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Old 1st October 2006, 05:25 AM   #1
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Join Date: Mar 2006
Location: Michigan
Default The 6b4g

Hello everybody!

I was poking around on the internet, and I found the 6b4G. this Power Triode runs the exact same cost as a 2a3, and is nearly identical except for the Voltage, and an octal base

I already have built a El34 SET and I was wondering if I simply adjust the bias circuitry a little bit, could I change from the El34 to a 6B4G? because this seems a whole lot cheaper than building a 2a3 amplifier.


-Moose
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Old 1st October 2006, 07:37 AM   #2
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6B4Gs do work with EL34 O/P "iron". However there are things you must watch out for.

Current production Sovtek 6B4Gs can take the B+ voltages found in EL34 circuits, while NOS 6B4Gs can't.

Hum levels rate to be unacceptably high with AC heating in a SE amp. So, current regulated DC filament supplies will be needed.

6B4Gs have tolerable (?) hum levels in PP circuits, where cancellation in the O/P trafo helps things out.
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Eli D.
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Old 1st October 2006, 07:54 PM   #3
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Best bet would be the Soviet 6C4C ( 6S4S), sold as a soviet 6B4G. These will take considerable abuse in terms of over voltage and higher Pa - plates don't glow until 21-22W Pa is exceeded...

James
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Old 1st October 2006, 09:33 PM   #4
DrRick is offline DrRick  United States
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Moose,

I hate to answer a question with a question, but with some additional data we can figure out the least painful way to implement the 6B4.
What is the existing operating point of the EL34?
What output xformer did you use? Are there multiple taps on the primary? What are the primary and secondary impedances and what is the current carrying capacity of the primary?
What is the B+ voltage, what voltage is available (i.e. did you have to reduce the B+ to achieve the target operating point?). How much current can the power supply provide? What filament windings are available?
How is the EL34 biased?
Do you have a schematic you can post?

While the Sovtek 6B4 can handle higher voltage and plate dissipation then the real 6B4s, the original sounds much better (my opinion, some may like the Sovtek better, but we probably would all agree that it sounds different).

Eli makes an important point regarding hum. To me, it's the #1 problem with filamentary tubes. Some engineers do manage to use AC heating and have very low noise. I think I would start out by adding a hum pot (a 100 ohm pot across the filament with the rotor grounded). If that doesn't yield an acceptable hum level then I would try DC. If you do end up using DC, Eli's on the money with the current regulated supply suggestion.

Some triodephiles insist on AC heated filaments. All these nuances are part of the "fun" of DIY, you can tune your designs to your own individual ears and system.

I have been lurking for years but have rarely posted, I hope you guys don't mind my comments. I spent part of my career as an engineer and I've been building audio related gear since I built my first crystal radio when I was five (I hate to think about it, but that was around the time of the Cuban Missile Crisis-I'm starting to feel old).

BTW Moose, I've accumulated 3000+ square feet of tube audio gear and parts, so if you need something for this project let me know. I'm always up for trades.

Peace.
DrRick
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Old 1st October 2006, 10:02 PM   #5
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Quote:
Originally posted by DrRick
Moose,

I hate to answer a question with a question, but with some additional data we can figure out the least painful way to implement the 6B4.
What is the existing operating point of the EL34?
What output xformer did you use? Are there multiple taps on the primary? What are the primary and secondary impedances and what is the current carrying capacity of the primary?
What is the B+ voltage, what voltage is available (i.e. did you have to reduce the B+ to achieve the target operating point?). How much current can the power supply provide? What filament windings are available?
How is the EL34 biased?
Do you have a schematic you can post?
DrRick
Its fine, I pretty much have the answers memorized

My OP is 360v on the plate, total dissapation is around 23 watts

Current is just about 65ma cathode bias resistor at 400 ohms putting 26volts across that resisitor

My B+ is probably around 370v, however I have been thinking of taking out a resistor to boost it a little.

I am using the Cheapo Hammond 125cse right now, it has multiple secondary taps.

I do have a shchem, i'll post it

My power Transformer is the Hammond 272hx
Pri- 117v
HV Sec: 300-0-300 200ma
Filament#1 2.5-0-2.5 3a (actual voltages are a little higher)
Filament#2 3.15-0-3.15 6a



Quote:
I have been lurking for years but have rarely posted, I hope you guys don't mind my comments. I spent part of my career as an engineer and I've been building audio related gear since I built my first crystal radio when I was five (I hate to think about it, but that was around the time of the Cuban Missile Crisis-I'm starting to feel old).
I first tapped two wires to a C battery, and to a christmas light when I was 6 or so, however this was right around the Time Bill Clinton took office, I'm not very old at all!


Quote:
BTW Moose, I've accumulated 3000+ square feet of tube audio gear and parts, so if you need something for this project let me know. I'm always up for trades.
I'll definatly think about it! thanks alot



A few things have changed about the design in the schem
Cathode Bias resisitor is at 400 ohms,

Primary impedance is at 5k

Cathode bypass cap is at 470uf
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