It is currently Mon Mar 16, 2020 9:34 pm

All times are UTC - 7 hours



Post new topic Reply to topic  [ 49 posts ]  Go to page Previous  1, 2, 3, 4  Next
Go to page Previous  1, 2, 3, 4  Next
Author Message
Post subject: Re: '65 TRR Dummy Load and Bias Range
Posted: Thu Sep 26, 2013 7:15 am
Offline
Hobbyist
Hobbyist
User avatar

Joined: Thu Jan 28, 2010 5:28 am
Posts: 39
shimmilou wrote:
You seem like a smart guy, so I'm sure that you will eventually figure it out. :wink:

This is what you posted first:

MichelG wrote:
...
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.


The comparison I am making is with 2 tubes (forget about the 4 tubes case I think it confuses the comparison). With 2 tubes but comparing 5 and 10 ohm loads, the difference is as you say and as measured 5W. That difference in load power means a significant difference in actual tube plate dissipation. (The 4 tube case has a power output of about 80Wrms into 4 ohm as expected. Did I state anywhere that the power output for the 4 tube case was half that? Maybe I'm not smart at all :-)).


Top
Profile
Fender Play Winter Sale 2020
Post subject: Re: '65 TRR Dummy Load and Bias Range
Posted: Thu Sep 26, 2013 7:16 am
Offline
Hobbyist
Hobbyist
User avatar

Joined: Thu Jan 28, 2010 5:28 am
Posts: 39
MichelG wrote:
shimmilou wrote:
You seem like a smart guy, so I'm sure that you will eventually figure it out. :wink:

This is what you posted first:

MichelG wrote:
...
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.


The comparison I am making is with 2 tubes (forget about the 4 tubes case I think it confuses the comparison). With 2 tubes but comparing 5 and 10 ohm loads, the difference is as you say and as measured 5W. That difference in load power means a significant difference in actual tube plate dissipation under that maximum signal swing. (The 4 tube case has a power output of about 80Wrms into 4 ohm as expected. Did I state anywhere that the power output for the 4 tube case was half that? Maybe I'm not smart at all :-)). Here are signals across output load at f=250Hz showing comparison of 3 cases: 4 tubes into 5 ohm load, 2 tubes into 5 ohm load and 2 tubes into 10 ohm load (10X probe used so voltage levels are 10x scale values). The signal levels in all 3 cases are the maximum levels just before hard clipping. The Po values for the 2 tube cases shown below are more accurate measurements than previous 40 & 45Wrms values:
Image


Top
Profile
Post subject: Re: '65 TRR Dummy Load and Bias Range
Posted: Thu Sep 26, 2013 10:40 am
Offline
Rock Icon
Rock Icon
User avatar

Joined: Tue Jun 29, 2010 1:20 pm
Posts: 9640
Location: Indiana
OK, more cool stuff, I like it! :)

So, we can agree that the "mismatched" load, does give less wattage out of the amp than a matched load as your results show, correct? If I'm looking at your readings correctly;
-Two tubes at 5 ohms (mismatched impedance) gives 36 watts,
-Two tubes at 10 ohms (closer to matched impedance) gives 42 watts.
Again, note that a lower ohm load does not result in more power out of the amp, as we would expect with a DC voltage.

Now, can you show me that the tubes, and/or tranny are actually dissipating more power with a mismatched load?

_________________
---> "The amp should be SWITCHED OFF AND UNPLUGGED before you do this!" <---

Por favor, disculpe mi español, no se llega a la práctica con mucha frecuencia.


Top
Profile
Post subject: Re: '65 TRR Dummy Load and Bias Range
Posted: Thu Sep 26, 2013 11:24 am
Offline
Hobbyist
Hobbyist
User avatar

Joined: Thu Jan 28, 2010 5:28 am
Posts: 39
shimmilou wrote:
OK, more cool stuff, I like it! :)

So, we can agree that the "mismatched" load, does give less wattage out of the amp than a matched load as your results show, correct? If I'm looking at your readings correctly;
-Two tubes at 5 ohms (mismatched impedance) gives 36 watts,
-Two tubes at 10 ohms (closer to matched impedance) gives 42 watts.
Again, note that a lower ohm load does not result in more power out of the amp, as we would expect with a DC voltage.

Now, can you show me that the tubes, and/or tranny are actually dissipating more power with a mismatched load?


I think we are converging now :-) I have also done the calculation to determine the actual plate dissipation (under max signal swing) for the 2 Tube case with either 5 or 10 ohms. Check out the last paragraph of the article:
"Estimating Tube Plate Dissipation from Measured Output Power"
http://www.jensign.com/TwinReverb
Although this is for pure resistive load, it still spells trouble if you run 2 tubes in a twin with the standard 4 ohm Jensens load if you crank it to full output swing.


Top
Profile
Post subject: Re: '65 TRR Dummy Load and Bias Range
Posted: Thu Sep 26, 2013 11:59 am
Offline
Rock Icon
Rock Icon
User avatar

Joined: Tue Jun 29, 2010 1:20 pm
Posts: 9640
Location: Indiana
MichelG wrote:
...it still spells trouble if you run 2 tubes in a twin with the standard 4 ohm Jensens load if you crank it to full output swing.


Yes, unless you have understanding neighbors, big trouble. :lol: I'll read that article now, thanks.

_________________
---> "The amp should be SWITCHED OFF AND UNPLUGGED before you do this!" <---

Por favor, disculpe mi español, no se llega a la práctica con mucha frecuencia.


Top
Profile
Post subject: Re: '65 TRR Dummy Load and Bias Range
Posted: Tue Oct 01, 2013 8:41 pm
Offline
Hobbyist
Hobbyist
User avatar

Joined: Thu Jan 28, 2010 5:28 am
Posts: 39
I've updated the article to show measurements of dynamic plate voltage and cathode current using the Groove Tube Bias tool and a home-made HV voltage divider.


Top
Profile
Post subject: Re: '65 TRR Dummy Load and Bias Range
Posted: Tue Oct 01, 2013 11:47 pm
Offline
Rock Icon
Rock Icon
User avatar

Joined: Tue Jun 29, 2010 1:20 pm
Posts: 9640
Location: Indiana
This is getting better all of the time, thanks. You're getting there. The counter EMF is going to be one of the main factors that limit the current through the primary of the transformer, the more current that you try to pull, the harder the tranny resists. That, and the fact that you can't have an ideal transformer, means that you will have much less power dissipated by the tubes than you might expect when halving the load impedance on the secondary. Conversely, when the output load impedance gets to be too high, the lack of counter EMF will allow the voltage to build to the point that the OT fails. Plus, we have more going on with a guitar signal than just one frequency, so these results are still only part of the picture, but still cool stuff.

It would be interesting to measure these same parameters with the OT secondary shorted, as would happen when no speaker is connected when a shorting jack is used to protect the OT (over several frequencies of course).

_________________
---> "The amp should be SWITCHED OFF AND UNPLUGGED before you do this!" <---

Por favor, disculpe mi español, no se llega a la práctica con mucha frecuencia.


Top
Profile
Post subject: Re: '65 TRR Dummy Load and Bias Range
Posted: Wed Oct 02, 2013 6:56 am
Offline
Hobbyist
Hobbyist
User avatar

Joined: Thu Jan 28, 2010 5:28 am
Posts: 39
shimmilou wrote:
This is getting better all of the time
------
It would be interesting to measure these same parameters with the OT secondary shorted, as would happen when no speaker is connected when a shorting jack is used to protect the OT (over several frequencies of course).


Yup it is fun learning this stuff and seeing how REAL circuits work. In particular with half the "correct design" load, i.e. 4 ohm with 2 tubes only, the measured results (at a nominal 250Hz and resistive output load) agrees pretty well with the basic power-output expectations. According to the '65 TRR schematic, Fender measure the "output power" of the amp using a 4 ohm resistive load and 1 kHz input signal into main channel with specified control settings.

In the '65TRR circuit, the OT speaker main output jack J5 is a 3 pin jack with the extra pin shorted to Gnd when nothing is connected to it as shown in output picture ... safest default I guess.
Image


Top
Profile
Post subject: Re: '65 TRR Dummy Load and Bias Range
Posted: Thu Oct 03, 2013 6:04 am
Offline
Hobbyist
Hobbyist
User avatar

Joined: Thu Jan 28, 2010 5:28 am
Posts: 39
but of course what use are all those measurements and theory unless you actually use that '65TRR for its sonic excellence? Playing clean or with a good OD pedal, I love the sonic depth of that amp! Love that bright incandescent T47 lamp too:
http://www.jensign.com/straightup/wgp_02_14_2013.html


Top
Profile
Post subject: Re: '65 TRR Dummy Load and Bias Range
Posted: Thu Oct 03, 2013 6:47 am
Offline
Rock Star
Rock Star
User avatar

Joined: Sat Jul 03, 2010 2:18 pm
Posts: 6544
MichelG,

I'm prolly mis-understanding ... but the outer portion of the the speaker jack (J5) has to be connected to the same ground as the secondary winding in order for the tranny to work.


Top
Profile
Post subject: Re: '65 TRR Dummy Load and Bias Range
Posted: Thu Oct 03, 2013 7:01 am
Offline
Hobbyist
Hobbyist
User avatar

Joined: Thu Jan 28, 2010 5:28 am
Posts: 39
BMW2002Ti wrote:
MichelG,

I'm prolly mis-understanding ... but the outer portion of the the speaker jack (J5) has to be connected to the same ground as the secondary winding in order for the tranny to work.


The "outer portion" of the J5 main speaker jack (or "Ring") is always connected to the Ground side of the OT (same as for the extension jack J6). The TIP connection of J5 is connected to Gnd if nothing is inserted into J5 which shorts the output of the OT. When you insert a plug connected to speaker into J5, that tip connection to Gnd is opened and the tip is still connected to the other side of OT and the OT is now properly connected to the speaker load. Worst thing you could do is put an OPEN plug into J5 which means an open (infinite) load which would generate a huge back-EMF and probably cause tubes to arc-over etc... very nasty. Note that putting a speaker-connected plug into the EXTENSION jack J6 with nothing connected to J5 will not work since the OT output will still be shorted by J5.


Top
Profile
Post subject: Re: '65 TRR Dummy Load and Bias Range
Posted: Tue Oct 08, 2013 7:46 am
Offline
Hobbyist
Hobbyist
User avatar

Joined: Thu Jan 28, 2010 5:28 am
Posts: 39
I have added a sketch to show how the various power-tube voltages, currents and Q point look based on my measurements. Provides a big-picture view of the push-pull dynamics. At very bottom of the page:
http://www.jensign.com/TwinReverb


Top
Profile
Post subject: Re: '65 TRR Dummy Load and Bias Range
Posted: Tue Oct 15, 2013 9:27 am
Offline
Hobbyist
Hobbyist
User avatar

Joined: Thu Jan 28, 2010 5:28 am
Posts: 39
article is updated and reorganized a bit, including a basic 6L6GC load-line analysis to estimate maximum expected power output (last section).


Top
Profile
Post subject: Re: '65 TRR Dummy Load and Bias Range
Posted: Tue Oct 22, 2013 10:19 am
Offline
Hobbyist
Hobbyist
User avatar

Joined: Thu Jan 28, 2010 5:28 am
Posts: 39
article has been updated with a push-pull calculator that some may find useful. Also some more stuff on maximum power calculations.


Top
Profile
Post subject: Re: '65 TRR Dummy Load and Bias Range
Posted: Tue Oct 22, 2013 2:00 pm
Offline
Rock Icon
Rock Icon
User avatar

Joined: Tue Jun 29, 2010 1:20 pm
Posts: 9640
Location: Indiana
Bookmarked. :)

Have you thought about changing the value of the screen grid resistors? Increasing the value can give some pleasing compression to the amp tone. I use 2k in my HRDlx.

Some more interesting info here that you might enjoy, not so math heavy either.

http://www.geofex.com/tubeampfaq/tubefaq.htm#transformers

_________________
---> "The amp should be SWITCHED OFF AND UNPLUGGED before you do this!" <---

Por favor, disculpe mi español, no se llega a la práctica con mucha frecuencia.


Top
Profile
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 49 posts ]  Go to page Previous  1, 2, 3, 4  Next
Go to page Previous  1, 2, 3, 4  Next

All times are UTC - 7 hours

Fender Play Winter Sale 2020

Who is online

Users browsing this forum: No registered users and 2 guests


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum

Search for:
Jump to: