Amp No 46

[Cressy Snr]

Oh dear.

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The transformer transplant has turned into a major rebuild.

[Daniel Quinn]

A screwdriver storage box , best use for a valve amp

[Cressy Snr]

Indeed!

[andrew Ivimey]

I've started using a dell optiplex p.c. as a 'collecting bin' to take to the re-cycling centre. First by crushing a broken de humidifier on top, took it off resolving that it should be merely the 'hat' on top of whatever electronic crap I've got; laptop psus , dvd player, multipurpose handset, several kilos of ptfe covered oxygen free copper wire, a couple of guitar valve amp chassis, a couple of homemade similar, a partridge and eight svetlana 6550s ( crunch!) a multimeter, a nine band shortwave radio, a hadron collider and a pair of 300bs.

But I still like what Steve is doing!

[Cressy Snr]

Rebuild completed:

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[Cressy Snr]

Haven’t seen the turntable for a week:

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Apart from that, the headphone system is now back up and running and sounding fine.
The system is louder for the same volume setting than before, possibly indicating a different step down ratio with the Pre II output transformers and maybe some degree of difficulty driving the 16R headphone impedance.
The Hammonds drive the headphones well on the 16R secondary as expected.

[Cressy Snr]

I've changed the valves to EH 6H30Pi. This valve produces slightly smoother treble than the ECC99. Very small difference but noticeable on my headphones. Below is the audio circuit and power supply:

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6H30Pi HPA PSU.png (40.28 KiB) Viewed 22867 times

[Cressy Snr]

I've collected a few bits from Simon, so a 45 headphone amp is now firmly on the cards. I'll get onto that at some point down the line and start a new thread.

[simon]

Can't wait to see how that works out Steve, got to love 45s

[Paul Barker]

Or, looking at thread title?



I would say that pair is a rare bargain and only UK postage costs.

[Cressy Snr]

Another week and another mod. The input stage is now choke loaded, using one Hammond 156C, 150H choke per channel:

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This necessitated making a new board from black perspex to mount the chokes on and cutting a hole in the bottom of the cabinet to provide access to the electronics for testing purposes. I also had to change a few component values in order to be able to fit the downsized components under the perspex plate. I'm using 0.22uF Russian PIO green caps because they had a smaller diameter than the Obbligato 0.33uF caps from before. Also the bypass cap on the input stage was reduced to 100uF, giving a -3dB cutoff of 5Hz, which is good enough.
The valves are 6N6P as opposed to the previous 6H30Pi so that I wouldn't saturate the 8mA capable choke. We're looking at 8.3mA through the choke, which is close enough.

Choke Loaded HPA.png (32.59 KiB) Viewed 20311 times

[Cressy Snr]

The sound quality has taken another step up. Nice.
All bodes well for the 45 HPA.

[Paul Barker]

Looks good but the bypass cap is small.

Ive calced a single hammond choke at 20hz below and two series chokes at 20hz. On the basis of the load Id use two in series. Dont forget two chokes in series half the capacitance. This is also better.

Ive also used my spread sheet I wrote to calculate the frequency -3db in both options for a 100 uF cap.

I’d raise the cap value.

[Paul Barker]

Sorry in the spread sheet I named the valve wrongly.

[Paul Barker]

Also, when calculating the gain the choke load impedance is parallel with 220k.