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loftin white : voltages

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Posted on August 4, 2015 at 07:21:31
genelex
Audiophile

Posts: 95
Joined: October 16, 2005



I have heard that the plate to cathode voltage of the output tube
in a loftin white should be about half of the B+. Is this true and why?
what happens if I have B+ 518v and 310v from plate to cathode?
thanks

 

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RE: loftin white : voltages, posted on August 4, 2015 at 08:02:44
Triode_Kingdom
Audiophile

Posts: 10049
Location: Central Texas
Joined: September 24, 2006
 Are you building this? I really dislike circuits where a driver failure (open) will cause the output tube to fry. Coupling caps aren't that bad. Anyway, I think the 50% rule you've heard must be only a very general guideline. It's probably intended to ensure that both tubes operate with enough voltage to be linear. I wouldn't rely on this for optimum results.

 

RE: loftin white : voltages, posted on August 4, 2015 at 12:40:42
drlowmu
Manufacturer

Posts: 9730
Location: East of Kansas City
Joined: January 10, 2005
 First of all, despite the name of the Sound Practices article, you are NOT building a Loftin-White.

A L-W involves feeding BOTH stages from a common B+ supply ( NO decoupling ) and feeding back the anti phase of the 60 HZ hum component back to the input tube, to cancel 60 HZ hum ( and some music maybe ). You have a simple two stage DC amp !!! You are showing us the Marzio-Jelasi schematic. Specifically, R5 ( and R6, R7 to ground ) and C7 serve to decouple the driver stage B+ from the final stage's B+, a good thing IMHO.

Questions :

(1) What is the value of your Final's tube's Rk, so we can compute current, and dissipation of the 2A3?

(2) Also, what TYPE of input tube do you use, and what is the input tube's B+ voltage, plate and cathode voltage, and Rk value ??

(3) What are your R values for R4, R5, R6 and R7? Same as in the article ???

This simple circuit you are doing is likely the very best sounding amplifier configuration in ALL of vacuum tube audio. You need to do it correctly, and it takes thought and time.

I have been DIY working with this topology exclusively for 9 years now, and only now am I getting it fairly correct. In my mind, and trusting my ears, its the right path to DIY and to hear !!

Why don't you correspond with me privately, my email is above, and we can share some ideas about this topology. Post your voltages so others may learn from this.

Jeff Medwin

 

RE: loftin white : voltages, posted on August 4, 2015 at 13:02:11
Paul Joppa
Industry Professional

Posts: 7296
Location: Seattle, WA
Joined: April 23, 2001
 The choice of 2A3 cathode voltage is a compromise between the following two considerations:

1) If that voltage is high, the resistances between 2A3 cathode and ground will dissipate (waste) excessive heat

2) If that voltage is low, the driver plate voltage will be low and the driver will distort more.

The compromise chosen by most designers is usually close to the rule of thumb (cathode voltage = half of plate voltage) you mentioned, but there is nothing special about the value. It's more an observation than a rule.

When I have designed such amplifiers, I usually choose the operating point of the 2A3 and the particular driver I may have chosen, and let the chips fall where they may. I prefer to have substantial headroom, both positive and negative, for the driver plate voltage. Most drivers are not as linear as the 2A3, and the extra headroom keeps the distortion low at the signal voltages in use. An alternative is to use a choke or current source for the driver plate load, which linearizes the driver and allows it to operate with less headroom.

 

RE: loftin white : voltages, posted on August 4, 2015 at 13:02:16
deafbykhorns
Audiophile

Posts: 1067
Location: Florida
Joined: October 17, 2003
 Give me some values and I'll SIM it for you.

 

RE: loftin white : voltages, posted on August 4, 2015 at 13:02:18
drlowmu
Manufacturer

Posts: 9730
Location: East of Kansas City
Joined: January 10, 2005
 I just looked up the parts values in the article. IF you used the specified 3.75K Rk for the 2A3 stage, you are pulling a whopping 82.66 mA. per tube, and, with your self-described 208 VDC P-K, is a whopping 17.19 Watts out of a 15 Watt rated tube. That is not too good - at all. The 2A3 WILL sound thermally STRESSED, because ....it is !!

You will need to re-do the R values, and shoot for a Golden Ratio in dissipation, 62 % of 15 Watts. Likely it will end up 5K or 5.5 K.

Since everything in your DC amp is interrelated, lets do this privately, via email.

Jeff Medwin

 

RE: what about a 40w JJ tube?, posted on August 4, 2015 at 13:37:41
deafbykhorns
Audiophile

Posts: 1067
Location: Florida
Joined: October 17, 2003
 .

 

RE: what about a 40w JJ tube?, posted on August 4, 2015 at 13:50:02
drlowmu
Manufacturer

Posts: 9730
Location: East of Kansas City
Joined: January 10, 2005
 Nice tube, "I" would not run it over 15 Watts. It will last FOREVER !!! I own some of them.

I will likely do 250 VDC P-K and 42-43 mA. and never have to think about it getting worn out. I don't resort to hot rodding a tube's dissipation, to get a circuit to sound great. There are simply far better paths to tube audio high performance IMHO. The performance MUST be sustainable over time, or its a poor design in my viewpoint.

Me thinks you next, need to "lose" the series coupling cap in your two stage 2A3 amp, replace it with three runs of 21 AWG silver, soft annealed.

Jeff Medwin

 

RE: what about a 40w JJ tube?, posted on August 4, 2015 at 13:57:46
deafbykhorns
Audiophile

Posts: 1067
Location: Florida
Joined: October 17, 2003
 But you said "shoot for a Golden Ratio in dissipation, 62 %"
Is the JJ not really rated for 40watts? He would be running it at approx 45% according to your calculation

 

RE: what about a 40w JJ tube?, posted on August 4, 2015 at 14:25:36
drlowmu
Manufacturer

Posts: 9730
Location: East of Kansas City
Joined: January 10, 2005
 So what. The JJ is an exception, a few years in existence. Nice tube too.

2A3s, many thousands of them, have been made for ... maybe a hundred years.

It does NOT hurt your car to run 45 MPH, rather than 62 MPH, and you will get there, safely and economically.

Jeff Medwin

 

RE: what about a 40w JJ tube?, posted on August 4, 2015 at 18:09:40
Garg0yle
Audiophile

Posts: 859
Joined: December 1, 2014
 If you run this golden ratio, (%62 / 37mA) you would get almost %6 distortion at one watt output with a 2.5K transformer.
%4 distortion with a 3.5K OPT.

If one wanted more then 1 watt output, the distortion would shoot up to %13 distortion with a 2.5K OPT and %8 with a 3.5K.

Is it just me, or does this seem a little excessive?


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RE: loftin white : voltages, posted on August 4, 2015 at 20:21:22
genelex
Audiophile

Posts: 95
Joined: October 16, 2005
 surprise surprise, I am working on a 300b amp

 

RE: what about a 40w JJ tube?, posted on August 5, 2015 at 07:03:44
drlowmu
Manufacturer

Posts: 9730
Location: East of Kansas City
Joined: January 10, 2005
 Use 42 mA., not 37 mA. like I do. Also, you are, as usual, full of c**p.

AVVT 2A3 was measured in a EE Lab at 6.8 Watts with under 1 % THD dude, using a 2.5 K output trannie.

No matter, under one Watt is REALLY where its at. With well-wired ALTEC A7-800s, 99% of my listening is under 1 Watt, typically 1/4 to 1/2 Watt.

Jeff Medwin

 

RE: loftin white : voltages, posted on August 5, 2015 at 07:05:28
drlowmu
Manufacturer

Posts: 9730
Location: East of Kansas City
Joined: January 10, 2005
 I feel sorry for you. Live and learn.

" After the 45, 2A3, and 845, everything else - is a hand-me-down "

Robert W. Fulton 1925-1988.

Jeff Medwin

 

RE: loftin white : voltages, posted on August 5, 2015 at 13:22:56
Cleantimestream
Audiophile

Posts: 7551
Location: Kentucky
Joined: June 30, 2005
 Robert may never have heard an 801 with a 16 K primary OPT.
The Mind has No Firewall~ U.S. Army War College.

 

RE: what about a 40w JJ tube?, posted on August 5, 2015 at 14:16:30
Garg0yle
Audiophile

Posts: 859
Joined: December 1, 2014
 That's not very nice.

42 mA is not %62 of 15watt, 37mA is. Try a calculator if you don't believe me.

AVVT tube?
As far as I can see, tubes under that name were only made for a couple years and are now unavailable. arguable a different tube altogether.
Hardly terms you could apply to regular branded 2A3s.

"No matter, under one Watt is REALLY where its at."
Fine, but doesn't %6 distortion at 1 watt seem excessive?


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RE: ( 1% THD at 6.8W ..., posted on August 6, 2015 at 06:20:38
deathtube 667
 That is really impressive distortion numbers for a SE 2A3 amp.

Is that DF's latest SS 2A3 mono?

 

RE: what about a 40w JJ tube?, posted on August 6, 2015 at 09:45:22
Al Noakes
 Please provide some proof of your statement as there are two issues which defy the laws of physics :

1.You cannot obtain 6.8 watts from a single ended 2A3

2.You cannot achieve 1% distortion from a single ended 2A3 with 2K5 OPTX

I can understand that some 2A3 have higher ratings so a tube with '2A3' written on the side may just be a 300B with a 2.5V filament but the claim of 1% distortion at 6.8 watts is BS unless feedback was used or other factors involved such as using in push pull . When you see published test results of DHT , 1% is easily achieved but it's usually using a flat load line with a CCS load to demonstrate the at-best linearity of the device

Al

 

RE: what about a 40w JJ tube?, posted on August 6, 2015 at 10:04:11
dave slagle
Manufacturer

Posts: 5430
Location: NYC
Joined: April 27, 2001
 the AVVT 2A3 had a 28W plate dissipation but still.... 7W from even a 300B at 1% distortion is tough.

dave

 

RE: loftin white : voltages, posted on August 6, 2015 at 10:07:19
mikeyb
Audiophile

Posts: 1807
Location: Minnesota
Joined: November 8, 2002
 Just one man's opinion.

And what exactly is wrong with a "hand me down", anyway?

 

RE: what about a 40w JJ tube?, posted on August 6, 2015 at 12:22:59
Al Noakes
 I think Jeff should steer well clear of the numbers game...

Even considering an amp as a whole , with cancellation , would be tough to hit that 1% .

I wouldn't be phased no matter what the measured distortion was at max power if it sounded good at listening levels :)

Al

 

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