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Old 10th January 2008, 10:16 AM   #1
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Default Suggestions needed for improving Rogers Cadet II PP power amplifier

Hello and a very happy new year to you all.

To begin with I must confess that I am not a technical person and do not understand circuit diagrams and other such complicated things… but I do enjoy listening to music – rock/pop and jazz.

I used to listen to music through a regular decent solid-state system till I discovered tubes amplifiers a couple of years ago and now there is no going back!!

With the help of a friend (all his good work) we got my father’s 1963 Rogers Cadet II push-pull power amplifier going which like the Rogers Cadet III uses 2 ECL86/6GW8 tubes per channel (from what I am given to understand, the power amplifier section of both the amplifiers are almost identical except that the Cadet II gives out ~6 watts/channel while the Cadet III gives out ~10 watts/channel on account of increased HT and slightly bigger O/P transformers – is that true?).

To get the Cadet II going we had to replace all the transformers (M4 grade core?) as they were all damaged as also replace all the resistors, capacitors (regular industrial quality). We introduced a 100K volume pot and fed a CD player directly to the power amplifier and it sounded very good and encouraging.

Then slowly the following modifications were made (not a technical person but will try and explain) –

• Separated the power supply and housed it in a separate chassis. The power section is now connected to the Rogers Cadet through a cable;
• Changed from solid-state to valve rectification (EZ81) and introducing a choke;
• Replaced coupling capacitors in the input and output stages with Sequa paper-in-oil capacitors;
• Replaced all other capacitors with Black Gates; audio signal resisters with Kiwame; and power related resisters with Mills;
• Decreased the global feedback (I think we are now using a 15k resistor instead of a 6.8k) (the sound opened up but the input sensitivity further increased); and
• Introduced a better quality volume pot in the power amplifier so that I can feed it directly from a source without going through its original per-amplifier (which was anyway a separate unit). I don’t think I am facing any impedance relate problems in this arrangement – no frequency roll-offs that I can make out. The volume pot is connected in a fixed series/variable shunt arrangement.

The end result - a brilliant sounding 6 watt/channel PP tube/valve amplifier - sounds airy, open, fast, with no harshness in the mids/high or softness in the bass.

Yes, we did find that the input sensitivity of the amplifier too high for the CD player and we had to introduced a resistor (don’t remember the value 100k?) on each channel on the signal path before the volume pot so as to reduce the input voltage (in the given circumstances, is there a better way to over come this high gain related problem?). Now at ~ 4 O clock volume position does the amplifier clip (estimated 6.5 watts/channel) with a 2 volt input signal.

The amplifier is being used with OB speakers (~96db sensitivity) and I feel the amplifier has enough juice to meet my humble needs.

However, there is a desire to improve it even more. I understand there is a ‘gingertubes variant of yvesm’s circuit’ which is supposed to be very good. Unfortunately I could not find a legible version of it – could someone help? How different is it from the Rogers Cadet II/III circuit? Will it mean major work in redoing the complete amplifier again (sorry, I do not understand circuit diagrams)?

Do you folks feel that by tweaking/improving the original circuit and/or upgrading the output transforms (this stuff is expensive!) the sound quality would greatly improve? Or should I stop here and not spend any more time & money on this amplifier and just keep enjoying it - honestly, I am quite happy and could live with this amplifier.

Look forward to all your suggestions/recommendations.

Cheers

Sanjiv
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Old 10th January 2008, 11:28 AM   #2
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Is the signal still going through the ECC807's based front end (? can't remember if thats the right valve). If it is then eliminating them will improve you gain problems and clean up the sound.

Shoog
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Old 10th January 2008, 01:19 PM   #3
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No, I am not using the pre-amplifier and only using the power amplifier with its 4 ECL86 tubes.

Cheers

Sanjiv
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Old 28th May 2008, 11:59 AM   #4
JPS is offline JPS  India
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Quote:
I understand there is a ‘gingertubes variant of yvesm’s circuit’ which is supposed to be very good. Unfortunately I could not find a legible version of it – could someone help?

Post#8

Cathode Feedback from Secondary for PP Amp
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Old 21st May 2012, 02:52 PM   #5
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Hello, does anyone have any ideal whether can a ecl86 be replace by a ecl84 on a rogers cadet 2 poweramp. Can they be replace with the change on pinout?

Thanks in advance.
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Old 21st May 2012, 03:49 PM   #6
DF96 is offline DF96  England
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I doubt it. You would have to review bias point etc. ECL84 was intended for video output.
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Old 21st May 2012, 06:38 PM   #7
Yvesm is offline Yvesm  France
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Quote:
Originally Posted by Sanjiv Sahgal View Post
. . .
However, there is a desire to improve it even more. I understand there is a ‘gingertubes variant of yvesm’s circuit’ which is supposed to be very good. Unfortunately I could not find a legible version of it – could someone help? How different is it from the Rogers Cadet II/III circuit? Will it mean major work in redoing the complete amplifier again (sorry, I do not understand circuit diagrams)? . . .
Do you mean this one ?

Push Pull ECL86/6GW8

Yves.
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Old 22nd May 2012, 07:58 AM   #8
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Ok thank, so I think its only sub is 6gw8,
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Old 22nd May 2012, 08:56 AM   #9
DF96 is offline DF96  England
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6GW8 is not a substitute but an exact equivalent; it is just the American name for the ECL86.
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Old 23rd May 2012, 03:59 AM   #10
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Yes ECL86 = 6GW8
and
ECL84 = 6DX8

I've built lovely amps using both but prefer the ECL86.

Your timing is bad, I had that Gingertubes variant of the Yves M's circuit amp back only 2 weeks ago so I could have looked to confirm what I did. It has now gone to my sister for her HiFi sytem. (I built systems for both of her daughters so she wanted one too).

I looked at that schematic linked above and believe it is accurate. Final shunt feedback resistor at 15K and global feedback resistor at 100K is correct. Note however that that I used output transformers with ultralinear taps (20% instead of the more usual 43%). If you don't have ultralinear taps use Yves original circuit which he linked above.

Having said all that, I confess I have a Rogers Cadet III which I have not bothered to modify in any way (even have one spare ECC807 unobtainium tube stashed). It drives a pair of vintage Warfedale Speakers and is driven by a vintage Pioneer FM Tuner. These items are hooked up to an electronic timer switch. This combination is in my bedroom and is the "Clock Radio" which wakes me up every morning.

Cheers,
Ian

EDIT: 6DX8/ECL84 make a seroiusly lovely little approximately 5 Watt push pull amp, for those wanting to experiment with other triode pentodes may I suggest the ECL85/6GV8. These were used in television and have slightly higher power output capability than the ECL84/6DX8. Uncertain as to their availability in the rest of the world but there is a serious glut of 6GV8 here in Oz, to the extent that guys were just throwing them out, because no one had bothered to tell them that they work well in a push pull audio amp, there must be 20 or more in my own tube collection.

Last edited by gingertube; 23rd May 2012 at 04:10 AM. Reason: Additional info
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