The Red Light District - another PP EL84 amp

[SY]

No reason you couldn't do an autobias version. I personally wouldn't bother- once the tubes burn in a bit, I find that the idle current only needs checking and adjustments every several months.

 

[TUBESMAN]

That is what I thought too. I'd like to do autobias just so the unit is always at peak performance. I will play with this design and see how far off "0" can you go before performance drops. That will help me decide if I need to bother.

 

[PWR RYD]

I'm nearly ready to start ordering all the parts to build a RLD amp. Couple of questions regarding the transformers. I can order the following Dynaclone power tranny and output trannies from Triode Electronics:

SCA35 replacement power trans $59.95
Z565-48 output trans $78.95 each

or I can order the following trannies from Edcor:

CXPP25-MS-8K/23% $56.52 each
XPWR005-120 $43.21

My question is which ones will perform/sound better? Saving money is a good thing but we're not talking about hundreds of dollars so I want to buy the ones that sound best.

 

[PWR RYD]

I read in your article that your Red Light District design is "scalable". I used the search function but didn't find anything. Have you scaled it up with any higher output tubes/parts?

 

[cjkpkg]

These are the edcors I used for my el84 pp amp...

works flawlessly...

XPWR007 120V 60Hz to 550 volt (275-0-275) @ 200mA,
6.3 volt @ 6A, 5.0 volt @ 3A
Power Transformer
RoHS Compliant

CXPP25-MS-7.6K 25Watt 7600 Z ohms to 4, 8, and 16 ohms
40% Screen Voltage
Classic Push Pull Transformer
RoHS Compliant

tube rectified / dc heaters

 

[SY]

I read in your article that your Red Light District design is "scalable". I used the search function but didn't find anything. Have you scaled it up with any higher output tubes/parts?

No, but stay tuned. Best way is paralleling EL84 and dropping the primary impedance.

 

[PWR RYD]

Oh Yeah! I am tuned in!!!!

 

[Tubelab_com]

Best way is paralleling EL84 and dropping the primary impedance.

Wire two EL84's in parallel on either side of the P-P pair. Total of 4 tubes per channel. Use matched tubes, or a seperate LED string for each cathode. If using a common LED string you must double the number of diode strings to handle the additional current of the extra tubes. Use an OPT that is half the impedance that you originally planned.

I was using 4 tubes in a parallel Simple P-P (similar circuit) with a 3300 ohm OPT and seperate white LED strings per tube. 30 WPC was easy. I got greedy and began to explore methods to extract more power. I replaced the LED's with a constant voltage circuit (search for the NLD thread). I was working on the screen regulator and extracting about 60 WPC when it blew up. That was last year and I have not had the time to work on it since.

 

[PWR RYD]

Sorry for the newbie questions... is it really that straight forward? No changes to the driver circuit if I parallel the EL84's and double the LED arrays? I assume I would need to double the output B+ ps current capability.

Would each EL84 get its own (matched) 47k grid stopper resistor or would it be a straight socket pin to socket pin parallel wiring?

Thanks.

 

[SY]

Yeah, it's really that simple. No driver stage changes. Each tube gets its own 47k grid stopper. You may or may not have to double up on the LED arrays, depending on the current rating of the particular LEDs you use. Power supply capacity will need to double. And the output transformer should present a 4k or so impedance. This will give you a solid 35 watts.

 

[PWR RYD]

So an output transformer like this one?

EDCOR Electronics Corporation. CXPP60-MS-4K

 

[SY]

Overkill for 35 watts, but it should work fine.

 

[PWR RYD]

The Edcor CXPP60-MS-4K is 4K pri, 4, 8, 16 sec, 60 watts, 20-20K, at $70.30

From Triode Electronics, a Dyna Clone A470S is 4.3 k pri, 4, 8, 16 sec, 35W, at $106.95

Hammond 1650N from Parts Express is 4.3k pri, 4, 8, 16 sec, 60 watts, at $135.87

Which has the best QC rep. and would sound the best??? Or am I totally missing something else that is better?

Thanks!

 

[SY]

I've used original Dyna from the SCA-35 (very good), UTC LS-52 (also very good), and James (excellent). My own bias is toward James, if you can get 'em. Very mixed experience with Hammond. Good experience with Edcor.

 

[PWR RYD]

I probably should not have listed each of those option's price as that is not my real concern. I am more concerned about getting a quality product that has the right specs to perform in that particular circuit design (i.e. 4k, 3.8k, 4.3k etc.).

I have purchased some Hammond products and was less than satisfied. I have no experience with Edcor but have read mostly good things and only a couple of questionable comments (mostly SET stuff). Dyna Clone products... no idea! I am sure the James transformers are great but I like to buy stuff that doesn't require international shipping.

Any more insight?

 

[SY]

If it were me, if I wanted to stay domestic, and if I didn't have an ex-wife taking all of my disposable income, I'd probably ping BudP and ask him what he could do for you. O-Netics have a great reputation.

 

[Mr_Zenith]

OK, so I've read and re-read almost the entire thread and I've decided to go ahead and build the RLD. While this is not my first DIY tube amp, it will mark my first foray into building one with what Tubelab calls "sand-based life forms" (other than using power supply diodes).

With what I've learned on this forum, I decided to put my $$ into some decent OPT's; it turns out I was able to snag the last two James PP OPT's from tctubes. The only problem is that they're the JS-6225HS model. I figure I can wire the plate circuit up for 10K by using the normal plate taps, then run my speakers through the 4-ohm taps. Since the RLD's output stage runs in pentode mode, I suppose I could also experiment with running the plate circuit through the screen taps and get a primary impedance somewhere around 7.2k. What do you guys think?

Also, I've got a couple of routes I could take for the power supply. The first is to use an old Tektronix 435 PT. This is extreme overkill, but I suppose its doable since it has multiple HV windings and enough heater capacity to light up Chicago. The second is to get one of the Antek toroids with a dual 300V winding.

Just wanted to throw this out there and get some advice on this little bit of phoolery before I did something completely retarded (Which I'm afraid I've probably already done...).

 

[SY]

They should work fine- the speaker's impedance is probably not too close to constant 8 ohms, so you might experiment with which taps to connect it for optimum performance in your setup.

 

[Mr_Zenith]

Thanks, SY. I sort of figured that was the case, so I wasn't going to lose any sleep over impedance ratios. The performance shouldn't be too bad, but if it is I can still swap out transformers later - and I've still got a nice pair o' James 5k jobs for a future project.

What I am concerned about is using a PT with dual HV windings. More specifically, do you suppose that the current fluctuations in the output stage winding could affect the linearity of the input stages? Please forgive my ignorance on this point; I would have done a simulation in PSUDII had there been an option to run parallel supplies from the same transformer. Thanks!

-Larry

 

[SY]

If you've got separate windings, there's probably not much to worry about unless you do some really gross overloading. But remember, you do want to have a higher B+ for the driver stage than for the output.