My First Tube Preamp

[fantom_mayonaise]

This is how I spent my Holy Week.

This preamp is designed by Tony Tecson; I even bought the basic kit (minus tubes) from him.




Even before easter, I've already installed the power socket and RCA jacks. Just eyeballed the location of the RCA jacks



Looking good.

[fantom_mayonaise]

Until I test fitted the rear panel on the chassis



Should have measured twice and cut once.

[fantom_mayonaise]

Test-fit and cutting of proto-board




The hole for the tube socket has been cut and surrounding pcb traces has been ground away

[fantom_mayonaise]

Populating the board




Partially completed board



The unglamorous side



A look at the project before calling it a day. I wasn't aware of the second mistake I committed during the build.

[fantom_mayonaise]

The nylon cutting board in the previous picture would be used as mounting platform. Here, slots have been milled away to hold the transformer, choke, caps and for ventilation.

[fantom_mayonaise]

Completed project




I routed the AC heater supply high above the board using magnetic copper wires.


Operating the unit at an ambient temperature of 30c, I could smell overheating components. I can't hold the large green capacitor for more than 10 seconds. I realized I should not have mounted these resistors under the output caps. They were at 100c measured using a contact temp probe attachment of my dmm! The capacitors were being toasted!



I was expecting the tube amp to sound warm, I didn't expect it to be this warm though. It's too warm hehehehe


I milled cooling slots next to the overheating resistors. You can see from the following picture that these are being pushed hard. I measured 90volts across one of the 6.8K resistors; it's dissipating more than its 1watt rating. A line is starting to appear at the middle of each resistor. I expect it to come apart soon. When they do, I'm going to replace them with 2 watters elevated as high as the leads will allow. Maybe increase the value a step or two. Sir Tony?



I have the top cover all cut and ready but until I relocated those cathode resistors, I'll leave the unit uncovered.


How does it sound?


Prior to this build, I acquired a yaqin tube buffer to tide me over until I finished. The DIY tube preamp sounds at least as good as the commercial unit. In fact, I notice slightly strong bass and that's without using the 1uf film cap provided by Sir Tony. I'm saving that for a future baby AKSA build The big green cap is only .82uf, perhaps a bigger value will yield deeper bass. Something to try on the next revision. My list includes

1. A power switch
2. Relay to bypass the preamp when power is off
3. Slightly higher cathode resistor values. Higher wattage definitely
4. Bigger output cap
                                                       

                                                      


          

[Disabled]

tulo laway mode

[zrehtiek/keitherz]

what is the total build cost sir. this is interesting, i feel like i want to build one too.

[Disabled]

^ same.

[RaffT]

@fantom

isa lang tanong ko: san mo nabili yung mini miller mo?

[fantom_mayonaise]

around 4k?  PM Sir Tony and ask if he still has a spare kit. I did not use the enclosure he provided. Ang importante yung power transformer. Pasadya kasi yung sa kanya.

I did a lot of reading on the net while waiting for the free time to build the kit and I realize the cost may be lowered by using what available transformers you have, but you will have to do calculations yourself, maybe even winding your own transformers and chokes. I went with the kit route since this is my first and I wanted the assurance that it will work. Plus I can count on Sir Tony for support in case a problem arises. Having said that, the preamp didn't work the first time I powered it. Turned out, one leg of the common mode choke is open. For now, I jumpered it. Sayang nga lang yung winding.

This DIY is actually more expensive than buying a second hand commercial unit 

[fantom_mayonaise]

@fantom

isa lang tanong ko: san mo nabili yung mini miller mo?

Sa panda construction supply sa binondo, sir. 3K yung original Dremel 220v unit nila, may kasama nang basic bits. Matagal mo rin magagamit ito sir, at madami mong mapaggagamitan. Mas mahal pa yung black and decker sa mga Ace, True Value and Handyman. Just now, I'm using it to clean up the swelas of my rubber shoes prior to glueing back the sole. Hehehehehe.

Meron ding mura na tig 1.5k sa gilid gilid ng raon, mohler yata ang tatak. Pwede na din yun, dremel lookalike.

[RaffT]

Sa panda construction supply sa binondo, sir. 3K yung original Dremel 220v unit nila, may kasama nang basic bits. Matagal mo rin magagamit ito sir, at madami mong mapaggagamitan. Mas mahal pa yung black and decker sa mga Ace, True Value and Handyman. Just now, I'm using it to clean up the swelas of my rubber shoes prior to glueing back the sole. Hehehehehe.

Meron ding mura na tig 1.5k sa gilid gilid ng raon, mohler yata ang tatak. Pwede na din yun, dremel lookalike.

ah ok... thanks sa info.. hehe nakita ko na yung sa raon, 1k6 price.. pangit quality.. try ko puntahan yan sa binondo pag makatime

[tony]

another tubero is born, congratulations on your first build fantom_mayonaise, lot's more tube amps to build...+1

[tony]

Slightly higher cathode resistor values. Higher wattage definitely

you will notice that the same value of plate resistor and cathode resistors were used, and the reason for this is so that the net "ac"  current draw from the power supply is zero....

you should install those 6.8k resistors as high off the board to allow air circulation....

so if you lower one, you lower the other.....but that is not all, the cathode bias of the first stage  will need readjustment...

lastly, if you can get your hands on a 7dj8, i have been told this sounded great on this preamp....

this is the link to my first post about this preamp: http://www.wiredstate.com/forum/viewtopic.php?f=11&t=24713

Tony’s 6H23 preamps

Circuit Highlights:

The circuit is composed of a common cathode direct coupled to a cathode follower capacitively coupled to the load using a Wima MKP 4.7ufd/250volt polypropylene cap or equivalent.
Since dc coupling between the input and output stage tubes is used, neon lamps NE-2 are installed from plate of first stage to cathode of second stage to protect the grid of the cathode follower. Upon power up, this lamp lights up and then extinguishes when tubes have properly attained operating conditions and stays off during operation of preamp.

The circuit is also an implementation of John Broskie’s zero ac current draw on the power supply. The use of common mode choke, a technique used on smps supplies to combat the effect of emi/rfi is put to good use here. To avoid hum and noise issues, the filaments are lifted to a dc voltage of approximately ¼ of B+ thru resistor voltage deviding networks......

Input impedance is about 100k and output impedance is about 300 ohms. Gain is approximately x5 and an input of about 0.7volts rms drives the amp to clipping, but since a 100k pot is used, this voltage limit is mitigated. Undistorted output of about 2.5 volts is enough to drive most amps into clipping.


The power supply:

The power supply is a typical full wave bridge. The addition of common mode filter choke however, ensures that the circuit sees very high impedance as far as AC is concerned while DC resistance is kept to low levels. so that the amplifier circuit itself is isolated from the psu.....Massive Capacitor filtering brings the psu impedance to a much lower level......

[tony]

did you series the 1n4001 cathode bias diodes? there should be 2 in series, if you just used 1, then the  plate draws lots of current, and that causes over dissipation in the 6.8k resistor....

you will also learn that tubes are more forgiving of mistakes, something you can not do with ss...

i will gladly replace that choke if you bring that to the house.....

[tony]

another tubero is born, congratulations on your first build fantom_mayonaise, lot's more tube amps to build...+1

meron pa nga palang isa pa...watch out for him.....

[fantom_mayonaise]

did you series the 1n4001 cathode bias diodes? there should be 2 in series, if you just used 1, then the  plate draws lots of current, and that causes over dissipation in the 6.8k resistor....
ng that to the house.....

Yes, there are two diodes on the first stage cathode. One is on top and another underneath.

i will gladly replace that choke if you bring that to the house.....

Thank you sir. Will do. I'll inform you ASAP.

[abeTIR]

huwaww a tube 

+1 fantom

[???]

Makikiusyoso lang muna.


San galing yung tubo?