How does that design get away with its push-pull driver stage using an E88CC (6922, or industrial 6DJ8) with +190V plate-cathode? I know the current is low, so the E88CC is only running at about 600mW dissipation, but still, that's a lot higher than the rated max plate voltage.
Maybe something like a 6CG7 would be better there?
--
Maybe something like a 6CG7 would be better there?
--
My experience as well. The 6AS7G is just a very noisy tube. The 5998A is better but, impossible find in NOS condition, instead they are thrashed usually.
It was earlier mentioned that having a front end with a higher B+ than the output is clumsy at best. But in this case it may not be? I have a 15 amp 6.3 volt filament transformer for everything. And a 230 volt 1.09 amp power transformer for 6N13S. All I need now is a power transformer for the front end. I'm going to use the topology from the posted schematic, (LTP pair feeding another pair for the second stage) for the front end. I've found a $16 Triad isolation transformer with a dual secondary for 230 volts. I could build a voltage doubler. The idea is to have a B+ in the 500 volt range to optimize the plate resistors and the tail resistors. When I receive the new transformer I'll know what my possible B+ voltages will be. If I don't have enough gain I could substitute 6SL7GT for the current schematic's 6SN7.
Last edited:
As for the noise problems with the 6N13S? I'm wondering if those problems are the result of driving the tubes very hot, 100ma 100volt? Along with hyper efficient speakers. My speakers are only 89 db efficient. And I listen to music louder than most so if there is a small amount of noise it may be tolerable?
One other question. In the earlier schematic the second stage 6SN7's use a CCS as the Bias/tail. If I use the conventional bias resistors and single tail resistor in the second stage the design would still benefit from the rising falling characteristic of the LTP and the CCS approximation of the tail resistor wouldn't they?
I've never encountered the noise issues that others seem to have. Maybe these tubes were gassy/improperly stored? Not sure. I've only ever used them push pull with reasonably efficient speakers and they were no worse than any other push pull amplifier build, even after several years of daily use. They're as quiet as the PSU and topology used.
Powering the heater without current flow leads to cathode interface resistance...
From Sweetwater: "When a tube cathode is fully heated but no anode current is allowed to flow for long periods of time (several hours), a high-resistance chemical layer can grow between the cathode tube and the oxide coating. This has an effect like an unbypassed cathode resistor; it increases noise and reduces the useful gain of the tube even though the oxide coating may have plenty of life left in it. The worst part is that once formed, this chemical layer cannot be removed."
From Sweetwater: "When a tube cathode is fully heated but no anode current is allowed to flow for long periods of time (several hours), a high-resistance chemical layer can grow between the cathode tube and the oxide coating. This has an effect like an unbypassed cathode resistor; it increases noise and reduces the useful gain of the tube even though the oxide coating may have plenty of life left in it. The worst part is that once formed, this chemical layer cannot be removed."
Ralph has built hundreds if not thousands of amps using this method. It eliminates flash over. You tie all of the pins to one side of the filament while heating. It works for these tubes. Not sure of any others. Also I have several pieces of amateur radio equipment (some which can rival much newer equipment) which have standby positions just to remove anode voltage with no ill effects. If you notice the Sweetwater statement says "can" not will.
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.