• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Hi-end 845 with only two stage

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#101
My experince with OTL is very old, many many years ago

I don't consider the OTL a good choice if you are searching a reference amplifier.

This because OTL need feedback and to use low global feedback it is necessary use many output tubes so the cost it is near to high power single ended.
Another point is that the OTL use bad vacuum tubes, ofter voltage regulator like 6080 and 6c33c-b with high distortion so other feedback it is necessary to compensate this.
Another point is that in the OTL is always used a bad power supply with no inductance and the power supply transformer are never enough big.
So for these aspect I don't like these amps and I don't susggest.
But many DIY have build my OTL with a relative low cost and now are happy.
So I suggest it only if you can get sulplus material like capacitors and transformers
 
#102
Thanks for your comments, appreciated.
I was under the impression that getting rid of the coupling transformer is the way to go in high end audio, but have never had the opportunity to play around with any single ended designs mostly due to the fact that they have received some negative publicity from some "gurus" for low power limited frequency range and impedence matching limitations.

Nevertheless I will take your word for it and go for a single ended design from your web site. Do you think the the 845 design surpasses the dual 300b reference amp you have built in sonic quality?
 
#105
Nick,
many commercial SE amplifiers give bad result because in the projects to get high power use low impedance transformers.
This give output impedance near 3-4ohm so the damping factor is terrible.
These amplifiers can give a good sound only if you use fullrange loudspeakers with impedance near to 16ohm.
All my vaccum tube amplifiers have an output impedance less than 2ohm so these can drive without problems any loudspeakers.
In my medium size room I am using this 845 amplifier to drive Martin Logan Aerius and this give a reference sound also with a good output level.
The 300b is not a good vaccum tube because the sound is not neutral and it have a characteristic. The medium frequency of the 300b is very natural but bass and high frequency have very large limitations.
I consider very well the 2A3 and 845, soon I will test GM70 (both versions).
The SV572-3 and SV811-3 are good like the 845 but these are not in the current production.
I hope my bad english can be understand.

Soon I will test and compare the Tango X-10S to a new Bartolucci output transformers designed for this project.
 
#107
The 6HV5a has a lot of gain. With that B+ it's also good for around 30ma plate current comfortably. If I have it right, a step down would provide more current to the output grid and a higher plate load to the driver in A2, as well as a lower DCR from grid to ground on the secondary for quicker recovery. The step down reduces input stage noise and, as you say, the capacitive load on the driver. If the circuit has gain to burn it seems worth considering.

BTW, that's not my quote in the signature, but my name is Richard so it still works out. 😉
 
#112
#113
1000v power supply - cheap version

with medium efficiency loudspeakers 88-89db it is not necessary have DC filaments or center trimmer for the 845 using Valve Art
 

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#116
JoshK,
if you read the first pages of this forum you will find some about this.
The input capacitor of the 6HV5A is not terrible and it can be driven with any line preampl., phono stage, DAC or CD with an output impedance less than 10Kohm so any.
Also for the volume control is not possible use value more than 10K.
Now I am driving it with my new phono stage.
http://www.audiodesignguide.com/Phono/pre-PhonoEnd.html
 
#117
audiodesign said:
1000v power - top version - not tested yet!
Click to expand...


Andrea,

Please DO NOT use rare and increasingly scarce 845's as rectifier tubes!!!! You are far better served using a pair of 866A mercury vapor rectifiers or the Xenon equivalent 3B28. You could also work very happily with a pair of single diode per envelope TV damper rectifier tubes. They would give you an automatic time delay on the B+ which would help increase the life of your 845 audio finals. I would also recommend that a choke input filter be used if the plate xfmer secondary voltage may be chosen at will as this will provide superior B+ stability. A bleeder R of sufficient wattage rating to handle 20% or so of the supply power (good design practise) is needed with a choke input to avoid skyrocketing first voltage, then droop as the load comes on.

Your project is very inspiring! I may use the 6HV5 or 6JD5 to drive another DH ouptut tube sometime. I do not have any 845's. I would have to find a couple of lumps of iron in my junk box that would work well enough as I.T.'s.

Do you happen to have any 6JD5 beam triodes that you could also run in your circuit (drop-in equivalent) and try the distortion analysis? I have both and wonder if one is marginally better than the other in this unique application.
 
#118
Bob,
have you some models about the single diode per envelope TV damper rectifier tubes ?

I have tested GZ34 NOS and new with good result but these cannot be used in this case for the high voltage, to check if in brigde connection are good (like Audionote Ongaku).

My experience with 6D22 is terrible, many many diodes in fault after few days in normal conditions.
 
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