Hi there,
that's my own KT88 design, actually based on a EAR-859.
Tubes:
- 1 x E88CC (military versions of ECC88) double-triode
- 1 x ECC99 double-triode (one triode-part used only as you can see)
- 1 x KT88 output pentode
All tubes are JJ (already bought).
I would be pleased if you could make any comments on the circuit. The anode (plate?) voltage was dropped to 320V - so a lower impedance and better output transformer can be achieved.
The schematics shows a positive feedback too - not only the negative. Please ignore it, I don't really want build the positive feedback too.
If you have any comments on this design, how could it be still a bit better, please, let me know here.
Performance is not a must have. This design will still give me in class-A 10-12 Watts of power. Quality of the sound is my first priority, without building conventional triode-based class-A. (It's a grid driven design, also called enhanced triode mode or simply ETM).
Thanks for the suggestions. (If no suggestions, just say "hey, nice, dude!")
PS: don't forget the non-existing positive feedback. I won't build it in.
that's my own KT88 design, actually based on a EAR-859.
Tubes:
- 1 x E88CC (military versions of ECC88) double-triode
- 1 x ECC99 double-triode (one triode-part used only as you can see)
- 1 x KT88 output pentode
All tubes are JJ (already bought).
I would be pleased if you could make any comments on the circuit. The anode (plate?) voltage was dropped to 320V - so a lower impedance and better output transformer can be achieved.
The schematics shows a positive feedback too - not only the negative. Please ignore it, I don't really want build the positive feedback too.
If you have any comments on this design, how could it be still a bit better, please, let me know here.
Performance is not a must have. This design will still give me in class-A 10-12 Watts of power. Quality of the sound is my first priority, without building conventional triode-based class-A. (It's a grid driven design, also called enhanced triode mode or simply ETM).
Thanks for the suggestions. (If no suggestions, just say "hey, nice, dude!")
PS: don't forget the non-existing positive feedback. I won't build it in.
An externally hosted image should be here but it was not working when we last tested it.
I´m not expert at all, but anyway, here I go.
Do you really need the 220n/63V capacitor after the volume? If there´s no DC from your source, I would leave it. If you need it, for whatever reason, and you are using a potentiometer for volume, put the cap before the potentiometer, because those devices don´t like DC at their wiper.
Don´t know if it´s necessary for small signal triodes (as the ECC88 and ECC99), but I always use gridstoppers (about 1k) soldered to the signal pin.
Have to admit I don´t understand the connections at the cathode (it shall be the cathode something connection used in the MC amplifiers) and neither the ones at the secondary.
Do you really need the 220n/63V capacitor after the volume? If there´s no DC from your source, I would leave it. If you need it, for whatever reason, and you are using a potentiometer for volume, put the cap before the potentiometer, because those devices don´t like DC at their wiper.
Don´t know if it´s necessary for small signal triodes (as the ECC88 and ECC99), but I always use gridstoppers (about 1k) soldered to the signal pin.
Have to admit I don´t understand the connections at the cathode (it shall be the cathode something connection used in the MC amplifiers) and neither the ones at the secondary.
Thanks, we'll think it through. 🙂
Itt will be fine for the KT88. Low voltage, not too high current, lower power. I don't need that lots of watts and it's better for the tube, too. According to the datasheets it's still a very good point, if not even better, than the "normal" use around the tube's limiting values.
The +18V signal to the grid of the top E88CC appears to be a DC servo loop to set the output tube bias. The gain of this loop may be a little low, have you simulated this? Variation of the +18V will affect current thru the bottom E88CC basically by varying the plate voltage on it.
The screen grid drive of the KT88 should be quite linear already, seem to be a lot of feedback loops here. The feedback(s) from the T1 secondary will require good bandwidth in the xfmer. But being single ended operation, T1 will need an air gap. Usually SE xfmers don't have good bandwidth because of the extra turns required to make up inductance for the air gap. The partial cathode drive, plate feedback (C8), and screen grid drive should be overkill for linearity already I would think. What % cathode feedback is T1?
Don
The screen grid drive of the KT88 should be quite linear already, seem to be a lot of feedback loops here. The feedback(s) from the T1 secondary will require good bandwidth in the xfmer. But being single ended operation, T1 will need an air gap. Usually SE xfmers don't have good bandwidth because of the extra turns required to make up inductance for the air gap. The partial cathode drive, plate feedback (C8), and screen grid drive should be overkill for linearity already I would think. What % cathode feedback is T1?
Don
>>Don´t know if it´s necessary for small signal triodes (as the ECC88 and ECC99), but I always use gridstoppers (about 1k) soldered to the signal pin.<<
Not only is a grid stopper on a 6922 (E88CC) section MANDATORY, a 100 Ohm plate stopper is a good idea too. The 6922 is a high gm VHF type that is HIGHLY prone to parasistic oscillation.
BTW, grid stopper resistors should be Carbon composition and the resistor's body should be mounted as close to the tube socket solder lug as possible.
Not only is a grid stopper on a 6922 (E88CC) section MANDATORY, a 100 Ohm plate stopper is a good idea too. The 6922 is a high gm VHF type that is HIGHLY prone to parasistic oscillation.
BTW, grid stopper resistors should be Carbon composition and the resistor's body should be mounted as close to the tube socket solder lug as possible.
hey, nice dude!
There are several appealing things about this amp. I like:
Screen grid Drive.
A2 operation
KT88 triode / ECC99 CF
Cathode feedback.
Good Luck with It.
Personally, something this complex I would build ''as is" first, but you may have more experience.
Cheers;
Doug
The output transformer will be an EI hypersil core design with about 15Hz - 50KHz bandwidth as planned. (The manufacturer is quite a professional, actual parameters will be measured and adjusted if needed to obtain these results).
Air gaps will be for efficient high-frequencies of corse used.
To your other questions will I reply tomorrow, my father is actually who's designing the tubey, so he knows the exact data, physics etc. of the planned design. (Electrician, he has built more tube amps since he was 21 yrs old. I hope he's doing well this time too.)
One more thing I asked him about: the positive feedback.
Actually, this - as you see - consists of R17, P2, R18. He told me, this is quite a nice thing, arguments were:
Controlling this feedback through adjusting P2, first this small circuit behaves as a negative feedback. At a point it reaches the "zero" level, so as if there were nothing at that point (= no R17, P2, R18). Further adjustments lead to a positive feedback.
The purpose of this all - he said - is the beautifying of the so called "damping factor", so the overall control of the speaker.
He told me, I shouldn't be afraid of it - despite many opinions regarding pisitive feedbacks - because we'll just adjust P2 while running the amp and we'll listen to the sound, how it performs, what it does with the sound, dynamics, etc.
Furthermore, it's an easy way to adjust / or compensate different speaker types..
Opinions on this R17-P2-R18 perhaps? (If needed at all or not, useful, no way ... any kind of arguments are welcome).
I don't know, if it matters at all, but the speakers will be (one side told here only) a Fostex FE208E sigma (also known as FE208EZ) in a TL-box, probably supported by a high quality ribbon tweeter (actually a magnetostat) like the Fostex FT7RP, Visaton MHT-12 or the Orca Raven R1. Consider this speaker setup, if it helps. 🙂Thank you for the helpful answers by the way, I hope, if someone is interested too in this design, he could build the monoblocks based on this circuitry too.
The positive feedback from P2 is a current derived feedback which many older amplifiers used for control of output impedance. (ElectroVoice is one that comes to mind.) The idea is to get the right output impedance for critical damping of the speaker. With the larger magnets on present day speakers to work with high damping factors, this may or may not be useful, but I would leave it in initially to try it out. No harm, since P2 lets one set the level anyway.
The air gap in the SE type transformers is required in most cases to avoid saturation of the magnetic core material by the steady uni-directional Class A plate current from just one output tube. Air gapping is something the transformer winder would dearly like to avoid for getting high bandwidth results. (P-P avoids the need for an air gap by using symetrically cancelling DC currents.) One way to avoid air gapping in a SE design is to use another (2nd) output tube (pentode) in psuedo P-P. The second tube gets no drive signal, just acts as an opposing current source for the benefit of the transformer. (requires a P-P, double primary, design xfmr. unless configured in Circlotron style mode.) Not very efficient power-wise however using a CCS. Another way is parafeeding the xfmr thru a cap. and using an inductor or CCS plate load.
Don
The air gap in the SE type transformers is required in most cases to avoid saturation of the magnetic core material by the steady uni-directional Class A plate current from just one output tube. Air gapping is something the transformer winder would dearly like to avoid for getting high bandwidth results. (P-P avoids the need for an air gap by using symetrically cancelling DC currents.) One way to avoid air gapping in a SE design is to use another (2nd) output tube (pentode) in psuedo P-P. The second tube gets no drive signal, just acts as an opposing current source for the benefit of the transformer. (requires a P-P, double primary, design xfmr. unless configured in Circlotron style mode.) Not very efficient power-wise however using a CCS. Another way is parafeeding the xfmr thru a cap. and using an inductor or CCS plate load.
Don
Thanks.
Another question, ther're two acually 🙂 :
1) Does this design need a stabilized power supply? My father told me, having a look at the characteristics, the transfer curves are quite flat, so as the voltage changes under load, the anode current will be still the same. Any comments on this ?
2) Heater voltage will be AC (6 Volts). He says, there's no need for rectifying the heater since in view of the heating it doesn't matter if a heater voltage changes 100 times per second it's polarity, or not. (It will be hot any way)..
What do you think ?
Another question, ther're two acually 🙂 :
1) Does this design need a stabilized power supply? My father told me, having a look at the characteristics, the transfer curves are quite flat, so as the voltage changes under load, the anode current will be still the same. Any comments on this ?
2) Heater voltage will be AC (6 Volts). He says, there's no need for rectifying the heater since in view of the heating it doesn't matter if a heater voltage changes 100 times per second it's polarity, or not. (It will be hot any way)..
What do you think ?
The proposed amplifier has lots of correction tweaks that will be critically dependent on the output tranformer. I would have thought that (if successful) a circuit like this would evolve from a given output transformer design, not the other way around.
I think it really doesn't matter, if we begin from the "end" (circuit for the OT) or at the beginning (OT for the circuit).
If an Output Transformer can be designed of really high quality, some basic parameters of it will be enough to give more attention to a well built-up circuitry.
If you have a good OT and a bad design, it won't be really nice. A good circuit plan and a bad OT even worse.
The best solution is to have a really good circuit plan, performing well on paper, and an exceptional OT, measured to meet the paper needs.
If an Output Transformer can be designed of really high quality, some basic parameters of it will be enough to give more attention to a well built-up circuitry.
If you have a good OT and a bad design, it won't be really nice. A good circuit plan and a bad OT even worse.
The best solution is to have a really good circuit plan, performing well on paper, and an exceptional OT, measured to meet the paper needs.
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