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6B4G Single End amp in a new suit ?

165.225.68.49
Posted on April 2, 2017 at 23:01:29
pix
Audiophile

Posts: 411
Location: sweden
Joined: March 18, 2006

I just picked up this sigle-end amplifier and it sound fine.
Hovever, since I am about to pick it apart for a new chassis, I might as well considdered some toplogy changes.
If this amp where yours, what changes would you considder trying?

I am using 2K5 Ohms sources (CD and phono-stage) with 1,5V output

 

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RE: 6B4G Single End amp in a new suit ?, posted on April 3, 2017 at 10:48:00
Chip647
Audiophile

Posts: 2652
Location: The South
Joined: December 24, 2012
 It looks like an interesting topology. No reason to build a whole new amp design. You could improve it by improving the filament supply and the B+ supply.

 

RE: 6B4G Single End amp in a new suit ?, posted on April 3, 2017 at 12:44:31
Triode_Kingdom
Audiophile

Posts: 10049
Location: Central Texas
Joined: September 24, 2006
 What are the resistors at the grids marked "332R"? If they are wirewound precision, they should be changed to carbon composition. I also suggest that you remove the 10,000uF electrolytic at the cathode of the driver. Not sure 120 ohms is appropriate for that tube in triode mode. Does this schematic represent a commercial product?









 

There is one other thing., posted on April 3, 2017 at 20:48:03
Michael Samra
Dealer

Posts: 36118
Location: saginaw michigan
Joined: January 30, 2005
 Isn't the B+ across the 6B4g supposed to be 250vdc? Using a 10 ohm resistor drops the entire B+ across the tube and at 50ma,he is at full dissipation for that tube.If it was setup for fixed bias,he could control the current at least when using a 10 ohm cathode resistor.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken

 

RE: 6B4G Single End amp in a new suit ?, posted on April 3, 2017 at 21:06:16
Paul Joppa
Industry Professional

Posts: 7296
Location: Seattle, WA
Joined: April 23, 2001
 It appears from the circuit that the 6B4G grid bias is positive, but everything else (including that the tube does not immediately melt down!) indicates that it must be negative. So, while I'm confident the circuit is mis-drawn, I am not confident aboout what the actual circuit really is.

Michael Samra, the 300v/50mA operating point is a good one; I use it in the Bottlehead Paramour/Stereomour - the basic design is 17 years old now and has never been a problem that I know of. The important thing is to use a higher load impedance - I use 4000 ohms. I don't know what impedance is used in this circuit.

 

RE: 6B4G Single End amp in a new suit ?, posted on April 3, 2017 at 22:13:23
Michael Samra
Dealer

Posts: 36118
Location: saginaw michigan
Joined: January 30, 2005
 Paul
I know you can do it but I always get a little nervous when you are at full dissipation of the tube's rating.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken

 

RE: 6B4G Single End amp in a new suit ?, posted on April 3, 2017 at 22:47:15
pix
Audiophile

Posts: 411
Location: sweden
Joined: March 18, 2006
 The Power supply is an "over the top" supply with separate supplies for each amplifier step. It has tube rectifiers for both B+ supplies and inductive filtering for each supply (even the filament supplies for the 6B4Gs.
I have not tear this appart, so I have not made an schematic of it yet.

 

No this is not an comersial product, posted on April 3, 2017 at 22:59:28
pix
Audiophile

Posts: 411
Location: sweden
Joined: March 18, 2006
 I am not sure about the grid stoppers. But due to the size of them I hardly think they are ww-resistors.
Still I have ha hugh stock of HOLCO-resistors, even though not carbon composition I think they are suitable for this application.

No, this is not an commercial Product. I bought it from a friend whom designed it severa years ago.
Initally my idea was to use some of the fine parts in it for my own design. But after using it for some weeks, I decided it was better to make new chassis for this one. And in the process, also make some improvents (if possible).

Whats worry me most is my fairly hi Zout frpm my sources, driving a fairly low Zin at this amp. Ideas such as replacing the 25K volyme for a 100K dito came to my mind. And also try the E280F as a pentode for decreasing the miller cap.
Anny ideas around that?

 

A matter of choises, posted on April 3, 2017 at 23:10:10
pix
Audiophile

Posts: 411
Location: sweden
Joined: March 18, 2006
 Some people uses 250V across the 6B4G (and often 60mA Idle current). This would give 15W Plate disipation. However, the OPTs often used is 2500 ohm load.
In this case the Lundahl OPT is a 3000 ohm making the designer to chose a slightly higer voltage. And to stay with the 15W dispation, the Idle current caused of that decreased to 50mA.

The 10 ohm resistor at the 6B4G cathode is just for measure purposes.
50mA current through it drops 0,5V which is negligable in this case.

 

the label bias is a negative voltage , posted on April 3, 2017 at 23:28:29
pix
Audiophile

Posts: 411
Location: sweden
Joined: March 18, 2006
 @Paul
Yes the label "bias" in this schematic is a negative voltage even though I have miss to wright it down.

The LL1664 is configured as a 3000:8 ohm load.

The amp sounds fine, no question about that. But that does´nt mean it could not be make sounding even better.
My curiosity was pointing at the input and the E280F driver, with the aim of increase sensitivity and decrease the source load.

Is it adviceable to try the driver as a pentode to lower the miller cap?
Change the 25K volume control for a 100K dito?
Increase (or remove) the grid leak resistor?



 

RE: A matter of choises, posted on April 4, 2017 at 03:45:31
Michael Samra
Dealer

Posts: 36118
Location: saginaw michigan
Joined: January 30, 2005
 You are good.I actually run a pair of my Fisher 100 amps which are identical to the 80AZ amps that take EL37s.I had built an outboard supply for separate filament transformers biased with a 1.2k 10 watt cathode resistor.It is actually running at 290v across each tube but these amps sound incredible and I'm making 16 watts output per amp.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken

 

RE: No this is not an comersial product, posted on April 4, 2017 at 06:05:02
Triode_Kingdom
Audiophile

Posts: 10049
Location: Central Texas
Joined: September 24, 2006
 "even though not carbon composition I think they are suitable for this application."

No, they're not. Only carbon composition is suitable for use as a grid stopper. All others risk creating oscillation that can be difficult to identify and troubleshoot.

No reason to have a 25K input Z. Remove the network and install a 100K pot to ground at the grid. Also, if you're interested in a pentode front end, the EF86 is one of the best. Internally shielded, stable, low distortion, minimal microphonics, etc. I won't use anything else.

 

RE: 6B4G Single End amp in a new suit ?, posted on April 4, 2017 at 08:24:06
amnesiac
Audiophile

Posts: 717
Joined: August 21, 2002
 I would leave alone what seems to be a nice amp built by someone with more knowledge then yourself. save some more money to build your own amp and use this as a reference.

 

RE: No this is not an comersial product, posted on April 4, 2017 at 08:52:16
PakProtector
Audiophile

Posts: 12366
Joined: May 14, 2002
 There are lots of good pentodes to use *AS* pentodes. The lower gm and plate dissipation of the small, nominally 'signal' ones require a fairly high value plate load resistor to deliver adequate gain. When working into the grid of a power triode, it is not only the Miller of the voltage amp, and its loading from the volume pot, but the Miller-amplified capacitance of the final you'll have to worry about.
cheers,
Douglas

Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.

 

RE: 6B4G Single End amp in a new suit ?, posted on April 5, 2017 at 13:04:16
Paul Joppa
Industry Professional

Posts: 7296
Location: Seattle, WA
Joined: April 23, 2001
 Yeah, that can be an issue. The classic RCA 2A3s are pretty tough (and/or conservatively rated), and many people now use the modern mini-300B single plate ones which are often quite happy at 20+ watts. I've used television tubes at full spec as well; they were developed to be reliable in a hot environment with many tubes in an enclosed space.

WE rated the 300B at 40 watts dissipation, but buried in the spec is the caution that this would significantly reduce their lifetime - they are not rated as conservatively. So while I don't think there is a universal rule, I do agree with the idea that it's an issue worth looking at closely.

 

RE: No this is not an comersial product, posted on April 6, 2017 at 08:35:16
pix
Audiophile

Posts: 411
Location: sweden
Joined: March 18, 2006
 Ok. I will change to carbon composition resistors.
Due to the input network I have a 100 K stepped ladder switch too try.
How high does the grid leak need to be for the E280F in triode mode?

 

RE: No this is not an comersial product, posted on April 6, 2017 at 11:30:15
pix
Audiophile

Posts: 411
Location: sweden
Joined: March 18, 2006
 So, what you are saying is that using the 6B4G as a penthode could cause drive loss compared to use it as a triode?
Or do you mean if changed to another driver tube?

 

Pio cap as bypass ?, posted on April 6, 2017 at 11:42:54
pix
Audiophile

Posts: 411
Location: sweden
Joined: March 18, 2006
 I also wondering what you guys think about the 2uF/325V Leclance pio cap at B2+?
The purpose of bypass e-lytic caps is for their incresed esr with frequency. But in this case I figure this pio cap must have higher esr than the e-lytic it is suppose to bypass.
Keep, change our remove?

 

RE: No this is not an comersial product, posted on April 6, 2017 at 13:09:02
Triode_Kingdom
Audiophile

Posts: 10049
Location: Central Texas
Joined: September 24, 2006
 I'm not familiar with the E280F specifically, but 100K is a safe value for a grid resistor.

 

RE: No this is not an comersial product, posted on April 7, 2017 at 04:57:21
PakProtector
Audiophile

Posts: 12366
Joined: May 14, 2002
 I don't think it is possible to use 6B4G as a pentode( though arranging them in Cascode is perhaps possible ). The late production 6AV5 Sylvania tubes were sold as 6B4's and I do not know where the connection of g2 to anode was made.

There are lots of small and medium-sized pentodes to use for voltage amplification. Try a small TV sweep tube...:)

Douglas

Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.

 

RE: No this is not an comersial product, posted on April 7, 2017 at 07:04:16
PakProtector
Audiophile

Posts: 12366
Joined: May 14, 2002
 For the input stage( or any other, for that matter), as low as possible w/o adversely effecting the preceding stage. I like a linestage that can drive low loads...and for amplifiers, have run the grid resistor as low as 8kOhms on its input stage. Makes for an interesting linestage testing rig; few will work well like that...:)
cheers,
Douglas

Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.

 

Ping Al Noakes!, posted on April 11, 2017 at 23:20:42
pix
Audiophile

Posts: 411
Location: sweden
Joined: March 18, 2006
 Knowing you build a similar amp using the E280F in pentode as well as triode-mode.
If I was to try the E280F in the topology above as a pentode. What needs to be changed (beside feeding the g2 with aprox 160V).
My guess is that the inductive load in this configuration is about 30-50H.
Brgds

 

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