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Confused by 6av5ga's sold as 6b4g's (don't work in my amp)

dogwan

dogwan

Dogwan
Picked up a NOS quad of those infamous 6AV5GA's that are labelled 6B4G.
Apparently the story goes that in the mid-70's Sylvania Philips had a military order to fill and they realized they could Triode strap the 6AV5's and give them the same pin out as the 6B4G and they would perform the same.

Anyway, I have a DIY PP 6B4G amp that I picked up a while ago. Have been using some '79 date codes Svetlana Winged C 6S4S tubes and it sounds great. But it is a little light on the bass. Wanted to try some different output tubes so picked up the aforementioned JAN (pseudo) 6B4G's.
Here's my original thread on the amp.

Recognize this diy 6B4G PP amp?

Did some horse trading recently and picked up this DIY push-pull 6B4G amp that looked interesting. Definitely needed some re-work but it has nice parts. Someone wrote in the inside "Built 1997, JB" or something close to that, in red sharpie. I don't want to comment too sharply on the...
audiokarma.org

When I installed them I get horrendous AC hum out of the amp. It's more than just an adjustment. It really is quite loud and overrides any music that might come out of the amp. With the Russian tubes, or the mis-matched real 6B4G's that came with the amp, it is nearly dead quiet.

I am guessing it's something to do with the the 6AV5's being IDHT vs. the 6B4G and 6S4S being true DHT? I don't really know enough about that. But, I would have thought that if the tubes were sold for military applications as a plug and play drop in for 6B4G that they should work in my amp?

Any thoughts?

I've attached the data sheets for both the 6B4G and the 6AV5.
Here's my hand drawn schematic of the amp (based on Eico ST70 iron).
DIY6b4gPP.amp.png

The tubes in question...

20241220_103622.jpg

The amp...
20241220_103650.jpg
 

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Interesting topic! I'm a 6AV5, 6B4G fan and have wondered about these "6B4G" tubes. Do you have a tube tester that can check the Sylvania 6B4G tubes that you received? If not, check pins 2 and 7 for filament continuity. Maybe the tubes you purchased at shorted?
 
dsndblm said:
Interesting topic! I'm a 6AV5, 6B4G fan and have wondered about these "6B4G" tubes. Do you have a tube tester that can check the Sylvania 6B4G tubes that you received? If not, check pins 2 and 7 for filament continuity. Maybe the tubes you purchased at shorted?
Click to expand...

I do have a tube tester (Sencore MU140). And one of the 1st things I did was test them. All 4 tested at 120% on emissions exactly. My gM function on the tester is unreliable due to a flaky multi-deck rotary switch (unobtanium) so I just use it as a basic Emissions and Grid Leakage tester. It has been rebuilt with one of Brice's replacement boards and has been calibrated.

Tube tester: Sencore MU-150

Hi Guys, I am not sure this belongs to the Tube Audio forum but I didn't find a more suitable category. I worked of this tester, a Sencore MU-150, that I found very nice so I am sharing this with you. The good points: - This tester is one of the last tube tester been produced, therefore is...
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I did notice that there were two positions that lit up the "Short" light. But since it did it on all 4 tubes I didn't think much of it as I figured the tube selection guide of the Sencore wouldn't be specific enough to these 6AV5's wired to 6B4G pinout. I.e., would it show shorts being strapped to Triode? Or somehow connecting the heaters to the Cathode to emulate the 6B4G?

I can retest and take pics to show which positions show shorts.
 
Don't know if it will help or not, but I found this "The 6AV5 absolutely requires G1 and G2 stopper resistors when triode strapped to prevent oscillation."
 
Hmm, I'm not sure how the 6AV5s were rewired to become a 6B4G. I would think that if the new 6B4Gs you received would work correctly as a 6B4G, they would test OK, with no shorts as a 6B4G. Test one of your old 6B4Gs in your tester, noting any position shorts and compare to the new 6B4G/6AV5 tubes that are giving you issues.
 
opnly bafld said:
Don't know if it will help or not, but I found this "The 6AV5 absolutely requires G1 and G2 stopper resistors when triode strapped to prevent oscillation."
Click to expand...

Hmmm. I just looked and Grids 1 and 2 on the 6AV5 are pins 1 and 8.

On the 6B4G pins 1 and 8 have no connections.

So, I need to figure out how they "Triode strapped" the 6AV5? I don't quite get how that works yet.
 
being indirectly heated, the balance pots aren't going to work. Those are meant to get the AC equal and opposite relative to ground so it cancels out. The cathode on the 6av5 cant tie to the middle of the heater, so its going to just be wired to one end or the other. Not surprised it hums. If these were actual 6av5 it would be an easy fix, simply disconnect the wiper of the balance pot from the rest of the bias stuff, add 50 ohms and connect it to the cathode of the 6av5. I'm assuming the cathode does not come out on it's own pin on these though since its meant to be a drop-in for a 6B4 .


triode strapping is simply connecting G2 / screen to the plate. At that point it acts like a triode. Possibly they put resistors inside the base of the tube, possibly not.
 
gadget73 said:
being indirectly heated, the balance pots aren't going to work. Those are meant to get the AC equal and opposite relative to ground so it cancels out. The cathode on the 6av5 cant tie to the middle of the heater, so its going to just be wired to one end or the other. Not surprised it hums. If these were actual 6av5 it would be an easy fix, simply disconnect the wiper of the balance pot from the rest of the bias stuff, add 50 ohms and connect it to the cathode of the 6av5. I'm assuming the cathode does not come out on it's own pin on these though since its meant to be a drop-in for a 6B4 .


triode strapping is simply connecting G2 / screen to the plate. At that point it acts like a triode. Possibly they put resistors inside the base of the tube, possibly not.
Click to expand...

Ah, I think I see. So at the end of the day these tubes would behave like a 6B4G, just not in this topology.

I don't think I am interested in altering the amp as it now stand since it works great with a true DHT 6B4G variant. Now what am I going to do with these tubes now?
There's a thread where someone performed a mod on the Dyna ST-70 to run these JAN 6B4G's on it successfully.

Angela Instruments Dyna St-70 6B4G

"To fill a military order Philips internally triode connected some 6AV5GA tubes, which when triode connected are pretty close to a 6B4G. These have been available for a number of years. It would seem Philips either couldn't or chose not to make a run of "real" 6B4G. IIRC these date from the 80s...
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Maybe I need to keep my ears and eyes open for a donor amp.
 
opnly bafld said:
Don't know if it will help or not, but I found this "The 6AV5 absolutely requires G1 and G2 stopper resistors when triode strapped to prevent oscillation."
Click to expand...
That would require either the addition of a stopper inside the base (in series with the screen) or the addition of a parasitic suppressor at each anode in the amplifier's wiring. Neither has an appreciable effect at audio frequencies. This is a typical suppressor for use at the anode:

Screen Shot 2024-12-20 at 6.10.22 PM.png

Jack
 
Ok, so I tried the JAN tubes again tonight and the hum (filament) pot had absolutely no affect on the loud hum the amp is producing with these tubes. No matter which way I turned them there was no affect.
 
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