I was reading the datasheet of the RCA 6F6 (metal) and was interested on those 18+ watts out of a pair in AB2 so I modified this schematic to somewhat fit the maximum ratings.
Injected 1Khz sine wave in input 1 and did the measure. Got a joke basically 5,9 volts AC, is about the output of a single valve with 250v in the plates so something must be wrong.
I'm thinking there's not enough voltage swing between the grids, accordingly to the datasheet for AB2 I need almost 100 volts but that I suppoe is p-p , my measure of 50 volts between the grid stoppers is closer to the RMS value.
The thing is I replaced for a moment the 6F6's with a pair of used 6V6GT and immediatly got about 15 watts (10.5 volts at the dummy load) only adjusting a bit the BIAS. So the amplifier IS working is just it doesn't like the 6F6's my question is why ? is the phase inverter isn't it?
Also I tried with a brand new 1613 pair and it was much worse, and I thought the 1613 is an industrial version of the 6F6, looks like it isn't.
Injected 1Khz sine wave in input 1 and did the measure. Got a joke basically 5,9 volts AC, is about the output of a single valve with 250v in the plates so something must be wrong.
I'm thinking there's not enough voltage swing between the grids, accordingly to the datasheet for AB2 I need almost 100 volts but that I suppoe is p-p , my measure of 50 volts between the grid stoppers is closer to the RMS value.
The thing is I replaced for a moment the 6F6's with a pair of used 6V6GT and immediatly got about 15 watts (10.5 volts at the dummy load) only adjusting a bit the BIAS. So the amplifier IS working is just it doesn't like the 6F6's my question is why ? is the phase inverter isn't it?
Also I tried with a brand new 1613 pair and it was much worse, and I thought the 1613 is an industrial version of the 6F6, looks like it isn't.
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
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You can NOT stay in AB2 mode with R-C coupling to the grids. It just grid-blocks.
Also probably can not use the huge screen resistor penciled on that plan.
6F6 suggestion dates from the time when interstage transformers were common to drive P-P grids. That should work OK.
Note that the 6F6 essentially vanished as soon as the 6V6 arrived. 6V6 is a "better" tube in most ways.
Pull out that old Sansui hi-fi amp with the dead channel. Connect the good channel to a P-P output transformer's speaker side. Connect the OT's plate side to the 6F6 grids. Connect plate-side CT to your negative bias, trimmer per suggestion. That should work OK for testing. It has not made sense for guitar amps since 1939 (well, into the 1950s when unbought grid transformers turned up cheap).
Also probably can not use the huge screen resistor penciled on that plan.
6F6 suggestion dates from the time when interstage transformers were common to drive P-P grids. That should work OK.
Note that the 6F6 essentially vanished as soon as the 6V6 arrived. 6V6 is a "better" tube in most ways.
Pull out that old Sansui hi-fi amp with the dead channel. Connect the good channel to a P-P output transformer's speaker side. Connect the OT's plate side to the 6F6 grids. Connect plate-side CT to your negative bias, trimmer per suggestion. That should work OK for testing. It has not made sense for guitar amps since 1939 (well, into the 1950s when unbought grid transformers turned up cheap).
6F6 plots. Note that at zero grid current we can only get a half-swing, 1/4 power, essentially what you got. When it tries to go into grid current, the uni-directional grid current instead charges-down the coupling capacitors and cuts-off the tubes. Transformer drive avoids this.
The peak grid Power is near 50V 30mA or 1.5 Watts Peak. Not an easy load!! The 6F6 datasheet suggests using a (third!) 6F6 as the driver.
The 6V6 will make about as much plate current as a positive-grid 6F6, without going positive grid.
The peak grid Power is near 50V 30mA or 1.5 Watts Peak. Not an easy load!! The 6F6 datasheet suggests using a (third!) 6F6 as the driver.
The 6V6 will make about as much plate current as a positive-grid 6F6, without going positive grid.
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Wow, I did not know all that, definitely thought the phase inverter was not suitable but didn't understand why, also realized the control grid at some point should draw current, which doesn't make sense to me also, now I'm getting it. I tried this because I read somewhere the tubes (6V6 & 6F6) were very similar, turns out they are not at all. Also read people talking highly of a 6F6 triode strapped pp driven by a couple of 6SN7's can't remember the author of such amp. Thanks for the insight PRR-
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6F6 is an Octal '42 which is derived from the '47, the first US power pentode. These were spiffy for the 1932 time frame, but there was a LOT of progress in the 1930s.
6V6 is a next-generation 6F6, but incorporating a lot of the improvements that were worked-out for the 6L6/807 project. Up to a point, 6F6 with more bias and drive will do many 6V6 chores. To get past that point you have to SLAM it. I think the 6F6 grid is wound with thicker wire than 6V6, so it will take a slam, but you need a slamming driver.
Once the 6V6 came out, many many "6F6" radios and amps switched. I think Ken-Rad was low-balling the price, and *everybody* was making 6V6 in their garages (you could buy all the tooling from the back of magazines). Drop-in with only a bias change. With one small triode for inverter you can lose the grid transformer (by then tubes were cheaper than iron). 6F6 were used into 1940 (and beyond), but I think in WWII the military preferred 6V6 and 6F6 kinda dried-up post-war. IIRC Fender used one in the standalone reverb, but his production was small and he probably found an unsold crate in a warehouse.
6V6 is a next-generation 6F6, but incorporating a lot of the improvements that were worked-out for the 6L6/807 project. Up to a point, 6F6 with more bias and drive will do many 6V6 chores. To get past that point you have to SLAM it. I think the 6F6 grid is wound with thicker wire than 6V6, so it will take a slam, but you need a slamming driver.
Once the 6V6 came out, many many "6F6" radios and amps switched. I think Ken-Rad was low-balling the price, and *everybody* was making 6V6 in their garages (you could buy all the tooling from the back of magazines). Drop-in with only a bias change. With one small triode for inverter you can lose the grid transformer (by then tubes were cheaper than iron). 6F6 were used into 1940 (and beyond), but I think in WWII the military preferred 6V6 and 6F6 kinda dried-up post-war. IIRC Fender used one in the standalone reverb, but his production was small and he probably found an unsold crate in a warehouse.
Hi, VMUNIX! I'm 8 months late for the thread but I've thought that my experience could help. I've got a quad of 1613 tubes (military grade 6F6's) and really wanted to give them a try.
So I used them on my last DIY tube amp project in a push-pull AB2 configuration based on the Fender Princeton 6G2.
I've added some mods such as triode/pentode mode and it helps to tame the power because it's way louder than I expected. I was looking for a bedroom practice/home recording amp but I'm pretty sure that it would make it for a small gig or to practice with a drummer (plugged to a 1x12 or 2x12 cab).
If you're interested I can measure what values are being handled in the output stage and pass them on.
From the PI I go as the same as the 6G2 schematic to the 1613's (with the addition 470ohm screen resistors and a master vol 1MA Push-pull pot wired for the triode/pentode mode). Also added a switch to able/disable the NFB and it's really nice.
IMO the 1613's deliver a boost in low-mids and mids when comparing with 6V6GT's in the same amp.
So I used them on my last DIY tube amp project in a push-pull AB2 configuration based on the Fender Princeton 6G2.
I've added some mods such as triode/pentode mode and it helps to tame the power because it's way louder than I expected. I was looking for a bedroom practice/home recording amp but I'm pretty sure that it would make it for a small gig or to practice with a drummer (plugged to a 1x12 or 2x12 cab).
If you're interested I can measure what values are being handled in the output stage and pass them on.
From the PI I go as the same as the 6G2 schematic to the 1613's (with the addition 470ohm screen resistors and a master vol 1MA Push-pull pot wired for the triode/pentode mode). Also added a switch to able/disable the NFB and it's really nice.
IMO the 1613's deliver a boost in low-mids and mids when comparing with 6V6GT's in the same amp.
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