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#1 A nice D3A---2A3 Monkey circuit

Posted: Sat Jul 03, 2010 4:41 pm
by nic
This one's going in a box :) Got this one up two days ago , made a few tweaks and it's now sounding very good , just need to tame a bit of bass bloom . Can run with E280F or D3A triode strapped with no changes , no electrolytics anywhere and also no sand devices of any kind . Plate chokes for the driver are 4 chamber/4x20H 1k DCR units giving 320mH at 30mA max in series , DCR works rather well with the D3A , gives a very nice sounding operating point around 12mA . Will try Osram PX4 and also LCL DC fils on both 2A3 and PX4 before deciding the chassis layout

cheers

Nic

#2

Posted: Sat Jul 03, 2010 5:03 pm
by Nick
Yep, nice simple circuit. Like it.

#3

Posted: Sat Jul 03, 2010 7:02 pm
by ed
how are you heating the 2a3s nic?

#4

Posted: Sat Jul 03, 2010 8:04 pm
by Cressy Snr
Nice Nic

I loved my PX25 monkey amp.

No nonsense sound...Fast and dynamic.

Just not really powerful enough for my baffles.

I'd love to build a 13E1 monkey but that would be very silly :wink:

Steve

#5

Posted: Sat Jul 03, 2010 8:27 pm
by andrew Ivimey
Yeah, I like it too - variation on a Monkey indeed. I'd love to hear it. My Monkey is locked in the shed (300b Thorsten version) Its not bad and an interesting design as were, for example, Steve's sperrymints.

This one is different!

The jury needs to hear this.

#6

Posted: Sat Jul 03, 2010 9:17 pm
by nic
It's AC heaters and AC filaments at present although I much prefer the clean sound of LCLCL DC fils on a DHT . I'm not 100% convinced regarding the 36 ohm cathode bias resistor , will need to do some hang some test kit off the amp when I get a chance and see if there's much of a signal at the E280F cathode . There's not a lot I can do with this short of implementing a stack of diodes or a bypassed 125r cathode res and bring the 5k resistor to 0V

cheers

Nic

#7

Posted: Sun Jul 04, 2010 2:47 pm
by nic
For a 2 hour lashup this amp measures very well . With unoptimsed layout and AC fils on the 2A3 there is 6.1mV and 6.5mV hum at the speaker terminals . Power output 3.6w at clip . With one channel's inputs shorted and signal in the other , there was slight crosstalk but nothing major . Into a resistive load a 1kHz squarewave was reproduced almost perfectly , a 10kHz square wave looked a bit ragged but the amp measured -3dB down at 50kHz , -1dB 27khz and 0.8dB down at 20kHz , at the low end -1dB down at 10Hz . Measurements taken at 1kHz intervals >10kHz , I did not feel the need to look for resonances further up from 50khz .

cheers

Nic

#8

Posted: Mon Oct 04, 2010 3:45 pm
by nic
Pictured wearing a pair of RG1-240A MV's but currently running paralleled CV31 for rectification . The CV378/GZ37 was ditched shortly after the last postings . What a low value valve . A £5 NOS 5Z3G wipes the floor with a £40 CV378 any day of the week ;)

cheers

Nic

#9

Posted: Mon Oct 04, 2010 6:39 pm
by thomas
Hhmmm fascinating, not sure entirely whats going on, like the minimal parts count though!

#10

Posted: Mon Oct 04, 2010 6:43 pm
by andrew Ivimey
Neither am I now I come to look at it again - the 2A3 is fed by g2 & 3 of the D3a - don't understand that but if there is enough signal and G3 isn't connected to the cathode well ... okay????!

#11

Posted: Mon Oct 04, 2010 7:05 pm
by nic
There is a little more complexity now with separate heater/filament switching (to allow pre-heating of MV's) and latching relay for the HT . Also a filament transformer per rectifier , so meatier types can be used . Anodes are crossed and paralleled between the rectifier sockets so just about any B4 base DH rectifier can be used including the topcapped U19 with added leads and MV's such as RG1-240 and RG1-250 can also be used with a ferrite choke added in series with each anode .

When triode strapping a pentode the G3 connection can be tied to the anode or to the cathode if the connection is brought out to the base . In this case I've tied the G3 up rather than down . This can make the valve draw slightly more grid current approaching 0V but I've not been able to get the spot galvonometer working to take a reading to confirm

cheers

Nic

#12

Posted: Mon Oct 04, 2010 7:09 pm
by pre65
Interesting, I had not studied the circuit up till now.

I have a 450V PSU, and four 40H chokes, but I've only got two Russian 6S4S (6.3V 2A3).

Are they worthy of this kind of circuit ?

#13

Posted: Mon Oct 04, 2010 8:26 pm
by nic
pre65 wrote:Interesting, I had not studied the circuit up till now.

I have a 450V PSU, and four 40H chokes, but I've only got two Russian 6S4S (6.3V 2A3).

Are they worthy of this kind of circuit ?
Those 40H chokes are probably too low in value to use as plate chokes for the driver unless it has low Ra . You could always run in series to achieve 80H . The chokes I'm using are 4 chamber 20H chokes (4 x 20H/1k) placed in series for 320H . This is overkill in the inductance department but the 4k DCR works rather well with the D3A 11mA operating point/2A3 bias point . You may need to add series resistance to the plate choke to achieve the required bias point if the DCR is too low . I've briefly tried the 6S4S and find these are good valves , well worth a punt :)

cheers

Nic

#14

Posted: Mon Oct 04, 2010 9:54 pm
by Mike H
andrew Ivimey wrote:Neither am I now I come to look at it again - the 2A3 is fed by g2 & 3 of the D3a - don't understand that but if there is enough signal and G3 isn't connected to the cathode well ... okay????!

Connection point blobs missing in drawing? Anode + g2 + g3 are actually all connected together? I.e. 'wires' don't cross, but connect


 

#15

Posted: Mon Oct 04, 2010 11:23 pm
by pre65
Mike H wrote:
andrew Ivimey wrote:Neither am I now I come to look at it again - the 2A3 is fed by g2 & 3 of the D3a - don't understand that but if there is enough signal and G3 isn't connected to the cathode well ... okay????!

Connection point blobs missing in drawing? Anode + g2 + g3 are actually all connected together? I.e. 'wires' don't cross, but connect

That's not how I read it.

G2 & G3 of the D3a go to 2A3 grid.

EDIT.

Perhaps Mike is right ! Would it work otherwise ?

D3a anode comes from 2A3 cathode.

Or have I got it wrong ?