so i get common tubes free... now i just need a project

[VictorG]

the last one being:

http://web.tiscali.it/audiofanatic3...P/EL34_EL84_Mullard/EL34_EL84_Mullard_PP1.jpg

with a parts list of

http://web.tiscali.it/audiofanatic3...P/EL34_EL84_Mullard/EL34_EL84_Mullard_PP2.jpg

if you just cut and paste these, you should get right thru.
B

 

[fdegrove]

Hi,

And to make your choice even harder....

BONAVOLTA.

This is what the French call "l'embarras du choix"...

Cheers,

 

[Jax]

You can ask me about this design. It's mine

The original web page it comes from is here which is my old site. There is a better drawing on my new site here

L63 is a British 6J5 which in turn is half a 6SN7. If you want, use a 6SN7 and share the two triodes between the channels.

 

[VictorG]

Unbelieveable!
To actually get to talk to a creator! Okay first question, have you ever plugged this in to electrostats? If not, how would you expect your amp to handle highly capacitive loads? Thnx,
B

Very interested in your amp, looking into parts now....

 

[VictorG]

is there an email address you could give me so as i research my amp I can address individual questions to you? Thnx

 

[Jax]

The feedback is lowish so it should not have too much of a problem driving electrostats but I have never tried it.

You can remove the feedback (R16, R17, C10 in the newer schematic) or add a 0.7 uH inductor in parallel with a 10 ohm resistor in series with your electrostats if there is stability problems.

C11 is not mounted in my amplifier.

Click the www button in my post, there is an instruction how to get my email address there

 

[VictorG]

When you added this:

"Still using my Quad II’s. I have redrawn the schematics above to make them more readable. The EL34 does not like being fed from a single 350V source so it will get a separate 400V supply for the prestages."

what was the observed problem when driving the EL34 from 350V? what was the improvment at 400V? Suggest just powering all off a 400V supply? why have two supplies?

B

 

[Jax]

I got low frequency oscillations due to coupling between the output stage and the phase splitter over the supply. It's called motorboating. Reason is the poor filtering in the old schematic. It may work with a 10H choke replacing the 1.2 kohm resistor in the old schematic but I never tried it.

The history of the amplifier is long, I intended to run the output at 435V and dropping to 400V with a resistor for the prestages. But the transformers are a bit too low in primary impedance so I chose an operating point with lower voltage and higher current for the output.

You can run the output at 435V if you use 5 kohm transformers, you may need to change the EL34 cathode resistors for a better bias point, 470 ohm I think. You must increase the screen dropping resistors from 68 to at least 470 also (which in my opinion sounds worse).

 

[VictorG]

A few more questions just to check on some things as I am pricing out parts....

love the picture album by the way, killer work....

1.) I am just gonna assume that those reisistors have different wattage ratings?

2.) In terms of the output transformers, I have seen several hammond output transformers in thier "classical" tube line that seem to have been designed with for example, the EL34 in PP in mind. If i get one of these rated to put out 25W, will this be an acceptable replacment for the Tango Tranny (these are a bit pricy for a college student.)

3.) Along the same line, for the power supply, i do not have a 600VA tranny around.... and you said that this was a beast for the needs anyway, got any reccomendation on something a little cheaper and a little smaller, and a little more mainstream (for obtaining purposes)

4.) For the caps, I take it I'll only need about 450V rated caps?

I appreciate your amp, and if I can make it fit in the budget, i look foward to buildin it soon. Thanks for all your help in this matter.
Brian

 

[Jax]

love the picture album by the way, killer work....

Thanks

1.) I am just gonna assume that those reisistors have different wattage ratings?

R1 and R2 are 4W wirewound.
R5 are two 36 kohm 2W MF in parallel to form a 18 kohm 4W resistor.
R9 and R10 are 2W MF.
R12 is a 2W MF.
R13 is a 4W wirewound.
R18 and R19 are 1/2W composition mounted close to the tube pins.
R3 and R4 are 1W composition mounted close to the tube pins.
The rest of the resistors can be 0.6W MF.

(MF = metal film)

2.) In terms of the output transformers, I have seen several hammond output transformers in thier "classical" tube line that seem to have been designed with for example, the EL34 in PP in mind. If i get one of these rated to put out 25W, will this be an acceptable replacment for the Tango Tranny (these are a bit pricy for a college student.)

The hammonds should do fine. With the bias point here, 3.5 kohm primary impedance with 40% UL taps should do. You may have to recalculate the feedback if there is no 16 ohm secondary available. For 8 ohm, change R16 to 1 kohm as a ballpark.

3.) Along the same line, for the power supply, i do not have a 600VA tranny around.... and you said that this was a beast for the needs anyway, got any reccomendation on something a little cheaper and a little smaller, and a little more mainstream (for obtaining purposes)

Nah
It is indeed a very stiff supply but way overkill. It was just what I had in the junk box. Any supply giving you around 350V 300mA for feeding both channels' output tubes should do. For 400V, either a separate supply designed for around 20mA or what I intend to try later: drop the 350V to 325V with a resistor and a filter capacitor, then connect the negative of a 75V 20mA supply to the 325V point.
You will need 6.3V 8A AC to feed the heaters as well. The heaters should either float above ground or biased to about 50V above ground. 6SN7 has a maximum of 100V rating between cathode and heater if I remember correctly and the cathodes on the splitter is at around 105V.

If your uncle have 6J5GT's, it may be easier to design 2 monoblocks instead.

4.) For the caps, I take it I'll only need about 450V rated caps?

C4 and C5 are 450V types.
C1 can be 16 or 25V.
C2 and C3 are 63V types.
C6 and C10 are 63V MKP.
C7, C8 and C9 are 630V MKP.

I appreciate your amp, and if I can make it fit in the budget, i look foward to buildin it soon. Thanks for all your help in this matter.
Brian [/B]

Build the input and splitter stages first, power them up and measure the voltages. V1 should have around 100V on its plate, V2A and B around 300V plate and 105V cathode.
Try to match the output tubes by measuring the cathode voltages.

You can experiment some, R6 and R7 could be increased to 220k for a deeper bass. You can adjust R10 for equal signal voltages on the EL34 grids.

 

[VictorG]

Thanks for the continued help.

I am almost done assembeling the parts list and things are looking good.

I want to include you in my thinking through, just to double check, the particular hammond i was thinking about using was the 1650F

Specs:

1650F
Audio Watts (RMS) 25
Primary Impedence: 7,600 ct
Max DC per side 128 ma.
Secondary Impedence(s) 4-8-16
"these transformers include 40% screen grid taps for Ultra-Linear operation"

The only reason I am double checking this transformer with you is that y your comments yo seemed to be pretty close to commenting on this peticualr tranny save tat you thought the primary impedence wouldbe 3.5k and you didnt think there was a secondary impedence of 16 avail. Were you looking at another tranny that I am unaware of, or did i just read to much into generalized statements?

Anyway, this transformer is in my range, not bad at all pricewise.

Thanks for the continued guidance. I am going to begin a pcb layout soon (just makes it easier for me)

Brian

 

[Jax]

7.6k impedance is a little too high for EL34's unless you are going to run them in class B with 800V plate

1645, 1650K or 1650N would be better. If money is tight, go for 1645.

 

[VictorG]

Power Supply

Jax-

You'll be happy to know (or at least I am) that i have aquired around 75% of the nessesary parts.

Now for the power supply-

Any thing I should know, or just get the votage through the bridge rectifyer and put some caps in parallel? Let me know what I should do here, as i dont see any info on this componenet of the system.

Thanks,
Brian

 

[Jax]

As a starter, for the 350V supply:

280-0-280V AC to a fullwave rectifier with 2 1N4007 diodes, a 22uF 450V cap, a 10H choke and a 100uF 450V cap forming a CLC pi filter.

or:

300-0-300V AC to a 5AR4 or GZ34 rectifier tube, same choke and capacitors above. You need a 5V 2A winding for the rectifier heater also.

You can use a 5U4 rectifier tube also but you need 320-0-320V and 5V 3A.

This is for one channel. If you want to share for both, go with the semiconductor diode version.

 

[VictorG]

last q

real quick, c11 on the schematic is missing a value. rating? Thanks a bunch, almost there.
B

 

[VictorG]

where would you recommend an output level control? 100ohm pot where? thnx again.

 

[Jax]

You can leave C11 out for the moment, it's only needed if you increase the feedback.

You mean a volume control? Just add a pot at the input.