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    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

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All DHT 845 SE
I have some 104D's.
Any problems with Microphonics with the 102D.
I have 100+Db speakers.
Phil

Hi Phil,

I'm coming back to you on this. I've just replaced my original set of 102d with premium matched ones. This new pair is acting differently to the first set. They are drawing more anode current, have higher gain and much more sensitive to vibration as a result. Tapping the chassis and the glass of one has the vibration passing to the speaker. Same setting as before, played even louder, does not provoke a microphonic reaction so not an issue in this case. (Some of my 76 on input duties do the same when tapped but are not excited by the speaker SPL, at my usual listening level, no more than 75db, 55-65db typical.)

Since the new set are 'Premium matched' as opposed to my first test mule pair which were cheap ebay 2nds (even if they don't advertise the fact!), I assume the behaviour of the latest pair is more typical.
 
tony said:
now playing.....amp can still use some tweaks...
EF86 buffered by 6N1P driving the 829b paralleled sections in triode mode



sad to say, this is a failed design, i can't find tubes that have sections that are equally matched,
and since the cathodes are common to both sections, they do not conduct equally so that
the resulting distortion is audible....i suppose i can tweak this by manipulating filament
voltage to get the transconductance equal on either side, but that will be too much hard work...
so i ditched the septar socket and put in an octal socket instead....
in came the 6AV5 horizontal output tube rated for 11 watts....but in this
amp it is running about 16 watts plate dissipation, 250volts plate at 60mA cathode,
does this ring a bell? yes, it does, it is the operating point for the 2A3....





the power factor of almost 90% and a power draw of about 80watts...
the power traffo is rated for 180VA, so that the power traffo will be
very cool to touch and will last a long long time....
the high power factor is indicative of a very good power supply...




for those interested to design their own SET,
this is how i sized the cathode bias resistor:
for triode mode, grid 1 bias is calculated as follows:
Vg = -(Vpx0.6)/mu
mu equals 4.5 and Vp is 250 volts,
so that Vg is 40volts,
looking for 60mA. Rk, the cathode resistor then becomes,
40/60 = 600ohms, this is achieved with a parallel combination of
1k and 1.5k 5 watt resistors, making for a 600 ohm 10 watt
cathode bias resistor....this resistor dissipates 2.4 watts
so that use of a 10 watt resistor is very much justified...

it would interest you to note the in the actual amplifier,
bias was at 39.5 volts, am i lucky or what? :tmi:
 
Hi. This is my first post here. I read this forum a long time, and now decided to write. So, I want to present you some of my DIY tube projects.

One of my first amps (~10 years ago):
SE A-class poweramp
DSC_5085.jpg
Circuit based on SRPP and SE-output, but I don't used chokes. Fail. This amp has so high level of noises.

And pair of last projects:
Headamp (2011) with built-in DAC. Each part (digital, low- and high-voltage analog, heat) of amp has its own power supply (PCB with transformers).
DSC_5076.jpg DSC_5078.jpg
Main concept is 6N23P (ECC88) + 2x 6N6P (7044) + transformer output. DAC based on AD1853+DIR9001.
P1210349.JPG


PP-poweramp (2014). 6SL7 as input and phaseinvertor, 6SN7 as drivers, 5881 as output.
1.jpg
I used Hammond transformers for output and custom-made toroidal transformers + 2x 10H chokes + 2x 5U4G in power supply. Sound much better then Onkyo Integra A-927 that I used to compare.
Some pics of chasis:
image~14609;5.jpg image~14533;2.jpg
It's steel with polymer paint (chasis) and TiN-coating (feet and plate)
I like Ongaku-style design so I tried to make it like Audio Note.
3D-model and result:
13.JPG 3.jpg 4.jpg
 
845 twins

Hello,
I decided to list the project I have been working on about one year.
Here they are my little baby-845 twins.
Regards,
Jarek, Poland
 

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Hi to All , this is my new amp , it's a hybrid one , the first gain stage consists of one PCC85 in LTP configuration followed by a second gain stage which is the triode section of the PCL86 and the pentode section ( of PCL86 ) is the cathode follower to drive the output stage which is a CFP ( compound output stage ) with transistors as drivers and mosfets as output devices .

And some technical characteristics :
Closed loop GAIN = 27dB
NFB = -17dB
POWER : 2 * 65W at 8Ω load
2 * 98W at 4Ω
Output impedance = 66mΩ
DAMPING FACTOR = 122 at 8Ω
HIGH FREQUENCY RESPONSE 260KHZ ( -3dB )
 

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