I'm getting 41v with 550R.
I'll have to put it on the meter and start turning it up and see if the filament voltage drops.
One thing I remembered last night is that I actually have a brand new 6.3v Hammond transformer. If there is a place to put it on the chassis it could be used to power the driver filaments. I could at least prototype with it to see what happens if it looks like filament voltage is a culprit. Also, the driver board is designed for 6DJ8, not 6N6P, so I could drop those back in to lower the current requirement some. The 6N6P is a better driver for a 300b though, but whatever works is worth a try.
Given that I can't seem to replicate any of the readings I get with the calculators, maybe I just run out of current from the transformer? It was always considered a weak part of the ST-70. OTOH, they also powered a crapload of amps in their time and they don't have a history of failure. If this turns out to be the case I'll look into the drop in replacements that are more robust. Or look for a purpose built one like the Hammond 300bx, specifically designed for the 300B, except that it doesn't have dual windings for stereo filaments.
I'll have to put it on the meter and start turning it up and see if the filament voltage drops.
One thing I remembered last night is that I actually have a brand new 6.3v Hammond transformer. If there is a place to put it on the chassis it could be used to power the driver filaments. I could at least prototype with it to see what happens if it looks like filament voltage is a culprit. Also, the driver board is designed for 6DJ8, not 6N6P, so I could drop those back in to lower the current requirement some. The 6N6P is a better driver for a 300b though, but whatever works is worth a try.
Given that I can't seem to replicate any of the readings I get with the calculators, maybe I just run out of current from the transformer? It was always considered a weak part of the ST-70. OTOH, they also powered a crapload of amps in their time and they don't have a history of failure. If this turns out to be the case I'll look into the drop in replacements that are more robust. Or look for a purpose built one like the Hammond 300bx, specifically designed for the 300B, except that it doesn't have dual windings for stereo filaments.
Hi zacster,
What value 'grid stoppers' are you using? This sounds an awful lot like one valve or the other is breaking into oscillation.
A few minutes spent on Oscillation in tube amps could be well worth the time.
Cheers
What value 'grid stoppers' are you using? This sounds an awful lot like one valve or the other is breaking into oscillation.
A few minutes spent on Oscillation in tube amps could be well worth the time.
Cheers
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The grid stoppers are 216k (or whatever value is close, 220k?). They are on the driver board, not at the tube socket. I know I've read to put those as close to the tube pin as possible, but this is where they are.
And in the meantime using the 6B4G is still possible and still sounds sweet. Some Sibelius symphony is playing right now since my wife is working from home and I can't just blast some music.
And in the meantime using the 6B4G is still possible and still sounds sweet. Some Sibelius symphony is playing right now since my wife is working from home and I can't just blast some music.
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There are no grid stopper resistors in the circuit then. There is a .47uf cap between the driver tube and the grid.
With a quad of Sovtek 6B4Gs in the sockets I have no problems at all, the amp has been working without problem for 10+ years now. It is only with a 300B I have any problems.
With a quad of Sovtek 6B4Gs in the sockets I have no problems at all, the amp has been working without problem for 10+ years now. It is only with a 300B I have any problems.
Tubes are different.😉 Just the fact that one of the 300B's seems to differ from the others gives some evidence. Maybe this circuit just isn't suitable for the 300B. Or perhaps the amp wouldn't be completely ruined by fitting grid stoppers.
Still -- good news concerning the 6B4G's giving satisfactory service.
koda, love your sign-off!
Regards
Still -- good news concerning the 6B4G's giving satisfactory service.
koda, love your sign-off!
Regards
All the 300Bs behave the same. It isn't just one tube.
I just wrote a long answer and then I touch something and it disappeared. Argh, I hate that.
To summarize, with just one side operating on my test speaker it worked. I then built the other side, tested it alone and it also worked. It was only when I put them together and ran it with my main speakers that I had a problem, so maybe there just isn't enough power from the supply to run this? Also, the cathode voltage and current are low, outside of what I see in the modeling tools or even using the paper curves. This is one reason I'm confused, I can't model what I see, so maybe I'm just asking too much from the old PA060? And probably the speculation I've read about the Sovtek 6B4G over the years just isn't true, it is NOT a 300B with 8 pins and 6.3v heaters. It is a 2A3 that can take more abuse, but not at the level of a 300B. Putting a 300B in its place just over stresses everything.
I just wrote a long answer and then I touch something and it disappeared. Argh, I hate that.
To summarize, with just one side operating on my test speaker it worked. I then built the other side, tested it alone and it also worked. It was only when I put them together and ran it with my main speakers that I had a problem, so maybe there just isn't enough power from the supply to run this? Also, the cathode voltage and current are low, outside of what I see in the modeling tools or even using the paper curves. This is one reason I'm confused, I can't model what I see, so maybe I'm just asking too much from the old PA060? And probably the speculation I've read about the Sovtek 6B4G over the years just isn't true, it is NOT a 300B with 8 pins and 6.3v heaters. It is a 2A3 that can take more abuse, but not at the level of a 300B. Putting a 300B in its place just over stresses everything.
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6BG4 heater voltage is 6.3V, current 1A, but 300B requires 5V/1.3A.
If you decrease voltage with resistors, the heater current would be inadequate.
If you decrease voltage with resistors, the heater current would be inadequate.
Current is drawn. As long as the resistor is 1 ohm, and the power transformer can provide the extra current, you will get 5V@1.3A at the tube.
The above comment from Paul Joppa, someone I trust (not that I don't trust you guys here, and thanks to Rick for sending me down this path). I was playing the amp earlier and even at the low volume it wasn't very distinct sounding, and I'm thinking that even if this didn't shut down at volume it wouldn't sound very good. And my voltage measurements don't make sense either. I'll give it a try and report back. 1k? Any other suggestions on value. I saw a formula somewhere too but it required knowing the output impedance of the driver, which I don't. At least I have an assortment of values of low wattage resistors.
And another thought, could the tube socket adapters be part of the problem with oscillation if that is indeed the case? The wires from input pin to output are forced into the adapter and probably twisted in an odd way in order to close it. That and the .27ohm dropping resistors are the only difference between using 6B4g and 300b.
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I looked through my stash of parts and my choices are 1K 1/4w or 470R 1/2w. From what I read 1k is kind of the standard used, are the 1/4w sufficient? I just checked and I see the 1/2w are carbon film but the 1k 1/4w I believe are carbon comp. I also think since they are so small I could actually use it to bridge the pins inside the adapter, this way it is as close as possible to the tube.
Would one be better than the other?
Would one be better than the other?
It probably would prefer the 1K.
Maybe the *real valve guys* could check me on this, but there shouldn't be enough current to trouble a 1/4W part, since it won't get hot 'til about 16 mA.
Fitting the resistor into the adapter sounds like a super idea -- you wouldn't have to hack up the amp! But once you reminded me that there's an adaptor, there's something I shoulda thunk of/mentioned earlier -- isn't the 300B a 'direct-heated' cathode? You'll need a separate filament supply for at least one of the output pairs; they can't both share the same one.😉
Sorry I didn't cover that earlier.
Cheers
Maybe the *real valve guys* could check me on this, but there shouldn't be enough current to trouble a 1/4W part, since it won't get hot 'til about 16 mA.
Fitting the resistor into the adapter sounds like a super idea -- you wouldn't have to hack up the amp! But once you reminded me that there's an adaptor, there's something I shoulda thunk of/mentioned earlier -- isn't the 300B a 'direct-heated' cathode? You'll need a separate filament supply for at least one of the output pairs; they can't both share the same one.😉
Sorry I didn't cover that earlier.
Cheers
I did the 1k 1/4w directly into the adapter earlier. The filaments are wired through the adapter and give me 5v. There are 2 filament windings on the transformer, one for each pair, with 550R cathode resistors on the center taps to ground. The adapters work as a switch, with them plugged in, the .27R dropping resistors are in line to each leg and result in 5v, without the adapter, the 6B4G take the 6.3v filament directly.
I need to test out the adapters for shorts, but assuming they're OK I'll give it a try soon. I only did one side.
I need to test out the adapters for shorts, but assuming they're OK I'll give it a try soon. I only did one side.
Of course the tests were bad. I had a broken joint in one of them and in the process of fixing it I broke another. On the second one the glue didn't hold. Oh well. Hopefully they'll both be OK tomorrow and I can test again with the tubes on one side.
I give up. Both adapters tested out ok but when I went to check them out something smoked, and I believe it was from the adapter. I tested the resistors and both still checked out at 1k, so I don't know what it is at this point.
Back to the drawing board. I have both LL1544a and LL1692a transformers and a quad of Genalex 300B. That should be a good start towards something! What I don't have are spare output and power transformers. Time to start hunting down a bargain somewhere. This time I'll follow a schematic.
Back to the drawing board. I have both LL1544a and LL1692a transformers and a quad of Genalex 300B. That should be a good start towards something! What I don't have are spare output and power transformers. Time to start hunting down a bargain somewhere. This time I'll follow a schematic.
One more thing to add, putting it all back together with the 6B4G works fine after a little scare on the one testing channel. I've had trouble with the pins on the driver tube on occasion, and it used this occasion to scare me. A little wiggle cleared it up. I'll deoxit later when I'm done listening.