDC Vgrid=-46V Ibias = 35mA
2 Tubes Vcc = 456VDC Vgrid = -46V Ibias = 36mA
...

2 Tubes with 5 ohm load: Peak output voltage(clip) = 20Vpeak so 40Wrms output
2 Tubes with 10 ohm load: Peak output voltage(clip) = 30Vpeak so 45Wrms output

That is 5 watts difference in output between two or four output tubes, according to your first numbers, which seems more realistic to me. :idea: Notice that with the lower resistance load, you have less wattage out, which is as I stated.

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