Back at the 45 SET

[mach1]

Try using the James opt in a conventional arrangement with dc passing through the primary (they were designed to be used this way, and may not operate optimally with no current). Then use the freed up plate choke so you have a separate choke on each 27. Use a separate 0C3 for each 27 and terminate it at the top of the 27 cathode resistor, reduce the value of the cathode resistor appropriately to obtain the same bias, then try dispensing with the cathode bypass cap.

You could also try a couple of Hammond 156c as plate chokes (150H) with increased voltage to compensate for their higher 3k5 resistance.

[mltube]

Thanks mach1,
The set up that I used sounds great. It was just somthing different that I wanted to try, and it worked. Just on breadboard still, funny you mention this, I was already unsoldering for that similar aproach. Thank you for the input and new ideas.... I'm a seven year noob and still like to try new things. Common modes of assembly have become same thing different day sort of thing. I just refuse to use any sand state stuff in my assemblies, even tho some may be great as ccs.
Gday from wisconsin, USA
By the way, your more than welcome to send any Ideas untried or not, i'll give it a go. Retired, and playing with tubes daily...

[mltube]

Hey mach1,
Wow,... I thought the parafeed setup sounded nice. I have found my new favorite here. I rewired the set to run direct with the james opt's and put in a set of NOS kenrad 45's that are matched within 5% of each other and it blew me away with a new level of listening pleasure. Like I said when I started this Thread "This is my first 45 Amp". I really like (LOVE) the sound transparancy. Next... I'll try some 26's up front. Then a thought toward new preamp.... using 10y's.. Happy day
I sort of figured the paraphase setup would work, but I knew running direct with james opt would be better. Any new or unusual preamp settups using DHT on your side of the pond?
Thank You,
Mike (mltube)

[mogliaa]

Hi Mike,
Thanks for the schematics and the time taking in sharing it was the world
I was not so sure about taking the parafeed option, however there are many around here recommending it, so may give it a try.
I will post the design I do have in mind when I get the time to upload it so you can give me your opinion if that is ok.

Thanks
Ale

[mltube]

Hey Ale,
The circuit posted with paraphase sounds great, but with the james opt, it sounds a little more open and airy by removing the plate load chokes of the 45's. The only difference that I have noticed is that after revising their is a ever so slight hum at low listening levels, but heard only with ear to the speaker. I use high eff. full range RCA's of 1936 circa.
Have a great day from
mike (mltube)

[mogliaa]

Brilliant. In my last amp (EL34 PP) I had a low hum noise that couldn't get rid of - you could only notice it when music was not playing. Therefore, I'm really keen to add a DC supply to the 45s.
I have two sets of 45 RCAs - have tested them but not for matching purposes.
At the moment I'm building a valve curve tracer circuit to feed the oscilloscope as per Alan Douglas' design. Very cool, but yet not finished.

Will send you my schematics when I get the chance....cheers
Ale

[kevinkr]

Have you measured your stereo separation? If I have interpreted your design correctly you will have quite a lot of internal channel cross-talk by virtue of the 1K plate load resistors for both channels sharing that 40H choke. (The choke is providing considerable reactance which is the lionshare of the plate loads for the 27s.) The simple solution would be to use a separate choke for each channel since you probably want to run 5 - 6mA through your 27s and don't really have the supply overhead to do it with your supply voltages. (You'd want something like a 27K plate load resistor running on about 300V..)

[mltube]

Kevin, Thanks, and good eye... The B+ is now set at 312 with some ps mods, and now slight hum has disappeared.
That issue has been resolved this afternoon with the removal of the 1k resistors and intalling a separate choke to load each 27. I do not have a lot of high tech test equipment to work with so it was tested by removing signal from each chan one at a time and listening for signal in speakers.

And the filaments are still AC... I am supprised how quiet this is "refering to hum".

I have set this up with a pair of Hepner 12" full range in OB and the sound that is produced is remarkable to say the least.

Kevin, a couple of months back I was putting this togather, then health stopped the progression of the build. It is nice to hear from you again, as I am a fan of your builds and designs.

Still building every day here, it's sooo addictive.
Have a great day
Mike

[mltube]

A note about the 27's, I have rotated 9 pairs of them through and the globe mesh plates have the most delicate transparent sound. The solid plate st's sound slightly harsh to my ears.
Cheers, Mike

[kevinkr]

Originally Posted by mltube

Kevin, Thanks, and good eye... The B+ is now set at 312 with some ps mods, and now slight hum has disappeared.
That issue has been resolved this afternoon with the removal of the 1k resistors and intalling a separate choke to load each 27. I do not have a lot of high tech test equipment to work with so it was tested by removing signal from each chan one at a time and listening for signal in speakers.

And the filaments are still AC... I am supprised how quiet this is "refering to hum".

I have set this up with a pair of Hepner 12" full range in OB and the sound that is produced is remarkable to say the least.

Kevin, a couple of months back I was putting this togather, then health stopped the progression of the build. It is nice to hear from you again, as I am a fan of your builds and designs.

Still building every day here, it's sooo addictive.
Have a great day
Mike

Yes, it is addictive.. Thanks for the kind words. My latest project is restoring a Thorens TD-124 MKII turntable and it has come a long way.. It's great that you are back on the amplifier project. I bet it sounds great too, I cut my first SE teeth on a 45 amplifier roughly 12 yrs ago, and had limited expectations, that project was a seminal moment in my design and listening experience. I found to my surprise that 2.5V filament output tubes can run on AC without significant hum issues even with very efficient speakers. The one issue that does exist is the intermodulation distortion spectra on the output caused by AC heating. I generally use CCS for heating output tubes except in most 2.5V applications. (Tubelab can probably elaborate further.) I do however recommend CCS heating for all DHTs regardless of filament voltage or application.