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Post subject: Re: '65 TRR Dummy Load and Bias Range
Posted: Wed Oct 23, 2013 8:06 am
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thanks for that faq. Very interesting reading. The screen-grid resistors in the TRR are 470 ohm ... which made me realize that I need to redo that "screen-grid sag" measurement and edit the comment I made based on the open-socket measurement with 2 tubes popped out.
My live V and I swing measurements on the TRR power tubes show that although it probably won't harm OT to run under-load by a factor of 2 (as mentioned in the faq), and you get less output power fed into OT primary, but the actual plate dissipation of the output tubes is exceeded so you can wear them out which is not mentioned in that faq.

Update: Here is a summary view of measurements at 1 kHz on the TRR with 4.0 ohm resistive load, 4 tubes at maximum output power including Ip, Vp, Vscreen_grid and Vout:
http://www.jensign.com/TwinReverb/twin_maxpow.jpg
The green T line in Vp and screen-grid V plots is the Vb quiescent line.
The screen-grid drop at max signal is ~ 30V but the screen supply drops by ~ 10V so the screen-grid current is (20V/470ohm) ~ 43 mA. OT efficiency is ~ 83%.


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Post subject: Re: '65 TRR Dummy Load and Bias Range
Posted: Wed Oct 23, 2013 6:06 pm
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Keep in mind that in an actual amp, operating normally, as tube Plate current rises, the Plate voltage drops. The same applies to the screen grid current and voltage. As screen grid current rises, the screen grid voltage drops, due to the increased voltage drop across the screen grid resistor, as well as some sag in the power supply. This is part of the equation not accounted for in the calculations, and another part of the reason that the mismatched output impedance is not going to cause much overload on the tubes themselves. :idea:

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Post subject: Re: '65 TRR Dummy Load and Bias Range
Posted: Thu Oct 24, 2013 5:51 am
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shimmilou wrote:
Keep in mind that in an actual amp, operating normally, as tube Plate current rises, the Plate voltage drops. The same applies to the screen grid current and voltage. As screen grid current rises, the screen grid voltage drops, due to the increased voltage drop across the screen grid resistor, as well as some sag in the power supply. This is part of the equation not accounted for in the calculations, and another part of the reason that the mismatched output impedance is not going to cause much overload on the tubes themselves. :idea:

Well the previous link shows actual MEASURED results so unless you don't believe the screen-grid measurements taken under full output power, they show very little screen grid droop. Similarly for B+ supply (at OT center-tap). Plate dissipation is easily determined by time average of MEASURED Vp and Ip irrespective of any droops in either supply. Results for under-load definitely show plate dissipation exceeded, lower output power (to OT primary) but higher total supplied power. Won't hurt OT but will shorten tube life if you played at max level continously .. which is obviously unlikely.


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Post subject: Re: '65 TRR Dummy Load and Bias Range
Posted: Thu Oct 24, 2013 8:52 am
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I believe the math (for the most part), but it isn't going to be sustained in actual use. Again, it appears that you are using peak current and peak voltage, at one particular frequency, which doesn't really cover what actually happens in real use of the amp. The power should be looked at as RMS, and the signal into the amp will be from a guitar, which will be a complex wave, covering many frequencies, which means you would have to calculate at each frequency, and then average, using RMS, not peak. I'll admit that the math is a little more than I care to deal with, but it seems to me that it is not representing real world operating conditions. Otherwise, you've gone to a great deal of trouble, and done a great job, and I appreciate your insight and information. :)

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Post subject: Re: '65 TRR Dummy Load and Bias Range
Posted: Thu Oct 24, 2013 9:49 am
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shimmilou wrote:
... Again, it appears that you are using peak current and peak voltage, at one particular frequency, which doesn't really cover what actually happens in real use of the amp. The power should be looked at as RMS... :)


The shape of the plate voltage swing (downward during conducting cycle) and plate current are quite close to half-sines between low and midrange frequencies (measured). You can always calculate exactly what the RMS power is by adding the voltagexcurrent at each instant of time and average that over the cycle which is what RMS power means means. Since the swings are pretty close to half-sines, at least for the clean TRR, the rms power is approximately Vpeak/Sqrt(2) X Ipeak/Sqrt(2) = 1/2 VpeakxIpeak (which is where that 1/2 factor comes from in several equations). That is the common design approach used as in the references I mention in article. Of course I realize that a guitar signal is multi-frequency and the net response of the amp is weighted over all frequencies, but design always starts somewhere. For example amp output power ratings, afaik are usually spec'd using a single frequency 1 kHz into a resistive load. Glad you enjoy the article ... give the calculator a spin .. it is just a simple design aid, using the basic equations that tube amp designers have used for years ... no compression or distortion included however.


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Post subject: Re: '65 TRR Dummy Load and Bias Range
Posted: Thu Oct 24, 2013 4:45 pm
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Cool. Being a tech for 30+ years now, I'm very familiar with the design (theory) vs what actually happens in use. I guess that this should make for some very well built, or at least very well designed equipment. However, with things like this, and gain circuits, and other circuits, you usually don't get the design values in actual use. As you pointed out, a single frequency will give a nominal value for things like speaker impedance, etc.

How does your computer scope compare to a traditional scope? Is it real time, or close? I haven't tried one since I didn't get along with the first digital/LCD scopes that I tried, like the Fluke scope-meters, as the lag always bugged me. I had a similar problem switching from an analog meter to a digital, but i got used to that.

I have a few scopes, but this is my favorite:

Image

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Post subject: Re: '65 TRR Dummy Load and Bias Range
Posted: Fri Oct 25, 2013 7:37 am
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shimmilou wrote:
....
How does your computer scope compare to a traditional scope? Is it real time, or close? I haven't tried one since I didn't get along with the first digital/LCD scopes that I tried, like the Fluke scope-meters, as the lag always bugged me. I had a similar problem switching from an analog meter to a digital, but i got used to that.
....


I've been fiddling with electronics since early '60s .... started trying to mod my parents' console stereo "audio in" to match my electric guitar when I knew nothing about electronics ... and got my fair share of HT shocks!
Nice HP scope there. A portable RT 150 MHz scope (used) is on my radar. Depending on the make, the USB compscope is almost real time. I love the portability and use with laptop, easy screen capture, data export and math of course :P . But if you want a USB compscope in BW ~ 100+MHz range, it gets pricy. (If I won a lottery I'd get one of those 300MHz Picolabs jobs). For audio use (and in fact nicely up to at least 10 MHz), the Digilent Analog Discovery USB compscope/fn generator unit is amazing ... powered directly from USB. Some examples of things I've used it for:
http://www.jensign.com/Discovery/
I used it quite extensively until the flimsy USB jack cracked off ... tiny solder bumps impossible to repair! Now L'm back to using my trusty (but lower performance) Syscomp Design USB comp-scope seen in most of those recent traces I've shown here.


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Post subject: Re: '65 TRR Dummy Load and Bias Range
Posted: Fri Oct 25, 2013 12:20 pm
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:lol: Same start here, with the guitar plugged into the stereo unit. It sounded fantastic.....for a few minutes. :oops:

I like the idea of a computer based scope, but I have always used a full-size even for travel.

I use a low-tech approach for screen capture, a Canon camera, which does a nice job. (Ripple on a DC supply)

Image

Test signal:

Image

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Post subject: Re: '65 TRR Dummy Load and Bias Range
Posted: Fri Oct 25, 2013 1:43 pm
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shimmilou wrote:
:lol: Same start here, with the guitar plugged into the stereo unit. It sounded fantastic.....for a few minutes. :oops:
...

Some years ago (check the date) I designed a totally passive EQ (~ reverse RIAA RC network) to adapt a typical electric guitar to a phono-input of typical older stereo. Sounds quite good and shows you what the "true" sound of your axe is through a clean (Hi-Fi) system. http://www.jensign.com/JavaScience/www/ ... neaxe.html

Nice scope traces, but I was expecting you to show me your tube amp's sag on the B+ and Vgrid supply voltages at max output level :wink:


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Post subject: Re: '65 TRR Dummy Load and Bias Range
Posted: Fri Oct 25, 2013 4:35 pm
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I am not that into it. :lol:

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Post subject: Re: '65 TRR Dummy Load and Bias Range
Posted: Tue Oct 29, 2013 11:48 am
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well, talking about dummy loads, I'm gonna see how limiting my '65 TRR to 25W output works at our next gig this Friday .. make sure nothing unexpected happends with this simple mod: Popping out 2 power tubes and adding a dummy series power resistor box to properly match. Obviously doesn't help with the REAL (physical) load ... but the Beatles got it right: "boy, you're gonna carry that weight"
http://www.jensign.com/TwinReverb/twin25watt.html


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Post subject: Re: '65 TRR Dummy Load and Bias Range
Posted: Tue Oct 29, 2013 12:14 pm
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I was wondering if you were ever going to get out from under the hood and play that thing. :lol:

I hope that all goes well.

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Post subject: Re: '65 TRR Dummy Load and Bias Range
Posted: Fri Nov 08, 2013 8:36 am
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shimmilou wrote:
I was wondering if you were ever going to get out from under the hood and play that thing. :lol:

I hope that all goes well.


It was a blast! Some pics from that (and other) gigs linked here:
http://www.jensign.com/straightup/
Decided to use all 4 tubes. Great dance crowd .. very noisy. Picked up 2 more gigs including a NYE "Dinner Party" thing. My '62 Jag areissue nd my unmodified '65 TRR .. stock speakers ... stock tubes, works perfectly for our mix of classic pop '60s-'80s. I like the sound depth you get with 2 x 12" Jensens. I do lead/rhythm plus comp'd in bass lines so need that sound depth. I use Boss BD2 pedal for about 4 pieces where I need reasonable blues overdrive sound (Cinnamon Girl, Heart Full of Soul etc).


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Post subject: Re: '65 TRR Dummy Load and Bias Range
Posted: Fri Nov 08, 2013 12:45 pm
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Awesome! It is interesting, after all of that work, you decided to go with the stock configuration of the TRRI. 8)

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Post subject: Re: '65 TRR Dummy Load and Bias Range
Posted: Fri Nov 08, 2013 3:04 pm
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shimmilou wrote:
Awesome! It is interesting, after all of that work, you decided to go with the stock configuration of the TRRI. 8)

Well it wasn't work but play and I am prepared for any tube or speaker eventuality. Maybe useful also to someone.


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